We theoretically investigate the electronic properties of twisted bilayer systems formed by two coupled anisotropic two-dimensional electron gases (2DEGs) with different effective masses along orthogonal directions, subjected to in-plane and out-of-plane magnetic fields, as well as a perpendicular electric bias. Within a two-band effective mass approximation, we derive and solve the $2\times 2$ Schrödinger Hamiltonian analytically, obtaining exact expressions for the energy spectra and layer-resolved wave functions. We demonstrate that the interlayer coupling energy breaks the energy degeneracy, creating bonding and antibonding subbands whose splitting competes with the bias voltage to control the degree of layer decoupling. The in-plane magnetic field induces momentum-space shifts, leading to the anticrossing of energy dispersion curves when coupling is present. In contrast, the combination of perpendicular electric and in-plane magnetic fields generates a two-valley structure with distinct Fermi surfaces. Remarkably, we identify a Rashba-like term acting on the pseudospin space that emerges exclusively from the twist between anisotropic layers, providing spin-orbit-like coupling without intrinsic spin-orbit interaction. For the Landau level spectrum, we derive analytical expressions revealing a twist-angle-dependent deviation from the conventional linear magnetic field dependence, arising from an effective complex interlayer coupling that introduces a unique twist-angle/magnetic-field coupling. This coupling enables tunable control of inter-Landau-level optical transition energies, with field- and level-dependent blueshifts and redshifts.

The spin dynamics of localized electrons in bulk semiconductors is governed by the interplay of effective nuclear field fluctuations, spin exchange between electrons, and spin transitions into the conduction band. Using spin noise spectroscopy, we reveal this interplay for donor-bound electrons in a CdTe/CdMgTe quantum well and spectrally separate electron spin relaxation and dephasing in zero magnetic field. We identify a specific regime of the electron spin dynamics, where temperature-induced activation of spin-independent hopping leads to a monotonic acceleration of electron spin relaxation. This behavior contrasts with bulk CdTe crystals, where the motional narrowing effect is observed. We attribute this difference to the significantly larger inhomogeneous broadening of the donor-related trion resonance in our quantum well compared to bulk samples. The theoretical analysis of the spin noise power and the strength of the spin exchange interaction provides the estimation of the donor concentration in our unintentionally doped structure.

We investigate the tunneling valley Hall (TVH) transport in a $n\text{−}i\text{−}n$ junction for two-dimensional tilted Dirac fermions modulated by ac gate voltages. In the static limit, the upper bound of the TVH angle is obtained, which depends only on the parameters of the Dirac cones. In the presence of periodic driving, we derive the TVH conductivity based on the Floquet scattering theory and calculate it numerically by means of the scattering-matrix method. It is found that photon-assisted tunneling at strong driving redistributes the transmission probability among multiple Floquet sidebands. Both the longitudinal and TVH conductivities can be several times larger than their static limits. Due to Floquet-sideband-related Fano resonances, a sideband-resolved transmission can be enhanced at the transverse momentum with a large transverse velocity so that the TVH angle exceeds its static maximum. The Floquet-enhanced TVH response can be tuned and optimized by both the Dirac-cone tilt parameter and the orientation of the $n\text{−}i\text{−}n$ interface relative to the crystalline axes. These findings could be helpful for the Floquet engineering of the TVH effect.

In this work, we patterned a two-dimensional electron gas (2DEG) on the surface of a ${\mathrm{SrTiO}}_3$ thin film grown homoepitaxially on ${\mathrm{SrTiO}}_3$ by hybrid molecular beam epitaxy (hMBE). We explored the superconducting dome in this material system by tuning electron density with ionic liquid gating. We found superconducting transitions up to 503 mK near an optimal electron density of approximately $3\times$ ${10}^{13}$ ${\mathrm{cm}}^{-2}$. This is a meaningful increase from the typical optimal transition near 350 mK in similar 2DEGs on ${\mathrm{SrTiO}}_3$ single crystal substrate surfaces. Systematic tuning of 2DEG electron density revealed a consistent BCS scaling between superconducting critical temperature, coherence length, and electron mean free path. Substantial variation of transition width across the dome was described by a paraconductivity model combining Aslamazov-Larkin and Maki-Thompson contributions.

We propose an interlayer-engineering scheme to realize a two-dimensional hybrid-order topological insulator, characterized by the coexistence of first-order and second-order topological phases, in a coupled trilayer Chern system. Starting from three quantum anomalous Hall layers with Chern numbers ${\mathcal{C}}_{1/2/3}=+1/-1/+1$ in the decoupled limit, interlayer tunneling hybridizes their edge states into a single chiral edge mode, while simultaneously opening a gap that supports corner states. Consequently, the system exhibits the coexistence of one-dimensional chiral edge states and zero-dimensional corner states within the same bulk gap, a hallmark of the hybrid-order topology. Furthermore, we map out the topological phase diagram, and show that the hybrid-order phase is robust against mass-type disorder. Our results identify interlayer hybridization as a minimal and broadly applicable strategy for engineering coexisting edge and corner states within a topological platform.

Defect spectroscopy in two-dimensional (2D) field-effect transistors (FETs) requires architectures that minimize contact and disorder-induced artifacts while preserving intrinsic carrier dynamics. We realize ${\mathrm{ReS}}_2\text{−}h\mathrm{BN}$ FETs with few-layer graphene (FLG) van der Waals contacts that form nearly barrier-free interfaces, yielding intrinsic transport with room-temperature mobilities of $18{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$. ${\mathrm{ReS}}_2$, a low-symmetry Group VII transition-metal dichalcogenide (TMD) with weak interlayer coupling, offers a distinct platform to explore defect-transport coupling beyond widely studied TMDs. The low-disorder ${\mathrm{ReS}}_2$-FLG architecture enables detection of random telegraph noise (RTN) even in micron-scale channels, revealing discrete two-level current fluctuations between 90 and 150 K arising from stochastic charge trapping at localized $h\mathrm{BN}$ defect sites. Interestingly, pronounced RTN persists over a broad gate-voltage range, consistent with disorder-dominated percolative transport in the ${\mathrm{ReS}}_2$ channel, where a single trap near a critical bottleneck can modulate the device conductance. With increasing temperature, the RTN evolves into a $1/f$ spectrum as multiple traps activate. Statistical analysis of RTN amplitudes and capture-emission kinetics uncovers defect energetics consistent with substitutional carbon-related centers in $h\mathrm{BN}$. These results establish FLG-contacted ${\mathrm{ReS}}_2\text{−}h\mathrm{BN}$ heterostructures as a sensitive and generalizable platform for probing dielectric-origin defect dynamics and charge trapping phenomena in van der Waals nanoelectronics.

The impact of projective lattice symmetry on electronic band structures has attracted significant attention in recent years, particularly in light of growing experimental studies of two-dimensional hexagonal materials in magnetic fields. Yet, most theoretical work to date has focused on the square lattice due to its relative simplicity. In this work, we investigate the role of projective lattice symmetry, particularly the translation, rotational, and sublattice symmetries, in a hexagonal lattice with rational magnetic flux, emphasizing the resulting topological constraints on the electronic band structure. We show that, at $\pi$ flux, the projective lattice symmetry in the hexagonal lattice enforces Dirac band touchings at $E\ne 0$, and for general rational flux it constrains the number of Dirac points at $E=0$. We further analyze the symmetry-imposed constraints on the Chern numbers of both isolated gapped bands and band multiplets connected by Dirac-point touchings. Our results demonstrate that these constraints in the hexagonal lattice differ substantially from those in the square lattice.

Topologically ordered states are among the most interesting quantum phases of matter that host emergent quasiparticles having fractional charge and obeying fractional quantum statistics. Theoretical study of such states is however challenging owing to their strong-coupling nature that prevents conventional mean-field treatment. Here, we demonstrate that an attention-based deep neural network provides an expressive variational wavefunction that discovers fractional Chern insulator ground states purely through energy minimization without prior knowledge and achieves remarkable accuracy. We introduce an efficient method to extract ground-state topological degeneracy—a hallmark of topological order—from a single optimized real-space wavefunction in translation-invariant systems by decomposing it into different many-body momentum sectors. Our results establish neural network variational Monte Carlo as a versatile tool for discovering strongly correlated topological phases.

Recent years have seen a vast increase in research into van der Waals magnetic materials. In many of these systems, magnetism is introduced via light $3d$ transition metal elements, combined with chalcogenides or halogens. Despite the great technological promise in the field of spintronics, the connection between the $d$-orbital configuration and the occurrence of low-dimensional magnetic order is currently unclear. Here we address the prototypical two-dimensional ferromagnet ${\text{CrI}}_3$, via complementary spectroscopies and density functional theory calculations. We reveal the electronic structure and orbital character of bulk ${\text{CrI}}_3$ in the paramagnetic and ferromagnetic phases, describing the couplings underpinning its energy diagram, and providing a robust experimental demonstration that the stabilization of ferromagnetism is attributable to orbital mixing between I $p$ and Cr $e_g$ states, and to the presence of strong Hund's coupling. These findings reveal the microscopic connection between orbital and spin degrees of freedom, providing fundamental insights into the behavior of low-dimensional magnetic materials.

We report a comprehensive study of the magnetic, magnetotransport, and Hall transport properties of the quaternary Heusler alloy CoRuFeSi. The compound has a saturation magnetization of 4.21 ${\mu }_B$/f.u. and is classified as a soft ferromagnet at room temperature. The longitudinal resistivity exhibits an upturn at low temperatures, a signature of electron-electron interactions. Further, half-metallic nature is supported by the absence of a quadratic temperature dependency at higher temperatures, which agrees well with first-principles electronic structure calculations. A key finding of this work is the observation of a nearly temperature-independent anomalous Hall conductivity of $\sim 74$ S/cm at room temperature, dominated by the intrinsic contribution. First-principles calculations reveal a nodal-line semimetallic electronic structure, and inclusion of Co-Ru antisite disorder yields intrinsic anomalous Hall conductivity in excellent agreement with experiment, highlighting the role of disorder in tuning Berry curvature. These results establish CoRuFeSi as a disorder-tolerant half-metal with a robust anomalous Hall response at room temperature, providing insight into the interplay between disorder, topology, and transport in quaternary Heusler systems and highlighting its potential for spintronic applications.

Superconducting resonators coupled to solid-state qubits offer a scalable architecture for long-range entangling operations and fast, high-fidelity readout. Realizing this requires low photon-loss rates and qubits with tunable electric dipole moments that couple strongly to the resonator's electric field while maintaining long coherence times. For spin qubits, spin-photon coupling is typically achieved via spin-charge hybridization. However, this introduces a fundamental tradeoff: a large spin-charge admixture enhances the coupling strength, which boosts readout and resonator-mediated gate speeds, but exposes the qubit to increased decoherence, thereby increasing the threshold required for strong coupling and limiting the time available for accurate state measurement. This makes it essential to identify optimal operating points for each qubit platform. We address this for the donor-based flip-flop qubit, the microwave-controllable electron-nuclear spin states of which make it suitable for coupling to microwave resonators. We demonstrate that, by choosing intermediate tunnel couplings that balance strong interaction with long qubit lifetimes, high-fidelity readout and strong coupling are simultaneously achievable. We also map out the respective charge-photon couplings and photon-loss rates required. Furthermore, we show that experimental constraints on charge-photon coupling and photon loss can be mitigated using squeezed input fields. As similar tradeoffs appear in quantum-dot-based qubits, our methods and insights extend naturally to these platforms, offering a potential route toward scalable architectures.

We present an analytical theory of second harmonic generation (SHG) in hybrid structures combining a nonlinear two-dimensional (2D) crystal with a dielectric metasurface waveguide. The theory describes the excitation spectrum and enhancement of SHG at both leaky mode and quasibound state in the continuum (quasi-BIC) resonances in terms of the material parameters. For low-loss systems, the SHG efficiency at leaky resonances is determined by their radiative broadening, governed by the relevant Fourier harmonics of the metasurface polarizability, whereas the SHG enhancement at quasi-BIC resonances is ultimately limited by inhomogeneous broadening and absorption in the system. We also describe the emergence and polarization properties of second harmonic diffracted beams. These beams appear even if both the 2D crystal and the metawaveguide are centrosymmetric owing to the nonlocal mechanism of SHG. The developed framework provides a systematic theoretical basis for optimizing the resonant nonlinear frequency conversion in hybrid 2D-material-metasurface platforms and identifies the fundamental limitations of the SHG efficiency.

The dispersion of topological surface states in three-dimensional topological insulators (TIs) is commonly attributed to the atomic spin-orbit coupling (SOC) of the constituent elements. Whether the observable surface-state dispersion reflects this bare atomic value, or instead an effective SOC renormalized by surface–bulk hybridization, remains an open question. Here, we investigate this issue for the occupied and unoccupied surface-state dispersion in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ by combining angle-resolved photoemission (ARPES) and time-resolved (tr-ARPES) at a fixed 1 ps delay time with ab initio one-step photoemission calculations. Comparison between experimental and calculated spectra indicates that the SOC governing the surface-state dispersion deviates from the atomic expectation, with this effect more clearly resolved in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ than in ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. The calculations, which explicitly include hybridization between topological surface states and bulk-derived surface resonance states, provide a microscopic framework for this behavior. These results indicate that the spin-orbit interaction governing topological surface states is effectively shaped by surface–bulk hybridization and can vary significantly across materials.

We study two-dimensional fermionic systems, displaying quadratic band touching in the normal state, in the presence of vortices and skyrmions of insulating and superconducting masses in the ordered phase. A prototypical example of such systems is the Bernal bilayer graphene that supports eight zero-energy modes in the presence of a mass vortex with the requisite U(1) symmetry. Near the vortex core, additional ten masses that close an SO(5) algebra can develop local expectation values by splitting the manifold of zero modes in five and ten different ways by lifting its SO(4) and SU(2) chiral symmetries, respectively. In particular, each SU(2) chiral symmetry can be broken by three distinct copies of chiral-triplet mass orders, giving rise to the notion of the color degeneracy among the competing orders near the vortex core. By contrast, a skyrmion of three anticommuting masses supports additional six masses in its core, and possesses an SU(2) isospin quantum number, aside from the usual generalized U(1) charge. Consequently, charge $4e$ Kekulé pair density waves can develop in the skyrmion core of Néel layer antiferromagnet, while a skyrmion of quantum spin Hall insulator in addition supports an $s$-wave pairing. We also analyze the internal algebra of competing orders in the core of these defects on checkerboard or kagome lattice that supports only a single copy of quadratic band touching in the normal state.

We investigate the stability of topological phases in Kitaev-Heisenberg ladders under local charge fluctuations by studying the competition with itinerant electrons in a Hubbard model on a honeycomb ribbon geometry. We analyze the evolution of string order parameters, spin correlations, and charge fluctuations as functions of hopping amplitude and interaction strength in a half-filled band. Our results from density matrix renormalization group calculations show that increasing electron bandwidth progressively suppresses the topological phases, shifting and narrowing their stability regions in the phase diagram. We identify the critical values of hopping where string order vanishes and characterize the interplay between magnetic order and charge fluctuations. These findings provide insight into the robustness of topological phases against doping and charge dynamics, with implications for candidate Kitaev materials and engineered quantum systems.

Anisotropic intersite exchange interactions in frustrated rare-earth magnets are difficult to assess both theoretically and experimentally. Here, we propose an ab initio force-theorem framework combining the quasiatomic Hubbard-I approach to $4f$ correlations with a static mean-field treatment of the on-site intershell Coulomb interaction between rare-earth $4f$ and $5d$ states to simultaneously capture both $4f$ superexchange and $5d$-mediated indirect exchange. Applying it to the triangular lattice Ce delafossites ${\mathrm{CsCeSe}}_2, {\mathrm{KCeS}}_2$, and ${\mathrm{RbCeO}}_2$, we find that the indirect exchange dominates in the selenide, the superexchange in the oxide, while both mechanisms contribute almost equally in the sulfide. The magnetic exciation spectra of ${\mathrm{CsCeSe}}_2$ and ${\mathrm{KCeS}}_2$ evaluated from the calculated spin Hamiltonains are in good qualitative and quantitative agreement with experimental data.