We report ferromagnetic resonance (FMR) experiments on thin magnetic Co layers either sandwiched symmetrically between Cu and Pt, or sandwiched asymmetrically between a Cu or Pt layer on one side and a ${\mathrm{SiO}}_x$ substrate on the other. In the symmetric samples, we find well-known behavior, namely, the FMR linewidth $\Delta H_{pp}$ is significantly larger for the case of Pt than for the case of Cu. This is due to the larger spin scattering in the Pt layer. However, for the asymmetric Co/Cu bilayers, the linewidth is much larger than for the symmetric Cu/Co/Cu trilayers and not much different from the linewidth of Co/Pt bilayers. We argue this to be due to the Rashba effect at the ${\mathrm{SiO}}_x/\mathrm{Co}$ interface, which gives rise to effective magnetic fields interacting with the electron spins in the Co layer and which can be measured without reverting to transport measurements.

Single crystals of ${\mathrm{EuCu}}_2{\mathrm{As}}_2$ and ${\mathrm{EuCu}}_2{\mathrm{Sb}}_2$ were grown from CuAs and CuSb self-flux, respectively. The crystallographic, magnetic, thermal, and electronic transport properties of the single crystals were investigated by room-temperature x-ray diffraction (XRD), magnetic susceptibility $\chi$ versus temperature $T$, isothermal magnetization $M$ versus magnetic field $H$, specific heat $C_{\mathrm{p}}\left(T\right)$, and electrical resistivity $\rho \left(T\right)$ measurements. ${\mathrm{EuCu}}_2{\mathrm{As}}_2$ crystallizes in the body-centered tetragonal ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$-type structure (space group $I4/mmm$), whereas ${\mathrm{EuCu}}_2{\mathrm{Sb}}_2$ crystallizes in the related primitive tetragonal ${\mathrm{CaBe}}_2{\mathrm{Ge}}_2$-type structure (space group $P4/nmm$). The energy-dispersive x-ray spectroscopy and XRD data for the ${\mathrm{EuCu}}_2{\mathrm{Sb}}_2$ crystals showed the presence of vacancies on the Cu sites, yielding the actual composition ${\mathrm{EuCu}}_{1.82}{\mathrm{Sb}}_2$. The $\rho \left(T\right)$ and $C_{\mathrm{p}}\left(T\right)$ data reveal metallic character for both ${\mathrm{EuCu}}_2{\mathrm{As}}_2$ and ${\mathrm{EuCu}}_{1.82}{\mathrm{Sb}}_2$. Antiferromagnetic (AFM) ordering is indicated from the $\chi \left(T\right),C_{\mathrm{p}}\left(T\right)$, and $\rho \left(T\right)$ data for both ${\mathrm{EuCu}}_2{\mathrm{As}}_2$ ($T_{\mathrm{N}}=17.5$ K) and ${\mathrm{EuCu}}_{1.82}{\mathrm{Sb}}_2$ ($T_{\mathrm{N}}=5.1$ K). In ${\mathrm{EuCu}}_{1.82}{\mathrm{Sb}}_2$, the ordered-state $\chi \left(T\right)$ and $M\left(H\right)$ data suggest either a collinear A-type AFM ordering of ${\mathrm{Eu}}^{+2}$ spins $S=\frac{7}{2}$ or a planar noncollinear AFM structure, with the ordered moments oriented in the tetragonal $ab$ plane in either case. This ordered-moment orientation for the A-type AFM is consistent with calculations with magnetic dipole interactions. The anisotropic $\chi \left(T\right)$ and isothermal $M\left(H\right)$ data for ${\mathrm{EuCu}}_2{\mathrm{As}}_2$, also containing ${\mathrm{Eu}}^{+2}$ spins $S=\frac{7}{2}$, strongly deviate from the predictions of molecular field theory for collinear AFM ordering and the AFM structure appears to be both noncollinear and noncoplanar.

We perform a numerical analysis of counterflow quantum turbulence of superfluid ${}^4\mathrm{He}$ with nonuniform flows by using the vortex filament model. In recent visualization experiments nonuniform laminar flows of the normal fluid, namely, Hagen-Poiseuille flow and tail-flattened flow, have been observed. Tail-flattened flow is a laminar flow in which the outer part of the Hagen-Poiseuille flow becomes flat. In our simulation, the velocity field of the normal fluid is prescribed to be two nonuniform profiles. This work addresses a square channel to obtain important physics not revealed in the preceding numerical works. In the studies of the two profiles we analyze the statistics of the physical quantities. Under Hagen-Poiseuille flow, inhomogeneous quantum turbulence appears as a statistically steady state. The vortex tangle shows a characteristic space-time oscillation. Under tail-flattened flow, the nature of the quantum turbulence depends strongly on that flatness. Vortex line density increases significantly as the profile becomes flatter, being saturated above a certain flatness. The inhomogeneity is significantly reduced in comparison to the case of Hagen-Poiseuille flow. Investigating the behavior of quantized vortices reveals that tail-flattened flow is an intermediate state between Hagen-Poiseuille flow and uniform flow. In both profiles we obtain a characteristic inhomogeneity in the physical quantities, which suggests that a boundary layer of superfluid appears near a solid boundary. The vortex tangle produces a velocity field opposite to the applied superfluid flow, and, consequently, the superfluid flow becomes smaller than the applied one.

The precise description of chemical bonds with different natures is a prerequisite for an accurate electronic structure method. The van der Waals density functional is a promising approach that meets such a requirement. Nevertheless, the accuracy should be assessed for a variety of materials to test the robustness of the method. We present benchmark calculations for weakly interacting molecular complexes and rare-gas systems as well as covalently bound molecular systems, in order to assess the accuracy and applicability of rev-vdW-DF2, a recently proposed variant [I. Hamada, Phys. Rev. B 89, 121103 (2014)] of the van der Waals density functional. It is shown that although the calculated atomization energies for small molecules are less accurate rev-vdW-DF2 describes the interaction energy curves for the weakly interacting molecules and rare-gas complexes, as well as the bond lengths of diatomic molecules, reasonably well.

Using the semiclassical Boltzmann transport theory, we analytically consider dc charge transport in gapless electron-hole (both chiral and nonchiral) systems in the presence of resistive scattering due to static disorder arising from random quenched impurities in the background. We obtain the dependence of the Boltzmann conductivity on carrier density and temperature for arbitrary band dispersion in arbitrary dimensionality assuming long-range $\left(\sim 1/r\right)$ Coulomb disorder and zero-range white-noise disorder $\left[\sim \delta \left(r\right)\right]$. We establish that the temperature and the density dependence of the Boltzmann conductivity manifests scaling behaviors determining, respectively, the intrinsic semimetallic or the extrinsic metallic property of the gapless system. Our results apply equally well to both chiral and nonchiral gapless systems, and provide a qualitative understanding of the dependence of the Boltzmann conductivity on the band dispersion in arbitrary dimensionality.

We present a theoretical description for a class of experimental setups that measure quantum Hall coefficients in ferromagnet/topological insulator/ferromagnet (FM-TI-FM) junctions. We predict that, varying the magnetization direction in ferromagnets, one can change the induced Hall voltage and transverse temperature gradient from the maximal values, corresponding to the quantized Hall coefficients, down to their complete suppression to zero. We provide detailed analysis of thermal and electrical Hall resistances as functions of the magnetization directions in ferromagnets, the spin scattering time in TI, and geometrical positions of FM leads and measurement contacts. We find a special symmetric configuration of the experimental setup, at which the quantum Hall coefficients are independent of spin scattering.

$\mathrm{Fe}{M}_2{X}_4$ spinels, with the magnetite structure, where $M$ is a transition metal and $X$ is oxygen or sulfur, are candidate materials for spin filters, one of the key devices in spintronics. We present here a computational study of the inversion thermodynamics and the electronic structure of these (thio)spinels for $M=\mathrm{Cr},\mathrm{Mn},\mathrm{Co},\mathrm{Ni}$, using calculations based on the density functional theory with on-site Hubbard corrections $\left(\mathrm{DFT}+U\right)$. The analysis of the configurational free energies shows that different behavior is expected for the equilibrium cation distributions in these structures: $\mathrm{FeC}{\mathrm{r}}_2{X}_4$ and $\mathrm{FeM}{\mathrm{n}}_2{\mathrm{S}}_4$ are fully normal, $\mathrm{FeN}{\mathrm{i}}_2{X}_4$ and $\mathrm{FeC}{\mathrm{o}}_2{\mathrm{S}}_4$ are intermediate, and $\mathrm{FeC}{\mathrm{o}}_2{\mathrm{O}}_4$ and $\mathrm{FeM}{\mathrm{n}}_2{\mathrm{O}}_4$ are fully inverted. We have analyzed the role played by the size of the ions and by the crystal field stabilization effects in determining the equilibrium inversion degree. We also discuss how the electronic and magnetic structure of these spinels is modified by the degree of inversion, assuming that this could be varied from the equilibrium value. We have obtained electronic densities of states for the completely normal and completely inverse cation distribution of each compound. $\mathrm{FeC}{\mathrm{r}}_2{X}_4,\mathrm{FeM}{\mathrm{n}}_2{X}_4,\mathrm{FeC}{\mathrm{o}}_2{\mathrm{O}}_4$, and $\mathrm{FeN}{\mathrm{i}}_2{\mathrm{O}}_4$ are half-metals in the ferrimagnetic state when Fe is in tetrahedral positions. When $M$ is filling the tetrahedral positions, the Cr-containing compounds and $\mathrm{FeM}{\mathrm{n}}_2{\mathrm{O}}_4$ are half-metallic systems, while the Co and Ni spinels are insulators. The Co and Ni sulfide counterparts are metallic for any inversion degree together with the inverse $\mathrm{FeM}{\mathrm{n}}_2{\mathrm{S}}_4$. Our calculations suggest that the spin filtering properties of the $\mathrm{Fe}{M}_2{X}_4$ (thio)spinels could be modified via the control of the cation distribution through variations in the synthesis conditions.

We study transport in a Weyl semimetal with donor and acceptor impurities. At sufficiently high temperatures transport is dominated by electron-electron interactions, while the low-temperature resistivity comes from the scattering of quasiparticles on screened impurities. Using the diagrammatic technique, we calculate the conductivity $\sigma (T,\omega ,n_A,n_D)$ in the impurities-dominated regime as a function of temperature $T$, frequency $\omega$, and the concentrations $n_A$ and $n_D$ of acceptors and donors and discuss the crossover behavior between the regimes of low and high temperatures and impurity concentrations. In a sufficiently compensated material $\left[|n_A-n_D|\ll (n_A+n_D)\right]$ with a small effective fine structure constant $\alpha ,\sigma (\omega ,T)\propto T^2/(T^{-2}-i\omega ·\text{const})$ in a wide interval of temperatures. For very low temperatures, or in the case of an uncompensated material, the transport is effectively metallic. We discuss experimental conditions necessary for realizing each regime.

The electronic structures of cubic and tetragonal ${\mathrm{MnV}}_2{\mathrm{O}}_4$ have been studied using hybrid-exchange density-functional theory. The computed electronic structure of the tetragonal phase shows an antiferro-orbital ordering on V sites and a ferrimagnetic ground state (the spins on V and Mn are antialigned). These results are in good agreement with the previous theoretical result obtained from the local-density approximation + $U$ methods [S. Sarkar et al., Phys. Rev. Lett. 102, 216405 (2009)]. Moreover, the electronic structure, especially the projected density of states of the cubic phase, has been predicted with good agreement with the recent soft x-ray spectroscopy experiment. Similar to the tetragonal phase, the spins on V and Mn in the cubic structure favor a ferrimagnetic configuration. Most interesting is that the computed charge densities of the spin-carrying orbitals on V in the cubic phase show an exotic orbital ordering, i.e., a ferro-orbital ordering along [110] but an antiferro-orbital ordering along $\left[\overline{1}10\right]$.

The standard theoretical approach to gapless spin liquid phases of two-dimensional frustrated quantum antiferromagnets invokes the concept of fermionic slave particles into which the spin fractionalizes. As an alternate we explore different kinds of gapless spin liquid phases in frustrated quantum magnets with $XY$ anisotropy where the vortex of the spin fractionalizes into gapless itinerant fermions. The resulting gapless fractionalized vortex liquid phases are studied within a slave particle framework that is dual to the usual one. We demonstrate the stability of some such phases and describe their properties. We give an explicit construction in an $XY$-spin-1 system on triangular lattice, and interpret it as a critical phase in the vicinity of spin-nematic states.

We explore the weak-strong-coupling Bose-Fermi duality in a model of a single-channel integer or fractional quantum Hall edge state with a finite-range interaction. The system is described by a chiral Luttinger liquid with nonlinear dispersion of bosonic and fermonic excitations. We use the bosonization, a unitary transformation, and a refermionization to map the system onto that of weakly interacting fermions at low temperature $T$ or weakly interacting bosons at high $T$. We calculate the equilibration rate which is found to scale with temperature as $T^5$ and $T^{14}$ in the high-temperature (“bosonic”) and the low-temperature (“fermonic”) regimes, respectively. The relaxation rate of a hot particle with the momentum $k$ in the fermonic regime scales as $k^7{T}^7$.

Recently, it has been shown that a metasurface of plasmonic nanospheres deposited on a highly refractive substrate requires a bianisotropic magnetoelectric coupling for its effective description. The effect has been coined substrate-induced bianisotropy. It leads to an asymmetric reflectance similar to bianisotropic metasurfaces. In this work, through a circuit model, we show that such bianisotropy does not necessarily emerge for all substrated metasurfaces. Indeed, we show that the thickness of the metasurface plays a crucial role to encounter substrate-induced bianisotropy. Moreover, by taking advantage of substrate-induced bianisotropy, we present the necessary conditions for the circuit model parameters to compensate the asymmetric reflectance generated by an intrinsically bianisotropic metasurface. We finally express that, in substrated metasurfaces, the asymmetric reflectance and the bianisotropic response are two separate issues albeit with interdependencies.

Magnesium oxide (MgO) is used in a variety of industrial applications due to its low cost and structural stability. In heterogeneous catalysis, MgO and Li-doped MgO have been studied as catalysts for the oxidative coupling of methane. In this work, we analyze the structure and stability of defect complexes comprising Li dopants and oxygen vacancies in MgO, combining scanning tunneling microscopy, photon-emission experiments, and density-functional theory computations. The experimental results strongly indicate that after annealing Li-doped MgO to temperatures of 600 K and higher, Li evaporates from the surface, but Li defects, such as substitutional defects, interstitials, or defect complexes comprising Li remain in the bulk. Our calculations show that bulk defect complexes containing ${\mathrm{F}}^{2+}$ color centers, that have donated their two electrons to two adjacent Li defects, are the most stable configurations at realistic pressure and temperature conditions.

We have investigated optical orientation in the vicinity of the direct gap of bulk germanium. The electron spin polarization is studied via polarization-resolved photoluminescence excitation spectroscopy unfolding the interplay between doping and ultrafast electron transfer from the center of the Brillouin zone towards its edge. As a result, the direct-gap photoluminescence circular polarization can vary from 30% to $-60\%$ when the excitation laser energy increases. This study provides also simultaneous access to the resonant electronic Raman scattering due to inter-valence-band excitations of spin-polarized holes, yielding a fast spectroscopic approach for the determination of the energy spectrum of holes in semiconducting materials.

The broken symmetry that develops below 17.5 K in the heavy fermion compound ${\mathrm{URu}}_2{\mathrm{Si}}_2$ has long eluded identification. Here we argue that the recent observation of Ising quasiparticles in ${\mathrm{URu}}_2{\mathrm{Si}}_2$ results from a spinor hybridization order parameter that breaks double time-reversal symmetry by mixing states of integer and half-integer spin. Such “hastatic order” (hasta: [Latin] spear) hybridizes Kramers conduction electrons with Ising, non-Kramers $5f^2$ states of the uranium atoms to produce Ising quasiparticles. The development of a spinorial hybridization at 17.5 K accounts for both the large entropy of condensation and the magnetic anomaly observed in torque magnetometry. This paper develops the theory of hastatic order in detail, providing the mathematical development of its key concepts. Hastatic order predicts a tiny transverse moment in the conduction sea, a colossal Ising anisotropy in the nonlinear susceptibility anomaly and a resonant energy-dependent nematicity in the tunneling density of states.

Time-domain thermoreflectance (TDTR) can be applied to metallic samples at high pressures in the diamond anvil cell and provide noncontact measurements of thermal transport properties. We have performed regular and beam-offset TDTR to establish the thermal conductivities of Si and ${\mathrm{Si}}_{0.991}{\mathrm{Ge}}_{0.009}$ across the semiconductor-metal phase transition and up to 45 GPa. The thermal conductivities of metallic Si and Si(Ge) are comparable to aluminum and indicative of predominantly electronic heat carriers. Metallic Si and Si(Ge) have an anisotropy of approximately 1.4, similar to that of beryllium, due to the primitive hexagonal crystal structure. We used the Wiedemann-Franz law to derive the associated electrical resistivity, and found it consistent with the Bloch-Grüneisen model.

We calculate the current as a function of applied voltage in a nontopological $s$-wave superconductor–quantum dot–topological superconductor (TS) tunnel junction. We consider the type of TS which hosts two Majorana bound states (MBSs) at the ends of a semiconductor quantum wire or of a chain of magnetic atoms in the proximity with an $s$-wave superconductor. We find that the $I-V$ characteristic of such a system in the regime of big voltages has a typical two-dot shape and is ornamented by peaks of multiple Andreev reflections. We also consider the other options when the zero-energy states are created by disorder (hereby Shiba states) or by Andreev zero-energy bound states at the surface of a quantum dot and a superconductor. The later are obtained by tuning the magnetic field to a specific value. Unlike the last two cases the MBS $I-V$ curves are robust to change the magnetic field. Therefore, the magnetic-field dependence of the tunneling current can serve as a unique signature for the presence of a MBS.

Probes that measure the local thermal properties of systems out of equilibrium are emerging as new tools in the study of nanoscale systems. One can then measure the temperature of a probe that is weakly coupled to a bias-driven system. By tuning the probe temperature so that the expectation value of some observable of the system is minimally perturbed, one obtains a parameter that measures its degree of local statistical excitation, and hence its local heating. However, one anticipates that different observables may lead to different temperatures and thus different local heating expectations. We propose an experimentally realizable protocol to measure such local temperatures and apply it to bias-driven quantum dots. By means of a highly accurate open quantum system approach, we show theoretically that the measured temperature is quite insensitive both to the choice of observable and to the probe-system coupling. In particular, even with observables that are distinct both physically and in their degree of locality, such as the local magnetic susceptibility of the quantum dot and the global spin-polarized current measured at the leads, the resulting local temperatures are quantitatively similar for quantum dots ranging from noninteracting to Kondo-correlated regimes, and are close to those obtained with the traditional “local equilibrium” definition.

We present results of the electronic band structure, Fermi surface, and electron transport property calculations in the orthorhombic $n$- and $p$-type SnSe, applying the Korringa-Kohn-Rostoker method and the Boltzmann transport approach. The analysis accounted for the temperature effect on crystallographic parameters in $Pnma$ structure as well as the phase transition to $CmCm$ structure at $T_c\sim 807$ K. Remarkable modifications of the conduction and valence bands were noticed upon varying crystallographic parameters within the structure before $T_c$, while the phase transition mostly leads to the jump in the band-gap value. The diagonal components of the kinetic parameter tensors (velocity, effective mass) and resulting transport quantity tensors [electrical conductivity $\sigma$, thermopower $S$, and power factor (PF)] were computed for a wide range of temperature (15–900 K) and hole $(p$-type) and electron $(n$-type) concentrations $\left({10}^{17}\text{–}{10}^{21}{\mathrm{cm}}^{-3}\right)$. SnSe is shown to have a strong anisotropy of the electron transport properties for both types of charge conductivity, as expected for the layered structure, with the generally heavier $p$-type effective masses compared to $n$-type ones. Interestingly, $p$-type SnSe has strongly nonparabolic dispersion relations, with the “pudding-mold-like” shape of the highest valence band. The analysis of $\sigma ,S$, and PF tensors indicates that the interlayer electron transport is beneficial for thermoelectric performance in $n$-type SnSe, while this direction is blocked in $p$-type SnSe, where in-plane transport is preferred. Our results predict that $n$-type SnSe is potentially even better thermoelectric material than $p$-type SnSe. Theoretical results are compared with the single-crystal $p$-SnSe measurements, and good agreement is found below 600 K. The discrepancy between the computational and experimental data, appearing at higher temperatures, can be explained assuming an increase of the hole concentration versus $T$, which is correlated with the experimental Hall data.

This study analyzes thermoelectric properties of a one-dimensional random conductor which shows localization effects and simultaneously includes resonant scatterers yielding sharp conductance resonances. These sharp features give rise to a distinct behavior of the Seebeck coefficient in finite systems and incorporate the degree of localization as a means to enhance thermoelectric performance, in principle. The model for noninteracting electrons is discussed within the Landauer-Büttiker formalism such that analytical treatment is possible for a wide range of properties, if a special averaging scheme is applied. The approximations in the averaging procedure are tested with numerical evaluations showing good qualitative agreement, with some limited quantitative disagreement. The validity of low-temperature Mott's formula is determined and a good approximation is developed for the intermediate temperature range. In both regimes the intricate interplay between Anderson localization due to disorder and conductance resonances of the disorder potential is analyzed.