We report multifrequency electron spin resonance spectroscopy studies of $\alpha$-MnTe in magnetic fields up to 16 T, applied along the easy anisotropy axis. At temperatures below $T_{\mathrm{N}}=310\mathrm{K}$, we observe a single resonance line corresponding to the pseudo-Goldstone mode of the antiferromagnetic resonance (AFMR). This mode exhibits the isotropic behavior with $g_{\mathrm{eff}}=2.01$, consistent with a complete quench of the orbital angular momenta for ${\mathrm{Mn}}^{2+}$ ions. At low temperatures, the resonance mode is remarkably narrow ($\sim 50\mathrm{mT}$ for the full width at the half-maximum at 5 K). The AFMR mode exhibits substantial broadening with increasing temperature, which can be understood in terms of the magnon-magnon scattering.

We present the theory and experimental demonstration of a topological classification of finite tight binding Hamiltonians with chiral symmetry. Our classification can be applied to randomly connected chiral networks, and regular lattices, giving a significant generalization of the known classification. Using the graph-theoretic notion of complete matchings, we show that many chiral tight binding structures can be divided into a number of sections, each of which has independent topological phases. Hence, the overall classification is $N{\mathbb{Z}}_2$, corresponding to $2^N$ distinct phases, where $N$ is the number of sections with a nontrivial ${\mathbb{Z}}_2$ classification. In our classification, distinct topological phases are separated by exact closures in the energy spectrum of the Hamiltonian, with degenerate pairs of zero-energy states. We show that these zero-energy states have an unusual localization across distinct regions of the structure, determined by the manner in which the sections are connected together. We use this localization to provide an experimental demonstration of the validity of the classification, through radio-frequency measurements on a coaxial cable network, which maps onto a tight-binding system. The structure we investigate is a cable analog of an ideal graphene ribbon, which divides into four sections and has a $4{\mathbb{Z}}_2$ topological classification.

The ability to control the magnetic state provides a powerful means to tune the underlying band topology, enabling transitions between distinct electronic phases and the emergence of novel quantum phenomena. In this work, we address the evolution of the ferromagnetic state in ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ upon applying external pressures up to 10.8 GPa using a combined experimental and theoretical study. The standard ab initio density functional theory computation, including ionic relaxations grossly overestimates the unit cell magnetization as a function of pressure. In our theoretical analysis we identify two possible mechanisms to remedy this shortcoming. Matching the experimental observations is achieved by a symmetry-preserving adjustment of the sulfur atom positions within the unit cell. Alternatively, we explore various combinations of the exchange and correlation parts of the effective potential that reproduce the experimental magnetization, the structural parameters and the measured optical conductivity spectra. Thus, the pressure-dependent behavior of the magnetization demands a careful theoretical treatment and analysis of both theoretical and experimental data.

We present a non-Markovian theory of muon spin relaxation that treats the implanted muon as an open quantum spin coupled to a temporally correlated local magnetic environment. Using a Schwinger-Keldysh influence-functional formulation, we derive a stochastic equation of motion for the muon spin in which the fluctuation kernel is fixed by the local-field correlation tensor, while the retarded memory torque describes an effective environmental backaction. This formulation extends conventional Markovian and strong-collision descriptions to complex magnetic environments where quasistatic broadening, ion-driven colored field fluctuations, and unresolved retarded environmental response coexist and cannot be reduced to a single local-field renewal process. Within the established limits, the theory reduces to standard Kubo-Toyabe forms. This unified framework enables quantitative global analysis of zero-field and weak longitudinal-field $\mu \mathrm{SR}$ spectra, providing a consistent description of field-dependent line shapes with single parameter set at each temperature. Applied to ${\mathrm{Li}}_{0.73}{\mathrm{CoO}}_2$, the model captures the interplay between quenched Gaussian width and a Li-driven dynamical component yielding fluctuation rates consistent with activated behavior. The fitted memory parameter is most visible in the crossover between quasistatic and fast-fluctuation limits, providing a systematic basis for modeling ion-driven magnetic fluctuations in functional materials.

We develop a quantum statistical theory of phonon-mediated dislocation drag in chemically disordered alloys. Starting from a configuration-dependent harmonic Hamiltonian with Kanzaki coupling, we integrate out the lattice exactly via a Keldysh influence functional and obtain, for each disordered realization, a causal retarded memory kernel for the collective glide coordinate. The drag coefficient required by continuum mobility laws is the zero-frequency limit of this kernel, rather than an empirical input, while the full kernel provides a principled non-Markovian mobility law. To make the theory tractable for large disordered supercells with ab initio methods and machine-learned interatomic potentials, we introduce a hybrid embedding scheme that retains the disorder-sensitive Kanzaki vertex in an inner atomistic region and couples it to an outer effective medium description for far-field phonon radiation. We prove that any procedure which homogenizes the lattice before evaluating the quadratic dissipation functional produces a strict lower bound on the disorder-averaged drag, missing a non-negative variance contribution that is discarded by construction. Calculations for bcc $\mathrm{AlWMo}+\mathrm{TiFe}$, and for face-centered cubic $\mathrm{FeCoNi}+\mathrm{CuCr}$ systems show that this variance term is typically dominant: virtual crystal estimates underpredict drag by factors of 2 to 8 and suppress the screw versus edge hierarchy predicted by the full theory. The composition dependence is strongly nonmonotonic, demonstrating that chemical complexity alone is not a reliable proxy for drag and that extrapolations based on bulk elastic descriptors or Leibfried-type scaling can fail. Temperature dependence follows analytically from Bose reweighting of a single equilibrium spectral density, recovering the high-temperature linear regime with a slope set by an infrared-weighted spectral moment that remains sensitive to core-level chemistry. The resulting Stieltjes pole parameters provide a complete, passive, non-Markovian mobility input for mesoscale dislocation dynamics in concentrated alloys, with explicit configurational uncertainty quantification and without driven molecular dynamics, thereby providing a complete representation of dislocation mobility in disordered alloys without the need for costly molecular dynamics simulations.

We develop a microscopic quantum theory of surface plasmon polaritons valid for arbitrary metal-dielectric geometries. Our framework is based on the Power-Zienau-Woolley representation of quantum electrodynamics, which provides an optimal separation between electronic and photonic degrees of freedom and is therefore particularly well suited for constructing quantum descriptions of polaritonic excitations in strongly dispersive media. Within this formulation, the fundamental electronic oscillator is identified as the bulk plasmon mode, which is nonperturbatively coupled to the radiative continuum of free photon modes. This coupling induces a geometry-dependent renormalization of the bulk plasma frequency, giving rise to confined plasmonic resonances. As specific applications, we recover the localized surface plasmon modes of metallic nanoparticles, including radiative frequency shifts and decay, as well as the exact dispersion relation of propagating surface plasmon polaritons at planar interfaces. Our quantum treatment further reveals that light-matter interactions at metal-dielectric interfaces are inherently in the ultrastrong coupling regime. As a result, in the quasistatic limit, the system exhibits unconventional ground-state quantum fluctuations that can be controlled through the refractive index. These results open new intriguing perspectives in the field of quantum plasmonics.

Environmental noise can enhance transport, an effect known as environment-assisted quantum transport. Most theoretical studies focus on optimizing system parameters under spatially uniform system–environment coupling. Here, instead, we optimize the environmental noise itself by allowing for site-specific dephasing. We investigate steady-state transport in finite-sized one-dimensional lattices with either ramped or disordered energy landscapes, considering both short- and long-range coherent tunneling. In the absence of environmental effects, these systems can exhibit localization in the thermodynamic limit, resulting in suppressed transport, arising from destructive interference. Using a Lindblad master equation framework, we implement local dephasing that is optimized to maximize the steady-state population flux. We find that for ramp potentials, short-range tunneling favors selective dephasing on alternating sites, whereas long-range tunneling benefits from a dephasing profile whose strength increases with distance from the injection site. In energetically disordered systems, strongly detuned sites require enhanced local dephasing under short-range tunneling to facilitate transport. In all cases, we find that site-optimized dephasing allows higher transport efficiency than uniform dephasing, and it is accompanied by increased spatial delocalization of the steady state. Our results provide microscopic insight into the interplay between coherent dynamics and environmental noise. Specifically, dephasing broadens energy levels locally, helping to overcome detuning and destructive interference. More generally, we establish spatially structured environmental noise as a strategy for controlling both quantum transport and state coherence in open systems.

The discovery of superconductivity in bilayer nickel oxides has revived an intense effort to understand the potential of high-temperature superconductivity in these materials and their relation to cuprate superconductors. In this work, we investigate the growth and properties of bilayer ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ thin films as a function of substrate, oxygen treatment, and applied pressure in order to study the evolution of transport properties. We report epitaxial growth of ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ thin films on ${\mathrm{LaAlO}}_3$ (LAO) (001) and ${\mathrm{SrLaAlO}}_4$ (SLAO) (001) substrates, and the effects of ex situ annealing in a high-pressure furnace under an oxygen-rich environment (15 bar). Transport measurements show that the ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ thin films on LAO(001) exhibit Fermi-liquid-like metallic behavior with a slight Kondo-like upturn at low temperatures, which evolves with the application of modest hydrostatic pressures toward non-Fermi-liquid behavior with a temperature dependence of resistance approaching $\sim T^{1.4}$ at 1.41 GPa. The ability to tune the normal state resistivity of ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ films to display non-Fermi-liquid behavior under such a modest hydrostatic pressure range—only 6–8 % of that typically applied via diamond anvil cell in ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ single crystals to achieve comparable effects—is both noteworthy and unexpected. These findings imply the strong tunability of ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ thin film under epitaxial strain and hydrostatic pressure leading to exotic transport behavior at low temperatures.

The noncollinear antiferromagnets ${\mathrm{Mn}}_{3}X$ $(X=\mathrm{Sn}, \mathrm{Ge})$ emerge as promising spin-current sources with both in-plane and out-of-plane spin polarizations, thereby enabling field-free magnetization switching. However, the microscopic origin of the out-of-plane spin polarization remains under debate, specifically whether it arises from the magnetic spin Hall effect (MSHE) or the spin swapping (SSW). Here, we comparatively evaluate the spin torques in single-crystal ${\mathrm{Mn}}_3\mathrm{Ge}/\mathrm{Py}$ bilayers with different crystallographic orientations using the ferromagnetic resonance technique. The distinct angular dependences of the measured spin-torque signals provide clear evidence for the bulk MSHE, which depends on antiferromagnetic order. In addition, we identify the antiferromagnetic-order independent component originating from the interfacial SSW. The coexisting MSHE and SSW, with comparable magnitudes, give rise to the out-of-plane spin polarization. Our study disentangles the origins of spin-torque generation in noncollinear antiferromagnets, providing valuable insights for their spintronic applications.

Magnonic excitations are investigated in chromium oxychloride (CrOCl), a van der Waals (vdW) antiferromagnet prone to a multitude of magnetic phase transitions, with absorption experiments in a broad continuous energy range. At low magnetic fields, the magnon spectra show a strong biaxial anisotropy and inform on the relative weights of the effective exchange coupling and the system anisotropies. As the magnetic field increases, magnons characteristic of a canted phase are first observed, with peculiarities attributed to in-plane anisotropies and magnon-magnon coupling. Subsequently, a hysteretic magnon spectrum appears as the system transitions to a ferrimagnetic state, with two new magnon branches partly coexisting with the lower-energy canted phase branch, indicating the formation of spatially separated magnetic phases. Further changes in the magnon spectrum in higher magnetic fields accompany transitions to the different canted magnetic phases previously reported. Our experiments show that competing exchange interactions and ground states broaden the options to generate different kinds of magnonic excitations in the same vdW material upon the variation of external parameters.

${\mathrm{ErMgGaO}}_4$ is a quantum antiferromagnet wherein the pseudospin-1/2 degrees of freedom of ${\mathrm{Er}}^{3+}$ decorate two-dimensional triangular planes separated by disordered nonmagnetic bilayers of ${\mathrm{Mg}}^{2+}$ and ${\mathrm{Ga}}^{3+}$. Its sister compound, ${\mathrm{YbMgGaO}}_4$, has attracted much interest as a quantum spin liquid ground state candidate, although the presence of the disordered Mg-Ga bilayers adds complexity to this description. In contrast, our powder ${\mathrm{ErMgGaO}}_4$ sample shows a clear spin glass transition near $T_g\sim 2.5\mathrm{K}$, about 1/6 of its Curie-Weiss temperature. We have carried out new inelastic neutron scattering measurements on these powder ${\mathrm{ErMgGaO}}_4$ samples. At high energies, these show the expected crystalline electric field (CEF) transitions within the $J=15/2$ multiplet of ${\mathrm{Er}}^{3+}$, but with the first excited CEF level sufficiently low in energy ($\sim 3$ meV) so as to allow the possibility that virtual CEF transitions influence the exchange couplings. At zero energy transfer, we observe diffuse elastic scattering which is analyzed using Warren line shapes appropriate for two- dimensional (2D) correlations. This reveals dominant 2D stripy correlations below $T_g$, coexisting with 2D ${120}^{\circ }$-type correlations that persist above $T_g$. At low temperatures, the low energy inelastic component of the scattering shows a continuum with bandwidth of $\sim 0.8$ meV. This dynamic magnetic spectral weight can be modeled at all $Q$, energies, and temperatures as the sum of high energy and low energy damped harmonic oscillators (DHO), with the high energy DHO defining the bandwidth of $\sim 0.8$ meV. We use linear spin wave theory to model this inelastic scattering and to estimate its spin Hamiltonian parameters in terms of a $J_1-J_2-\mathrm{\Delta }$ model on the triangular lattice. This gives a good description of the low lying spectral weight for ErMgGaO4, and allows us to place it on the theoretical $J_1-J_2-\mathrm{\Delta }$ phase diagram with $\frac{J_2}{J_1}=0.13\pm 0.03$ and $\mathrm{\Delta }=0.4\pm 0.1$, which is close to the expected quantum phase boundary between the spin liquid and the stripy ordered phases.

In this work, we investigate zero-temperature energy transport in a gapless quantum magnet. We find that suppressed energy transport can arise at low frequencies due to transient localization from fractionalization, even in the absence of extrinsic defects or disorder. Concretely, we consider a Kitaev ladder model, whose spin degrees of freedom fractionalize into visons and spinons, in a uniform magnetic field. For moderate magnetic fields, visons are heavy and act as quasistatic disorder that induces transient localization of light spinons even in the translation-invariant model and at zero temperature, which strongly suppresses the residual transport.

Motivated by the notion that a preponderance of Coulomb interactions might lead to hydrodynamics, we carry out an ab initio calculation of the charge-carrier transport properties of the electron-hole gas of doped graphene. We include both the phonon and Coulomb interactions within a momentum and band-resolved Boltzmann transport formalism. We find that under suitable conditions, the strong Coulomb drag effect induces phenomena like negative conductivity and joint electron-hole hydrodynamics (bifluidity) in the system. We also identify the exclusive electron or hole hydrodynamics. We find that there is a strong violation of the Wiedemann-Franz law in the low-doped regimes. Our work elucidates the roles of the microscopic scattering mechanisms that drive the hydrodynamic phenomena.

Low-frequency (0.05–40 Hz) dynamic elastic measurements and resonant ultrasound spectroscopy measurements (100–600 kHz) of RbSCN were performed in the temperature region of the order-disorder improper ferroelastic phase transition at $T_c\approx$ 435 K. Quite similar to KSCN, the low-frequency data show, in addition to the intrinsic phase transition anomalies, superelastic softening in the $a$ and $b$ directions, resulting from movements of ferroelastic domain walls under dynamic stress. However, in contrast to KSCN, a sudden discontinuous increase of Young's modulus appears in RbSCN at $T^{*}<T_c$, which is accompanied by a frequency-dependent damping peak. This behavior is reminiscent of a first-order phase transition. Heating RbSCN slightly above $T^{*}$, followed by subsequent cooling, removes all signs of domain wall dynamics. The results demonstrate that the anomalies in RbSCN around $T^{*}$ result from collective domain switching events that are induced when the temperature-dependent critical pinning stress ${\sigma }_c\left(T\right)$ falls below the applied external stress $\sigma$, implying that $T^{*}(\sigma ={\sigma }_c)$. This interpretation is supported by calculations of the temperature dependences of twin boundary widths $w$ and energies $F_w$, as well as the Peierls potential $V_0$ using a compressible pseudospin model, which leads to a critical pinning stress ${\sigma }_c\left(T\right)$ that is in excellent agreement with experimental values of $T^{*}\left({\sigma }_c\right)$.

Two-dimensional magnets provide fertile platforms for achieving high-temperature ferromagnetism, desirable perpendicular magnetic anisotropy, and tunable topological transport, enabling robust magnetic stability and efficient spin-charge conversion in spintronic devices. However, these properties are usually weakened by substrates. Here, we systematically study supported $\mathrm{Gd}{M}_2/M$(111) ($M=\mathrm{Cu}$, Ag, Au) surface alloys and freestanding monolayer $\mathrm{Gd}{M}_2$, using first-principles calculations. For supported systems, substrate-induced structural buckling and charge redistribution weaken their $M$-Gd (s, p)-d-mediated exchange interactions, leading to weakened ferromagnetism and low Curie temperatures (below 100 K). Upon removing the substrates, charge redistribution enhances the effective exchange couplings in freestanding monolayer $\mathrm{Gd}{M}_2$, and the elimination of buckling further strengthens ferromagnetism, thereby yielding Curie temperatures above room temperature. Notably, freestanding monolayer $\mathrm{Gd}{\mathrm{Au}}_2$ exhibits a pronounced out-of-plane magnetic anisotropy driven by the orbital redistribution near the Γ point. Combined with symmetry-tunable band topology, this anisotropy gives rise to a sizable anomalous Hall conductivity in freestanding monolayer $\mathrm{Gd}{\mathrm{Au}}_2$. Our results establish the connection between interfacial coupling, lattice geometry, electronic structure, and magnetic and topological properties, enabling the engineering of two-dimensional high-temperature spintronic functionalities.

With the advent of atomically thin and tunable van der Waals materials, a two-dimensional electronic Wigner crystal has recently been observed. The smoking gun signal was the appearance of an umklapp branch in optical exciton spectroscopy coming from the periodic potential generated by the Wigner crystal assumed to be static. Vibrations of the Wigner crystal, however, lead to a gapless phonon spectrum, which may affect the exciton spectrum. To explore this, we develop a field-theoretical description of an exciton interacting with electrons forming a Wigner crystal including the coupling to the phonons. We show that the importance of the exciton-phonon coupling scales with the exciton-electron interaction strength relative to the typical phonon energy squared. The motion of the exciton leads to two kinds of scattering processes, where the exciton emits a phonon either staying within the same Bloch band (intraband scattering) or changing its band (interband scattering). Using a nonperturbative self-consistent Born approximation, we demonstrate that these scattering processes lead to the formation of quasiparticles (polarons) consisting of the exciton in Bloch states dressed by Wigner crystal phonons. The energy shift and damping of these polarons depend on the electron density in a nontrivial way since it affects both the exciton-phonon interaction strength as well as the phonon and exciton spectra. In particular, the damping is strongly affected by whether the polaron energy is inside the gapless phonon scattering continuum or not. Using these results, we finally analyze their effects on the observed spectral properties of the exciton.

Superconducting diodes promise dissipationless rectification, yet equilibrium platforms without engineered junctions typically exhibit modest efficiencies. We identify a general thermodynamic origin of this behavior that is largely independent of microscopic details. Denoting $\epsilon =|I_c^{-}/I_c^{+}|$, where $I_c^{\pm }$ are critical currents in opposite directions with $|I_c^{+}|>|I_c^{-}|$ by convention, we show that an exact perfect equilibrium diode $(\epsilon =0)$ is forbidden by continuity of the free energy. Asymptotically perfect behavior $\epsilon \to 0$ is possible within a global equilibrium description, but requires free-energy singularities to enable softening of the unfavorable current-carrying branch. We demonstrate this explicitly in an exactly solvable Ising superconductor model. For smooth single-branch superconductors, standard finite-order polynomial Landau theory yields finite lower bounds on $\epsilon$, while Josephson systems obey analogous bounds set by finite harmonic content of the current-phase relation. The high efficiencies often reported in Josephson platforms arise because such devices commonly operate under phase constraints, metastable switching, or external drive, thereby evading unconstrained equilibrium limits. Thus, our results provide a unified framework for interpreting diode efficiencies across superconducting platforms.

It has been proposed that Floquet topological states can be used to generate period-doubled oscillations that break discrete time-translation symmetry. This behavior originates from topological $\pi$ modes—boundary states pinned at half the driving frequency. Although such oscillations resemble those in time crystals, the underlying mechanisms are different: the period-doubled Floquet edge states arise from a single-particle Hamiltonian and are protected by the space-time symmetries of the lattice. Recently, period-quadrupled oscillations have been realized in acoustic and photonic systems, governed by $\pi /2$ modes—boundary states pinned at one quarter of the driving frequency. These studies establish the sequence $2T$ to $4T$, where $T$ is the driving period, and further generalizations within the same framework lead to $8T, 16T$, and other powers of two, rather than to $3T$ or $6T$. In this work, we realize another set of edge states—$2\pi /3$ and $\pi /3$ modes, with quasienergies pinned at $\pm 2\pi /3T$ and $\pm \pi /3T$. Using designed acoustic waveguides, we observe $3T$- and $6T$-periodic responses from an initially localized boundary excitation, providing evidence for the existence of these states.

Periodically driven systems host topological phases without static analogs, such as the anomalous Floquet phase characterized by trivial bulk bands yet robust boundary modes. In this work, we investigate the scattering problem of a Floquet Chern insulator and reveal the non-Hermitian skin effect (NHSE) of reflected waves. Using a discrete-time scattering formalism, we demonstrate how the non-Hermitian winding number of the reflection matrix is linked to the bulk Floquet invariant via boundary resonances. This reflected-wave NHSE relies on which quasienergy gap the incident wave resides in, leading to a gap-dependent Goos-Hänchen (GH) shift. We further show that the momentum-integrated GH shift quantitatively yields the Floquet topological invariant of the corresponding gap. Our work highlights a frequency-dependent NHSE of reflected waves in driven systems and provides a real-space scattering approach to identify nonequilibrium topology.

Even-denominator quantum Hall states are leading candidates for realizing non-Abelian topological orders, with the $\nu =\frac{5}{2}$ plateau in GaAs the first and most-studied example. Recent experiments in GaAs and bilayer graphene (BLG) have observed many ‘next-generation’ even-denominator states at filling factors such as $\nu =\frac{3}{4}, \frac{3}{8}$, and $\frac{3}{10}$. We develop the theory of these states, including analyses of their bulk quasiparticles, of methods for distinguishing between pairing channels in edge transport measurements, and of their trial wave functions. As part of this study, we derive general relations of how flux attachment affects many universal properties of states. In particular, we prove that the topological stability of interface modes is invariant under flux attachment. We compare next-generation paired states to Bonderson-Slingerland states at the same filling factors, and demonstrate that their quasiparticles carry identical charges and obey the same exchange statistics. The next-generation and Bonderson-Slingerland states still describe distinct phases, and we find that the former are energetically favored in the lowest Landau level, while the latter are favored in the first excited level.