A microelectromechanical system vibrating in its shear mode was used to study the viscosity of normal liquid ${}^3\mathrm{He}$ from 20 to 770 mK at 3, 21, and 29 bars. The damping coefficient of the oscillator was determined by frequency sweeps through its resonance at each temperature. Using a slide film damping model, the viscosity of the fluid was obtained. Our viscosity values are compared with previous measurements and with calculated values from Fermi-liquid theory. The crossover from the classical to the Fermi-liquid regime is manifest in the temperature dependence of viscosity. In the Fermi-liquid regime, the temperature dependence of viscosity changes from $T^{-1}$ to $T^{-2}$ on cooling, indicating a transition from the Stokes flow to the Couette flow regime.

The magnetoelectric effect predicted in topological insulators makes heterostructures that combine magnetic materials and such insulators promising candidates for spintronics applications. Here, we theoretically consider a setup that exhibits two well-separated interfaces between a topological insulator and a ferromagnetic insulator. We show that there is a topological magnetic dipole-dipole interaction stemming from long-range Coulomb interactions. We analytically derive the magnetization dynamics at the two interfaces and discuss how the long-range coupling can be applied to nonlocally induce the formation of a magnetic texture at one interface by suitably gating the other interface.

In this paper, we study the rotated ferromagnetic Heisenberg model (RFHM) in two different transverse fields, $h_x$ and $h_z$, which can be intuitively visualized as studying spin-orbit coupling (SOC) effects in two-dimensional (2D) Ising or anisotropic $XY$ model in a transverse field. At a special SOC class, it was found in our previous work [Phys. Rev. A 92, 043609 (2015)] that the RFHM at a zero field owns an exact spin-orbit coupled ground state called the $Y\text{−}x$ state. It supports not only the commensurate magnons (called $\mathrm{C}\text{−}{\mathrm{C}}_0$ and $\mathrm{C}\text{−}{\mathrm{C}}_{\pi })$, but also the incommensurate magnons (called C-IC). These magnons are nonrelativistic, not embedded in the exact ground state, so need to be thermally excited or generated by various external probes. Their dramatic response under a longitudinal $h_y$ field was recently worked out by Sun et al. [arXiv:1502.05338]. Here we find they respond very differently under the two transverse fields. Any $h_x$ $\left(h_z\right)$ introduces quantum fluctuations to the ground state and changes the collinear $Y\text{−}x$ state to a canted coplanar $YX\text{−}x$ $\left(YZ\text{−}x\right)$ state. The $\mathrm{C}\text{−}{\mathrm{C}}_0,\mathrm{C}\text{−}{\mathrm{C}}_{\pi }$, and C-IC magnons become relativistic and sneak into the quantum ground state. We determine the competing boundaries among the $\mathrm{C}\text{−}{\mathrm{C}}_0,\mathrm{C}\text{−}{\mathrm{C}}_{\pi }$, and C-IC magnons, especially the detailed dispersions of the C-IC magnons inside the canted phases, which can be mapped out by the transverse spin structure factors. As $h_x$ $\left(h_z\right)$ increases further, the $\mathrm{C}\text{−}{\mathrm{C}}_0$ magnons always win the competition and emerge as the seeds to drive a transition from the $YX\text{−}x$ (or $YZ\text{−}x)$ to the ferromagnetic along the $X$ ($\text{or}Z)$ direction called the $X$-FM (or $Z$-FM) phase. We show that the transition is in the 3D Ising universality class and it becomes the 3D $XY$ transition at the two Abelian points. We evaluate these magnons' contributions to magnetization and specific heat at low temperatures which can be measured by various established experimental techniques. The nature of the finite-temperature transitions are also studied. Some analogies with quantum fluctuations generated multiple vortices and multiple landscapes in quantum spin glass are mentioned. The implications to cold-atom systems and some $4d$ and $5d$ materials with strong SOC are briefly discussed.

Complex-oxide interfaces can give rise to two-dimensional electron gases (2DEGs) with extremely high densities: for ${\mathrm{SrTiO}}_3/{\mathrm{GdTiO}}_3$ (STO/GTO), a density of 1/2 electron per unit-cell area is found within the STO. In this work we use first-principles calculations to study GTO/STO/GTO heterostructures, where both interfaces contribute electrons to the STO. We find that for a thick STO layer the electrons from the interfaces delocalize over multiple ${\mathrm{TiO}}_2$ planes. Once the STO thickness is reduced below three layers, we find that the electrons localize on every other interfacial Ti atom, leading to an insulating phase. We attribute this localization to the combination of high electron density and distortions at the interface. This is further confirmed by a model of the transition based on electron doping of bulk STO, allowing for the same type of distortions as at the interface with GTO. These findings elucidate previous observations [P. Moetakef et al., Phys. Rev. B 86, 201102 (2012)], but our proposed physical mechanisms are general and should apply to other complex-oxide interfaces as well.

We employ the Gutzwiller variational approach to investigate the interplay of Coulomb interaction and spin-orbit coupling in a three-orbital Hubbard model. Already in the paramagnetic phase we find a substantial renormalization of the spin-orbit coupling that enters the effective single-particle Hamiltonian for the quasiparticles. Only close to half band-filling and for sizable Coulomb interaction do we observe clear signatures of Hund's atomic rules for spin, orbital, and total angular momentum. For a finite local Hund's rule exchange interaction we find a ferromagnetically ordered state. The spin-orbit coupling considerably reduces the size of the ordered moment, it generates a small ordered orbital moment, and it induces a magnetic anisotropy. To investigate the magnetic anisotropy energy, we use an external magnetic field that tilts the magnetic moment away from the easy axis $(1,1,1)$.

We study excitonic processes second order in the electric fields in noncentrosymmetric crystals. We derive formulas for shift current and second harmonic generation produced by exciton creation, by using the Floquet formalism combined with the Keldysh Green's function method. It is shown that (i) the steady dc shift current flows by exciton creation without dissociation into free carriers and (ii) second harmonic generation is enhanced at the exciton resonance. The obtained formulas clarify topological aspects of these second order excitonic processes which are described by Berry connections of the relevant valence and conduction bands.

The $GW$ approach of many-body perturbation theory has become a common tool for calculating the electronic structure of materials. However, with increasing number of published results, discrepancies between the values obtained by different methods and codes become more and more apparent. For a test set of small- and wide-gap semiconductors, we demonstrate how to reach the numerically best electronic structure within the framework of the full-potential linearized augmented plane-wave (FLAPW) method. We first evaluate the impact of local orbitals in the Kohn-Sham eigenvalue spectrum of the underlying starting point. The role of the basis-set quality is then further analyzed when calculating the $G_0{W}_0$ quasiparticle energies. Our results, computed with the exciting code, are compared to those obtained using the projector-augmented plane-wave formalism, finding overall good agreement between both methods. We also provide data produced with a typical FLAPW basis set as a benchmark for other $G_0{W}_0$ implementations.

We develop an analytically solvable model able to qualitatively explain nonhydrogenic exciton spectra observed recently in two-dimensional (2D) semiconducting transition-metal dichalcogenides. Our exciton Hamiltonian explicitly includes additional angular momentum associated with the pseudospin degree of freedom unavoidable in 2D semiconducting materials with honeycomb structure. We claim that this is the key ingredient for understanding the nonhydrogenic exciton spectra that was missing so far.

We calculate the anomalous Hall conductivity ${\sigma }_{xy}$ of the surface states in cubic topological Kondo insulators. We consider a generic model for the surface states with three Dirac cones on the (001) surface. The Fermi velocity, the Fermi momentum, and the Zeeman energy in different Dirac pockets may be unequal. The microscopic impurity potential mediates mixed intra- and interband extrinsic scattering processes. Our calculation of ${\sigma }_{xy}$ is based on the Kubo-Streda diagrammatic approach. It includes diffractive skew scattering contributions originating from the rare two-impurity complexes. Remarkably, these contributions yield anomalous Hall conductivity that is independent of impurity concentration, and thus is of the same order as other known extrinsic side jump and skew scattering terms. We discuss various special cases of our results and the experimental relevance of our study in the context of the recent hysteretic magnetotransport data in ${\mathrm{SmB}}_6$ samples.

We have measured the scattering strength of charged impurities on a semiconducting single-walled carbon nanotube with known chirality. The resistivity of the nanotube is measured as a function of the density of adsorbed potassium atoms, enabling the determination of the resistance added by an individual potassium atom. Holes are scattered 37 times more efficiently than electrons by an adsorbed potassium atom. The determined scattering strength is used to reveal the spatial extent and depth of the scattering potential for potassium, a model Coulomb adsorbate. Our result represents an essential experimental input to understand adsorbate-induced scattering and provides a crucial step for paving the way to rational design of nanotube-based sensors.

We investigate theoretically the energy exchange between the electrons of two copropagating, out-of-equilibrium edge states with opposite spin polarization in the integer quantum Hall regime. A quantum dot tunnel coupled to one of the edge states locally injects electrons at high energy. Thereby a narrow peak in the energy distribution is created at high energy above the Fermi level. A second downstream quantum dot performs an energy-resolved measurement of the electronic distribution function. By varying the distance between the two dots, we are able to follow every step of the energy exchange and relaxation between the edge states, even analytically under certain conditions. In the absence of translational invariance along the edge, e.g., due to the presence of disorder, energy can be exchanged by non-momentum-conserving two-particle collisions. For weakly broken translational invariance, we show that the relaxation is described by coupled Fokker-Planck equations. From these we find that relaxation of the injected electrons can be understood statistically as a generalized drift-diffusion process in energy space for which we determine the drift velocity and the dynamical diffusion parameter. Finally, we provide a physically appealing picture in terms of individual edge-state heating as a result of the relaxation of the injected electrons.

We have determined positron annihilation characteristics (lifetime and Doppler broadening) in six basic vacancy-type defects of $6H$-SiC and two nitrogen-vacancy complexes using ab initio calculations. The positron characteristics obtained allow us to point out which positron technique in the most adapted to identify a particular defect. They show that the coincidence Doppler broadening technique is the most relevant for observing the silicon vacancy–nitrogen complexes, ${\mathrm{V}}_{\mathrm{Si}}{\mathrm{N}}_{\mathrm{C}}$, and carbon vacancy–carbon antisite ones, ${\mathrm{V}}_{\mathrm{C}}{\mathrm{C}}_{\mathrm{Si}}$. For the other studied defects, the calculated positron characteristics are found to be too close for the defects to be easily distinguished using a single positron annihilation technique. Then it is required to use complementary techniques, positron annihilation based or other.

We study the role of dipolar interactions in the standard protocol used to achieve dynamic nuclear polarization (DNP). We point out that a critical strength of interactions is required to obtain significant nuclear hyperpolarization. Otherwise, the electron spins do not thermalize among each other, due to the incipient many-body localization transition in the electron spin system. Only when the interactions are sufficiently strong, in the so-called spin-temperature regime, they establish an effective thermodynamic behavior in the out-of-equilibrium stationary state. The highest polarization is reached at a point where the spin temperature is just not able to establish itself anymore. We provide numerical predictions for the level of nuclear hyperpolarization and present an analytical technique to estimate the spin temperature as a function of interaction strength and quenched disorder. We show that, at sufficiently strong coupling, nuclear spins perfectly equilibrate to the spin temperature that establishes among the spins of radicals.

We have used time-of-flight inelastic neutron scattering to measure the spin wave spectrum of the canonical half-doped manganite ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$ in its magnetic and orbitally ordered phase. The data, which cover multiple Brillouin zones and the entire energy range of the excitations, are compared with several different models that are all consistent with CE-type magnetic order, but arise through different exchange coupling schemes. The Goodenough model, i.e., an ordered state comprising strong nearest-neighbor ferromagnetic interactions along zigzag chains with antiferromagnetic interchain coupling, provides the best description of the data, provided that further neighbor interactions along the chains are included. We are able to rule out a coupling scheme involving formation of strongly bound ferromagnetic dimers, i.e., Zener polarons, on the basis of gross features of the observed spin wave spectrum. A model with weaker dimerization reproduces the observed dispersion but can be ruled out on the basis of discrepancies between the calculated and observed structure factors at certain positions in reciprocal space. Adding further neighbor interactions results in almost no dimerization, i.e., recovery of the Goodenough model. These results are consistent with theoretical analysis of the degenerate double exchange model for half-doping, and provide a recipe for how to interpret future measurements away from half-doping, where degenerate double exchange models predict more complex ground states.

Broadband microwave spectroscopy has been performed on single-crystalline ${\mathrm{GaV}}_4{\mathrm{S}}_8$, which exhibits a complex magnetic phase diagram including cycloidal and Néel-type skyrmion lattice phases below 13 K. At small magnetic fields two collective modes with large frequency separation are found that reflect the cycloid state subject to a large uniaxial anisotropy. In the skyrmion lattice phase three modes are resolved. Analyzing the spin-precessional motion microscopically, we explain the low frequency of the breathing mode in these Néel skyrmions by the hard-axis orientation of relevant spins.

The size limitation of ab initio calculations impedes first-principles simulations of crystal defects at nanometer sizes. Considering clusters of self-interstitial atoms as a paradigm for such crystal defects, we have developed an ab initio–accuracy model to predict formation energies of defect clusters with various geometries and sizes. Our discrete-continuum model combines the discrete nature of energetics of interstitial clusters and continuum elasticity for a crystalline solid matrix. The model is then applied to interstitial dislocation loops with $\langle 100\rangle$ and $1/2\langle 111\rangle$ Burgers vectors, and to C15 clusters in body-centered-cubic crystals Fe, W, and V, to determine their relative stabilities as a function of size. We find that in Fe the C15 clusters were more stable than dislocation loops if the number of self-interstitial atoms involved was fewer than 51, which corresponds to a C15 cluster with a diameter of $1.5$ nm. In V and W, the $1/2\langle 111\rangle$ loops represent the most stable configurations for all defect sizes, which is at odds with predictions derived from simulations performed using some empirical interatomic potentials. Further, the formation energies predicted by the discrete-continuum model are reparametrized by a simple analytical expression giving the formation energy of self-interstitial clusters as a function of their size. The analytical scaling laws are valid over a very broad range of defect sizes, and they can be used in multiscale techniques including kinetic Monte Carlo simulations and cluster dynamics or dislocation dynamics studies.

Lead fluoride, $\mathrm{Pb}{\mathrm{F}}_2$, was investigated experimentally in the laser-heated diamond anvil cell by x-ray diffraction to pressures of 75 GPa at room temperature and to 64.5 GPa and 2430 K, as well as through first-principles density functional theory calculations up to 70 GPa. During room temperature compression, no discontinuous changes in the x-ray diffraction pattern or volume were observed, but the lattice parameters displayed highly anomalous trends between 10–22 GPa with enhanced compressibility along the $a$ direction and reduced or even negative compressibility along $b$ and $c$. Theoretical calculations of valence electron densities at 22 GPa showed that $\alpha -\mathrm{Pb}{\mathrm{F}}_2$ underwent a pressure-induced isosymmetric phase transition to a postcotunnite $\mathrm{C}{\mathrm{o}}_2\mathrm{Si}$ structure and also revealed the detailed atomic rearrangements associated with the development of an extra Pb-F bond in the high-pressure phase. Our x-ray results and theoretical calculations are consistent with an isosymmetric phase transition smoothly occurring over 10–22 GPa rather than abruptly as previously suggested. The characteristic values for the cell constants $a/c$ and $\left(a+c\right)/b$, which are used to distinguish among cotunnite-, $\mathrm{C}{\mathrm{o}}_2\mathrm{Si}$-, and $\mathrm{N}{\mathrm{i}}_2\mathrm{In}$-type phases, require modification based on our results. An equation of state fit yields a bulk modulus, ${\mathrm{K}}_0$, of 72(3) GPa for the cotunnite-type, and an ambient-pressure volume, ${\mathrm{V}}_0$, of $182\left(2\right){\AA }^3$, and ${\mathrm{K}}_0=81\left(4\right)\mathrm{GPa}$ for the $\mathrm{C}{\mathrm{o}}_2\mathrm{Si}$-type phase when fixing the pressure derivative of the bulk modulus, ${K_0}^{\prime }=4$. Upon heating above 1200 K at pressures at or above 25.9 GPa, $\mathrm{Pb}{\mathrm{F}}_2$ partially transformed to the hexagonal $\mathrm{N}{\mathrm{i}}_2\mathrm{In}$-type phase but wholly or partially reverted back to $\mathrm{C}{\mathrm{o}}_2\mathrm{Si}$-type phase upon temperature quench. From 43–65 GPa, nearly complete transformation to the $\mathrm{N}{\mathrm{i}}_2\mathrm{In}$-type $\mathrm{Pb}{\mathrm{F}}_2$ was observed at high temperature, but the material partially transformed back to the orthorhombic phase upon temperature quench. Our results show that high-pressure behavior of $\mathrm{Pb}{\mathrm{F}}_2$ is distinct from that of the alkaline earth fluorides with similar ionic radii. Our results also have relevance to understanding the behavior of lanthanide and actinide dioxides, which have been predicted theoretically to exhibit similar isosymmetric transitions at Mbar pressures.