Session X62: Correlations - Measurement and Control

Domain wall spin-texture in quantum Hall magnets

Recent experiments [Stepanov Petr, et al. Nature Physics 14.9,907; Wei et.al Science 362.229-233; Haoxin,arXiv:1904.11485] reported long range transport of spinful collective modes in single-layer graphene based quantum Hall magnets(QHMs), including transport across junctions between states with different Landau level filling factors. These experiments motivate a detailed study of the various junctions that can be formed between N=0 QHMs since the microscopic electronic and spin-texture structure near the junction influences the collective mode transmission rate. Using a self-consistent Hartree-Fock calculation, we show that the spin-textures around the junction of \nu=0 and \nu=\pm 1 QHMs are wide, reaching ~140nm at B=8T. For the junction of \nu=-1 and \nu=1 QHMs, however, the domain wall is narrower and the spin-texture depends on the sublattice potential. I will explain the microscopic physics that controls these junction properties.   

Spin transport in quantum Hall magnets

Recent nonlocal electrical measurements [Stepanov Petr, et al. Nature Physics 14.9, 907; Wei et.al Science 362, 229-233; Haoxin et.al ,arXiv:1904.11485] have revealed remarkable spin transport properties in devices that host various graphene quantum Hall magnet (QHM) states. I will discuss our efforts to understand several different aspects of these experiments: 1) using an electric circuit model, we provide an explanation for the finite range of bias voltages over which nonlocal voltages are observed across insulating magnetic states and 2) using a microscopic spin-wave theory we estimate the transmission probabilities of magnon-like collective modes across junctions between QHMs that have different Landau level filling factors.   

Enhanced thermal Hall effect in nearly ferroelectric insulators

In the context of recent experimental observations of an unexpectedly large thermal Hall conductivity in insulating La_2-xSr_xCuO₄ and SrTiO₃, we theoretically explore conditions under which acoustic phonons can give rise to such a large thermal Hall effect. Both the intrinsic and extrinsic contributions to the thermal Hall conductivity are large in proportion to the dielectric constant and the flexoelectric coupling. While the intrinsic contribution is still orders of magnitude smaller than the observed effect, an extrinsic contribution proportional to the phonon mean free path appears likely to account for the observations, at least in SrTiO₃. We predict a larger intrinsic contribution to thermal Hall effect in certain insulating perovskites.   

Probing quantum spin liquids in equilibrium using the inverse spin Hall effect

We propose an experimental method utilizing a spin orbit coupled metal to quantum magnet bilayer that will probe quantum magnets lacking long range magnetic order, e.g., quantum spin liquids, via a dc resistance measurement across the metal. The bilayer is held in thermal and chemical equilibrium, and spin fluctuations arising across the single interface are converted into voltage fluctuations in the metal as a result of the inverse spin Hall effect. In thermal equilibrium, changes to the voltage noise in the metal are measurable as changes to the resistance via the fluctuation dissipation theorem. We examine the theoretical workings of the proposed bilayer system, and offer predictions for the temperature scaling of the enhancement to the dc resistance measured across the metal for three quantum spin liquid models. We consider the Heisenberg spin-$1/2$ kagom{\'e} lattice model and extract the spinon gap. We find characteristic $T^3$ scaling of the dc resistance enhancement for the Kitaev model in the gapless phase. Finally, for fermionic spinons coupled to a $U(1)$ gauge field we find subdominant $T^{4/3}$ scaling of the dc resistance enhancement. We therefore show that our proposed bilayer can test the relevance of a quantum spin liquid model to a given candidate material.   

Improved quantum transport calculations for interacting nanostructures

Nanoelectronics devices such as semiconductor quantum dots and single molecule transistors exhibit a rich range of physical behavior due to the interplay between orbital complexity, strong electronic correlations and device geometry. Understanding and simulating the quantum transport through such nanostructures is essential for rational design and technological applications. In this talk I present theoretical reformulations of the Kubo and Meir-Wingreen formulae for mesoscopic quantum transport calculations in linear response, and demonstrate the improvement over standard methods with several example applications using the numerical renormalization group.   

Resistive switching duality: insulator-to-metal vs. metal-to-insulator switching

Electrical triggering of metal-insulator transitions offers an opportunity to build energy-efficient and scalable electronics. While insulator-to-metal (I-M) switching in materials such as VO₂ is thoroughly studied, much less is known about an opposite metal-to-insulator (M-I) switching. We present a detailed study of M-I switching in (La,Sr)MnO₃ (LSMO). Negative differential resistance (NDR) region in the I-V characteristic is necessary for the resistive switching. By comparing LSMO to VO₂, we observe an interesting duality: the N-type NDR in LSMO is a “mirror reflection” of the S-type NDR in VO₂. Using Kerr effect imaging, we found that an insulating blocking domain perpendicular to the current flow forms in the LSMO during the NDR. This behavior is reciprocal to VO₂ in which a conducting filament parallel to the current flow emerges during the NDR. M-I and I-M resistive switchings complement each other providing a broad range of nonlinear electrical properties, which could allow designing of complex electronic devices.   

Quantum aspects of “hydrodynamic” transport from weak electron-impurity scattering

Recent experimental observations of apparently hydrodynamic electronic transport have generated much excitement. However, theoretical understanding of the observed non-local transport (whirlpool) effects and parabolic current profiles has remained at the level of a phenomenological analogy with classical fluids. A more microscopic account of genuinely hydrodynamic electronic transport is difficult because such behavior requires strong interactions to diffuse momentum. Here, we show that the non-local conductivity effects can indeed occur for fermion systems in the presence of disorder. By explicit calculation of the conductivity at finite wavevector σ(q) for selected weakly disordered free fermion systems, we propose experimental strategies for demonstrating distinctive quantum effects in non-local transport at odds with the expectations of classical kinetic theory. Our results imply that the observation of whirlpools or other "hydrodynamic" effects does not guarantee the dominance of electron-electron scattering over electron-impurity scattering.   

Integrable systems of heterogeneous qubits

We explore an unusual type of quantum spin matter that can be realized by qubits having different physical origin, such as in bound states near defects in Dirac materials. Interactions of different qubits in this matter are described by essentially different coupling operators that do not commute with each other. We show that at least the simplest such models satisfy integrability conditions that we use to describe pseudospin dynamics in a linearly time-dependent magnetic field. Generalizing to arbitrary numbers of qubits, we construct a spin Hamiltonian, which we call the gamma-magnet. For arbitrarily strong interactions, nonadiabatic dynamics, and any initial eigenstate, we find that quantum interference suppresses spin-flips, so that the system remains close to the initial state. This effect may not have a counterpart in classical physics and can be a signature of a new type of spin ordering, which is different from both disordered spin glasses and ordered phases of spin lattices.   

Looking into thermalization and localization through the lens of quantum coherence

Quantum coherence quantifies the amount of superposition a quantum state can have in a basis. Since there is a difference in the structure of eigenstates of the ergodic and many-body localized systems, we expect them also to differ in terms of their coherences. Here, we numerically calculate different measures of quantum coherence in the excited eigenstates of an interacting disordered Hamiltonian as a function of the disorder. We show that these can be used as an order parameter to detect the well-studied ergodic to the many-body localized phase transition. We also perform quantum quench studies to distinguish the behavior of coherence in thermalized and localized phases. We present a protocol to calculate measurement-based localized coherence to investigate the thermal and many-body localized phases. The protocol allows looking at the correlation in a non-destructive way since tracing out a subsystem always destroys coherence and correlation.   

Measurement-induced criticality in random quantum circuits

We investigate the critical behavior of the entanglement transition induced by projective measurements in (Haar) random unitary quantum circuits. Using a replica approach, we map the calculation of the entanglement entropies in such circuits onto a two-dimensional statistical mechanics model. In this language, the area- to volume-law entanglement transition can be interpreted as an ordering transition in the statistical mechanics model. We derive the general scaling properties of the entanglement entropies and mutual information near the transition using conformal invariance. We analyze in detail the limit of infinite on-site Hilbert space dimension in which the statistical mechanics model maps onto percolation. In particular, we compute the exact value of the universal coefficient of the logarithm of subsystem size in the nth Rényi entropies for n≥1 in this limit using relatively recent results for conformal field theory describing the critical theory of 2D percolation, and we discuss how to access the generic transition at finite on-site Hilbert space dimension from this limit, which is in a universality class different from 2D percolation.   

Temperature dependence of quantum information scrambling in gapped local systems

We study temperature dependence of quantum information scrambling, specifically, in systems with a gap. Firstly, we perform large scale tensor network based numerics in gapped chaotic one dimensional spin chains to obtain scrambling data at different temperatures. We find that our numerics work very well even at low temperatures, and we are able to determine the temperature dependence of butterfly velocity to be √(T/m), where m is the mass, for T<m. From our numerics, we also observe a broadening of the operator wavefront at finite temperatures, which had been observed in the context of infinite T previously. Secondly, we perform a perturbative calculation to study scrambling in the paramagnetic phase of a 2+1 D non-linear sigma model to analytically understand the temperature dependence of the butterfly velocity. Using the ladder diagram techniques, we verify the √(T/m) behavior of the butterfly velocity at low temperatures, T<<m. Thirdly, we discuss these results in the context of a recently proposed state dependent bound on scrambling, and show that our results are in accordance with the bound, and in fact provides a clear physical picture of scrambling at low temperatures.   

Magnetic Hamiltonian Engineering by Light-Matter Interaction

In this work, we explore the flexibility and magnitude of Hamiltonian engineering by considering photon-electron coupling and photon-phonon coupling. We compare the pros and cons of both routes in simple toy models, and extend our results to more realistic models.   

Hydrogenated Rare-Earth Nickelate Nanojunctions with Synaptic Behavior Studied by X-ray Nanodiffraction

Rare-earth nickelates have been the target of intense fundamental and application-focused studies due to their rich phase diagram and possible utilization of these materials as novel sensors, memory devices and hardware elements for artificial intelligence. For example, hydrogenation of SmNiO₃ (SNO) drastically changes its electronical properties giving rise to a new insulating phase. Moreover, application of voltage pulses allows one to repeatedly switch the hydrogenated SNO films between the lower and higher resistance states. This allows one to create a device with synaptic behavior, which is a crucial hardware element for neuromorphic computing. With nanofocused x-ray diffraction and spectroscopy we performed in-situ spatially-resolved studies of the microscopic mechanism behind the potentiation and depression of an SNO-based device. We revealed changes in both electronic structure and crystal lattice between pristine and hydrogenated SNO films, as well as before and after electrical switch of the device.   

Unveiling electron correlation in semiconducting Heusler FeVSb by angle-resolved photoemission and dynamical mean field theory

Electron-electron correlations are responsible for many of the exotic properties of transition metal oxides and chalcogenides; however, the role of correlations in transition metal Heusler compounds is often overlooked. Here, combining angle-resolved photoemission spectroscopy (ARPES) and dynamical mean field theory (DMFT), we directly probe the single particle spectral function of the Heusler FeVSb. ARPES measurements on epitaxial FeVSb films reveal a mass renormalization of m^* / m_bare = 1.4, where m_bare is the mass from DFT-LDA calculations that do not include a Hubbard U. This mass renormalization lies in dramatic contrast to other Heuslers LnPt(Sb,Bi) (Ln = lanthanide) [1-3] and CoTiSb [4], for which bare DFT calculations are in quantitative agreement with ARPES. By treating the many-body interactions more accurately at the level of DMFT, we quantitatively reproduce the measured electronic structure and comment on the differences between FeVSb and other Heuslers. Our work calls for a re-thinking of the role of correlations in FeVSb and in Heuslers more generally.

[1] H. Kim et. al., Sci. Adv., 4, 4 eaao4513 (2018)
[2] J. Logan et. al., Nature Comm. 7, 11993 (2016)
[3] Z. K. Liu et. al. , Nature Comm. 7, 12924 (2016)
[4] J. K. Kawasaki, et. al. Sci. Adv. 4, 6, eaar5832 (2018)