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Seminars 2019
| Title: | Light propagation beyond standard optics in a uniform-density polarizable medium |
| Speaker: | Professor Janne Ruostekoski |
| Date: | 10 December 2019 |
| Time: | 3pm - 4pm |
| Venue: | Hilbert Space (SPMS-PAP-02-02) |
| Host: | Associate Professor David Wilkowski |
| Abstract: | The interaction of light with ensembles of resonant emitters is becoming increasingly important for both fundamental research and technological applications as experimentalists realize a growing number of such systems. We perform microscopic numerical simulations of light propagation in a uniform-density polarizable medium on an atom-by-atom basis. Comparisons between these essentially exact simulations for cold and dense atomic ensembles and the predictions obtained from the standard electrodynamics of a polarizable medium (EDPM) reveal that the more than a century-old wisdom of conventional textbook optics can dramatically and qualitatively fail. The failure of EDPM is not due to quantum effects, but reflects emergent cooperative phenomena and strong light-induced correlations between the atoms. However, incorporating the effects of thermal motion in hot atom vapours or inhomogeneous resonance broadening restores the usual phenomenology of effective continuous medium electrodynamics. These strong cooperative interactions can be utilized in arrays of atoms and other dipolar emitters, e.g., in preparation of giant subradiant states. |
| Title: | Axions, Dark Matter and Light Polarization |
| Speaker: | Dr Liu Hongwan |
| Date: | 10 December 2019 |
| Time: | 11am - 12pm |
| Venue: | MAS Executive Classroom 1 (SPMS-MAS-03-06) |
| Host: | Assistant Professor Yang Bo |
| Abstract: | The Standard Model of particle physics is one of the most well-tested theories in science. Numerous experimental results, however, strongly indicate that the Standard Model is incomplete. Measurements of the neutron electric dipole moment give a value that is at least ten billion times smaller than expected; at the same time, we know that 85% of all matter in the universe is made up of dark matter, an as yet undiscovered particle unlike any of the ones we know. In this talk, I will give a review of these two mysteries, and explain how both problems can be solved simultaneously by a hypothetical particle known as the axion. In the presence of axion dark matter, Maxwell's equations are modified, leading to new and unexpected electromagnetic behavior that can be detected in the lab. I will discuss a new experimental proposal to search for axion dark matter [1], based on detecting an anomalous rotation of the polarization of light as it propagates within a birefringent laser cavity. [1] H. Liu, B. D. Elwood, M. Evans, and J. Thaler, Phys. Rev. D100, 023548 (2019), arXiv:1809.01656 [hep-ph]. |
| Title: | Recent Developments in Magnetic Materials -High Magnetic Anisotropy in L1_0 -MnGaAl and LTP-MnBi Thin Films- |
| Speaker: | Dr. Takao Suzuki |
| Date: | 9 December 2019 |
| Time: | 1pm - 2pm |
| Venue: | MAS Executive Classroom 1 (SPMS-MAS-03-06) |
| Host: | Associate Professor S.N. Piramanayagam |
| Abstract: | Magnetic materials are classified into three types; “Hard”, “Soft” and “Semi-hard”. One of the key physical quantities to govern such properties is “Magnetic anisotropy”. The energy of magnetic anisotropy varies over a range of 10^-3 to 10^2 μeV/atom, much smaller than atomic binding energy (a few eV/atom) and exchange energy (10~10^3 meV/atom). Nevertheless, it plays a key role in magnetic characteristics. There has been much development in basic and applied magnetism, leading to the significant technological impacts in magnetic recording, spintronics and permanent magnets. Those developments were only made possible through developing and controlling magnetic anisotropy in magnitude and direction for magnetization. The talk presents first the overview of the recent development of high magnetic anisotropy materials. Some of our works performed on rare-earth free permanent magnets such as L1_0 -Mn-Ga and LTP Mn-Bi thin films will be discussed. |
| Title: | Learning from History of Magnetism |
| Speaker: | Dr. Takao Suzuki |
| Date: | 9 December 2019 |
| Time: | 9am - 11am |
| Venue: | MAS Executive Classroom 1 (SPMS-MAS-03-06) |
| Host: | Associate Professor S.N. Piramanayagam |
| Abstract: | This lecture aims to provide the knowledge on basic magnetism and magnetic materials through introducing the history of magnetism to undergraduate- and graduate-level students in all disciplines. 1. Earth Magnetic Field
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| Title: | The stochastic thermodynamics of computation |
| Speaker: | Professor David Hilton Wolpert |
| Date: | 4 December 2019 |
| Time: | 2pm - 3pm |
| Venue: | Conference Room, Research & Graduate Office (SPMS-CBC-02-01) |
| Host: | Assistant Professor Mile Gu |
| Abstract: | One of the central concerns of computer science is how the resources needed to perform a given computation depend on that computation. Moreover, one of the major resource requirements of computers—ranging from biological cells to human brains to high-performance (engineered) computers— is the energy used to run them, i.e. the thermodynamic costs of running them. Those thermodynamic costs of performing a computation have been a long-standing focus of research in physics, going back (at least) to the early work of Landauer and colleagues. However, one of the most prominent aspects of computers is that they are inherently non-equilibrium systems. Unfortunately, the research by Landauer and co-workers on the thermodynamics of computation was done when nonequilibrium statistical physics was still in its infancy, severely limiting the scope and formal detail of their analyses. The recent breakthroughs in non-equilibrium statistical physics hold the promise of allowing us to go beyond those limitations. Here I present some initial results along these lines, concerning the entropic costs of running (loop-free) digital circuits and Turing machines. These results reveal new, challenging engineering problems for how to design computers to have minimal thermodynamic costs. They also allow us to start to combine computer science theory and stochastic thermodynamics at a foundational level, thereby expanding both |
| Title: | Orbital order and nematic superconductivity in twisted bilayer graphene |
| Speaker: | Dr Xingyu Gu |
| Date: | 4 December 2019 |
| Time: | 11am - 12pm |
| Venue: | John Bardeen Room |
| Host: | Assistant Professor Justin Song |
| Abstract: | Recent experiments in twisted bilayer graphene (TBG)^(1-3) show spontaneously rotaGonal symmetry breaking (nemaGc state) in the strongly correlated region. I this talk, I will show that this nemaGc state can be idenGfied as a ferro-orbital state. A strong ferro-orbital order with addiGonal anGferromagneGc order leads to Mo_ insulaGng state at half-filling. The effect of the nemaGc order on pairing symmetry is also studied. It is found that strong exchange interacGon together with the ferro-orbital order leads to a nemaGc superconducGng phase. By diagonalizing the bogoliubov mean field Hamiltonian, the nemaGc superconducGng state is found to be fully gapped in momentum space. References: [1] Jiang, Y., et al. Charge order and broken rotaGonal symmetry in magic-angle twisted bilayer graphene. Nature 573, 91–95 (2019) [2] Choi, Y., et al. Electronic correlaGons in twisted bilayer graphene near the magic angle. Nat. Phys. 15, 1174–1180 (2019) [3] Kerelsky, A., et al. Maximized electron interacGons at the magic angle in twisted bilayer graphene. Nature 572, 95–100 (2019) |
| Title: | Present and Future trends in Metasurface Antennas |
| Speaker: | Professor Stefano MACI |
| Date: | 2 December 20219 |
| Time: | 10am - 11am |
| Venue: | SPMS-LT5 (SPMS-03-08) |
| Host: | Associate Prof Ranjan Singh |
| Abstract: | The word “Metasurface” denotes a surface constituted by PCB or 3D printed elements small in terms of wavelengths that collectively exhibits equivalent homogeneous boundary conditions to the Electromagnetic fields. Metasurfaces (MTSs) are used in microwave as well as in Terahertz and Optics. MTSs have had and are having a strong impact in Antenna applications. In the years 2000-2010 MTS for antennas were essentially uniform in space and realized by periodic printed elements. This was the first generation of MTS. In the present second generation (2010-2020) MTS for antennas are constructed in such a way to change boundary conditions in space to interact and shape the field launched by a feed. Today we are facing a transition to the third generation of MTS antennas, where MTS change boundary conditions in space and time, opening new scenarios in 5G communications and beyond. In this presentation, MTS antennas of past and present generation are reviewed with ideas on possible future communication scenarios. |
| Title: | Two-dimensional monoelemental materials beyond graphene |
| Speaker: | Professor Zdeněk Sofer |
| Date: | 28 November 2019 |
| Time: | 2pm - 3pm |
| Venue: | Hilbert Space (PAP-02-02) |
| Host: | Associate Professor Cesare Soci |
| Abstract: | Monoelemental two-dimensional (2D) materials are at the forefront of current material research. Beyond graphene, which is intensively studied over more than one decade, the other related materials remain almost unexplored. The research activities in the field of other layered materials like phosphorene, black phosphorus monolayer, are rapidly growing in the last few years. Compare to graphene, all these materials are non-zero band-gap semiconductors. This property opens new application possibilities in electronic and optoelectronic devices. Also, the research in the field of energy storage and conversion, as well as sensors and other fields, is rapidly growing. The important properties are based on electronic structure and are strongly related to the material morphology, especially the number of layers. The band-gap energy increases for monolayer materials. It is very important since the rapid increase of band-gap energy allows to utilize the visible region of spectra. The properties of 2D materials can be further controlled by their functionalization. The non-covalent functionalizations are well known in phosphorene chemistry in order to modify its electronic properties and increase its chemical stability towards oxidation. The covalent functionalizations for monoelemental layered material are much less explored and currently exist only a few reports. |
| Title: | Size controlled Silicon QDs: Basic properties & optoelectronic applications |
| Speaker: | Professor M. Zacharias |
| Date: | 28 November 2019 |
| Time: | 10am - 11am |
| Venue: | Hilbert Space (PAP-02-02) |
| Host: | Professor Fan Hongjin |
| Abstract: | The fabrication of SiOx/SiO2 superlattices combined with thermal annealing enables the size and density control of Si quantum dots. The layered-arranged Si quantum dots represent a model system to systematically study the photonic and electronic properties of indirect gap quantum dots prepared in a CMOS compatible way. Hence, the model system is used to understand the interplay of absorption and recombination, the carrier kinetics and the electronic transport properties for matrix embedded Si quantum dots. Starting with the temperature dependence of the size depending band gap the interplay of radiative and non-radiative recombination will be discussed for high quantum yield. Doping of quantum dots and the respective experimental techniques for its quantification are at the very limit of the nowadays experimental possibilities. Systematic doping experiments with P and B will be presented which will be analyzed for doping efficiency into the Si NCs and self-purification effects. |
| Title: | Ultra-low switching energy memories to artificial atoms |
| Speaker: | Professor T Venky Venkatesan |
| Date: | 25 November 2019 |
| Time: | 11am - 12pm |
| Venue: | Conference Room, Research & Graduate Office (SPMS-CBC-02-01) |
| Host: | Associate Professor Elbert Chia |
| Abstract: | Memory devices are responsible for a significant fraction of the energy consumed in electronic systems- typically 25% in a laptop and 50% in a server station. Reducing the energy consumption of memories is an important goal. For the evolving field of artificial intelligence the compatible devices must simulate a neuron. We are working on three different approaches towards these problems- one involving an organic metal centred azo complex, the other involving oxide based ferroelectric tunnel junctions and the last involving real live neuronal circuits. In the organic memristors that we have built on oxide surfaces the device performance exceeds the ITRS roadmap specification significantly demonstrating the viability of this system for practical applications. More than that, these organic memories exhibit multiple states arising from interplay of redox states, counter ion location (studied by in-situ Raman and UV-Vis measurements) and molecular self-assembly leading to the possibility of neuronal systems. These molecular devices are extremely stable and reproducible- a significant departure from conventional organic electronics. On the oxide- front the significant results are that ferroelectric tunnelling is seen even in barriers with single and two atomic layers of BaTiO3 or BiFeO3. Oxygen vacancy motion can also play an important role in changing the device characteristics leading to synaptic characteristics. Last but not the least, oxide surfaces can be utilized to force neurons to grow at specific places on a surface giving the potential for fabricating live neuronal circuits. |
| Title: | Simulation of Spin Chains Using Room-Temperature Polariton Condensates |
| Speaker: | Professor Zhanghai Chen |
| Date: | 21 November 2019 |
| Time: | 11am - 12pm |
| Venue: | Hilbert Space (SPMS-PAP-02-02) |
| Host: | Professor Xiong Qihua |
| Abstract: | Most of the optimization problems of complex systems in real-world remain challenging for conventional digital computers. Some of such problems can be mapped into the Ising model, and then efficiently solved by searching the globe minimum of the Ising Hamiltonian. Finding an effective physical system for the simulation of Ising model is a recently emerged way of addressing such optimization problems. In this talk, analog-spin chains of exciton-polariton condensates at room temperature based on a one-dimensional polariton lattice induced by exciting ZnO microrod with a controllable periodic laser pattern will be presented. Depending on the chosen lattice constant, the spontaneously phase-locked condensates reveal either anti-phase (π) or in-phase (zero) steady-state, which mimics the antiferromagnetic or ferromagnetic one-dimensional classical Ising spin-lattice. In addition, once the external excitation power increases, a chain of coupled condensate pairs with a small phase shift induced by the tunneling effect arises at a lower energy state. These observations pave the way to the realization of analog spin simulators based on the periodic condensates of exciton-polaritons. |
| Title: | Excitons in Two-Dimensional Semiconductor Systems |
| Speaker: | Professor Kai Chang |
| Date: | 20 November 2019 |
| Time: | 11am - 12pm |
| Venue: | Hilbert Space (SPMS-PAP-02-02) |
| Host: | Professor Xiong Qihua |
| Abstract: | Excitons, typical quasi-particles in semiconductors, have been widely studied in the past decades. With the progresses of semiconductor fabrication techniques, there are emerging new physics and phenomena in semiconductor quantum structures, such as semiconductor quantum structures, two-dimensional materials and perverskite materials. In this talk, I will talk about the exciton insulator, which was firstly proposed by Prof. Mott in 1961. When the binding energy of exciton in these systems is larger than the single-particle bandgap, the systems becomes unstable, and open a bandgap forming an exciton insulator phase. This concept has been widely studied theoretically and confirmed experimentally in recent years. Here, We demonstrate theoretically and experimentally the existence of topological exciton insulating phases in two-dimensional (2D) semiconductor systems, based on the multi-band k*p theory and the BCS-like many-body theory. We consider two kinds of systems: InAs/GaSb quantum wells and 2D Van der Waals heterostructures. In InAs/GaSb quantum wells, i.e., a 2D topological insulator, we demonstrate theoretically that the ground state of the system is no longer the 2D topological insulator, but a topological exciton insulator when the Coulomb interaction between electrons and holes is included. For a 2D material, we find that a perpendicular electric field can decrease the bandgap, which even becomes smaller than the exciton binding energy, leading to the formation of exciton insulator phase. Due to large exciton binding energy, the exciton insulator phase in the 2D VdH system could be observed at room temperature. Finally, I will present the exciton BEC in a 2D material under an electric field, which induces a Rashba spin-orbit interaction(RSOI). We find that, due to the small exciton Bohr radius in these materials, high BEC critical temperature becomes reachable. The RSOI couples the bright and dark excitons, and induces exciton vortices in TMD monolayers. The exciton condensation at the K and K’ valleys show mirror-symmetric patterns composed of reversely rotating exciton vortices. |
| Title: | Percolative phase transition in the dynamics of quantum entanglement |
| Speaker: | Dr Brian Skinner |
| Date: | 18 November 2019 |
| Time: | 11am - 12pm |
| Venue: | Conference Room, Research & Graduate Office (SPMS-CBC-02-01) |
| Host: | Assistant Professor Justin Song |
| Abstract: | When left unobserved, many-body quantum systems tend to evolve toward states of higher entanglement. Making a measurement, on the other hand, tends to reduce the amount of entanglement in a many-body system by collapsing one of its degrees of freedom. In this talk I discuss what happens when a many-body quantum system undergoes unitary evolution that is punctuated by a finite rate of projective measurements. Using numerical simulations and theoretical scaling arguments, we show that for a 1D spin chain there is a critical measurement rate separating two dynamical phases. At low measurement rate, the entanglement grows linearly with _me, producing a volume-law entangled state at long times. When the measurement rate is higher than the critical value, however, the entanglement saturates to a constant as a function of _me, leading to area-law entanglement. We map the dynamical behavior of the entanglement onto a problem of classical percolation, which allows us to obtain the critical scaling behavior near the transition. I briefly discuss generalizations of our result to higher dimensions, and its implications for the difficulty of simulating quantum systems on classical computers. |
| Title: | Excited Carriers in Metals: from icy cold to comfortably warm to scalding hot |
| Speaker: | Professor Jacob B Khurgin |
| Date: | 15 November 2019 |
| Time: | 1.30pm - 2pm |
| Venue: | MAS EC ROOM 1(SPMS-MAS-03-06) |
| Host: | Professor Xiong Qihua |
| Abstract: | The field of plasmonics in recent years has experienced a certain shift in priorities. Faced with undisputable fact that loss in metal structures cannot be avoided, or even mitigated (at least not in the optical and near IR range) the community has its attention to the applications where the loss may not be an obstacle, and, in fact, can be put into productive use. Such applications include photodetection, photo-catalysis, and others where the energy of plasmons is expended on generation of hot carriers in the metal. Hot carriers are characterized by short lifetimes, hence it is important to understand thoroughly their generation, transport, and relaxation in order to ascertain viability of the many proposed schemes involving them. In this talk we shall investigate the genesis of hot carriers in metals by investigating rigorously and within the same quantum framework all four principle mechanisms responsible for their generation: interband transitions, phonon-and-defect assisted intraband processes, carrier-carrier scattering assisted transitions and Landau damping. For all of these mechanisms we evaluate generation rates as well as the energy (effective temperature) and momenta (directions of propagation) of the generated hot electrons and holes. We show that as the energy of the incoming photons increases towards the visible range the electron-electron scattering assisted absorption becomes important with dire consequences for the prospective “hot electron” devices as four carriers generated in the process of the absorption of a single photon can at best be characterized as “lukewarm” or “tepid” as their kinetic energies may be too small to overcome the potential barrier at the metal boundary. Similarly, as the photon energy shifts further towards blue the interband absorption becomes the dominant mechanism and the holes generated in the d-shell of the metal can at best be characterized as “frigid” due to their low velocity. It is the Landau damping process occurring in the metal particles that are smaller than 10nm that is the most favorable on for production of truly “hot” carriers that are actually directed towards the metal interface. We also investigate the relaxation processes causing rapid cooling of carriers. Based on our analysis we make predictions about performance characteristics of various proposed plasmonic devices. |
| Title: | A fully error-corrected logical quantum bit encoded in grid states of a superconducting cavity |
| Speaker: | Dr Steven Touzard |
| Date: | 14 November 2019 |
| Time: | 11am - 12pm |
| Venue: | Hilbert Space (SPMS-PAP-02-02) |
| Host: | Associate Professor David Wilkowski |
| Abstract: | The operation of universal quantum computers is easily derailed by noise that modifies the state of physical qubits, causing logical errors. Fortunately, such errors can be detected and corrected if quantum information is encoded non-locally. Applying this idea to hardware efficient bosonic codes, Gottesman Kitaev and Preskill proposed to encode a protected qubit into states forming grids in the phase-space of a harmonic oscillator. Here, we prepare and stabilise such a qubit using repeated applications of a novel gate sequence on a superconducting microwave cavity. We demonstrate significant suppression of all logical errors, in quantitative agreement with a theoretical estimate based on the measured imperfections of the experiment. Our results are applicable to other continuous variable systems and, in contrast with previous implementations of quantum error correction, can mitigate the impact of a wide variety of noise processes and open a way towards fault-tolerant quantum computation. |
| Title: | Quantum excitations of "hidden orders" and thermal transports of "hidden particles" |
| Speaker: | Dr Chen Gang |
| Date: | 11 November 2019 |
| Time: | 11am - 12pm |
| Venue: | Conference Room, Research & Graduate Office (SPMS-CBC-02-01) |
| Host: | Assistant Professor Yang Bo |
| Abstract: | "Hidden order", that was first proposed for URu2Si2, has been a long-standing issue in modern condensed matter physics. Here we provide new insights about the identification of the nature of the hidden orders. We apply our insights to the understanding of the peculiar quantum orders and excitation spectra in the triangular lattice magnet TmMgGaO4. In the second part of the talk, we will provide new insights about the emergent Lorentz force on the neutral particles in spin liquids and explain the physical origin of the thermal Hall effects of these topological excitations. |
| Title: | 2D Semiconductor optoelectronic devices |
| Speaker: | Dr Goki Eda |
| Date: | 4 November 2019 |
| Time: | 11am - 12pm |
| Venue: | Conference Room, Research & Graduate Office (SPMS-CBC-02-01) |
| Host: | Assistant Professor Justin Song |
| Abstract: | New materials are of fundamental importance for developing disruptive technologies. A large family of two-dimensional (2D) materials, each having unique properties associated with its ultimate thinness and often distinct symmetry, offer tremendous opportunities to explore novel physics and device concepts. In particular, 2D layered semiconductors and their heterostructures are building blocks for realizing unconventional nano-electronic, photonic and optoelectronic devices. In this talk, I will primarily discuss the unique optoelectronic response of 2D semiconductor devices that reveal the fundamental role of many-body interaction and ultrafast interlayer charge transfer dynamics. I will start by discussing the conduction mechanism of light-emitting tunnel diode consisting of few-layer graphene (FLG), hexagonal boron nitride (hBN), and monolayer WS2. We find that this device exhibits electrically assisted upconversion of near-infrared (NIR) to visible light, which evidences the role of hot carrier tunnelling. We further investigate the role of exciton–exciton annihilation (EEA), a four-body interaction involving the energy and momentum transfer between two holes and two electrons. Our tunnel diode devices exhibit unexpected photoresponse due to generation of non-equilibrium high energy carriers derived from EEA in monolayer semiconductors. In the last part, I will discuss our recent findings on the peculiar magnetic properties of Cr2Ge2Te6, a ferromagnetic semiconductor. We show how heavy electron doping of this material by ionic gating leads to substantial modulation of its Curie temperature and magnetic anisotropy. |
| Title: | Landscape and generalisation in deep learning |
| Speaker: | Associate Professor Matthieu Wyart |
| Date: | 22 October 2019 |
| Time: | 2pm - 3pm |
| Venue: | Hilbert Space (SPMS-PAP-02-02) |
| Host: | Assoc Prof Massimo Pica Ciamarra |
| Abstract: | Deep learning is very powerful at a variety of tasks, including self-driving cars and playing go beyond human level. Despite these engineering successes, why deep learning works remains unclear; a question with many facets. I will discuss two of them: (i) Deep learning is a fitting procedure, achieved by defining a loss function which is high when data are poorly fitted. Learning corresponds to a descent in the loss landscape. Why isn’t it stuck in bad local minima, as occurs when cooling glassy systems in physics? What is the geometry of the loss landscape? (ii) in recent years it has been realised that deep learning works best in the over-parametrised regime, where the number of fitting parameters is much larger than the number of data to be fitted, contrarily to intuition and to usual views in statistics. I will propose a resolution of these two problems, based on both an analogy with the energy landscape of repulsive particles and an analysis of asymptotically wide nets. |
| Title: | Charge injection for 2D materials contact and fractional modeling |
| Speaker: | Professor Ricky LK Ang |
| Date: | 21 October 2019 |
| Time: | 11am - 12pm |
| Venue: | Conference Room, Research & Graduate Office (SPMS-CBC-02-01) |
| Host: | Assistant Professor Justin Song |
| Abstract: | Electron injection from a material through an interface to another material (including vacuum) is a fundamental process in device physics. Depending on the energy used for the charge injection, it can be broadly characterized into 3 different processes known as thermionic emission TE (by thermal energy), field emission FE (by quantum tunneling) and photoemission PE (by absorption of photons or optical tunneling). The basic models for these processes (TE, PE, PE) have been formulated many decades ago, known as the Richardson law, Fowler-Nordheim (FN) law, and photo-electric effect or Keldysh model. With the development of two-dimensional (2D) atomic scale materials in the 2000’s, the above-mentioned traditional laws are unable to explain experimental results, which require new revision to account for the novel 2D material properties, as well as structure at nanometer dimensions and dynamics at ultrashort time scales. In the first part of this talk, I will present such revised models in order to show smooth transition from the traditional models and to show good agreements with experimental results. On the second part, we will share some recent works in using fractional calculus is solving some complicated objects in physics and engineering to indicate some advantages in using fractional models to provide useful scaling laws that agree with more expensive computational approaches. |
| Title: | Perturbative Frictional Jamming and its relation to electron transport in disordered media |
| Speaker: | Dr Mahesh Bandi |
| Date: | 21 October 2019 |
| Time: | 11am - 12pm |
| Venue: | Hilbert Space (PAP-02-02) |
| Host: | Assistant Professor Yong Ee Hou |
| Abstract: | It is well known that external perturbations evolve a frictional granular pack jammed in an initial metastable configuration to an eventual stable one Beneficial in achieving efficient packing, athermal perturbations can also cause failure Understanding pack response to perturbations naturally carries both fundamental and applied significance In a related context, the power law pressure P increase against packing fraction phi is considered one signature of the frictionless jamming transition In contrast, independent studies reveal frictional jamming exhibits an initial exponential pressure rise before deviating towards the putative power law The range of phi over which pressure rises exponentially is marked by a marginally stable solid (fragile state) sensitive to perturbations In this talk, I report experiments on frictional granular pack pressure response to controlled perturbations in this fragile state. In particular, I will deduce an empirical result from the experimental data which establishes a close correspondence between this classical (frictional jamming) problem and a quantum effect, viz. Hopping conduction mechanism for electron transport in amorphous semiconductors. |
| Title: | Multilayer networks |
| Speaker: | Professor Mikko Kivelä |
| Date: | 15 October 2019 |
| Time: | 11am - 12pm |
| Venue: | Hilbert Space (SPMS-PAP-02-02) |
| Host: | Associate Prof Chew Lock Yue |
| Abstract: | Network science has been very successful in investigations of a wide variety of applications from biology and the social sciences to physics, technology, and more. In many situations, it is already insightful to use a simple (and typically naive) representation as a simple, binary graph in which nodes are entities and unweighted edges encapsulate the interactions between those entities. This allows one to use the powerful methods and concepts for example from graph theory, and numerous advances have been made in this way. However, as network science has matured and (especially) as ever more complicated data has become available, it has become increasingly important to develop tools to analyse more complicated structures. For example, many systems that were typically initially studied as simple graphs are now often represented as time-dependent networks, networks with multiple types of connections, or interdependent networks. This has allowed deeper and more realistic analyses of complex networked systems, but it has simultaneously introduced mathematical constructions, jargon, and methodology that are specific to research in each type of system. Recently, the concept of "multilayer networks“ was developed in order to unify the aforementioned disparate language (and disparate notation) and to bring together the different generalised network concepts that included layered graphical structures. In this talk, I will introduce multilayer networks and discuss how to study their structure. Generalisations of the clustering coefficient for multiplex networks and graph isomorphism for general multilayer networks are used as illustrative examples. |
| Title: | Noncentrosymmetric superconductivity |
| Speaker: | Professor Yuan Huiqiu |
| Date: | 4 October 2019 |
| Time: | 11am - 12pm |
| Venue: | Conference Room, Research & Graduate Office (SPMS-CBC-02-01) |
| Host: | Associate Professor Elbert chia |
| Abstract: | Noncentrosymmetric superconductivity has attracted considerable
attention in recent years [1]. While a superconductor lacks an inversion
center in its crystal structure, the an[symmetric spin-orbital coupling
(ASOC) may split the spin degeneracy of conduction electrons and
allows the admixture of spin-singlet and spin-triplet pairing state,
leading to rich physical properties. In this talk, I will overview the recent
progress on the study of noncentrosymmetric superconductors,
covering heavy fermions systems, weakly correlated systems with
different spin-orbit coupling strength and the more recent work of
superconductivity with broken [me reversal symmetry. |
| Title: | Parton paradigm for the quantum Hall effect |
| Speaker: | Dr Ajit C. Balram |
| Date: | 23 September 2019 |
| Time: | 11am - 12pm |
| Venue: | Conference Room, Research & Graduate Office (SPMS-CBC-02-01) |
| Host: | Assistant Professor Yang Bo |
| Abstract: | Two-dimensional electrons subjected to a strong perpendicular magnetic field and cooled to low-temperatures exhibit the marvelous phenomena of the fractional quantum Hall effect (FQHE). The strong magnetic field quenches the kinetic energy and FQHE essentially arises from interactions between the electrons. Alongside superconductivity, Bose-Einstein condensation and spin-liquids, FQHE provides a paradigm in our understanding of collective behavior of quantum many-body systems. FQHE in the lowest Landau level (LLL) is understood in a unified manner in terms of composite fermions, which are bound states of electrons and vortices. The strongest states in the LLL are understood as integer quantum Hall states of composite fermions and the compressible 1/2 state as a Fermi liquid of composite fermions. For the FQHE in the second LL, such a unified description does not exist: experimentally observed states are described by different physical mechanisms. In this talk, I will discuss our first steps towards a unified understanding of states in the second LL using the ``parton" theory. I will elucidate in detail our recent work on the parton construction of wave functions to describe many of the FQH states observed in the second LL. |
| Title: | Resistivity from extremely correlated Fermi liquid theory |
| Speaker: | Professor Sriram Shastry |
| Date: | 20 September 2019 |
| Time: | 11am - 12pm |
| Venue: | Conference Room, Research & Graduate Office (SPMS-CBC-02-01) |
| Host: | Associate Professor Pinaki Sengupta |
| Abstract: | A qualitative overview of the problem of the strongly correlated
problem is given with emphasis on the electrical resistivity. This is one
of the important unsolved problems in Condensed Matter physics. The
recently advanced Extremely Correlated Fermi liquid (ECFL) theory has
yielded encouraging results, which will be reviewed.
The talk is specially geared at non-specialists and the technical details
are avoided as far as possible. Details of the work can be found in the
web-based reprint collection: |
| Title: | Recent advances in van der Waals heterostructure-based Spintronics |
| Speaker: | Professor Stephan Roche |
| Date: | 9 September 2019 |
| Time: | 11am - 12pm |
| Venue: | Conference Room, Research & Graduate Office (SPMS-CBC-02-01) |
| Host: | Associate Professor Gao Weibo and Assistant Professor Justin Song |
| Abstract: | I will discuss how physical properties of graphene can be strongly enriched and manipulated by harvesting the large amount of possibilities of proximity effects with magnetic insulators, strong spin-orbit coupling SOC materials such as transition metal dichalcogenides (TMD) and topological insulators (TI). First I will introduce some foundations of spin transport for Dirac fermions propagating in supported graphene devices or interfaced with strong SOC materials, with a particular emphasis on how spin dynamics is monitored by the nature of SOC induced in graphene by nearby TMDs and TIs. Such proximity effect will be revealed by giant spin lifetime anisotropy, with spins oriented in the graphene plane relaxing much faster than spins pointing out of the plane. This anisotropy, arising from the specific nature of the SOC induced in the graphene layer and crucially on the symmetry of the graphene/TMD & TI interfaces, also inspires ways for manipulating spin properties using proximity effects, such as inducing and tailoring Spin Hall effect by proximity effects. Finally, I will present some spin transport results in quasiballistic graphene devices, as well as some universal features in polycrystalline graphene, all results reinforcing the promising future of graphene and other 2D materials in improving mainstream spin-based memories or advancing spin logics technologies. |
| Title: | Optical Absorption, Photothermal Effect and Thermal Radiation at Subwavelength Scale |
| Speaker: | Professor Min Qiu |
| Date: | 3 September 2019 |
| Time: | 2pm - 3pm |
| Venue: | Hilbert Space (SPMS-PAP-02-02) |
| Host: | Professor Xiong Qihua |
| Abstract: | The ability to control optical absorption and thermal emission facilitates a wide range of applications including infrared detectors, thermophotovoltaics, radiative cooling and infrared camouflage. For optical absorption, different methods have been proposed to realize narrowband absorbers with high quality factor for sensing and broadband absorbers for energy harvesting. For thermal emission, the dynamic control of the emissivity has been an attractive research topic recently. Incorporating with materials such as semiconductor quantum wells, graphene and phase change materials, the emissivities of thermal emitters can be continuously tuned. Here, we will discuss some progresses in both optical absorption enhancement and thermal emission control. First, both plasmonic and dielectric nanostructures are proposed to realize narrowband and broadband optical absorbers. Next, dynamic control of the emissivities of thermal emitters with zero static power consumption based on phase-change material Ge2Se2Te5 (GST) are illustrated. Finally, we introduce some inspiring applications of thermal emission control, such as thermal camouflage and thermal management. |
| Title: | Spatio-temporal electronic correlations: From quantum criticality to π-tons |
| Speaker: | Dr Karsten Held |
| Date: | 2 September 2019 |
| Time: | 10.30am - 11.30am |
| Venue: | Conference Room, Research & Graduate Office (SPMS-CBC-02-01) |
| Host: | Assistant Professor Marco Battiato |
| Abstract: | Electronic correlations give rise to fascinating physical phenomena such as high temperature superconductivity and (quantum) criticality, but their theoretical description remains a grand challenge. Dynamical mean field theory has been a big step forward: it accurately describes the local electronic correlations including their quantum, temporal dynamics. In recent years diagrammatic extensions of dynamical mean field theory, such as the dynamical vertex approximation, have been developed. These methods not only include the dynamics but also non-local correlations on all length scales [1]. After a brief introduction to these methods, I will present some recent highlights: the discovery of a new universality class of quantum critical exponents in the Hubbard model [2], the description of quantum criticality in the periodic Anderson model [3], and the discovery of new polaritons in strongly correlated electron systems, coined π-tons[4]. [1] G. Rohringer, H. Hafermann, A. Toschi, A. A. Katanin, A. E. Antipov, M. I. Katsnelson, A.
I. Lichtenstein, A. N. Rubtsov, and K. Held, Rev. Mod. Phys. 90, 025003 (2018)
|
| Title: | Failure of Nielsen-Ninomiya theorem and fragile band topology of twisted bilayer graphene |
| Speaker: | Dr Bohm Jung Yang |
| Date: | 29 August 2019 |
| Time: | 11am - 12pm |
| Venue: | Conference Room, Research & Graduate Office (SPMS-CBC-02-01) |
| Host: | Assistant Professor Justin Song |
| Abstract: | I am going to talk about novel topological properties of twisted bilayer graphene at magic angle, a representative spinless fermion system with space time inversion IST symmetry. We show that the Wannier obstruction and the fragile topology of the nearly flat bands in twisted bilayer graphene at magic angle are manifestations of the nontrivial topology of two-dimensional real wave functions characterized by the Euler class. To prove this, we examine the generic band topology of two dimensional real fermions in systems with space-time inversion I_{ST} symmetry. The Euler class is an integer topological invariant classifying real two band systems. We show that a two-band system with a nonzero Euler class cannot have I_{ST}-symmetric Wannier representation. Moreover, a two-band system with the Euler class e_{2} has band crossing points whose total winding number is equal to 2e_{2}. Thus the conventional Nielsen-Ninomiya theorem fails in systems with a nonzero Euler class. We propose that the topological phase transition between two insulators carrying distinct Euler classes can be described in terms of the pair creation and annihilation of vortices accompanied by winding number changes across Dirac strings. When the number of bands is bigger than two, there is a Z_{2} topological invariant classifying the band topology, that is, the second Stiefel Whitney class (w_2). Two bands with an even (odd) Euler class turn into a system with w_2=0 (w_2=1) when additional trivial bands are added. Although the nontrivial second Stiefel-Whitney class remains robust against adding trivial bands, it does not impose Wannier obstruction when the number of bands is bigger than two. However, when the resulting multiband system with the nontrivial second Stiefel-Whitney class is supplemented by additional chiral symmetry, a nontrivial second-order topology and the associated corner charges are guaranteed. |
| Title: | Topological and nonreciprocal dynamics in non-Hermitian systems |
| Speaker: | Dr Xu Haitan |
| Date: | 26 August 2019 |
| Time: | 11am - 12pm |
| Venue: | Conference Room, Research & Graduate Office (SPMS-CBC-02-01) |
| Host: | Assistant Professor Yang Bo |
| Abstract: | Non-Hermitian systems exhibit rich physical phenomena that open the door to qualitatively new forms of control. Here I will introduce our recent works on topological and nonreciprocal dynamics in non-Hermitian optomechanical systems [Nature 537, 80 (2016); Nature 568, 65 (2019)]. Specifically, we realized topological energy transfer by adiabatically encircling an exceptional point (a singularity of the complex spectrum). We also demonstrated that this energy transfer is non-reciprocal: a given topological operation can only transfer energy in one direction. Furthermore, we realized stable nonreciprocal coupling between phonon modes with optomechanical interference. We achieved phonon isolation which can be tuned continuously over a wide range from -30dB to 30dB. We showed the nonreciprocal phonon transfer as a new way to control the thermal fluctuation of phonon modes by controlling laser phases. |
| Title: | Using electron transport in few-layer graphene to probe role of symmetry and quantum oscillation phases |
| Speaker: | Professor Mandar Deshmukh |
| Date: | 19 August 2019 |
| Time: | 11am - 12pm |
| Venue: | Conference Room, Research & Graduate Office (SPMS-CBC-02-01) |
| Host: | Assistant Professor Justin Song |
| Abstract: | Two dimensional materials are of great interest for fundamental science and applications. There have been extensive studies on monolayer graphene where unique properties result from the symmetry of the honeycomb lattice. Few layer graphene systems are of interest as they offer interesting opportunities to study effect of electronic interacJons1 while monolayer graphene was largely understood in terms of independent electron picture. In addition, few layer graphene offer an opportunity to break simple symmetries and study their consequence^2. Our recent experiment reveals that, surprisingly, a trivial band can inherit non-trivial quantum oscillation phase from Berry’s phase of the linear Dirac band due to strong filling-enforced constraints between the linear and quadratic band Fermi surfaces that co-exist3. Given that many topological materials contain multiple bands, our work indicates how additional bands, which are thought to obscure the analysis, can actually be exploited to tease out the subtle effects of Berry’s phase. 1. Datta et al. Nature Communications 8, 14518 (2017). 2. Datta et al. Physical Review Letters 121, 056801 (2018). 3. Datta et al. arxiv 1902.04264 (Science Advances in press). |
| Title: | Revealing local and nonlocal relaxation processes in 2D glass-forming liquids |
| Speaker: | Assistant Professor Hayato Shiba |
| Date: | 15 August 2019 |
| Time: | 11am - 12pm |
| Venue: | Hilbert Space (SPMS-PAP-02-02) |
| Host: | Associate Professor Massimo Pica Ciamarra |
| Abstract: | If a liquid is cooled rapidly to form a glass, its structural relaxation becomes retarded. In
two dimensions, however, strong fluctuations make it difficult to evaluate such
retardation in the dynamics. In this talk, it will be demonstrated that the dynamics in 2D
glass-forming liquids can be fully understood as a superposition of relaxation processes
coming from local and nonlocal fluctuations.
References [1] H. Shiba, Y. Yamada, T. Kawasaki, and K. Kim, Phys. Rev. Lett. 117, 245701/1-6 (2016) [2] H. Shiba, P. Keim, and T. Kawasaki, J. Phys.: Cond. Matter 30, 094004/1-9 (2018) [3] H. Shiba, T. Kawasaki, and K. Kim, preprint arXiv:1905.05458 (2019) |
| Title: | RECREATIONAL PHYSICS an Odyssey student - initiative for physics undergraduates |
| Speaker: | Mr Ha Quang Trung |
| Date: | 14 August 2019 |
| Time: | 1.30pm |
| Venue: | MAS Executive Classroom 2 (MAS-03-08) |
| Host: | - |
| Abstract: | Recreational Physics is a semester-long workshop designed to challenge students' understanding of physics concepts and encourage collaborative learning and problem solving. This program is entirely run by students, for students, therefore those who are keen on pushing their limits can look forwards to intensive yet fun discussion sessions. |
| Title: | Quantum simulation of charge transfer and atomic motion at the atomic scale |
| Speaker: | Dr Yongqing Cai |
| Date: | 5 August 2019 |
| Time: | 11am - 12pm |
| Venue: | Conference Room, Research & Graduate Office (SPMS-CBC-02-01) |
| Host: | Assistant Professor Justin Song |
| Abstract: | Flow of energy in functional materials and devices is always accompanied with the transport of charge. Under various external field stimuli like the thermal and electric fields also tend to trigger the motion of nuclei away from their equilibrium positions. Therefore, understanding the transport mechanism and response of charge carriers and nuclei with external perturbation are critical to the design, synthesis, characterization and application of functional materials. However, such atomic-scale processes and phenomena tend to be hard to be measured and understood with traditional experimental measurements. With the aid of state-of-art modern supercomputer and the progress of advanced algorithm, nowadays computer simulation becomes an increasingly important and effective approach in the materials research, complementing the traditional experimental studies. In this talk, first-principles calculations based on density functional theory for exploring the charge transfer at interface of materials, and vibrational and defective properties of two-dimensional materials such as MoS2 and phosphorene will be presented. |
| Title: | 2D Atomic and Molecular Lattices: Rational Synthesis and New Properties |
| Speaker: | Professor Thomas J. Kempa |
| Date: | 25 July 2019 |
| Time: | 10.30am - 11.30am |
| Venue: | Hilbert Space (SPMS-PAP-02-02) |
| Host: | Professor Xiong Qihua |
| Abstract: | Our group develops rational chemical solutions to challenges in materials research. This talk will focus on our recent efforts in synthesizing 2D atomic and molecular lattices and in discovering new properties within them. Atomic Lattices: 2D transition metal dichalcogenides (TMD) have enjoyed widespread adoption in opto-electronic, catalytic, and device studies. However, methods offering explicit control over the dimensionality, morphology, and crystalline phase of TMDs are rare. We show that Si substrates bearing phosphide moieties can control the dimensionality and morphology of MoS2 crystals grown over them. On surfaces with a high density of Si–P dimers, MoS2 crystals form 1D-like ribbons, which are largely singlelayer, exceptionally uniform, and of the semiconducting 2H phase. The widths of these 1D crystals can be tuned from the nano- to the micron-scale. Cluster expansion and density functional theory calculations support a mechanism for the substrate-directed growth responsible for transforming MoS2 from a 2D to 1D crystal morphology. Moreover, 1D MoS2 crystals exhibit a significantly blue shifted photoluminescence (PL), compared to 2D crystals, at room temperature. Notably, this PL is precisely and progressively tunable through synthetic control of the 1D crystal width. Molecular Lattices: Metalorganic frameworks (MOFs) are versatile materials that have been used as tunable scaffolds for energy storage, catalysis, and separations. We are focused on developing new approaches for the synthesis and characterization of hierarchically structured and stimuli responsive MOFs. We demonstrate the synthesis of 2D MOFs composed of molecular complexes containing strongly-coupled di-Mo cores. These materials exhibit anomalous gas adsorption characteristics, redox activity, and photo-tunable charge transport. Notably, we demonstrate the versatility of chemical vapor deposition and the unique opportunities this method presents for the preparation of layered 2D MOFs. We show that single crystal device studies allow for not only detailed investigation of charge transport mechanisms within these materials, but also in situ identification of the unique response of these MOFs to optical, electronic, and chemical stimuli. Collectively, our studies underscore the importance of rational synthesis in elaborating materials with unique and prescribed properties. |
| Title: | Role of interlayer interactions in stacked 2D crystals |
| Speaker: | Professor Young Woo Son |
| Date: | 22 July 2019 |
| Time: | 11am - 12pm |
| Venue: | Conference Room, Research & Graduate Office (SPMS-CBC-02-01) |
| Host: | Professor Yu Ting / Assistant Professor Justin Song |
| Abstract: | Interlayer interactions in stacked 2D crystals are one of key ingredients in exhibiting their qualitative different physical properties. For example, a well-known 2D quantum spin Hall insulating transition metal dichalcogenide can show either topological Weyl metallic phase or trivial one depending on a minute stacking order difference. Recent advances in fabricating stacked 2D crystals enable us to perform controlled studies on interesting electronic, magnetic and topological properties in low dimensional heterostructures. In this talk, I will first discuss interplay between interlayer interactions and electronic properties in graphene bilayer systems. And I will also briefly discuss a possible modification in its phonon spectrum. Regarding on electronic properties, I will show that the system has a quasicrystalline order through a perfect incommensurate interlayer interaction when two graphene rotate 30 degrees with respect to each other and shows localized 12-fold resonant states with fractal scaling. In addition to existing 2D materials, if time allowed, I will also introduce a new computational scheme to search a new family of 2D crystals that will expand both material and property spaces of layered crystals. |
| Title: | Novel photonics for optical beam steering and novel 3D fabrication toward complex metamaterials |
| Speaker: | Dr Josué J. López |
| Date: | 22 July 2019 |
| Time: | 11am - 12pm |
| Venue: | Hilbert Space (PAP-02-02) |
| Host: | Associate Professor Zhang Baile / Associate Professor Chong Yidong |
| Abstract: | Progress in nanophotonic design and nanofabrication techniques has fueled strong interest in designing novel and compact devices for imaging, sensing, and optical communication. This talk will discuss two developments. The first is a new planar-lens photonic integrated circuit design for optical beam steering. The second is a new 3D nanofabrication technique that allows for complex nanostructures with feature sizes in the tens of nanometers. Current leading LIDAR solutions use one-dimensional (1D) phased-array antennas to steer a coherent beam bi-directionally. Interestingly, the RF Radar literature contains lens-based beam steering solutions (e.g. Rotman lenses) that overcome major challenges found in phased arrays. Their photonic analogs have not yet been investigated and are a viable solution for optical beam steering. Herein, we demonstrate the first planar-lens-based beam steering device that functions at λ = 1550 ± 50 nm and is fabricated with all-dielectric materials. The 1st generation design has a total angular range of 41.0° x 12.0°. Moreover, we present a 2nd generation design for a planar generalized Luneburg. The Luneburg Lens device has a theoretical in-plane field of view of 160° with no off-axis aberrations. This planar-lens-based approach opens a path towards chip-scale beam steering at low size, weight, power consumption, and cost (SWaP-C). In the second part of the talk, we discuss the potential of a new 3D nanopatterning technique with feature sizes in the tens of nanometers. This Implosion Fabrication (ImpFab) uses the anchoring of nanoparticles with nanoscale precision in a hydrogel (and its subsequent volumetric shrinking) to create complex 3D nanostructures. Potential metamaterial applications will be discussed. |
| Title: | Remarkable simplicity out of complexity: role of the 2nd law of thermodynamics in kinetic models for complex systems |
| Speaker: | Dr. Rho Shin Myong |
| Date: | 17 July 2019 |
| Time: | 11am - 12pm |
| Venue: | Hilbert Space (PAP-02-02) |
| Host: | Assistant Professor Marco Battiato |
| Abstract: | Kinetic models were developed to describe the complex systems. They include the Boltzmann, quantum Boltzma nn, Boltzmann-Curtiss, generalized Boltzmann, Vlasov-Landau, Balescu-Lenard, and Fokker-Planck equations. The applications of these complicated mathematical models are indeed diverse: molecular motion in gases and li quids, electron transport in semiconductors, viscoelastic fluids, granular flows, multi-phase flows, soft matter, biol ogical system, and non-physical systems like stock market. Nonetheless, there is a remarkable simplicity; all those models share microscopic interactions among particles a nd their interplay with the kinematic motion of particles in the macroscopic framework, and the closure problem t o close the open high-order hierarchical systems. In the case of closure problem, the 2 nd law of thermodynamics is supposed to play a key role. This talk will review various approaches developed in the past to solve these problems and then present a recent approach based on the closing-last 2 nd-order closure theory and its implementation to the practical multidimensional rarefied and microscale gas flows within the framework of the 2 nd-order nonlinear coupled constitutive relations (NCCR) and the mixed-type modal discontinuous Galerkin method |
| Title: | Quantum Nanoscience: Atoms on Surfaces |
| Speaker: | Professor Andreas J. Heinrich |
| Date: | 28 June 2019 |
| Time: | 11am - 12pm |
| Venue: | Hilbert Space (SPMS-PAP-02-02) |
| Host: | Assistant Professor Bent Weber |
| Abstract: | The scanning tunneling microscope is an amazing tool because of its atomic-scale spatial resolution. This can be combined with the use of low temperatures, culminating in precise atom manipulation and spectroscopy with microvolt energy resolution. In this talk we will apply these techniques to the investigation of the quantum spin properties of magnetic atoms sitting on thin insulating films. We will start our exploration with the understanding of the quantum spin states (also called the magnetic states) of these adsorbates. To measure these states, we combined scanning tunneling with x-ray absorption spectroscopy and found amazing agreement of those vastly Different Techniques (Science 2014, PRL 2015). Next, we will investigate the lifetimes of excited states. Surprisingly, we find life times that vary from nanoseconds to hours, a truly amazing consequence of the quantum states of different adsorbates. Finally, we will explore the superposition of quantum states which is inherent to spin resonance techniques. We recently demonstrated the use of electron spin resonance on single Fe atoms on MgO (Science 2015). This technique combines the power of STM of atomic scale spectroscopy with the unprecedented enery resolution of spin resonance techniques, which is about 10,000 times better than normal spectroscopy. |
| Title: | New Schemes of Room-Temperature Solution-Processes for High Performance Organic and Perovskite Optoelectronic Devices |
| Speaker: | Professor Wallace C.H. Choy |
| Date: | 26 June 2019 |
| Time: | 10.30am - 11.30am |
| Venue: | Hilbert Space (SPMS-PAP-02-02) |
| Host: | Professor Sum Tze Chien |
| Abstract: | While high temperature evaporation and sputtering are commonly used for forming films of optoelectronic devices, we will discuss our approaches toward room-temperature solution process for forming various devices. Regarding the room-temperature processed metal-oxide based carrier transport layer, we demonstrate their good electron and hole transport properties for all solution processed organic/inorganic optoelectronics such as organic solar cells (OSCs), perovskite solar cells, dye sensitized solar cells, organic light emitting diodes, etc which can favor the efficient transport of carriers between the photoactive layer and electrode as well as high optical transparency. With the incorporation of metal nanoparticles, the electrical and optical properties can be enhanced. The interesting features of the novel carrier extraction layers are low temperature, solution process and water free for high performance optoelectronics such as OSCs with power conversion efficiency (PCE) of 15.8%. In addition, we have developed some room temperature processed Ag nano-network which can serve as transparent flexible electrodes. With the knowledge of solution processed organic and inorganic materials, we also propose different low/room temperature approaches for highly stable and efficient perovskites in solar cells, LEDs and photodetectors in which the solar cells show no hysteresis and most recent PCE of 21.5%. |
| Title: | Observation of Majorana bound states and conductance plateau in an iron-based superconductor |
| Speaker: | Professor Gao Hong-Jun |
| Date: | 26 June 2019 |
| Time: | 9.30am - 10.15am |
| Venue: | SPMS-LT5 (SPMS-03-08) |
| Host: | Professor Xiong Qihua / Assistant Professor Bent Weber |
| Abstract: |  |
| Title: | Gap-dependent scanning tunneling microscopy: from point contact and Josephson junction to orbital selective imaging |
| Speaker: | Professor Yukio Hasegawa |
| Date: | 26 June 2019 |
| Time: | 10.20am - 11.05am |
| Venue: | SPMS-LT5 (SPMS-03-08) |
| Host: | Professor Xiong Qihua / Assistant Professor Bent Weber |
| Abstract: | In scanning tunneling microscopy one probes decaying wave functions of electronic states from a sample surface using a sharp
needle tip located at very proximate distance from the surface. We usually do not care how the tip is close to the surface as far
as the tunneling current flowing between them. There are, however, several cases where the tip-sample gap distance matters,
and by carefully analyzing the gap variation one can extract information that cannot be accessible by other methods. Here
some of such examples are demonstrated in my presentation.
|
| Title: | Topological materials for low-energy electronics |
| Speaker: | Professor Michael S. Fuhrer |
| Date: | 26 June 2019 |
| Time: | 11.10am - 12pm |
| Venue: | SPMS-LT5 (SPMS-03-08) |
| Host: | Professor Xiong Qihua / Assistant Professor Bent Weber |
| Abstract: |  |
| Title: | Delivery Strategies of Nanomedicine for Cancer Treatment [CANCELLED] |
| Speaker: | Professor Yang Xiangliang |
| Date: | 25 June 2019 |
| Time: | 4pm - 5pm |
| Venue: | MAS Executive Classroom 1 (MAS-03-06) |
| Host: | Professor Shen Zexiang |
| Abstract: | To ameliorate complex physiological barriers of tumor tissues, improve targeting delivery efficiency and PK/PD behavior of antitumor nanomedicine, we put forward “Five De” strategies, which included long circulation, Targeting, Penetration, Internalization, Release. Based on “Five De”, we developed 4 novel targeting strategies in tumor therapy. First, non-PEGylation hydrophobicity reverse strategy. According to the unique microenvironment of tumor tissues, we developed temperature- and pH-responsive nanogels. These nanogels could realize hydrophobicity reverse, overcome PEG dilemma, enhance tumor targeting efficiency and tumor therapy effects finally. Second, mechano-nano oncology strategy. We develop tumor cells-derived MPs drug delivery system. Through modulation in stiffness of MPs, the PK/PD behavior of MPs was enhanced significantly. Thirdly, hyperbaric oxygen (HBO) Strategy. HBO was a common adjuvant therapy method in clinic. HBO could improve tumor hypoxia microenvironment, enhance accumulation and penetration of nanomedicine in tumor tissues. Meanwhile, HBO also make tumor cells sensitive against antitumor drug. Fourthly, HES strategy. In the basis of RES block, drug co-delivery and drug covalent coupling, we developed various novel HES drug delivery system, and realized enhanced tumor chemotherapy effects. |
| Title: | Advanced optimization methods for metasurfaces of various functionalities for nano-optics applications |
| Speaker: | Dr Kseniia Baryshnikova |
| Date: | 17 June 2019 |
| Time: | 3pm - 4pm |
| Venue: | Hilbert Space (SPMS-PAP-02-02) |
| Host: | Professor Boris Lukiyanchuk |
| Abstract: | All-dielectric nanophotonics is a rapidly growing field of modern science. Metasurfaces and other planar devices based on all-dielectric nanoparticles lead to manage the light propagation at the nanoscale. Impressive effects such as perfect absorption, invisibility, chirality effects, negative refraction, light focusing in the area with size smaller than wavelength, nano-lasing etc - can be achieved with all-dielectric technologies. While it is needed to use more and more complicated designs for solution of modern nanophotonics' currents tasks, non-classical methods of optimization become relevant. For example, to design metamaterials evolutionary or genetic algorithms show their high applicability. Designs of devices based on metamaterials become more and more complicated with the increasing of functionality of these devices. One of examples of metadevices which need complicated optimization is a metalens. In this report, a new approach to design metalenses with evolutionary and genetic algorithms is discussed. |
| Title: | Multipartite entanglement in Floquet Ising spin models |
| Speaker: | Dr Sunil Kumar Mishra |
| Date: | 14 June 2019 |
| Time: | 11am - 12pm |
| Venue: | Hilbert Space (SPMS-PAP-02-02) |
| Host: | Assistant Professor Tomasz Paterek |
| Abstract: | In this talk, I will discuss a method for generation of genuine multipartite entangled states in a kicked Ising spin chain. We consider an integrable and a nonintegrable Floquet system that is periodic in time and has constant quasienergy gaps with degeneracies. We start with all spins polarized along one direction and show that they evolve into states with high entanglement by calculating the average entanglement entropy and geometric measure of entanglement. We will also see the number of parties involved in the entanglement can be obtained by calculating the quantum Fisher information. |
| Title: | The power of vortices – dual perspectives on exotic quantum phases |
| Speaker: | Professor David Mross |
| Date: | 6 June 2019 |
| Time: | 11am - 12pm |
| Venue: | Conference Room, Research & Graduate Office (SPMS-CBC-02-01) |
| Host: | Assistant Professor Justin Song |
| Abstract: | The analysis of topological excitations in interacting many-body systems often provides important insights into quantum phases and phase transitions. Seminal examples are the (thermal) BKT transition as well as the (quantum) superfluid-Moa insulator transition, which are naturally described in terms of vortices. More recently, such a ‘dual’ formulation has proven extremely illuminating in the study of exotic phases of matter that host fractional excitations, e.g., in topological phases and quantum magnets. In my talk, I will review the dual description of conventional phases of matter and explain how exotic, fractionalized phases are captured within this approach. I will then generalize these dualities to fermionic systems, and discuss the implications for the half-filled Landau level and quantum spin liquids. |
| Title: | Workshop on Particle Physics and analyzing data from
CERN and LIGO experiments Topic: Introduction to Particle Physics and CERN; Matlab Basics. |
| Speaker: | Professor Duncan Carlsmith |
| Date: | 21 May 2019 |
| Time: | 9.30am - 1pm |
| Venue: | The Hive TR+ 53 LHS-02-07 |
| Host: | - |
| Abstract: | - |
| Title: | Workshop on Particle Physics and analyzing data from
CERN and LIGO experiments Topic: CERN CMS data analysis lab, hunt for and fit bumps in di-lepton data. |
| Speaker: | Professor Duncan Carlsmith |
| Date: | 22 May 2019 |
| Time: | 9.30am - 1pm |
| Venue: | The Hive TR+ 53 LHS-02-07 |
| Host: | - |
| Abstract: | - |
| Title: | Workshop on Particle Physics and analyzing data from
CERN and LIGO experiments Topic: LIGO gravitational wave data analysis, reproducing the figure in the publication leading to the 2017 Nobel prize |
| Speaker: | Professor Duncan Carlsmith |
| Date: | 23 May 2019 |
| Time: | 9.30am - 1pm |
| Venue: | The Hive TR+ 53 LHS-02-07 |
| Host: | - |
| Abstract: | - |
| Title: | Designer Chalcogenides |
| Speaker: | Assistant Professor Robert E Simpson |
| Date: | 16 May 2019 |
| Time: | 2pm - 3pm |
| Venue: | MAS Executive Classroom 2 (MAS-03-07) |
| Host: | Associate Professor Lew Wen Siang |
| Abstract: | Phase change materials have important applications in photonics and data storage. However, new phase change materials must be discovered, designed, and optimized to overcome (1) the high switching energy, (2) the low cycleability, and (3) the high optical absorption. Although phase change chalcogenides of the GeTe-Sb2Te3 family have been successfully applied to both electrical and optical data storage, their high switching energy and optical absorption limits their application in universal memories and visible photonics devices. We have used a combination of strain engineering, evolutionary algorithm-led crystal structure optimization, statistical experiment designs, and high throughput combinatorial materials screening to search, design, and optimize phase change materials. In particular, we discovered that Sb2S3 exhibits superior properties for reprogrammable and tunable photonics in the visible and near infrared spectrum. In this talk I will discuss our phase change chalcogenide materials design approach, and show how these methods have allowed us to demonstrate prototype phase change data storage and tunable photonics devices with low switching energies and low optical absorption respectively. |
| Title: | Tilt dependent Hall effect in a Weyl semimetal |
| Speaker: | Dr Anirban Kundu |
| Date: | 13 May 2019 |
| Time: | 11am - 12pm |
| Venue: | Conference Room, Research & Graduate Office (SPMS-CBC-02-01) |
| Host: | Assistant Professor Justin Song |
| Abstract: | A Weyl semimetal (WSM) can be understood by the availability of low energy quasiparticle states called Weyl fermions (WFs) with linear in momentum dispersion near the crossing points of two non-degenerate bands in momentum space, when the time-reversal or/and inversion symmetry is broken. A WSM is called tilted when the energy dispersion becomes tilted by addition of a symmetry breaking term in the Hamiltonian. Based on the strength of tilt the WSMs can be sub-grouped into two: type-I and type-II. We calculate current density for both the type-I and type-II WSM using the Boltzman transport theory to the second order in magnetic field. In case of type-I WSM with a non-zero tilt, the first order term produces Hall conductivity, where the magnitude varies linearly at the small tilt limit. The zeroth and second order current density terms also increase with the tilt parameter but exhibit no Hall signal. For the type-II WSM, our calculations show a linear in B dependence of the Hall signal whose magnitude increases with the tilt parameter. The magnitude of the tilt-induced Hall signal depends on the chiral dependence of the tilt, i.e. whether the tilt parameter changes sign between two valleys of opposite chirality or not. We provide possible applications of our results. |
| Title: | Flexible polymer waveguides for nonlinear and quantum photonics |
| Speaker: | Dr James A Grieve |
| Date: | 3 May 2019 |
| Time: | 11.30am - 12.30pm |
| Venue: | Hilbert Space (SPMS-PAP-02-02) |
| Host: | Associate Professor Rainer Dumke |
| Abstract: | Low cost polymer waveguides optimized for single mode guiding of visible light can be an ideal tool to study nonlinear and quantum photonics. In this talk, I will trace the development of a waveguide platform based on an elastic, optical-grade polymer, and show how the elastic response of these materials can be used to create tunable devices. We showcase this tunability by mapping a coherent quantum walk in a waveguide lattice, using a scheme which can be extended to multiphoton experiments. Simple, few-step fabrication based on a casting technique also enables us to embed a wide range of materials and micro- or nano-scale objects inside our devices. When combined with polarization insensitive single-mode guiding this may be a powerful tool to probe nonlinear optical processes, as we demonstrate by encapsulating a monolayer of the transition metal dichalcogenide MoS2. In the future, these polymer chips may host large scale devices with diverse optical properties, with applications from quantum simulation to classical signal processing. |
| Title: | 3/2 fractional quantum Hall plateau in a single layer two-dimensional electron gas |
| Speaker: | Dr Xi Lin |
| Date: | 30 April 2020 |
| Time: | 11am - 12pm |
| Venue: | Conference Room, Research & Graduate Office (SPMS-CBC-02-01) |
| Host: | Assistant Professor Yang Bo |
| Abstract: | The fractional quantum Hall states arise from strong interaction between electrons subject to a strong magnetic field in two-dimension. Such systems host exotic topological phases with intriguing theoretical properties, leading to potential applications in robust quantum devices. After the observation of 1/3 fractional quantum Hall (FQH) state, around 100 FQH states have been found. Most of them are odddenominator states and can be understood under the framework of composite fermion theory. The 5/2 state and its hole conjugate 7/2 state are the rare examples of evendenominator states in single layer two-dimensional electron gas, and even-denominator states might bring us non-Abelian statistics. In a single layer two-dimensional electron gas, we observed a new even-denominator fractional quantum Hall plateau quantized at (h/e^2 )/(3/2) under confinement, at a bulk filling factor of 5/3. This unexpected plateau develops below 300 mK with a quantization of 0.02%. The conductance transmitting through the confined region is also quantized at 3/2 e^2/h, and the conductance of 1/6 e^2/h is backscattered. This talk will also introduce how we achieved an ultra-low electron temperature environment. Because of the weak electron-phonon coupling at ultra-low temperature, the electron temperature is usually higher than the environment temperature typically measured with a resistor thermometer. Even with well thermalized sample located at 10 mK, the temperature of the conducting electrons in it can be above 100 mK. We have accomplished electron thermometry down to 20 mK in our lab. |
| Title: | Electron hydrodynamics and thermal transport in graphene-based materials |
| Speaker: | Professor Giovanni Vignale |
| Date: | 29 April 2019 |
| Time: | 11am - 12pm |
| Venue: | Conference Room, Research & Graduate Office (SPMS-CBC-02-01) |
| Host: | Assistant Professor Justin Song |
| Abstract: | Electric and thermal transport in electronic systems has long been described in terms of a single-particle picture, which emphasizes the role of collisions between electrons and impurities or phonons, with electron-electron interactions playing a secondary role. It is only in the past two decades that advances in the fabrication of ultra clean samples have refocused the interest on collective hydrodynamic transport - a transport regime which is controlled by the nearly conserved quantities: number, momentum, and energy, and by electron-electron interactions. In this talk I review some of the recent theoretical and experimental progress in our understanding of electronic hydrodynamics in graphene-based materials. I focus on thermal transport and its relation to electric transport, epitomized by the Wiedemann-Franz law which, in its conventional form, predicts a universal ratio between electric and thermal resistivities. Significant deviations from this prediction are found in single layer and double layer graphene, both in the doped case, where the Wiedemann-Franz ratio is reduced, and in the undoped case, where it is greatly enhanced. In the latter case an interesting scenario emerges, in which a small amount of disorder helps to expose an underlying singularity of the transport coefficients: vanishing thermal resistivity, finite electric resistivity, and diverging Wiedemann-Franz ratio and Seebeck coefficient. |
| Title: | Have we found excitonium? Charge density waves and superconductivity in TiSe2 driven by exciton condensation |
| Speaker: | Professor Vitor Perreira |
| Date: | 22 April 2019 |
| Time: | 11am - 12pm |
| Venue: | Conference Room, Research & Graduate Office (SPMS-CBC-02-01) |
| Host: | Assistant Professor Justin Song |
| Abstract: | The interplay of charge-density wave (CDW) order and superconductivity (SC) is of perennial interest in condensed matter physics since the underlying physics might unlock the promise of high- temperature SC. Experimental research on these correlated phases has been explosively revived recently in different realizations of two-dimensional crystals, which have highly tunable carrier density by field effect. Their nature in the semimetal TiSe2 presents a notable case since it had been conjectured, from as early as 1976, as a candidate excitonic insulator. Despite long-standing theoretical proposals for their existence, unequivocal identification of an excitonic insulator material has remained elusive. Recent experiments have changed that, and reinforce the view that the CDW phase in TiSe2 is a direct manifestation of its intrinsic excitonic character. I will present an encompassing theoretical framework that describes how the excitonic instability and excitonic fluctuations likely underpin the entirety of the experimental phase diagram in this compound. First, I will show how the excitonic mechanism captures the experimental suppression of the CDW phase upon doping with very good quantitative agreement. In particular, fixing the only parameter in the theory to the undoped state, the predicted reduction of critical temperature with doping follows the experimental curve without further adjustment. Subsequently, a model of coupled CDW and SC order parameters will be shown to naturally harbour a dome-shaped SC phase at finite doping that tallies with experiments. A novel and unusual SC phase is predicted, characterized by spatial nonuniformity and multiple dimensional crossovers that have well defined experimental implications. Finally, excitonic fluctuations are shown to mediate SC pairing among electronic quasiparticles with the appropriate energy scales to explain the SC transition temperatures, which further reinforces the view that SC is boosted by loss of CDW commensurability, as seen experimentally. By tackling different microscopic aspects of both the CDW and SC phases, in particular their interplay, this work is able to, for the first time, reproduce the experimental phase diagram with extremely good quantitative agreement. It thus represents an important theoretical counterpart of recent experiments in establishing TiSe2 as an example of a correlated excitonic material. |
| Title: | Modal Galerkin methods with applications for transport problems |
| Speaker: | Dr Satyvir Singh |
| Date: | 8 April 2019 |
| Time: | 11am - 12pm |
| Venue: | Conference Room, Research & Graduate Office (SPMS-CBC-02-01) |
| Host: | Assistant Professor Justin Song |
| Abstract: | In this talk, we consider the development of the numerical methods for solving Boltzmann-Curtiss kinetic equation for gas flows and Quantum Boltzmann kinetic equation for solids in semiconductor devices. The classical description based on the first-order Navier-Stokes-Fourier (NSF) constitutive laws has serious limitations in predicting the correct flow behavior of gases in thermal nonequilibrium. As a consequence, simple modification of transport coefficients in the classical NSF theory or introduction of the velocity-slip and temperature-jump boundary conditions cannot solve the current bottleneck of problems in the study of gas flows in non-equilibrium. In order to cope with these deficiencies, a non-classical theory based on the second-order Boltzmann–Curtiss constitutive relations for diatomic and polyatomic gases was studied. An important result obtained in these studies is that constitutive relations between stresses and the strain rate are generally nonlinear and coupled in states far from thermal equilibrium. On the other hand, Quantum Boltzmann kinetic equation represents a way of describing the time evolution of a system consisting of a large number of particles (electrons or phonons). Due to the high number of dimensions and their intrinsic physical properties, the construction of numerical methods represents a challenge and requires a careful balance between accuracy and computational complexity. Among traditional high-order methods, the discontinuous Galerkin methods have received increasing attention as a numerical technique for predicting the flow behavior of gas dynamics problems. In this present work, we proposed a multi-dimensional explicit modal discontinuous Galerkin method based on structured/unstructured meshes for solving Boltzmann-Curtiss kinetic equation as well Quantum-Boltzmann transport equation. The performance of this numerical scheme is assessed by solving several well-known problems |
| Title: | Broadband Ferromagnetic Resonance Spectroscopy: The “Swiss Army Knife” for Understanding Spin-Orbit Phenomena |
| Speaker: | Dr Justin M. Shaw |
| Date: | 2 April 2019 |
| Time: | 3.30pm - 4.30pm |
| Venue: | Hilbert Space (SPMS-PAP-02-02) |
| Host: | Associate Professor S.N. Piramanayagam |
| Abstract: | Modern spin-based technologies rely on multiple, simultaneous phenomena that originate from the spin-orbit interaction in magnetic systems. These include damping, magnetic anisotropy, orbital moments, and spin-orbit torques that are manifested in the spin-Hall and Rashba-Edelstein effects. While cavity based ferromagnetic resonance (FMR) spectroscopy has been used to characterize magnetic materials for many decades, recent advances in broadband and phase-sensitive FMR techniques have allowed further refinement, improved accuracy, and new measurement capability. In fact, broadband FMR techniques can now precisely measure spin-orbit torques at the thin-film level without the requirement of device fabrication. Broadband FMR measurements have also improved our fundamental understanding of magnetic damping. Numerous extrinsic relaxation mechanisms can obscure the measurement of the intrinsic damping of a material. This created a challenge to our understanding of damping because experimental data were not always directly comparable to theory. As a result of the improved ability to quantify all of these relaxation mechanisms, many theoretical models have been refined. In fact, this has recently led to both the prediction and discovery of new materials with ultra-low magnetic damping that will be essential for future technologies based on spintronics, magnonics, spin-logic and high-frequency devices. I will begin this lecture with a basic introduction to spin-orbit phenomena, followed by an overview of modern broadband FMR techniques and analysis methods. I will then discuss some recent successes in applying broadband FMR to improve our ability to control damping in metals and half-metals, quantify spin-orbit torques and spin-diffusion lengths in multilayers, and determine the interrelationships among damping, orbital moments, and magnetic anisotropy. The impact of these result on specific technologies will also be discussed. |
| Title: | The phase diagram of a two-dimensional electron-phonon model with long range interactions |
| Speaker: | Professor George Batrouni |
| Date: | 1 April 2019 |
| Time: | 11am - 12pm |
| Venue: | Conference Room, Research & Graduate Office (SPMS-CBC-02-01) |
| Host: | Associate Professor Pinaki Sengupta |
| Abstract: | I present a quantum Monte Carlo algorithm for electron-phonon systems based on the Langevin equation. I will discuss some interesting optimizations such as Fourier acceleration, which greatly speeds up convergence, and I show that the execution [me scales almost linearly with system size while it scales as the cube of the size for other algorithms. I then use this algorithm to calculate, for the first [me, the phase diagram of the strongly interacting electron-phonon system in two dimensions with long range interactions. I discuss future plans. |
| Title: | Ionic-electronic Processes at Low Frequency in Perovskite Solar Cells |
| Speaker: | Professor Juan Bisquert |
| Date: | 27 March 2019 |
| Time: | 2pm - 3pm |
| Venue: | Hilbert Space (SPMS-PAP-02-02) |
| Host: | Professor Sum Tze Chien |
| Abstract: | The development of organic-inorganic lead halide perovskites with very large efficiency requires us to understand the operation of the solar cell. We describe the results of analysis of kinetic phenomena using frequency modulated techniques. The capacitance reveals a very large charge accumulation at the electron contact, which has a great impact in the cell measurements, both in photovoltage decays, recombination, and hysteresis. We show the identification of the impedance of ionic diffusion by measuring single crystal samples. Working in samples with lateral contacts, we can identify the effect of ionic drift on changes of photoluminescence, by the creation of recombination centers in deffects of the structure. We describe the application of IMPS lightmoduled techniques, in combination with impedance spectroscopy. |
| Title: | The magic and physics of twisted bilayer graphene |
| Speaker: | Dr Fanqi Yuan |
| Date: | 25 March 2019 |
| Time: | 11am - 12pm |
| Venue: | Conference Room, Research & Graduate Office (SPMS-CBC-02-01) |
| Host: | Assistant Professor Justin Song |
| Abstract: | Recently, unconventional superconducting phase and correlated insulating phase in twisted bilayer graphene have attracted a lot of attention, which occurs at specific fillings and within a narrow range of twist angles (so-called magic angle). In this talk, I would like to address the following questions. 1. What are the suitable models to describe the electronic states in twisted bilayer graphene? 2. Why is the “magic angle” so special? 3. What are the possible superconducting and insulating phases at half filling? Related topics such as strain effects and other superladces will be discussed |
| Title: | What the structure of neutron stars can reveal? |
| Speaker: | Professor Paramasivan Arumugam |
| Date: | 21 March 2019 |
| Time: | 3pm - 4pm |
| Venue: | Hilbert Space (SPMS-PAP-02-02) |
| Host: | Assistant Professor Tomasz Paterek |
| Abstract: | Neutron stars weigh upto twice the Sun and they are quite compact with a radius around 10 km. I will show that the phase transitions in highly dense matter (like nucleons to quarks or other exotic particles) can be predicted from careful observation of neutron stars. For this study, we require precise equation of state (EoS) of dense matter given that the implications of general relativity are understood. The relevant EoS is dominated by the nuclear phase and hence the inputs from nuclear interactions are crucial. Consequently, there are strong correlations between the size and mass of neutron star and the measurable properties of atomic nuclei like compressibility, neutron skin thickness, and symmetry energy. Recent developments in this regard will be highlighted. |
| Title: | Current induced magnetization switching of ferri-magnetic rare earth transition metal alloy films |
| Speaker: | Dr Takeshi KATO |
| Date: | 20 March 2019 |
| Time: | 10.30am - 11.30am |
| Venue: | Hilbert Space (SPMS-PAP-02-02) |
| Host: | Assoc Prof Lew Wen Siang |
| Abstract: | Current induced magnetization switching (CIMS) is known as a promising technology to realize a Gbit-class magnetic
random-access memory (MRAM), since this method provides low power consumption and scalability compared to an
old field-writing method. Two types of CIMS are known at present: spin transfer torque (STT) switching and spin orbit
torque (SOT) switching. STT utilizes the spin-polarized current which exerts a torque to the magnetization in the
memory layer, and currently 256 Mbit STT-MRAM products are commercially available. SOT utilizes the pure spin
current generated through spin Hall effect in the heavy metal layer adjacent to the memory layer, and faster switching
and lower power consumption are expected compared to those of STT switching. Most studies on STT and SOT
switching are for ferro-magnetic memory layer(1), (2), while few reports for ferri-magnetic memory layer(3), (4). In this
talk, STT and SOT switching of ferri-magnetic rare earth (RE) - transition metal (TM) alloy films are reviewed. The
RE-TM alloys are technologically attractive, since magnetic properties, such as magnetization, perpendicular
anisotropy, and Curie temperature, are easily tuned by the alloy compositions. Variations of the STT and SOT with the
magnetic properties of RE-TM alloy films are also reviewed.
(1) Z. Diao, Z. Li, S. Y. Wang, Y. Ding, A. Panchula, E. Chen, L. C. Wang, and Y. Huai, J. Phys.: Condens. Matter, 19, 165209 (2007). (2) L. Liu, C.-F. Pai, Y. Li, H. W. Tseng, D. C. Ralph, and R. A. Buhrman, Science 336, 555 (2012). (3) B. Dai, T. Kato, S. Iwata, S. Tsunashima, IEEE Trans. Magn., 49, 4359 (2013). (4) N. Roschewsky, T. Matsumura, S. Cheema, F. Hellman, T. Kato, S. Iwata, S. Salahuddin, Appl. Phys. Lett., 109, 112403 (2016). |
| Title: | Quantum control of ultracold dipolar molecules |
| Speaker: | Professor Huanqian Loh |
| Date: | 18 March 2019 |
| Time: | 11am - 12pm |
| Venue: | Conference Room, Research & Graduate Office (SPMS-CBC-02-01) |
| Host: | Associate Professor David Wilkowski / Assistant Professor Justin Song |
| Abstract: | Polar molecules offer long-range anisotropic interactions, which are fundamental to a wide variety of phenomena from protein folding to ferrofluid behavior. Recent efforts to cool and trap polar molecules have brought these particles into the quantum regime, making them highly attractive for applications like quantum information storage and exploring novel condensed matter phases. In this talk, I will discuss the creation and quantum control of dipolar fermionic NaK molecules in the ground rovibronic state and at ultracold temperatures as low as 300 nK. Using microwaves, we have coherently manipulated both the rotational states and the nuclear spin states of the molecules. I will report our observation of nuclear spin coherence times on the scale of 1 second, and discuss its implications for quantum memory and probing new physics via Hertz-level precision spectroscopy. Finally, I will present a new experiment I am setting up to achieve single-quantum-state control with single-molecule resolution for quantum simulation. |
| Title: | Materials by design: Machine learning and data-driven discovery |
| Speaker: | Dr Kedar Hippalgaonkar |
| Date: | 11 March 2019 |
| Time: | 11am - 12pm |
| Venue: | Conference Room, Research & Graduate Office (SPMS-CBC-02-01) |
| Host: | Assistant Professor Justin Song |
| Abstract: | Materials innovation is accelerating rapidly with the advent of high performance and high-throughput computing and automation. New tools provided by artificial intelligence, specifically machine and deep learning, are available to decipher large volumes of data towards predictive materials discovery. Thanks to initiatives in materials informatics, large databases such as those from Materials Project, Aflow and Material Information Platform etc. are now publicly available and can be appended to quickly with DFT and similar calculations in different fields. In this talk, I will first discuss the state-of-the art at the intersection of high-throughput computing, robotics/automation and machine learning applied to materials science and engineering. Then, I will address the key challenges in adding to, and applying machine learning to, such large databases. Specifically, I will talk about the case of thermoelectrics and how we have devised a new experimental technique to directly measure thermoelectric transport and material descriptors towards generating large, high quality datasets with high throughput. We are able to extract new scientific insights paving the way for future discovery of high performance functional materials, including inorganic-organic hybrids, 2D materials and bulk inorganics. |
| Title: | Scalable quantum information processing with trapped-ion atomic clock |
| Speaker: | Dr Tan Ting Rei |
| Date: | 27 February 2019 |
| Time: | 4pm - 5pm |
| Venue: | Hilbert Space (SPMS-PAP-02-02) |
| Host: | Associate Professor Rainer Dumke |
| Abstract: | With excellent isolation from environmental perturbation and exquisite controls over quantum system made up of individual particles, trapped-ion systems have been instrumental in the advancement of precision metrology (especially in the field of time and frequency standard), and is currently being development into a promising platform for quantum information processing (QIP). In this talk, I will discuss a number of QIP experiments performed in an ion trap designed to implement a quantum charged-coupled device (CCD) architecture. In particular, I will describe an implementation of a “hybrid” quantum entangling gate between two atomic ions of different species. Such an entangling gate is an important step towards scaling up quantum information processor and quantum network as it allows quantum information to be transferred between different kinds of qubit in a hybrid architecture. The second topics of this talk is about the progress of establishing a multi-ion optical clock based on lutetium ion (176Lu+) currently being pursued at Centre for Quantum Technologies (CQT) in National University of Singapore (NUS). Lutetium ion possesses a combination of unique properties making it a favourable candidate for the new generation of state-of-the-art atomic clock that offer unprecedented accuracy. Then, I will briefly discuss how lutetium ion and techniques developed for Lu-based atomic clock can be translated and potentially used for QIP. |
| Title: | Novel Types of Nodal-loop Metals |
| Speaker: | Dr Shengyuan Yang |
| Date: | 25 February 2019 |
| Time: | 11am - 12pm |
| Venue: | Conference Room, Research & Graduate Office (SPMS-CBC-02-01) |
| Host: | Associate Professor Yidong Chong/Assistant Professor Justin Song |
| Abstract: | Topological states of matter have been attracting great interest. Following the studies on topological insulators, the recent focus shifts towards the various topological metals (or semimetals), in which nontrivial band crossings appear near the Fermi level, endowing the low-energy electrons with novel emergent properties. In this talk, I discuss a class of metals where the band crossings form 1D loops in the Brillouin zone. Our recent works reveal that the loops can be characterized by several properties. (1) Depending on how the loop winds around the Brillouin zone, it has a Z3 characterization. (2) The type of dispersion around the loop classifies the loop into type-I, type-II, or the hybrid type. The type-II loop shows suppressed low-frequency optical absorption, and the hybrid loop shows zero-field magnetic breakdown and distinct features in the magnetic quantum oscillations. (3) The loop may also be classified based on the order of dispersion, and we can have quadratic and cubic loops protected by crystalline symmetry. (4) Under spin-orbit coupling, some loops are gapped out, but some are robust owing to certain nonsymmorphic symmetries. Under proper conditions, multiple loops can exist, connect each other, and form a nodal chain. |
| Title: | Development of Universal Transfer Technique for Large Area 2-Dimensional Materials onto Target Substrates |
| Speaker: | Professor Dae Joon Kang |
| Date: | 21 February 2019 |
| Time: | 2pm - 3pm |
| Venue: | Hilbert Space (SPMS-PAP-02-02) |
| Host: | Professor Xiong Qihua |
| Abstract: | Transfer and integration of nanostructures onto desirable substrates is the prerequisite for their fundamental studies and practical applications. Conventional transfer techniques involving stamping, lift-off and/or striping are greatly limited by the process-specific shortcomings, including the requirement for chemical etchant or high-temperature annealing and the introduction of surface discontinuity and/or contamination that can greatly deteriorate the intrinsic properties of the transferred materials. We have developed a universal transfer method implementable at mild conditions to transfer large area 2-Dimensional (2D) materials grown by chemical vapor deposition method onto various substrates. This technique not only allows the effective transfer to an arbitrary target substrate with a high degree of freedom, but also avoids PMMA etching thereby maintaining the high quality of the transferred 2D materials with minimum contamination. We applied this method to transfer various 2D materials grown on different rigid substrates of general interest, such as graphene on copper foil, bilayer graphene on platinum, h-BN on platinum, MoS2 on SiO2 /Si, MAXenes. We believe that our method can facilitate the development of nanoelectronics by accelerating the clean transfer and integration of low-dimensional materials into multidimensional structures. |
| Title: | Real-time approach to optical properties of 2D materials: time-dependent Bethe-Salpeter Equation |
| Speaker: | Dr Paolo Emilio Trevisanutto |
| Date: | 28 January 2019 |
| Time: | 11am - 12pm |
| Venue: | Conference Room, Research & Graduate Office (SPMS-CBC-02-01) |
| Host: | Assistant Professor Justin Song |
| Abstract: | The exciton, an electron-hole (e-h) quasiparticle has an important role in applied physics, especially for 2D materials where its binding energy is high. This peculiar effect determines the stability and localization of excitons that can be exploited either for qubits or, by using self-trapping effects, for Single Light Emitter devices. Another effect of current interest is non-linear harmonic generation in 2D materials where it can be used to extract important information regarding, e.g., the number of layers and crystallographic orientation of few-layered MoS2. In this talk, we review that current state of art of ab initio many-body perturbation theory, the Bethe-Salpeter Equation in the linear regime and we describe a theoretical approach (Time-dependent Bethe-Salpeter Equation) to study the nonlinear optical response of electronic systems based on a real-time solution of the electronic dynamics in the presence of time-dependent electromagnetic fields. Using accurately parameterized tight-binding models and electronic interactions, this allows expedited and accurate calculations of the nonlinear response to arbitrary order for realistic systems. We demonstrate its capabilities by computing the excitonic spectrum and high-harmonic generation (SHG, THG, FHG) in MoS2 and BN monolayers |
| Title: | Electrons: Strange Particles with an Intelligent Spirit |
| Speaker: | Professor Xuechu Shen |
| Date: | 25 January 2019 |
| Time: | 3.30pm - 4.30pm |
| Venue: | Hilbert Space (SPMS-PAP-02-02) |
| Host: | Professor Xiong Qihua |
| Abstract: | Electrons are strange and naughty particles with intelligent spirit. Electrons in solids are the footing stone of modern microelectronics, information technology, etc. In this talk, the discovery, history, and wave-Particle duality of electrons will be reviewed. The electronic motion in solids, impurity electrons, chaotic electrons, and chaotic motion of electron in solids will be discussed. |
| Title: | Spin-orbit technologies: from magnetic memory to terahertz generation |
| Speaker: | Dr Hyunsoo Yang |
| Date: | 21 January 2019 |
| Time: | 11am - 12pm |
| Venue: | Conference Room, Research & Graduate Office (SPMS-CBC-02-01) |
| Host: | Associate Professor Elbert Chia and Assistant Professor Justin Song |
| Abstract: | Spintronic devices utilize an electric current to alter the state of a magnetic material and thus find great applications in magnetic memory. Over the last decade, spintronic research has focused largely on techniques based on spin-orbit coupling, such as spin-orbit torques (SOTs), to alter the magnetic state. The phenomenon of spin-orbit coupling in magnetic heterostructures was also recently used to generate terahertz emission and thus bridge the gap between spintronics and optoelectronics research. I will introduce the basic concepts of SOTs, such as their physical origin, the effect of SOTs on a magnetic material, and how to quantitatively measure this effect. Next, I will discuss the latest trends in SOT research, such as the exploration of novel material systems like topological insulators and two-dimensional materials to improve the operation efficiency. Following this, some of the technical challenges in SOT-based magnetic memory will be highlighted. Moving forward, I will introduce the process of terahertz generation in magnetic heterostructures, where the spin-orbit coupling phenomenon plays a dominant role. I will discuss the details of how this terahertz emission process can be extended to novel material systems such as ferrimagnets and topological materials. The final section will focus on how the terahertz generation process can be used to measure SOTs in magnetic heterostructures, thus highlighting the interrelation between terahertz generation and the SOTs, which are linked by the underlying spin-orbit coupling. |
| Title: | Quantum Computing for the Near Future |
| Speaker: | Dr. Man-Hong Yung |
| Date: | 21 January 2019 |
| Time: | 2pm - 3pm |
| Venue: | Hilbert Space (SPMS-PAP-02-02) |
| Host: | Assistant Professor Mile Gu |
| Abstract: | In the near future, it is possible that quantum devices with 50 or more high quality qubits can be engineered. On one hand, these quantum devices could potentially perform specific computational tasks that cannot be simulated efficiently by classical computers. On the other hand, the number of qubits would not be enough for implementing textbook quantum algorithms. An immediate question is how one might exploit these near term quantum devices for really useful tasks? In addition, one may also expect that these powerful quantum devices are accessible only through cloud services over the internet, which imposes the question of how might one verify the server, behind the internet, does own a quantum computer instead of a classical simulator? In this talk, I will share my thoughts over these questions based on my recent works. |
| Title: | Dark resonance fringes and dressed matter-waves: quantum technologies from atomic physics |
| Speaker: | Dr Thomas Zanon-Willette |
| Date: | 17 January 2019 |
| Time: | 11am - 12pm |
| Venue: | Hilbert Space (SPMS-PAP-02-02) |
| Host: | Associate Prof David Wilkowski |
| Abstract: | Quantum technology is now becoming an active field mixing atomic physics and quantum hamiltonian engineering based on state superpositions and entanglement for a new generation of extreme accurate sensors. I will focused during this talk on a few applications from atomic physics emerging as successful methods to realize compact atomic CPT clocks based on dark resonance fringes and developing new concepts with atomic systems manipulated trough off-resonant time-periodically modulated electromagnetic fields. Dark resonance fringes in double lambda atomic systems have been realized in atomic vapor cells with buffer gas paving the way to compact stable atomic clock sensors for unprecedented metrological application. Dark resonances have also been suggested as new frequency standards in optical lattice clock with forbidden transitions of alkaline earth atoms using a combination of Electromagnetically Induced Transparency (EIT)/Raman and pulsed spectroscopy techniques to accurately cancel frequency shifts arising from laser fields. The proposed scheme for optical clocks using bosonic systems was offering an efficient population transfer up to 60% with potential inaccuracy 10-17 . Quantum engineering of phase-shifts for an hyper Raman-Ramsey optical clock will be also presented to produce an ultra-narrow optical transition in bosonic alkali-earth systems free from light shifts and with a significantly reduced sensitivity to laser parameter variations. I will finish the talk with the Floquet dressed atom and matter-wave concept in order to renormalize and control atomic properties for quantum sate engineering leading to a new class of Stern-Gerlach interferometers and Zeeman-free optical clocks with nuclear spin where the atomic Landé factor is modified. |
| Title: | On a Quest for Novel Functional Materials: Theory and Computation Guided Discovery and Design |
| Speaker: | Dr Vladan Stevanović |
| Date: | 15 January 2019 |
| Time: | 2pm - 3pm |
| Venue: | Hilbert Space (SPMS-PAP-02-02) |
| Host: | Associate Professor Pinaki Sengupta |
| Abstract: | Discovery and design of novel functional materials is of paramount importance for accelerating development of disruptive technologies needed to help secure a more sustainable future. However, searching for and designing new materials requires knowledge of a number of properties that are relevant applications and that are typically not available for a large number of systems. Hence, computational approaches, which are able to provide access to these properties with the required accuracy are instrumental in accelerating the pace at which materials discovery and design occurs. In this talk I will present our recent work, which draws on the solid-state and semiconductor theory to develop predictive, computationally tractable and experimentally validated approaches to search for novel functional materials. The application space covers photovoltaic, thermoelectric, as well as materials for transparent and power electronics. The main focus is on methods to predict transport properties of semiconductors including the charge carrier and heat transport as well as the tolerance to defects and the ability to be doped. In many of the application areas our recent developments offer quantitative predictions of relevant properties, which, in turn, allow large-scale calculations and identification of new candidate systems. I will present examples of our material searches and discuss experimental realization of the computationally identified material systems. |
| Title: | Zero mode of QFT and superluminal signalling in relativistic quantum information |
| Speaker: | Mr Erickson Tjoa |
| Date: | 14 January 2019 |
| Time: | 12.30pm - 1.30pm |
| Venue: | Hilbert Space (SPMS-PAP-02-02) |
| Host: | Dr Koh Teck Seng |
| Abstract: | In this presentation we will see how the “zero mode” of a quantum field --- that is, the constant component of the Fourier series decomposition of the field --- which arises when certain boundary conditions are imposed, can lead to superluminal signaling (causality violation) when it is ignored during quantization. This phenomenon can be operationally understood by coupling two spacelike-separated detectors (“atoms”) to the quantum field and see how their transition probabilities influence one another. In this presentation we will first review the theoretical minimum of quantum field theory for a massless scalar field and motivate operational approach of understanding quantum field using particle detectors known technically as Unruh-DeWitt model. We will then use this model to understand the zero-mode problem. Background knowledge in special relativity, quantum mechanics and electromagnetism is desirable but it is hoped that the basic ideas of the presentation are accessible to general physics undergraduates. |
| Title: | Quantum many-body scarring: a weak ergodicity-breaking phenomenon |
| Speaker: | Dr. Wen Wei Ho |
| Date: | 11 January 2019 |
| Time: | 11am - 12pm |
| Venue: | MAS Executive Classroom 1 (MAS-03-06) |
| Host: | Assistant Professor Justin Song |
| Abstract: | A central postulate of statistical mechanics is that of ergodicity -- a generic state prepared out of equilibrium is believed to explore its allowed phase phase and eventually thermalize. In interacting quantum systems, known exceptions to this behavior include strongly disordered, many-body localized (MBL) systems, and finely-tuned, integrable systems. Recently, quench experiments with Rydberg atom arrays [Nature 551, 579 (2017)] demonstrated non-thermalizing dynamics of a new kind: surprisingly long-lived, periodic revivals from certain simple initial states, while quick relaxation and equilibriation from others as expected in a quantum chaotic system. In this talk, I will show that these observations are attributed to the presence of a small number of exceptional, nonthermal many-body eigenstates dubbed "quantum many-body scars" that violate the eigenstate thermalization hypothesis (ETH), which are embedded in a sea of otherwise thermal eigenstates [1]. I will furthermore demonstrate that underlying the long-lived many-body revivals is an unstable, periodic orbit, captured in a suitable variational "semiclassical" description of the dynamics using matrix-product-states, which suggests a firm connection to the similarly named phenomenon of quantum scarring in single-particle quantum chaos [2]. Lastly, I will discuss recent work on how a weak, quasilocal deformation can stabilize revivals, leading to virtually perfect oscillations with emergent SU(2) dynamics, and which suggests an underlying Hamiltonian with exact quantum many-body scarring [3]. Quantum many-body scarring represents a new universality class of quantum dynamics in strongly interacting systems resulting from a weak form of ergodicity breaking, with direct experimental signatures. Refs:
|