We present simulation results on the structure and dynamics of micromagnetic point singularities with atomistic resolution. This is achieved by embedding an atomistic computational region into a standard micromagnetic algorithm. Several length scales are bridged by means of an adaptive mesh refinement and a seamless coupling between the continuum theory and a Heisenberg formulation for the atomistic region. The code operates on graphical processing units and is able to detect and track the position of strongly inhomogeneous magnetic regions. This enables us to reliably simulate the dynamics of Bloch points, which means that a fundamental class of micromagnetic switching processes can be analyzed with unprecedented accuracy. We test the code by comparing it with established results and present its functionality with the example of a simulated field-driven Bloch-point motion in a soft-magnetic cylinder.

The galvanomagnetic and thermogalvanomagnetic transport of the prototypical ferromagnetic transition-metal alloy system fcc ${\mathrm{Co}}_x{\mathrm{Pd}}_{1-x}$ has been investigated on the basis of Kubo's linear response formalism. The results for the full electric conductivity tensor allow discussing the spin-orbit-induced anisotropic magnetoresistance and the anomalous Hall effect. These are complemented by results for the corresponding thermogalvanomagnetic transport properties anisotropy of the Seebeck effect and anomalous Nernst effect. The relation between the respective response coefficients is discussed with the underlying electronic structure calculated relativistically within the Korringa-Kohn-Rostoker coherent potential approximation band structure method for disordered alloys.

We investigate the effects of quantum (zero-temperature) long-wavelength fluctuations of free-standing crystalline membranes, which are two-dimensional objects embedded into three-dimensional space. The fluctuations produce logarithmic renormalization of elasticity and bending moduli of the membranes. We find one-loop RG equations to demonstrate that the system is in the “asymptotic freedom” regime; that is, the quantum fluctuations destabilize the flat membrane phase.

The $n$-doped and undoped silicon nanowires grown by chemical vapor deposition on quartz substrate were characterized using optical-pump terahertz-probe transmission experiments. Temporal decays of the differential transmission measurements are reproduced using a biexponential function with an initial photocarrier lifetime of $\sim$2 ps and a longer decay time of 10 ps to a few tens of picoseconds. Based on the influence of the laser fluence, a carrier capture and recombination scenario is proposed to explain these temporal decay curves. For both samples, the capture of photocarriers by the traps present on the surface of the nanowires plays an important role in the observed photoconductivity dynamics. Frequency-dependent complex photoconductivity data curves are extracted from the terahertz (THz) traces taken at different optical-pump THz-probe delays. These data curves are reproduced using a plasmon resonance model. The fitting procedure allows us to determine carrier scattering times of about $28\pm 6$ and $14\pm 4$ fs for the undoped and doped samples, respectively. Our results show that defects and ionized impurities introduced by $n$ doping the silicon nanowires reduce the photocarrier mobility and lifetime.

Ballistic spin resonance was experimentally observed in a quasi-one-dimensional wire by Frolov et al. [Nature (London) 458, 868 (2009)]. The spin resonance was generated by a combination of an external static magnetic field and the oscillating effective spin-orbit magnetic field due to periodic bouncings of the electrons off the boundaries of a narrow channel. An increase of the D'yakonov-Perel spin relaxation rate was observed when the frequency of the spin-orbit field matched that of the Larmor precession frequency around the external magnetic field. Here we develop a model to account for the D'yakonov-Perel mechanism in multisubband quantum wires with both the Rashba and Dresselhaus spin-orbit interactions. Considering elastic spin-conserving impurity scatterings in the time-evolution operator (Heisenberg representation), we extract the spin relaxation time by evaluating the time-dependent expectation value of the spin operators. The magnetic field dependence of the nonlocal voltage, which is related to the spin relaxation time behavior, shows a wide plateau, in agreement with the experimental observation. This plateau arises due to injection in higher subbands and small-angle scattering. In this quantum mechanical approach, the spin resonance occurs near the spin-orbit-induced energy anticrossings of the quantum wire subbands with opposite spins. We also predict anomalous dips in the spin relaxation time as a function of the magnetic field in systems with strong spin-orbit couplings.

In a recent article, Grover [Phys. Rev. Lett. 111, 130402 (2013)] introduced a simple method to compute Renyi entanglement entropies in the realm of the auxiliary field quantum Monte Carlo algorithm. Here, we develop this approach further and provide a stabilization scheme to compute higher-order Renyi entropies and an extension to access the entanglement spectrum. The method is tested on systems of correlated topological insulators.

We used first-principles calculations to investigate the existence and origin of the ferroelectric instability in the ABF${}_3$ fluoroperovskites. While the ground states of most $AB$F${}_3$ compounds are paraelectric ($Pnma$ phase), we find that many fluoroperovskites have a ferroelectric instability in their high-symmetry cubic structure that is of similar amplitude to that commonly found in oxide perovskites. In contrast to the oxides, however, the fluorides have nominal Born effective charges, indicating a different mechanism for the instability. We show that the instability originates from ionic size effects, and is therefore in most cases largely insensitive to pressure and strain, again in contrast to the oxide perovskites. An exception is NaMnF${}_3$, where coherent epitaxial strain matching to a substrate with equal $in$-plane lattice constants destabilizes the bulk Pnma structure, leading to a ferroelectric, and indeed multiferroic, ground state with an unusual polarization/strain response.

We demonstrate gate control of the electronic $g$ tensor in single and double quantum dots formed along a bend in a carbon nanotube. From the dependence of the single-dot excitation spectrum on magnetic field magnitude and direction, we extract spin-orbit coupling, valley coupling, and spin and orbital magnetic moments. Gate control of the $g$ tensor is measured using the splitting of the Kondo peak in conductance as a sensitive probe of Zeeman energy. In the double-quantum-dot regime, the magnetic field dependence of the position of cotunneling lines in the two-dimensional charge stability diagram is used to infer the real-space positions of the two dots along the nanotube.

We study a two-dimensional frustrated Heisenberg antiferromagnet on the windmill lattice consisting of triangular and dual honeycomb lattice sites. In the classical ground state, the spins on different sublattices are decoupled, but quantum and thermal fluctuations drive the system into a coplanar state via an “order from disorder” mechanism. We obtain the finite temperature phase diagram using renormalization group approaches. In the coplanar regime, the relative U$\left(1\right)$ phase between the spins on the two sublattices decouples from the remaining degrees of freedom, and is described by a six-state clock model with an emergent critical phase. At lower temperatures, the system enters a ${\mathbb{Z}}_6$ broken phase with long-range phase correlations. We derive these results by two distinct renormalization group approaches to two-dimensional magnetism: Wilson-Polyakov scaling and Friedan's geometric approach to nonlinear sigma models where the scaling of the spin stiffnesses is governed by the Ricci flow of a 4D metric tensor.

The key feature of a quantum spin coupled to a harmonic bath—a model dissipative quantum system—is competition between oscillator potential energy and spin tunneling rate. We show that these opposing tendencies cause environmental entanglement through superpositions of adiabatic and antiadiabatic oscillator states, which then stabilizes the spin coherence against strong dissipation. This insight motivates a fast-converging variational coherent-state expansion for the many-body ground state of the spin-boson model, which we substantiate via numerical quantum tomography.

The complex optical properties of a single crystal of hexagonal FeCrAs ($T_N\simeq 125$ K) have been determined above and below $T_N$ over a wide frequency range in the planes (along the $b$ axis) and along the perpendicular ($c$-axis) direction. At room temperature, the optical conductivity ${\sigma }_1\left(\omega \right)$ has an anisotropic metallic character. The electronic band structure reveals two bands crossing the Fermi level, allowing the optical properties to be described by two free-carrier (Drude) contributions consisting of a strong, broad component and a weak, narrow term that describes the increase in ${\sigma }_1\left(\omega \right)$ below $\simeq$15 meV. The dc resistivity of FeCrAs is “nonmetallic,” meaning that it rises in power-law fashion with decreasing temperature, without any signature of a transport gap. In the analysis of the optical conductivity, the scattering rates for both Drude contributions track the dc resistivity quite well, leading us to conclude that the nonmetallic resistivity of FeCrAs is primarily due to a scattering rate that increases with decreasing temperature, rather than the loss of free carriers. The power law ${\sigma }_1\left(\omega \right)\propto {\omega }^{-0.6}$ is observed in the near-infrared region and as $T\to T_N$ spectral weight is transferred from low to high energy ($\gtrsim$0.6 eV); these effects may be explained by either the two-Drude model or Hund's coupling. We also find that a low-frequency in-plane phonon mode decreases in frequency for $T<T_N$, suggesting the possibility of spin-phonon coupling.

We present data for the energy dissipation factor $Q^{-1}$ over a broad temperature range at various pressures of a torsion pendulum setup used to study ${}^3\mathrm{He}$ confined in a 98% open silica aerogel. Values for $Q^{-1}$ above $T_c$ are temperature independent and have weak pressure dependence. Below $T_c$, a deliberate axial compression of the aerogel by 10% widens the range of metastability for a superfluid equal spin pairing (ESP) state; we observe this ESP phase on cooling and the B phase on warming over an extended temperature region. While the dissipation for the B phase tends to zero as T $\to$ 0, $Q^{-1}$ exhibits a peak value greater than that at $T_c$ at intermediate temperatures. Values for $Q^{-1}$ in the ESP phase are consistently higher than in the B phase and are proportional to ${\rho }_s/\rho$ until the ESP to B phase transition is attained. We apply a viscoelastic collision-drag model, which couples the motion of the helium and the aerogel through a frictional relaxation time ${\tau }_f$. We conclude that unless ${\tau }_f$ is an order of magnitude larger than expected, an additional mechanism to dissipate energy not captured in the collision-drag model and related to the emergence of the superfluid order must exist. The extra dissipation below $T_c$ is possibly associated with mutual friction between the superfluid phases and the clamped normal fluid. The pressure dependence of the measured dissipation in both superfluid phases is likely related to the pressure dependence of the gap structure of the “dirty” superfluid. The large dissipation in the ESP state is consistent with the phase being the A or the Polar with the order parameter nodes oriented in the plane of the cell and perpendicular to the aerogel anisotropy axis.

Using methods based on conformal field theory, we construct model wave functions on a torus with arbitrary flat metric for all chiral states in the abelian quantum Hall hierarchy. These functions have no variational parameters, and they transform under the modular group in the same way as the multicomponent generalizations of the Laughlin wave functions. Assuming the absence of Berry phases upon adiabatic variations of the modular parameter $\tau$, we calculate the quantum Hall viscosity and find it to be in agreement with the formula, given by Read, which relates the viscosity to the average orbital spin of the electrons. For the filling factor $\nu =2/5$ Jain state, which is at the second level in the hierarchy, we compare our model wave function with the numerically obtained ground state of the Coulomb interaction Hamiltonian in the lowest Landau level, and find very good agreement in a large region of the complex $\tau$ plane. For the same example, we also numerically compute the Hall viscosity and find good agreement with the analytical result for both the model wave function and the numerically obtained Coulomb wave function. We argue that this supports the notion of a generalized plasma analogy that would ensure that wave functions obtained using the conformal field theory methods do not acquire Berry phases upon adiabatic evolution.

The behavior of the polarization switching by applying electric fields ($E$) along the $\langle 100\rangle$ and $\langle 111\rangle$ directions has been investigated for the single crystals of ferroelectric (${\mathrm{Bi}}_{0.5}$${\mathrm{Na}}_{0.5}$)${\mathrm{TiO}}_3$ (BNT) using high-energy synchrotron radiation x-ray diffraction (SR-XRD) and density functional theory (DFT) calculations. Single-crystal SR-XRD analyses reveal that the $E$-induced switching of spontaneous polarization ($P_{\mathrm{s}}$) occurs via the non-180° (71° and/or 109°) $P_{\mathrm{s}}$ rotation. The DFT calculations show that the energy barrier for the 71° $P_{\mathrm{s}}$ rotation is much lower than those for the 109° $P_{\mathrm{s}}$ rotation and for the direct 180° $P_{\mathrm{s}}$ reversal. These experimental and DFT calculation results lead to the conclusion that the 71° $P_{\mathrm{s}}$ rotation is the dominant pathway for the $E$-induced polarization switching in the BNT crystals.

We investigate emergent impurity-induced states arising from pointlike scatterers in the spin-density wave phase of iron-based superconductors within a microscopic five-band model. Independent of the details of the band structure and disorder potential, it is shown how stable magnetic $(\pi ,\pi )$ unidirectional nematogens are formed locally by the impurities. Interestingly, these nematogens exhibit a dimer structure in the electronic density, are directed along the antiferromagnetic $a$ axis, and have typical lengths of $\sim$10 lattice constants, in excellent agreement with recent scanning tunneling experiments. These electronic dimers provide a natural explanation of the dopant-induced transport anisotropy found, e.g., in the 122 iron pnictides.

A nonlocal supercurrent was observed in mesoscopic planar SNS Josephson junctions with additional normal-metal electrodes, where nonequilibrium quasiparticles were injected from a normal-metal electrode into one of the superconducting banks of the Josephson junction in the absence of a net transport current through the junction. We claim that the observed effect is due to a supercurrent counterflow, appearing to compensate for the quasiparticle flow in the SNS weak link. We have measured the responses of SNS junctions for different distances between the quasiparticle injector and the SNS junction at temperatures far below the superconducting transition temperature. The charge-imbalance relaxation length was estimated by using a modified Kadin, Smith, and Skocpol scheme in the case of a planar geometry. The model developed allows us to describe the interplay of charge imbalance and Josephson effects in the nanoscale proximity system in detail.

The large spread of published values for spin Hall angles, even for the most studied normal-metal platinum, reflects the difficulty of investigating the inverse spin Hall effect. In this Rapid Communication we examine the inverse spin Hall effect and anisotropic magnetoresistance for different ${\text{Ni}}_{81}{\text{Fe}}_{19}$(Py)/normal-metal bilayers operated by spin pumping using a coplanar waveguide. Angle, frequency, and temperature dependent measurements show that spin pumping and the inverse spin Hall effect can be used to quantify spin Hall angles only when the anisotropic magnetoresistance as a parasitic voltage generating effect is avoided. In this case we obtain spin Hall angles of $0.12$ and $0.011$ for Pt and Au, respectively. Furthermore, these measurements can be used to determine the spin diffusion length of Pt as a function of temperature.

The electrical and structural characteristics of SmTiO${}_3$/SrTiO${}_3$/SmTiO${}_3$ and GdTiO${}_3$/SrTiO${}_3$/GdTiO${}_3$ heterostructures are compared. Both types of structures contain narrow SrTiO${}_3$ quantum wells, which accommodate a confined, high-density electron gas. As shown previously [Phys. Rev. B 86, 201102(R) (2012)] SrTiO${}_3$ quantum wells embedded in GdTiO${}_3$ show a metal-to-insulator transition when their thickness is reduced so that they contain only two SrO layers. In contrast, quantum wells embedded in SmTiO${}_3$ remain metallic down to a single SrO layer thickness. Symmetry-lowering structural distortions, measured by quantifying the Sr-column displacements, are present in the insulating quantum wells, but are either absent or very weak in all metallic quantum wells, independent of whether they are embedded in SmTiO${}_3$ or in GdTiO${}_3$. We discuss the role of orthorhombic distortions, orbital ordering, and strong electron correlations in the transition to the insulating state.

We demonstrate the basic features of an all-optical spin tomography on picosecond time scale. The magnetization vector associated with a mode-locked electron spin ensemble in singly charged quantum dots is traced by ellipticity measurements using picosecond laser pulses. After optical orientation the spins precess about a perpendicular magnetic field. By comparing the dynamics of two interacting ensembles with the dynamics of a single ensemble we find buildup of a spin component along the magnetic field in the two-ensemble case. This component arises from a Heisenberg-like spin-spin interaction.