# PSSCMP 2021: Virtual Poster Sessions

We will be having a Poster Session, for participants to share their research on Thursday, June 10th, 4pm. The Poster Session will be separated by Breakout Rooms. Join the Poster Session Zoom link, and freely explore Breakout Rooms to listen, discuss, and ask questions.

Breakout Room 1:

Shakti Shankar Acharya, Ravenshaw University, “Mixed Ground State in Fe-Ni Invar Alloys”

We investigate the ground state properties of Invar alloys via detailed study of the electronic structure of Fe1-xNix alloys (x = 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.9) employing x-ray photoelectron spectroscopy (XPS). While all the alloys exhibit soft ferromagnetic behavior with Curie temperature much higher than the room temperature, the results for invar alloy, Fe0.6Ni0.4 exhibit anomalous behavior. Moreover, the magneto-resistance of the Invar alloy becomes highly negative while the end members possess positive magneto-resistance. The core level spectra of the Invar alloy exhibit emergence of a distinct new feature below 20 K while all other Fe-Ni alloys exhibit no temperature dependence down to 10 K. Interestingly, the shallow core level spectra (3s, 3p) of Fe and Ni of the Invar alloy reveal stronger deviation at low temperatures compared to the deep core levels (2s, 2p) indicating crystal field effect. It appears that there is a large precipitation of antiferromagnetic phase below 20 K possessing low magnetic moment (0.5μB) on Fe within the Î± phase. The discovery of negative magneto-resistance, anomalous magnetization at low temperature and the emergence of unusual new features in the core levels at low temperature provide an evidence of mixed phase in the ground state of Invar alloys.

https://doi.org/10.1016/j.jallcom.2021.158605

Key words: Invar alloys, ground state, photoemission, core level, magnetoresistance

Breakout Room 2:

Iman Ahmadabadi, Joint Quantum Institute, University of Maryland, College Park, “Electronic Floquet Vortex States Induced by Light with the Orbital Angular Momentum”

We propose a scheme to create an electronic Floquet vortex state by irradiating the circularly-polarized laser light carrying non-zero orbital angular momentum on the two-dimensional semiconductor. We study the properties of the Floquet vortex states analytically and numerically using methods analogous to the techniques used for the analysis of superconducting vortex states, while we exhibit that the Floquet vortex created in the current system has the wider tunability. To illustrate the impact of such tunability in quantum engineering, we demonstrate how these vortex state can be used for quantum information processing.

Keywords: Floquet, vortex states, quantum engineering

Breakout Room 3:

Nikita Astrakhantsev, University of Zurich, “Broken-Symmetry Ground States of the Heisenberg model on the Pyrochlore Lattice”

The spin-1/2 Heisenberg model on the pyrochlore lattice is an iconic frustrated three-dimensional spin system with a rich phase diagram. Besides hosting several ordered phases, the model is debated to possess a spin-liquid ground state when only nearest-neighbor antiferromagnetic interactions are present. Here, we contest this hypothesis with an extensive numerical investigation using both exact diagonalization and complementary variational techniques. Specifically, we employ a RVB-like many-variable Monte Carlo ansatz and convolutional neural network quantum states for (variational) calculations with up to 4×43 and 4×33 spins, respectively. We demonstrate that these techniques yield consistent results, allowing for reliable extrapolations to the thermodynamic limit. Our main results are (1) the determination of the phase transition between the putative spin-liquid phase and the neighboring magnetically ordered phase and (2) a careful characterization of the ground state in terms of symmetry-breaking tendencies. We find clear indications of spontaneously broken inversion and rotational symmetry, calling the scenario of a featureless quantum spin-liquid into question. Our work showcases how many-variable variational techniques can be used to make progress in answering challenging questions about three-dimensional frustrated quantum magnets. https://arxiv.org/abs/2101.08787

Keywords: pyrochlore, neural quantum states, quantum spin liquid, valence bond solid

Breakout Room 4:

Francisco Brito, University of York, “Simulation of Quantum Spin Liquid Phases Using Spectral Methods”

We carry out accurate Chebyshev polynomial expansions [1-3] and thermal pure quantum state (TPQ) [4] simulations of quantum spin models with highly entangled ground states. Then, we use a hybrid framework combining the two methods to map out in a numerically exact fashion the phase diagram of the Kitaev-Heisenberg model on the honeycomb lattice. Our results agree with previously obtained results with exact diagonalization [5]. Spin correlations are calculated with spectral accuracy in large systems with 24 spins. Our approach can be extended to realistic spin models accommodating impurities, defects and external perturbations. Our findings suggest that a hybrid spectral-TPQ approach can be used to probe complex spin systems, such as Kitaev systems with disorder, which are currently a topic of widespread interest [6,7].

[1] A. Ferreira, ER. Mucciolo, Phys. Rev. Lett. 115, 106601 (2015)

[2] A. Braun, P. Schmitteckert, Phys. Rev. B 90, 165112 (2014)

[3] S. M. JoÃ£o et al., Royal Society Open Science, 7, 2 (2020)

[4] S. Sugiura and A. Shimizu, Phys. Rev. Lett. 108, 240401 (2012)

[5] J. Chaloupka, G. Jackeli, and G. Khaliullin, Phys. Rev. Lett. 105, 027204 (2010)

[6] J. Knolle, R. Moessner, N. B. Perkins, Phys. Rev. Lett. 122, 047202 (2019)

[7] W-H. Kao, J. Knolle, G.B. HalÃ¡sz, R. Moessner, N. B. Perkins (2021)

Francisco Brito is supported by a DTP studentship funded by the Engineering and Physical Sciences Research Council.

F. Brito, A. Ferreira, in preparation (2021)

Keywords: Quantum Spin Liquids, Kitaev Model, Spectral Methods, Thermal Pure Quantum States, Chebyshev Polynomial Green Function

Breakout Room 5:

Joseph Cuozzo, The College of William & Mary, “Missing Shapiro Steps in Topologically Trivial Josephson Junction on InAs Quantum Well”

Josephson junctions hosting Majorana fermions have been predicted to exhibit a 4π periodic current phase relation. One experimental consequence of this periodicity is the disappearance of odd steps in Shapiro steps experiments. Experimentally, missing odd Shapiro steps have been observed in a number of materials systems with strong spin-orbit coupling and have been interpreted in the context of topological superconductivity. Here we report on missing odd steps in topologically trivial Josephson junctions fabricated on InAs quantum wells. We ascribe our observations to the high transparency of our junctions allowing Landau-Zener transitions. The probability of these processes is shown to be independent of the drive frequency. We analyze our results using a bi-modal transparency distribution which demonstrates that only few modes carrying 4Ï€ periodic current are sufficient to describe the disappearance of odd steps. Our findings highlight the elaborate circumstances that have to be considered in the investigation of the 4Ï€ Josephson junctions in relationship to topological superconductivity.

https://www.nature.com/articles/s41467-020-20382-y

Keywords: Shapiro, topological, Josephson

Breakout Room 6:

Andrew Cupo, Dartmouth College, “Driving Quantum-Confined Massless Dirac Fermions: Floquet Graphene Antidot Lattices”

We establish the theoretical foundation of the Floquet graphene antidot lattice. In this system, massless Dirac fermions are driven periodically by circularly polarized electromagnetic radiation, and their motion is excluded from an array of nanoholes. The interesting emergent properties are encoded in the quasienergy spectra, which are computed within the Floquet formalism. First, we find that the Dirac dispersion can be restored in real time as compared to the gapped equilibrium state, which may enable the creation of an optoelectronic switch or a dynamically tunable electronic waveguide. Second, the ability to shift the energy gap between high symmetry points can change which crystal momenta are dominant in the scattering processes that determine electronic transport and optical emission. Third, the bands can be flattened near the $\Gamma$ point, pointing to selective dynamical localization and enabling a low-wavelength-pass electronic filter. Lastly, quadratic and linear dispersions emerge in orthogonal directions at the M point, which is the signature of a Floquet semi-Dirac material. All predictions are valid for experimentally accessible, near IR radiation. This corresponds to the high-frequency (above bandwidth) limit for the graphene antidot lattice. By contrast, for standard graphene, the same limit is only reached for ionizing, extreme-UV photon energies. Cycling between laser-induced Floquet electronic phases may play an important role in the development of next-generation on-chip devices for optoelectronic applications.

Keywords: graphene, antidot lattice, massless Dirac fermions, quantum confinement, Floquet, dynamical localization, semi-Dirac

Breakout Room 7:

Michael Denner, University of Zurich, “Electronic Instabilities of the Kagome Metals AV3Sb5”

The recent discovery of AV3Sb5 (A=K,Rb,Cs) has uncovered an intriguing arena for exotic Fermi surface instabilities in a kagome metal, displaying charge ordered and superconducting phases with unconventional properties. In this poster session I will discuss the understanding of these instabilities that emerges from a range of  – partially complementary and partially controversial –  experiments and our recent theoretical studies. As a first step, we develop a theory of electronically mediated charge density wave formation. Additionally, we show that the sublattice interference mechanism is central to understanding the formation of superconductivity in a kagome metal. Thus, while many questions remain to be answered in forthcoming studies, the existing body of work already establishes AV3Sb5 as platform for correlated quantum phases of great promise.

arXiv:2103.14045, arXiv:2104.05671, arXiv:2012.15709

Keywords: kagome metal, AV3Sb5, superconductivity, charge density wave

Breakout Room 8:

Bivas Dutta, Weizmann Institute of Science, “Unique Way to Distinguish between Non-abelian Topological Orders: Probing the Majorana-mode”

Quantum Hall states – the progenitors of the growing family of topological insulators -- are a rich source of exotic quantum phases. The nature of these states, being abelian or non-abelian, supporting electrons, fractionally charged quasiparticles, and neutral Majorana quasiparticles, is reflected in the gapless edge modes due to ‘bulk-edge’ correspondence. The most-studied putative non-abelian state is the spin-polarized filling factor ν =5/2, whose charge e/4 quasiparticles are accompanied by neutral modes. This filling, however, permits different possible topological orders, which can be abelian or non-abelian. While numerical calculations favor the non-abelian Pfaffian (Pf) and anti-Pfaffian (A-Pf) orders to have the lowest ground-state energy, recent thermal conductance measurements suggested the experimentally realized order to be the particle-hole Pfaffian (PH-Pf) order. It has been suggested that lack of thermal equilibration among the different edge modes of the A-Pf order can account for this discrepancy. Identification of the topological order is crucial in fundamental experiments; such as quasiparticle interference (braiding), in future of fault-tolerant topological-qubits, and in testing the theoretical predictions. We developed a powerful new method that interfaces the studied quantum state with another state, thus controlling the structure of the internal modes. Here, we employ such construction to identify the topological order of the ν =5/2 state. Interfacing two half-planes, one hosting the ν =5/2 state and the other an integer ν =3 state, the formed interface supports a fractional ν =1/2 charge mode with ½ quantum conductance and a neutral Majorana mode, while the integer modes are localized. The counter-propagating chirality of the Majorana mode, probed by measuring partition noise, agreed with the PH-Pf topological order and thus excluding the A-Pf order.

arXiv:2101.01419

Keywords: non-abelian topological orders, \nu=5/2 quantum Hall state, Majorana-mode

Breakout Room 9:

Ashish Gangshettiwar, University of Texas at Austin, “Direct Imaging of Phase Separation in Ti Doped Bilayer Calcium Ruthenate”

We report the nanoscale imaging of Ti-doped bilayer calcium ruthenates during the Mott metal-insulator transition by microwave impedance microscopy. Different from a typical first-order phase transition where coexistence of the two terminal phases takes place, a new metallic stripe phase oriented along the in-plane crystalline axes emerges inside both the G-type antiferromagnetic insulating state and the paramagnetic metallic state. The effect of this electronic state can be observed in macroscopic measurements, allowing us to construct a phase diagram that takes into account the energetically competing phases. Our work provides a model approach to correlate the macroscopic properties and mesoscopic phase separation in complex oxide materials.

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.101.201106

Keywords: Metal Insulator transition, Strongly correlated systems, Scanning probe microscopy, Microwave Impedence Microscopy, ruthenates

Breakout Room 10:

Augusto Ghiotto, Columbia University, “Quantum Criticality in Twisted Transition Metal Dichalchogenides”

In moiré heterostructures, gate-tunable insulating phases driven by electronic correlations have been recently discovered. Here, we use transport measurements to characterize the gate-driven metal-insulator transitions and the metallic phase in twisted WSe2 near half filling of the first moiré subband. We find that the metal-insulator transition as a function of both density and displacement field is continuous. At the metal-insulator boundary, the resistivity displays strange metal behavior at low temperature with dissipation comparable to the Planckian limit. Further into the metallic phase, Fermi-liquid behavior is recovered at low temperature which evolves into a quantum critical fan at intermediate temperatures before eventually reaching an anomalous saturated regime near room temperature. An analysis of the residual resistivity indicates the presence of strong quantum fluctuations in the insulating phase. These results establish twisted WSe2 as a new platform to study doping and bandwidth-controlled metal-insulator quantum phase transitions on the triangular lattice.

Nature Materials volume 19, pages861–866(2020)

arXiv:2103.09796

Keywords: Quantum Criticality, Planckian dissipation, TMDs, Moiré

Breakout Room 11:

Franzisca Gorniaczyk, Weizmann Institute of Science, “Thermal Transport in Amorphous Indium Oxide Thin Films”

Thin films of amorphous indium oxide are widely studied for undergoing a zero-temperature superconductor-to-insulator phase transition. In this work I focus on the insulating side of this transition which exhibits the so-called hot-electron effect, where electrons form their own heat bath, whose temperature defines the electronic conductivity. Due to its exponential temperature dependence the insulator can be driven out of equilibrium by a comparatively small applied Joule power, resulting in discontinuities in the current-voltage characteristics. Thermal transport measurements in this regime a thermal conductivity exceeding the Wiedemann-Franz law by several orders of magnitude.

Keywords: thermal conductivity, superconductor-insulator transition, hot-electron effect

Breakout Room 12:

Zeyu Hao, Harvard University, “Electric Field-tunable Superconductivity in Alternating-Twist Magic-Angle Trilayer Graphene”

Engineering moiré superlattices by twisting layers in van der Waals (vdW) heterostructures has uncovered a wide array of quantum phenomena. We constructed a vdW heterostructure that consists of three graphene layers stacked with alternating twist angles ±θ. At the average twist angle θ ~ 1.56°, a theoretically predicted “magic angle” for the formation of flat electron bands, we observed displacement field–tunable superconductivity with a maximum critical temperature of 2.1 kelvin. By tuning the doping level and displacement field, we found that superconducting regimes occur in conjunction with flavor polarization of moiré bands and are bounded by a van Hove singularity (vHS) at high displacement fields. Our findings display inconsistencies with a weak coupling description, suggesting that the observed moiré superconductivity has an unconventional nature.

DOI: 10.1126/science.abg0399

Keywords: Moire superlattices, twisted graphene

Breakout Room 13:

Wen-Yu He, Massachusetts Institute of Technology, “Quantum Oscillation of Thermally Activated Conductivity in a Monolayer WTe$_2$-like Excitonic Insulator”

Recently, quantum oscillation of the resistance in insulating monolayer WTe_2 was reported, but the origin of the quantum oscillation with no electron Fermi surfaces is unknown. Here we propose that the observed quantum oscillation can arise from the gap modulation in the hybridized Landau levels formed in the excitonic insulating monolayer WTe_2. We show that for system like monolayer WTe_2, the excitonic insulating state arising from the coupled one hole and two electron pockets possesses a finite region in interaction parameter space that shows gap modulation in a magnetic field. In this region, the thermally activated conductivity displays the 1/B periodic oscillation and it can further develop into discrete peaks at low temperature, in agreement with the experimental observation. We show that the relative anisotropy of the bands is a key parameter and the qunatum oscillations decrease rapidly if the anisotropy increases further than the realistic value for monolayer WTe_2.

https://arxiv.org/abs/2105.02411

Keywords: quantum oscillation, excitonic insulator.

Breakout Room 14:

Joshuah Heath, Boston College, “Luttinger's Theorem-Violating Fermi Liquids and Power-law Green's Functions”

The failure of Landau-Fermi liquid theory is often considered a telltale sign of universal, scale-invariant behavior in the emergent field theory of interacting fermions. Nevertheless, there exist borderline cases where weak scale invariance coupled with particle-hole asymmetry can coexist with the Landau quasiparticle paradigm. After briefly introducing the necessary conditions for Luttinger's theorem, I'll show explicitly that a Landau-Fermi liquid can exist for weak power-law scaling of the retarded Green's function. Such an exotic variant of the traditional Fermi liquid, although exhibiting a finite quasiparticle weight and large quasiparticle lifetime, is shown to always be incompatible with Luttinger's theorem for any non-trivial scaling.

J.T. Heath and K.S. Bedell, New J. Phys. 22 063011 (2020)

J.T. Heath, J. Low Temp. Phys. 201, p. 200â€“212 (2020)

Keywords: Fermi liquids, Luttinger's theorem

Breakout Room 15:

Tobias Holder, Weizmann Institute of Science, “Anomalous Acceleration and Quantum Geometry in Semimetals and Flat Bands”

While transport properties are often determined by the semiclassical velocity of the quasiparticle, here I will talk about gapped and semimetallic materials which depart from this canonical picture and where it is necessary to account for additional components of the electronic motion which are most easily envisaged as quasiparticle accelerations. This is particularly pronounced in the bulk photovoltaic effect, a second-order optical response in which incident light is converted into a dc-current. This current can be viewed as the result of acceleration terms which give access to many details of the band structure geometry. As examples I discuss the role of Berry charges in Weyl semimetals and a large current response in the terahertz regime in twisted bilayer graphene.

arXiv:2101.07539

Phys. Rev. Research 2, 033100 (2020)

Keywords: nonlinear optical response, bulk photovoltaic effect, quantum metric

Breakout Room 16:

Goktug Islamoglu, Independent Researcher, “Observance of Hund’s Rules in the Same Setting: Coexistence of Intra-Atomic and Inter-Atomic Exchanges in Cellular Automata”

A cellular automaton is modified to generate long range interactions uniaxially, and short range interactions in its normal direction. The neighborhood density is tuned above a certain threshold to magnetize the system. This is achieved through coupling of cells to its neighborhood. Similarity of coupling in this 2D simulation to a Hund’s coupling can be extended to the ferromagnetic alignment of the cells to intra-atomic exchanges and antiferromagnetic layering in the normal axis to inter-atomic exchanges. Paramagnetic state, the energy-efficient ground-state not related to the long-range ordering and short-range ordering, is arrived at the checkerboard formation of atoms.

Keywords: Hund's Rule, Long Range Interactions, Dipole, Ferromagnetic, Antiferromagnetic, Layered Structures, Cellular Automata

Breakout Room 17:

Fabian Jerzembeck, Max-Planck-Institute for Chemical Physics of Solids, “Uniaxial pressure studies on Sr2RuO4”

The unconventional superconductor Sr2RuO4 has been extensively studied under in-plane uniaxial pressure. $a$-axis pressure deforms one of the Fermi surfaces elliptically, so that at a compressive strain of $\varepsilon = -0.44~\%$, one of the Fermi surfaces undergoes a Lifshitz transition at a single point in $k$-space, dramatically altering the low-temperature electronic properties. In contrast, $c$-axis pressure causes charge transfer from the $d_{xz}$ and $d_{yz}$ to the $d_{xy}$-orbital, resulting in an expansion of the main Fermi surface and finally in Lifshitz transitions at the $X$ and $Y$ points simultaneously. DFT calculations suggest that this occurs at $\varepsilon \approx -2.5~\%$. We present new data on in-plane uniaxial pressure and first results on uniaxial pressure along the c-axis, which surprisingly show a decreasing $T_c$ even as the density of states increases.

[1] KJA Franke et al., Phys. Rev. B 98, 054428, 2018

[2] TJ Hicken et al., Phys. Rev. Research 2, 032001(R), 2020

[3] TJ Hicken et al., Phys. Rev. B 103, 024428, 2021

[4] TJ Hicken et al., arXiv:2105.01393, 2021

Keywords: skyrmion, muon-spin spectroscopy, topological magnetism, chiral soliton lattice, density functional theory, magnetometry

Breakout Room 18:

Abhishek Kumar, Indiana University Bloomington, “Floquet Gauge Pumps as Sensors for Spectral Degeneracies Protected by Symmetry or Topology”

We introduce the concept of a Floquet gauge pump whereby a dynamically engineered Floquet Hamiltonian is employed to reveal the inherent degeneracy of the ground state in interacting systems. We demonstrate this concept in a one-dimensional XY model with periodically driven couplings and transverse field. In the high frequency limit, we obtain the Floquet Hamiltonian consisting of the static XY and dynamically generated Dzyaloshinsky-Moriya interaction (DMI) terms. The dynamically generated magnetization current depends on the phases of complex coupling terms, with the XY interaction as the real and DMI as the imaginary part. As these phases are cycled, the current reveals the ground state degeneracies that distinguish the ordered and disordered phases. We discuss experimental requirements needed to realize the Floquet gauge pump in a synthetic quantum spin system of interacting trapped ions

Floquet Gauge Pump : arXiv:2012.09677 (accepted in PRL)

Keywords: Floquet, Topological, Gauge Pump, Ground state degeneracies

Breakout Room 19:

Ajesh Kumar, University of Texas at Austin, “Heavy Fermion Physics in a Transition Metal Dichalcogenide Moire Heterostructure”

We propose a trilayer transition metal dichalcogenide moire heterostructure that realizes the Kondo lattice model. We identify gating conditions required to achieve the Kondo lattice regime using a self-consistent Hartree-Fock calculation. We also obtain a phase diagram containing the heavy-fermion quantum critical point using a parton mean-field calculation. Further, we predict experimental signatures of heavy-fermion criticality in this setup.

Keywords: moire, strong correlations, Kondo lattice

Breakout Room 20:

Mariia Labendik, Weizmann Institute of Science, “Shot noise detection in ν = 12/5 quasiparticle charge”

The fractional quantum Hall effect is known to reveal peculiar anyonic topological states of matter, such as parafermions that are thought to have non-Abelian exchange statistics. Among those states there are 5/2 and 12/5 parafermions that can be experimentally observed. A particular interest is aimed towards the state 12/5 as it is predicted to be described by exotic parafermionic statistics suggested by Read and Rezayi. One way to probe the state experimentally is measuring its fractional charge using quantum shot noise. My study is intended to experimentally detect the fragile fractional state 12/5 statistics by means of quantum shot noise.

Keywords: quantum shot noise, non-Abelian exchange statistics, fractional quantum Hall effect, parafermions

Breakout Room 21:

Yu-Ping Lin, University of Colorado Boulder, “Complex Charge Density Waves at Van Hove Singularity on Hexagonal Lattices: Haldane-model Phase Diagram and Potential Realization in Kagome Metals AV3Sb5”

I show that the interplay of real and imaginary charge density waves support a rich Haldane-model phase diagram at Van Hove singularity on hexagonal lattices. Based on a Ginzburg-Landau analysis, the 3Q complex orders develop at three nesting momenta under a total phase condition. The phase diagram contains the trivial and Chern insulator phases, which are the deformations of purely real and imaginary orders with site/bond and current density modulations, respectively. The gapless phase boundary may host a Dirac semimetal and an exotic single-Dirac-point semimetal. Our theoretical model offers transparent interpretations of experimental observations in the kagome metals AV3Sb5 with A = K, Rb, Cs.

Y.-P. Lin and R. M. Nandkishore, arXiv:2104.02725

Keywords: Complex 3Q charge density wave, Van Hove singularity, hexagonal lattice, Haldane-model phase diagram, kagome metal AV3Sb5

Breakout Room 22:

Zhu-Xi Luo, University of California, Santa Barbara, “Magic Continuum in Twisted Bilayer Square Lattice with Staggered Flux”

We derive the general continuum model for a bilayer system of staggered-flux square lattices, with arbitrary elastic deformation in each layer. Applying this general continuum model to the case where the two layers are rigidly rotated relative to each other by a small angle, we obtain the band structure of the twisted bilayer staggered-flux square lattice. We show that this band structure exhibits a "magic continuum" in the sense that an exponential reduction of the Dirac velocity and bandwidths occurs in a large parameter regime. We show that the continuum model of the twisted bilayer system effectively describes a massless Dirac fermion in a spatially modulating magnetic field, whose renormalized Dirac velocity can be exactly calculated. We further give an intuitive argument for the emergence of flattened bands near half filling in the magic continuum and provide an estimation of the large number of associated nearly-zero-energy states. We also show that the entire band structure of the twisted bilayer system is free of band gaps due to symmetry constraints.

https://arxiv.org/abs/2104.02087

Keywords: moire physics, twisted bilayer, flat band, continuum field theory

Breakout Room 23:

Philippa McGuinness, Max Planck Institute for the Chemical Physics of Solids, “Symmetries of Non-local Transport in Delafossite Metals”

The delafossites PdCoO2 and PtCoO2 are layered oxide metals comprised of highly-correlated insulating layers and highly-conducting metallic layers. Their extremely long in-plane mean free paths of up to 20 Âµm enable the rare non-local ballistic transport regime to be reached at low temperature. In addition, these delafossite metals have a broadly hexagonal Fermi surface, in contrast to the circular Fermi surfaces present within monolayer graphene and the semiconductor heterostructures previously widely studied in this regime. We perform transport measurements within square-shaped microstructures, demonstrating novel anisotropies in the bend resistance and asymmetries in the Hall resistance which are a product of the hexagonal Fermi surface and cannot be observed in materials with a circular Fermi surface or within the Ohmic regime. Despite their non-intuitive nature, we determine through Landauer-BÃ¼ttiker modeling that these signals do not violate fundamental symmetries or conservation laws. In addition, asymmetries can be observed up to a square size nearly 20 times the mean free path, establishing the extended nature of this regime within delafossite metals.

Keywords: electrical transport, non-local transport, delafossite

Breakout Room 24:

Johannes Mitscherling, Max Planck Institute for Solid State Research, “Interband Contributions to the Electrical Conductivity for a General Two-Band Model: Quantum Metric and Berry Curvature”

In recent years, there is an increasing interest in transport properties of multiband systems due to advances in experimental techniques. We focus on the longitudinal, the anomalous and the ordinary Hall conductivity for a general two-band model. This model captures a broad spectrum of systems with very different and rich physics like Chern insulators, ferromagnets, and spiral spin density waves. Within a systematic microscopic framework, we identify and discuss a decomposition of the conductivity formulas into intra- and interband contributions. We further relate the interband contributions to concepts of quantum geometry, namely the quantum metric and the Berry curvature, and discuss their properties. We exemplify the general analysis by several applications ranging from spiral magnetic order in cuprates to the quantum anomalous Hall effect in Chern insulators.

Mitscherling, Longitudinal and anomalous Hall conductivity of a general two-band model, Phys. Rev. B 102, 165151 (2020)

Keywords: Electrical Conductivity, Multiband systems, Anomalous Hall, Berry curvature, Quantum metric

Breakout Room 25:

Tusaradri Mohapatra, National Institute of Science Education and Research, Odisha, India, “Probing Pairing Symmetry of Unconventional Superconductor via Hanbury-Brown and Twiss Correlations”

One of the outstanding problems is the unambiguous detection of pairing symmetry in unconventional superconductors like in Iron pnictide. The most probable candidates are the two-band $s_{++}$ and sign-reversed $s_{\pm}$ wave pairing. Andreev conductance and shot noise can be used as a probe for the pairing symmetry of Iron pnictide superconductors. Clear differences emerge in both the zero bias differential conductance and the shot noise in the tunneling limit for the two cases enabling an effective distinction between the two. Hanbury Brown and Twiss (HBT) shot noise correlations and the non-local differential conductance the distinct character of helical and chiral superconductors as well as their difference from non-topological superconductors. The topological nature of unconventional superconductors can be clearly distinguished regardless of the setup whether symmetric, non-local or asymmetric by observing HBT correlations. This is significant because at metal-chiral superconductor junctions stable Majorana bound states (MBS), which persist even in presence of magnetic field have been predicted to occur. Our study will help in distinguishing chiral from helical superconductors and even in the search for stable MBS, which have potential importance in topological quantum computation. Further, motivation for our work arises from inconsistency in previous experiments which have probed the different topological and non-topological pairing symmetries via Knight shift or scanning SQUID microscopy.

[1] C. Benjamin and T. Mohapatra, EPL 132, 47002 (2020);  [2] T. Mohapatra, S. Pal and C. Benjamin, arXiv:2103.14920.

Keywords: Unconventional Superconductors, Shot noise, HBT correlation, Iron pnictide, Topological superconductors, Andreev conductance

Breakout Room 26:

Nicolás Morales-Durán, University of Texas at Austin, “Relevance of Non-local Interactions in Semiconductor moiré Heterobilayers”

We show that non-local interaction terms, such as interaction-assisted hopping and intersite-exchange play a significant role in the extended Hubbard model describing semiconductor moiré materials. We discuss the influence of the non-local terms on the metal-insulator transition and on the magnetic properties of insulating states at half-filling, by means of an exact diagonalization study of the electronic properties of narrow moiré bands in TMD heterobilayers.

arXiv:2011.13558

Keywords: Generalized Hubbard model, semiconductor moiré materials, exact diagonalization

Breakout Room 27:

Daniel Muñoz-Segovia, Donostia International Physics Center (DIPC), “Superconducting Collective Modes in Single-layer Niobium Diselenide”

Single-layer niobium diselenide (NbSe2) has attracted considerable attention during the last years due to the combination of the lack of inversion symmetry and the so-called Ising spin-orbit coupling, which confer interesting properties to its superconducting ground state. In particular, they result in the so-called singlet-triplet mixing, i.e., the superconductivity has a symmetry-allowed triplet contribution even if the dominant pairing interaction would be in the singlet channel. Moreover, recent experiments observing nematic superconductivity under an applied in-plane magnetic field point to the fact that the triplet has a condensation energy close to the singlet [2,3], which is also consistent with the ab-initio calculations showing proximity to a ferromagnetic state [4,5]. Taking this into account and motivated by a recent STM experiment which has observed signatures of a bosonic mode intrinsic to the superconducting state [1], in this work we have computed the Bardarsis-Schrieffer collective mode using a simplified continuum model of NbSe2. This mode consists of the fluctuation from the singlet state to the triplet channel with opposite phase (imaginary). In this poster, I will discuss how our result compares to the experiment [1] by analyzing the dependence of the collective mode energy with the superconducting gap and the temperature. Finally, I will propose some possible experiments that might help better characterize this collective mode.

References:

[1] W. Wan, et al., arXiv:2101.04050 (2021).

[2] A. Hamill, et al., arXiv:2004.02999 (2020).

[3] C.-W. Cho, et al., arXiv:2003.12467 (2020).

[4] D. Wickramaratne, et al., Phys. Rev. X 10, 041003 (2020).

[5] S. Divilov, et al., arXiv:2005.06210 (2020).

Keywords: NbSe2, unconventional superconductivity, collective modes

Breakout Room 28:

Sen Niu, Peking University, “Emergent Gapless Topological Luttinger Liquid”

Topological phases are quantum states characterized by bulk topological invariants and exhibit nontrivial edge properties. Gapless Luttinger liquid is a low energy effective description of one dimensional electrons with interaction. Conventionally if a Luttinger liquid is topological, it must possess features such as degenerate edge modes and entanglement spectrum, otherwise it is viewed as trivial. Here we predict an emergent gapless topological Luttinger liquid characterized by the nontrivial many-body spin texture in a one dimensional interacting spin-orbit coupled system at generic filling factor. Interestingly, at appropriate parameter regime bulk spin textures with nontrivial winding can emerge from a conventionally trivial phase of which the low energy physics is described by a simple spinless Luttinger liquid. We use winding number of the spin texture as a topological invariant to characterize the effects of filling factor and interaction on the ground state. We find the winding of spin texture is determined by global properties in energy and momentum spaces, instead of low energy physics near fermi points. The observable spin texture can be generalized to ﬁnite temperature directly and measured in optical lattice experimentally.

This work will appear in arXiv in May or June.

Keywords: Topological phase; topological spin texture; emergent topology; Luttinger liquid.

Breakout Room 29:

Pavel Volkov, Rutgers University, “Magic Angles and Topology in Twisted Nodal Superconductors”

We propose twisted bilayers of two-dimensional nodal superconductors as a new platform to realize topological and correlated superconducting phases. We show that the Fermi velocity of the Dirac excitations in the Bogoliubov-De Gennes quasiparticle dispersion is strongly renormalized by the interlayer hopping, vanishing at a "magic angle”, where interactions between quasiparticles lead to the formation of correlation-induced phases. We demonstrate that magnetic field, electric gating, and current bias can be used for versatile control of the system. In particular, we show that current bias can open a topological gap, with the system being characterized by a non-zero Chern number that is equal to the number of nodes. This produces a quantized thermal Hall effect with gapless thermal currents on the boundary.

https://arxiv.org/abs/2012.07860

Keywords: Unconventional/topological superconductivity; Twistronics

Breakout Room 30:

Haina Wang, Princeton University, “Sensitivity of Pair Statistics on Pair Potentials in Many-body Systems”

We study the sensitivity and practicality of Henderson’s theorem in classical statistical mechanics, which states that the pair potential v(r) that gives rise to a given pair correlation function g2(r) [or equivalently, the structure factor S(k)] in a classical many-body system at number density ρ and temperature T is unique up to an additive constant. While widely invoked in inverse-problem studies, the utility of the theorem has not been quantitatively scrutinized to any large degree. We show that Henderson’s theorem has practical shortcomings for disordered and ordered phases for certain densities and temperatures. Using proposed sensitivity metrics, we identify illustrative cases in which distinctly different potential functions give very similar pair correlation functions and/or structure factors up to their corresponding correlation lengths. Our results reveal that due to a limited range and precision of pair information in either direct or reciprocal space, there is effective ambiguity of solutions to inverse problems that utilize pair information only, and more caution must be exercised when one claims the uniqueness of any resulting effective pair potential found in practice. We have also identified systems that possess virtually identical pair statistics but have distinctly different higher-order correlations. Such differences should be reflected in their individually distinct dynamics (e.g., glassy behaviors). Finally, we prove a more general version of Henderson’s theorem that extends the uniqueness statement to include potentials that involve two- and higher-body interactions.

https://aip.scitation.org/doi/abs/10.1063/5.0021475

Keywords: Probability theory, Monte Carlo methods, Metric geometry, Many body systems, Optimization algorithms, Descriptive statistics, Interatomic potentials, Classical statistical mechanics, Phase transitions, Crystal lattices

Breakout Room 31:

Tsz Chun Wu, Rice University, “Topological Anomalous Skin Effect in Weyl Superconductors”

We show that a Weyl superconductor can absorb light via a novel surface-to-bulk mechanism, which we dub the topological anomalous skin effect. This occurs even in the absence of disorder for a single-band superconductor, and is facilitated by the topological splitting of the Hilbert space into bulk and chiral surface Majorana states. In the clean limit, the effect manifests as a characteristic absorption peak due to surface-bulk transitions.  We also consider the effects of bulk disorder, using the Keldysh response theory. For weak disorder, the bulk response is reminiscent of the Mattis-Bardeen result for $s$-wave superconductors, with strongly suppressed spectral weight below twice the pairing energy, despite the presence of gapless Weyl points. For stronger disorder, the bulk response becomes more Drude-like and the $p$-wave features disappear.  We show that the surface-bulk signal survives when combined with the bulk in the presence of weak disorder. The topological anomalous skin effect can therefore serve as a fingerprint for Weyl superconductivity.

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.103.104517

Keywords: Weyl superconductors, surface states, disorders, skin effect

Breakout Room 32:

Ya-Hui Zhang, Harvard University, “Chiral Spin Liquid and Exciton Metal in Moire Bilayer”

We propose to simulate the spin physics using pseudo-spin in moire bilayer, a generalization of the well-studied quantum Hall bilayer.  We offer theoretical and numerical evidences for existence of chiral spin liquid and exciton metal (spinon fermi surface) in the system.  As the layer pseudo-spin here carries electric polarization moment, it is easy to probe the "spin" physics with conventional electric measurements.  Smoking gun evidence of these spin liquids could be obtained by counter-flow transport.

arXiv:2103.09825, arXiv:2005.12925

Keywords: spin liquid, moire bilayer

Breakout Room 33:

Hossein Dehghani, University of Maryland, "Light-Induced topological superconductivity vis Floquet interaction engineering."

We propose a mechanism for light-induced unconventional superconductivity in a two-valley semiconductor with a massive Dirac-type band structure. The superconducting phase results from the out-of-equilibrium excitation of carriers in the presence of Coulomb repulsion and is stabilized by coupling the driven semiconductor to a bosonic or fermionic thermal bath. We consider a circularly polarized light pump and show that by controlling the detuning of the pump frequency relative to the band gap, different types of chiral superconductivity would be induced. The emergence of novel superconducting states, such as the chiral p-wave pairing, results from the Floquet engineering of the interaction. This is realized by modifying the form of the Coulomb interaction by projecting it into the states that are resonant with the pump frequency. We show that the resulting unconventional pairing in our system can host topologically protected chiral bound states. We discuss a promising experimental platform to realize our proposal and detect the signatures of the emergent superconducting state.

https://journals.aps.org/prresearch/abstract/10.1103/PhysRevResearch.3.023039

Keywords: Superconductivity, Floquet engineering, topological matter, non-equilibrium phases.

(Added at a later date, does not follow the prior alphabetical format)