${\mathrm{V}}_{1-x}{\mathrm{Al}}_x$ is a representative example of highly resistive metallic alloys exhibiting a crossover to a negative temperature coefficient of resistivity (TCR), known as the Mooij correlation. Despite numerous proposals to explain this anomalous behavior, none have provided a satisfactory quantitative explanation thus far. In this work, we calculate the electrical conductivity using an ab initio methodology that combines the Kubo-Greenwood formalism with the coherent potential approximation (CPA). The temperature dependence of the conductivity is obtained within a CPA-based model of thermal atomic vibrations. Using this approach, we observe the crossover to the negative TCR behavior in ${\mathrm{V}}_{1-x}{\mathrm{Al}}_x$, with the temperature coefficient following the Mooij correlation, which matches experimental observations in the intermediate-to-high temperature range. Analysis of the results allows us to clearly identify a non-Boltzmann contribution responsible for this behavior and describe it as a function of temperature and composition.

The non-Hermitian skin effect (NHSE)—the anomalous boundary accumulation of an extensive number of bulk modes—has emerged as a hallmark of non-Hermitian physics, with broad implications for transport, sensing, and topological classification. A central open question is how magnetic or synthetic gauge fields influence this boundary phenomenon. Here, we develop a theoretical framework for magnetic control of the NHSE along line boundaries in two-dimensional single-band lattices. Using a non-Hermitian extension of the anisotropic Harper–Hofstadter model as a representative example, we show that magnetic fields suppress the geometric skin effect in reciprocal models, whereas skin localization can persist in nonreciprocal systems. The analysis disentangles the interplay of flux, nonreciprocity, and boundary geometry, revealing that magnetic fields mitigate or suppress the NHSE through distinct physical mechanisms—such as bulk localization via Landau or Anderson physics or the restoration of effective reciprocity. In particular, the geometry-dependent skin effect in reciprocal systems is found to be fragile against even weak magnetic fields.

We report on the structural evolution, spin-phonon interactions, and magnetoelastic effects in bulk ${\mathrm{LaFeO}}_3$ perovskite across its antiferromagnetic transition. Our investigation is motivated by anomalies in the zone-center phonon bandwidth and peak positions which, as superexchange precursors, deviate in far-infrared emission and high-temperature Raman scattering spectra from the behavior expected in a purely anharmonic lattice. High-resolution synchrotron x-ray diffraction (SXRD) patterns provide intriguing insights into the interplay between the perovskite lattice and the magnetic transition. We found that, while the lattice constants exhibit slight deviations at the Néel temperature ($T_{\mathrm{N}}$), due to spin-phonon interactions, individual interatomic bonds display distinct behavior. The octahedral B-site basal plane, involving oxygen ion pairs that mediate superexchange interactions, displays bond contraction (or elongation) assimilable to that observed in x-ray diffraction for negative thermal expansion. These bond deviations reach a singular point near $T_{\mathrm{N}}$, where a sharp change in bond length is observed. A similar abrupt change is also detected in the nearest-neighbor La–O2 basal plane distances, which we interpret as arising from ion size differences within the $3d\text{−}2p$ hybridized states associated with the superexchange mechanism. High-resolution SXRD measurements of the c axis $\mathrm{F}{\mathrm{e}}^{3+}\text{–}\mathrm{O}1$ and $\mathrm{L}{\mathrm{a}}^{3+}\text{–}\mathrm{O}1$ apex distances reveal a distinct inflection at the Néel temperature. This behavior supports the role of lattice distortions and crystal field modifications in the emergence of noncollinear weak ferromagnetism in $R\text{Fe}{\mathrm{O}}_3$ ($R=$ rare earth) perovskites. Furthermore, our SXRD patterns show an increase in diffraction line intensities peaking at $T_{\mathrm{N}}$, suggesting a straightforward indicator for spin-phonon interactions that also resolves into lattice metastability deduced by nonlinear thermal expansion in the paramagnetic phase. Preliminary results across the full $R\text{Fe}{\mathrm{O}}_3$ ($R=$ rare earth) series indicate a shared structural framework among all members, potentially offering insights into previously inconclusive structural interpretations in oxides with common octahedral sublattices.

The dynamic nuclear polarization (DNP) in a system of ${\mathrm{Mn}}^{2+}$ spins is studied in a weakly ferromagnetic matrix formed in heavily cobalt-doped ZnO single crystals. It is established that the thermal mixing mechanism of DNP leads to cooling of the common reservoir of electron spin-spin interactions of the ${\mathrm{Mn}}^{2+}$ and ${\mathrm{Co}}^{2+}$ spins to a single spin temperature in the rotating frame. We show the variation of the polarization amplification of the common nuclear subsystem as a function of microwave irradiation frequency.

Altermagnets present a class of fully compensated collinear magnetic order, where the two sublattices are not related merely by time-reversal combined with lattice translation or inversion, but require an additional lattice rotation. This distinctive symmetry leads to a characteristic splitting of the magnon bands; however, the splitting is only partial—residual degeneracies persist along certain lines in the Brillouin zone as a consequence of the underlying altermagnetic rotation. We consider a two-dimensional $d$-wave altermagnetic spin model on the checkerboard lattice and introduce additional interactions such as an external magnetic field and Dzyaloshinskii-Moriya interactions that lift these degeneracies. The resulting magnon bands become fully gapped and acquire nontrivial topology, characterized by nonzero Chern numbers. We demonstrate the crucial role of altermagnetism for the generation of the Berry curvature. As a direct consequence of the topological magnons, we find finite thermal Hall conductivity ${\kappa }_{xy}$, which exhibits a characteristic low-temperature scaling, ${\kappa }_{xy}\propto T^4$. Moreover, ${\kappa }_{xy}$ changes signs under reversal of the magnetic field, exhibiting a sharp jump across zero field at low temperatures. We also demonstrate topologically protected chiral edge modes in a finite strip geometry.

Low-dimensional crystalline materials exhibiting large atomic fluctuations enable low thermal conductivity for many applications, yet the critical interplay between thermal anisotropy and lattice anharmonicity remains elusive. Here, we combine first-principles calculations with unified thermal transport theory to uncover the unconventional thermal transport anisotropy correlated with the striking quartic anharmonicity in a quasi-two-dimensional compound $\mathrm{C}{\mathrm{u}}_2{\mathrm{P}}_2{\mathrm{O}}_7$. We find that the unique quasiplanar atomic network and hierarchical bonding create a permissive environment for intense oscillations of interlayered O atoms, triggering giant quartic anharmonicity. This, combined with small phonon interbranch spacing, renders wavelike coherences the dominant mechanism for heat transport, resulting in a nonmonotonic temperature dependence of lattice thermal conductivity and a reversal of its anisotropy between in-plane and out-of-plane directions. Our findings yield fundamental insights into the coherence-driven thermal transport anomaly in highly anharmonic low-dimensional crystals, which would guide the rational design of advanced thermal materials.

Rare-earth-based double perovskites exhibit a large magnetocaloric effect, but often undergo first-order magnetic transitions because of the field-induced metamagnetic behavior. Since the first-order transitions involve a large hysteresis loss in a cooling cycle, suppression of the metamagnetic behavior is essential to improve the efficiency of such double perovskites. A promising approach for negating the loss of hysteresis involves modulating strain-sensitive magnetic interactions via thin films. In this work, we demonstrate that epitaxial strain significantly suppresses the metamagnetic behavior in epitaxial ${\mathrm{Dy}}_2{\mathrm{NiMnO}}_6$ grown on ${\mathrm{SrTiO}}_3$ and ${\mathrm{LaAlO}}_3$ compared to its bulk counterpart. Such epitaxial strain modifies the magnetic transition from the first order in bulk to the second order in thin films and significantly impacts the magnetocaloric properties. This difference is primarily attributed to strain-induced modifications in the Dy-(Ni/Mn) exchange interactions, driven by changes in the Ni-Mn exchange strength. Density functional theory calculations further confirm the role of strain in modifying the exchange interaction. Manipulating the order of magnetic phase transitions via strain engineering opens up the possibility of enhancing the magnetocaloric cooling efficiency in double perovskites and other first-order transition materials.

The relation of the Aharonov-Casher (AC) effect and the force on a particle having a magnetic moment is explored. The general form of the AC Hamiltonian is derived using the Foldy-Wouthuysen transformation to the Dirac equation. Geometries in which an analytic expression for the phase can be obtained are examined, as well as the relation of the AC phase to the Berry phase. The AC phase is determined for an arbitrary homogeneous electric field; it is quadratic (linear) in the field strength for small (large) electric field strengths.

Charge density wave (CDW), a symmetry-breaking collective phenomenon in condensed matter systems, exhibits periodic modulations of electron density coupled with lattice distortions, where the lattice plays a critical role via electron-phonon coupling. In transition metal dichalcogenides (TMDs) $1T\text{−}\mathrm{Ta}{\mathrm{S}}_2/\mathrm{TaS}{\mathrm{e}}_2$, experiments reveal rich temperature- and pressure-dependent CDW phase behaviors, along with metastable CDW states induced by ultrafast optical excitation. Nevertheless, the underlying mechanisms governing thermal/pressure-driven transitions and particularly the microscopic evolution of CDW phases remain incompletely understood. Here, we perform first-principles anharmonic phonon calculations and machine-learning force-field molecular dynamics at finite temperatures/pressures to investigate the CDW transitions in $1T\text{−}\mathrm{Ta}{\mathrm{S}}_2/\mathrm{TaS}{\mathrm{e}}_2$. The calculated CDW transition temperature $T_{\mathrm{CDW}}$ and critical pressure $P_{\mathrm{c}}$ are in quantitative agreement with experimental values. Our results demonstrate that the melting of CDW originates from phonon anharmonicity, with ionic fluctuations dominating the transition dynamics. We observe the microscopic evolution of CDW under varying temperature/pressure, revealing an ultrafast nucleation process of CDW (∼3 ps). Our results emphasize the essential role of phonon anharmonicity in elucidating CDW transition mechanisms underlying, and advance fundamental understanding of CDW-related phenomena in TMDs.

The conventional description of thermal noise in conductors (the Nyquist scenario) assumes that this noise has local origin and, therefore, the noise in the emf (electromotive force) in a conductor with nonuniform temperature can be described as a sum of contributions from uncorrelated regions, each in local thermal equilibrium. Here we propose an alternative scenario for the propagation of thermal noise in a conductor and suggest experiments that could distinguish between different scenarios. To explore the proposed scenario, we introduce a workable 1D model for a gas of particles that undergo stochastic collisions with the lattice and exert distance-dependent forces on each other. We define current, voltage, and emf in a manner appropriate to a wire with limited number of particles. For uniform temperature and within appropriate length and temperature ranges, we verify that our simulations comply with Nyquist's result. Our numerical simulations indicate that (1) thermal noise in a resistor at uniform local temperature is influenced not only by its own temperature but also by the temperatures of other resistors in the circuit; (2) for sufficiently long circuits, the deviation from the Nyquist prediction is inversely proportional to the distance between the centers of the resistors; and (3) if the resistors differ in temperature, their emf can be correlated, even if they are detached. Analytic results can be obtained in the limit of strong interparticle interaction. The long-range repulsion between charges in electrically connected resistors may have conceptual and technological impact on nanodevices.

Stabilizing magnetic skyrmions typically requires a Dzyaloshinskii–Moriya interaction or competing interactions that produce magnetic frustration. An alternative route relies on external fields. In conventional magnetic materials, spatially inhomogeneous magnetic fields—such as demagnetizing or Oersted fields—can stabilize skyrmions. Here, we propose using a superconducting nanoring to locally control and tune the magnetic field in an adjacent ferromagnetic layer. The field generated by a persistent supercurrent stabilizes a Néel skyrmion. We analyze the stabilization conditions in ferromagnetic layers with perpendicular magnetic anisotropy as functions of nanoring size and induced supercurrent. We show that skyrmion stability requires the supercurrent to exceed a critical threshold. Consistent results are presented from analytical modeling and micromagnetic simulations for thin Co and Ga:YIG films.

We report a spin-$(1/2,5/2)$ three-leg ladder realized in a radical-Mn polymer, exhibiting an antiferromagnetic transition and magnetization curves accurately described by classical mean-field theory. Although the underlying spin model intrinsically supports strong quantum fluctuations, as confirmed by quantum Monte Carlo simulations, the real system shows an anomalously complete suppression of quantum behavior. These findings provide a key experimental benchmark for the quantum-classical crossover and suggest that lattice topology can play a crucial role in tuning the balance between quantum and classical physics in strongly correlated systems.

Domain-wall bimerons are composite topological structures formed by embedding bimerons within domain walls in ferromagnets with in-plane anisotropy. These hybrid textures have recently attracted significant attention due to their promise for racetrack memory applications. In this work, we systematically investigate the current-driven dynamics of single domain-wall bimerons and bimeron chains under Zhang-li torque (ZLT) and spin-orbit torque (SOT). We show that when the spin current is injected or polarized perpendicular to the domain wall, the bimeron Hall effect facilitates efficient motion along the wall. In contrast, spin currents injected or polarized parallel to the wall suppress transverse motion, leading to a significant reduction in the bimeron Hall angle. This anisotropic response is observed for both ZLT and SOT driving mechanisms. Furthermore, we find that increasing the number of bimerons within a domain wall diminishes their collective mobility. These results provide key insights into domain-wall bimeron dynamics and offer guidance for their integration into bimeron-based spintronic devices.

We present a theory of fractional Chern insulator stabilization against charge-ordered states. We argue that the phase competition is captured by an effective interaction range, which depends on both the bare interaction range and quantum geometric properties. We claim that short effective interaction ranges stabilize fractional states while longer-range interactions favor charge-ordered states. To confirm our hypothesis, we conduct a numerical study of the generalized Hofstadter model using the density matrix renormalization group. Our theory offers an interpretation of the geometric stability hypothesis and generalizes it, providing a unifying framework for several approaches to fractional phase stabilization. Finally, we propose a route toward experimental verification of the theory and possible implications for fractional states in bands with higher Chern numbers.

The interplay of high-order Van Hove singularities and topology plays a central role in determining the nature of the electronic correlations governing the phase of a system with unique signatures characterizing their presence. Layered van der Waals heterostuctures are ideal systems for band engineering through the use of twisting and proximity effects. Here, we use symmetry to demonstrate how twisted kagome bilayers can host topological high-order Van Hove singularities. We study a commensurate system with a large twist angle and demonstrate how the initial choice of high-symmetry stacking order can greatly influence the electronic structure and topology of the system. Furthermore, we study the possibility of sublattice interference in the system. Our results illustrate the rich energy landscape of twisted kagome bilayers and unveil large Chern numbers (of order 10), establishing twisted bilayer kagome as a natural playground for probing the mixing of strong correlations and topology.

We propose a nanobeam resonator incorporating an active monolayer, designed to achieve a high Purcell enhancement. The resonator is fully composed of transition-metal-dichalcogenide materials and intended to operate as a high-$\beta$-factor nanolaser. A theoretical framework that models and optimizes the Purcell enhancement associated with the emission from atomically thin layers is developed. This framework is based on a resonance expansion, enabling spectral resolution of physical quantities governed by high-$Q$ resonances. The numerical optimization of the resonator leads to the presence of a high-$Q$ resonance supporting a strong electric field confinement in the monolayer to maximize the modal gain.

We present a symmetry-based framework for the analysis of excitonic states, incorporating both time-reversal and space-group symmetries. We demonstrate the use of time-reversal and space-group symmetries to obtain exciton eigenstates at symmetry-related center-of-mass momenta in the entire Brillouin zone from eigenstates calculated for center-of-mass momenta in the irreducible Brillouin zone. Furthermore, by explicitly calculating the irreducible representations of the little groups, we classify excitons according to their symmetry properties across the Brillouin zone. Using projection operators, we construct symmetry-adapted linear combinations of electron-hole product states, which block diagonalize the Bethe-Salpeter equation (BSE) Hamiltonian at both zero and finite exciton center-of-mass momenta. This enables a transparent organization of excitonic states and provides direct access to their degeneracies, selection rules, and symmetry-protected features. As a demonstration, we apply this formalism to monolayer ${\mathrm{MoS}}_2$, where the classification of excitonic irreducible representations and the block structure of the BSE Hamiltonian show excellent agreement with compatibility relations derived from group theory. Beyond this material-specific example, the framework offers a general and conceptually rigorous approach to the symmetry classification of excitons, enabling significant reductions in computational cost for optical spectra, exciton-phonon interactions, and excitonic band structure calculations across a wide range of materials.

Phonons are essential quasiparticles of all crystals and play a key role in fundamental properties such as thermal transport and superconductivity. Acoustic phonons can be interpreted as Goldstone modes that emerge due to the spontaneous breaking of translational symmetry. In this article, we investigate the quantum geometric contribution to the phonon spectrum in the absence of Holstein phonons. Using graphene as a case study, we decompose the dynamical matrix into distinct terms that exhibit different dependencies on the electron energy and wave function. We then examine the role of quantum geometry in shaping the phonon spectrum of the material, and we find that removing the nontrivial quantum geometric contribution from the dynamical matrix causes the acoustic phonon modes to behave in a nonanalytic fashion.

This work presents a numerical framework for modeling and solving time-modulated responses of complex bianisotropic metasurfaces. First, Lorentz-dispersive surfaces are implemented as impedance sheet models for the surface-wave-assisted transmissive bianisotropic metasurface, and the results are validated with prior analytical solutions. Next, a finite-difference time-domain (FDTD)-based numerical solution for time-modulated media is developed within the MIT Electromagnetic Equation Propagation (meep) framework using a sampled time-varying material function approach, and is verified through comparisons with circuit-based numerical methods, analytical solutions, and a reference FDTD solver. The results show good agreement in terms of harmonic frequencies, power levels, and phase-coherent transmission response. The method is then applied to simulate the time-modulated metasurface modeled with Lorentz-dispersive multilayers, demonstrating nonreciprocal transmission and unidirectional absorption under relatively low-frequency modulation. The proposed numerical approaches offer efficient and practical frameworks for modeling complex electromagnetic media in the time domain and for performing dynamic full-wave simulations, providing a viable solution path for analyzing functionalities such as isolation, unidirectional amplification, and absorption—phenomena that are difficult to achieve in time-invariant systems.