In modern nanostructures with very low defect densities, a hydrodynamic regime of electric transport has recently been realized in which two-dimensional (2D) electrons form a viscous fluid due to frequent electron-electron collisions. Many bright transport phenomena have been observed in these systems. Of particular interest are two-component hydrodynamic electron systems where a richer variety of phenomena becomes possible than in one-component systems. The simplest way to implement and control a two-component 2D electron system is to place a structure with 2D electrons in a magnetic field with a large component in the 2D plane, which leads to a Zeeman splitting of the electron energy spectrum into two subbands. Here we develop a microscopic model of hydrodynamic transport in such systems. By solving the kinetic equation, we calculate the electron-electron relaxation rates of the first and second angular harmonics of the two-component distribution function. Then we derive the hydrodynamic balance equations with the kinetic coefficient containing these rates. Namely, we take into account the shear viscosity in each fluid component and the effect of the friction between the two components. The latter leads to equalization of the hydrodynamic velocities in the two subbands. The obtained equations can be used to explain the results of puzzling magnetotransport experiments in ultrapure nanostructures in a strong oblique magnetic field.

We study the low-energy effective theory of spin-1/2 quantum ice on the planar pyrochlore lattice from the perspective of its global symmetries. We show that the Fourier transform of the spin operator $S_{\mathbf{Q}}^z$ can be expressed in terms of conserved quantities associated with a one-form U(1) symmetry along two high-symmetry momentum paths, leading to constraints on both the static and dynamical structure factors. We also derive Lieb–Schultz–Mattis type constraints that forbid the existence of a unique gapped ground state in the low-energy effective theory. Our findings also apply to the $(2+1)$-dimensional U(1) quantum link model.

We investigate the emergence of altermagnetic (AM) phases on the square-kagome lattice. Our analysis reveals that matrix element effects due to an orthogonal sublattice weight decomposition of Fermi level eigenstates known as sublattice interference enable decoupled magnetic ordering tendencies on distinct sublattices. Depending on which sublattice undergoes a magnetic instability, we identify a $d_{xy}$-type AM phase and a $d_{x^2-y^2}$-type AM phase originating from different sublattice polarization patterns. Using the Kotliar-Ruckenstein slave boson formalism, we explore the stability of these AM phases as a function of interaction strength. Our findings demonstrate that sublattice-selective magnetic instabilities provide a versatile route to engineer the nature of AM order.

Topological kink states are inherently tied to valley-based topological charges, and extending this framework is critical for enhancing quantum transport efficiency. Using tight-binding models, we demonstrate that even-spin-Chern (ESC) insulators offer a versatile platform for realizing new classes of topological kink states when integrated with quantum spin/valley Hall (QSH/QVH) insulators. A buckled ESC/QVH junction hosts topological multifold kink (TMK) states arising from multiple topological charge sources, yielding highly spin-polarized, dissipationless chiral channels. A planar ESC/QSH junction supports a mirror-protected topological crystalline kink (TCK) state with six channels that markedly enhance the transport efficiency. Interfacial hopping governs the emergence of gapless topological kink states. Density functional theory calculations identify the Pb/${\mathrm{SbAsH}}_2$ junction as an exemplary candidate for realizing disparate TMK states. Our work establishes ESC insulators as a promising foundation for high-performance quantum topological devices and provides insights into the classification and microscopic origins of topological kink states.

Three-dimensional (3D) quantum Hall effect (QHE) of Fermi arcs in Weyl semimetals has attracted significant interest. In this work, we explore the 3D QHE of Fermi arc surface states in tilted Weyl semimetals. We systematically consider both the symmetric the and asymmetric tilts. For the asymmetric tilt, the Fermi arcs form a straight line, which cannot support the 3D QHE irrespective of the value of the tilt. However, for the symmetric tilt, when the tilt increases from zero to finite values in type I, the intersection geometry of Fermi arcs undergoes a transformation from null crossing to finite-area overlap, which corresponds to a nonzero Hall conductivity. Meanwhile, the Landau levels exhibit asymmetric energy distribution with respect to the Fermi level at Weyl nodes, which prevent the cancellation between contributions from electronic states above and below the Weyl nodes, leading to the nonvanishing Hall conductivity. This topological reconstruction suggests that the nonzero symmetric tilt can modulate the 3D QHE in Weyl semimetals. We also demonstrate that the intersection area between Fermi arcs increases with the symmetric tilt strength, while the Hall plateau width decreases. The sheet Hall conductivity follows the conventional $1/B$ dependence, while its slope shows tilt-enhanced behavior. Our study provides critical insights into the interplay between the tilt and the 3D QHE of Fermi arc surface states in tilted Weyl semimetals.

We investigate the electron and heavy hole spin dynamics as a function of magnetic field in ensembles of indirect band gap (In,Al)As/AlAs quantum dots (QDs) with type-I band alignment. Employing a comprehensive model that accounts for both the exciton level quartet and the magnetic-field-driven redistribution of excitons between these states via spin relaxation processes, we extract the electron (${\tau }_{se}$) and heavy hole (${\tau }_{sh}$) spin relaxation times as a function of magnetic field for QDs of varying sizes. Our analysis reveals that both ${\tau }_{se}\left(B\right)$ and ${\tau }_{sh}\left(B\right)$ exhibit power-law scaling behavior, yet the scaling exponents for electrons and heavy holes show markedly different evolutions with QD size. For QDs with a diameter of about 9 nm, we find ${\tau }_{se}\left(B\right)\propto B^{-5}$ and ${\tau }_{sh}\left(B\right)\propto B^{-3}$. Remarkably, increasing the QD diameter to about 16 nm results in a drastic change of the scaling laws, with both ${\tau }_{se}\left(B\right)$ and ${\tau }_{sh}\left(B\right)$ following a $\propto B^{-9}$ dependence. We discuss the underlying mechanisms responsible for this size-dependent transformation of the magnetic field scaling behavior of carrier spin relaxation.

We investigate the emergence of topological superconductivity and Majorana zero modes in the two-dimensional electron gas formed at the (001) ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interface. Using a realistic multiband tight-binding model that incorporates the $t_{2g}$ orbital structure together with atomic and Rashba spin-orbit couplings, we determine the topological phase diagrams for both fully two-dimensional and quasi-one-dimensional geometries. In the two-dimensional limit, we show that a finite out-of-plane magnetic-field component is required to drive a topological phase transition. In this case, the critical field is strongly band dependent, and for higher-lying bands it is controlled by the interplay of spin and orbital Zeeman effects, as well as atomic spin-orbit coupling. Although a purely in-plane field is insufficient to induce the topological transition in a full 2D system, we demonstrate that a lateral confinement relaxes this constraint. In this case, the character of the edge modes depends sensitively on the field orientation, with out-of-plane fields producing conventional counterpropagating chiral modes and transverse in-plane fields giving rise to copropagating antichiral modes. Finally, Majorana zero modes in LAO/STO nanowires with varying widths are analyzed. We demonstrate that subbands predominantly composed of $d_{yz/xz}$ orbitals exhibit exceptionally long localization lengths, which may preclude the observation of Majorana bound states in nanowires of typical experimental dimensions.

We study the performance of Kitaev-Heisenberg (KH) clusters as working media realizing a quantum Otto engine (QOE). An external Zeeman field with linear time dependency is used as the driving mechanism. The efficiency strongly depends on Kitaev $\left(\kappa \right)$ and Heisenberg $\left(J\right)$ exchange interaction. Interestingly, the efficiency is comparable when the relative magnitude of $\kappa$ and $J$ is the same but of opposite signs. The above results are explained due to a subtle interplay of frustration, quantum fluctuation, and duality of eigenspectra for the KH system when both the signs of $\kappa$ and $J$ are reversed. The maximum efficiency is shown to be dynamically related to eigenspectra forming discrete narrow bands, where total spin angular momentum becomes a good quantum number. We relate this optimum efficiency to the realization of weakly interacting magnons, where the system reduces to an approximate eigensystem of the external drive. Further, we show that consideration of negative bath temperature can increase the efficiency. Finally, we examine the generic spin-$S$ Kitaev model, which shows that efficiency decreases as $S$ is increased. We relate this dependency to a measure of quantum fluctuation which also decreases as $S$ is increased, establishing a possible advantage in efficiency for $S=1/2$ due to stronger quantum fluctuations. The results could be of practical interest for materials with KH interactions as a platform for QOE.

The exotic electronic structure of materials usually hosts emergent quantum states and unconventional transport behaviors, which not only advance our understanding of fundamental physics but also offer promise for future technological applications. In the topological system ${\mathrm{TaCo}}_2{\mathrm{Te}}_2$, the observed nonlinear field-dependent Hall resistivity exhibits a pronounced plateau in the absence of magnetic ordering. Through detailed analysis, we develop a four-band model to understand this behavior, rather than attributing it to an anomalous Hall effect or a nontrivial topological mechanism. Angle-dependent quantum oscillation measurements, combined with density functional theory calculations, reveal multiple intricately shaped Fermi surfaces which contribute to anisotropic magnetoresistance driven by distinct scattering rates across different Fermi sheets. Under high magnetic fields, a significant spin splitting emerges, exhibiting strong angular dependence. Our results demonstrate that in a nontrivial topological system the intrinsic complexity of the electronic structure can give rise to exotic transport phenomena predominantly governed by the Lorentz force.

Motivated by the continued interest in Fermi-surface reconstruction without symmetry breaking, we present an analytically tractable microscopic model of a fractionalized Fermi liquid on a square lattice and discuss its potential relevance to the cuprates. As in ancilla-qubit constructions, the model is related to Kondo lattice systems, but in this case, the conduction electrons interact with a ${\mathbb{Z}}_2$ spin liquid of the Yao-Lee type, with a Majorana Fermi surface. The associated ${\mathbb{Z}}_2$ gauge theory is static so that the model can be analytically solved to leading-logarithic accuracy. There are two phases: one in which the fractionalized fermions of the spin liquid hybridize with conduction electrons to form a common Fermi surface violating the naive Luttinger count, and one in which they remain decoupled. We discuss the salient features of the small Fermi-surface phase, including analytically derived momentum dependent coherence factors responsible for the appearance of Fermi arcs à la Yang-Rice-Zhang. We further discuss the impact of quantum and thermal fluctuations, including a strong diamagnetic response and a logarithmically divergent Sommerfeld coefficient at the onset of the pseudogap.

Quantum phase transitions in itinerant ferromagnets and the associated anomalous metallic behavior near quantum critical points constitute a central topic in condensed matter physics. In this work, we employ large-scale determinant quantum Monte Carlo simulations to investigate a two-dimensional fermion–Ising coupling model on a square lattice at half filling, in which itinerant electrons are longitudinally coupled to quantum Ising spins. We map out the complete temperature–transverse field ($T–h$) phase diagram and determine the ferromagnetic–paramagnetic phase boundary through a finite-size scaling analysis, yielding a ferromagnetic quantum critical point at $h_c=3.47\left(1\right)$ with a shift exponent $\psi =0.64\left(2\right)$. Assuming $z=1$, this shift exponent is consistent with the $(2+1)$-dimensional Ising universality class within error bars. In the paramagnetic regime above this quantum critical point, we observe a marginal Fermi-liquid phase characterized by linear-in-$T$ resistivity. We quantitatively determine, in large-scale numerical simulations, the crossover boundary from marginal Fermi-liquid to bad-metal behavior, defined by a dip in the local single-particle spectral function at the Fermi level. Multiple independent probes—including stochastic analytic continuation spectra, imaginary-time Green's function midpoint values, specific heat peaks, self-energy crossings, and direct-current resistivity minima—consistently cross-validate this crossover boundary. We further suggest that bad-metal behavior is plausibly governed by strong short-range antiferromagnetic fluctuations enhanced by Fermi-surface nesting, while persistently large double occupancy across the crossover region is less indicative of conventional Mott-type charge localization as the dominant mechanism.

We theoretically investigate signatures of exciton condensation and the underlying interband coherence in scanning tunneling microscopy. We consider both monolayer and bilayer condensates in the regime of a dilute condensate of tightly bound excitons. For monolayer condensates, interband coherence is directly encoded in spatially oscillating contributions to the tunneling conductance, which break the underlying lattice symmetry. We show how scanning tunneling microscopy allows one to extract the exciton wave function. For bilayer condensates, we show that the formation of the exciton insulator is signaled by the emergence of a characteristic peak in the tunneling conductance, which can be used to extract the (local) exciton density. Our results are based on analytical considerations using a systematic solution of the mean-field equations in powers of the exciton density as well as numerical calculations.

The Edelstein effect in magnetic systems enables magnetization switching via the coupling between current-induced spin accumulation and intrinsic magnetic order and is therefore highly promising for next-generation spintronic devices. Realizing and manipulating the Edelstein effect in two-dimensional (2D) magnetic systems is particularly desirable for achieving high-efficiency and multifunctional spintronic applications. In this work, based on first-principles calculations and symmetry analysis, we demonstrate that the Edelstein effect can intrinsically arise in the 2D in-plane ferromagnetic semiconductor $2H\text{−}{\mathrm{VTe}}_2$, with its behavior strongly dependent on the magnetization orientation. For monolayer $2\mathrm{H}\text{−}{\mathrm{VTe}}_2$ with $D_{3h}$ crystal symmetry, under an applied current along the $+x$ direction, only the time-reversal-even $z$ component and the time-reversal-odd $y\left(x\right)$ component of the spin accumulation are allowed when the magnetization is aligned along $+x$ $(+y)$. For ferromagnetic bilayer $2H\text{−}{\mathrm{VTe}}_2$ in AB or BA stacking, where the crystal symmetry is reduced to $C_{3v}$, additional spin components emerge with the presence of in-plane magnetization. Specifically, for magnetization along $+x$ $(+y)$, besides $d{S}_z^{\mathrm{even}}$ and $d{S}_y^{\mathrm{odd}}(d{S}_z^{\mathrm{even}} \text{and} d{S}_x^{\mathrm{odd}})$, extra components such as $d{S}_y^{\mathrm{even}}\mathrm{and}$ $d{S}_z^{\mathrm{odd}}(d{S}_y^{\mathrm{even}})$ become allowed. Notably, these additional components can be reversibly switched by changing the stacking configuration from AB to BA via interlayer sliding, which is equivalent to applying a $M_{z}T$ operation. The parity dependence of Edelstein coefficients on the in-plane magnetization is also evaluated. Our results not only deepen the understanding of current-induced spin accumulation in 2D magnetic systems from both symmetry and first-principles perspectives but also identify $2H\text{−}{MX}_2$ materials as a promising platform for realizing intrinsic and tunable Edelstein effects in high-efficiency spin-orbit torque devices.

We investigate electronic transport across a magnetic domain wall (DW) in a three-dimensional (3D) second-order topological insulator subject to Anderson disorder. In the clean limit, the DW hosts two copropagating one-dimensional (1D) topological edge states that act as the two arms of an effective Aharonov-Bohm (AB) interferometer, inducing a sinusoidal conductance oscillation. Upon the introduction of disorder, the AB oscillations are suppressed, while a half-quantized plateau of $0.5e^2/h$ for the ensemble-averaged conductance emerges. Notably, within this plateau, the conductance fluctuation exhibits a distinctive two-step plateau structure, with values of $\sim 0.35e^2/h$ at moderate disorder, followed by a second plateau at $\sim 0.29e^2/h$ under strong disorder. By developing theoretical frameworks that account for the random-phase interference and intermode mixing of the two arms, we identify the first fluctuation plateau as a signature of the phase-averaging regime (PAR) and the second as a signature of the mode-mixing regime (MMR). Furthermore, we show that, in the PAR, the conductance follows a U-shaped beta distribution, while it evolves into a uniform distribution in the MMR. The Fano factor associated with shot noise is also computed, which exhibits a similar two-step plateau structure at $1/4$ and $1/3$, corresponding to the PAR and MMR, respectively. Our work provides a clear demonstration of the disorder-induced crossover from PAR to MMR, and highlights the crucial role of second-order conductance cumulants in identifying these transport regimes. The results suggest disorder-engineering as a powerful route for controlling electronic transport across DW-based devices.

We investigate Majorana edge-mode distributions and nonlocal transport in a minimal phenomenological model of coupled Kitaev chains with prescribed Fibonacci modulation. The model is not intended as a self-consistent description of a quasiperiodic superconductor; rather, it provides a controlled setting for comparing an integer topological index with local and nonlocal conductances. Our central result is that the $\nu =1$ sector contains distinct phases that cannot be distinguished by the topological index alone. Although these phases have the same integer invariant, they have different Majorana edge-mode distributions, which are resolved by nonlocal differential conductance. We also identify a topological phase transition induced by Fibonacci quasiperiodicity in this minimal coupled-chain setting.

We report muon-spin rotation measurements of the pressure dependence of the oxygen-isotope $\left({}^{16}\mathrm{O}\text{/}{}^{18}\mathrm{O}\right)$ effect on the spin-density wave (SDW) transition in the trilayer Ruddlesden-Popper nickelate ${\mathrm{Pr}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$. At ambient pressure, the SDW transition shows a finite isotope shift, with ${}^{16}T_{\mathrm{SDW}}=158.04\left(5\right)\mathrm{K}$ and ${}^{18}T_{\mathrm{SDW}}=159.81\left(6\right)\mathrm{K}$. Under hydrostatic pressure, $T_{\mathrm{SDW}}$ decreases linearly at rates that are equal within uncertainty for the two isotope compositions, $\mathit{d}{}^{16}{T}_{\mathrm{SDW}}/\mathit{d}p=-4.93\left(5\right)$ K/GPa and $\mathit{d}{}^{18}{T}_{\mathrm{SDW}}/\mathit{d}p=-4.90\left(7\right)$ K/GPa, such that the isotope shift remains constant within uncertainty under compression. The absence of pressure enhancement of the isotope effect points to a predominantly electronic origin of the SDW transition and is consistent with recent inelastic x-ray scattering results, suggesting a regime of intertwined order in trilayer RP nickelates, which is stabilized by strong spin interactions.

The mechanism driving the charge density wave (CDW) transition in ${\mathrm{TiSe}}_2$ has been debated for decades, with proposals ranging from an excitonic insulator to a lattice-driven instability. A central question remains whether the transition involves an opening or enhancement of the bulk band gap. Using high-resolution angle-resolved photoemission spectroscopy, we directly track the temperature evolution of the bulk band edges across the CDW transition at $T_{\text{CDW}}\approx 200\mathrm{K}$. Contrary to the expectation of a gap-opening transition, we find that the size of the fundamental band gap remains constant from the high-temperature normal phase down to 160 K. While the CDW induces clear band-folding signatures and spectral weight redistribution, the underlying band extrema are unperturbed. These results demonstrate that ${\mathrm{TiSe}}_2$ does not undergo a temperature-driven electronic gap opening. Instead, they support a scenario where the transition is governed by a lattice symmetry-breaking reconstruction that folds, but does not gap, the electronic structure of a pre-existing band insulator.

Electronic correlations in graphene are generally considered weak due to the large bandwidth of its $\pi$ electrons. Here we show that tensile expansion of the honeycomb lattice provides a direct route to enhancing correlation effects. Using variational and diffusion quantum Monte Carlo, we compare a conventional Jastrow-Slater determinant wave function with a resonating-valence-bond (RVB) Jastrow-antisymmetrized geminal product ansatz for a series of stretched graphene lattices. We find that the energy gain of the RVB state relative to the single-determinant description increases with bond expansion up to a critical strain ${\delta }_{\mathrm{cr}}$, and decreases beyond it, revealing a nonmonotonic evolution of electronic correlations. A quantitative analysis places ${\delta }_{\mathrm{cr}}\approx 14$–19% across the system sizes studied, extrapolating to $13.7\pm 0.8\%$ in the thermodynamic limit. Analysis of the optimized many-body pairing amplitude further reveals that this crossover is accompanied by a sharp, specific enhancement of $\pi$-orbital character in the nearest-neighbor pairing between adjacent carbon atoms, providing direct microscopic evidence for the emergence of RVB-like bonding. This behavior indicates a transition from a weakly correlated Dirac semimetal to a regime with enhanced nondynamic correlation and short-range singlet pairing. Our results provide direct many-body evidence that lattice expansion drives graphene into a regime where RVB-like correlations become energetically favorable, offering a simple route to tuning correlation effects in Dirac materials.

We investigated the spin polarization of Rb atoms undergoing light-induced desorption from a spin-polarized Co (110) surface. Desorption of Rb atoms was induced via pulsed UV irradiation of the Rb-adsorbed Co surface. The number of desorbed atoms, their velocity distribution, and their spin polarization were measured via the absorption of circularly polarized light resonant with the desorbed Rb atoms. The results were analyzed in conjunction with density functional theory calculations, indicating that the desorbed Rb atoms were spin polarized. This was explained by a nonthermal desorption mechanism accompanying spin-polarized electron transfer between the Co surface and adsorbed Rb.