We explore theoretically the magnetoresistance of Weyl semimetals in transversal magnetic fields away from charge neutrality. The analysis within the self-consistent Born approximation is done for the two different models of disorder: (i) short-range impurities and (ii) charged (Coulomb) impurities. For these models of disorder, we calculate the conductivity away from charge neutrality point as well as the Hall conductivity, and analyze the transversal magnetoresistance (TMR) and Shubnikov–de Haas oscillations for both types of disorder. We further consider a model with Weyl nodes shifted in energy with respect to each other (as found in various materials) with the chemical potential corresponding to the total charge neutrality. In the experimentally most relevant case of Coulomb impurities, we find in this model a large TMR in a broad range of quantizing magnetic fields. More specifically, in the ultraquantum limit, where only the zeroth Landau level is effective, the TMR is linear in magnetic field. In the regime of moderate (but still quantizing) magnetic fields, where the higher Landau levels are relevant, the rapidly growing TMR is supplemented by strong Shubnikov–de Haas oscillations, consistent with experimental observations.

We discuss generation of series expansions for Ising spin glasses with a symmetric $\pm J$ (i.e., bimodal) distribution on $d$-dimensional hypercubic lattices using linked-cluster methods. Simplifications for the bimodal distribution allow us to go to higher order than for a general distribution. We discuss two types of problems, one classical and one quantum. The classical problem is that of the Ising spin glass in a longitudinal magnetic field $h$, for which we obtain high temperature series expansions in variables $tanh(J/T)$ and $tanh(h/T)$. The quantum problem is a $T=0$ study of the Ising spin glass in a transverse magnetic field $h_T$ for which we obtain a perturbation theory in powers of $J/h_T$. These methods require (i) enumeration and counting of all connected clusters that can be embedded in the lattice up to some order $n$, and (ii) an evaluation of the contribution of each cluster for the quantity being calculated, known as the weight. We discuss a general method that takes the much smaller list (and count) of all no free-end (NFE) clusters on a lattice up to some order $n$ and automatically generates all other clusters and their counts up to the same order. The weights for finite clusters in both cases have a simple graphical interpretation that allows us to proceed efficiently for a general configuration of the $\pm J$ bonds and at the end perform suitable disorder averaging. The order of our computations is limited by the weight calculations for the high-temperature expansions of the classical model, while they are limited by graph counting for the $T=0$ quantum system. Details of the calculational methods are presented.

We report a combined analytical and density matrix renormalized group study of the antiferromagnetic $XXZ$ spin-$1/2$ Heisenberg chain subject to a uniform Dzyaloshinskii-Moriya (DM) interaction and a transverse magnetic field. The numerically determined phase diagram of this model, which features two ordered Ising phases and a critical Luttinger liquid, one with fully broken spin-rotational symmetry, agrees well with the predictions of Garate and Affleck [I. Garate and I. Affleck, Phys. Rev. B 81, 144419 (2010)]. We also confirm the prevalence of the $N^z$ Néel Ising order in the regime of comparable DM and magnetic field magnitudes.

The magnetic structure of tetragonal ${\mathrm{UAu}}_2{\mathrm{Si}}_2$ was investigated by single-crystal neutron-diffraction experiments. Below $T_{\mathrm{N}}$= 20 K it orders antiferromagnetically with a propagation vector of $\mathit{k}=(2/3,0,0)$ and magnetic moments of uranium ions pointing along the tetragonal $c$ axis. Weak signs of the presence of a ferromagnetic component of magnetic moment were traced out. Taking into account a group theory calculation and experimental results of magnetization and ${}^{29}\mathrm{Si}$ nuclear magnetic resonance, the magnetic structure is determined to be a squared-up antiferromagnetic structure, with a stacking sequence ($++-$) of the ferromagnetic $ac$-plane sheets along the $a$ axis. This result highlights similar magnetic correlations in ${\mathrm{UAu}}_2{\mathrm{Si}}_2$ and isostructural ${\mathrm{URu}}_2{\mathrm{Si}}_2$.

In quasi-two-dimensional (quasi-2D) quantum magnets, the ratio of Néel temperature $T_{\text{N}}$ to Curie-Weiss temperature ${\mathrm{\Theta }}_{\text{CW}}$ is frequently used as an empirical criterion to judge the strength of frustration. In this work, we investigate how these quantities are related in the canonical quasi-2D frustrated square or triangular $J_1\text{−}{J}_2$ model. Using the self-consistent Tyablikov approach for calculating $T_{\text{N}}$ we show their dependence on the frustration control parameter $J_2/J_1$ in the whole Néel and columnar antiferromagnetic phase region. We also discuss approximate analytical results. In addition, the field dependence of $T_{\text{N}}\left(H\right)$ and the associated possible reentrance behavior of the ordered moment due to quantum fluctuations are investigated. These results are directly applicable to a class of quasi-2D oxovanadate antiferromagnets. We give clear criteria to judge under which conditions the empirical frustration ratio $f={\mathrm{\Theta }}_{\text{CW}}/T_{\text{N}}$ may be used as measure of frustration strength in the quasi-2D quantum magnets.

The possibility of a fast and precise Abrikosov vortex manipulation by a focused laser beam opens the way to create laser-driven Josephson junctions. We theoretically demonstrate that a vortex pinned in the vicinity of the Josephson junction generates an arbitrary ground state phase which can be equal not only to 0 or $\pi$ but to any desired ${\phi }_0$ value in between. Such ${\phi }_0$ junctions have many peculiar properties and may be effectively controlled by the optically driven Abrikosov vortex. Also we theoretically show that the Josephson junction with the embedded vortex can serve as an ultrafast memory cell operating at sub THz frequencies.

We study the differential conductance for charge tunneling into a semiconductor wire–superconductor hybrid structure, which is actively investigated as a possible scheme for realizing topological superconductivity and Majorana zero modes. The calculations are done based on a tight-binding model of the heterostructure using both a Blonder-Tinkham-Klapwijk approach and a Keldysh nonequilibrium Green's function method. The dependence of various tunneling conductance features on the coupling strength between the semiconductor and the superconductor, the tunnel barrier height, and temperature is systematically investigated. We find that treating the parent superconductor as an active component of the system, rather than a passive source of Cooper pairs, has qualitative consequences regarding the low-energy behavior of the differential conductance. In particular, the presence of subgap states in the parent superconductor, due to disorder and finite magnetic fields, leads to characteristic particle-hole asymmetric features and to the breakdown of the quantization of the zero-bias peak associated with the presence of Majorana zero modes localized at the ends of the wire. The implications of these findings for the effort toward the realization of Majorana bound states with true non-Abelian properties are discussed.

The ability to controllably manipulate the laser-induced ultrafast magnetic dynamics is a prerequisite for future high-speed spintronic devices. The optimization of devices requires the controllability of the ultrafast demagnetization time ${\tau }_M$ and intrinsic Gilbert damping ${\alpha }_{\mathrm{intr}}$. In previous attempts to establish a relationship between ${\tau }_M$ and ${\alpha }_{\mathrm{intr}}$, the rare-earth doping of a permalloy film with two different demagnetization mechanisms was not a suitable candidate. Here, we choose Co/Ni bilayers to investigate the relations between ${\tau }_M$ and ${\alpha }_{\mathrm{intr}}$ by means of the time-resolved magneto-optical Kerr effect (TR-MOKE) via adjusting the thickness of the Ni layers, and obtain an approximately proportional relation between these two parameters. The remarkable agreement between the TR-MOKE experiment and the prediction of a breathing Fermi-surface model confirms that a large Elliott-Yafet spin-mixing parameter $b^2$ is relevant to the strong spin-orbital coupling at the Co/Ni interface. More importantly, a proportional relation between ${\tau }_M$ and ${\alpha }_{\mathrm{intr}}$ in such metallic films or heterostructures with electronic relaxation near the Fermi surface suggests the local spin-flip scattering dominates the mechanism of ultrafast demagnetization, otherwise the spin-current mechanism dominates. It is an effective method to distinguish the dominant contributions to ultrafast magnetic quenching in metallic heterostructures by simultaneously investigating both the ultrafast demagnetization time and Gilbert damping. Our work can open an avenue to manipulate the magnitude and efficiency of terahertz emission in metallic heterostructures such as perpendicular magnetic anisotropic Ta/Pt/Co/Ni/Pt/Ta multilayers, and then it has an immediate implication for the design of high-frequency spintronic devices.

The coexistence of a new-type antiferromagnetic (AFM) state, the so-called hedgehog spin-vortex crystal (SVC), and superconductivity (SC) is evidenced by an ${}^{75}\mathrm{As}$ nuclear magnetic resonance study on single-crystalline $\mathrm{CaK}{\left({\mathrm{Fe}}_{0.951}{\mathrm{Ni}}_{0.049}\right)}_4{\mathrm{As}}_4$. The hedgehog SVC order is clearly demonstrated by the direct observation of internal magnetic induction along the $c$ axis at the As1 site (close to K) and a zero net internal magnetic induction at the As2 site (close to Ca) below an AFM ordering temperature $T_{\mathrm{N}}\sim 52$ K. The nuclear spin-lattice relaxation rate $1/T_1$ shows a distinct decrease below $T_{\mathrm{c}}\sim 10$ K, providing also unambiguous evidence for the microscopic coexistence. Furthermore, based on the analysis of the $1/T_1$ data, the hedgehog SVC-type spin correlations are found to be enhanced below $T\sim 150$ K in the paramagnetic state. These results indicate the hedgehog SVC-type spin correlations play an important role for the appearance of SC in the new magnetic superconductor.

We report comprehensive x-ray absorption spectroscopy studies at both the Ga $K$ edge and Yb $L_2$ edge to elucidate the local structure of Ga and Yb dopants in ${\mathrm{Yb}}_x{\mathrm{Ga}}_y{\mathrm{Co}}_4{\mathrm{Sb}}_{12}$. Our extended x-ray absorption fine structure (EXAFS) data confirm that Ga atoms occupy two crystallographic sites: one is the $24g$ site replacing Sb, and the other is the $2a$ site in the off-center void position. We find that the occupancy ratio of these two sites varies significantly as a function of the filling fraction of additional Yb, which exclusively occupies the $2a$ on-center site. At low concentrations of Yb, ${\mathrm{Ga}}_{24g}$ and ${\mathrm{Ga}}_{2a}$ dopants coexist and they form a charge-compensated compound defect proposed by Qiu et al. [Adv. Funct. Mater. 23, 3194 (2013)]. The ${\mathrm{Ga}}_{24g}$ occupancy increases gradually with increasing Yb concentration, and almost all Ga occupies the $24g$ site for the highest Yb concentration studied ($x=0.4$). In addition to the local structural evidence provided by our EXAFS data, we also present x-ray absorption near-edge structure (XANES) spectra, which show a small Ga $K$-edge energy shift as a function of Yb concentration consistent with the change from predominantly ${\mathrm{Ga}}_{2a}$ to ${\mathrm{Ga}}_{24g}$ states. Our result suggests that the increased solubility of Yb in Yb-Ga co-doped ${\mathrm{Co}}_4{\mathrm{Sb}}_{12}$ skutterudites is due to the increased ${\mathrm{Ga}}_{24g}$ electron acceptor, and thus provides an important strategy to optimize the carrier concentration in partially filled skutterudites.

We report $\mu \mathrm{SR}$ experiments on the ternary compounds ${\mathrm{Sm}}_{1-x}{\mathrm{La}}_x{\mathrm{NiC}}_2$ ($x=0.85$, 0.92, and 0.96), crossing from a ferromagnetic to a superconducting phase. Zero-field $\mu \mathrm{SR}$ measurements of the ferromagnetic sample ($x=0.85$) unveil a glassylike character of the ferromagnetically ordered state. At the putative quantum critical compound ($x$ = 0.92), fluctuations of the Sm moments slow down below 2 K and remain dynamic down to 30 mK, showing persisting spin dynamics. Moreover, we find a time-field scaling ($t/H^{\gamma }$) of the $\mu \mathrm{SR}$ asymmetry function, evidencing quantum critical fluctuations. As to the superconducting material ($x$ = 0.96), the muon spin-relaxation rate displays a $\lambda$-like peak at $T=250$ mK, indicating the coexistence of weak magnetism and filamentary superconductivity. Our results demonstrate that ${\mathrm{Sm}}_{1-x}{\mathrm{La}}_x{\mathrm{NiC}}_2$ constitutes a model system for studying a ferromagnetic quantum critical point tuned by chemical pressure.

We studied the structural, electronic, and magnetic properties of Mn-doped topological insulators ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ within the density-functional theory (DFT) using the generalized gradient approximation (GGA) in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital band-structure method. The x-ray absorption spectra (XAS) and x-ray magnetic circular dichroism at the Mn $K$ and $L_{2,3}$ edges were investigated theoretically from first principles. The calculated results are in good agreement with experimental data. The complex fine structure of the Mn $L_{2,3}$ XAS in Mn-doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ was found to be not compatible with a pure ${\mathrm{Mn}}^{3+}$ valency state. Its interpretation demands mixed valent states. The theoretically calculated x-ray emission spectra at the Mn $K$ and $L_{2,3}$ edges are also presented and analyzed.

Microscopic theory of the normal-to-superconductor coexistence line of a multiband Weyl superconductor subjected to magnetic field is constructed. It is shown that the Weyl semimetal that is nonsuperconducting or having a small critical temperature $T_c$ at zero field might become a superconductor at higher temperatures when the magnetic field is tuned to a series of quantized values $H_n$. The pairing occurs on Landau levels. It is argued that the phenomenon is detectable much easier in Weyl semimetals than in parabolic band metals since the quantum limit already has been approached in several Weyl materials. The effect of Zeeman coupling leading to splitting of the reentrant superconducting regions on the magnetic phase diagram is considered. An experimental signature of the superconductivity on Landau levels is the reduction of magnetoresistivity. This has been observed already in ${\mathrm{Cd}}_3{\mathrm{As}}_2$ and several other compounds. The novel kind of quantum oscillations of magnetoresistance detected in ${\mathrm{ZrTe}}_5$ is discussed along these lines.

When magnetic field is applied to metals and semimetals quantum oscillations appear as individual Landau levels cross the Fermi level. Quantum oscillations generally do not occur in superconductors (SC) because magnetic field is either expelled from the sample interior or, if strong enough, drives the material into the normal state. In addition, elementary excitations of a superconductor—Bogoliubov quasiparticles—do not carry a well-defined electric charge and therefore do not couple in a simple way to the applied magnetic field. We predict here that in Weyl superconductors certain types of elastic strain have the ability to induce chiral pseudomagnetic field, which can reorganize the electronic states into Dirac-Landau levels with linear band crossings at low energy. The resulting quantum oscillations in the quasiparticle density of states and thermal conductivity can be experimentally observed under a bending deformation of a thin film Weyl SC and provide new insights into this fascinating family of materials.

We present an efficient exact diagonalization scheme for the extended dynamical mean-field theory and apply it to the extended Hubbard model on the square lattice with nonlocal charge-charge interactions. Our solver reproduces the phase diagram of this approximation with good accuracy. Details on the numerical treatment of the large Hilbert space of the auxiliary Holstein-Anderson impurity problem are provided. Benchmarks with a numerically exact strong-coupling continuous-time quantum Monte Carlo solver show better convergence behavior of the exact diagonalization in the deep insulator. Special attention is given to possible effects due to the discretization of the bosonic bath. We discuss the quality of real axis spectra and address the question of screening in the Mott insulator within extended dynamical mean-field theory.

This paper introduces a self-biased broadband magnet-free and linear isolator based on one-way space-time coherency. The incident wave and the space-time-modulated medium share the same temporal frequency and are hence temporally coherent. However, thanks to the unidirectionally of the space-time modulation, the space-time-modulated medium and the incident wave are spatially coherent only in the forward direction and not in the opposite direction. As a consequence, the energy of the medium strongly couples to the propagating wave in the forward direction, while it conflicts with the propagating wave in the opposite direction, yielding strong isolation. We first derive a closed-form solution for the wave scattering from a spatiotemporally coherent medium and then show that a perfectly coherent space-time-modulated medium provides a moderate isolation level which is also subject to one-way transmission gain. To overcome this issue, we next investigate the effect of space-coherency imperfection between the medium and the wave, while they are still perfectly temporally coherent. Leveraging the spatial-coherency imperfection, the medium exhibits a quasiarbitrary and strong nonreciprocal transmission. Finally, we present the experimental demonstration of the self-biased version of the proposed broadband isolator, exhibiting more than $122\%$ fractional operation bandwidth.

We describe a general analytical framework of a nanoplasmonic cavity-emitter system interacting with a dielectric photonic waveguide. Taking into account emitter quenching and dephasing, our model directly reveals the single-photon extraction efficiency $\eta$ as well as the indistinguishability $I$ of photons coupled into the waveguide mode. Rather than minimizing the cavity modal volume, our analysis predicts an optimum modal volume to maximize $\eta$ that balances waveguide coupling and spontaneous emission rate enhancement. Surprisingly, our model predicts that near-unity indistinguishability is possible, but this requires a much smaller modal volume, implying a fundamental performance trade-off between high $\eta$ and $I$ at room temperature. Finally, we show that maximizing $\eta I$ requires that the system has to be driven in the weak coupling regime because quenching effects and decreased waveguide coupling drastically reduce $\eta$ in the strong coupling regime.