Transport of charge and valley (axial) degrees of freedom coupled to order-parameter dynamics in magnetic Weyl semimetals is studied in the framework of nonequilibrium thermodynamics. In addition to the established valley-related transport anomalies that are rooted in band-structure topology, we construct dissipative couplings between the three dynamic constituents of the system driven out of equilibrium by electromagnetic perturbations. We show how the valley degree of freedom mediates an effective coupling between the charge and magnetic sectors of the system, through a combination of the chiral anomaly, on the electric side, and the Onsager-paired valley torque and pumping, on the magnetic side. This work complements previous studies of magnetic Weyl semimetals by a more systematic analysis of collective dissipation. We discuss several concrete examples of the valley-mediated current-driven magnetic instabilities and charge pumping, and extend the theory to the antiferromagnetic case.

We report a resonant elastic x-ray scattering study of a thin-film sample of ${\mathrm{Sr}}_2{\mathrm{CrReO}}_6$, which has one of the highest ferrimagnetic transition temperatures among ordered double perovskites. We found resonantly enhanced magnetic Bragg peaks at $\mathbf{Q}=$ (odd, odd, odd) at both the rhenium $L_2$ and $L_3$ edges, which coincide with the structural Bragg peaks of ${\mathrm{Sr}}_2{\mathrm{CrReO}}_6$. By analyzing the widths of these Bragg peaks, we extracted very different structural and magnetic correlation lengths. The former is about 15 nm, while the latter is constrained by the instrumental resolution to be at least 90 nm. We argue that a finite structural correlation length is consistent with the existence of antiphase nanodomains in our sample. On the other hand, a much larger magnetic correlation length indicates that the magnetic correlation extends far beyond the boundaries of individual domains and is consistent with strong antiferromagnetic coupling between different antiphase domains. Last, from the azimuthal dependence of the magnetic intensity, we show that the magnetic moments lie perpendicular to the $c$ axis, which explains the earlier bulk magnetization data.

The structure inversion asymmetry at surfaces and interfaces gives rise to the Rashba spin-orbit interaction (SOI), that breaks the spin degeneracy of surface or interface states. Hence, when an electric current runs through a surface or interface, this Rashba effect generates an effective magnetic field acting on the electron spin. This provides an additional tool to manipulate the spin state in materials such as Si and Ge possessing inversion symmetry in their bulk form. The existence of Rashba states could be demonstrated by photoemission spectroscopy at the interface between different metals and Ge(111) and by spin-charge conversion experiments at the Fe/Ge(111) interface even though it is made of two light elements. In this paper, we identify the fingerprint of the Rashba states at the Fe/Ge(111) interface by magnetotransport measurements in the form of a large unidirectional magnetoresistance of up to $0.1\%$. From its temperature dependence, we find that the Rashba energy splitting is larger than in pure Ge(111) subsurface states.

Using an analytically solvable model, we show that a qubit array-based detector allows one to achieve the fundamental Heisenberg limit in detecting single photons. In the case of superconducting qubits, this opens other opportunities for quantum sensing and communications in the important microwave range.

In a tilted doped Weyl semimetal there are well-defined ranges of photon energies where there is dichroism. These correspond to regions of energy where the imaginary part of the dynamic Hall conductivity is finite. Such regions can involve contributions from both negative and positive chirality nodes or just one of them. As the temperature $\left(T\right)$ is increased, the boundaries of the regions of finite dichroism become smeared out in energy and extend beyond their original range for $T=0$ and at the same time the dichroism is reduced. When $T$ is increased to be of the order of the chemical potential associated with the doping, the dichroism vanishes completely. We have also extended the work to include the dynamic Hall angle which is found to behave in an analogous way. We treat the case when both chirality nodes have the same value of chemical potential, as well as when the two nodes are displaced in energy by an amount $\pm {\mathcal{Q}}_0$ due to broken inversion symmetry. Further, we have included the impurity effect within the framework of the first-order Born approximation to analyze the absorptive parts of longitudinal and Hall conductivity at a finite temperature.

In this paper we construct a one-dimensional insulator with an approximate chiral symmetry belonging to the AIII class and discuss its properties. The construction principle is the intentional pollution of the edge of a two-dimensional quantum spin Hall insulator with magnetic impurities. The resulting bound states hybridize and disperse along the edge. We discuss under which circumstances this chain possesses zero-dimensional boundary modes on the level of an effective low-energy theory. The main appeal of our construction is the independence on details of the impurity lattice: the zero modes are stable against disorder and random lattice configurations. We also show that in the presence of Rashba coupling, which changes the symmetry class to A, one can still expect localized half-integer boundary excess charges protected by mirror symmetry although there is no nontrivial topological index. All of the results are confirmed numerically in a microscopic model.

We introduce a procedure to determine accurate and precise force-distance curves in dynamic force measurements utilizing a sharp tip. While single force curves are prone to systematic, often unnoticed errors, we present their self-consistent retrieval by an alignment procedure using repetitive measurements with the force probe oscillating at varied amplitude. By processing model data, we show that the procedure provides the valid force curve, the actual oscillation amplitude, and fully compensates thermal drift. The benefit of the method is demonstrated by application to experimental data.

We develop an ab initio formalism for dipolar electron-phonon interactions (EPIs) in two-dimensional (2D) materials. Unlike the purely longitudinal Fröhlich model, we show that the out-of-plane dipoles also contribute to the long-wavelength nonanalytical behavior of EPIs. And the 2D dipolar EPI plays an important role not only in the typical polar material ${\text{MoS}}_2$, but also in graphane and fluorinated graphene. By incorporating this formalism into Wannier-Fourier interpolation, we enable accurate EPI calculations for 2D materials and subsequent intrinsic carrier mobility prediction. The results show that the Fröhlich model is inadequate for 2D materials and a correct long-wavelength interaction must be included for a reliable prediction.

The crystal structure, electronic structure, and transport properties of crystals with the nominal composition ${\mathrm{Nb}}_{0.25}{\mathrm{Bi}}_2{\mathrm{Se}}_3$ are investigated. X-ray diffraction reveals that the as-grown crystals display phase segregation and contain major contributions of BiSe and the superconducting misfit layer compound ${\left(\mathrm{Bi}\mathrm{Se}\right)}_{1.1}\mathrm{Nb}{\mathrm{Se}}_2$. The inhomogeneous character of the samples is also reflected in the electronic structure and transport properties of different single crystals. Angle-resolved photoemission spectroscopy (ARPES) reveals an electronic structure that resembles poor-quality ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ with an ill-defined topological surface state. High-quality topological surface states are instead observed when using a highly focused beam size, i.e., nanoARPES. While the superconducting transition temperature is found to vary between 2.5 and 3.5 K, the majority of the bulk single crystals does not exhibit a zero-resistance state suggesting filamentary superconductivity in the materials. Susceptibility measurements of the system together with the temperature dependence of the coherence length extracted from the upper critical field are consistent with conventional BCS superconductivity of a type II superconductor.

We use Ru $L_3$-edge resonant inelastic x-ray scattering to study the full range of excitations in ${\mathrm{Ca}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ from meV-scale magnetic dynamics through to the eV-scale interband transitions. This bilayer $4d$-electron correlated metal expresses a rich phase diagram, displaying long-range magnetic order below 56 K followed by a concomitant structural, magnetic, and electronic transition at 48 K. In the low-temperature phase, we observe a magnetic excitation with a bandwidth of $\sim 30$ meV and a gap of $\sim 8$ meV at the zone center, in excellent agreement with inelastic neutron scattering data. The dispersion can be modeled using a Heisenberg Hamiltonian for a bilayer $S=1$ system with single-ion anisotropy terms. At a higher energy loss, $dd$-type excitations show heavy damping in the presence of itinerant electrons, giving rise to a fluorescencelike signal appearing between the $t_{2g}$ and $e_g$ bands. At the same time, we observe a resonance originating from localized $t_{2g}$ excitations, in analogy to the structurally related Mott insulator ${\mathrm{Ca}}_2\mathrm{Ru}{\mathrm{O}}_4$. But whereas ${\mathrm{Ca}}_2\mathrm{Ru}{\mathrm{O}}_4$ shows sharp separate spin-orbit excitations and Hund's-rule driven spin-state transitions, here we identify only a single broad asymmetric feature. These results indicate that local intraionic interactions underlie the correlated physics in ${\mathrm{Ca}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$, even as the excitations become strongly mixed in the presence of itinerant electrons.

The exciton valley dynamics in van der Waals heterostructures with transition metal dichalcogenide monolayers is driven by the long-range exchange interaction between the electron and the hole in the exciton. It couples the states active in the opposite circular polarizations resulting in the longitudinal-transverse splitting of excitons propagating in the monolayer plane. Here we study theoretically the effect of the dielectric environment on the long-range exchange interaction and demonstrate how the encapsulation in hexagonal boron nitride modifies the exciton longitudinal-transverse splitting. We calculate the exciton spin-valley polarization relaxation due to the long-range exchange interaction and demonstrate that the variation of the monolayer environment results in significant, up to fivefold, enhancement of the exciton valley polarization lifetime.

A recent work [Balram, Jain, and Barkeshli, Phys. Rev. Res. 2, 013349 (2020)] has suggested that an unconventional state describing ${\mathbb{Z}}_n$ superconductivity of composite bosons, which supports excitations with charge $1/\left(3n\right)$ of the electron charge, is energetically better than the Laughlin wave function at $\nu =7/3$ in GaAs systems. All experiments to date, however, are consistent with the latter. To address this discrepancy, we study the effect of finite width on the ground state and predict a phase transition from an unconventional ${\mathbb{Z}}_n$ state at small widths to the Laughlin state for widths exceeding $\sim 1.5$ magnetic lengths. We also determine the parameter region where an unconventional state is stabilized in the one-third filled zeroth Landau level in bilayer graphene. The roles of Landau level mixing and spin are also considered.

The interplay between electron-electron correlations and disorder has been a central theme of condensed matter physics over the past several decades, with particular interest in the possibility that interactions might cause delocalization of an Anderson insulator into a metallic state, and the disrupting effects of randomness on magnetic order and the Mott phase. Here we extend this physics to explore electron-phonon interactions and show, via exact quantum Monte Carlo simulations, that the suppression of the charge density wave correlations in the half-filled Holstein model by disorder can stabilize a superconducting phase. Our simulations thus capture qualitatively the suppression of charge ordered phases and emergent superconductivity recently seen experimentally.

Monolayer graphene provides an ideal material to explore one of the fundamental light-field driven interference effects: Landau-Zener-Stückelberg interference. However, direct observation of the resulting interference patterns in momentum space has not proven possible, with Landau-Zener-Stückelberg interference observed only indirectly through optically induced residual currents. Here we show that the transient electron momentum density (EMD), an object that can easily be obtained in experiment, provides an excellent description of momentum resolved charge excitation. We employ state-of-the-art time-dependent density function theory calculations, demonstrating by direct comparison of EMD with conduction band occupancy, obtained from projecting the time propagated wave function onto the ground state, that the two quantities are in excellent agreement. For even the most intense laser pulses we find the electron dynamics to be almost completely dominated by the $\pi$ band, with transitions to other bands strongly suppressed. Simple model based tight-binding approaches can thus be expected to provide an excellent description for the laser induced electron dynamics in graphene.

Ramp wave study involving graded impactors with discrete or continuous variation of material density is of immense interest for investigation of shock induced structural changes in solids. Hydrodynamic simulation of ramp wave generated by continuously changing functionally graded materials (FGM) requires accurate knowledge of its equation of state (EOS) parameters. However, theoretical as well as experimental EOS data for the same are not readily available. Current work is an attempt towards development of semiempirical EOS model for binary FGM (bFGM) wherein the density is a continuous function of position, either linearly or quadratically. This has been achieved by deriving analytical functions for density dependence of hugoniot parameters using the recently proposed kinetic energy average (KEA) model as well as commonly employed interpolation based methods. Six bFGMs with Al, Mg, and paraffin as low-impedance components and Cu and W as high-impedance ones are considered for the present study. It is shown that shock impedance of linear and quadratic bFGM can be expressed as superlinear/superquadratic functions of position. Subsequently, hydrodynamic simulation results of impact loading of thus constructed bFGMs on ultrathin Ta target are reported. The time profile of target pressure displayed signatures of quasi-isentropic compression (shock ramp). Ramp profiles obtained in our simulation are compared with those generated by recently reported [Phys. Rev. B 99, 214105 (2019)] alternate material bilayer graded density impactor (blGDI) with programmable layer thicknesses. Time profile of ramp pulse is shown to have direct correlation with spatial profile of shock impedance. Influence of associated physical parameters, such as impedance of front layer material, impedance ratio of two components, impact velocity, and mixture model of EOS on shock-ramp adiabats are explored in great detail. The merits of using low and high impedance material as one component of bFGM or blGDI in lowering target heating and enhancing peak ramp pressure are investigated theoretically. Study reveals that paraffin based bFGM with quadratic density function produces the lowest temperature and entropy.

We present a detailed study of the magnetic and electronic properties of ${\mathrm{U}}_2{\mathrm{Rh}}_3{\mathrm{Si}}_5$, a material that has been demonstrated to exhibit a first-order antiferromagnetic phase transition. From a high-magnetic-field study, together with extensive experiments in moderate fields, we establish the magnetic phase diagrams for all crystallographic directions. The possibility of an electronic phase in a narrow interval above the Néel temperature as a precursor of a magnetic phase is discussed.

Ultrathin two-dimensional (2D) ferromagnets with intrinsic half-metallicity are highly prospective in designing nanoscale spintronics devices. In this work, we systematically investigate the spin transport and dynamical properties of one such group of promising 2D ferromagnets—monolayer iron dihalides (${\mathrm{FeX}}_2$, X = Cl, Br, I)—using density functional theory (DFT). First, we explore the spin transport properties of these ${\mathrm{FeX}}_2$ monolayers by combining the nonequilibrium Green's function (NEGF) technique with DFT. This study shows an inherent half-metallicity with a large spin gap that offers $100\%$ spin-polarization over a wide Fermi window (>1 eV). We then focus on understanding their magnetocrystalline anisotropy, Gilbert damping, and exchange interactions, in-depth, which are the key aspects in controlling the spin dynamics. We use force theorem to determine the magnetocrystalline anisotropy and Kambersky's torque-torque correlation model for Gilbert damping. Our calculations reveal a sizable perpendicular anisotropy (0.04 to 0.25 mJ/${\mathrm{m}}^2$) along with a relatively low Gilbert damping ($7.9\times {10}^{-5}$ to $3.7\times {10}^{-4}$) in these materials. Using spin-polarized Green's function formalism, we finally explore the effective exchange interactions in these materials and determine their spin-wave stiffness, exchange stiffness constants, and Curie temperatures. All these calculations, collectively, provide significance of these 2D ${\mathrm{FeX}}_2$ ferromagnets for next-generation spintronics applications.

The celebrated Kitaev honeycomb model provides an analytically tractable example with an exact quantum spin liquid ground state. While in real materials, other types of interactions besides the Kitaev coupling ($K$) are present, such as Heisenberg ($J$) and symmetric off-diagonal ($\mathrm{\Gamma }$) terms, these interactions can also be generalized to a triangular lattice. Here, we carry out a comprehensive study of the $J-K-\mathrm{\Gamma }$ model on a triangular lattice covering the whole parameter region, using a combination of exact diagonalization, classical Monte Carlo, and analytic methods, with an emphasis on the effects of the $\mathrm{\Gamma }$ term. In the HK limit ($\mathrm{\Gamma }=0$), we find five quantum phases which are quite similar to their classical counterparts. Among them, stripe-A and dual Néel phases are robust against the introduction of the $\mathrm{\Gamma }$ term, in particular, stripe A extends to the region connecting $K=-1$ and $K=1$ for $\mathrm{\Gamma }<0$. Though the ${120}^{\circ }$ Néel phase also extends to a finite $\mathrm{\Gamma }$, its region is largely reduced compared to the previous classical result. Interestingly, the ferromagnetic (dubbed FM-A) and stripe-B phases are unstable in response to an infinitesimal $\mathrm{\Gamma }$ interaction. Moreover, we find five additional phases for $\mathrm{\Gamma }\ne 0$ which are elaborated by both the quantum and classical numerical methods. Part of the parameter space previously identified as the ${120}^{\circ }$ Néel phase in the classical study is found to give way to the modulated stripe phase. Depending on the sign of the $\mathrm{\Gamma }$ term, the FM-A phase transits into FM-B ($\mathrm{\Gamma }>0$) and FM-C ($\mathrm{\Gamma }<0$) phases with different spin orientations. Similarly, the stripe-B phase transits into stripe-C ($\mathrm{\Gamma }>0$) and stripe-A ($\mathrm{\Gamma }<0$) phases. Around the positive $\mathrm{\Gamma }$ point, due to the interplay of the Heisenberg, Kitaev, and $\mathrm{\Gamma }$ interactions, we find a possible quantum spin liquid in a noticeable region with a continuum in spin excitations.

We report a systematic study of the temperature and field dependences of the spin Seebeck effect (SSE) in a bilayer of $\mathrm{Pt}/{\mathrm{Gd}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$. An anomalous structure is observed in the magnetic field dependent measurements at temperatures between ∼60 and ∼210 K. Unlike the ordinary SSE signal originating from the bare magnons, which changes sign at ∼95 and ∼266 K, the sign of the anomalies remains unchanged with increasing temperature. Moreover, the anomalies are found to show a temperature-sensitive double-peak structure between ∼116 and ∼143 K. We attribute these anomalies to the contribution from the quasiparticles hybridized between the Gd moment dominated spin wave (α mode) and the transversal acoustic phonon, known as the magnon polarons. Given that the magnon polaron induced anomalies occur at the field where the linear phonon dispersion is tangential to the magnon dispersion curve, we explain these rich phenomena by an increase of the group velocity of the α-mode magnon with increasing temperature and the nonparabolic magnon dispersion of ${\mathrm{Gd}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$. Our results demonstrate that the magnon polaron induced SSE is helpful for the investigation of the magnon dispersion evolution with a simple transport approach.

This study examines the effect of distorted triangular magnetic interactions in the kagome lattice. Using a Holstein-Primakoff expansion, we determine the analytical solutions for classical energies and the spin-wave modes for various magnetic configurations. By understanding the magnetic phase diagram, we characterize the changes in the spin waves and examine the spin distortions of the ferromagnetic, antiferrimagnetic, and ${120}^{\circ }$ phases that are produced by variable exchange interactions and lead to various noncollinear phases, which provides a deeper understanding of the magnetic fingerprints of these configurations for experimental characterization and identification.