Topological phase transitions in photonic systems are traditionally realized by geometrically modifying the unit cell. Here, we introduce a cavity-based mechanism that enables dynamic control of topological phases without structural alterations. Specifically, we study the topological phase of electric dipolar arrays placed between two parallel metal plates. Using the image-charge method, we demonstrate that the coupling strength between dipoles is strongly influenced not only by their distances to the two plates but also by the orientation of the dipoles themselves. Exploiting this feature, we construct an equally spaced Su-Schrieffer-Heeger (SSH) chain composed of dipoles with tailored orientations. We show that when the chain is closer (farther) to the top plate than to the bottom plate, the system exhibits a topologically trivial (nontrivial) phase. Hence, the topological phase of the array can be controlled simply by tuning the relative position of the chain between the two plates. We further generalize this approach to two-dimensional dipolar arrays, demonstrating its applicability to a Kagome lattice. This cavity-enabled strategy offers a practical route for achieving topological phase transitions in photonic systems without the need for lattice geometry modification, paving the way for more versatile implementations of topological photonics.

The emergence of altermagnets offers a promising platform for studying spin-charge coupling scenarios. In this study, we investigate the significant regulatory influence of uniaxial strain on the electronic and plasmonic properties of two-dimensional $d$-wave altermagnets, specifically, monolayer ${\text{V}}_2{\text{Se}}_2\text{O}$. Our analysis, based on density functional theory calculations and a tight-binding model of an altermagnetic Lieb lattice, reveals that applying strain along the [100] direction effectively lifts the degeneracy between the X and Y valleys, enabling the generation of spin-polarized carriers at optimal hole-doping concentrations. Compressive strain along the [100] direction promotes directional propagation of spin plasmons along the $x$ direction, whereas tensile strain shifts this behavior to the $y$ direction. Moreover, applying tensile strain along the [110] direction retains the $d$-wave altermagnetic characteristics while resulting in distinct anisotropic responses in spin plasmons that diverge from those induced by [100] strain. The observation of low-loss spin plasmons and their strain-tunable propagation in monolayer ${\text{V}}_2{\text{Se}}_2\text{O}$ greatly enhance the application potential of spin plasmons and pave the way for exploration of mechanical-electric and mechanical-optical couplings in altermagnetic materials.

We present a unified machine-learning-based framework to investigate the electronic structure and plasmonic response of graphene/hexagonal boron nitride (hBN) moiré heterostructures. This framework integrates machine-learning-derived interatomic potentials and a data-driven Hamiltonian model, facilitating a fully ab initio-trained and parameter-free strategy that effectively links atomic relaxation processes to electronic property predictions. By applying this methodology, we conduct a systematic exploration of the graphene/hBN moiré heterostructures at four representative twist angles, revealing distinct out-of-plane corrugations in the relaxed structures predicted by the machine-learned potential. Our findings uncover a twist-angle-dependent reduction in the Dirac-point gap within the electronic bands. Additionally, the calculated plasmon spectra exhibit pronounced electron-hole asymmetry due to the presence of the secondary Dirac-point gap, consistent with previous theoretical expectations. Importantly, our approach eliminates the necessity for empirical parameter fitting, thus enhancing its applicability to a broader range of two-dimensional heterostructures with complex interlayer geometries. This work paves the way for a generalized framework that enables automated, high-accuracy modeling of moiré systems, effectively integrating ab initio data, machine learning, and the study of collective excitations.

Two-dimensional van der Waals (vdW) ferroelectric $\alpha \text{−}{\mathrm{In}}_2{\mathrm{Se}}_3$, renowned for its structural adaptability and electrically switchable polarization, presents a transformative platform for advanced nanoelectronics. Here, employing first-principles calculations based on density functional theory, we systematically explore the pressure-modulated electronic states of twisted $\alpha \text{−}{\mathrm{In}}_2{\mathrm{Se}}_3$ moiré superlattices, including conventional stacking configurations constructed from ferroelectric $\alpha \text{−}{\mathrm{In}}_2{\mathrm{Se}}_3$ quintuple layers. In contrast to the conventional stackings, key findings are revealed in twisted $\alpha \text{−}{\mathrm{In}}_2{\mathrm{Se}}_3$ moiré superlattices: First, interlayer twist engineering induces localized interfacial charge states and generates flat electronic bands near the Fermi level, suggesting potential emergence of correlated quantum phases. Second, externally applied out-of-plane pressure drives a spatial redistribution of interfacial charges toward intralayer regions, accompanied by progressive band gap closure and a reversible semiconductor-to-metal transition, demonstrating robust mechano-electronic coupling. Third, strategic introduction of surface selenium vacancies amplifies the system's responsiveness to mechanical stimuli, accelerating band gap closure under pressure through localized strain and defect-mediated charge reorganization. These results establish a unified framework linking twist engineering, ferroelectrics, and defect control in vdW heterostructures and offer unprecedented avenues for designing adaptive nanoelectronics, including mechanically reconfigurable devices and strain-engineered quantum platforms.

We study the degree of band flatness and anisotropic quantum geometry in magic-angle twisted bilayer graphene by varying the twist angle and the lattice relaxation through optical conductivity. We show that the degree of band flatness and its quantum geometry can be revealed through optical absorption and its resulting optical bounds, which are based on the trace condition in quantum geometry. More specifically, the narrow and isolated peak of optical absorption in the low-energy region provides information about the bandwidth between two flat bands. When this value is smaller than the electron interaction, it serves as a critical condition for the emergence of flat-band superconductivity. Furthermore, optical absorption also provides the gap value between the flat band and the dispersive band, and when this gap is larger than the electron interaction, it facilitates the realization of fractional Chern insulating phases. We show that the narrow and isolated peak of optical bound near zero energy decreases as lattice relaxation increases. Meanwhile, we demonstrate that the imaginary part of generalized optical Hall conductivity reveals the vanishing of the negative part of Berry curvature, which is enforced by the refined trace-determinant inequality. Accordingly, we show that the total amount of the negative part and component of the Berry curvature approaches zero in the single ideal flat-band case. In contrast, when considering all occupied bands, the total amount of the negative component is slightly different from zero. Finally, we demonstrate that the condition of vanishing of flat-band velocities and the emergent chiral symmetry are sufficient for the saturation of the trace condition, which pertains to the isotropic case. In contrast, the determinant condition can be saturated in anisotropic systems, and the quantum metric and Berry curvature are related by a “saturation matrix” whose elements can be measured through the optical response. Additionally, the nonvanishing difference between the optical bound and the imaginary part of the generalized optical Hall conductivity also provides an anisotropic signature in the fractional Chern insulating phases.

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.