Confined electrons in low dimensions host desirable material functions for downscaled electronics as well as advanced energy technologies. Thermoelectricity is a most fascinating example since the dimensionality modifies the electron density of states dramatically, leading to enhanced thermopower as experimentally examined in artificial two-dimensional (2D) structures. However, it is still an open question whether such an enhanced thermopower in low dimensions is realized in layered materials with strong 2D characters, such as cuprates. Here, we report unusual enhancement of the thermopower in the layered organic compound $\alpha$-(BEDT-${\mathrm{TTF})}_2{\mathrm{I}}_3$, where BEDT-TTF stands for bis(ethylenedithio)-tetrathiafulvalene. We find that the slope in the Jonker plot (thermopower $S$ vs logarithm of electrical conductivity $ln\sigma$) for $\alpha$-(BEDT-${\mathrm{TTF})}_2{\mathrm{I}}_3$ is significantly larger than that of conventional semiconductors. Moreover, the large slope is also seen in the related layered salt, demonstrating the impact of the 2D-confined carriers in the layered organics on thermoelectricity.

The moiré Hubbard model describes correlations in certain homobilayer twisted transition metal dichalcogenides. Using exact diagonalization and density matrix renormalization group methods, we find magnetic Mott insulating and metallic phases which, upon doping, exhibit intertwined charge and spin ordering and, in some regimes, pair binding of holes. The phases are highly tunable via an interlayer potential difference. Remarkably, the hole-binding energy is found to be highly tunable, revealing an experimentally accessible regime where holes become attractive. In this attractive regime, we study the superconducting correlation function and point out the possibility of weak superconductivity.

Liquids realize a highly complex state of matter in which strong competing kinetic and interaction effects come to life. As such, liquids are, generally, more challenging to understand than either gases or solids. In weakly interacting gases, the kinetic effects dominate. By contrast, low temperature solids typically feature far smaller fluctuations about their ground state. Notwithstanding their complexity, with the exception of quantum fluids (e.g., superfluid helium) and supercooled liquids (including glasses), various aspects of common liquid dynamics such as their dynamic viscosity are often assumed to be given by rather simple, Arrhenius-type, activated forms with nearly constant (i.e., temperature independent) energy barriers. In this paper, we analyze experimentally measured viscosities of numerous liquids far above their equilibrium melting temperature to see how well this assumption fares. We find, for the investigated liquids, marked deviations from simple activated dynamics. Even far above their equilibrium melting temperatures, as the temperature drops, the viscosity of these liquids increases more strongly than predicted by activated dynamics dominated by a single uniform energy barrier. For metallic fluids, the scale of the prefactors of the best Arrhenius fits for the viscosity is typically consistent with that given by the product $\left(nh\right)$ with $n$ the number density and $h$ Planck's constant. More generally, in various fluids (whether metallic or nonmetallic) that we examined, $\left(nh\right)$ constitutes a lower bound scale on the viscosity. We find that a scaling of the temperature axis (complementing that of the viscosity) leads to a partial collapse of the temperature dependent viscosities of different fluids; such a scaling allows for a functional dependence of the viscosity on temperature that includes yet is far more general than activated Arrhenius form alone. We speculate on relations between non-Arrhenius dynamics and thermodynamic observables.

Here, we report first-principles calculations of laser-induced photocurrents at the surface of a prototype Rashba system. By referring to Keldysh nonequilibrium formalism combined with the Wannier interpolation scheme, we perform first-principles electronic structure calculations of a prototype ${\mathrm{BiAg}}_2$ surface alloy, which is a well-known material realization of the Rashba model. In addition to the nonmagnetic ground state situation, we also study the case of in-plane magnetized ${\mathrm{BiAg}}_2$. We calculate the laser-induced charge photocurrents for the ferromagnetic case and the laser-induced spin photocurrents for both the nonmagnetic and the ferromagnetic cases. Our results confirm the emergence of very large in-plane photocurrents as predicted by the Rashba model and are in agreement with previous experimental measurements of THz emission generated at Bi/Ag interfaces. The resulting photocurrents satisfy all the symmetry restrictions with respect to the light helicity and the magnetization direction. We provide microscopic insights into the symmetry and magnitude of the computed currents based on the ab initio multiband electronic structure of the system, and scrutinize the importance of resonant two-band and three-band transitions for driven currents, thereby establishing a benchmark picture of photocurrents at Rashba-like surfaces and interfaces. Our work contributes to establishing the interfacial Rashba spin-orbit interaction as a major mechanism for the generation of in-plane photocurrents, which are of great interest in the field of ultrafast and terahertz spintronics.

Moiré materials have become one of the most active fields in material science in recent years due to their high tunability, and their unique properties emerge from the moiré-scale structure modulation. Here, we propose twisted bilayer ${\mathrm{Bi}}_2{\left({\mathrm{Te}}_{1-x}{\mathrm{Se}}_x\right)}_3$ as a moiré material where the moiré-scale modulation induces a topological phase transition. We show, in twisted bilayer ${\mathrm{Bi}}_2{\left({\mathrm{Te}}_{1-x}{\mathrm{Se}}_x\right)}_3$, a topological insulator domain and a normal insulator domain coexist in the moiré lattice structure, and edge states on the domain boundary make nearly flat bands that dominate the material properties. The edge states further contribute to a moiré-scale band inversion, resulting in moiré-scale topological states. There are corresponding moiré-scale edge states and they are so to speak “edge state from edge state,” which is a unique feature of twisted bilayer ${\mathrm{Bi}}_2{\left({\mathrm{Te}}_{1-x}{\mathrm{Se}}_x\right)}_3$. Our result not only proposes characteristic quantum phases in twisted bilayer ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ family, but also suggests the twisting of stacking-sensitive topological materials paves an avenue in the search for novel quantum materials and devices.

We present a study of strain modulation effects on electronic structures of a two-dimensional single-band chiral $p\text{−}\mathrm{wave}$ superconductor within the BCS mean-field scheme. We employ a lattice model and solve the Bogolyubov-de Gennes equations numerically. Implementing strain modulations through spatially varying hopping matrix elements, we observe the appearance of spontaneous supercurrents from the resulting spatial modulation of order parameter amplitudes. Moreover, sufficiently strong strain modulation induces the formation of topologically distinct domains within the system and causes the appearance of chiral edge modes, which is captured by spectral functions and local Chern markers.

We report a voltage-controlled critical behavior observed in a GaAs epitaxial structure containing a dense array of ErAs nanoparticles. When fabricated with metal electrodes, the structure displays a voltage- and temperature-dependent metal-to-insulator transition and strong hysteresis in the current versus voltage and versus temperature characteristics, with critical temperatures as high as 77 K. Furthermore, we observed a diverging rms deviation of the electrical conductance with respect to the critical bias voltage, which further supports the existence of a phase transition. The insulating phase is governed by Efros-Shklovskii variable range hopping, and the conductance reduces beyond instrument limits as the temperature drops toward zero; supporting it is an Anderson insulator. The metallic phase displays a conductance minimum at a critical temperature behaving similarly to that of single quantum dot due to Kondo resonance, and then the conductance increases as the temperature continues to drop. Furthermore, the metallic phase displays a colossal magnetoresistance under a weak magnetic field at 77 K while the insulating phase does not. And the metal-to-insulator phase transition can be induced by the magnetic field showing a critical discontinuity, similar to that versus temperature and voltage. We propose that the metal-to-insulator phase transition can be explained by the concept of an Anderson insulator with Kondo impurities.

We demonstrate the presence of an optical phase transition with frustration-induced spontaneous symmetry breaking in a triangular planar atomic array due to cooperative light-mediated interactions. We show how the array geometry of triangle unit cells at low light intensities leads to degenerate collective radiative excitations forming nearly flat bands. We drive degenerate pairs of collective excitations to be equally populated in both the case of the atomic polarization being in the lattice plane and the case of the atomic polarization being perpendicular to it. At higher intensities, above specific threshold values, this symmetry in the populations is spontaneously broken. We also develop an effective few-mode model that provides semianalytic descriptions of the symmetry-breaking threshold and infinite-lattice limit phase transition. Surprisingly, we find how excitations due to dipolar interactions correspond to optical analogs of those found in frustrated magnets and superfluids, with closely related symmetry-breaking mechanisms despite the significant physical differences between these systems, opening the potential for simulating even quantum magnetism. Transmitted light through the array conveys information about symmetry breaking in the hysteresis behavior of the spectrum. Moreover, in a Mott-insulator state, the atomic positions are subject to zero-point quantum fluctuations. Interpreting each stochastic realization as a light-induced quantum measurement of the atomic position configuration, we find how strong nonlinearities and even weak position uncertainties lead to considerable measurement-induced symmetry breaking, while ensemble averaging over many realizations restores the original symmetry and the unbroken state. Larger position uncertainty results in the formation of domains of different broken symmetries.

We propose an improved quantum algorithm to calculate the Green's function through real-time propagation, and use it to compute the retarded Green's function for the two-, three-, and four-site Hubbard models. This protocol significantly reduces the number of controlled operations when compared to those previously suggested in literature. Such reduction is quite remarkable when considering the two-site Hubbard model, for which we show that it is possible to obtain the exact time propagation of the $|N\pm 1\rangle$ states by exponentiating one single Pauli component of the Hamiltonian, allowing us to perform the calculations on an actual superconducting quantum processor.

Widespread processes in nature and technology are governed by the dynamical transition whereby a material in an initially solid-like state, whether soft or hard, then yields. Major unresolved questions concern whether any material will yield smoothly and gradually (“ductile” behavior) or fail abruptly and catastrophically (“brittle” behavior); the roles of sample annealing, disorder, and shear band formation in the onset of yielding and failure; and, most importantly from a practical viewpoint, whether any impending catastrophic failure can be predicted before it happens. We address these questions by studying theoretically the yielding of slowly sheared athermal amorphous materials, within a minimal mesoscopic lattice elastoplastic description. Our contributions are fourfold. First, we elucidate whether yielding will be ductile or brittle, for any given level of sample annealing prior to shear. For highly annealed samples, we find brittle yielding for all samples sizes. For poorly annealed samples we uncover an important dependence on the size of the sample of material being sheared, with ductile yielding for small samples, and brittle yielding only for large system sizes. Second, we show that yielding comprises two distinct stages: a prefailure stage, in which small levels of strain heterogeneity slowly accumulate within the material, followed by a catastrophic brittle failure event, in which a shear band quickly propagates across the sample via a cooperating line of (individually) localized plastic events. Third, we provide an exact expression for the slowly growing level of strain heterogeneity in the prefailure stage, expressed in terms of the macroscopically measured stress-strain curve and the sample size, and in excellent agreement with our simulation results. Fourth, we elucidate the basic mechanism via which a shear band then nucleates, in terms of the onset of cooperativity between plastic events. We furthermore provide an expression for the probability distribution of shear strains at which failure occurs, expressed in terms of the sample size and the disorder inherent in the sample, as determined by the degree of annealing prior to shear.

We present a computational study based on density functional plus dynamical mean-field theory calculations for the normal-state electronic reconstruction of pure and hole-doped ${\mathrm{NdNiO}}_2$ superconductors. Our results capture the $T$ dependence of the electrical resistivity, providing a many-particle interpretation of the emergence of pseudogaplike features at low energies as well as the weakly insulating regime seen in experiment. We show how Mottness manifests itself in the weak localization to bad-metallicity crossover due to strong correlation in the $e_g$-shell one-particle spectral functions and self-energies.

Twisted bilayer graphene (TBG) near “magic angles” has emerged as a rich platform for strongly correlated states of two-dimensional (2D) Dirac semimetals. Here we show that twisted bilayers of thin film magnetic topological insulators (MTIs) with large in-plane magnetization can realize flat bands near 2D Dirac nodes. Using a simple model for thin films of MTIs, we derive a continuum model for two such MTIs, twisted by a small angle with respect to each other. When the magnetization is in plane, we show that interlayer tunneling terms act as effective SU(2) vector potentials, which are known to lead to flat bands in TBG. We show that by changing the in-plane magnetization, it is possible to tune the twisted bilayer MTI band dispersion to quadratic band touching or to flat bands, similar to the TBG. If realized, this system can be a highly tunable platform for strongly correlated phases of two-dimensional Dirac semimetals.

Genuinely two-dimensional robust crosscurrents—which flow against the natural direction of heat flux— have been missing since the discovery of their one-dimensional counterpart. We provide a setup to realize them on a cubic three-dimensional (3D) lattice hosting a Hofstadter model coupled to two heat baths with different temperatures. We show that these currents exhibit dissipative robustness; they are stable against the presence of impurities and tilting of the gauge field in certain nonequilibrium configurations. Moreover, we find protected boundary currents with genuinely 3D robustness, i.e., they are only stable if tunneling can occur in all three spatial directions. The model also presents generic surface currents, which are robust for both bosonic and fermionic systems. We identify the underlying qualitative mechanism responsible for the robustness of the surface currents and the crucial role played by certain discrete symmetries.

We report neutron depolarization measurements of the suppression of long-range ferromagnetism and the concomitant emergence of magnetic irreversibilities and spin freezing in ${\mathrm{CePd}}_{1-x}{\mathrm{Rh}}_x$ around $x^{*}\approx 0.6$. Tracking the temperature versus field history of the neutron depolarization, we find clear signatures of long-range Ising ferromagnetism below a Curie temperature $T_{\mathrm{C}}$ for $x=0.4$ and a spin freezing of ferromagnetic clusters below a freezing temperature $T_{\mathrm{F}1}$ for $x>x^{*}$. Under zero-field-cooling/field-heating and for $x>x^{*}$ a reentrant temperature dependence of the neutron depolarization between $T_{\mathrm{F}2}<T_{\mathrm{F}1}$ and $T_{\mathrm{F}1}$ is microscopically consistent with a thermally activated growth of the cluster size. The evolution of the depolarization as well as the reentrant temperature dependence as a function of Rh content are consistent with the formation of a Kondo cluster glass below $T_{\mathrm{F}1}$ adjacent to a ferromagnetic quantum phase transition at $x^{*}$.

The Kitaev model is an exactly solvable quantum spin model within the language of constrained real fermions. In spite of numerous studies for magnetic fields along special orientations, there is a limited amount of knowledge on the complete field-angle characterization, which can provide valuable information on the existence of fractionalized excitations. For this purpose, we first study the pure ferromagnetic and antiferromagnetic Kitaev models for arbitrary external magnetic field directions via a mean-field theory, showing that there are many topological phases with different (or zero) Chern numbers, depending on the magnetic field strength and orientations. However, a realistic description of the candidate Kitaev materials, within the edge-sharing octahedra paradigm, requires additional coupling terms, including a large off-diagonal term $\mathrm{\Gamma }$ along with possible anisotropic corrections ${\mathrm{\Gamma }}_p$. It is therefore not sufficient to rely on the topological properties of the bare Kitaev model as the basis for the observed thermal Hall-conductivity signals, and an understanding of these extended Kitaev models with a complete field response is demanded. Starting from the zero-field phase diagram of $\mathrm{K}\text{−}\mathrm{\Gamma }\text{−}{\mathrm{\Gamma }}_p$ models, we identify, besides the Kitaev spin liquid phase, antiferromagnetic zigzag, ferromagnetic phases, as well as an unusual Kitaev(-$\mathrm{\Gamma }$) spin liquid phase. The magnetic field response of these phases for arbitrary field orientations provides a remarkably rich phase diagram. For an extended parameter range and just above the critical field where the zigzag phase is suppressed, there is an intermediate phase region with suppressed energy gaps and substantial spin fractionalization. To comply our findings with experiments, we also reproduce a large asymmetry in the extent of this intermediate phases specifically for the two different field directions $\theta =\pm {60}^o$ with respect to the normal to the plane of the honeycomb lattice.

We uncover a non-Hermitian skin mode in which the eigenmodes of a nonreciprocal topoelectrical (TE) heterojunction circuit array are localized at the heterojunction interface under both open boundary condition (OBC) and periodic boundary condition (PBC). This is in marked contrast to the conventional non-Hermitian skin effect (NHSE) in homogenous non-Hermitian systems that exists under OBC but vanishes in PBC. The interfacial non-Hermitian skin effect is induced when two non-Hermitian circuit chain segments with dissimilar NHSE decay lengths are cascaded together. It is shown analytically that the interfacial NHSE decay rates are directly associated with the asymmetric intracell couplings in the TE segments. Such nonreciprocal directional couplings can be realized in practical circuits by means of negative impedance converters with current inversion (INIC). In contrast to the conventional NHSE in homogeneous systems, the TE heterojunction circuit allows for a voltage profile localized at the interface, that is independent of the boundary conditions and whose decay length can be modulated by circuit parameters. Finally, the results presented are general and applicable to other analogous non-Hermitian platforms such as photonic and condensed matter nanoscale systems.

The Heisenberg antiferromagnet on the kagome lattice is an archetypal instance of how large ground-state degeneracies arise, and how they may get resolved by thermal and quantum fluctuations. Augmenting the Heisenberg model by chiral spin interactions has proved to be of particular interest in the discovery of certain chiral quantum spin liquids. Here we consider the classical variant of this chiral kagome model and find that it exhibits, similar to the classical Heisenberg antiferromagnet, a remarkably large and structured ground-state manifold, which combines continuous and discrete degrees of freedom. This allows for a rich set of order-by-disorder phenomena. Degeneracy lifting occurs in a highly selective way, choosing already at the harmonic level specific triaxial states, which however retain an emergent ${\mathbb{Z}}_2$ degree of freedom (absent in the conventional Heisenberg model). We also study the competition of entropic and energetic ground-state selection as the model interpolates between the purely chiral and Heisenberg cases. For this mixed model, we find a “proximate ordered-by-disorder” finite-temperature regime where fluctuations overcome the energetic ground-state preference of the perturbation. Finally, a semiclassical route to a spin liquid is provided by quantum order by disorder in the purely chiral models, where the aforementioned ${\mathbb{Z}}_2$ degrees of freedom are elevated to the role of an emergent gauge field.

We systematically investigate the ground-state properties of self-bound droplets of quasi-two-dimensional binary Bose gases by using the Gaussian state theory. We find that quantum droplets consists of two macroscopic squeezed phases and a macroscopic coherent phase. We map out the phase diagram and determine all phase boundaries via both numerical and nearly analytical methods. In particular, we find three easily accessible signatures for the quantum phases and the stabilization mechanism of the self-bound droplets by precisely measuring their radial size. Our studies indicate that binary droplets represent an ideal platform for in-depth investigations of the quantum nature of the droplet state.