We investigate resonant localization and reflection-phase dynamics in graded phononic superlattices with spatially varying band edges. Starting from the elastic wave equation, we derive, using the local periodic approximation and the multiple-scale method, an effective Schrödinger-type equation for the envelope function, whose coefficients are expressed in terms of unit-cell averages involving the local Bloch mode. Spatial modulation of the local band edge then acts as an effective potential containing an internal turning point, allowing the system to be interpreted as an open quasibound scattering structure. For parabolic band-edge profiles, the effective potential becomes approximately harmonic and supports a series of quasibound resonances with a half-harmonic-like character. We show that the resonance frequencies form a nearly equally spaced series consistent with WKB quantization, and that the spatial mode profiles are accurately described by Hermite-function envelopes. The turning-point region is described by an Airy-function connection that consistently links the oscillatory and evanescent regions of the wave field. We further show that the semiclassical round-trip phase determines the quasibound resonance condition, while the Wigner reflection delay, defined from the total scattering phase, exhibits pronounced peaks near these resonances. These results establish a semiclassical description of resonant localization, quasibound scattering, and temporal delay in graded periodic wave systems.

The intrinsic structural asymmetry of Janus MoSSe offers a promising platform for stabilizing magnetic skyrmions; however, its relatively weak Dzyaloshinskii-Moriya interaction (DMI) hinders practical applications. Here, we reveal that ${\mathrm{Mn}}_m{\mathrm{Rh}}_n$ ($m+n=3$) cluster modification and strain engineering can effectively enhance both the DMI and exchange interactions by first-principles calculations. Specifically, at 4% and 6% strain, the DMI and exchange parameter ($J$) for $\mathrm{R}{\mathrm{h}}_3@\mathrm{MoSSe}$ rise significantly, reaching peak values of 4.123 meV and 18.268 meV, respectively. This enhancement stems from strain-induced modifications in the $d$-orbital distribution of Rh atoms near the Fermi level, which optimize both spin-orbit coupling and exchange interactions. Under 2% tensile strain, skyrmions with a diameter of 22 nm are stabilized in $\mathrm{R}{\mathrm{h}}_3@\mathrm{MoSSe}$. A moderate external field of 1 T reduces their size slightly to 20 nm, while a 5 T field induces the formation of 14 nm ferromagnetic skyrmions in the same system. Our findings highlight the critical role of cluster adsorption and strain engineering in modulating DMI and advancing the study of FM skyrmions in two-dimensional Janus materials.

Moiré minibands in twisted homobilayer semiconductors can, under suitable approximations, be modeled as a pair of Landau levels with opposite Chern numbers. This provides a minimal model for searching novel topological states in a time-reversal-symmetric Hamiltonian. In this work, we investigate the effects of an external magnetic field in this model. We study the many-body ground state in the density–magnetic-field ($n\text{−}B$) plane along the $dn/dB=\pm 1/{\mathrm{\Phi }}_0$ Středa line with Hartree-Fock approximation. Away from charge neutrality, we find the Chern-insulating (incompressible) state is very robust while toward charge neutrality, we find a transition from incompressible phase to compressible phase as the interaction strength $\kappa$ decreases. Using time-dependent mean-field theory, we further analyze spin-flip excitations and find that the incompressible state along the Středa line toward charge neutrality becomes unstable even at large $\kappa$ when magnetic field is sufficiently large. Finally, we solve the two-body problem in a given Landau level exactly where the two particles experience unequal magnetic fields using a basis called center-of-charge basis. This basis allows any isotropic interaction to be parametrized by a single quantum number, the relative angular momentum, thereby extending the Haldane pseudopotentials to the unequal-magnetic-fields case. As the difference of the two magnetic fields varies, these pseudopotentials show a sequence of level crossings, leading to nonmonotonic structure of pseudopotentials that is absent in ordinary Landau-level systems. Our formulation provides a useful starting point for studying weak-field physics in moiré flat bands, where magnetic Bloch-state basis becomes computationally impossible due to the large basis sizes.

Two-dimensional transition metal dichalcogenides offer unprecedented opportunities for quantum photonics through strain-mediated exciton engineering. However, the paradoxical coexistence of strain-gradient-driven exciton funneling and efficient trion formation has remained unresolved. Here, we address this challenge by demonstrating that multistrain domains enable room-temperature exciton-trion interconversion in strain-engineered ${\mathrm{MoSe}}_2$ monolayers. This interconversion is most likely driven by flexoelectric fields arising from interfacial strain gradients. Using a thermal strain-modulated chemical vapor deposition approach, we have engineered interfacial strain gradients ($\sim 2.0\%\mu {\mathrm{m}}^{-1}$) that induce flexoelectric polarization, generating symmetry-broken quantum wells and driving near-unity quantum yield exciton-trion interconversion at room temperature. Crucially, through multimodal characterization techniques, we have unraveled the long-standing paradox in exciton dynamics under heterogeneous strain fields. Our findings demonstrate that the flexoelectric field likely plays a decisive role in decoupling electron-hole pair dynamics while maintaining charge neutrality—a critical mechanism mediated through electric effects rather than direct strain influences. This breakthrough significantly bridges the gap between two apparent contradictions in quantum phenomena: the competing processes of exciton funneling and trion formation observed in strained semiconductor systems. Further, by validating this model in flower-shaped samples with alternating curvature-induced field gradients, we establish a unified framework for deterministic exciton engineering. Our work provides fundamental insights into strain-mediated excitonics and enables scalable quantum photonic architecture through deterministic strain engineering.

The non-Hermitian skin effect, where bulk states accumulate at system boundaries, challenges the conventional bulk-boundary correspondence. Here we propose a scheme to realize hybrid skin-topological states in exciton polariton honeycomb lattices by introducing sublattice-dependent gain and loss. This non-Hermiticity couples with the intrinsic topological edge modes, leading to relocalization of edge states. We show two distinct regimes: hybrid skin-Chern states with reversible localization controlled by TE-TM splitting (characterized by a change in spectral winding number from $w=2$ to $w=-1$), and hybrid skin-antichiral states which preserves the spin-polarized property. Our results bridge polariton spin physics and non-Hermitian topology, opening routes toward controllable nonreciprocal and spin-polarized transport.

We investigate the DC conductivity of two-dimensional tilted Dirac systems with short- and long-range impurity scattering, across the subcritical (Type I), critical, and overcritical (Type II) tilt regimes. Using the Kubo formalism, we uncover transport signatures that depend sensitively on both the tilt regime and the impurity type. For short-range impurities, the conductivity as a function of the tilt parameter $t$ exhibits a minimum at the critical point $t=1$, increasing away from it, with pronounced anisotropy emerging in the Type-II regime. Long-range impurities instead produce an enhanced conductivity near the critical point. The energy dependence of the conductivity also differs sharply between the two cases: it is negligible to moderate for short-range impurities, but strong for long-range impurities, scaling nearly quadratically for $t<1$ and linearly for $t>1$. We further show that vertex corrections are negligible at the Lifshitz transition ($t=1$). Finally, we interpret these results within a unified geometric framework, establishing the tilt parameter as a powerful knob for engineering the transport properties of disordered Dirac materials.

We present an SU(2) gauge theory of fluctuating stripe order in the two-dimensional Hubbard model. The theory is based on a fractionalization of the electron operators in fermionic chargons with a pseudospin degree of freedom, and charge-neutral spinons capturing fluctuations of the spin orientation. The chargons are treated in a renormalized mean-field theory. We focus on regions of the phase diagram where they undergo stripe order. The spinons are described by a nonlinear sigma model with pseudospin stiffnesses determined by the chargons. They prevent breaking of the physical SU(2) spin symmetry at any finite temperature, resulting in a charge-ordered pseudogap phase with a reconstructed Fermi surface and a spin gap. The spectral function for single-particle excitations exhibits a collection of Fermi arcs and other structures. The arcs appear in various regions of the Brillouin zone, but never exclusively near the Brillouin zone diagonals. For a suitable choice of the bare dispersion relation, the form of the low energy spectral function resembles experimental observations from angular resolved photoemission on cuprates in which stripe fluctuations play an important role.

Odd-parity altermagnetism (ALM) extends compensated collinear magnetism beyond the even-parity spin splitting of conventional altermagnets, but its role in correlated topological phases remains largely unexplored. Using the cluster slave-spin method, we show that the odd-parity ALM appearing in the ALM Chern-insulating phase of the Haldane-Hubbard model significantly reconstructs the local distribution of the Berry curvature in the conventional Chern-insulating phase, while the total Chern number remains unchanged compared to the Chern-insulating phase. The Berry curvature becomes spin and valley selective, zigzag ribbons develop nonsymmetric chiral edge states, while armchair ribbons remain inversion symmetric. The optical response mirrors this separation between the local reconstruction and the global topology: low-energy spectra are governed by quasiparticles near the gap, whereas the low-frequency Hall conductivity stays quantized, ${\sigma }_{\mathrm{T}\uparrow }(\mathrm{\Omega }\to 0)={\sigma }_{\mathrm{T}\downarrow }(\mathrm{\Omega }\to 0)=e^2/h$. These results establish the Haldane-Hubbard model as a minimal correlated platform for odd-parity altermagnetic topology.

Topological chiral crystals have emerged as a fertile material platform for investigating optical phenomena derived from the distinctive Fermi surface Berry curvature and orbital magnetic moment textures around multifold chiral band crossings pinned at the time-reversal invariant momenta. In this work, by means of a tight-binding model and first-principles-based calculations, we investigate metallic electro-optic responses stemming from the Berry curvature and orbital magnetic moment of Bloch electrons across 37 materials belonging to space group 198 (SG198). Previously thought to vanish in SG198, our findings reveal a nonzero Berry curvature dipole attributed to the energetic misalignment between topologically charged point nodes of opposite chirality. Moreover, BeAu's large Fermi-level magnetoelectric electro-optic coefficient and small Fermi-level gyrotropic magnetic coefficient yield the lowest crossover bias field among the materials examined, well below the Berry curvature dipole critical field and favorable for isolating the magnetoelectric electro-optic effect.

Unlike broken time-reversal-symmetric (TRS) systems with a well-defined Chern number, directly measuring the bulk $Z_2$ invariant and Berry curvature (if nonzero) in topological insulators and their higher-order topological families remains an unsolved problem. Here, based on the refined trace-determinant inequality (TDI) involving the trace and determinant of the quantum metric and maximal Berry curvature (MBC), we propose an optical bound on the MBC for two-dimensional TRS insulators. As a result, using experimental data from a series of measurements—where the band-inversion parameter is tuned and the optical conductivity is measured over a certain energy range—one can identify the $Z_2$ topological signature and construct the topological phase diagram by integrating the optical bound over frequency. This is supported by the momentum integration of the refined TDI, $f$-sum rule, and its topological extension, which provide a topological lower bound. To clearly identify the $Z_2$ topological signature, the faster decay of the optical weight in the topologically trivial region is crucial; this faster decay can be controlled by the optical gap and the inverse mass tensor. Crucially, the MBC we introduced enables us to prove the existence of tight topological lower bounds for the optical weight, quantum weight, and the double quantum volume. Based on the tight topological lower bounds, their physical meaning can be interpreted as an upper bound on the number of boundary states. We illustrate our approach using three representative topological models: the Kane-Mele model, mirror-protected insulator, and quadrupole insulator. Our results demonstrate that the MBC can reveal symmetry-protected topology and plays a role analogous to that of the original Berry curvature. Our framework opens an avenue for extracting the $Z_2$ topological signature of TRS insulators from optical conductivity data.

We report a detailed de Haas–van Alphen (dHvA) study of the recently discovered kagome metal ${\mathrm{LuNb}}_6{\mathrm{Sn}}_6$ using torque magnetometry, electrical transport, magnetization, and heat-capacity measurements. Temperature-dependent torque, resistivity, and heat capacity data reveal a charge density wave (CDW) transition at $T_{\mathrm{CDW}}=85\mathrm{K}$. The thermal hysteresis observed in both measurements establishes the first-order nature of the transition. Quantum oscillation measurements identify two major dHvA frequencies: $F_{\alpha }\approx 20\mathrm{T}$ and $F_{\beta }\approx 200\mathrm{T}$ for a field applied within the ab plane, and their angular dependence is consistent with ellipsoidal Fermi surface (FS) pockets. Landau fan diagram analysis reveals evidence for a nontrivial Berry phase associated with the $F_{\alpha }$ pocket, indicating possible nontrivial electronic topology in ${\mathrm{LuNb}}_6{\mathrm{Sn}}_6$. Analysis of the temperature and magnetic field dependence of the oscillations using the Lifshitz-Kosevich formula yields electronic parameters that suggest anisotropic quantum transport properties. First-principles calculations provide further insight into the electronic structure, revealing Dirac-like band crossings, a flat band, and multiple van Hove singularities near the Fermi level. Our calculations based on the pristine phase cannot fully reproduce the experimentally observed quantum oscillation frequencies, suggesting that CDW-induced Fermi surface (FS) reconstruction plays a crucial role in the ground-state electronic structure of ${\mathrm{LuNb}}_6{\mathrm{Sn}}_6$. These results provide new insight into the FS topology and electronic structure of ${\mathrm{LuNb}}_6{\mathrm{Sn}}_6$, enriching our understanding of the electronic properties of kagome materials.

Controlling spin dynamics at ultrafast timescales is crucial for next-generation spintronic and quantum devices, yet achieving precise control and understanding the underlying microscopic mechanisms remain challenging. Here, we demonstrate that ferroelectric polarization in $\mathrm{Cr}{\mathrm{I}}_3/\alpha -\mathrm{I}{\mathrm{n}}_2\mathrm{S}{\mathrm{e}}_3$ heterostructures serves as a multifunctional switch that selectively regulates spin-relaxation behavior. Using time-dependent density functional theory combined with nonadiabatic molecular dynamics, we show that femtosecond laser excitation induces similar ultrafast demagnetization in both polarization states, while the subsequent relaxation dynamics of photoexcited carriers diverge dramatically. Polarization-controlled interfacial electric fields and band alignments steer photoexcited electrons along two distinct pathways: a stepwise interlayer transfer in the polarization-up ($P\uparrow$) configuration and a direct intralayer spin-flip in the polarization-down ($P\downarrow$) configuration. These bifurcated dynamics originate from the ferroelectric-tuned interplay between spin-orbit coupling and electron-phonon interactions. These findings establish a microscopic link between ferroelectric control, spin-orbital coupling, electron-phonon coupling, and ultrafast spin dynamics, offering a route for all-optical, non-volatile manipulation of spins in two-dimensional heterostructures.

Chemical substitution can tune $1T\text{−}\text{Ta}{\mathrm{S}}_2$ from the commensurate charge-density-wave (CCDW) phase to the nearly commensurate CDW phase (NCCDW), but the microscopic mechanism remains unclear. Using first-principles calculations, we compare two prototypical cases: Se-for-S and Ti-for-Ta substitutions, which reveal dopant site selectivity as the key difference. We show that the CCDW-to-NCCDW phase transition can be understood from the angle of domain wall (DW) formation. The trend can be rationalized by the competition between the energy gain from locally reregistering the SD pattern around quenched dopants and the cost of creating DWs. A statistical estimation based on the first-principles relative substitution energy and DW formation energy captures the much lower critical concentration for Ti than for Se to create DWs. Isolated Se- or Ti-substituted SD clusters remain insulating, whereas DW reconstruction generates metalliclike low-energy spectral weight. This comparison indicates that the metallic behavior experimentally associated with the CCDW-to-NCCDW transition originates primarily from DW regions rather than from direct dopant-induced metallization.

Bismuth-induced $(1\times 2)$ and $(1\times 3)$ surface reconstructions on InSb(110) exhibit a tunable giant Rashba splitting associated with one-dimensional atomic chain structures. Remarkably, the lower Bi-coverage Bi/InSb(110)-$(1\times 3)$ phase shows a larger Rashba splitting parameter than the $(1\times 2)$ phase, revealing a counterintuitive dependence of spin splitting on adsorbate concentration. Combining surface-science experiments and density functional theory, we show that the magnitude of band splitting is governed by the interplay between atomic-scale surface corrugation and spin-orbit coupling from Bi. Our calculations further indicate that, while Bi is essential to induce the Rashba effect, the significant contribution arises from the InSb substrate. The Bi/InSb(110) reconstructed surface consists of Bi chains bonded selectively to In or Sb of the substrate. These results establish Bi/InSb(110) as a platform for engineering the strength of spin splitting and for enabling the ordered growth of InBi nanostructures.

Conventional valleytronics relies on two momentum-separated valleys characterized by opposing optical selection rules. However, this bivalley architecture encounters challenges due to intervalley scattering and static selection rules. In this study, we propose a single-valley architecture capable of generating two orthogonal polarization responses, which represent two distinct nonequivalent states. Focusing on the $M$-point valley in a tetragonal lattice with a little group of $D_{2d}$, we demonstrate that uniaxial strain functions as a dual switch. In its unstrained state, the valley is optically inactive, exhibiting no polarized absorption. When strain is applied along the $x$ direction, the symmetry is reduced to $C_{2v}$, activating $x$-polarized absorption and defining one valley state. Conversely, applying strain along the $y$ direction activates $y$-polarized absorption, thereby defining the other state. Crucially, since only one valley is involved, intervalley scattering is entirely absent. Using first-principles calculations on monolayer ${\text{Zr(CuN)}}_2$, we demonstrate this strain-reconfigurable linear dichroism and uncover frequency-encoded orthogonal dichroism. Our findings pave the way for programmable valleytronics anchored in a single valley, effectively overcoming the inherent limitations associated with traditional bivalley systems.

Nonlinear optical (NLO) effects have drawn considerable attention in condensed matter physics. In magnetically ordered materials, NLO responses are widely believed to be time-reversal odd ($T$-odd). In particular, the widely studied $PT$-symmetric collinear antiferromagnetic (AFM) materials have shown that their NLO responses depend critically on spin-orbit coupling (SOC). In this work, we demonstrate that $E$-type AFM materials, as a class of type-II multiferroic materials independent of SOC, exhibit NLO responses that break the traditional conception. Using both magnetic group and spin group methods, we compare the symmetry-breaking mechanisms between $PT$-symmetric AFM systems and $E$-type AFM systems. Through first-principles calculations on representative $E$-type AFM materials (${\mathrm{MnS}}_2, {\mathrm{LuMnO}}_3$, and ${\mathrm{NdNiO}}_3$), we find that the NLO coefficients of the two spin sublattices are identical in both magnitude and sign. Therefore, these NLO responses persist in the absence of SOC effect, and are time-reversal even ($T$-even). Additionally, we discover that the NLO responses can be effectively tuned across distinct magnetic polymorphs. The above results provide new insights into the symmetry origins and microscopic mechanisms of NLO effects in SOC-free magnetically ordered materials.

In magnetically frustrated systems, competing exchange interactions suppress long-range magnetic order and give rise to nontrivial ground states. In this study, we investigate the magnetic semiconductor ${\text{YbCuS}}_2$, which crystallizes in the orthorhombic $Pnma$ structure. In this structure, the Yb ions form an effective spin-1/2 zigzag chain along the $b$ axis. ${\text{YbCuS}}_2$ exhibits the first-order phase transition at $T_{\mathrm{O}}$ $=$ 0.95 K, accompanied by an incommensurate modulation along the $b$ axis with reduced Yb moments. We synthesize twinned crystals composed of three domains that share the $c$ axis through nearly 120-degree rotations, to conduct specific heat and magnetization measurements. Below $T_{\mathrm{O}}$, we observe an increase in magnetization between 3 and 7 T only for $B$ $\perp$ $c$, reminiscent of a magnetization plateau, while no such feature is observed for $B\parallel c$. We attribute this anisotropic behavior to on-site anisotropy arising from the crystalline electric field of the Yb ion. Accordingly, the 1/3 magnetization plateau must manifest itself for the magnetic fields applied along the $a$ axis, which is consistent with the anisotropic magnetic-field and temperature phase diagrams for $B$ $\perp$ $c$ and $B\parallel c$.

We study the nonequilibrium spin-splitter effect in superconducting altermagnets and superconductor altermagnet hybrids by computing the alternating spin current and the edge spin density in the presence of an alternating electric field. We show that while in the normal state the effect is not sensitive to the field frequency, in the superconducting state, there is a strong effect for frequencies on the scale of ${\mathrm{\Delta }}_0$ or lower. We contrast the effect with the spin accumulation induced by the spin-Hall effect by showing that for the altermagnet spin-splitter effect the out-of-phase spin density does not diverge in the adiabatic limit. This difference is attributed to the absence of any equilibrium spin-splitter effect in altermagnets. In fact, the out-of-phase component vanishes below the gap excitation frequency $2{\mathrm{\Delta }}_0$, because below this frequency the absence of dissipation and the behavior of the system under time-reversal directly determine the relative phase between the charge current, spin current, and spin accumulation. The nonequilibrium effect can be tuned by external parameters like temperature. In fact, it has a nonmonotonic temperature dependence, taking its largest value for temperatures around $0.8T_c$. The value at this temperature can be significantly larger than the normal state spin density or the low temperature spin density. Thus, besides using the nonequilibrium spin-splitter effect to identify altermagnets, its tunability makes it also suitable for applications.