In this work we investigate how the low-energy unoccupied electronic structure of bulk ${\mathrm{BaBiO}}_3$ is modulated by temperature and oxygen vacancies. Using oxygen $K$-edge x-ray absorption spectroscopy (XAS), we find a sizable temperature dependence of the measured spectra, with an important modification of the preedge peak line shape, which might be associated with the coupling between the excited electron and the anharmonic lattice dynamics of ${\mathrm{BaBiO}}_3$. Thermal treatment significantly affects the spectral profile, particularly by suppressing the preedge peak, but our density-functional-theory calculations suggest that oxygen vacancies alone cannot fully account for the observed spectral modifications.

In this study, using density functional theory, we investigate the structural, electronic, and magnetic properties of Fe adatom, interstitial, and substitutional configurations in monolayer ${1T}^{\prime }{\text{-WTe}}_2$. We find that under Te-poor conditions, Fe preferentially occupies topographically distinct tetrahedral interstitial sites, donating electrons to the host and stabilizing local magnetic moments, whereas under Te-rich conditions the preferred defect configuration switches to substitutional. Simulated scanning tunneling microscopy images of the interstitial defects reproduce key features found in experiment, suggesting the coexistence of multiple Fe-related defect configurations. The spin-orbit-coupled electronic properties with the computed Hubbard $U$ parameter reveal metallic behavior for the interstitial configuration, while the substitutional configuration largely retains the semimetallic electronic structure of pristine ${1T}^{\prime }{\text{-WTe}}_2$. The emergence of localized Fe ($3d$) states near the Fermi level, together with calculated work-function shifts and the local potential profile, indicates the $n$-type character, consistent with experimental observations. These findings provide a microscopic understanding of Fe-induced defect formation and the resulting electronic and magnetic properties of ${1T}^{\prime }{\text{-WTe}}_2$, guiding further experimental investigations and the development of defect-engineered two-dimensional quantum materials for future spintronic and magnetic-device applications.

The coherence of free electrons sets fundamental limits on the resolution and contrast of phase-sensitive electron microscopy because coherence is lost whenever the electron leaves a distinguishing excitation in its environment. We develop a quantitative theory of fast-electron decoherence in two canonical geometries: a cylindrical hole drilled through a realistic metal and a circular aperture in a thin, perfectly conducting film. By combining an electromagnetic Green-tensor formulation with the fluctuation-dissipation theorem while fully retaining retardation, we obtain the decoherence probability and elastic phase as functions of the electron trajectory, temperature, and material response. For the cylindrical hole, we obtain a closed-form, azimuthally resolved expression that separates the dependencies on path position, temperature, and conductivity. In the high-conductivity and high-temperature limits, this result reduces to a universal expression that is linear in temperature and independent of both conductivity and electron velocity. For the circular aperture, which we solve using a cylindrical-wave modal expansion, the energy-loss probability diverges as $1/\omega$ at low frequency, whereas the decoherence probability remains finite and reaches a maximum when the aperture radius is comparable to the relevant electromagnetic wavelength. Because the interaction is spatially localized, aperture-induced decoherence is generally weaker than that produced by a cylindrical hole, except for sufficiently large apertures at high temperature. Finally, we show that, for sufficiently large holes, decoherence can dominate the spatial broadening of a focused electron probe and must therefore be incorporated into the design of coherent electron-beam instruments.

The current-induced vibrational dynamics is a key factor determining the mechanical stability of molecular nanojunctions. Beyond conventional Joule heating, a different mechanism leading to vibrational instability, which is induced by nonconservative current-induced forces, has been reported at low bias voltage. Given reports that this mechanism can induce extremely large vibrational excitation in models in which the unperturbed vibrational potentials are harmonic, a natural question is whether it can also be a source of current-induced bond rupture and dissociation in anharmonic systems. Consequently, in this work, we extend previous investigations of two-level, two-mode molecular nanojunctions to various anharmonic unperturbed vibrational potentials. Using a mixed quantum-classical approach based on electronic friction and Langevin dynamics, we first explore the impact of nonconservative forces on dissociation dynamics in Morse-type potentials. Furthermore, using quartic vibrational potentials, we use steady-state vibrational excitation and electric current to explore the impact of anharmonicity on this mechanism of vibrational instability, investigating whether it survives beyond the purely harmonic limit in this system.

Orthorhombic boron nitride ($o\text{−}{\mathrm{B}}_2{\mathrm{N}}_2$) has recently emerged as a promising two-dimensional semiconductor, yet most studies have so far focused on its monolayer. Using first-principles calculations, we systematically investigate the stacking-dependent electronic structure and optical response of bilayer $o\text{−}{\mathrm{B}}_2{\mathrm{N}}_2$. We demonstrate that different stacking configurations substantially modify the electronic structure, enabling the tunability of the quasiparticle band gap between slightly below 2 and about 3 eV while preserving a direct-gap character. The optical spectra exhibit pronounced anisotropy and are dominated by strong excitonic effects in the visible region, with exciton binding energies reaching up to 0.39 eV. Analysis of the exciton wave functions reveals a stacking-driven crossover in exciton character. While the ${\mathrm{AA}}^{\prime \prime }$, AB, and ${\mathrm{AB}}^{\prime }$ stackings host tightly bound intralayer excitons, AA, ${\mathrm{AA}}^{\prime }$, and ${\mathrm{AB}}^{\prime \prime }$ favor mixed intralayer-interlayer excitons, characterized by smaller binding energies. These findings establish a direct connection between stacking configuration, quasiparticle electronic structure, and excitonic properties in bilayer $o\text{−}{\mathrm{B}}_2{\mathrm{N}}_2$, highlighting stacking engineering as an effective route for tailoring the material's optoelectronic response.

Quantum states defined over a parameter space form a Grassmann manifold. To capture the geometry of the associated gauge structure, gauge-invariant quantities are essential. We employ the projector of a multilevel system to quantify the quantum distance between states. Using the multidimensional scaling method, we transform the quantum distance into a reconstructed manifold embedded in Euclidean space. This approach is demonstrated with examples of topological systems, showcasing their topological features within these manifolds. Our method provides a comprehensive view of the manifold rather than focusing on local properties.

Geometry and topology are fundamental to modern condensed matter physics, but their precise connection in quantum systems remains incompletely understood. Here, we develop an analytical scheme for calculating the curvature of the quantum metric of Bloch bands. Using a gauge-invariant formulation based on eigenprojectors, we construct the full Riemannian geometry of the quantum-state manifold and apply it to a two-dimensional two-band model. We find that the Gauss curvature is constant over regular regions, but the manifold inevitably develops a closed curve of singular points where the metric tensor degenerates. These singularities obstruct the conventional Gauss-Bonnet theorem. By introducing the notion of a front and a signed area form, we derive a generalized Gauss-Bonnet relation that includes a singular curvature term defined along the fold curve. This result establishes a direct, quantized link between the total signed Gauss curvature and the Chern number, providing a unified geometric interpretation of Berry curvature and quantum metric. This framework bridges differential geometry and topological band theory, revealing how singular folds mediate the discrepancy between quantum volume and topological charge.

The interactions among lattice, charge, and spin degrees of freedom fundamentally shape material properties. In FeSe, symmetry-breaking perturbations serve as highly sensitive probes of these couplings. Previous work has shown that defects and isoelectronic substitution can substantially alter these interactions, giving rise to additional phonon modes. In this study, uniaxial strain is employed as a tunable symmetry-breaking control parameter to probe the intrinsic lattice response in the absence of disorder. The temperature evolution of phonon excitations was examined with a temperature resolution of 3 K in the vicinity of the nemato-structural transition temperature $T_{\mathrm{s}}$, under strain applied along the $\langle 110\rangle$ and $\langle 100\rangle$ crystallographic directions. A subtle asymmetry of the $A_{1g}^{ph}$ mode appears in the unstrained crystal within a narrow temperature window around $T_{\mathrm{s}}$, originating from the emergence of an additional mode in the fully symmetric channel. With applied strain, this feature becomes more distinctly resolved. The anomaly is attributed to modifications of the coupling between lattice and electronic degrees of freedom driven by the ordering fluctuations right above the nematic transition. These fluctuations may enhance the susceptibility of the electron-lattice system and enable two-phonon Raman scattering involving the acoustic mode near the $X$ and $R$ points. The presence of this two-phonon scattering depends on both the strength and the direction of the applied strain, indicating a high sensitivity of FeSe to local symmetry breaking.

Using quantum distance, magnetic susceptibility in the noninteracting limit can be rigorously split into two contributions: one arises solely from band dispersion while the other stems from quantum geometric contributions. In this paper, we apply this decomposition to two materials, LaFeAsO and ${\mathrm{Pb}}_9\mathrm{Cu}{\left({\mathrm{PO}}_4\right)}_6\mathrm{O}$, and demonstrate that their dominant magnetic fluctuations originate from the geometric contribution. In LaFeAsO, stripe-type antiferromagnetic fluctuations arise primarily from quantum geometry, while in ${\mathrm{Pb}}_9\mathrm{Cu}{\left({\mathrm{PO}}_4\right)}_6\mathrm{O}$ the geometric term suppresses antiferromagnetic fluctuations and stabilizes ferromagnetic fluctuations. Our findings highlight the essential role of quantum geometry in governing magnetic fluctuations in multiband systems, and provide a unique and quantitative framework to disentangle band-structure and wave-function-geometry effects that have often been discussed collectively as multiorbital effects.

Multipole order parameters with spin-orbit coupling in the $t_{2g}$ subspace are systematically investigated based on symmetry analysis and tensor-coupling methods. Through a classification according to their space-time symmetry, ranks, as well as point groups ($O_h$ and $D_{3d}$), the distinct physical significance of these multipoles is clarified. Subsequently, their induced parameters under external vector fields are derived, which uncover the various novel physical effects such as magnetodielectric and perpendicular polarization responses, serving as fingerprints for identifying the underlying multipole orders. These results not only resolve ambiguities in the physical significance of $t_{2g}$ multipoles, but also provide a concrete roadmap for their experimental detection and manipulation through external field responses.

We investigate the topology of the different phases of the extended Su-Schrieffer-Heeger (eSSH) model, which includes hopping processes between translationally inequivalent atoms beyond nearest neighbors. Exact analytical expressions for the edge states of a semi-infinite eSSH chain are derived, with wave functions that decay exponentially from the boundary with a unit-cell decay factor $z$. From the winding number of the bulk Hamiltonian under periodic boundary conditions, we determine the topological phase diagram and establish the bulk-boundary correspondence: changes in the winding number coincide with bulk gap closings and with the condition $\left|z\right|=1$ for the edge-state solutions. For finite chains, we further obtain analytical, approximate expressions for the low-energy edge states, which are shown to be highly accurate.

We study the effects of a perpendicular magnetic field on the excitonic insulator (EI) phase in the semiconductor regime using an extended planar four-Fermi model. Within the large-$N$ approximation, we determine the coupled scalar and excitonic condensates at finite temperature, chemical potential, and magnetic field. The field enhances the EI condensate and raises its critical temperature, providing an excitonic realization of magnetic catalysis, while the scalar condensate remains constant throughout the EI phase. By contrast, the critical chemical potential depends nonmonotonically on the field because of the successive occupation of Landau levels. The magnetic field also shifts the mean-field tricritical point and enlarges the first-order region of the temperature-chemical-potential phase diagram. We further analyze the Hall conductivity and find that increasing the field reduces the number of plateaus and modifies the threshold for a finite Hall response. For the parameters considered, the emergence of the EI condensate is accompanied by a characteristic change in the Hall conductivity, including a field-dependent change of slope near a continuous transition. These results show that the combined phase structure and Hall response can provide complementary signatures of excitonic ordering in planar fermionic systems.

We develop a transfer-controlled finite-size theory of the regular positive-frequency absorption of isolated, half-filled, open Su–Schrieffer–Heeger chains and their static Rice-Mele extension. The central result is a quantitative spectroscopy framework that connects a projected boundary doublet to the exact finite chain. Exact spectral pairing yields the edge-edge, edge-bulk, and bulk-bulk thresholds, and exponentially localized terminal states give closed expressions for the hybridization, transition frequency, dipole matrix element, and integrated Kubo weight. A finite-dimensional spectral-perturbation theorem supplies explicit bounds on the boundary-energy shift, spectral-projector error, line-frequency error, and integrated-weight error in terms of the boundary-complement coupling and spectral separation. The current operator is generated from the physical orbital embedding, and a causal Kubo construction retains the complete retarded pole structure before phenomenological homogeneous broadening and instrumental convolution. Exact diagonalization tests the threshold taxonomy, scaling laws, and transfer bounds over the stated parameter domains. Conditional on correct line assignment, known channel multiplicity and boundary separation, absolute response calibration, occupation correction, and transfer admissibility, the line center and area determine the magnitudes of the boundary hybridization and Rice-Mele detuning. Hopping reconstruction requires a common-coupling length series, a resolved extended-sector feature, or a full-spectrum fit. Topological attribution additionally requires independent bulk, symmetry, or termination evidence.

Field-induced quantum spin liquids (QSLs) and related phenomena in Kitaev honeycomb systems are under intensive investigation. Here, we investigate the anisotropic antiferromagnetic Kitaev model under magnetic field $h$ along the [111] direction by using state-of-the-art tensor network simulations. By varying the anisotropy parameter $\alpha$ from 1 to 1.25, we map out the $\alpha \text{−}h$ phase diagram through a comprehensive analysis of ground-state and finite-temperature properties, including entanglement spectrum, magnetization and its derivatives, spin structure factors, specific heat, and thermal entropy. Our results reveal two distinct field-induced phase transitions, with an intermediate QSL phase persisting up to $\alpha <1.125$. This provides a clear constraint for observing such QSL in real Kitaev magnets. Moreover, for $\alpha >1$, thermodynamic calculations indicate that the intermediate-temperature Kitaev fractional liquid regime extends down to significantly lower temperatures than the isotropic Kitaev case, forming a broader fractional plateau in entropy curve. Upon adiabatic demagnetization or pressurization process, a topological cooling effect can be achieved, demonstrating a potential application of anisotropic Kitaev magnets.

We employ a general method, known as anomaly matching, to derive no-go theorems of fermionic lattice models. For our main result, we show that time-reversal invariant $(3+1)$-dimensional lattice systems (such as Dirac and Weyl semimetals) can never admit a lone low-energy symmetry-protected Dirac fermion (or node), i.e., it must always come in higher multiplets or be fine tuned. This theorem holds for both noninteracting and interacting systems as long as the electromagnetic $U\left(1\right)$ symmetry is a normal subgroup of the microscopic symmetry group $G_{\mathrm{UV}}$, a condition that is ubiquitous in physical $U\left(1\right)$ preserving lattice models. To show that our theorems are tight, we also explore both well-known and new systems that are converses of the no-go theorem, obtained by forfeiting certain assumptions such as a broken time-reversal symmetry (magnetic Weyl semimetal), a nononsite $\mathbb{R}$ symmetry (almost local Dirac node model), no-symmetry protection (fine-tuned Dirac semimetal), or multiple low-energy Dirac nodes (time-reversal invariant Weyl and Dirac semimetals). We will also explicitly demonstrate that, while this theorem strictly prohibits single time-reversal invariant symmetry-protected Dirac node, it does allow for other odd numbers of Dirac nodes under certain circumstances, such as three Dirac nodes in the Fu-Kane-Mele diamond lattice model. This is akin to the Nielsen-Ninomiya theorem for an odd number of differently charged chiral fermions, whose lattice realizations are allowed if certain anomaly cancellation conditions are met.

Defects in semiconductors have recently attracted renewed interest owing to their potential in novel quantum applications. Here we investigate the electronic and magnetic properties induced by 60° dislocations in Ge. Using large-scale density functional theory calculations, we determine the band structure for both the shuffle and glide sets in their lowest-energy configurations. The band structure for the shuffle set reveals defect-induced dispersive bands localized within the band gap near the $\mathrm{\Gamma }$ point, whereas for the glide set we observe strong overlap with the conduction band. Defect-induced band splitting evident away from $\mathrm{\Gamma }$ reveals Rashba-Dresselhaus spin-orbit coupling, an effect previously reported only for screw dislocations. Remarkably, we find evidence that specific dislocation arrangements can stabilize antiferromagnetic ordering with sizable local magnetic moments and considerable exchange splitting between opposite spin states. These results uncover rich physics in Ge dislocations through the combination of spin-orbit coupling and magnetic ordering, potentially enabling novel defect-based functionalities in Ge devices.

The Shastry-Sutherland compound ${\mathrm{SrCu}}_2{\left({\mathrm{BO}}_3\right)}_2$ has attracted considerable interest as a platform for exploring quantum phases and quantum phase transitions driven by magnetic frustration. The pressure-induced structural and magnetic phase transitions in ${\mathrm{SrCu}}_2{\left({\mathrm{BO}}_3\right)}_2$, however, remain controversial. To address this issue, we performed high-pressure ${}^{11}\mathrm{B}$ nuclear magnetic resonance (NMR) measurements on ${\mathrm{SrCu}}_2{\left({\mathrm{BO}}_3\right)}_2$ up to 14 GPa. The NMR spectra reveal two pressure-induced monoclinic phases. With pressure above 4 GPa and with temperature below 10 K, the rapid broadening of the NMR spectrum and the power-law behavior of the spin-lattice relaxation rate $1/T_1$ provide clear evidence for a gapless 3D antiferromagnetic (AFM) phase in the monoclinic phase. At an intermediate temperature range around 20 K, the emergence of the field-dependent NMR line splitting resolves a two-dimensional, short-range ordered AFM phase; at temperature above 30 K, the sublinear power-law behavior of $1/T_1$ identifies an extended correlated paramagnetic regime.

We solve the problem of the spin quantum Hall transition on random networks by mapping it to a loop model. Using tools of two-dimensional quantum gravity, we compute critical exponents that characterize this transition and confirm that these are related to the exponents for the regular (square) network through the Knizhnik-Polyakov-Zamolodchikov relation. Our results demonstrate the relevance of the geometric randomness of the networks and support conclusions of numerical simulations of random networks for the integer quantum Hall transition.

The superconducting diode effect (SDE) enables nonreciprocal dissipationless transport when inversion and time-reversal symmetries are simultaneously broken. Rashba nanowires proximitized by conventional $s$-wave superconductors provide a minimal setting in which spin-orbit coupling and Zeeman fields generate asymmetric finite-momentum pairing. While most studies focus on the single-channel limit, which yields small diode efficiencies and requires multiple Zeeman-field components, realistic devices host multiple transverse subbands. Here, we investigate the SDE in multichannel Rashba nanowires with harmonic and rectangular quantum-well confinement using a self-consistent Bogoliubov–de Gennes formalism. Both geometries support asymmetric Fulde-Ferrell (FF) states that drive pronounced nonreciprocal supercurrents and stabilize a topological phase with Majorana zero modes, where the Cooper-pair momentum is controlled by an externally injected supercurrent. Pairing-susceptibility analysis shows that field-induced asymmetry favors directional Cooper pairing, explaining the nonmonotonic Zeeman-field dependence of the diode response. Harmonic confinement yields diode efficiencies of $\sim 60\%$ (coupled channels) and $\sim 55\%$ (independent channels). In contrast, rectangular confinement sustains $\sim 60\%$ efficiency in both regimes and uniquely exhibits a tunable sign reversal in the coupled-channels case. Moreover, interchannel coupling allows a transverse Zeeman field alone to generate a finite diode response. These results establish the robustness of the SDE and topological FF states against transverse confinement variations, highlighting multichannel nanowires as powerful platforms for high-efficiency nonreciprocal transport and current-controlled topological superconductivity.