The magnetovolume effect is a well established yet frequently overlooked phenomenon in magnetic materials that may affect a wide range of physical properties. Our study explores the influence of the magnetovolume effect on the transient reflectivity of MnSi, a well-known chiral magnet with strong magnetoelastic coupling. We observe a unipolar reflectivity transient in the paramagnetic phase, contrasting with a bipolar response in phases with magnetic long-range order. Comparing our findings with thermal expansion from literature, we establish that the bipolar response originates in the magnetovolume effect which dominates the thermal expansion and influences the optical reflectivity. Our results highlight not only that the magnetovolume effect must be considered when discussing transient reflectivity measurements of magnetic materials but also that such measurements permit to study the characteristic time scales of the magnetovolume effect itself, contributing to a deeper understanding of this often-neglected phenomenon.

Quantum magnonics, an emerging field focusing on the study of magnons for quantum applications, requires precise measurement methods capable of resolving single magnons. Existing techniques introduce additional dissipation channels and are not apt for magnets in free space. Brillouin light scattering (BLS) is a well-established technique for probing the magnetization known for its high sensitivity and temporal resolution. The coupling between magnons and photons is controlled by a laser input, so it can be switched off when a measurement is not needed. In this article, we theoretically investigate the efficacy of BLS for quantum tomography of magnons. We model a finite optomagnonic waveguide, including the optical noise added by the dielectric, to calculate the signal-to-noise ratio (SNR). We find that the SNR is typically low due to a small magneto-optical coupling; nevertheless, it can be significantly enhanced by injecting a squeezed vacuum into the waveguide. We reconstruct the density matrix of the magnons from the statistics of the output photons using a maximum likelihood estimate. The classical component of a magnon state, defined as the regions of positive Wigner function, can be reconstructed with a high accuracy while the nonclassical component necessitates either a higher SNR or a larger data set. The latter requires more compact data structures and advanced algorithms for postprocessing. The SNR is limited partially by the input laser power that can be increased by designing the optomagnonic cavity with a heat sink.

Realization of noncentrosymmetric magnetic Weyl metals is expected to exhibit anomalous transport properties stemming from the interplay of unusual bulk electronic topology and magnetism. Here, we present spin-valve-like magnetoresistance at room temperature in ferrimagnetic Weyl metal ${\mathrm{Mn}}_2\mathrm{PdSn}$ that crystallizes in the inverse Heusler structure. Anomalous magnetoresistance displays dominant asymmetric component attributed to domain wall electron scattering, indicative of spin-valve-like behavior. Ab initio calculations confirm the topologically nontrivial nature of the band structure, with Weyl nodes proximate to Fermi energy, providing deeper insights into the observed intrinsic Berry curvature mediated substantial anomalous Hall conductivity. Our results underscore the inverse Heusler compounds as promising platform to realize magnetic Weyl metals/semimetals and leverage emergent transport properties for electronic functionalities.

Previously, it was shown that a superconducting state dominated by singlet $d_{x^2-y^2}$ intraband pairs emerges from the fluctuation exchange approximation (FLEX) applied to a realistic model for ${\text{Sr}}_2{\text{RuO}}_4$, a result that is increasingly aligned with experimental data. Here we apply FLEX to model the strain-dependent phase diagram of ${\text{Sr}}_2{\text{RuO}}_4$ and show that we are able to reproduce its unusual features. This adds weight to the argument that a predominantly $d_{x^2-y^2}$ singlet pairing state represents a reasonable starting point for describing the superconducting properties of ${\text{Sr}}_2{\text{RuO}}_4$.

We study elastic electron scattering and localization by ubiquitous isotopic disorder in one-dimensional systems appearing due to an interaction with phonon modes localized at the isotope impurities. By using a tight-binding model with an intersite hopping matrix element dependent on the interatomic distance, we find a mass-dependent backscattering probability by single and pairs of isotopic impurities. For the pairs, in addition to the mass, the distance between the isotopes plays a critical role. Single impurities effectively attract electrons and can produce localized weakly bound electron states. In the presence of disorder, the electron free path at positive energies becomes finite and the corresponding Anderson localization at the spatial scale greatly exceeding the distance between the impurities becomes possible.

The spin Seebeck effect (SSE) is sensitive to thermally driven magnetic excitations in magnetic insulators. Vanadium dioxide in its insulating low-temperature phase is expected to lack magnetic degrees of freedom, as vanadium atoms are thought to form singlets upon dimerization of the vanadium chains. Instead, we find a paramagnetic SSE response in $\mathrm{V}{\mathrm{O}}_2$ films that grows as the temperature decreases below 50 K. The field and temperature-dependent SSE voltage is qualitatively consistent with a general model of paramagnetic SSE response and inconsistent with triplet spin transport. Quantitative estimates find a spin Seebeck coefficient comparable in magnitude to that observed in strongly magnetic materials. The microscopic nature of the magnetic excitations in $\mathrm{V}{\mathrm{O}}_2$ requires further examination.

Superior to ferromagnetic (FM) materials, antiferromagnetic (AFM) materials do not have any net magnetic moment and are robust to external magnetic perturbation with ultrahigh dynamic speed. To achieve spontaneous valley polarization and anomalous valley Hall effect (AVHE) in AFM materials is of great significance for potential applications in spintronics and valleytronics. Here, we predict an A-type AFM monolayer ${\mathrm{Fe}}_2{\mathrm{CF}}_2$ with large spontaneous valley polarization. Monolayer ${\mathrm{Fe}}_2{\mathrm{CF}}_2$ has zero Berry curvature in momentum space but nonzero layer-locked hidden Berry curvature in real space, which provides the basic conditions for the realization of AVHE. Because ${\mathrm{Fe}}_2{\mathrm{CF}}_2$ possesses the combined symmetry ($PT$ symmetry) of spatial inversion $\left(P\right)$ and time reversal $\left(T\right)$ symmetry, the spin is degenerate, which prevents the AVHE. An out-of-plane external electric field can be used to produce spin splitting due to the introduction of layer-dependent electrostatic potential, and then layer-locked AVHE can be realized in ${\mathrm{Fe}}_2{\mathrm{CF}}_2$. Moreover, the spin order of spin splitting can be reversed when the direction of electric field is reversed. It is proved that the AVHE can be achieved in Janus ${\mathrm{Fe}}_2\mathrm{CFCl}$ without external electric field due to intrinsic built-in electric field. Our works provide an AFM monolayer with excellent properties to realize AVHE.

An electrodynamic (ED) theory for 2D particle arrays that undergo a slow rigid rotation $\mathit{\Omega }$ and observed in their rest frame of reference ${\mathcal{R}}^{\mathrm{\Omega }}$ is developed. The theory holds for both transverse electric (TE) and transverse magnetic (TM) excitations. Analytical and numerical techniques are applied, in conjunction of a suitable Greens function, to study the effect of rotation on electrically small 2D scatterers such as dielectric cylinders of arbitrary isotropic material and cross-section geometry, and to derive the polarizability matrix in ${\mathcal{R}}^{\mathrm{\Omega }}$ and the corresponding discrete dipole formulation. The formulation is then used for a preliminary study of particle arrays and tested against full wave numerical solutions of rotating structures that hold in ${\mathcal{R}}^{\mathrm{\Omega }}$. The basic physical mechanisms that govern the rotation footprint on the array response are explored and discussed, and their ramifications on TE and TM cases are contrasted. These mechanisms hold in general and are not idiosyncratic to 2D. It is shown that the ED is mainly due to the excitation of a large number of Sagnac loops created inside the structure as a result of the multiple scattering events between the array inclusions. However, the collective interactions of a large number of loop exhibit a net dynamics that is far more intricate than that of a single Sagnac loop, and even contradicts it in some aspects. The rotation footprint on the collective ED properties of a 2D periodic structure such as stop-band location, induced nonreciprocity, and the role of the structure symmetries in these effects are explored.

We investigate the spin-$\frac{1}{2}$ antiferromagnetic Heisenberg model with a Dzyaloshinskii-Moriya interaction on kagome lattice, making use of the variational Monte Carlo technique. An exotic quantum spin state is found to arise from a melting of the $\mathit{Q}=0$ long-range magnetic order by a topological transition, when a small anisotropic third-nearest-neighbor antiferromagnetic Heisenberg interaction is turned on. This state is a gapped quantum spin liquid, characterized by a topological order with ground-state degeneracy $n_g=4$ and topological entanglement entropy $\gamma =ln2$, suggesting it is an Abelian topological phase. Furthermore, the Chern numbers of the spin-up (-down) spinon occupied bands of this state are $C_{\uparrow \downarrow }=\pm 1$, respectively. From this perspective, this state is also a time-reversal symmetric (total Chern number $C_{\text{total}}=0$) topological insulator with spinons as the chiral edge states, which carry opposite spin and move in the opposite direction. It is analogous to the quantum spin Hall state but the spin current is carried by deconfined spinons in a quantum spin liquid, and so is dubbed as the spinon quantum spin Hall state.

Lifshitz transitions are topological transitions of a Fermi surface, whose signatures typically appear in the conduction properties of a host metal. Here, we demonstrate, using an extended Falicov-Kimball model of a two-flavor fermion system, that a Lifshitz transition which occurs in the noninteracting limit impacts interaction-induced insulating phases, even though they do not host Fermi surfaces. For strong interactions we find a first-order transition between states of different polarization. This transition line ends in a very unusual quantum critical endpoint, whose presence is stabilized by the onset of interflavor coherence. We demonstrate that the surfaces of maximum coherence in these states reflect the distinct Fermi-surface topologies of the states separated by the noninteracting Lifshitz transition. Experimental realizations of our results are discussed for both electronic and optical lattice systems.

We study viscoelastic response in Weyl semimetals with broken time-reversal symmetry. The principal finding is that topology and anisotropy of the Fermi surface are manifested in the viscoelasticity tensor of the electron fluid. In the dynamic (interband) part of this tensor, the anisotropy leads to a qualitatively different, compared with isotropic models, scaling with frequency and the Fermi energy. The components of the viscosity tensor determined by the Fermi-surface properties agree in the Kubo and kinetic formalisms; the latter, however, misses the anomalous Hall viscosity originating from filled states below the Fermi surface. The anisotropy of the dispersion relation is also manifested in the acceleration and relaxation terms of the hydrodynamic equations providing means to probe the anisotropy in transport experiments.

Topological band theory predicts a $\mathbb{Z}$ classification of three-dimensional (3D) Dirac semimetals (DSMs) at the single-particle level. Namely, an arbitrary number of identical bulk Dirac nodes will always remain locally stable and gapless in the single-particle band spectrum, as long as the protecting symmetry is preserved. In this work we find that this single-particle classification for $C_n$-symmetric DSMs will break down to ${\mathbb{Z}}_{n/\text{gcd}(2,n)}$ in the presence of symmetry-preserving electron interactions. Our theory is based on a dimensional reduction strategy which reduces a 3D Dirac fermions to one-dimensional building blocks, i.e., vortex-line modes, while respecting all the key symmetries. Using bosonization technique, we find that there exists a minimal number $N=n/\text{gcd}(2,n)$ such that the collection of vortex-line modes in $N$ copies of DSMs can be symmetrically eliminated via four-fermion interactions. While this gapping mechanism does not have any free-fermion counterpart, it yields an intuitive “electron-trion coupling” picture. By developing a topological field theory for DSMs and further checking the anomaly-free condition, we independently arrive at the same classification results. Our theory paves the way for understanding topological crystalline semimetallic phases in the strongly correlated regime.

Efficient $p$-type doping is essential and challenging for applications of ultra-wide-bandgap AlGaN with a high aluminum content. Although the band-edge-assisted ionization enabled by the superlattice of [0001] origination can reduce the ionization level due to the raised valence band maximum (VBM), the indispensable electric field induced by the spontaneous and piezoelectric polarization is in the entire range of alloy and quite negative to the photoelectric applications. In this work, we propose that band-edge-assisted ionization can be activated even without the universal electric field, which forms an ultralow ionization level of $-0.1$ eV for the ${\mathrm{Al}}_{0.8}{\mathrm{Ga}}_{0.2}\mathrm{N}\left[11\overline{2}0\right]$ digital alloy. The energy reduction is triggered by two factors: the intensive VBM states dominated by the coupling between GaN layers and the intriguing decrease in the unoccupied $p_z$ level, which is induced by a symmetry reduction from $C_{3v}$-like to $C_s$ at the dopant site. The transition of symmetry is triggered by a perturbation from the GaN layers, which is caused by a chainlike coupling of $p_x$ states between nitrogen at the VBM and dopant sites and can be partially canceled depending on the dopant sites. In addition to the conventional band bending in band-edge-assisted ionization, the reduction of ionization level activated by long-ranged orbital coupling offers a new view for efficient $p$-type doping in AlGaN systems.

Employing density functional theory, we explore the energetics, geometries, and electronic properties of Janus WSSe in terms of curvature effects. Total energy calculations on tubular forms of Janus WSSe reveal that Janus WSSe intrinsically possesses a scrolled conformation with a radius of curvature of 2.3 nm or larger. Scrolling releases compressive and tensile strains on the Se and S atoms, respectively, in the flat conformation. Curvature associated with scrolling does not affect the band edge alignment of the Janus WSSe. The internal electric field across a WSSe layer decreases monotonically with increasing curvature.

Fano resonance features asymmetric spectral line shape and holds numerous potential applications such as filters, sensors, lasers, nonlinear optics, and slow-light devices. However, general methods to achieve Fano resonance face substantial challenges because of their limited dynamic parametric tunability. Here, we present a theoretical demonstration of an alternate method to realize dynamically reversing the magnitude and line shape of the Fano profile in anisotropic bilayer metallic gratings by simply manipulating the polarization angle of incident light. By introducing an in-plane shift in one grating layer, the resonant phase of the hybridized plasmon modes formed within the bilayer system transforms between in phase and out of phase with the polarization direction, leading to the asymmetry reversal of the Fano line shape. This concept can be demonstrated and explained by applying a deduced temporal coupled-mode theory, which reveals that the asymmetry parameter $q$ undergoes a sign reversal along with the reversal of Fano line shape. This theory is further examined with the results of numerical simulations, finding excellent agreement between them. Our proposed approach enables completely and dynamically reversing the spectral asymmetry of the Fano resonance in an anisotropic bilayer grating, showing potential for applications involving active control and manipulation of Fano resonance.

In this work we investigated the effects of nanoscale roughness on Derjaguin-Landau-Verwey-Overbeek (DLVO) forces using the proximity force approximation. To that end, the force between a gold-coated sphere and a plate, both immersed in pure and salt-added $\left[\mathrm{NaCl}\right]=0.01M$ ethanol, was measured as a function of surface roughness with atomic force microscopy. The force data were compared with the predictions of DLVO theory with and without incorporating the contribution of surface roughness. It was observed that the surface roughness does not modify the qualitative behavior of the force-displacement curve but rather has a secondary influence by altering the values of the free parameters of the DLVO theory, namely, the Debye length and the electrostatic potential of the interacting bodies. In fact, it was found that the repulsive component of the force in pure ethanol increased with increasing roughness. These observations coincide with a positive correlation of the absolute value of the surface potentials with increasing surface roughness, suggesting that roughness effectively enhances the electric field near the surfaces, resulting in stronger electrostatic forces. For the salt-added solution, the contributions of the electrostatic forces are heavily suppressed, and the Casimir contribution plays the dominant role. Finally, we have shown that failure to incorporate the complete optical properties of the interacting metal bodies yields unphysical results for the free parameters of the electrostatic double-layer force.

The impact of the growth orientation on spin dynamics in GaAs/AlGaAs quantum wells (QWs) is explored through magnetophotoluminescence measurements. Samples grown on both (100) and (311)A GaAs surfaces exhibit intensity oscillations at high magnetic fields, characteristic of magnetoabsorption arising from interband transitions between Landau levels when excited near resonance. Notably, the (311)A surface reveals optical responses rarely observed on the conventional (100) surface, including an inversion in circular polarization degree influenced by both excitation power and temperature. To elucidate these findings, we develop an eight-level model encompassing electron and hole excited states. Our analysis demonstrates that thermalization is the primary and sufficient factor responsible for reproducing spin inversion with external parameters.

Pump-probe experiments with femtosecond time resolution can be used to disentangle the electronic dynamics from the lattice response and thus provide valuable insights into the nonequilibrium behavior of correlated materials. In Cr-doped ${\mathrm{V}}_2{\mathrm{O}}_3$, a multiorbital Mott-Hubbard material which has been intensively investigated for decades, time-resolved experiments reported a photoinduced insulator-metal transition leading to a transient metal state with nonthermal properties. Here, we combine nonequilibrium dynamical mean-field theory with realistic first-principles modeling to simulate the ultrafast response of this material to a laser excitation. Our calculations reproduce the insulating initial state, with orbital occupations in agreement with experiment, and reveal an ultrafast pump-induced gap filling associated with a charge reshuffling between the $e_g^{\pi }$ and $a_{1g}$ orbitals. However, in contrast to the related compound ${\mathrm{VO}}_2$, the electronic system thermalizes within a few tens of femtoseconds and we find no evidence for the existence of a metastable nonthermal metal. This suggests that the reported nonthermal behavior in the experiments may be associated with the mismatch between the electronic and lattice temperatures.

We study a relationship between conformally invariant boundary conditions and anomalies of conformal field theories (CFTs) in $1+1$ dimensions. For a given CFT with a global symmetry, we consider symmetric “gapping potentials”, which are relevant perturbations to the CFT. If a gapping potential is introduced only in a subregion of the system, it provides a certain boundary condition to the CFT. From this equivalence, if there exists a Cardy boundary state, which is invariant under a symmetry, then the CFT can be gapped with a unique ground state by adding the corresponding gapping potential. This means that the symmetry of the CFT is anomaly free. Using this approach, we systematically deduce the anomaly-free conditions for various types of CFTs with several different symmetries. They include the free compact boson theory, Wess-Zumino-Witten models, and unitary minimal models. When the symmetry of the CFT is anomalous, it implies a Lieb-Schultz-Mattis type ingappability of the system. Our results are consistent with, where available, known results in the literature. Moreover, we extend the discussion to other symmetries including spin groups and generalized time-reversal symmetries.