Raman scattering spectra of zigzag spin chain ${\mathrm{TaFe}}_{1.25}{\mathrm{Te}}_3$ single crystal are presented in a temperature range from 80 to 300 K. Nine Raman active modes of $A_g$ and $B_g$ symmetry are clearly observed and assigned by probing different scattering channels, which is confirmed by lattice dynamics calculations. Temperature dependence of the Raman modes linewidth is mainly governed by the lattice anharmonicity. The only deviation from the conventional behavior is observed for $A_g$ symmetry modes in a vicinity of the magnetic phase transition at $T_N\approx 200$ K. This implies that the electron-phonon interaction weakly changes with temperature and magnetic ordering, whereas small changes in the spectra near the critical temperature can be ascribed to spin fluctuations.

Driven-dissipative systems define a broad class of nonequilibrium systems where an external drive (e.g., laser) competes with a dissipative environment. The steady state of dynamics is generically distinct from a thermal state characteristic of equilibrium. As a representative example, a driven-dissipative system with a continuous symmetry is generically disordered in two dimensions in contrast with the well-known algebraic order in equilibrium XY phases. In this paper, we study a two-dimensional driven-dissipative model of weakly interacting bosons with a continuous $U\left(1\right)$ symmetry. Our aim is twofold: First, we show that an effectively equilibrium XY phase emerges despite the driven nature of the model, and that it is protected by a natural ${\mathbb{Z}}_2$ symmetry of the dynamics. Second, we argue that this phase is unstable against symmetry-breaking perturbations as well as static disorder, whose mechanism in most cases has no analog in equilibrium. In the language of renormalization group theory, we find that, outside equilibrium, there are more relevant directions away from the XY phase.

The techniques of propagating spin-wave spectroscopy and current-induced spin-wave Doppler shift are applied to a 20-nm-thick Fe/MgO(001) film. The magnetic parameters extracted from the position of the spin-wave resonance peaks are very close to those tabulated for bulk iron. From the zero-current propagating wave forms, a group velocity of 4 km/s and an attenuation length of about 6 $\mu \mathrm{m}$ are extracted for 1.6-$\mu \mathrm{m}$-wavelength spin wave at 18 GHz. From the measured current-induced spin-wave Doppler shift, we extract a surprisingly high degree of spin polarization of the current of $83\%$, which constitutes the main finding of this work. This set of results makes single-crystalline iron a promising candidate for building devices utilizing high-frequency spin waves and spin-polarized currents.

In the quest for exotic superconducting pairing states, the Rashba effect, which lifts the electron-spin degeneracy as a consequence of strong spin-orbit interaction (SOI) under broken inversion symmetry, has attracted considerable interest. Here, to introduce the Rashba effect into two-dimensional (2D) strongly correlated electron systems, we fabricate noncentrosymmetric (tricolor) superlattices composed of three kinds of $f$-electron compounds with atomic thickness; $d$-wave heavy fermion superconductor ${\mathrm{CeCoIn}}_5$ sandwiched by two different nonmagnetic metals, ${\mathrm{YbCoIn}}_5$ and ${\mathrm{YbRhIn}}_5$. We find that the Rashba SOI-induced global inversion symmetry breaking in these tricolor Kondo superlattices leads to profound changes in the superconducting properties of ${\mathrm{CeCoIn}}_5$, which are revealed by unusual temperature and angular dependencies of upper critical fields that are in marked contrast with the bulk ${\mathrm{CeCoIn}}_5$ single crystals. We demonstrate that the Rashba effect incorporated into 2D ${\mathrm{CeCoIn}}_5$ block layers is largely tunable by changing the layer thickness. Moreover, the temperature dependence of in-plane upper critical field exhibits an anomalous upturn at low temperatures, which is attributed to a possible emergence of a helical or stripe superconducting phase. Our results demonstrate that the tricolor Kondo superlattices provide a new playground for exploring exotic superconducting states in the strongly correlated 2D electron systems with the Rashba effect.

The electronic structure of the reentrant superconductor $\mathrm{Eu}({\mathrm{Fe}}_{0.86}{\mathrm{Ir}}_{0.14}$)${}_2{\mathrm{As}}_2$ ($T_c=22$ K) with coexisting ferromagnetic order ($T_M=18$ K) is investigated using angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy. We study the in-plane and out-of-plane band dispersions and Fermi surface (FS) of $\mathrm{Eu}{\left({\mathrm{Fe}}_{0.86}{\mathrm{Ir}}_{0.14}\right)}_2{\mathrm{As}}_2$. The near-$E_F$ Fe-$3d$-derived band dispersions near the $\mathrm{\Gamma }$ and $X$ high-symmetry points show changes due to Ir substitution, but the FS topology is preserved. From momentum-dependent measurements of the superconducting gap measured at $T=5$ K, we estimate an essentially isotropic $s$-wave gap ($\mathrm{\Delta }\sim 5.25\pm 0.25$ meV), indicative of strong-coupling superconductivity with $2\mathrm{\Delta }/k_B{T}_c\simeq 5.8$. The gap gets closed at temperatures $T\ge 10$ K, and this is attributed to the resistive phase which sets in at $T_M=18$ K due to the ${\mathrm{Eu}}^{2+}$-derived magnetic order. The modification of the FS with Ir substitution clearly indicates an effective hole doping with respect to the parent compound.

The single-parameter scaling hypothesis relating the average and variance of the logarithm of the conductance is a pillar of the theory of electronic transport. We use a maximum-entropy ansatz to explore the logarithm of the particle, or energy density $ln\mathcal{W}\left(x\right)$ at a depth $x$ into a random one-dimensional system. Single-parameter scaling would be the special case in which $x=L$ (the system length). We find the result, confirmed in microwave measurements and computer simulations, that the average of $ln\mathcal{W}\left(x\right)$ is independent of $L$ and equal to $-x/\ell$, with $\ell$ the mean free path. At the beginning of the sample, $\mathrm{var}[ln\mathcal{W}(x\left)\right]$ rises linearly with $x$ and is also independent of $L$, with a sublinear increase and then a drop near the sample output. At $x=L$ we find a correction to the value of $\mathrm{var}[lnT]$ predicted by single-parameter scaling.

We use neutron polarization analysis to study spin excitation anisotropy in the optimally isovalent-doped superconductor ${\mathrm{BaFe}}_2$(${\mathrm{As}}_{0.7}{\mathrm{P}}_{0.3}$)${}_2$ ($T_c=30$ K). Different from optimally hole- and electron-doped ${\mathrm{BaFe}}_2{\mathrm{As}}_2$, where there is a clear spin excitation anisotropy in the paramagnetic tetragonal state well above $T_c$, we find no spin excitation anisotropy for energies above 2 meV in the normal state of ${\mathrm{BaFe}}_2$(${\mathrm{As}}_{0.7}{\mathrm{P}}_{0.3}$)${}_2$. Upon entering the superconducting state, significant spin excitation anisotropy develops at the antiferromagnetic (AF) zone center ${\mathbf{Q}}_{\mathrm{AF}}=(1,0,L=\mathrm{odd})$, while the magnetic spectrum is isotropic at the zone boundary $\mathbf{Q}=(1,0,L=\mathrm{even})$. By comparing the temperature, wave vector, and polarization dependence of the spin excitation anisotropy in ${\mathrm{BaFe}}_2$(${\mathrm{As}}_{0.7}{\mathrm{P}}_{0.3}$)${}_2$ and hole-doped ${\mathrm{Ba}}_{0.67}{\mathrm{K}}_{0.33}{\mathrm{Fe}}_2{\mathrm{As}}_2$ ($T_c=38$ K), we conclude that such anisotropy arises from spin-orbit coupling and is associated with the nearby AF order and superconductivity.

The layered transition metal dichalcogenide ${\mathrm{WTe}}_2$ has recently attracted significant attention due to the discovery of an extremely large magnetoresistance, a predicted type-II Weyl semimetallic state, and a pressure-induced superconducting state. By a careful measurement of the superconducting upper critical fields as a function of the magnetic field angle at a pressure as high as 98.5 kbar, we provide the first detailed examination of the dimensionality of the superconducting condensate in ${\mathrm{WTe}}_2$. Despite the layered crystal structure, the upper critical field exhibits a negligible field anisotropy. The angular dependence of the upper critical field can be satisfactorily described by the anisotropic mass model from 2.2 K ($T/T_c\sim 0.67$) to 0.03 K ($T/T_c\sim 0.01$), with a practically identical anisotropy factor $\gamma \sim 1.7$. The temperature dependence of the upper critical field, determined for both $H\perp ab$ and $H\parallel ab$, can be understood by a conventional orbital depairing mechanism. A comparison of the upper critical fields along the two orthogonal field directions results in the same value of $\gamma \sim 1.7$, leading to a temperature-independent anisotropy factor from near $T_c$ to $<0.01T_c$. Our findings thus identify ${\mathrm{WTe}}_2$ as a nearly isotropic superconductor, with an anisotropy factor among one of the lowest known in superconducting transition metal dichalcogenides.

We use inelastic light scattering to study ${\mathrm{Sr}}_{1-x}{\mathrm{Na}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ ($x\approx 0.34$), which exhibits a robust tetragonal magnetic phase that restores the fourfold rotation symmetry inside the orthorhombic magnetic phase. With cooling, we observe splitting and recombination of an $E_g$ phonon peak upon entering the orthorhombic and tetragonal magnetic phases, respectively, consistent with the reentrant phase behavior. Our electronic Raman data reveal a pronounced feature that is clearly associated with the tetragonal magnetic phase, suggesting the opening of a different electronic gap. The energy of this gap is substantially smaller than the gap observed in the orthorhombic magnetic phase, and the two gaps compete only weakly for spectral weight. These observations are consistent with the notion that the reentrant phase behavior is driven by spin-orbit interactions, which seem to dictate the manifestation of magnetism in Fe-based superconductors. No phonon back folding can be detected above the noise level, which implies that any lattice translation symmetry breaking in the tetragonal magnetic phase must be very weak.

We report a high-resolution resonant inelastic extreme ultraviolet (EUV) scattering study of the quantum Heisenberg antiferromagnet ${\mathrm{KCoF}}_3$. By tuning the EUV photon energy to the cobalt $M_{23}$ edge, a complete set of low-energy $3d$ spin-orbital excitations is revealed. These low-lying electronic excitations are modeled using an extended multiplet-based mean-field calculation to identify the roles of lattice and magnetic degrees of freedom in modifying the resonant inelastic x-ray scattering (RIXS) spectral line shape. We have demonstrated that the temperature dependence of RIXS features upon the antiferromagnetic ordering transition enables us to probe the energetics of short-range spin correlations in this material.

We investigate the Heisenberg $XXZ$ chain with long-range interactions in the $Z$ dimension. By applying two magnetic boundary reservoirs, we drive the system out of equilibrium and induce a nonzero steady-state current. The long-range coupled chain shows nearly ballistic transport and linear response for all potential differences of the external reservoirs. In contrast, the common isotropic nearest-neighbor coupling shows negative differential conductivity and a transition from diffusive to subdiffusive transport for a far-from-equilibrium driving. Adding disorder, the change in the transport for nearest-neighbor coupling is therefore highly dependent on the driving. We find for the disordered long-range coupled $XXZ$ chain, any change in the transport behavior is independent of the potential difference and the coupling strengths of the external reservoirs.

We calculate a low-frequency surface impedance of a dirty, $s$-wave superconductor with an imperfect surface incorporating either a thin layer with a reduced pairing constant or a thin, proximity-coupled normal layer. Such structures model realistic surfaces of superconducting materials which can contain oxide layers, absorbed impurities, or nonstoichiometric composition. We solved the Usadel equations self-consistently and obtained spatial distributions of the order parameter and the quasiparticle density of states which then were used to calculate a low-frequency surface resistance $R_s\left(T\right)$ and the magnetic penetration depth $\lambda \left(T\right)$ as functions of temperature in the limit of local London electrodynamics. It is shown that the imperfect surface in a single-band $s$-wave superconductor results in a nonexponential temperature dependence of $Z\left(T\right)$ at $T\ll T_c$ which can mimic the behavior of multiband or $d$-wave superconductors. The imperfect surface and the broadening of the gap peaks in the quasiparticle density of states $N\left(\epsilon \right)$ in the bulk give rise to a weakly temperature-dependent residual surface resistance. We show that the surface resistance can be optimized and even reduced below its value for an ideal surface by engineering $N\left(\epsilon \right)$ at the surface using pair-breaking mechanisms, particularly by incorporating a small density of magnetic impurities or by tuning the thickness and conductivity of the normal layer and its contact resistance. The results of this work address the limit of $R_s$ in superconductors at $T\ll T_c$, and the ways of engineering the optimal density of states by surface nanostructuring and impurities to reduce losses in superconducting microresonators, thin-film strip lines, and radio-frequency cavities for particle accelerators.

The bulk-boundary correspondence establishes a connection between the bulk topological index of an insulator or superconductor, and the number of topologically protected edge bands or states. For topological superconductors in two dimensions, the first Chern number is related to the number of protected bands within the bulk energy gap, and is therefore assumed to give the number of Majorana band states in the system. Here we show that this is not necessarily the case. As an example, we consider a hexagonal-lattice topological superconductor based on a model of graphene with Rashba spin-orbit coupling, proximity-induced $s$-wave superconductivity, and a Zeeman magnetic field. We explore the full Chern number phase diagram of this model, extending what is already known about its parity. We then demonstrate that, despite the high Chern numbers that can be seen in some phases, these do not strictly always contain Majorana bound states.

In muon-spin-rotation experiments, positive muons are implanted in the material and come to rest in the unrelaxed host lattice. The formation of the final configuration requires a lattice relaxation which does not occur instantly. The present paper is concerned with the transition from the initial stopping state to the final muon configuration. We identify the often observed fast relaxing signal in muon experiments (e.g., in several oxides studied recently) with the transition state in this conversion process. This state is paramagnetic with a small hyperfine interaction (in the order of MHz) which fluctuates and averages to almost zero. Because of its apparent diamagnetic frequency behavior, the fast signal was in the past assigned to ${\mathrm{Mu}}^{+}$ or ${\mathrm{Mu}}^{-}$. We present evidence that this state is actually paramagnetic. The model presented in this paper is of importance for the interpretation of past and future $\mu \mathrm{SR}$ measurements.

The current-induced spin polarization and momentum-dependent spin-orbit field were measured in ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ epilayers with varying indium concentrations and silicon doping densities. Samples with higher indium concentrations and carrier concentrations and lower mobilities were found to have larger electrical spin generation efficiencies. Furthermore, current-induced spin polarization was detected in GaAs epilayers despite the absence of measurable spin-orbit fields, indicating that the extrinsic contributions to the spin-polarization mechanism must be considered. Theoretical calculations based on a model that includes extrinsic contributions to the spin dephasing and the spin Hall effect, in addition to the intrinsic Rashba and Dresselhaus spin-orbit coupling, are found to reproduce the experimental finding that the crystal direction with the smaller net spin-orbit field has larger electrical spin generation efficiency and are used to predict how sample parameters affect the magnitude of the current-induced spin polarization.

This work addresses the problem of elastic scattering on a localized impurity in a one-dimensional crystal with sublattice degrees of freedom. The impurity yields long-range interferences in the local density of states known as Friedel oscillations. Here we show that the internal degrees of freedom of Bloch waves are responsible for a geometrical phase shift in Friedel oscillations. The Fourier transform of the energy-resolved interference pattern reveals a topological property of this phase shift, which is intrinsically related to a wave-function topological property (Zak phase) in the absence of impurity. As a result, Friedel oscillations in the local density of states can be regarded as a probe of wave topological properties in a broad class of classical and quantum systems, such as acoustic and photonic crystals, ultracold atomic gases in optical lattices, and electronic compounds.