Spin dynamics of two-dimensional electrons in moderate in-plane electric fields is studied theoretically. The streaming regime is considered, where each electron accelerates until reaching the optical phonon energy, then it emits an optical phonon, and a new period of acceleration starts. Spin-orbit interaction and elastic scattering result in anisotropic relaxation of electron-spin polarization. The overall spin dynamics is described by a superposition of spin modes in the system. The relaxation time of the most long-living mode depends quasiperiodically on the inverse electric field. The spin modes can be conveniently revealed by means of spin noise spectroscopy. It is demonstrated that the spectrum of spin fluctuations consists of peaks with the low-frequency peak much narrower than satellite ones, and the widths of the peaks are determined by the decay times of the modes.

We investigate Landau level structures of semimetals with nodal ring dispersions. When the magnetic field is applied parallel to the plane in which the ring lies, there exist almost nondispersive Landau levels at the Fermi level ($E_F=0$) as a function of the momentum along the field direction inside the ring. We show that the Landau levels at each momentum along the field direction can be described by the Hamiltonian for the graphene bilayer with fictitious interlayer couplings under a tilted magnetic field. Near the center of the ring where the in-terlayer coupling is negligible, we have Dirac Landau levels which explain the appearance of the zero modes. Although the interlayer hopping amplitudes become finite at higher momenta, the splitting of zero modes is exponentially small and they remain almost flat due to the finite artificial in-plane component of the magnetic field. The emergence of the density of states peak at the Fermi level would be a hallmark of the ring dispersion.

We consider an array of isotropic particles possessing electric polarizability located in the sites of a cubic lattice. The dispersion properties of the structure and the polarization of eigenmodes are studied employing the discrete dipole model. We reveal the features beyond the effective medium model, namely, the anisotropy of the structure induced by spatial dispersion, and suggest a simple experiment allowing one to observe the effect. Additionally, we demonstrate that the eigenmodes of the structure are neither transverse nor longitudinal but have a “mixed” polarization state in the general case.

The existence of an excitation gap in the bulk spectrum is one of the most prominent fingerprints of topological phases of matter. In this paper, we propose a family of two-dimensional Hamiltonians that yield an unusual class $D$ topological superconductor with a gapless bulk spectrum but well-localized Majorana edge states. We perform a numerical analysis for a representative model of this phase and suggest a concrete physical realization by analyzing the effect of magnetic impurities on the surface of a strong topological insulator.

We develop a theory of thermal fluctuations of spin density emerging in a two-dimensional electron gas. The spin fluctuations probed at spatially separated spots of the sample are correlated due to Brownian motion of electrons and spin-orbit coupling. We calculate the spatiotemporal correlation functions of the spin density for both ballistic and diffusive transport of electrons and analyze them for different types of spin-orbit interaction, including the isotropic Rashba model and the persistent spin helix regime. The measurement of spatial spin fluctuations provides direct access to the parameters of spin-orbit coupling and spin transport in conditions close to thermal equilibrium.

Retardation effects (REs) are known to cause a crossover from linear to sublinear behaviors in the dispersion relation of two-dimensional (2D) plasma waves at long wavelengths. In the present work, we systematically analyze REs on plasma waves in both 2D and 1D electron gases, and we clarify the experimental conditions for observing the crossover. We show that graphene is the only material that might gratify the conditions with current technology. Although it closely imitates graphene, the surface of 3D topological insulators is found to be unsuitable for this purpose due to strong electronic scattering by intrinsic disorder. We also show that the crossover, though theoretically possible, virtually never occurs in 1D plasma. In addition, the relevance of our results to terahertz plasmonics is discussed.

Experiments with one-dimensional arrays of Josephson junctions in the regime of dominating charging energy show that the Coulomb blockade is lifted at the threshold voltage, which is proportional to the array's length and depends strongly on the Josephson energy. We explain this behavior as depinning of the Cooper-pair-charge-density by the applied voltage. We assume strong charge disorder and argue that physics around the depinning point is governed by a disordered sine-Gordon-like model. This allows us to employ the well-known theory of charge density wave depinning. Our model is in good agreement with the experimental data.

The Jahn-Teller effect is a fascinating and ubiquitous phenomenon in modern quantum physics and chemistry. We propose a class of oxides with a melilite structure ${\mathrm{Ba}}_{2}T{\mathrm{Ge}}_2{\mathrm{O}}_7$ $(T=\text{V},\text{Ni})$ where Jahn-Teller distortions are mainly responsible for the appearance of electric polarization. At the heart of the proposed mechanism lies the lack of inversion symmetry displayed by tetrahedrally coordinated transition-metal ions, which allows for the condensation of polar Jahn-Teller distortions, at odds with octahedral coordination typical of conventional ferroelectric oxides with a perovskite structure. Since the noncentrosymmetric local environment of transition-metal ions also activates the proposed spin-dependent hybridization mechanism for magnetically induced electric polarization, proper multiferroic phases with intrinsic magnetoelectric interactions could be realized in this class of low-symmetry materials.

We present a microscopic theory of magnon transport in ferromagnetic insulators (FIs). Using magnon injection through microwave pumping, we propose a way to generate magnon dc currents and show how to enhance their amplitudes in hybrid ferromagnetic insulating junctions. To this end, focusing on a single FI, we first revisit microwave pumping at finite (room) temperature from the microscopic viewpoint of magnon injection. Next, we apply it to two kinds of hybrid ferromagnetic insulating junctions. The first is the junction between a quasiequilibrium magnon condensate and magnons being pumped by microwave, while the second is the junction between such pumped magnons and noncondensed magnons. We show that quasiequilibrium magnon condensates generate ac and dc magnon currents, while noncondensed magnons produce essentially a dc magnon current. The ferromagnetic resonance (FMR) drastically increases the density of the pumped magnons and enhances such magnon currents. Lastly, using microwave pumping in a single FI, we discuss the possibility that a magnon current through an Aharonov-Casher phase flows persistently even at finite temperature. We show that such a magnon current arises even at finite temperature in the presence of magnon-magnon interactions. Due to FMR, its amplitude becomes much larger than the condensed magnon current.

We study the phase diagram of the Bose-Hubbard model on the kagome lattice with a broken sublattice symmetry. Such a superlattice structure can naturally be created and tuned by changing the potential offset of one sublattice in the optical generation of the frustrated lattice. The superstructure gives rise to a rich quantum phase diagram, which is analyzed by combining quantum Monte Carlo simulations with the generalized effective potential Landau theory. Mott phases with noninteger filling and a characteristic order along stripes are found, which show a transition to a superfluid phase with an anisotropic superfluid density. Surprisingly, the direction of the superfluid anisotropy can be tuned by changing the particle number, the hopping strength, or the interaction. Finally, we discuss characteristic signatures of anisotropic phases in time-of-flight absorption measurements.

We establish that Majorana fermions on the boundary of topological superconductors have only spin-triplet superconducting correlations independent of whether the bulk superconducting gap is spin singlet or triplet. This is universal for time-reversal broken (respected) topological superconductors (TSCs) with an odd number of (pairs of) Majorana fermions on the boundary. Consequently, resonant Andreev reflection induced by Majorana fermions only occurs in spin-triplet channels and always injects spin-triplet Cooper pairs into the leads. This spin-triplet condensate results in a spin-orbit coupling (SOC) controlled oscillatory critical current without a $0-\pi$ transition in the TSC/SOC-semiconductor/TSC Josephson junction. The observation of this unique current-phase relation can serve as the definitive signal for Majorana fermions. Our study shows a technique for manipulating Majorana fermions based on their spin-triplet superconducting correlations.

In this manuscript, I present new hybrid devices based on multiwire/channel Rashba semiconductors, which harbor Majorana fermions (MFs) without a Zeeman field. In contrast, magnetic fluxes, supercurrents, or electric fields can be employed, yielding an enhanced device manipulability. The generic topological phase diagram for two-nanowire/channel systems exhibits features of quantum criticality and a rich interplay of phases with 0, 1, or 2 MFs per edge. The most prominent and experimentally feasible implementation relies on the already existing platforms of InAs two-dimensional electron gas (2DEG) on top of a Josephson junction. Appropriate design of the latter device allows phases with 1 or 2 MFs, both detectable in zero-bias anomaly peaks with a single or double unit of conductance.

We investigate sound propagation in a moving fluid confined in a randomly corrugated tube. For weak randomness and small fluid velocities $v^{\left(0\right)}$, the localization length $\xi$ shows extreme sensitivity to the variation of $v^{\left(0\right)}$. In the opposite limit of large fluid velocities, $\xi$ acquires a constant value which is independent of the frequency of the incident sound wave, the degree of randomness, and $v^{\left(0\right)}$ itself. Finally, we find that the variance ${\sigma }_{lnT}^2$ of the logarithm of transmittance $ln\left(T\right)$ is a universal function of the ensemble average $\langle lnT\rangle$, which is not affected by the fluid velocity.

Superconductors derived from topological insulators and topological crystalline insulators by chemical doping have long been considered to be candidates as topological superconductors. ${\mathrm{Pb}}_{0.5}{\mathrm{Sn}}_{0.5}\mathrm{Te}$ is a topological crystalline insulator with mirror symmetry protected surface states on (001)-, (011)-, and (111)-oriented surfaces. The superconductor $({\mathrm{Pb}}_{0.5}{\mathrm{Sn}}_{0.5}{)}_{0.7}{\mathrm{In}}_{0.3}\mathrm{Te}$ is produced by In doping in ${\mathrm{Pb}}_{0.5}{\mathrm{Sn}}_{0.5}\mathrm{Te}$, and is thought to be a topological superconductor. Here we report scanning tunneling spectroscopy measurements of the superconducting state as well as the superconducting energy gap in $({\mathrm{Pb}}_{0.5}{\mathrm{Sn}}_{0.5}{)}_{0.7}{\mathrm{In}}_{0.3}\mathrm{Te}$ on a (001)-oriented surface. The spectrum can be well fitted by an anisotropic $s$-wave gap function of $\mathrm{\Delta }\left(\theta \right)=0.72+0.18cos4\theta$ meV using Dynes model. The results show that the superconductor seems to be a fully gapped one without any in-gap states, in contradiction with the expectation of a topological superconductor.

We study the dynamics of magnetic correlations in the half-filled fermionic Hubbard model following a fast ramp of the repulsive interaction. We use Schwinger-Keldysh self-consistent second-order perturbation theory to investigate the evolution of single-particle Green's functions and solve the nonequilibrium Bethe-Salpeter equation to study the dynamics of magnetic correlations. This approach gives us new insights into the interplay between single-particle relaxation dynamics and the growth of antiferromagnetic correlations. Depending on the ramping time and the final value of the interaction, we find different dynamical behavior which we illustrate using a dynamical phase diagram. Of particular interest is the emergence of a transient short-range ordered regime characterized by the strong initial growth of antiferromagnetic correlations followed by a decay of correlations upon thermalization. The discussed phenomena can be probed in experiments with ultracold atoms in optical lattices.

The thermally induced nonlinear vibration of single-walled carbon nanotubes (SWNTs) was investigated, focusing on the peak broadening and/or multiple peaks of the frequency spectra. From Poincaré maps of SWNT tip trajectories, we observed two distinguishable patterns of vibration: planar and nonplanar (whirling) motion which appeared repeatedly. The alternation of the pattern is sometimes accompanied by a change of rotational direction. Proposed approximate solutions of nonlinear beam equations can reproduce well the alternation of the patterns for both cantilevered and suspended SWNTs. By using this analytical approach, we have found that multiple peaks of experimentally observed frequency spectra are due to the repeated and sudden alternations of two modes, induced by nonlinear effects.

With scanning transmission x-ray microscopy we study six magnetostatically coupled vortices arranged in a ring that resembles a benzene molecule. Each vortex is contained in a ferromagnetic microdisk. When exciting one vortex of the ring molecule with an alternating magnetic high-frequency field, all six vortices perform gyrations around the equilibrium center positions in their disks. In a rigid particle model, we derive the dispersion relation for these modes. In contrast to carbon atoms, magnetic vortices have a core polarization that strongly influences the intervortex coupling. We make use of this state parameter to reprogram the dispersion relation of the vortex molecule experimentally by tuning a homogeneous and an alternating polarization pattern. In analogy to the benzene molecule, we observe motions that can be understood in terms of normal modes that are largely determined by the symmetry of the system.

Using both broadband ferromagnetic resonance (FMR) spectroscopy and ab initio calculations, we study the magnetodynamic properties of permalloy $(Py,{\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20})$ and ${\mathrm{Py}}_{100-x}{\mathrm{M}}_x$ films with M as platinum (Pt), gold (Au), or silver (Ag). From the uniform FMR mode, we extract the saturation magnetization $\left(M_S\right)$, damping $\left(\alpha \right)$, and inhomogeneous broadening $\left(\mathrm{\Delta }{H}_0\right)$; from the first perpendicular standing spin-wave (PSSW) mode, we extract the exchange stiffness $\left(A\right)$. $M_S$ and $A$ are found to decrease with increasing alloying, most strongly for Au and less so for Pt. On the other hand, $\alpha$ increases rapidly with both Pt and Au content, while being virtually independent of Ag content. The physical origins of the observed trends in $\alpha$, $M_S$, and $A$ are analyzed and explained using density functional theory calculations in the coherent potential approximation. The calculated trends quantitatively agree with the experimental observations. The drastically different impacts of Pt, Au, and Ag on the various fundamental magnetodynamic properties will allow for significant design freedom, where different properties can be varied independently of others through careful combinations of the Pt, Au, and Ag contents of ${\mathrm{Py}}_{100-x}{\mathrm{M}}_x$ films. By empirical approximations of each property's concentration dependence, we can dial in any desired combination of magnetodynamic properties within this parameter space. As a proof-of-principle demonstration we design a set of ${\mathrm{Py}}_{100-x-y}{\mathrm{Pt}}_x{\mathrm{Ag}}_y$ films, where the saturation magnetization stays constant throughout the set and the damping can be tuned by a factor of 4.

In this Comment we discuss recent results presented by Nguyen et al. on shock compressed molybdenum up to 438 GPa [Phys. Rev. B 89, 174109 (2014)]. The aim of Nguyen's article is to show that there is no phase transition near 210 GPa. We propose instead an interpretation of their data that this material shows the onset of partial melting along the Hugoniot at $240\left(20\right)\mathrm{GPa}$, which is evident from abrupt changes in the pressure dependence of the shear modulus. This interpretation may solve the significant controversy in the melting slopes derived from shock and static experiments.