Seminars 2020

One can observe terahertz (THz) radiation from various materials when excited with a femtosecond laser. The excited carriers travel by their diffusion or drift according to the density gradient, the mobility difference, or built-in/external fields, inducing ultrafast photocurrent. THz waves reflect various kinds of ultrafast spatiotemporal carrier dynamics in their THz emission waveforms. The observation of the waveforms enables us to explore the ultrafast nature of electronic materials and devices as THz emission spectroscopy. Since 1994, we have been working on THz emission spectroscopy and microscopy, and we named the system as a Laser THz Emission Microscope known as LTEM widely . We have applied LTEM to evaluate materials and devices for real use in the field of semiconductor R&D as well as fundamental material characterization. Here we explain some examples on semiconductors. The applications are MIS surface potential, solar cells, and wide bandgap semiconductors.

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In this talk, I propose to focus on two different topics that we have recently investigated with our BEC experiment using phase or/and amplitude modulation of a 1D optical lattices:

1) In the low frequency regime, we have studied the kinetics of the quantum transition to staggered states for phase-modulated lattice. Our data once combined with numerics enables us to identify the regime under which quantum fluctuations act as the triggering mechanism for the phase transition [1].

2) In the second part, I will focus on a relatively strong and quasi-resonant amplitude modulation of the lattice to observe a quantum chaos transport mechanism called chaos-assisted tunneling. Under such a modulation, the classical phase space exhibits stable islands surrounded by a large chaotic sea. The chain of islands mimics an effective superlattice, with new controllable tunneling properties. The coupling between islands is indeed mediated by delocalized Floquet states that spread over the chaotic sea. As a result, the transport between the islands exhibit sharp resonances where the tunneling rate varies by orders of magnitude over a short range of parameters. We experimentally demonstrate and characterize these resonances for the first time in a quantum system. This opens the way to new kinds of quantum simulations with long-range transport [2].

[1] E. Michon, C. Cabrera-Gutiérrez, A. Fortun, M. Berger, M. Arnal, V. Brunaud, J. Billy, C. Petitjean, P. Schlagheck, and D. Guéry-Odelin, New Journal of Physics 20, 053035 (2018).

[2] M. Arnal, G. Chatelain, M. Martinez, N. Dupont, O. Giraud, D. Ullmo, B. Georgeot, G. Lemarié, J. Billy and D. Guéry-Odelin, Science Advances 6, eabc4486 (2020).

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Advances in scaling down heterostructures and atomically-thin two-dimensional (2D) materials suggest a novel approach to systematically design materials as well as to realize exotic states of matter. A given material can be transformed through proximity effects whereby it acquires properties of its neighbors, for example, becoming superconducting, magnetic, topologically nontrivial, or with an enhanced spin-orbit coupling. Such proximity effects not only complement the conventional methods of designing materials, but can also overcome their various limitations. In proximitized materials it is possible to realize properties that are not present in any constituent region of the considered heterostructure. After providing some background on proximity effects we discuss implications of magnetism leaking into initially a non-magnetic region. We show that gate-tunable band topology allows helicity reversal of the emitted light and novel paths to spin lasers . Inspired by the 1937 prediction of Majorana fermions which are their own antiparticles, there is an intensive effort to realize their condensed-matter analogs. Combined magnetic and superconducting proximity effects could enable elusive topologically-protected Majorana bounds states (MBS) for fault-tolerant quantum computing. We discuss our proposal for realizing such MBS in 2D platforms and the challenges for their experimental demonstration. Recent measurements of proximity-induced topological superconductivity provide novel opportunities for controlling MBS.

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Cell migration plays a crucial role in both the proper development of an organism as well as the progression of certain diseases.  For example, white blood cells circulate in the bloodstream and then migrate through the tissue to reach the site of an infection.  Skin cells migrate collectively to close a wound during healing.  Cancer cells can shed from a tumor and travel long distances to a distant site and seed a secondary tumor in a process called metastasis.  In this talk, I will present several examples of how physical principles can be used to elucidate the mechanisms behind cell migration.  In particular, I will talk about how cells can migrate by changing their shapes in what has been termed amoeboid migration.  I will discuss experimental techniques to probe cell shape changes and to measure the forces that generate such deformations.  I will also discuss computational models developed to simulate such processes.

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We will present our recent developments in the realization of ultrasensitive vectorial force field sensors based on suspended silicon carbide nanowires, at room and dilution temperatures [4]. We will introduce an universal measurement method where the lateral 2D force field gradients are determined from the modifications of the nanowire eigenfrequencies and eigenvector orientations they induce [1,2]. We will then present how those exceptional force sensors can be implemented in a cavity nano-optomechanical experiment [3] where we giant coupling strength achieved opens the road towards optomechanical experiments in the single photon regime.

[1] L. Mercier de Lépinay et al, Nature Nanotech. 12, 156 (2017)
[2] L. Mercier de Lépinay et al, Nature Comm. 9, 1401(2018)
[3] F. Fogliano et al, arxiv 1904.01140 (2019)
[4] F. Fogliano et al, arxiv:200902912

Models of systems biology, climate change, ecology, complex instruments, and macroeconomics have parameters that are hard or impossible to measure directly. If we fit these unknown parameters, fiddling with them until they agree with past experiments, how much can we trust their predictions? We have found that predictions can be made despite huge uncertainties in the parameters – many parameter combinations are mostly unimportant to the collective behavior. We will use ideas and methods from differential geometry and approximation theory to explain sloppiness as a ‘hyperribbon’ structure of the manifold of possible model predictions. We show that physics theories are also sloppy – that sloppiness may be the underlying reason why the world is comprehensible. We will present new methods for visualizing this model manifold for probabilistic systems – such as the space of possible universes as measured by the cosmic microwave background radiation. 

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I will discuss our work on thin epitaxial films of Na3Bi (a topological Dirac semimetal) which, when thinned to a few atomic layers, becomes a large bandgap (>300 meV) quantum spin Hall insulator, with an electric-field tuned topological-to-conventional transition, making it a promising platform for topological electronics. We use STM to demonstrate that thick (3D) epitaxial films of Na3Bi have potential fluctuations comparable to high-quality graphene on hBN. Quasi-particle interference mapping via STM allows us to measure the valence and conduction band structure, where we find only two Dirac cones over a large energy range, and a surprisingly high electron velocity which suggests interactions are important in determining the bandstructure. When thinned to a few atomic layers Na3Bi is a large gap (>300 meV) 2D topological insulator with topologically protected edge modes observable in STM[3]. Electric field applied perpendicular to the Na3Bi film, by potassium doping or by proximity of an STM tip, closes the bandgap completely and reopens it as a conventional insulator. Electrical transport experiments in the edge conduction regime show a novel giant negative magnetoresistance due to suppression of spin-flip scattering by magnetic field. Comparison to a simple theoretical model indicates >98% of the resistance is due to spin-flip scattering, well beyond the limit of 2/3 for a generic non-helical metal with exchange-mediated scattering, providing an unambiguous signature of helical transport.

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Complex, stochastic processes underpin quantitative science. It is therefore of paramount importance to study and understand the behaviour of such processes for the crucial twin purposes of modelling and prediction. These tasks are typically resource-intensive, motivating the need for methods that ameliorate these requirements. A promising recent development to this end [1,2], using a cross-disciplinary blend of tools from quantum and complexity science, has highlighted that quantum simulators can operate with much smaller memories than the minimal possible classical models [3,4], while providing equally accurate predictions. Presently, these efficient quantum models are designed with prior knowledge of the minimal classical model, necessitating the use of classical inference algorithms when applied to real data. Here, we introduce the Quantum Inference Protocol, an inference algorithm specifically tailored to construct quantum models. It avoids certain drawbacks that the classical models contain [5]. We show that our protocol is robust to statistical noise arising from finite data, and does not require smoothing techniques for imperfect probability distributions. Our results form a key step in the application of this emerging field to real world systems.

References:
[1] M. Gu, K. Wiesner, E. Rieper, and V. Vedral, Nature Communications 3, 762 (2012).
[2] F. C. Binder, J. Thompson, and M. Gu, Physical Review Letters 120, 240502 (2018).
[3] J. P. Crutchield and K. Young, Physical Review Letters 63, 105 (1989).
[4] C. R. Shalizi and J. P. Crutchfield, Journal of Statistical Physics 104, 817 (2001).
[5] M. Ho, M. Gu, and T. J. Elliott, Physical Review A 101 (3), 032327 (2020).

After decades of explorations, suffering from the subtle nature and sample quality, whether a metallic ground state exists in a two-dimensional (2D) system beyond Anderson localization is still a mystery. Our work reveals how quantum phase coherence evolves across bosonic superconductor-metal-insulator transitions via magneto-conductance quantum oscillations in high-Tc superconducting films with patterned nanopores. A robust intervening anomalous metallic state characterized by both resistance and oscillation amplitude saturations in the low temperature regime is detected, which suggests that the saturation of phase coherence plays a prominent role in the formation of the anomalous metallic state. Furthermore, we carried out a systematic transport study on the macro-size ambient-stable ultrathin crystalline PdTe2 films grown by molecular beam epitaxy (MBE). Remarkably, at ultralow temperatures, the film undergoes superconducting state and anomalous metallic state with increasing perpendicular magnetic field. The high quality filters are used to exclude the influence from external high frequency noise. Our findings offer the reliable evidences on the existence of anomalous quantum metallic ground states in 2D systems, which could be of fundamental importance for the understanding of quantum materials. Ising superconductor is a kind of superconducting system with strong spin-orbit coupling (SOC). It is reported that the broken in-plane inversion symmetry gives rise to Zeeman-type SOC, which polarizes the spins of the electrons to the outof-plane direction and leads to a huge in-plane critical magnetic field much larger than Pauli limit. The Pauli limit is defined as the magnetic field required to destroy the Cooper pairs via the spin pair breaking effect in conventional superconductors. This special superconductivity with strong Zeeman-type SOC is called Ising superconductivity. Because of Zeeman-type SOC and spin polarizations, Ising superconductors exhibit large in-plane critical field up to several times of the Pauli limit. For the first time, we reported the observation of Ising superconductivity in macro-size monolayer NbSe2 films grown by MBE [3] and the interface induced Ising superconductivity in ultrathin crystalline Pb films [4]. Furthermore, the 6-monolayer (ML) (around 3 nm) PdTe2 film exhibits a large in-plane critical field more than 7 times of the Pauli limit, which is the characteristic of Ising superconductivity. Different from the previously reported Ising superconductors, the PdTe2 film keeps the in-plane inversion symmetry, which indicates that there exists a new mechanism of Ising superconductivity, so-called type-II Ising superconductivity

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Since the discovery of two-dimensional van der Waals magnets, the field of studying 2D magnetic phenomena has been rapidly developed. In this talk, I will present our recent progress along this direction. I will firstly discuss the observation of antiferromagnetic exciton and multiple exciton phonon bound states in zigzag antiferromagnet NiPS3. I will then discuss the emergent orbital ferromagnetism in twisted monolayer-bilayer graphene. The ferromagnetism occurs at one-quarter filling of the conduction band, with a large associated anomalous Hall effect. Uniquely, the magnetization direction can be switched purely with electrostatic doping at zero magnetic field.

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The high precision and accuracy possible with atomic systems make them attractive for a range of applications including quantum sensing, quantum computation, and quantum simulation. I will discuss two experiments at UCLA towards sensing motion with laser-cooled atoms. The first borrows techniques from cavity QED to use an optical cavity to directly read out the motion of an ensemble of atoms over subwavelength scales on a microsecond timescale. The second experiment combines techniques from matter wave interferometry and quantum computation towards the creation of a single ion gyroscope.

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Billions of years of natural evolution have created today's colorful biological world. Biomimetics have given us many important ideas for solving scientific and engineering problems for more than 100 years. From a material perspective, many biological structures are made of nanomaterials with intriguing capabilities to manipulate light propagation. In this talk, the speaker will firstly briefly summarize some inspirations from micro-nano biomimetics that have led to unique designs of optical and optoelectronic devices. Then the speaker will focus on one example which is biomimetic eye using a hemispherical nanowire array as an artificial retina.

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Surfaces of topological materials, such as topological insulators or Weyl semimetals, are known to host metallic states due to the bulkedge correspondence. In addition to the usual topologically protected chiral surface states, which do not depend on the specific form of the interface, several massive states appear if the interface width is larger than a particular intrinsic length (given by the bulk gap and the Fermi velocity). These states, first described by Volkov and Pankratov (VP) in the 1990ies [1], are intrinsically relativistic and can be related to Landau bands of relativistic fermions. We show that the gap variation can be interpreted precisely as a vector potential that is affected by an additional electric field in a relativistic manner [2]. The electric field can thus be used not only to dope electronically these massive surface states, but they become even more accessible due to the reduction of the Landau gap in the presence of an electric field. The effect is at the origin of an oscillating resistance measured as a function of the electric field in high-frequency experiments at ENS, Paris [3]. Furthermore, VP are expected to have a clear signature in magneto-optical spectroscopy [4]. We finish with a short discussion of how this "Landau-level approach" can also be used in the framework of Weyl semimetals and the description of Fermi arcs that play the role of chiral Landau bands here [5,6].

[1] V. Volkov and O. Pankratov, JETP Lett. 42, 178 (1985).
[2] S. Thoumakov et al., Phys. Rev. B 96, 201302 (2017).
[3] A. Inhofer et al., Phys. Rev. B 96, 195104 (2017).
[4] X. Lu and M. O. Goerbig, EPL 126, 67004 (2019).
[5] S. Tchoumakov et al., Phys. Rev. B 95, 125306 (2017).
[6] D. K. Mukherjee et al., arXiv:1907.01295

Networks of interacting polariton condensates have been shown to offer a versatile platform for engineering and studying complex systems such as phase or spin-synchronized lattices [1, 2]. In this talk, we will present an in-depth experimental study of the nature of interaction, synchronization and coherence between spatially separated, nontrapped and ballistically expanding polariton condensates. We show that this driven-dissipative system differs from a conventional Josephson-junction of trapped condensates since the coupling is not mediated by a tunneling current but by radiative coupling inherently connected with finite time of particle transfer [3]. Synchronization of polariton condensates is observed for macroscopic distances (>100 µm) in systems of only two spatially separated condensates, as well as in 1D and 2D lattices of polariton condensates. We demonstrate that interactions in-between condensates can be optically controlled [4] and are described by delay-differential equations which makes networks of non-trapped polariton condensates a promising platform to study time-delay coupled systems [5], that arise in many areas of nature. Using ordered arrangement of multiple lasers, we demonstrate our ability to synthesise various polariton crystal landscapes. Specifically, we investigate polyacetylene-like lattices (Su-Schrieffer-Heeger system) where laser engineered dimerised interactions allow us to observe period doubling of the crystal bands and multimodal condensation. When lattice defects are implanted flat-bands appear in the band structure with subsequent polariton condensation into solitonic solutions. Moreover, in periodic polygon structures, characterized by discrete rotational symmetry, we find modes supporting persistent circulating currents along the polygon edges. Lastly, we briefly discuss the prospect and challenges of using polariton condensate networks as neuromorphic hardware [6].

References:
[1] N.G. Berloff, et al., Nature Materials 16, 1120-1126 (2017).
[2] H. Ohadi, et al., Phys. Rev Letters 119, 067401 (2017).
[3] J.D. Töpfer, et al., arXiv:1905.09092 (2019).
[4] S. Alyatkin, et al., arXiv:1907.08580 (2019).
[5] H.G. Schuster, et al., Progress of Theoretical Physics 81, 939 (1989).
[6] A. Opala, et al., Phys. Rev. Applied, 2019, 11, 064029.