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Seminars 2021
| Title: | The role of 2D Materials in Fusion with Silicon ICs |
| Speaker: | Professor Peng Zhou |
| Date: | 5 April 2021 |
| Time: | 10.30am - 11.30am |
| Venue: | Zoom (ID and PW will be given upon registration) |
| Host: | Associate Professor Gao Weibo |
| Abstract: | As the feature size of silicon-based integrated circuits (ICs) approaches the physical limit, short-channel effects appear, gate control attenuates, and leakage current increases, which seriously affects transistor performance and causes chip failure. Due to the inherent thickness of bulk silicon, the physical area cannot be further reduced, which restricts the area efficiency of silicon-based ICs. In addition, the speed mismatch between memory readout and logic operation, and the separation of memory and computing units together form the memory wall bottleneck in silicon-based ICs. With unique characteristics, including no dangling bond surface, atomic-level thickness, abundant adjustable energy bands, excellent optical electrostatic properties etc., twodimensional (2D) materials have the potential to enhance gate control, reduce leakage, improve energy and area efficiency, and realize the integration of perception, memory and computing. This report discusses the roadmap for the fusion of 2D materials and silicon ICs, including alleviating the problems faced by silicon ICs from the application of 2D materials in gate-all-around, memory and logic transistors, and enabling the creation of an all-in-one sensing, memory and computing system. Finally, it provides an outlook on the challenges and promising paths to fusing 2D materials with silicon ICs for large-scale applications. |
| Title: | Dirac or Weyl Points in Fe3Sn2? An Investigation from ARPES Perspective |
| Speaker: | Dr Ekahana Sandy Adhitia |
| Date: | 29 March 2021 |
| Time: | 4pm - 5pm |
| Venue: | Zoom (ID and PW will be given upon registration) |
| Host: | Associate Professor Elbert Chia |
| Abstract: | Magnetic Weyl Semimetals have been sought recently as a logical addition to the topological material database. One particular searching-path narrow down to the Kagome lattice family where the Co3Sn2S2 has been well reported to host an odd-number of Weyl pairs indicating its time-reversal breaking feature. At the same time, ferromagnetic Fe3Sn2 is also reported to host Weyl points despite of its lacking experimental report on the Fermi arcs. However, a competing view is also reported that claims Fe3Sn2 to host Dirac points originating from its bi-layer Kagome structure. This talk will present the overview of this issue and how the latest result in laser micro-ARPES opens a new guide for another ab-initio calculation to be done. |
| Title: | Engineering and Measurement of Thermal Radiation |
| Speaker: | Professor Mikhail Kats |
| Date: | 22 March 2021 |
| Time: | 10.30am - 11.30am |
| Venue: | Zoom (ID and PW will be given upon registration) |
| Host: | Assistant Professor Justin Song |
| Abstract: | Thermal radiation is the phenomenon responsible for most of the light in the universe. Though understanding of thermal radiation dates back over a century, recent advances have encouraged the re-examination of this phenomenon and its applications. This talk will describe our group’s advances and outline future work in the measurement and manipulation of thermal radiation. First, I will discuss our innovations in thermal-radiation metrology, especially for low-temperature thermal emitters, emitters with temperaturedependent emissivity, and emitters out of equilibrium. Such improvements can enable techniques such as our recently demonstrated technique of depth thermography, in which measurements of thermal radiation yield temperature information below the surface of objects. I will also describe our invention of a minimalistic spectroscopy technique that requires no gratings, interferometers, or any other wavelength-selective components. Then, I will describe the use of phase-transition materials including vanadium dioxide and rareearth nickelates to demonstrate new phenomena, including negative- and zero-differential thermal emittance. I will also discuss our recent demonstration of nanosecond-scale modulation of emissivity and thermal-radiation pulses down to picosecond scales. The talk will include discussion of exciting opportunities of thermal-radiation engineering for infrared-privacy and thermoregulation technologies. |
| Title: | Light-lattice Interactions in Atomically Thin Van der Waals Semiconductors Toward Integrated Two-dimensional Circuitry |
| Speaker: | Professor Moon-Ho Jo |
| Date: | 18 March 2021 |
| Time: | 5pm |
| Venue: | Zoom (ID and PW will be given upon registration) |
| Host: | Associate Professor Gao Weibo |
| Abstract: | In this talk, we discuss a series of our recent works on selective light-lattice interactions in atomically thin van der Waals (vdW) semiconductors toward realization of two-dimensional (2D) integrated circuitry (ICs). Complementary doping on a semiconductor is an elementary process to build monolithic ICs. However, for 2D vdW semiconductors, which offer diverse emerging electronic and optical properties, the construction of monolithic ICs remains challenging because of the absence of a locally selective doping method [1]. Toward this end, we first demonstrated a simple method of “programmable writing” of various 2D ICs on atomically thin vdW semiconductor host lattice by exploiting a novel concept of self-aligned doping with a scanning light probe, in analogy to ion implantation in Si CMOS technology [2]. Then, we discovered that such direct lightlattice interactions can be selective and reversible with the choices of light colors, i.e. “reconfigurable” photo-induced doping using different photon energies, which were supported by visual and spectroscopic evidence of individual n- and p-dopants at the atomic scale [3]. This simple doping enables one to repeatedly inscribe and erase the carrier types and concentrations of an identical semiconductor channel at room temperature with conventional light sources. We indeed showed diverse CMOS ICs using such light probes, including n-p-n (p-n-p) bipolar junction transistor amplifiers, radial p-n photovoltaic cells and reconfigurable CMOS inverter-switches [2-3]. At the end, we discuss another recent example of atomically thin photomemtransistors, which can be viewed as an atomistic synapse networks in hardware-based artificial neural networks technology [4]. |
| Title: | From Silicon Valley to Dark Matter: Discovering and Utilizing Quantum Materials and Effects |
| Speaker: | Dr Mazhar Ali |
| Date: | 8 March 2021 |
| Time: | 4pm - 5pm |
| Venue: | Zoom (ID and PW will be given upon registration) |
| Host: | Assistant Professor Justin Song |
| Abstract: | In recent years, solid state physics research has rapidly expanded due to the
increasing availability and diversity of materials showing quantum effects. These
quantum materials range from strongly correlated systems to topological systems and
more. The successes of finding new electronic states, including novel fermions and
quasiparticles in stable materials, has helped prepare the “quantum revolution” –
where next generation computing and sensing technologies aim to utilize these
exotic properties. In this talk I will briefly discuss how, using the concepts of topology,
symmetry, and understanding of crystal and electronic structures, we discovered a
couple of new topological materials (WTe2
, KV3
Sb5
) and what their important
properties are. We then show how the lessons we learned are used in ongoing
fundamental research on novel Hall effects and superconductivity as well as applied
research – Spintronics, Josephson-Junctions, and even novel Dark Matter detectors. View Seminar Recording |
| Title: | The Impact and Pursuit of Magnetic Topological Insulator States |
| Speaker: | Professor Simin Nie |
| Date: | 22 February 2021 |
| Time: | 10.30am - 11.30am |
| Venue: | Zoom (ID and PW will be given upon registration) |
| Host: | Assistant Professor Guoqing Chang |
| Abstract: | The introduction of concepts from topology greatly deepens our understanding of the
electronic states in solids. In the last decade, due to both the high scientific interest and the
promising applications in novel quantum devices, the study of topological states has been
one of the most active and fruitful fields in the condensed matter physics. Although the
nonmagnetic topological states have been extensively studied, the study of magnetic
topological materials is just beginning. In this talk, I will report our two recent works in
magnetic topological states. In the first work, guided by the study of a specific honeycomb
lattice, we show that LuSI (YSI) is a 3D strong topological insulator with the right-handed
helical surface states, while GdSI is the long-pursuing ideal magnetic Weyl semimetal with
only two pairs of Weyl nodes residing at the Fermi level. The ideal Weyl semimetal is
beneficial to the large negative magnetoresistance, large anomalous Hall conductivity and
large anomalous Hall angle, which are important for the related device designs. In the second
work, we propose that multiple topological semimetal phases can be achieved in “soft”
ferromagnetic material EuB6 by simply tuning the direction of the magnetic moment. The
corresponding topological phase transitions can be monitored by the measurement of
topological surface states or anomalous Hall conductivity. Moreover, large-Chern number
quantum anomalous Hall effect can be realized in its [111]-oriented quantum-well structures. View Seminar Recording |
| Title: | Observation of Tensor Monopoles with a Superconducting Quantum Circuit |
| Speaker: | Professor Yang Yu |
| Date: | 18 February 2021 |
| Time: | 5pm |
| Venue: | Zoom (ID and PW will be given upon registration) |
| Host: | Associate Professor David Wilkowski |
| Abstract: | Monopoles play a center role in gauge theories and topological matter. There are two fundamental types of monopoles in physics:
vector monopoles and tensor monopoles. Examples of vector monopoles include the Dirac monopole in three dimensions and Yang
monopole in five dimensions, which have been extensively studied and observed in condensed matter or artificial systems. However,
tensor monopoles are less studied, and their observation has not been reported. On the other hand, superconducting quantum
circuits provide an excellent platform for quantum computation and quantum simulation due to their scalability, controllability, and
flexibility. Here we experimentally construct a tunable spin-1 Hamiltonian to generate a tensor monopole and then measure its
unique features with superconducting quantum circuits. The energy structure of a 4D Weyl-like Hamiltonian with threefold
degenerate points acting as tensor monopoles is imaged. Through quantum-metric measurements, we report the first experiment
that measures the Dixmier-Douady invariant, the topological charge of the tensor monopole. Moreover, we observe topological
phase transitions characterized by the topological Dixmier-Douady invariant, rather than the Chern numbers as used for
conventional monopoles in odd-dimensional spaces. View Seminar Recording |
| Title: | Creating and measuring the elusive Majorana fermions |
| Speaker: | Professor Vidya Madhavan |
| Date: | 8 February 2021 |
| Time: | 10.30am - 11.30am |
| Venue: | Zoom (ID and PW will be given upon registration) |
| Host: | Assistant Professor Bent Weber |
| Abstract: | A Majorana fermion is a special type of fundamental particle which is its own
antiparticle. The possible realization of these exotic Majorana fermions as
quasiparticle excitations in condensed matter physics has created much
excitement. Most recent studies have focused on Majorana bound states which
can serve as topological qubits. More generally, akin to elementary particles,
Majorana fermions can propagate and display linear dispersion. These excitations
have not yet been directly observed, and can also be used for quantum
information processing. This talk is focused on our recent work in realizing
dispersing Majorana modes. I will describe the conditions under which such
states can be realized in condensed matter systems and what their signatures are.
Finally, I will describe our scanning tunneling experiments of domain walls in the
superconductor FeSe0.45Te0.55, which might potentially be first realization of
dispersing Majorana states in 1D. View Seminar Recording |
| Title: | Dynamic Orders in Quantum Matter |
| Speaker: | Professor Alexander Balatsky |
| Date: | 4 February 2021 |
| Time: | 10am - 11am |
| Venue: | Zoom (ID and PW will be given upon registration) |
| Host: | Associate Professor Christos Panagopoulos |
| Abstract: | Quantum matter out of equilibrium emerge as an important platform to induce correlations and transient orders. Broad basic
questions about orders that are inherently dynamic have been addressed in the context of driven cold atoms, spins, magnetic states
and superconductors. I will discuss the example of emergent dynamic and entangled states in quantum paralelectrics where driven
electric fluctuations induce magnetization, the *dynamic multiferroicity* phenomenon [1]. We see a rapidly growing list of unusual
quantum states in time domain to be discovered. I will point to other examples of *dynamic orders* out of equilibrium, e.g. Berezinski
pairing and coherent states of magnons in Dirac materials [2].
[1] Dynamic multiferroicity of a ferroelectric quantum critical point, K Dunnett, JX Zhu, NA Spaldin, V Juričić, AV Balatsky, Physical review letters 122 (5), 057208 (2020); Dynamical multiferroicity, DM Juraschek, M Fechner, AV Balatsky, NA Spaldin, Physical Review Materials 1 (1), 014401(2017) [2] Bose-Einstein condensate of Dirac magnons: pumping and collective modes, PO Sukhachov, S Banerjee, AV Balatsky, arXiv preprint arXiv:2008.01328 (2020) View Seminar Recording |
| Title: | Femto-magnetism meets spintronics: towards integrated magneto-photonics |
| Speaker: | Professor Bert Koopmans |
| Date: | 2 February 2021 |
| Time: | 4pm - 5pm |
| Venue: | Zoom (ID and PW will be given upon registration) |
| Host: | Assistant Professor Marco Battiato |
| Abstract: | Novel schemes for optically controlling ferromagnetic order at a femtosecond time scale receive great scientific interest. In the strongly non-equilibrium regime, it has become possible not only to quench magnetic order, but even to deterministically switch the magnetic state by a single femtosecond laser pulses. Moreover, it has been shown that pulsed laser excitation can induce spin currents over several to tens of nanometers. This development triggered a merge of the fields of ‘femto-magnetism’ and spintronics – opening up a fascinating playground for novel physical phenomena. In this lecture I will discuss the underlying principles, but also envision their exploitation in THz magnonics and integrated spintronic-photonic memories. After a brief review of the field, mechanisms for ultrafast loss of magnetic order upon fs laser heating as well as all-optical switching will be explained. Next, different processes that give rise to laser-induced spin currents will be distinguished. In particular I will address experiments that have demonstrated laser-induced spin transfer torque on a free magnetic layer. These fs spin currents are absorbed within a few nanometers, providing ideal conditions for exciting and exploring THz spin waves. Finally, it will be argued that synthetic, layered ferrimagnets provide an ideal platform for combining fs optical control with advanced spintronic functionality. It will be shown how magnetic bits can be written ‘on-the-fly’ by fs laser pulses in a so-called magnetic racetrack, where they are immediately transported by a dc current. Such schemes may lead to a novel class of integrated photonics, in which information is transferred back and forth between the photonic and magnetic domain without any intermediate electronic steps. |
| Title: | Spin-Orbit Physics for Low-power Memory and Logic Devices |
| Speaker: | Dr Peng Song |
| Date: | 29 January 2021 |
| Time: | 4pm - 5pm |
| Venue: | Zoom (ID and PW will be given upon registration) |
| Host: | Associate Professor SN Piramanayagam |
| Abstract: | The development of materials science and physical concepts has made spintronic a research field that is not only able to reveal fundamental science, but also of great technical relevance. A key concept in spintronics, the spin-orbit coupling (SOC), has proven itself as a mine to explore for rich physics and device applications during the last decade. The emergence of atomically-thin two-dimensional (2D) crystals provides a fantastic playground to further explore the possibilities of tuning device physics through SOC engineering. In this talk, I will first discuss how we use different methods to modulate SOC in 2D crystals and as a result, to tune spin-charge interconversion, magnetism and superconductivity in the crystals. In the second part, I will discuss my research plan to develop novel and intrinsic spin-orbit physics to address longstanding challenges in spintronic memory and logic devices. specifically, I will present my strategies, mostly relied on interactions between SOC and other ordered states, to develop more efficient spin-orbit torque enabled magnetization switch devices and spin field-effect transistors. |
| Title: | Integration of 2D materials on silicon photonics towards a novel platform of information processing |
| Speaker: | Dr Sanghoon Chae |
| Date: | 27 January 2021 |
| Time: | 10am - 11am |
| Venue: | Zoom (ID and PW will be given upon registration) |
| Host: | Assistant Professor Justin Song |
| Abstract: | By using optical platforms instead of metallic interconnects, photonic devices can achieve both high speed and low power consumption suitable for next generation information processing. Although the state-of-the-art silicon (Si) photonic chips are outstanding optical platforms for light propagation, it requires external active optical components such as light sources and photodetectors. A potential solution comes in the form of atomically thin two-dimensional (2D) materials. Their remarkable optoelectronic properties are widely tunable by doping, strain, and external fields, owing to their atomic thickness and unique characteristics. In this presentation, I will discuss my current endeavors of novel photonics and optoelectronics functions using 2D materials integrated Si photonic, including ultra-low loss phase modulations, light emissions, and photodetections. Turning discussion towards a long-term goal, I will demonstrate new 2D-Si photonic applications for quantum information processing and neuromorphic computing that can outperform classical computers. |
| Title: | Unconventional superconductivity in two-dimensional Van der Waals materials |
| Speaker: | Professor Miguel M. Ugeda |
| Date: | 25 January 2021 |
| Time: | 4pm - 5pm |
| Venue: | Zoom (ID and PW will be given upon registration) |
| Host: | Assistant Professor Bent Weber |
| Abstract: | Van der Waals materials provide an ideal platform to explore superconductivity in the presence of strong
electronic correlations, which are detrimental of the conventional phonon-mediated Cooper pairing in the BCSEliashberg theory and, simultaneously, promote magnetic fluctuations. Despite recent progress in understanding
superconductivity in layered materials, the glue pairing mechanism remains largely unexplored in the singlelayer limit, where electron- electron interactions are dramatically enhanced. In this talk, I will present
experimental evidence of unconventional Cooper pairing mediated by magnetic excitations in monolayers of Se
-based transition metal superconductors (NbSe2 and TaSe2), two model strongly correlated 2D materials. Our
high-resolution spectroscopic measurements (STS) reveal a characteristic spin resonance excitation in the
density of states that emerges from the quasiparticle coupling to a collective bosonic mode. This resonance
gradually vanishes by increasing the temperature and upon applying a magnetic field up to the critical values,
which sets an unambiguous link to the superconducting state. Furthermore, we find clear anticorrelation
between the energy of the spin resonance and the local superconducting gap, which invokes pairing of electronic
origin associated with spin fluctuations.
2D TMD materials will reduce the enormous complexity associated with the investigation of unconventional
superconductivity, and will rapidly allow us to expand our current limited knowledge of non-phononic Cooper
pairing. They offer unprecedented simplicity for modelling as compared to the most studied bulky
unconventional superconductors, i.e., cuprates, Fe-pnictides and heavy-fermion compounds. In two dimensions,
TMD superconductors are even simpler to model than twisted bilayer graphene, where superconductivity is
intrinsically linked to specific magic angles. From the experimental point of view, our work opens the
tantalizing possibility to explore unconventional superconductivity in simple, scalable and widely accessible 2D
materials. View Seminar Recording |
| Title: | A multiscale approach to magnetisation dynamics |
| Speaker: | Professor Olle Eriksson |
| Date: | 11 January 2021 |
| Time: | 3.15pm - 4.15pm |
| Venue: | Zoom (ID and PW will be given upon registration) |
| Host: | Assistant Professor Marco Battiato |
| Abstract: | In this talk a theoretical multiscale approach of magnetization dynamics is presented. The method bridges length scales and involves an atomistic description coupled in a seamless way to micromagnetism. This allows to describe the dynamics of magnets, using simulation cells with a size that is comparable to experimental sample sizes, while enabling an atomistic resolution where needed. The method is illustrated by e.g. the interaction of skyrmions with atom sized lattice defects. |