We obtain an analytical expression for the heat current between two overdamped quantum oscillators interacting with local thermal baths at different temperatures. The total heat current is split into classical and quantum contributions. We show how to evaluate both contributions by taking advantage of the timescale separation associated with the overdamped regime and without assuming the usual weak-coupling and Markovian approximations. We find that nontrivial quantum corrections survive even when the temperatures are high compared to the frequency scale relevant for the overdamped dynamics of the system.

Anomalous behavior of spin stiffness is revealed in ZnO-based two-dimensional electron systems (2DESs) with strong Coulomb interaction and Wigner-Seitz parameter $r_s>6$. The spin stiffness is extracted directly from the quadratic $k$ dispersion of spin excitons at $\nu =1$ probed by inelastic light scattering. The resulting values are found to be dramatically rescaled compared to the case of weakly interacting 2DESs—spin stiffness turned out to be of the order of the cyclotron energy with the effective mass of Fermi-liquid quasiparticles. This result is also confirmed by the exact diagonalization simulations.

We report on the direct measurement of the electron spin splitting and the accompanying nuclear Overhauser field, and thus the underlying nuclear spin polarization (NSP) and fluctuation bandwidth, in a single InAs quantum dot under resonant excitation conditions with unprecedented spectral resolution. The dot consists of ${10}^4\text{−}{10}^5$ nuclei, and is electrically biased to quantum confine an additional single electron. The electron spin splitting is measured directly via resonant spin-flip single-photon Raman scattering detected by superconducting nanowires to generate excitation-emission energy maps. The observed two-dimensional maps reveal an Overhauser field that has a nonlinear dependence on excitation frequency. This study provides new insight into earlier reports of so-called avoidance and tracking, showing two distinct NSP responses directly by the addition of an emission energy axis. The data show that the polarization processes depend on which electron spin state is optically driven, with surprising differences in the polarization fluctuations for each case: In one case, a stabilized field characterized by a single-peaked distribution shifts monotonically with the laser excitation frequency resulting in a nearly constant optical interaction strength across a wide detuning range, while in the other case the previously reported avoidance behavior is actually the result of a nonlinear dependence on the laser excitation frequency near zero detuning leading to switching between two distinct mesoscopic nuclear spin states. The magnitude of the field, which is as large as 400 mT, is measured with sub-100 nuclear spin sensitivity. Stable and unstable points of the Overhauser field distribution are observed, resulting from the nonlinear feedback loop in the electron-trion-nuclear system. Nuclear spin polarization state switching occurs between fields differing by 160 mT at least as fast as 25 ms. Control experiments indicate that the strain-induced quadrupolar interaction may explain the measured Overhauser fields.

A valley-contrasting Berry curvature in bilayer transition metal dichalcogenides with spin-orbit coupling can generate valley magnetization when the inversion symmetry is broken, for example, by an electric field, regardless of time-reversal symmetry. A nontrivial Berry curvature can also lead to anomalous transport responses, such as the anomalous Hall effect and the anomalous Nernst effect. Applied to a bilayer $\mathrm{W}{\mathrm{Se}}_2$, an electric field can tune the Berry curvature and orbital magnetic moment, which has important consequences for the orbital magnetization and the anomalous Nernst responses. The orbital magnetization and its two contributions, one due to the magnetic moment and one due to the Berry curvature, are calculated and interpreted in terms of opposite circulating currents of the bands in the two layers. The valley anomalous Nernst coefficient and spin Nernst coefficient are also calculated. We find that a finite electric field leads to peaks and dips in the Nernst responses that have the signs of the Berry curvatures of the bands and are proportional to their magnitudes; it also enhances the valley Nernst responses. These experimentally verifiable findings may be promising for caloritronic applications.

We propose a quantum absorption refrigerator using the quantum physics of resonant tunneling through quantum dots. The cold and hot reservoirs are fermionic leads, tunnel coupled via quantum dots to a central fermionic cavity, and we propose configurations in which the heat absorbed from the (very hot) central cavity is used as a resource to selectively transfer heat from the cold reservoir on the left to the hot reservoir on the right. Heat transport in the device is particle-hole symmetric; we find two regimes of cooling as a function of the dot energies—symmetric with respect to the Fermi energy of the reservoirs—and we associate them with heat transfer by electrons above the Fermi level and holes below the Fermi level. We also discuss optimizing the cooling effect by fine-tuning the energy of the dots as well as their linewidth and characterize regimes where the transport is thermodynamically reversible such that the Carnot coefficient of performance is achieved with zero cooling power delivered.

Topological phase transitions occur when the electronic bands change their topological properties, typically featuring the closing of the band gap. While the influence of topological phase transitions on electronic and optical properties has been extensively studied, its implication on phononic properties and thermal transport remains unexplored. In this paper, we use first-principles simulations to show that certain phonon modes are significantly softened near topological phase transitions, leading to increased phonon-phonon scattering and reduced lattice thermal conductivity. We demonstrate this effect using two model systems: pressure induced topological phase transition in ${\mathrm{ZrTe}}_5$ and chemical composition induced topological phase transition in ${\mathrm{Hg}}_{1-\mathrm{x}}{\mathrm{Cd}}_{\mathrm{x}}\mathrm{Te}$. We attribute the phonon softening to emergent Kohn anomalies associated with the closing of the band gap. Our paper reveals the strong connection between electronic band structures and lattice instabilities, and opens up a potential direction towards controlling heat conduction in solids.

The recent discovery of a nonsaturating linear magnetoresistance in several correlated electron systems near a quantum critical point has revealed an interesting interplay between the linear magnetoresistance and the zero-field linear-in-temperature resistivity. These studies suggest a possible role of quantum criticality on the observed linear magnetoresistance. Here we report our discovery of a nonsaturating, linear magnetoresistance in ${\mathrm{Mo}}_8{\mathrm{Ga}}_{41}$, a nearly isotropic strong electron-phonon coupling superconductor with a linear-in-temperature resistivity from the transition temperature to $\sim 55$ K. The growth of the resistivity in field is comparable to that in temperature, provided that both quantities are measured in the energy unit. Our data sets are remarkably similar to magnetoresistance data of the optimally doped ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$, despite the clearly different crystal and electronic structures, and the apparent absence of quantum critical physics in ${\mathrm{Mo}}_8{\mathrm{Ga}}_{41}$. A new empirical scaling formula is developed, which is able to capture the key features of the low-temperature magnetoresistance data of ${\mathrm{Mo}}_8{\mathrm{Ga}}_{41}$, as well as the data of ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$.

We study the many-body electronic structure of the stoichiometric and electron-doped trilayer nickelate ${\mathrm{Pr}}_4{\mathrm{Ni}}_3{\mathrm{O}}_8$ in comparison to that of the stoichiometric and hole-doped infinite layer nickelate ${\mathrm{NdNiO}}_2$ within the framework of density functional plus dynamical mean field theory, noting that ${\mathrm{Pr}}_4{\mathrm{Ni}}_3{\mathrm{O}}_8$ has the same nominal carrier concentration as ${\mathrm{NdNiO}}_2$ doped to a level of 1/3 holes/Ni. We find that the correlated Ni-$3d$ shells of both of these low valence nickelates have similar many-body configurations with correlations dominated by the $d_{x^2-y^2}$ orbital. Additionally, when compared at the same nominal carrier concentration, the materials exhibit similar many-body electronic structures, self energies, and correlation strengths, but differ in Fermiology. Compared to cuprates, these materials are closer to the Mott-Hubbard regime due to their larger charge transfer energies. Moreover, doping involves the charge reservoir provided by the rare earth $5d$ electrons, as opposed to cuprates where it is realized via the oxygen $2p$ electrons.

We study possible Weyl semimetals of strongly correlated electrons by investigating magnetotransport properties in pyrochlore $R_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ ($R$ denotes rare-earth ions), choosing three types of $R$ ions to design the exchange coupling scheme between $R4f$ and Ir $5d$ moments: nonmagnetic Eu $\left(4f^6\right)$, isotropic Gd $\left(4f^7\right)$, and anisotropic Tb $\left(4f^8\right)$. In the doping-induced semimetallic state, distinctive features of magnetoresistance and the Hall effect are observed in $R=\mathrm{Gd}$ and Tb compounds due to the effects of the exchange-enhanced isotropic and anisotropic Zeeman fields, respectively, exemplifying the double-Weyl semimetal and the two-in two-out line-node semimetal as predicted by theories. In particular, the Hall angle of an $R=\mathrm{Gd}$ compound is strongly enhanced to 1.5% just above the critical doping for the Mott transition. Furthermore, an unconventional Hall contribution is discerned for a lower doping regime of the $R=\mathrm{Gd}$ compound, which can be ascribed to the emergence of Weyl points with the field-distorted all-in all-out order state. These findings indicate that the hole-doping-induced Mott transition as well as the characteristic $f\text{−}d$ exchange interaction stabilizes versatile topological semimetal states in a wide range of material parameter space.

We study the classical Heisenberg model on the geometrically frustrated Shastry-Sutherland (SS) lattice with additional Dzyaloshinskii-Moriya (DM) interaction in the presence of an external magnetic field. We show that several noncollinear and noncoplanar magnetic phases, such as the flux, all-in/all-out, 3-in–1-out/3-out–1-in, and canted-flux phases are stabilized over wide ranges of parameters in the presence of the DM interaction. We discuss the role of DM interaction in stabilizing these complex magnetic phases. When coupled to these noncoplanar magnetic phases, itinerant electrons experience a finite Berry phase, which manifests in the form of topological Hall effect, whereby a nonzero transverse conductivity is observed even in the absence of a magnetic field. We study this anomalous magnetotransport by calculating the electron band structure and transverse conductivity for a wide range of parameter values, and demonstrate the existence of topological Hall effect in the SS lattice. We explore the role of the strength of itinerant electron-local moment coupling on electron transport and show that the topological Hall features evolve significantly from strong to intermediate values of the coupling strength, and are accompanied by the appearance of a finite spin Hall conductivity.

We introduce a non-Abelian kagome lattice model that has both time-reversal and inversion symmetries and study the flat band physics and topological phases of this model. Due to the coexistence of both time-reversal and inversion symmetries, the energy bands consist of three doubly degenerate bands whose energy and conditions for the presence of flat bands could be obtained analytically, allowing us to tune the flat band with respect to the other two dispersive bands from the top to the middle and then to the bottom of the three bands. We further study the gapped phases of the model and show that they belong to the same phase as the band gaps only close at discrete points of the parameter space, making any two gapped phases adiabatically connected to each other without closing the band gap. Using the Pfaffian approach based on the time-reversal symmetry and parity characterization from the inversion symmetry, we calculate the bulk topological invariants and demonstrate that the unique gapped phases belong to the ${\mathbb{Z}}_2$ quantum spin Hall phase, which is further confirmed by the edge state calculations.

We present a methodology to quantify polarization and electron affinity changes at interfaces by combining scanning tunneling spectroscopy, off-axis electron holography in transmission electron microscopy (TEM), and self-consistent calculations of the electrostatic potential and electron phase change. We use a precisely known grown-in doping structure to calibrate the surface potential of the TEM lamella and thereby achieve a quantitative analysis of electron phase changes measured by off-axis electron holography. Using this calibration, we deduce quantitatively polarization and electron affinity changes for ${\mathrm{Al}}_{\text{0.06}}{\mathrm{Ga}}_{\text{0.94}}\mathrm{N}/\mathrm{GaN}$ and ${\mathrm{In}}_{\text{0.05}}{\mathrm{Ga}}_{\text{0.95}}\mathrm{N}/{\mathrm{Al}}_{\text{0.06}}{\mathrm{Ga}}_{\text{0.94}}\mathrm{N}$ interfaces. The latter interface reveals, as expected, biaxial relaxation as well as polarization and electron affinity changes. However, at the ${\mathrm{Al}}_{\text{0.06}}{\mathrm{Ga}}_{\text{0.94}}\mathrm{N}/\mathrm{GaN}$ interface anomalous lattice relaxations and vanishing polarization and electron affinity changes occur, whose underlying physical origin is anticipated to be total energy minimization by the minimization of Coulomb interactions between the polarization-induced interface charges.

We derive the formula for the near-field radiative heat transfer between dipole particles and a multilayered slab by combing the transmission Green's function and transfer matrix method. In this framework we study the radiative heat transfer (RHT) from a multilayered slab to a particle and the many-body effect of particles. By adjusting the multilayered structure, we optimize heat exchange and achieve a significant enhancement (as high as 7.6 times) in heat transfer from slab to particle. Enhancement occurs at the particle resonance, owing to the surface resonance modulated to couple with the localized resonance of the particle. The roles of the number of layers and particle-slab distance on the enhancement are discussed. For a system of multiple particles horizontally placed above a multilayered slab, we demonstrate that the RHT from slab to particle is significantly inhibited due to strong particle-particle interaction. Analysis of the particle-particle interaction shows that the match of surface resonance and localized resonance of a particle intensifies the interactions among particles, inhibiting heat transfer from slab to particle. This inhibitory effect plays an important role in the tuning of near-field radiation and near-field thermal microscopy.

Anisotropy is a general feature in materials. Strong anisotropy could lead to interesting physical properties and useful applications. Here, based on first-principles calculations and theoretical analysis, we predict a stable two-dimensional (2D) material—the monolayer ${\mathrm{MoOCl}}_2$—and show that it possesses intriguing properties related to its high anisotropy. Monolayer ${\mathrm{MoOCl}}_2$ can be readily exfoliated from the van der Waals layered bulk, which has already been synthesized. We show that a high in-plane anisotropy manifests in the structural, phononic, mechanical, electronic, and optical properties of monolayer ${\mathrm{MoOCl}}_2$. The material is a metal with highly anisotropic Fermi surfaces, giving rise to open orbits at the Fermi level, which can be probed in magnetotransport. Remarkably, the combination of high anisotropy and metallic character makes monolayer ${\mathrm{MoOCl}}_2$ an almost ideal hyperbolic material. It has two very wide hyperbolic frequency windows from 0.41 eV (99 THz) to 2.90 eV (701 THz), and from 3.63 eV (878 THz) to 5.54 eV (1340 THz). The former window has a large overlap with the visible spectrum, and the dissipation for most of this window is very small. The window can be further tuned by the applied strain, such as that at a chosen frequency, a transition between elliptic and hyperbolic character can be induced by strain. Our work discovers a highly anisotropic 2D metal with extraordinary properties, which holds great potential for electronic and optical applications.

We examine the heat and charge transport of a driven topological superconductor. Our particular system of interest consists of a Y-junction of topological superconducting wires, hosting non-Abelian Majorana zero modes at their edges. The system is contacted to two leads which act as continuous detectors of the system state. We calculate, via a scattering matrix approach, the full counting statistics of the driven heat transport, between two terminals contacted to the system, for small adiabatic driving and characterize the energy transport properties as a function of the system parameters (driving frequency, temperature). We find that the geometric, dynamic contribution to the pumped heat statistics results in a correction to the Gallavotti-Cohen type fluctuation theorem for quantum heat transfer. Notably, the correction term to the fluctuation theorem extends to cycles which correspond to topologically protected braiding of the Majorana zero modes. This geometric correction to the fluctuation theorem differs from its analogs in previously studied systems in that (i) it is nonvanishing for adiabatic cycles of the system's parameters, without the need for cyclic driving of the leads and (ii) it is insensitive to small, slow fluctuations of the driving parameters due to the topological protection of the braiding operation.

It was recently shown that phonon spectral mapping of atomic structure is possible in scanning transmission electron microscopy. These results were obtained by deflecting the transmitted beam to detect probe electrons that had been scattered through large momentum transfer into a detector centered at an angle more than twice the radius of the bright-field disk. Atomic-scale spectral maps are still obtained for electrons scattered to angles smaller than the probe convergence angle, albeit at lower contrast and despite the smaller momentum transfers associated with these electrons. It is reported here that apparent reversals of contrast can be observed for these smaller momentum transfers, which is not the case when imaging using probe electrons scattered through larger momentum transfers. This phenomenon can be explained in terms of the channeling of the incident probe. We discuss how such contrast reversals may be avoided and atomic resolution maps reliably obtained.

We report the creation of stable fractionalized polar skyrmionlike structures via flexoelectric effect in a prototypical multiferroic ${\mathrm{BiFeO}}_3$ thin film, which are directly imaged by angle-resolved piezoresponse force microscopy technique. We also demonstrate an approach to enhance piezoresponse at the nanoscale. Phase-field simulations reveal that a tip-driven flexoelectric effect allows the formation of an ordered polar skyrmionlike configuration that is topologically protected by the converse flexoelectric effect. Our findings open up avenues for designing and controlling spatially localized topological textures in rhombohedral ferroelectrics, and also could contribute to future developments for ferroelectric-based nanoelectronic devices.

It is well known that the electronic thermal conductivity of clean compensated semimetals can be greatly enhanced over the electric conductivity by the availability of an ambipolar mechanism of conduction, whereby electrons and holes flow in the same direction experiencing negligible Coulomb scattering as well as negligible impurity scattering. This enhancement—resulting in a breakdown of the Wiedemann-Franz law with an anomalously large Lorenz ratio—has been recently observed in two-dimensional monolayer and bilayer graphene near the charge neutrality point. In contrast to this, three-dimensional compensated semimetals such as ${\mathrm{WP}}_2$ and Sb are typically found to show a reduced Lorenz ratio. We investigate the reasons for this difference, focusing on the low-temperature regime where the electron-electron scattering is expected to dominate over other scattering mechanisms. We show that the different regimes of Fermi statistics (nondegenerate electron-hole liquid in graphene versus degenerate electron-hole liquid in compensated semimetals) are not sufficient to explain the reduction of the Lorenz ratio in the latter. We propose that the solution of the puzzle lies in the large separation of electron and hole pockets in momentum space, which allows compensated semimetals to sustain sizable regions of electron-hole accumulation near the contacts. These accumulations suppress the ambipolar conduction mechanism and effectively split the system into two independent electron and hole conductors. We present a quantitative theory of the crossover from ambipolar to unipolar conduction as a function of the size of the electron-hole accumulation regions, and show that it naturally leads to a sample-size-dependent thermal conductivity.