Aspen Center for Physics - 2024 Winter Conference Disorder and Quantum Phases of Matter
Lightning Talks and Posters
1. Christopher Baldwin
Michigan State University
Email: baldw292@msu.edu
Title: Self-averaging & the replica trick in quantum spin glasses & SYK
Abstract: The structure of the Sachdev-Ye-Kitaev (SYK) model is strikingly similar to the canonical mean field models of spin glass theory, yet the resulting behavior is quite different --- the SYK model is "maximally chaotic" and has connections to quantum gravity at low temperature, whereas spin glasses instead have frozen dynamics and an intricate equilibrium structure. In the first part of this talk, we present some explanation as to why, by proving the following result: in any bosonic model with all-to- all random interactions, there must be a breakdown of self-averaging of the partition function, and thus the analysis used to study the (fermionic) SYK model cannot directly apply. In this sense, the fermionic nature of the SYK model is a necessary ingredient for its physics. In the second part of the talk, we consider the physical implications of a breakdown of self-averaging in more detail. We show that, surprisingly, self-averaging transitions need *not* be related to any spin glass physics. Doing so requires resolving some ambiguities in the replica trick that appear to have gone unaddressed until now.
2. Simon Billinge
Columbia University
Email: sb2896@columbia.edu
Title: Intrinsic quantum textures: quantifying local symmetry breaking from the atomic pair distribution function analysis of x-ray and neutron diffraction data
Abstract: For a number of years now it has been apparent from local structural probes such as the atomic pair distribution (PDF) analysis of x-ray and neutron data, that a significant number of interesting quantum materials have local structures that have distinctively lower symmetry than the average structure. These can be extrinsic, due to quenched disorder, but we are also finding evidence for intrinsic textures in quantum materials. I will describe the experimental approaches and present results for some exemplar intrinsic quantum textures.
3. Saptarshi Biswas
Northwestern University
Email: saptarshibiswas531@gmail.com
Title: Spin-charge separation and spin-Chern number of disordered higher-order topological insulators
Abstract: According to the ten-fold classification scheme of topological insulators, only the time-reversal symmetric, spin-orbit-coupled systems (class AII) can support stable, quantum spin Hall states at two spatial dimensions. In this work, we employ magnetic flux tube as a real-space probe to show that all three Wigner Dyson symmetry classes (A, AI, and AII) can display stable, quantum spin (or pseudo- spin) Hall insulators in the presence of disorder. This goal is accomplished by demonstrating spin- charge separation in the presence of flux tube. Our analysis is a manifestly gauge invariant method for addressing the magnitude of spin Chern number of first order and higher order topological insulators.
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Aspen Center for Physics - 2024 Winter Conference Disorder and Quantum Phases of Matter
4. Noah Bray-Ali
National Institute of Standards and Technology
Email: Nbrayali@gmail.com
Title: New Sum Rule for Infrared Electrodynamics of Polar Materials
Abstract: A wide range of polar materials including transition metal chalcogenides, cuprates, and apatites have a diffuse collective mode in the mid-infrared frequency range with a quantum relaxational form for the self-energy. Meanwhile, in the far infrared, a sharp polar mode dominates the electrodynamics of polar materials. Remarkably, the sum of the resonance frequency of the mid-infrared collective mode and the far-infrared polar mode seems to give a material-independent, universal result.
Reference: N. Bray-Ali, arXiv.org:2108.12243 (2023)
5. Lu Chen
Universite de Sherbrooke
Email: lu.chen@usherbrooke.ca
Title: Effect of Disorder on the Phonon Thermal Hall Effect
Abstract: Phonons are known to generate a thermal Hall effect in various insulators including multiferroic materials and cuprate Mott insulators, but the underlying mechanism is still unclear. Theoretical proposals for the phonon thermal Hall effect fall into two categories: intrinsic scenarios based on the coupling of phonons to their environment, and extrinsic scenarios based on the skew scattering of phonons by disorders like impurities or defects. Our observation of a large phonon thermal Hall conductivity in Cu3TeO6
indicates that the phonon thermal Hall effect is likely to be a fairly common property of solids. To investigate the effect of disorders on the phonon thermal Hall effect, we studied the thermal Hall effect in Rh doped Sr2IrO4 and observed a 30-fold enhancement of the thermal Hall angle with just 2% of Rh doping. This work highlights the effect of disorders on the phonon thermal Hall effect and evidences an impurity-induced phonon thermal Hall effect in the antiferromagnetic phase of Sr2IrO4.
6. Alisa Danilenko
New York University
Email: alisa.danilenko@nyu.edu
Title: Diagnosing disorder in epitaxial Al-InAs heterostructures
Abstract: Two-dimensional superconductor-semiconductor hybrid heterostructures have been utilized in a range of quantum devices, from voltage tunable gatemon qubits to attempts at realizing topological phases. It is becoming clear that the presence of disorder in these structures plays a key role in any experiment involving them. We aim to study the sources and strength of this disorder directly, building up a toolkit to analyze heterostructures immediately after growth, considering high and low carrier density transport signatures, fabrication procedures, and surface morphology. We discuss strategies for improved material growth and sample preparation based on these results.
7. Sanjib Kumar Das
Lehigh University
Email: skd321@lehigh.edu
Title: Quantized electrical and thermal responses of two-dimensional disordered (non-)Hermitian topological insulators and superconductors
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Aspen Center for Physics - 2024 Winter Conference Disorder and Quantum Phases of Matter
Abstract: Altland-Zirnbauer (AZ) topological classification of quantum phases of matter shows that there are five topologically nontrivial classes in every spatial dimension. Particularly in two spatial dimensions, there are two insulating (namely quantum Hall and quantum spin Hall insulators) and three superconducting (namely p+ip, d+id, and p±ip pairings) AZ phases. The bulk topological invariant of these phases of matter manifests quantized electrical and thermal responses that can be experimentally measured in multi-terminal Hall bar geometries. This work demonstrates such quantized electric and thermal responses of disordered topological insulators and superconductors computed within the scattering matrix formalism using the software package Kwant.
8. Alex Eaton
University of Cambridge
Email: flexeaton@gmail.com
Title: Disorder sensitivity of multiple spin-triplet superconducting phases in UTe2
Abstract: The acute sensitivity of odd-parity superconductivity to the presence of impurities is a textbook example of the important role crystalline disorder can play in correlated materials. The spin triplet superconductor UTe2 exhibits two distinct magnetic field-induced superconducting phases, one of which spectacularly persists to B > 70 T. This poster discusses recent measurements on a new generation of pristine quality UTe2 samples grown by a molten salt flux technique, which possess residual resistivities over an order of magnitude lower than prior growth batches, enabling the attainment of quantum oscillation measurements to probe the underlying electronic structure. Remarkably the two field-induced superconducting phases exhibit markedly different sensitivities to crystalline disorder in their temperature and angular domains, suggesting that there are at least two separate microscopic mechanisms giving rise to distinct p-wave superconducting phases in UTe2.
9. Augusto Ghiotto
Columbia University
Email: ag3310@columbia.edu
Title: Stoner instabilities and Ising excitonic states in twisted transition metal dichalcogenides
Abstract: Moiré transition metal dichalcogenide (TMD) systems provide a tunable platform for studying electron-correlation driven quantum phases and phase transitions, including metal-insulator transitions and anomalous quantum Hall states. Such phases have been exclusively found at rational fillings of the moiré superlattice, and it is believed that lattice commensurability plays a key role in their stability. In this work, we show via magnetotransport measurements on twisted WSe2 that new correlated electronic phases can exist away from commensurability. The first of these phases is an antiferromagnetic metal that is driven by proximity to the van Hove singularity, which occurs at a range of incommensurate dopings. The temperature, magnetic field and density dependence of the Hall effect carry signatures of the reconstructed Fermi surface due to itinerant magnetic ordering. The second is a re-entrant magnetic field-driven metal insulator phase transition in the vicinity of half-filling of the moiré superlattice. Magnetic field dependence of the longitudinal resistance shows metallic behavior at fields above 5 T, but transitions to an insulating state above ∼ 24 T. A detailed analysis of the Landau fans and the high field ρxy rules out the possibility of a trivial insulator and suggests that orbital effects are relevant to the transition and the proximal metallic phase, indicating a new class of correlation-driven metal-insulator transitions perhaps involving strong excitonic correlations among the carriers. These results establish twisted WSe2 as a new platform to study incommensurate quantum phases of interacting electrons on the triangular lattice.
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Aspen Center for Physics - 2024 Winter Conference Disorder and Quantum Phases of Matter
10. Christopher Homes
Brookhaven National Laboratory
Email: homes@bnl.gov
Title: The effects of disorder on the lattice dynamics and superconductivity in the cuprates
Abstract: The optical conductivity provides a wealth of information in complex oxide materials, for instance the gapping of electronic excitations and the transfer of spectral weight due the formation of a superconducting or charge-density wave. While the infrared-active lattice vibrations are also present, their contribution to the physical properties of these systems is often overlooked; however, they are excellent probes of local bonding and disorder. The effects of chemical doping and substitutional disorder can be clearly seen in the vibrational line widths of the in-plane infrared modes in the cuprates; some materials such as hole-doped YBCO and some of the electron-doped materials have remarkably sharp absorptions, while in other materials such as LSCO the vibrational modes are extremely broad.
11. Pavan Hosur
University of Houston
Email: phosur@central.uh.edu
Title: Superconductor vortices in Weyl semimetals: quantum oscillations, supersymmetry, and disorder induced Majorana fermions
Abstract: Vortices in type-II superconductors carry critical information about the parent metal. For example, ordinary metals produce equally spaced levels with finite zero-point energy, whereas massless Dirac metals yield exotic Majorana fermions at precisely zero energy. Weyl semimetals are gapless topological materials defined by accidental band intersections or Weyl nodes in the bulk and a bizarre surface metal composed of open Fermi arcs instead of closed Fermi surfaces. We ask, “1) What is the spectrum of superconductor vortices in Weyl semimetals? 2) Can it trap Majorana fermions?" Restricting to non-magnetic Weyl semimetals and superconductivity that is gapped when uniform, we show that the spectrum is generically gapped and follows from semiclassical quantization of closed orbits consisting of Fermi arcs on opposite surfaces connected by one-way bulk conduits. We predict a slew of exotic behaviors such as periodic oscillations in the density of states as the vortex is tilted and transmutation between bosonic, fermionic, and supersymmetric natures. Moreover, if the semiclassical orbits are shorted by quantum tunneling in the bulk, surface Majorana modes appear under simple conditions based on the Fermi arc connectivity and bulk Weyl node positions. We propose well-studied materials NbP, TaP, LifeCoAs and FeSeTe for realizing different parts of our proposals.
12. Yusuke Iguchi
Stanford University
Email: yiguchi@stanford.edu
Title: Superconducting vortices carrying a temperature-dependent fraction of the flux quantum
Abstract: In a single crystal of multicomponent superconductors, it is theoretically expected that an isolated vortex carries only a part of the flux quantum at the level of Ginzburg-Landau theory. This unquantized vortex, however, had never been observed experimentally since its first theoretical discovery in 2002. We investigate isolated vortices in the multiband superconductor Ba1-xKxFe2As2(x=0.77) by using scanning superconducting quantum interference device (SQUID) magnetometry. In many locations, we observed vortices that carried only part of a flux quantum, with a magnitude that varied continuously with temperature. We interpret these features as vortices with unquantized magnetic flux, whose magnitude is
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Aspen Center for Physics - 2024 Winter Conference Disorder and Quantum Phases of Matter
determined by the temperature-dependent parameters of a multiband superconductor. We further characterize the mobility and manipulability of the unquantized vortices.
13. Jonas Felix Karcher
Institution: Pennsylvania State University
Email: jmk7851@psu.edu
Title: Extended critical phase in quasiperiodic quantum Hall systems
Abstract: We consider the effects of quasiperiodic spatial modulation on the quantum Hall plateau transition, by analyzing the Chalker-Coddington network model for the integer quantum Hall transition with quasiperiodically modulated link phases. In the conventional case (uncorrelated random phases), there is a critical point separating topologically distinct integer quantum Hall insulators. Surprisingly, the quasiperiodic version of the model supports an extended critical phase for some angles of modulation. We characterize this critical phase and the transitions between critical and insulating phases. For quasiperiodic potentials with two incommensurate wavelengths, the transitions we find are in a different universality class from the random transition. Upon adding more wavelengths they undergo a crossover to the uncorrelated random case. We expect our results to be relevant to the quantum Hall phases of twisted bilayer graphene or other Moiré systems with large unit cells.
14. Sergey Kravchenko
Northeastern University
Email: s.kravchenko@northeastern.edu
Title: Collective depinning and sliding of a quantum 2D Wigner solid
M. Yu. Melnikov, A. A. Shashkin, S.-H. Huang, C. W. Liu and S. V. Kravchenko
Abstract: We report the observation of two-threshold voltage-current characteristics accompanied by a peak of broadband current noise between the two threshold voltages in the insulating state at low densities in the 2D electron system in ultra-high mobility SiGe/Si/SiGe heterostructures. The observed results can be described by a phenomenological theory of the collective depinning of elastic structures, which naturally generates a peak of a broadband current noise between the dynamic and static thresholds and changes to sliding of the solid over a pinning barrier above the static threshold. This gives compelling evidence for the electron solid formation in this electron system.
15. Katharina Laubscher
University of Maryland
Email: klaubsch@umd.edu
Title: Majorana zero modes in gate-defined germanium hole nanowires
Abstract: We theoretically study proximitized gate-defined one-dimensional channels in planar Ge hole gases as a potential platform for Majorana zero modes. One of the main advantages of the Ge-based platform is that ultra-clean planar Ge quantum wells with hole mobilities exceeding one million and mean free paths on the order of tens of microns already exist, such that gate-defined Ge hole channels may be able to overcome some of the problems caused by the presence of substantial disorder in more conventional Majorana platforms. To investigate the prospects for the experimental realization of Ge-based Majorana zero modes, we first calculate the topological phase diagrams for pristine Ge hole nanowires of different wire geometries. Next, by numerically calculating the local tunneling conductance spectra in the topological phase for realistic disorder strengths, we show that the reduced disorder-to-gap ratio leads to less ambiguity in the experimental transport data compared to what is expected and reported in InAs or
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Aspen Center for Physics - 2024 Winter Conference Disorder and Quantum Phases of Matter
InSb based platforms, where the presence of substantial disorder is likely the most challenging obstacle for future progress.
16. Carlos Lima
California State University Long Beach
Email: LimaCD1618@gmail.com
Title: The construction of a realistic Hamiltonian for the bosonic Moore-Read Pfaffian at ν=1
Abstract: Physical systems that support topologically ordered phases have garnered interest due to the possibility of fractional braiding statistics among the low-energy quasi-particle excitations. The fermionic case of the fractional quantum Hall effect at filling factor 5/2, thought to support non-Abelian excitations, is experimentally challenging. Alternatively, the setting which can engender bosonic fractional quantum Hall states in different realistic physical systems has been established.
References: Peterson, M. R., Park, K., Das Sarma, S., PRL 101, 156803 (2008),
Qi, X., Ranard, D., Quantum 2019,
Chertkov, E., Clark, B. K. PRX 8, 031029 (2018).
17. Pavel Nosov
Stanford University
Email: nosov@stanford.edu
Title: Interplay of superconductivity and localization near a 2D ferromagnetic quantum critical point
Abstract: We study the superconducting instability of a two-dimensional disordered Fermi liquid weakly coupled to the soft fluctuations associated with proximity to an Ising-ferromagnetic quantum critical point. We derive interaction-induced corrections to the Usadel equation governing the superconducting gap function and show that diffusion and localization effects drastically modify the interplay between fermionic incoherence and strong pairing interactions.
18. Akshat Pandey
Stanford University
Email: akshatp@stanford.edu
Title: Random geometry at an infinite-randomness fixed point
Abstract: We study the low-energy physics of the critical (2+1)-dimensional random transverse-field Ising model. The one-dimensional version of the model is a paradigmatic example of a system governed by an infinite-randomness fixed point, for which many results on the distributions of observables are known via an asymptotically exact renormalization group (RG) approach. In two dimensions, the same RG rules have been implemented numerically, and demonstrate a flow to infinite randomness. However, analytical understanding of the structure of this RG has remained elusive due to the development of geometrical structure in the graph of interacting spins. To understand the character of the fixed point, we consider the RG flow acting on a joint ensemble of graphs and couplings. We propose that the RG effectively occurs in two stages: (1) randomization of the interaction graph until it belongs to a certain ensemble of random triangulations of the plane, and (2) a flow of the distributions of couplings to infinite randomness while the graph ensemble remains invariant. This picture is substantiated by a numerical RG in which one obtains a steady-state graph degree distribution and subsequently infinite- randomness scaling distributions of the couplings. Both of these aspects of the RG flow can be approximately reproduced in simplified analytical models.
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Aspen Center for Physics - 2024 Winter Conference Disorder and Quantum Phases of Matter
19. J. Clayton Peacock
New York University
Email: jcp9552@nyu.edu
Title: Many-body delocalization: thermal inclusions and operator growth in disordered spin chains
Abstract: Recent studies have questioned the stability of the many-body localized (MBL) phase in one dimensional disordered spin chains in the thermodynamic limit. Thermalization in this limit is thought to be due to “quantum avalanches", in which rare Griffiths regions of lower disorder serve as thermalizing baths. In this poster we present evidence of avalanches in numerical studies using biased sampling, as well as an analysis of operator growth in unbiased spin chains using the recursion method, and in both cases show these systems are unstable at disorders much larger than commonly believed.
20. Enrico Rossi
William & Mary
Email: enrico.rossi@gmail.com
Title: Interplay of Quantum Metric and Disorder in 2D Superfluid States
Abstract: In multi-orbital systems with flat bands, the quantum metric of the Bloch states can give a non negligible contribution to several physical quantities. In particular, it can provide a significant contribution to the stiffness of superfluid and superconducting states that can be realized in these systems. I will briefly discuss the general connection between quantum metric and the properties of 2D correlated states that break a continuous symmetry and then present results to show how the interplay of quantum metric and disorder affect the stiffness of superconducting states in a variety of 2D models and discuss their relevance to systems like twisted bilayer graphene.
21. Bitan Roy
Lehigh University
Email: bir218@lehigh.edu
Title: How dirty topological insulators become trivial
Abstract: Topological insulators featuring bulk gap with nontrivial topological invariant are robust against weak disorder. However, at strong disorder, the topological property of the system becomes fragile and the system becomes a trivial insulator. A question of deep fundamental importance then arises in this context. How do dirty topological insulators become a trivial one in the presence of disorder? We address this question by computing the local Chern marker of a lattice-regularized model for two-dimensional Chern insulators in the presence of random charge impurities. Our numerical computation shows that the area of the bulk topological regime with quantized local Chern number continuously shrinks as disorder strength increases and in the trivial insulating phase at strong disorder, all the sites of the system have zero local Chern number. We also show that a similar outcome holds for time-periodic Floquet Chern insulator.
22. Xueyang Song
Massachusetts Institute of Technology
Email: xueyangs@mit.edu
Title: Transition out of Fractional quantum anomalous Hall states in Moire systems
Abstract: Motivated by the recent experimental breakthrough on the observation of the fractional quantum anomalous Hall (FQAH) effects in twisted semiconductor and pentalayer graphene, we explore the rich
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Aspen Center for Physics - 2024 Winter Conference Disorder and Quantum Phases of Matter
physics and phase transitions associated with the FQAH phase. We explore proximate phases including the charge density wave (CDW) orders, and topological ordered states. At filling -2/3, a quantum Ginzburg Landau critical theory, as well as quantum electrodynamics (QED), are proposed to describe various FQAH CDW transitions and the competition of CDW order and FQAH phases. When CDW coexists with FQAH, a direct transition from FQAH to trivial CDW insulator is possible. Experiment signatures, such as universal jump of resistivities and CDW halo around anyons in FQAH, follow from the beyond Landau critical theory.
23. Tyler Sterling
University of Colorado Boulder
Email: ty.sterling@colorado.edu
Title: Introduction of oxygen vacancy planes into rutile TiO2 by passing electric current revealed by neutron scattering
Abstract: We have shown that a recently developed method for processing materials called flash introduces ordered oxygen vacancies into rutile TiO2. In flash, a material is raised to high temperature and an electric field is applied across it; at fixed temperature there is a critical field where metallic current starts to flow which signals an onset of a new current-driven phase called flash that is characterized by rapid ionic diffusion, enhanced electronic conductivity, and photoluminescence. In the state of flash, the current density is tuned to control the concentration of defects. The defects can be frozen in by quenching. We used a newly developed sample environment to perform in-situ diffuse neutron scattering on flashing rutile TiO2 crystals on the CORELLI instrument at the SNS. We also measured crystals with defects frozen in by quenching. We found sea-urchin-like diffuse scattering features which correspond to oxygen vacancies ordering into planes. The planes are consistent with a certain Magnéli phase of slightly reduced rutile. The in-situ data allow us to correlate the concentration of defects with the current density, showing that flash can be used to control the concentration of non-trivial defects in rutile TiO2.
24. Hanna Terletska
Middle Tennessee State University
Email: Hanna.Terletska@mtsu.edu
Title: Quantum Embedding Methods for Disordered Materials In and out of Equilibrium
Abstract: Significant progress has been made in recent years in comprehending the properties of disordered electronic systems, largely owing to the critical role that quantum embedding numerical methods simulating many-body disordered correlated electron systems. In this presentation, we present our newly developed quantum embedding tools designed for disordered systems under both equilibrium and non
equilibrium conditions. These tools include DCA, cDMFT, nonequilibrium DMFT+CPA, and the Typical medium TMDCA. We demonstrate how these tools effectively capture phenomena such as disorder induced localization, strong nonlocal effects near critical transitions, and accurately model screening effects. Additionally, we provide examples of recent applications of these developed tools across a variety of models and systems, including single and multi-band Anderson models, models featuring local and off-diagonal disorder, and the Anderson-Hubbard model in non-equilibrium scenarios. Lastly, we showcase the application of these methods to realistic systems within the framework of density functional theory, highlighting their versatility and potential impact.
25. Eduardo Jonathan Torres Herrera
Benemerita Universidad Autonoma de Puebla, Mexico
Email: ejtorres@gmail.com
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Aspen Center for Physics - 2024 Winter Conference Disorder and Quantum Phases of Matter
Title: Effects of correlated random disorder on the dynamics of many-body quantum systems
Abstract: We numerically study the effects of autocorrelated disorder on the static and dynamical properties of a one-dimensional many-body quantum system exhibiting many-body localization. Specifically, by means of some standard measures of energy level repulsion and localization of energy eigenstates, we show that a strong degree of correlations between the on-site potentials in the one-dimensional spin-1/2 Heisenberg model leads to suppression of the many-body localization phase, while level repulsion is mitigated for small disorder strengths, although energy eigenstates remain well extended. Our findings are also remarkably manifested in the time domain, on which we put the main emphasis, as shown by the time evolution of experimentally relevant observables, like the return probability and the spin-spin autocorrelation function.
26. Jukka Vayrynen
Purdue University
Email: jvayryne@purdue.edu
Title: Influence of disorder on antidot vortex Majorana states in 3D topological insulators
Abstract: Topological insulator/superconductor two-dimensional heterostructures are promising candidates for realizing topological superconductivity and Majorana modes. In these systems, a vortex pinned by a pre-fabricated antidot in the superconductor can host Majorana zero-energy modes (MZMs), which are exotic quasiparticles that may enable quantum information processing. However, a major challenge is to design devices that can manipulate the information encoded in these MZMs. One of the key factors is to create small and clean antidots so that the MZMs localized in the vortex core have a large gap to other excitations. If the antidot is too large or too disordered, the level spacing for the subgap vortex states may become smaller than temperature. In this paper, we numerically investigate the effects of disorder, chemical potential, and antidot size on the subgap vortex spectrum using a two-dimensional effective model of the topological insulator surface. Our model allows us to simulate large system sizes with vortices up to 1.8 μm in diameter. We also compare our disorder model with the transport data from existing experiments. We find that the spectral gap can exhibit a non-monotonic behavior as a function of disorder strength, and that it can be tuned by applying a gate voltage.
27. Yuxin Wang
Northwestern University
Email: yuxinwang2023@u.northwestern.edu
Title: Topological real-space responses and stable integer invariants from dimensional reduction
Abstract: Dimensional reduction of topological response is a powerful tool to understand topological field theory of insulating states at various spatial dimensions. Thus far, this procedure has been successfully implemented by Qi et al. only for time-reversal symmetric spin-orbit coupled insulators (symplectic class AII) in the absence of disorder. In this work, we show that the dimensional reduction of four-dimensional quantum Hall effect can be performed for all three Wigner Dyson classes in the presence and absence of disorder to understand stable topological states at three and two dimensions which are not predicted by ten-fold classification scheme. We accomplish this goal by using magnetic monopoles and flux tubes as non perturbative real-space probes. We show that many topological phases of matter which are often called "fragile topological insulators" actually support stable invariants and quantized responses.
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Aspen Center for Physics - 2024 Winter Conference Disorder and Quantum Phases of Matter
28. Xinghai Zhang
Rice University
Email: xz51@rice.edu
Title: Magnetic instability and spin-glass order beyond Anderson-Mott transition in interacting power law random banded matrix fermions
Abstract: There is longstanding confusion regarding the interplay between Anderson localization, interactions, and magnetism at the zero-temperature metal-insulator transition (MIT), such as that observed in two-dimensional electron gases or doped bulk semiconductors. Analytical calculations long ago predicted strong magnetic fluctuations upon approaching such a MIT from the metallic side due to quantum interference effects. The nature of any incipient magnetic instability was unclear, although local-moment
physics has long been suspected to play a role. Here we perform complementary analytical and numerical calculations for fermions with power-law random-banded matrix (PRBM) hopping subject to repulsive Hubbard interactions. Analytical results on the metallic side are nearly identical to those in finite dimensions, signaling the onset of a magnetic instability at the MIT. Hartree-Fock numerics confirms and clarifies this picture, showing that local moments smoothly nucleate at the Fermi level from the MIT as we tune into the interacting Anderson insulator. We show that these moments form a spin glass. Our work shows how interference physics and frustrated local-moment magnetism can arise in concert as two sides of the same MIT coin.
29. He Zhao
Brookhaven National Lab
Email: hzhao@bnl.gov
Title: A tale of two Pair Density Waves in a magnetic superconductor
Abstract: Most known superconductors are characterized in equilibrium by a complex order parameter that is uniform in space, i.e., . Yet, under specific conditions, exotic Cooper pairing state can emerge with a non-uniform equilibrium order parameter, possessing a finite center-of-mass momentum. Such spatially dependent order parameters ( ) can generically feature variations in either their amplitude or phase — or a combination of both. Utilizing Spectroscopic Imaging Scanning Tunneling Microscopy (SI STM), we've identified two primary PDW states in the iron pnictide superconductor, EuRbFe4As4 (ER-1144), a material that features co-existing superconductivity (Tc ≈ 37 kelvin) and magnetism (Tm ≈ 15 kelvin). In the ferromagnetic superconducting phase, the first type is characterized by a superconducting gap that has a long-range, unidirectional spatial modulation ( ) in the absence of any other translational symmetry breaking density-wave orders. The second type features an anisotropic Doppler energy shift in reciprocal space due to the phase winding ( ). In contrast, all the striking features are completely suppressed crossing the magnetic transition. Additionally, both PDW states are also impacted by an out-of-plane external magnetic field, evidenced by the Bogoliubov quasiparticle spectrum imaging. Our findings provide valuable insights into the intricate nature of the PDW states and the interplay between magnetism and superconducting order.
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