Condensed Matter / Quantum Information

MAY 24

“Plasma-based Gas Aggregation Sources of Nanoparticles: Perspectives and Challenges“

ONDREJ KYLIAN

Department of Macromolecular Physics, Charles University, Prague, Czech Republic

ABSTRACT: 

Magnetron-based gas aggregation sources (m-GAS) have become a highly interesting alternative to more commonly used chemical techniques for the synthesis of nanoparticles. The increasing popularity of these deposition systems relates to their advantageous features: relative simplicity, no or limited use of potentially harmful solvents or chemical precursors and the possibility of deposition of nanoparticles on virtually any substrate compatible with high-vacuum conditions. However, despite the unique features that m-GAS deposition systems offer, their use is still limited to laboratories. As it will be discussed, this situation is, besides fundamental questions related to the formation, growth, and transport of NPs, also due to technological issues connected with a relatively low production yield of NPs that is unpractical for real use and the limited range of materials from which nanoparticles may be produced by m-GAS systems. Various novel concepts that address the current needs will be introduced that include the use of cylindrical post magnetrons, full face erosion magnetrons, magnetrons with segmental targets or systems based on the in-flight coating/modification/decoration of NPs.

APRIL 19

“Superconductivity in the Pressurized Nickelate La3Ni2O7 in the Vicinity of a BEC-BCS Crossover“

HENNING SCHLOEMER

Theoretical Nanophysics, Ludwig-Maximilians-University of Munich, Germany

ABSTRACT: 

Ever since the discovery of high-temperature superconductivity in cuprates, gaining microscopic insights into the nature of pairing in strongly correlated, repulsively interacting fermionic systems has remained one of the greatest challenges in modern condensed matter physics. Following recent experiments reporting superconductivity in the bilayer nickelate La3Ni2O7 (LNO) with remarkably high critical temperatures of Tc = 80 K, it has been argued that the low-energy physics of LNO can be described by a strongly correlated, bilayer t-J model. In this talk, I present our recent investigations of this bilayer system, where we utilize density matrix renormalization group techniques to establish a thorough understanding of the model and the magnetically induced pairing through comparison to the perturbative limit of dominating inter-layer spin couplings. In particular, this allows us to explain appearing finite-size effects, firmly establishing the existence of long-range pairing order in the thermodynamic limit. As the effective model in the perturbative limit is known to show linear resistivity above the superconducting transition temperature, we propose a pair-based interpretation of the extended strange metal phase observed in LNO. By analyzing binding energies, we predict a BEC-BCS crossover as a function of the Hamiltonian parameters, whereas LNO is anticipated to lie on the BCS side in vicinity of the transition. Lastly, I discuss how binding energies in the system are of the order of the inter-layer coupling, which suggest strikingly high critical temperatures of the Berezinskii-Kosterlitz-Thouless transition and raise the question whether nickelate superconductors possibly facilitate critical temperatures above room temperature.

APRIL 12

“Rotation Symmetry Protected Boundary Modes in Abelian Topological Phases“

ABHINAV PREM

Theoretical Physics, IAS Princeton

ABSTRACT: 

Spatial symmetries can enrich the topological classification of interacting quantum matter and endow additional “weak” topological indices upon systems with non-trivial strong topological invariants (protected by internal symmetries). In this talk, I will discuss the boundary physics of charge conserving systems with a non-zero shift invariant, which is protected by either a continuous U(1) or discrete CN rotation symmetry. In particular, I will discuss an interface between two systems with the same Chern number but distinct shift invariants and show that the interface hosts protected gapless edge modes. For general Abelian topological orders in 2D, I will prove sufficient conditions for gapless edge states protected by continuous rotation symmetry. For the case of discrete rotation symmetries, I will show that the Chern-Simons field theory for systems with gappable edges predicts fractional corner charges. These can also be computed when the system is placed on the two-dimensional surface of a Platonic solid, which relates to the fractional charge bound at disclination defects. Time permitting, I will discuss recent results regarding the relation between the shift and many-body real-space invariants for 2D systems with crystalline symmetry.

APRIL 5

“Probing Matter in Space-Time with the Quantum Superposition Microscope“

WILSON HO 

UC Irvine

ABSTRACT:

The quantum superposition microscope (QSM) combines ultrafast coherent light with a scanning tunneling microscope (STM) to induce the superposition of two levels in a molecular sensor for advancing precision measurement of matter with simultaneous femtosecond and Ångström resolutions. The sensing molecule in the STM junction is irradiated with femtosecond pump-probe pulses of radiation and coherent oscillations of its two levels are measured by the tunneling electrons. The oscillation frequency exhibits greatly enhanced sensitivity to the sensor’s environment compared to microscopes that do not rely on the superposition principle. The decoherence time probes further the coupling of the sensor to the sample. The common occurrence of systems with two levels as eigenstates in a double-well potential suggests broad applications of the QSM in sensing the static and dynamic properties of matter in space-time. The operational principles of the QSM reveal its unique applications as exemplified by using femtosecond THz radiation in exciting a single hydrogen molecule in the STM junction to sense sample’s electric field at the atomic scale.

CANCELLED – MARCH 22 

“Micro Thermoelectric Devices: Bridging Thermal Management and Empowering the Internet-of-Things”

HEIKO REITH

Leibniz Institute of Solid State and Materials Research, Dresden, Germany

ABSTRACT:

Electrochemical deposition is an excellent choice for the fabrication of micro thermoelectric devices (μTED) that can be used for hot spot cooling and self-powered sensing applications in combination with fabrication methods that are fully compatible with on-chip integration.1 Electrochemical deposition offers advantages over other techniques due to its compatibility with lithography, its high deposition rates, its tunable material composition, its cost efficiency and its large scalability. In order to achieve maximum device performance, the geometry of the device needs to be optimized to match the electrical and thermal properties of the thermoelectric materials and to maximize the ZT value of the device. This is usually done by adjusting the cross-sectional areas. However, this limits the maximum packing density. Here we present a new design strategy for the fabrication of μTEDs in which we optimize the heights to enable higher packing densities, to increase the cooling power density and the output power, and to reduce the amount of thermoelectric materials required, which are often toxic and expensive.
To fabricate the μTEDs, we use photolithography combined with electrochemical deposition of Bi2(TexSe1-x)3 and Te as n-type and p-type materials, respectively.2,3 Injecting N2 into the electrolyte prior to deposition reduces the amount of oxygen in the electrodeposited material, resulting in lower contact resistance, higher ΔT and increased cooling power density. To increase the ZT of the device, which is crucial for better device performance, we have also implemented geometry optimization in the fabrication process. The maximum cooling of the area-optimized and height-optimized μTEDs is 10.8 K and 10.5 K at room temperature, respectively. Finite element simulations show both can achieve cooling densities of hundreds of watts per square centimeter. The height-optimized μTEDs exhibit maximum cooling of 21 K at an ambient temperature of 343 K and require 75% less Te than area-optimized devices.2 As generators, the μTEDs exhibit high open-circuit voltages and power densities in the range of a few milliwatts per square centimeter, even at low temperature differences. Fine-tuned for efficiency, stability and seamless scalability, these embedded μTEDs highlight their exceptional suitability for hotspot cooling. They also enable higher circuit integration density and provide power for energy-autonomous Internet-of-Things sensors, paving the way for diverse applications in areas such as biomedicine, IoT device operation and local thermal management.

MARCH 1

“The Anomalous Hall Nano-Oscillator”

ERIC ARTURO MONTOYA

University of Utah

ABSTRACT:

Spin-orbit torques (SOTs) enable the energy-efficient manipulation of magnetization by electric current which is promising for future information technology – with applications in non-volatile memory, microwave-assisted magnetic recording, spin torque nano-oscillators (STNO), and neuromorphic computing. SOTs can be generated in bilayers of ferromagnetic (FM) and non-magnetic (NM) materials with broken inversion symmetry. The most explored SOTs are the spin Hall torque originating from spin-orbit coupling in in NM layers and the Rashba torque induced by spin-orbit coupling at NM/FM interfaces. Recently, we have demonstrated that the planar Hall current in a FM can lead to a giant SOT applied to the same FM, an example of a new class of self-generated spin orbit torques. In this talk, I will report the discovery of a giant self-generated SOT induced by anomalous Hall current in FM conductors. This anomalous Hall torque displays an unusual angular symmetry that is distinct from the previously discovered spin Hall, Rashba, and planar Hall torques. We demonstrate that the strength of the anomalous Hall SOT can be tuned by filling of the FM electronic bands in the same manner as the AHE magnitude. Finally, we demonstrate a new type of spin torque oscillator ­– the anomalous Hall nano-oscillator.

FEBRUARY 16

“Van der Waals scanning probe tips”

ABHISEK KOLE

Peter Grunberg Institut (PGI-3), Forschungszentrum Julich, Germany

ABSTRACT:

Van der Waals materials are known for their intriguing emergent 2D physics, such as correlated phenomena and topological effects. Here, we report the methodical fabrication of van der Waals scanning tunnelling tips from exfoliated graphite flakes with a graphene nanoribbon like edge as a scanning tunnelling tip. The principle of STM is based on the quantum mechanical tunneling between the tip and sample, revealing the convoluted underlying electronic structure. We characterize the tip by performing atomically resolved STM of an Ag(111) surface. The tip-sample differential conductance 𝑑𝐼𝑑𝑉⁄ revealed direct evidence of tunnelling through a graphene nanoribbon with zigzag edge state. Furthermore, analysis of Friedel oscillation around atomic defects on Ag(111) surface reveals the signature of momentum-dependent tunneling between the tip and sample, which we support by tight binding calculations. Our results extend the scanning probe toolkit and establish a new method for the characterization of novel systems.

FEBRUARY 9

“Infrared characterization of laser-synthesized metal clusters and metal fullerene complexes: From vibrations to intra d-band transitions”

JOOST BAKKER

Radboud University, Institute for Molecules and Materials, HFML-FELIX Laboratory, Nijmegen, The Netherlands

ABSTRACT:

Atomic clusters uniquely bridge the gap between the discrete electronic levels of atoms and the correlated band structure of bulk matter. As a well-defined and fully controllable system free from outer influences, gas phase clusters are used as model systems for the active site in heterogeneous catalysis, as potential seed species form the formation of more complex materials in the interstellar medium, and as models for the emergence of bulk properties. Because of their quantum nature, cluster properties can drastically vary, with order of magnitude changes upon the addition or elimination of a single atom. The FELIX infrared free-electron lasers provide broadly tunable and intense light in the THz to near-IR spectral range. In this talk, I will discuss how utilize this radiation for structural characterization of on-the-fly synthesized metal-fullerene complexes as well as for cluster systems of pure metal atoms, where we seek to characterize low-lying electronic states, indicative for the still molecule-like nature of these particles.

JANUARY 26

“Physical Limits to Evolutionary and Engineering Optimization of Bacterial Motility”

VICTOR SOURJIK

Max Planck Institute for Terrestrial Microbiology and Center for Synthetic Microbiology, Marburg

ABSTRACT:

Although all biological systems must obey the laws of physics, specific examples of physical limitations on the performance of biological systems remain sparce. Bacterial motility is among the quantitatively best-understood biological behaviors, as it has long served as a model of how physics can help to understand bacterial ability to move and follow chemical gradients in the environment (chemotaxis). Using the example of Escherichia coli, I will discuss how physical limits might have shaped the evolution of bacterial motility and of the chemotaxis system. Our recent work suggests that physical limitations on bacterial swimming, along with fitness tradeoffs associated with investment of limited cellular resources in motility, can be sufficient to quantitatively explain regulation of motility gene expression in E. coli. Moreover, physics of motility also determines performance of bacterial microswimmers that could be used for various biotherapeutic applications, and taking physics into account is important for their rational engineering.

JANUARY 19

“Solar-cycle variation of solar meridional flow and its implications on the magnetic fields in the convection zone”

CHIA-HSIEN LIN

Department of Space Science and Engineering, National Central University, Taiwan

ABSTRACT: Solar magnetic fields are the main driver of many observed activities on the Sun and in the interplanetary space. While observations have indicated that the magnetic fields come from below the solar surface, there has been no convincing detection of the magnetic fields in the deep convective zone because the magnetic pressure is much smaller than the gas presure inside the Sun. As a result, where the sources of the variable solar magnetic fields are located and how they change with solar cycle remain as some of the biggest mysteries in the solar physics. In this study, the aim is to probe the variation of the magnetic fields below the surface using solar meridional flows. Solar meridional flows are axisymmetric flows on the meridional planes, and exist in the entire convective zone. We use SOHO/MDI helioseismic data from 1996 to 2010, which includes two solar minima and one maximum. The time-distance method is first applied to the data to determine the travel-time difference between northward and southward propagating waves. The travel-time difference is then related to the meridional flow based on the ray path theory. Finally, an inversion procedure is applied to the travel-time difference to obtain the meridional flow speed at the solar minimum and maximum. The results show that the flow pattern in the entire convective zone changes significantly from solar minimum to maximum and that the flow change is related to the active latitudes, which are the centroid locations of the surface magnetic fields. This suggests that the change of the meridional flow pattern is closely related to the change of the magnetic fields. Therefore, the solar cycle variation of the meridional flow is a promising tool to probe the solar cycle variation of the solar magnetic fields in the convective zone.

*Wednesday, December 6, 2023 

3:00 pm in SSL 104, Viz Lab

Lindsay Bassman Oftelie, Marie Curie Fellow, CNRNANO, Pisa, Italy

Algorithmic Cooling: Theory and Implementation on Quantum Computers

While a variety of physical implementations of quantum computers are still being explored, all must fulfill a fundamental set of requirements, one of which is the ability to initialize the qubits into a pure, fiducial quantum state. Such purified, or cooled, qubits are needed for both the initialization of registers for computation, as well as for ancillary qubits required for quantum error correction. Thus, a key hurdle in the success of quantum computers is developing methods to generate extremely cold qubits. Algorithmic cooling (AC), a promising technique for purifying qubits beyond what physical cooling methods can achieve, lowers the temperature of a subset of qubits by applying certain logic gates to the entire system of qubits. Here we investigate AC with the typical qubits used in contemporary quantum computers. We identify a family of algorithms which optimally cools a target qubit and find the analytical expression for its final energy. Furthermore, we identify the sub-family of algorithms that achieves maximal cooling with minimal work cost. Finally, we investigate implementing these cooling algorithms on noisy quantum computers.

November 17, 2023

Nai-Chang Yeh, Caltech

Engineering Topological Electronics, Photonics and Optoelectronics Based on Two-Dimensional van der Waals Materials

Topology has been recognized to play an important role in providing novel routes to understanding and creating new quantum phases and phenomena in condensed matter physics. Recent advances in the development of two-dimensional (2D) atomic crystals from various van der Waals (vdW) materials have also simulated intense research efforts because of their unique properties for exploring the effects of different degrees of freedom on the quantum states of matter and their great promises for a wide range of technological applications. In this talk, I will describe our recent expeditions of engineering topological electronics, photonics, and optoelectronics based on 2D-vdW materials. For topological electronics, we develop valley-Hall transistors based on nanoscale strain-engineered single layer graphene ^[1] and discover a variety of new quantum phenomena associated with the topological electronic states in the presence of strain-induced giant pseudo-magnetic fields. In the case of topological photonics, we explore strong light-matter interactions in single-layer transition metal dichalcogenides (TMDs) with topological photons generated by optical and plasmonic vortices. ^[2,3] We will further describe how novel topological optoelectronic phenomena may be generated by applying spin-orbit coupling of light to the 2D-vdW materials and discuss potential technological applications of these topological electronics/photonic/optoelectronics.

References:

^[1] “Nanoscale engineering of giant pseudo-magnetic fields, valley polarization and topological channels in strained graphene”, C.-C. Hsu, M. L. Teague, J.-Q. Wang, and N.-C. Yeh^*, Science Advances 6, aat9488 (2020).

^[2] “Dramatically enhanced valley-polarized emission of monolayer WS₂ at room temperature with plasmonic Archimedes spiral nanostructures and gated control”, W.-H. Lin^*, P. C. Wu, H. Akbari, G. R. Rossman, N.-C. Yeh^*, and H. A. Atwater^*, Advanced Materials 34 (3), 2104863 (2022).

^[3] “Control of trion-to-exciton conversion in monolayer WS₂ by orbital angular momentum of light”, R. Kesarwani, K. B. Simbulan, T.-D. Huang, Y.-F. Chiang, N.-C. Yeh^*, Y.-W. Lan^*, and T.-H. Lu^*, Science Advances 8, eabm0100 (2022).

October 27, 2023 – *Seminar Begins at 1:00 pm*

Sergei Urazhdin, Emory University

Orbital Liquid in Ultrathin Magnetic Films

Electron’s orbital moment has recently emerged as an important degree of freedom which can be generated, transported, and used to control magnetic systems [1]. However, its role in magnetism remains poorly understood. I will discuss surprising experimental observations revealing a crucial role of electron’s orbital moment in ferromagnetism and elucidating a previously unrecognized connection between magnetism and unconventional superconductivity.

Our magnetoelectronic measurements of heterostructures based on ultrathin transition metal ferromagnets revealed two separate magnetic order parameters: one associated with spin ordering at the Curie point T_C1, and another “anomalous” order parameter with a critical point T_c2 about 50K above the Curie temperature. Remarkably, magneto-optical measurements are not sensitive to the anomalous contribution, suggesting that the origin of the latter is qualitatively different from spin magnetism. X-ray magnetic circular dichroism (XMCD) measurements show that the “anomalous” order parameter is associated with incipient orbital ferromagnetism whose signatures vanish below T_c2 without the onset of ferromagnetic orbital ordering. Electric current applied to micropatterned structures in this regime reveals that orbital magnetism is “hidden” but does not disappear below T_c2.

I will show that these anomalous behaviors are captured by a simple Hubbard model of orbital correlations among nearest neighbor sites in an ultrathin ferromagnetic layer, leading to the conclusion that orbital moments form an orbital liquid [2] – a long-range correlated orbital state that lacks ordering due to the geometric orbital frustration, analogous to quantum spin liquids formed by frustrated spins and believed to hold the key to high-temperature superconductivity [3]. In the studied orbital liquid, orbital moments are ferromagnetically coupled, which would be impossible for spin liquid due to spin conservation. I will discuss the implications of these results for our understanding of the mechanisms of magnetism and for the emerging field of orbitronics.

[1] D. Go, et al. “Orbitronics: Orbital currents in solids”, EPL 135, 37001 (2021).

[2] S. Ivanov, J. Peacock, S. Urazhdin “Orbital correlations in ultrathin films of late transition metals”, Phys. Rev. Mater. 7, 01440 (2023)

[3] P. W. Anderson “The Resonating Valence Bond State in La₂CuO₄ and Superconductivity”, Science 235, 1196 (1987).

October 6, 2023

Paolo Biagioni, Department of Physics, Politecnico di Milano

Plasmonic and Dielectric Antennas in the Mid Infrared with Semiconductor Materials

Plasmonics has not yet made its way to the microelectronic industry, mostly because of the lack of compatibility of typical plasmonic materials with foundry processes. In this context, we have undertaken the development of heavily-doped Ge films as novel plasmonic materials, grown on Si wafers with CMOS-compatible processes [1].

We will review the plasmonic applications developed in the framework and with the foreground of the European project GEMINI (‘Germanium mid-infrared plasmonics for sensing’), including antenna arrays for sensing [2], optically-activated antennas [3], nanoantennas for nonlinear frequency conversion [4], and time-resolved experiments [5].

Moreover, we will discuss our recent approaches to sensing based on the use of epsilon-near-zero InAs materials to boost dielectric antennas, which compared to plasmonic antennas offer lower material losses and a richer palette of multipolar resonances that can find e.g. application in chiral light-matter interactions.

[1] J. Frigerio, A. Ballabio, G. Isella, E. Sakat, G. Pellegrini, P. Biagioni, M. Bollani, E. Napolitani, C. Manganelli, M. Virgilio, A. Grupp, M.P. Fischer, D. Brida, K. Gallacher, D.J. Paul, L. Baldassarre, P. Calvani, V. Giliberti, A. Nucara, and M. Ortolani, Phys. Rev. B 94, 085202 (2016).

[2] L. Baldassarre, E. Sakat, J. Frigerio, A. Samarelli, K. Gallacher, E. Calandrini, G. Isella, D.J. Paul,  M. Ortolani, and P. Biagioni, Nano Lett. 15, 7225 (2015).

[3] M. P. Fischer, Ch. Schmidt, E. Sakat, J. Stock, A. Samarelli, J. Frigerio, M. Ortolani, D.J. Paul, G. Isella, A. Leitenstorfer, P. Biagioni, and D. Brida, Phys. Rev. Lett. 117, 047401 (2016).

[4] M. P. Fischer, A. Riede, K. Gallacher, J. Frigerio, G. Pellegrini, M. Ortolani, D.J. Paul, G. Isella, A. Leitenstorfer, P. Biagioni, and D. Brida, Light: Science & Applications 7, 106 (2018).

[5] M.P. Fischer, N. Maccaferri, K. Gallacher, J. Frigerio, G. Pellegrini, D.J. Paul, G. Isella, A. Leitenstorfer, P. Biagioni, and D. Brida, Optica 8, 898 (2021).

September 29, 2023

Brad Marston, Brown Theoretical Physics Center, Brown University

Waves of Topological Origin in the Fluid Earth System and Beyond

Symmetries and topology are central to our understanding of physical systems. Topology, for instance, explains the precise quantization of the Hall effect and the protection of surface states in topological insulators against scattering from disorder or bumps. However discrete symmetries and topology have not, until recently, contributed much to our understanding of the fluid dynamics of oceans and atmospheres. In this talk I show that, as a consequence of the rotation of the Earth that breaks time reversal symmetry, equatorial Kelvin and Yanai waves emerge as topologically protected edge modes. The non-trivial topology of the bulk Poincaré waves is revealed through their winding number in frequency – wavevector space. Bulkinterface correspondence then guarantees the existence of the two equatorial waves. I discuss our recent direct detection of the winding number in observations of Earth’s stratosphere. Thus the oceans and atmosphere of Earth naturally share basic physics with topological insulators [1]. As equatorially trapped Kelvin waves in the Pacific ocean are an important component of El Niño Southern Oscillation, the largest climate oscillation on time scales of a few years, topology plays a surprising role in Earth’s climate system. We also predict thatwaves of topological origin will arise in magnetized plasmas [2]. The waves may appear in laboratory plasma experiments, and they may also arise in the solar system and beyond.

[1] Delplace, P., Marston, J. B. & Venaille, A. Topological origin of equatorial waves. Science 358,
1075–1077 (2017).

[2] Parker, J. B., Marston, J. B., Tobias, S. M. & Zhu, Z. Topological Gaseous Plasmon Polariton in
Realistic Plasma. Physical Review Letters 124, 195001 (2020)

September 22, 2023 

Thomas Schaepers, Juelich Forschungszentrum, Germany

Phase-Coherent Transport in Multi-terminal Topological Insulator-Based Nanostructures 

Networks of three-dimensional nanoribbons of topological insulators (TI) in combination with superconducting electrodes are promising building blocks for topoelectronic applications and topological quantum computations. In our approach, these structures are fabricated by a dedicated fabrication method that uses selective-area growth in combination with in-situ shadow evaporation of the superconducting electrodes. On single straight TI nanoribbons and TI ring structures, we have found pronounced Aharonov-Bohm oscillations in magnetoresistance, indicating transport via topologically protected surface states [1,2,3]. In three-terminal TI nanoribbon T- and Y-junctions, a dependence of the current on the in-plane magnetic field has been observed, with the current in the surface states being clearly steered toward a preferred output at different magnetic field orientations. The origin of this steering effect is interpreted in terms of orbital effects in combination with spin-momentum locking [4]. In in-situ prepared superconductor-topological insulator nanoribbon Josephson junctions a pronounced supercurrent was observed [5]. For multi-terminal TI hybrid junction the interplay of the Josephson supercurrent in the different branches is investigated. Here, we found a clear coupling in the supercurrent between the different electrodes.

Work done in collaboration with: D. Rosenbach, J. Kölzer, G. Behner, E. Zimmermann, J. Teller, A. Rupp, J. Karthein, A.R. Jalil, K. Moors, T.W. Schmitt, M. Schleenvoigt, M. Vaßen-Carl, G. Bihlmaier, H. Lüth, G. Mussler, P. Schüffelgen, D. Grützmacher.
[1] J. Kölzer, et al., Nanotechnology 31, 325001 (2020).
[2] D. Rosenbach, et al., Sci. Post. Phys. Core 5, 17 (2022).
[3] G. Behner et al., Nano Letters, 23, 6347 (2023).
[4] J. Kölzer, et al., Communications Materials 2, 1 (2021).
[5] D. Rosenbach, et al., Science Advances 7, eabf1854 (2021).

March 11, 2022

Patrick Ayotte
Fulbright fellow, Département de Chimie
Université de Sherbrooke

Shining light, and beams of molecules, to look at chemistry on ice

Abstract:

Ice surfaces play an important role in atmospheric and interstellar chemistry, catalyzing reactions affecting air quality in the polar boundary layer, upper tropospheric and stratospheric chemistry, and the formation of complex molecules in the interstellar medium (ISM).  Molecular beam and surface science techniques, along with in situ optical/electronic spectroscopy/microscopy, can help quantify the complex coupled interfacial kinetics while molecular modeling and numerical simulations yield insight into the rich and complex ice surface chemistry. Since the mid 80’s, the astrophysical community has used the abundance of the nuclear spin isomers (NSI) of water molecules to provide insight into the formation mechanism of the molecular constituents of the ISM, the history of comets as well as other celestial bodies. Water samples enriched in the ortho-H₂O NSI are also foreseen to yield NMR applications in surface science. Motivated by these perspectives, methodological bottlenecks need to be overcome: 1-the development of efficient enrichment protocols, and 2-the improvement of storage strategies for water samples enriched in either of its NSI. We use magnetic focussing in a supersonic molecular beam to prepare water vapour strongly enriched (i.e., OPR>50:1) in the ortho-H₂O NSI. These samples can be stored in rare gas matrices and endofullerenes however, NSI interconversion, a process that is strongly forbidden by quantum mechanics, causes sample enrichments to decay over several hours.  We will delve into the isotope, matrix and temperature dependence of the NSI inter-conversion kinetics of isolated water molecules to look for clues of the interconversion mechanism thereby revealing the role of confinement on the intramolecular hyperfine couplings responsible for ortho-H₂O«para-H₂O inter-conversion.  Finally, perspectives for the application of ortho-H₂O to study chemical dynamics at ice surfaces will be discussed.

February 18, 2022

Dr. Bikash Kanungo

Research Scientist, Dept. of Mechanical Engineering

University of Michigan

A data driven approach to improved exchange-correlation functionals in density functional theory

Abstract

The need for improved exchange-correlation (XC) functionals in density functional theory (DFT) that can provide quantum accuracy can hardly be over-emphasized. Traditionally, the development of XC functionals have involved a mixture of physical intuition and semi-empirical fitting. The existing XC functionals, despite their successes in prediction of various material properties, exhibit some notable failures: under-prediction of band-gaps, inaccurate bond-dissociation curves, inaccurate reaction barriers, to name a few. To remedy the shortcomings of the existing XC function, we envisage a data driven approach to construct XC functionals, using accurate groundstate densities from many-body calculations. The key idea is to first obtain the exact XC potential corresponding to a groundstate density via the inverse DFT method. Subsequently, the density and XC potential pairs from various atoms and molecules can form the training data to learn the functional dependence between them, and hence, approximate the XC functional. In this talk, I will present an accurate and robust approach to solve the inverse DFT problem which had, heretofore, remained an open challenge owing to various numerical instabilities in previous attempts. I will present the exact XC potential for several molecules, ranging from weakly to strongly correlated systems, computed from full configuration interaction (CI) reference densities. I will also provide a comparison of the exact XC potential against model XC potentials from non-local (B3LYP, HSE06, SCAN0, and M08-HX) and semilocal/local (SCAN,PBE, and PW92) XC functionals. This comparative study establishes substantial qualitative and quantitative differences between the exact and model XC potentials, despite small differences in their densities. As a result, it underlines the utility of the XC potentials in the design and development of future XC functionals, which remains an unexplored route to developing XC functionals. Lastly, I will discuss some of the ongoing efforts in the group towards developing neural network (NN) based local and non-local XC functionals, utilizing the exact XC potentials from our inverse DFT calculations.

February 4, 2o22

Bjorn Kjellstrand 

Arizona State University

The PMC Turbo Mission: Studying Gravity Wave and Instability Dynamics in the Polar Summer Mesosphere

Abstract:

PMC Turbo is a balloon-borne experiment designed to observe gravity waves, instabilities, and turbulence traced by polar mesospheric clouds. In the middle and upper atmosphere, dynamics with scales ranging from tens of meters to several thousand kilometers primarily arise due to the influence of gravity waves propagating from lower altitudes. While gravity waves play an important role in the structure and variability of the middle atmosphere at local and global scales, their effects are imprecisely parameterized by climate models.

PMC Turbo flew for nearly six days at an altitude of 38 kilometers while observing dynamics above 80 kilometers. We launched it from the Esrange Space Center in Sweden and it landed near the Northwest Passage in Canada. The experiment included seven pressure vessels, each of which contained an optical camera and a computer controlling image capture, flight control, and ground communication. The payload also included the first high-powered lidar to successfully fly aboard a balloon-borne platform.

In addition to the instrument, I will discuss Kelvin-Helmholtz instabilities we observed. These instabilities play a major role in energy dissipation and have a close relationship with gravity waves. Imaged Kelvin-Helmholtz instabilities include complicated billow interactions and secondary instabilities. Comparison between our data and direct numerical simulations reveals an elevated rate of energy dissipation associated with these dynamics. I will also discuss future plans, including analysis of data from a “piggyback” instrument that flew over Antarctica.

January 28, 2022

Samuele Sanna

Università di Bologna

Nuclei and muons as local magnetic probes to study quantum materials

Abstract:

Quantum materials display emergent quantum macroscopic properties which are the result of the many-body collective interactions of their particles and quasi-particles components typically acting on the atomic or nanoscopic length scale. Their behavior is often enriched and dominated by the competition of different concurrent couplings such as exchange, spin-orbit, electron-phonon, etc, of comparable energy scale.

Suitable nuclei, when naturally present in the original material, or implanted positive muons mu⁺ can be exploited as local probes of the quantum properties arising from these collective interactions and competing energy couplings. The local character of nuclear and mu⁺ spectroscopies relies on the short-range magnetic hyperfine and/or the electric quadrupolar interactions with the environment.

In this talk I will illustrate some of the potentials of the nuclear magnetic and quadrupole resonances (NMR/NQR) and muon spin spectroscopy (mu⁺SR) techniques to study collective phenomena occurring in magnetic, superconducting and charge ordered quantum materials. Recent selected examples will consider quasi-2D unconventional superconductors and/or materials with strong spin-orbit coupling displaying multipolar-magnetic or charge density wave ordering. The examples will include both static or quasi-static and low-frequency dynamical phenomena, with respect to the timescale of these techniques (10^-12-10^-4 s and 10^-10-10^-2 s for mu⁺SR and NMR respectively).

December 10, 2021

Raffaella Calarco

Institute for Microelectronic and Microsystems

Chalcogenide Phase-Change Materials: Basic Research and Applied Perspectives

Abstract:

For twenty years chalcogenide (PCMs) have found their major application in re-writable optical storage media. Nowadays, PCMs are employed also in microelectronics for non-volatile electrical random access memories (PCRAM). The success of these materials is related to the rapid phase-change from the crystalline to the amorphous state, both exhibiting dissimilar optical contrast which is also accompanied by a huge difference in resistivity. Many different compounds exhibit PCM properties; however the most interesting, from both a fundamental and practical perspective for PCRAM application are the alloys along the GeTe-Sb₂Te₃ (GST) pseudo-binary line.

The understanding of the switching process in PCM, dating back to the 1970s, was based upon a picture in which an amorphous phase is generated by a rapid cooling of the melt and the corresponding crystalline phase is created by annealing the amorphous phase above the crystallization temperature. While this simple idea has a strong appeal, the subtle nature of PCM bonding was shown to offer the interpretation to the switching.

Ge₂Sb₂Te₅ displays a cubic and a trigonal crystalline phase. Such materials are lamellar and display a pronounced bond hierarchy, featuring strong bonds within quasi 2D building blocks, while weak bonds link adjacent blocks. These weak bonds are frequently referred to as van der Waals (vdW) bonds. The weak interlayer interaction – which causes the 2D nature of these materials – is both a blessing and a curse. On the one hand, it allows the growth of heterostructures and superlattices of dissimilar 2D materials without epitaxial guidance (vdW epitaxy). Yet, it also creates adverse side-effects such as poor adhesion and wetting.

Here after a broad introduction to the PCM and their applications for electrical memory, I will give an overview of some of our recent and past achievements in this filed, from basic research on 2D properties of GST to memory applications.

December 3, 2021

Hermann Kohlstedt 

Chair of Nanoelectronics Institute for Electrical Engineering and Information Engineering

Kiel University

Bio-inspired Information Processing: The Future of Artificial Intelligence?

Abstract:

Information processing in biological nerve system is characterized by highly parallel, energy efficient and adaptive architectures in contrast to clock driven digital Turing machines. Even simple creatures outperform supercomputers when it comes to pattern recognition, failure tolerant systems and cognitive tasks. Fundamental building blocks leading to such remarkable properties are neurons as central processing units, which are (with variable strengths) interconnected by synapses to from a complex dynamical three-dimensional network. The field of neuromorphic engineering aims to mimic such biological inspired information pathways by electronic circuitries. Up to today, this approach is hindered by an inadequate understanding how to link the information pathways on the local, synaptic and neuron level to the global functionality of the entire brain network. In the talk I will show restrictions of conventional IT and present alternative ways for computing based on bio-inspired circuits comprising memristives devices. Examples including an electronic version of Pavlov ́s dog, Amoeba anticipation, pattern recognition, chaos, and memory, and pulsed-coupled oscillators The challenges and possible limitations of bio-inspired computing approach will be discussed.

November 26, 2021

Felix Flicker

Cardiff University

Dimer Models on Quasicrystals

Abstract:

How many ways are there of tiling a chess board with dominoes? This is an example of a classical dimer model. Removing one domino reveals the two squares it covered: one black, one white. These can be thought of as a particle-antiparticle pair. Further domino re-arrangements allow the particles to move around on the board. Dimer models provide a general setting to study physical systems in which strong correlations emerge from local constraints. Examples include emergent magnetic monopoles in the spin ice materials, spin liquids, and resonating valence bond states. Despite the simplicity of dimer models they exhibit a range of exotic phenomena, including fractionalisation and topological order. A central question is whether the emergent quasiparticles — black and white squares in the chess board analogy — can become deconfined, separating to arbitrary distance at finite energy cost. The answer depends on the symmetries, or otherwise, of the dimer configurations.

Dimer models have a long and exalted history, with many exact results proven in both finite, and infinite periodic, settings. In this talk I will outline exact results in a new setting: infinite aperiodic tilings based on two-dimensional ‘quasicrystals’. Focussing on the Ammann-Beenker tiling, with an 8-fold rotational symmetry and discrete scale invariance, I provide evidence of quasiparticle deconfinement. This is seemingly at odds with the received wisdom that deconfined phases cannot exist in 2+1D compact U(1) gauge theories. I will outline our understanding of how that result likely survives. The behaviour emerges from a rich set of dimer correlations which exhibit a natural scale invariance.

November 19, 2021

Riccardo Mazzarello 

Sapienza University

Anderson Localization in Disordered Chalcogenides

Abstract:

Disorder-induced Anderson localization and metal-insulator transition (MIT) have been a central topic in both condensed matter physics and materials science in the past 60 years. Recently, a disorder-driven MIT was observed experimentally in the crystalline state of Ge-Sb-Te alloys upon annealing. These alloys are employed in phase-change memories, which exploit their ability to switch rapidly and reversibly between a crystalline and an amorphous phase exhibiting resistivity contrast. The occurrence of a MIT in the crystalline phase might enable the realization of multilevel memories and neuromorphic devices by proper control of the degree of disorder.
In this talk, I will first present our ab initio study of Ge-Sb-Te, which helped to elucidate the localization properties of the electronic states in the insulating phase and the microscopic mechanisms driving the transition. Then, I will discuss our recent work showing that Sb2Te3 – the parent compound of Ge-Sb-Te alloys and a well-known topological insulator – can form an insulating, highly-disordered rocksalt-like phase that displays a MIT upon annealing, similarly to Ge-Sb-Te.
In the last part of my talk, I will present our ab initio computational screening over binary and ternary chalcogenides forming disordered rocksalt structures, potentially leading to Anderson localization. We have identified the factors that affect the stability of the rocksalt structure, and have defined proper indicators to distinguish (meta-)stable compounds from unstable ones in a rationalized materials map.

November 12, 2021

Jake Bringewatt 

Joint Quantum Institute (JQI)/University of Maryland

Lefschetz Thimble Quantum Monte Carlo for Spin Systems

Abstract:

Monte Carlo simulations are often useful tools for modeling quantum systems, but in some cases they suffer from a sign problem, which manifests as an oscillating phase attached to the probabilities being sampled. This sign problem generally leads to an exponential slow down in the time taken by a Monte Carlo algorithm to reach any given level of accuracy, and it has been shown that completely solving the sign problem for an arbitrary quantum system is NP-hard. However, a variety of techniques exist for mitigating the sign problem in specific cases; in particular, the technique of deforming the Monte Carlo simulation’s plane of integration onto Lefschetz thimbles (that is, complex hypersurfaces of stationary phase) has seen success for many problems of interest in the context of quantum field theories. We extend this methodology to discrete spin systems by utilizing spin coherent state path integrals to re-express the spin system’s partition function in terms of continuous variables. This translation to continuous variables introduces additional challenges into the Lefschetz thimble method, which we address. We show that these techniques do indeed work to lessen the sign problem on some simple spin systems.

Based on: arXiv:2110.10699

November 5, 2021

Mohsen Yarmohammadi

University of Dortmund

Non-equilibrium dynamics of a driven-dissipative dimerized spin-1/2 chain

Abstract:
Due to the rise in experimental progress in several photonic platforms, theoretical addressing the non-equilibrium behavior in driven-dissipative quantum systems has triggered considerable interest in recent times. This talk is devoted to the analysis of dynamics of a dimerized spin-1/2 chain model which is driven out-of-equilibrium by the presence of a classical steady laser field. A particular study is given on the spin-phonon coupling effect treated as weak-to-strong perturbations, that the infrared-active phonons are driven by the laser. We employ the quantum master equation, which follows the construction of the dissipation path to a phononic bath for both phonon and spin sectors in the driven coupled spin-lattice system. This talk includes a detailed study of novel non-equilibrium dynamics of driven-dissipative quantum magnetic materials.

October 29, 2021

David J. Norris

ETH Zurich

The Dream of the Perfect Nanocrystal

Abstract:

Quantum dots are nanometer-sized crystallites of semiconductor that have a roughly spherical shape. Due to a world-wide effort, quantum dots are now commercially used as a robust fluorescent material in displays and lighting. However, despite decades of research, state-of-the-art samples still contain particles with a distribution in size and shape. Because this causes variations in their optical properties, their performance for applications is reduced. This leads to a fundamental question: can we achieve a sample of semiconductor nanocrystals in which all the particles are exactly the same? In this talk we will discuss this possibility by examining two classes of nanomaterials. First, we will consider thin rectangular particles known as semiconductor nanoplatelets. Amazingly, nanoplatelet samples can be synthesized in which all crystallites have the same atomic-scale thickness (e.g. 4 monolayers). This uniformity in one dimension suggests that routes to monodisperse samples might exist. After describing the underlying growth mechanism for nanoplatelets, we will then move to a much older nanomaterial—magic-sized clusters (MSCs). Such species are believed to be molecular-scale arrangements (i.e. clusters) of semiconductor atoms with a specific (“magic”) structure with enhanced stability compared to particles slightly smaller or larger. Their existence implies that MSC samples can in principle be the same size and shape. Unfortunately, despite three decades of research, the formation mechanism of MSCs remains unclear, especially considering recent experiments that track the evolution of MSCs to sizes well beyond the “cluster” regime. Again, we will discuss the underlying growth mechanism and its implications for nanocrystal synthesis. Finally, we will present an outlook if perfect nanomaterials can be obtained.

October 22, 2021

Christian Klinke

University of Rostock and Swansea University

The Wondrous World of Lead Sulphide

Abstract:

The colloidal synthesis as a chemical method provides an enormous wealth of means to manipulate nanomaterials’ size and shape. In turn, this has a tremendous impact on the structures’ physical properties. I will discuss synthetic, crystallographic and electronic aspects of colloidal lead sulphide nanomaterials with surprising results.

October 8, 2021

Yi-Hsiang Chen

USC

Quantum Algorithm for Time-Dependent Hamiltonian Simulation by Permutation Expansion

Abstract:

Understanding how a quantum system evolves has been a central task ever since the development of quantum mechanics. Yet, simulating quantum dynamics on classical computers is formidable due to the ex- ponentially large parameter space of a quantum many-body system. Quantum computers, on the other hand, provide a promising route to solving this problem. A majority of existing quantum simulation algorithms tackle systems with time-independent particle interactions. In this work, we consider the most general cases with time-dependent interactions, where the noncommutativity between the interaction operators at different times adds another layer of complexity to the simulation problem.

Specifically, our algorithm involves a permutation and exponential expansion for the interaction Hamiltonian, a switch to the interaction picture, and the incorporation of the linear combination of unitaries technique (LCU). Combining the permutation expansion with the Dyson series leads to an integral-free representation with efficiently computable coefficients involving the notion of divided differences with complex inputs. Under this representation, we perform a quantum simulation for the time-evolution operator by means of the LCU technique. The full evolution is divided into variable time steps with optimized durations, where each step is determined based on the norm of the interaction Hamiltonian at the beginning of the step, leading to a gate count that scales with an L1-norm-like scaling with respect only to the norm of the interaction Hamiltonian, rather than that of the total Hamiltonian. This feature amounts to a cost that does not scale with the total evolution time asymptotically for time-decaying systems. In addition, since the integral-free expansion form has coefficients with norms that are independent of the frequencies of the Hamiltonian, the overall simulation cost is frequency-independent. This is in stark contrast with existing algorithms that their costs all depend on ||dH/dt||, which increases with frequencies. Therefore, the frequency-independent feature provides a significant speedup over existing algorithms for systems with highly oscillating components. This can be of practical use for various purposes. Lastly, our algorithm retains the near optimal scaling with simulation error. Overall, this algorithm can simulate any quantum dynamics and provide speedup over existing quantum algorithms for a large class of systems.

October 1, 2021

Alexander B. Khanikaev

City College of New York

Topological polaritonics with Van der Waals materials

Abstract: 

Topological photonics offers enhanced control over electromagnetic fields by providing a platform for robust trapping and guiding topological states of light [1,2]. By combining capabilities offered by topological photonics with strong light-matter interactions in polaritonic Van der Waals systems one can further expand possibilities to manipulate both light and matter degrees of freedom of these half-light half-matter quasiparticles.

In this talk I will first present an approach to spin-Hall topological polaritonics based on the versatile platform of polaritonic metasurfaces containing monolayer transition metal dichalcogenides (TMDs) [3]. Our approach leverages the large exciton dipole moment in a monolayer semiconductor and the remarkable compatibility of 2D materials with various photonic structures to realize strong coupling between light and matter. We show that the strong coupling regime between a topological spin-Hall photonic crystal and a TMD monolayer featuring a pair of degenerate TR partner excitons gives rise to a topological transition and the formation of a topological polaritonic phase characterized by nonvanishing spin-Chern numbers. Introduction of domain walls separating topological and trivial phases is then shown to produce spin-polarized polaritonic boundary modes. Spin-locking of these modes and their selective coupling to circularly polarized light of opposite handedness enables unique polaritonic spin-Hall phenomena that we demonstrate experimentally. In addition, by studying photoluminescence of WSe₂ topological nanostructure, we confirm valley polarization of edge polaritons.

In the second part of my talk, I will show that strong coupling between topological photons with phonons in hexagonal boron nitride (hBN) offers a new platform to control and guide hybrid states of light and lattice vibrations [4]. The observed topological edge-states of phonon-polaritons are found to carry nonzero angular momentum locked to their propagation direction, which enables their robust transport. Thus, these topological quasiparticles enable robust funneling of helical infrared phonons along arbitrary pathways and across sharp bends, thus offering unprecedented opportunities for applications, from Raman and vibrational spectroscopy with structured phonons, to directional heat dissipation along topologically resilient heat sinks.

September 17, 2021

Xiao Mi

Google

Observation of Time-Crystalline Eigenstate Order on a Quantum Processor 

Abstract:

Quantum processors of today are already capable of surpassing classical supercomputers on certain specialized tasks [1]. A current milestone for the quantum information science community is the fulfilment of quantum computational advantage on a practical problem of interest. Studying many-body phases of matter offers unique opportunities toward this coveted goal since many outstanding questions remain surrounding the critical behaviors of quantum phases. Here we report on the first experimental observation of a non-equilibrium phase of matter, the discrete time crystal (DTC). A DTC breaks time-translational symmetry and displays spatio-temporal quantum order in all of its eigenstates, a feature dubbed “eigenstate order”. We implement Floquet dynamics on a 1D chain of 20 superconducting qubits [2]. Engineered disorders in the two-qubit couplings allow many-body localization (MBL) to occur despite strong external drive, thereby stabilizing the non-equilibrium phase [3]. We carefully validate the phase structure of the DTC by probing the average response of all eigenstates belonging to the Floquet unitary. Using a suitable choice of order parameter, we further identify the location of the MBL-ergodicity crossover via experimentally observed finite-size effects. These results open a direct path to studying quantum phase transitions and critical phenomena on NISQ quantum processors.

*During the Covid Pandemic, the following seminars were held via Zoom*

May 14, 2021

Nick Hunter-Jones

Perimeter Institute

Quantum pseudorandomness from domain walls and spectral gaps

Abstract:

Random quantum circuits (RQCs) are valuable constructions in both quantum information and quantum many-body physics. They are a solvable model of strongly-interacting quantum dynamics, rapid scramblers of quantum information, and have been the central focus of recent demonstrations of quantum computational advantage. In this talk we’ll discuss some very useful techniques for studying RQCs involving both a statistical-mechanical mapping to lattice model as well as bounding the spectral gap of a local Hamiltonian. Using these approaches, we’ll compute the depth at which RQCs anti-concentrate (where the distribution over possible measurement outcomes is sufficiently spread-out) and the rate at which they generate quantum pseudo-randomness.

May 7, 2021

Kang L Wang

UCLA

Topological phase transitions and Magnetoelectric effects in Heterostructures

Abstract:

Topological quantum materials are a class of compounds featuring electronic band structures, which are topologically distinct from common metals and insulators. These materials have emerged as an exceptionally fertile ground for material science research and the studies have been expanded to Weyl semimetals and 2-D materials.  One of most notable applications is spin orbit toque, giving rise to an energy efficiency magnetic memory.  We will focus on recent material discoveries and experimental advancements of topological materials and their heterostructures.  This talk will discuss the topological phase transitions of various topological heterostructures, beginning with quantum anomalous Hall in magnetic doped topological insulators as controlled by magnetic fields.  Then the electric field is used to illustrate the phase diagrams, displaying quantum anomalous Hall and normal insulators.  With the interface engineering of different magnetic layers or antiferromagnets, the axial electromagnetic or axion phase in a sandwiched structure is demonstrated and an additional phase, the parity Hall phase with ½ quantization of quantum resistance, was then theoretically interpreted.  When interfaced with an s-wave superconductor, the chiral Majorana mode can be studied, in addition to the previously pursued Majorana zero mode.  Finally, we conclude with a prospect for the discovery of additional topological materials for studying quantum processes, quasi-particles, and their composites as well as exploiting potential applications for these materials.

April 30, 2021

Fabrizio Messina

University of Palermo

Carbon dots: an emerging family of functional nanomaterials for applications in photo‐nanotechnology

Abstract:

Carbon nanodots (CDs) are an emerging family of zero‐dimensional carbon nanomaterials displaying optical properties akin to semiconductor nanocrystals, such as a bright and tunable fluorescence in the visible spectral range. However, CDs display several key advantages with respect to other optical nanomaterials, such as non‐toxicity, low cost and ease of synthesis. Since their original discovery in 2006, CDs have been attracting a large and interdisciplinary research interest in nanoscience. Many groups are currently pursuing their application in multiple fields, such as optoelectronics, photocatalysis, bioimaging and sensing. Yet, the fundamental nature of CD optical transitions remains hardly understood.

In this talk I will present a selection of the studies on CDs conducted by my group in the last five years. We have been addressing the fundamental photophysics of CDs, with the goal of pinpointing the microscopic mechanisms responsible of their bright fluorescence and relating them to their underlying, variable chemical structures. In recent times, we have been focusing more and more on devising strategies to couple CDs to other nanomaterials, such as carbon nanotubes, polyoxometalates, metallic and magnetic nanoparticles. The results are very promising and confirm the excellent properties of CDs as artificial light harvesters and photo‐excited charge donors in the design of functional nanocomposites.

April 23, 2021

Stephan Wirths

ABB Research Center

Group IV Semiconductors: From Photonics to Power Electronics

Abstract:

The monolithic, large-scale integration of photonics on Si is limited by the inability of group IV materials to emit light efficiently. In this context, Sn-based group IV semiconductors have attracted increasing scientific interest during the last decades due to the exciting possibility to pass the indirect-to-direct band gap transition by alloying Ge with Sn1. Whereas this transition has been predicted already in the early 1980’s, the quality of epitaxially grown GeSn and SiGeSn layers on Si and Ge substrates has been limited mainly owing to the low solid solubility of Sn in (Si)Ge (< 1 at.%) and the large lattice mismatch (> 15 %). Hence, the enormous potential of this material system regarding optoelectronic applications, i.e. laser diodes, has not been fully exploited. We have synthesized the first group IV materials – (Si)GeSn epilayers – with direct band gap on Si and optical quality never shown before. These developments in epitaxial growth have paved the route towards the first direct band gap group IV laser sources monolithically integrated on Si(001)2. We have investigated this novel gain material using photo- and electroluminescence in order to gain insights into fundamental group IV laser physics. The emission wavelength range of these novel group IV laser sources can be tuned between 2 mm and 2.6 mm enabling photon transmission for short distances. Furthermore, we have demonstrated the first group IV microdisk laser suitable for small footpring monolithic integration on Si platform3.
Nowadays, power electronics is undergoing an exciting and profound technology shift driven by the steadily growing demand for energy of our digital society and the urgent requirement for low carbon emission transport infrastructures. Si based power electronics reaches its performance limits regarding high energy-efficient power converters for e-mobility and renewable energy applications. SiC MOSFETs have entered the power devices arena and are the frontrunners to replace traditional Si IGBT technology due to their higher breakdown voltage and thermal conductivity. Despite their successful market entry, several challenges that are strongly connected to the state-of-the-art gate stack technology are still to be solved though in order to fully exploit the enormous potential of SiC power MOSFETs. Conventional SiO2 gate oxides for example suffer from highly defective oxide/SiC interfaces and the intense electric field across the gate oxide negatively impacts device performance as well as reliability. At Hitachi ABB Power Grids, we have developed a novel MOS gate stack technology based on high-k dielectrics for power electronic devices. For the first time, we have demonstrated vertical power SiC MOSFETs using high-k-based MOS interfaces4. We have successfully fabricated fully functional vertical high-k power SiC MOSFETs for several voltage classes, namely 1.2kV, 1.7kV and 3.3kV. We demonstrated devices with superior SiC/high-k dielectric interface trap density and, thus, boosted performance by 35% compared to SiO2 gate stack technology.

April 16, 2021

Nai-Chang Yeh

Caltech

Investigating and controlling different quantum degrees of freedom in two-dimensional van der Waals materials and nano-metamaterials

Abstract:

Van der Waals (vdW) materials, ranging from insulators to semiconductors, semi-metals and superconductors, are layered quantum materials that exhibit novel optical, electronic, orbital, magnetic, topological and superconducting properties. Varying the interfacial stacking order of vdW homo/heterostructures further yields Moiré bands and new physical phenomena. Nanoscale strain-engineering of vdW materials (e.g., graphene, transition-metal dichalcogenides, TMDs) can induce novel quantum and topological phenomena, such as giant pseudo-magnetic fields (PMF) and flat bands associated with quantized Landau levels in monolayer graphene, and strain-dependent energy gaps and exciton lifetimes in 2H-phase TMDs. Quantum confinement due to reduced spatial dimensions and broken symmetry is known to induce new quantum phenomena and nonlinear responses that are non-existent in the bulk. Spin-orbit coupling of light by optical vortices (a.k.a. twisted light, Laguerre-Gaussian beam) and plasmonic vortices produces photons with nontrivial spin/orbital-angular-momentum, which can impart integer and fractional topological charges onto condensed matter and atomic/molecular systems through light-matter interactions. Therefore, investigating and controlling the light-matter interactions in vdW nanomaterials/heterostructures/metastructures represents a pathway to explore new quantum phenomena and to advance opto-electronic/valleytronic/spintronic applications.

We employ scalable CVD and PECVD fabrication techniques to synthesize a variety of vdW nanomaterials and their heterostructures, including monolayer and twisted bilayer graphene,^[1-3] quasi one-dimensional (1D) graphene nanostripes,^[1,4] h-BN,^[5] and monolayer-to-multilayer TMDs MX₂ and MX_2-yX¢_y (M = Mo, W; X, X¢ = S, Se, Te; 0 £ y £ 2).^[6,7] We investigate their properties by gated electrical transport measurements, spatially resolved and polarization-dependent photoluminescence, time-resolved photoluminescence, Raman spectroscopy, x-ray and ultraviolet photoelectron spectroscopy, Kelvin-probe force microscopy, conductive atomic force microscopy, scanning tunneling microscopy/spectroscopy.^[6-9] We perform nanoscale strain engineering on monolayer graphene to control the strain-dependent PMF, electronic correlation, and valley/spin polarization;^[8,9] and on TMDs to control the energy gaps and exciton lifetimes. We develop plasmonic vortices on TMDs by metasurfaces together with circularly polarized light to control the light-matter interaction, valley/spin polarization and topological charge in TMD. Our approach leads to various novel findings, including: valley splitting, quantum oscillations, spontaneous symmetry breaking and spin polarization, quantum valley-Hall and quantum anomalous-Hall effects in valley Hall transistors based on nanoscale strain-engineered monolayer graphene;^[8,9] exceedingly long and temperature independent optically excited carrier lifetimes (~ 10 ns) as well as broadband (from IR to UV) photoluminescence in quasi-1D graphene nanostripes; much enhanced valley polarization in monolayer WS₂ by combining plasmonic vortices with circularly polarized light; and compositional dependent opto-electronic/valleytronic properties in WS_2-yTe_y.^[7] Our results suggest a new paradigm of vdW materials/heterostructures/metastructures for exploring novel quantum and topological states of matter and for advancing quantum devices and quantum information technology.

April 9, 2021

HongWen Jiang

UCLA

Harnessing valley states in silicon for quantum information processing

Abstract:

Semiconductor quantum dots are a leading approach for the implementation of solid-state based quantum computing, as the coherence time of the qubits can be extremely long and various interactions, inherent to semiconductors, can be harvested to precisely control superposition and entanglement. Although spin and charge of single electrons are normally used for encoding qubits, valley states in silicon represent another quantum degree of freedom to store and process quantum information, with a string of desirable properties. In this talk, I present our experiments at UCLA to explore valley states in Si quantum dot device. In particular, coherent manipulation and projective read-out of valley states in a Si/SiGe quantum dot device showed promise of fast electrical control as well as protection against charge noise [1]. Two-axis quantum control of a valley qubit using gate pulse sequences with X and Z rotations, occurring within a fast operation time of 300 ps, has been accomplished [2]. A set of gate operations and quantum measurements completely map out the surface of the Bloch sphere in a single phase-space plot, which is subsequently used to evaluate various qubit operation fidelities by state and process tomography. In addition, we found a very long intervalley relaxation time of 12 ms, that is comparable to the spin relaxation time at the spin-valley hybridization “hot spot” [3].
[1] J. S. Schoenfield, B. M. Freeman and H. W. Jiang, “Coherent manipulation of valley states at multiple charge configurations of a silicon quantum dot device,” Nature Communication, 8, 64 (2017).
[2] Nicholas E. Penthorn, Joshua S. Schoenfield, John D. Rooney, Lisa F. Edge, and HongWen Jiang, “Two-axis quantum control of a fast valley qubit in silicon”, npj Quantum Information, 5, 94 press (2019).
[3] Nicholas E. Penthorn, Joshua S. Schoenfield, Lisa F. Edge, and HongWen Jiang, “Direct measurement of electron intervalley relaxation in a Si/SiGe quantum dot”, Phys. Rev. Applied 14, 054015 (2020).

March 19, 2021

Robert Jones

Juelich Research Center

What are phase change memory materials, and why do Ge/Sb/Te alloys dominate?

Abstract: 

Phase change materials (PCM) have dominated the world of optical storage media, particularly rewritable media like DVD-RW and Blu-ray Disc RE, for at least 30 years. Nanosized bits in a thin polycrystalline layer are switched reversibly and on a timescale of nanoseconds) between amorphous (a-) and crystalline (c-) states, which can be identified by measuring the resistivity or optical reflectivity. The elements involved are almost always from groups 13-16: Te (group 16) is almost always present, followed by Ge (14) and Sb (15). Ge/Sb/Te alloys in the composition range (GeTe)1-x (Sb2Te3)x  (0<x<1) have been the materials of choice in commercial devices: all have metastable rock salt structures that change little over decades at archival temperatures, and all contain vacancies (cavities). The prototype Ge2Sb2Te5 (GST) is the most widely studied member of this family.

The rock salt structure of GST can be understood using a model [1] with simple  ingredients: (1) Among elements of groups 13-16, Ge, Sb, and Te stand out for the pronounced similarity between their valence (outermost s- and p-orbitals) atomic orbitals. This is a consequence of “secondary periodicity” among elements [2]. (2) The average number of valence electrons per site, including vacant sites, is close to 5, and we may adopt the arguments of H. Jones [3], who explained the structure and semimetallic properties of Bi as arising from a (“Peierls) distortion of a simple cubic structure. (3) The average occupancy of five valence electrons leads locally to vacancies close to Te atoms and to the near-perfect Te sublattice.

These observations allow us to rationalize the structures of Ge/Sb/Te alloys and to discuss alternative compositions of phase change materials. An alternative approach involving a change in bonding mechanism between crystalline and amorphous states will be mentioned.

March 5, 2021

Xufeng Kou

Shanghai Tech University

Magnetic Topological Insulators-Based Emerging Physics and Applications

Abstract: 

When magnetic order is introduced into topological insulators (TIs), the time-reversal-symmetry (TRS) is broken, and the non-trivial topological surface is driven into a new massive Dirac-fermions state. By adjusting the spin splitting with respect to the intrinsic spin-orbit coupling strength, the constituent band structure leads to the formation of quantum anomalous Hall t (QAH) effect, where scale-invariant chiral edge-state current can conduct coherently without energy dissipation at zero magnetic field. Such salient quantum transport features, along with versatile magnetic order-driven topology manipulation strategies, have resulted in unprecedented advancements of TRS-breaking topological quantum physics in the past decade.

In this talk, I will present our work on the magnetic topological insulators (MTIs). Owning to the precise control of the Fermi level position and doping profile, we report the observation of QAH effect in the Cr-doped (BiSb)2Te3 samples with film thickness across the 2D limit. By further manipulating the topological surface gap, we realize the metal-to-insulator QAH phase transition and identify the unique thickness-tailored magnetism in the MTI system. In addition to the uniform MTIs, I will also present our recent work on several MTI based heterostructures. First, in the TI/MTI system, we demonstrate that the spin-orbit torque is highly efficient that the critical charge current density required for the magnetization switching is three orders of magnitude smaller than that of heavy metals. In addition, by constructing novel FM/TI and AFM/TI heterostructures, we realize emergent interfacial magnetic effects, which can be tailored through artificial structural engineering. Our results highlight the great potential for MTI-based systems as a suitable platform to explore energy-efficient spintronics applications.

*During the Covid Pandemic, the following seminars were held via Zoom*

March 27, 2020

Chia Wei Hsu

USC EE

Photonics in complex systems

Abstract:

The interactions between light and complex systems provide vast opportunities for exploring fundamental physics and for a wide range of applications. Materials, structures, and the light fields themselves can all be designed to tailor the optical properties of a system. Inversely, when a system’s optical properties are fully characterized, one may infer its underlying material composition and structure. In this talk, I will describe some of our work on photonics in complex systems. The first part concerns “bound states in the continuum,” which are special eigenstates that remain perfectly confined even when its eigenfrequency lies in the continuous spectrum of the extended states of an open system. I will describe the first experimental realization of bound states in the continuum not protected by symmetry, their topological property, and potential realization in fiber Bragg gratings. The second part concerns non-Hermitian degeneracies (called exceptional points) in periodic structures, where we predict and experimentally realize rings of exceptional points in the momentum space as well as paired exceptional points connected by bulk Fermi arcs. In the third part, I will describe the control of light transport via wavefront shaping, the long-range correlations between multiply scattered photons, and a transverse localization phenomenon for the transmission eigenchannels that is distinct from Anderson localization.

March 6, 2020

Mercehdeh Khajavikhan

USC EE

Non-Hermitian Photonics: Optics at an Exceptional Point

Abstract:

In recent years, non-Hermitian degeneracies, also known as exceptional points (EPs), have emerged as a new paradigm for engineering the response of optical systems. At such points, an N-dimensional space can be represented by a single eigenvalue and one eigenvector. As a result, these points are associated with abrupt phase transitions in parameter space. Among many different non-conservative photonic configurations, parity-time (PT) symmetric systems are of particular interest since they provide a powerful platform to explore and consequently utilize the physics of exceptional points in a systematic manner. In this talk, I will review some of our recent works in the area of non-Hermitian (mainly PT-symmetric) active photonics. For example, in a series of works, we have demonstrated how the generation and judicial utilization of these points in laser systems can result in unexpected dynamics, unusual linewidth behavior, and improved modal response. On the other hand, biasing a photonic system at an exceptional point can lead to orders of magnitude enhancement in sensitivity- an effect that may enable a new generation of ultrasensitive optical sensors on-chip. Non-Hermiticity can also be used as a means to promote or single out an edge mode in photonic topological insulator lattices. Rotation sensors play a crucial role in a diverse set of applications associated with navigation, positioning, and inertial sensing. Most optical gyroscopes rely on the Sagnac effect induced phase shift that scales linearly with the rotational velocity. In ring laser gyroscopes (RLGs), this shift manifests itself as a resonance splitting in the emission spectrum that can be detected as a beat frequency. The need for ever-more precise RLGs has fueled research activities towards devising new approaches aimed to boost the sensitivity beyond what is dictated by geometrical constraints. In this respect, attempts have been made in the past to use either dispersive or nonlinear effects. Here, we propose a new scheme for ultrasensitive laser gyroscopes that utilizes the physics of exceptional points. By exploiting the properties of such non-Hermitian degeneracies, we show that the rotation-induced frequency splitting becomes proportional to the square root of the gyration speed- thus enhancing the sensitivity to low angular rotations by orders of magnitudes. We will then describe a possible modification of a standard RLG to support an exceptional point, and measure the resulting enhanced sensitivity in the proposed system.

February 25, 2020

Adrian Lupascu

University of Waterloo

Investigation of connectivity in quantum annealers

Abstract:

Tntum annealing is a computational paradigm for solving combinatorial problems based on finding the ground state of a Hamiltonian. The common approach is to encode problems using Ising spin Hamiltonians. The connectivity of the spins in the Ising system, which is the number of controllable spin-spin interactions, is important for the computational power of an annealer. We present our work on the implementation of long-range interactions in a quantum annealer. This work is done in the context of an implementation of quantum annealers based on superconducting flux quantum bits. The usual approach for implementation of spin-spin (qubit-qubit) interactions is based on the use of superconducting loops interrupted by a Josephson junction (RF-SQUIDs). This approach breaks down for coupling qubits at large distances. The coupler tree architecture is a proposal for implementation of long-range interactions between superconducting flux qubits using a network of coupled RF-SQUIDs. We present the results of experiments with two capacitively shunted flux qubits connected by a chain of RF-SQUIDs. We demonstrated propagation of a magnetic flux signal through the chain, a first step towards demonstration of long range qubit-qubit interactions. We will discuss future prospects for this work and other directions for implementation of high connectivity.

January 31, 2020

Clarice Aiello

UCLA

From nanotech to living sensors: unraveling the spin physics of biosensing at the nanoscale

Abstract:

Substantial in vitro and physiological experimental results suggest that similar coherent spin physics might underlie phenomena as varied as the biosensing of magnetic fields in animal navigation and the magnetosensitivity of metabolic reactions related to oxidative stress in cells. If this is correct, organisms might behave, for a short time, as “living quantum sensors” and might be studied and controlled using quantum sensing techniques developed for technological sensors. I will outline our approach towards performing coherent quantum measurements and control on proteins, cells and organisms in order to understand how they interact with their environment, and how physiology is regulated by such interactions. Can coherent spin physics be established – or refuted! – to account for physiologically relevant biosensing phenomena, and be manipulated to technological and therapeutic advantage?

January 24, 2020

Michal Farnik

J. Heyrovsky Institute of Physical Chemistry, Prague

Condensation and interactions of molecules on nanoparticles

Abstract:

Clusters represent a bridge between individual molecules and bulk. Their investigations provide a detailed molecular-level understanding of bulk properties and complex processes in condensed matter. Our versatile cluster beam apparatus (CLUB) in Prague allows for a large variety of experiments with clusters and nanoparticles: adsorption of molecules; photoionization mass spectrometry; electron ionization and attachment; photodissociation and velocity map imaging including IR-UV pump-probe experiments, etc. Some of these experiments will be introduced in the talk. The first part will concentrate on molecular adsorption in relevance to aerosol particle formation in the atmosphere. The second part will show how intermolecular reactions are affected by the cluster medium. Using examples, we will demonstrate how the investigated phenomena are relevant to atmospheric processes, to astronomy and astrochemistry, and even to surface-assisted technologically relevant processes such as FEBID (focused electron beam induced deposition).

January 17, 2020

Victor Sourjik

Max Planck Institute Marburg

Environmental regulation, benefits and costs of bacterial motility

Abstract:

Microorganisms possess diverse mechanisms to regulate investment into individual cellular processes according to their extra- and intracellular environment. How these regulatory strategies reflect the inherent tradeoff between the benefit and cost of resource investment remains largely unknown, particularly for many cellular functions that are not immediately related to growth. I will present our recent work on understanding the physiological importance and environmental regulation of motility and chemotaxis of Escherichia coli, one of the most complex and costly bacterial behaviors. I will particularly focus on the regulation of motility as a function of bacterial growth rate. Our work showed that in poor nutritional conditions bacteria increase their investment in motility in proportion to the selective advantage provided by chemotaxis. Thus, bacteria appear to pre-invest resources into the motile behavior in proportion to the anticipated benefit that can be provided by chemotaxis when gradients of secondary nutrients are either introduced in their environment or created by bacterial communities through excretion and consumption of metabolites.