The magnetic properties of the electride compound ${\mathrm{Y}}_2\mathrm{C}$ were investigated by muon spin rotation and magnetic susceptibility on two samples with different forms (polycrystalline and single crystalline), to examine the theoretically predicted Stoner ferromagnetism for electride bands. There was no evidence of static magnetic order in both samples even at temperatures down to 0.024 K. For the polycrystalline sample, the presence of a paramagnetic moment at the Y sites was inferred from the Curie-Weiss behavior of the muon Knight shift and susceptibility, whereas no such tendency was observed in the single-crystalline sample. These observations suggest that the electronic ground state of ${\mathrm{Y}}_2\mathrm{C}$ is at the limit between weak-to-strong electronic correlation, where on-site Coulomb repulsion is sensitive to a local modulation of the electronic state or a shift in the Fermi level due to the presence of defects and/or impurities.

We present a theory of molecular motors based on the Ehrenfest dynamics for nuclear coordinates and the adiabatic limit of the Kadanoff-Baym equations for current-induced forces. Electron-electron interactions can be systematically included through many-body perturbation theory, making the nonequilibrium Green's function formulation suitable for first-principles treatments of realistic junctions. The method is benchmarked against simulations via real-time Kadanoff-Baym equations, finding an excellent agreement. Results on a paradigmatic model of a molecular motor show that correlations can change dramatically the physical scenario by, e.g., introducing a sizable damping in self-sustained van der Pol oscillations.

We report an asymmetry in the $R$ vs $T$ characteristics across the first-order metal-insulator transition (MIT) of ${\mathrm{V}}_2{\mathrm{O}}_3$ nanowires. The resistance changes in a few, large jumps during cooling through the MIT, while it does it in a smoother way during warming. The asymmetry is greatly enhanced as the width of the nanowire approaches a characteristic domain size. Our results, together with previous reports on ${\mathrm{VO}}_2$ [W. Fan et al., Phys. Rev. B 83, 235102 (2011)] and FeRh [V. Uhlíř et al., Nat. Commun. 7, 13113 (2016)] imply that asymmetry is a generic feature of first-order phase transitions in one-dimensional systems. We show that this behavior is a simple, elegant consequence of the combined effects of the transition hysteresis and the temperature dependence of the insulating gap in this case (and generically, the order parameter relevant for the physical observable). We conclude that our proposed asymmetry mechanism is universally applicable to many electronic first-order phase transitions.

Spin- and angle-resolved photoelectron spectroscopy is commonly used to determine the spin texture of the occupied electronic states. If spin-orbit coupling is strong, the spin polarization of the photoelectrons and that of the initial states may deviate significantly. To alleviate part of this problem we propose a recipe for improved spin retrieval. The basic idea is to combine photoemission intensities from (at least) two different photoemission experiments in a way which reflects the symmetry of the photoemission setups; the procedure avoids group-theoretical analyses or relativistic photoemission calculations. In this paper we introduce the approach, motivated by the example of photoemission from W(110) illuminated by circularly polarized light. Limitations of the method are discussed.

Motivated by neutral excitations in disordered electronic materials and systems of trapped ultracold particles with long-range interactions, we study energy-level statistics of quasiparticles with the power-law hopping Hamiltonian $\propto 1/r^{\alpha }$ in a strong random potential. In solid-state systems such quasiparticles, which are exemplified by neutral dipolar excitations, lead to long-range correlations of local observables and may dominate energy transport. Focusing on the excitations in disordered electronic systems, we compute the energy-level correlation function $R_2\left(\omega \right)$ in a finite system in the limit of sufficiently strong disorder. At small energy differences, the correlations exhibit Wigner-Dyson statistics. In particular, in the limit of very strong disorder the energy-level correlation function is given by $R_2(\omega ,V)=A_3\frac{\omega }{{\omega }_V}$ for small frequencies $\omega \ll {\omega }_V$ and $R_2(\omega ,V)=1-(\alpha -d)A_1{\left(\frac{{\omega }_V}{\omega }\right)}^{\frac{d}{\alpha }}-A_2{\left(\frac{{\omega }_V}{\omega }\right)}^2$ for large frequencies $\omega \gg {\omega }_V$, where ${\omega }_V\propto V^{-\frac{\alpha }{d}}$ is the characteristic matrix element of excitation hopping in a system of volume $V$, and $A_1,A_2$, and $A_3$ are coefficients of order unity which depend on the shape of the system. The energy-level correlation function, which we study, allows for a direct experimental observation, for example, by measuring the correlations of the ac conductance of the system at different frequencies.

The dynamics of the order fluctuations in the ${\mathrm{AuAgZn}}_2$ close to the critical point ($T_c\simeq 609$ K) was observed by coherent x-ray scattering. With the high beam intensity of the ID10 ESRF beamline and with the new pixel detector, the dynamics was measured with a few tens of millisecond resolution. The intensity connected to the diffuse scattering corresponding to fluctuations was unambiguously distinguished from the surface pretransitional ordering occurring in this system close to $T_c$. The variations of the fluctuation time with temperature and wave vectors were measured in this system belonging to the universality class of Ising second order transition with a nonconserved order parameter. The direct observation of the critical slowing down in the vicinity of the second-order transition led to an estimate of the dynamic exponent $z\simeq 1.96\left(11\right)$, in rough agreement with theory (model “A” of Hohenberg and Halperin [Rev. Mod. Phys. 49, 435 (1977)]).

We study many-body localization for a disordered chain of spin-1/2 fermions. When both down and up components are exposed to the same strong disorder, a power-law growth of the entanglement entropy was observed [P. Prelovšek et al., Phys. Rev. B 94, 241104 (2016)] that suggests that many-body localization is not complete; the density (charge) degree of freedom is localized, while the spin degree of freedom is apparently delocalized. We show that this powerlike behavior is only a transient effect and that, for longer times, the growth of the entanglement entropy is logarithmic in time. At the same time, the decay of the spin correlation slows down, showing that the spin transport is strongly reduced at long times. The dynamics of the system is quite similar to the one of many-body localized systems. Because of the limited duration of the numerical experiment, it is not possible to determine whether the system is truly many-body localized or diffusing very slowly at long times.

A classic Onsager reciprocity relation for Fourier heat conduction in the absence of magnetic fields states that the thermal conductivity tensor in bulk anisotropic solids is symmetric. However, since Fourier's law fails in thin dielectric films due to ballistic phonon transport effects, it is natural to ask whether an analogous Onsager relation can be identified in the boundary scattering regime. To answer this question, we solve the Boltzmann transport equation (BTE) under the relaxation time approximation for in-plane and cross-plane heat transport for thin films with anisotropic phonon dispersion relations and scattering rates. We use these BTE solutions to show that the thermal conductivity tensor of thin films is symmetric from the diffusive regime through the boundary scattering regime. We illustrate this reciprocity by calculating thermal conductivity suppression functions for a model anisotropic material. We compare our BTE solution to previous atomistic simulations of arbitrarily aligned graphite thin films, and use published first-principles calculations to model anisotropic in-plane heat flow in aligned black phosphorus. Our derivation shows how Onsager reciprocity for anisotropic heat conduction extends into the boundary scattering regime, and reduces the number of independent measurements required to fully characterize heat transport in anisotropic thin films.

In the theory of antiferromagnetism, the staggered field—an external magnetic field that alternates in sign on atomic length scales—is used to select the classical Néel state from a quantum magnet but justification is missing. This work examines, within the decoherence framework, whether repeated local measurement can replace a staggered field. Accordingly, the conditions under which local decoherence can be considered a continuous measurement are studied. The dynamics of a small magnetic system is analyzed to illustrate that local decoherence can lead to (symmetry-broken) order similar to order resulting from a staggered field.

In this paper we present a comprehensive study of the spontaneous currents in time-reversal symmetry breaking (TRSB) multicomponent superconductors with cubic crystalline symmetry. We argue, not limiting to cubic lattices, that spontaneous current on certain high-symmetry surfaces can exist only if the TRSB pairing simultaneously breaks a certain pair of mirror symmetries. This is shown to have exact correspondence with the Gingzburg-Landau (GL) theory and is verified by numerical Bogoliubov de-Gennes (BdG) calculations. In the course we extend the BdG to include effects of gap anisotropy and surface disorder, both of which could lead to much suppressed current. The GL theory has been known to describe well the spontaneous current. However, we highlight a special case where it becomes less adequate and show that a refined effective theory for low temperatures is needed. These results could shed light on the phenomenology of cubic superconductors such as ${\mathrm{U}}_{1-x}{\mathrm{Th}}_x{\mathrm{Be}}_{13}$, the filled skutterudites ${\mathrm{PrOs}}_4{\mathrm{Sb}}_{12}$, ${\mathrm{PrPt}}_4{\mathrm{Ge}}_{12}$, and related compounds.

Intra-unit cell magnetic order has been observed in four different families of high-temperature superconductors from polarized neutron diffraction experiments and supported by several other techniques. That order, which does not break translation symmetry, is consistent with the predicted orbital moments generated by two microscopic loop currents in each ${\mathrm{CuO}}_2$ cell. Recently, using polarized neutron diffraction, Croft et al. [Phys. Rev. B 96, 214504 (2017)] claim to find no evidence for such orbital loop currents in charge ordered ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$. Their experiment is done with detwinned samples at least 100 times smaller than in previous experiments without counting much longer. We show by a detailed quantitative analysis of their data that contrary to their conclusion, the magnetic signal falls below their threshold of detection. None of the data reported by Croft et al. challenge the universality of the intra-unit cell order in cuprates.

The issues raised in the preceding comment of Bourges et al. [Phys. Rev. B 98, 016501 (2018)], are shown to be unfounded. We highlight the complications caused by inhomogeneous beam polarization that can occur when using polarized neutron diffraction to detect small magnetic moments.

We derive a quantum-mechanical formula of the orbital magnetic quadrupole moment (MQM) in periodic systems by using the gauge-covariant gradient expansion. This formula is valid for insulators and metals at zero and nonzero temperature. We also prove a direct relation between the MQM and magnetoelectric (ME) susceptibility for insulators at zero temperature. It indicates that the MQM is a microscopic origin of the ME effect. Using the formula, we quantitatively estimate these quantities for room-temperature antiferromagnetic semiconductors ${\mathrm{BaMn}}_2{\mathrm{As}}_2$ and ${\mathrm{CeMn}}_2{\mathrm{Ge}}_{2-x}{\mathrm{Si}}_x$. We find that the orbital contribution to the ME susceptibility is comparable with or even dominant over the spin contribution.

In an antiferromagnet (AF) with uniaxial anisotropy, spin-up and spin-down magnons coexist and form an intrinsic degree of freedom resembling electrons. When polarized by an adjacent ferromagnet (F), a magnonic pure spin current can be thermally generated in an AF. We explore thermal magnon transport in an insulating F/AF/F trilayer where propagating magnons inside the AF spacer can transfer angular momenta between the two Fs. We find that a sufficiently large temperature gradient can switch the downstream F via a magnonic spin-transfer torque if it is initially antiparallel with the upstream F. A reciprocal switching is achievable by reversing the temperature gradient. In typical materials, we estimate the threshold to be 1 K/nm at room temperature, which can be reduced by enhancing the interfacial exchange coupling and by increasing the temperature.

The coherence of an electronic spin in a semiconductor quantum dot decays due to its interaction with the bath of nuclear spins in the surrounding isotopes. This effect can be reduced by subjecting the system to an external magnetic field and by applying optical pulses. By repeated pulses in long trains the spin precession can be synchronized to the pulse period $T_{\mathrm{R}}$. This drives the nuclear spin bath into states far from equilibrium leading to nuclear frequency focusing. In this paper, we use an efficient classical approach introduced in Fauseweh et al. [Fauseweh, Schering, Hüdepohl, and Uhrig, Phys. Rev. B 96, 054415 (2017)] to describe and to analyze this nuclear focusing. Its dependence on the effective bath size and on the external magnetic field is elucidated in a comprehensive study. We find that the characteristics of the pulse as well as the nuclear Zeeman effect influence the behavior decisively.

Anomalous Hall effect in epitaxially grown $L{1}_0$-ordered FePd and FePt films on MgO(001) has been investigated as a function of the film thickness ($d$). It is found that the anomalous Hall resistivity can be described by conventional scaling law, i.e., ${\rho }_{\mathrm{AH}}=a{\rho }_{xx}+b{\rho }_{xx}^2$ with longitudinal resistivity ${\rho }_{xx}$. It is interesting to find that both $a$ and $b$ parameters approximately change as a linear function of the inverse film thickness. The linear $1/d$ dependencies can be attributed to the symmetry breaking at the surface. Moreover, the bulk and in particular the surface contributions to the parameters $a$ and $b$ in $L{1}_0$ FePt films are enhanced, compared with those of $L{1}_0$ FePd films, due to larger spin orbit interaction of heavier Pt atoms. It is indicated that not only the bulk but also the interface components of $a$ and $b$ are tuned by the spin-orbit interaction. The present results might stimulate further theoretical investigation of the mechanism of the anomalous Hall effect.