In the thermally driven superfluid ${}^4\mathrm{He}$ turbulence, the counterflow velocity $U_{\text{ns}}$ partially decouples normal and superfluid turbulent velocities. Recently, we suggested [J. Low. Temp. Phys. 187, 497 (2017)] that this decoupling should tremendously increase the turbulent energy dissipation by mutual friction and significantly suppress the energy spectra. Comprehensive measurements of the apparent scaling exponent $n_{\text{exp}}$ of the second-order normal-fluid velocity structure function $S_2\left(r\right)\propto r^{n_{\text{exp}}}$ in the counterflow turbulence [J. Gao et al., Phys. Rev. B 96, 094511 (2017)] confirmed our scenario of gradual dependence of the turbulence statistics on flow parameters. We develop an analytical theory of the counterflow turbulence, accounting for a twofold mechanism of this phenomenon: (i) a scale-dependent competition between the turbulent velocity coupling by mutual friction and the $U_{\text{ns}}$-induced turbulent velocity decoupling and (ii) the turbulent energy dissipation by mutual friction enhanced by the velocity decoupling. The suggested theory predicts the energy spectra for a wide range of flow parameters. The mean exponents of the normal-fluid energy spectra ${〈m〉}_{\text{10}}$, found without fitting parameters, qualitatively agree with the observed $n_{\text{exp}}+1$ for $T\gtrsim 1.85\mathrm{K}$.

In a one-dimensional weak-link wire the spin-orbit interaction (SOI) alone cannot generate a nonzero spin current. We show that a Zeeman field acting in the wire in conjunction with the Rashba SOI there does yield such a current, whose magnitude and direction depend on the direction of the field. When this field is not parallel to the effective field due to the SOI, both the charge and the spin currents oscillate with the length of the wire. Measuring the oscillating anisotropic magnetoresistance can thus yield information on the SOI strength. These features are tuned by applying a magnetic and/or an electric field, with possible applications to spintronics.

We present inelastic neutron scattering results of phonons in ${\left({\mathrm{Pb}}_{0.5}{\mathrm{Sn}}_{0.5}\right)}_{1-x}{\mathrm{In}}_x\mathrm{Te}$ powders, with $x=0$ and 0.3. The $x=0$ sample is a topological crystalline insulator, and the $x=0.3$ sample is a superconductor with a bulk superconducting transition temperature $T_c$ of 4.7 K. In both samples, we observe unexpected van Hove singularities in the phonon density of states at energies of 1–2.5 meV, suggestive of local modes. On cooling the superconducting sample through $T_c$, there is an enhancement of these features for energies below twice the superconducting-gap energy. We further note that the superconductivity in ${\left({\mathrm{Pb}}_{0.5}{\mathrm{Sn}}_{0.5}\right)}_{1-x}{\mathrm{In}}_x\mathrm{Te}$ occurs in samples with normal-state resistivities of order 10 $\mathrm{m}\mathrm{\Omega }\mathrm{cm}$, indicative of bad-metal behavior. Calculations based on density functional theory suggest that the superconductivity is easily explainable in terms of electron-phonon coupling; however, they completely miss the low-frequency modes and do not explain the large resistivity. While the bulk superconducting state of ${\left({\mathrm{Pb}}_{0.5}{\mathrm{Sn}}_{0.5}\right)}_{0.7}{\mathrm{In}}_{0.3}\mathrm{Te}$ appears to be driven by phonons, a proper understanding will require ideas beyond simple BCS theory.

Lattice thermal conductivities of two ${\mathrm{SiO}}_2$ polymorphs, i.e., $\alpha$ quartz (low) and $\alpha$ cristobalite (low), were studied using first-principles anharmonic phonon calculation and linearized phonon Boltzmann transport equation. Although $\alpha$ quartz and $\alpha$ cristobalite have similar phonon densities of states, phonon frequency dependencies of phonon group velocities and lifetimes are dissimilar, which results in largely different anisotropies of the lattice thermal conductivities. For $\alpha$ quartz and $\alpha$ cristobalite, distributions of the phonon lifetimes effective to determine the lattice thermal conductivities are well described by energy and momentum conservations of three phonon scatterings weighted by phonon occupation numbers and one parameter that represents the phonon-phonon interaction strengths.

Static and dynamic magnetic properties of P$\overline{3}$1$m$-phase ${\mathrm{MnSb}}_2{\mathrm{O}}_6$ have been studied by means of muon-spin relaxation ($\mu \mathrm{SR}$), high-frequency electron-spin resonance (HF-ESR), specific heat, and magnetization studies in magnetic fields up to 25 T. The data imply onset of long-range antiferromagnetic order at $T_{\mathrm{N}}$ = 8 K and a spin-flop-like transition at $B_{\mathrm{SF}}$ $\approx$ 0.7–1 T. Below $T_{\mathrm{N}}$, muon asymmetry exhibits well-defined oscillations indicating a narrow distribution of the local fields. A competing antiferromagnetic phase appearing below $T_{\mathrm{N}2}$ = 5.3 K is evidenced by a step in the magnetization and a slight kink of the relaxation rate. Above $T_{\mathrm{N}}$, both $\mu \mathrm{SR}$ and HF-ESR data suggest short-range spin order. HF-ESR data show that local magnetic fields persist up to at least $12T_{\mathrm{N}}\approx 100$ K. Analysis of the antiferromagnetic resonance modes and the thermodynamic spin-flop field suggest zero-field splitting of $\mathrm{\Delta }\approx 18$ GHz which implies small but finite magnetic anisotropy.

The magnetic phase diagram of the quaternary borocarbide ${\mathrm{TbNi}}_2{\mathrm{B}}_2\mathrm{C}$ is investigated by direct means and by studying magnetically induced modifications of the crystal structure. Detailed superconducting quantum interference device measurements reveal a complex phase diagram with five distinct magnetic phases. The phase boundaries are mapped out comprehensively. Synchrotron hard x-ray measurements in applied magnetic fields are employed to probe the magnetoelastic distortions throughout the phase diagram. The determination of the wave vectors of these field-induced lattice deformations suggests a range of commensurate spin-slip-type magnetic structures at low temperatures with wave vectors of the form $(q,0,0)$ with $q=6/11$ and $5/9$. The proposed magnetic structures yield values of magnetization well in-line with observations. The scattering intensity due to the magnetoelastic deformations exhibits a drastic jump at the phase boundary at 1.3 T and low temperatures.

In Chen et al. [Chen et al., Phys. Rev. B 94, 180104(R) (2016)], an observed increase in x-ray diffraction intensity from $\mathrm{BaTi}{\mathrm{O}}_3$ after an ultrafast terahertz pulse is attributed to enhanced vibrations of the Ti atoms based on a simplified diffraction model. We show that standard Debye-Waller theory leads to more appropriate modeling of time-resolved diffraction intensities, at least in the absence of a fully coherent vibrational response.

In this reply to S. Durbin's comment on our original paper “Ultrafast terahertz-field-driven ionic response in ferroelectric ${\mathrm{BaTiO}}_3$,” we concur that his final equations 8 and 9 more accurately describe the change in diffracted intensity as a function of Ti displacement. We also provide an alternative derivation based on an ensemble average over unit cells. The conclusions of the paper are unaffected by this correction.

The copper-oxygen planes in cuprates have been at the center of the search for a theory of high-temperature superconductivity. We conduct an extensive study of the ground state of the three-band Hubbard (Emery) model in the underdoped regime. We focus on the magnetic and charge orders, and present results from generalized Hartree-Fock (GHF) calculations. The ground-state properties at the thermodynamic limit are challenging to pin down because of sensitivity to computational details, including the shapes and sizes of the supercells. We employ large-scale computations with various technical improvements to determine the orders within GHF. The ground state exhibits a rich phase diagram with hole doping as the charge transfer energy is varied, including ferromagnetic domain walls embedded in an antiferromagnetic background, spin spirals, and nematic order.

The magnetoelectric response in inversion-breaking two-dimensional Dirac systems induced by strain is analyzed. It is shown that, in the same way that the piezoelectric response in these materials is related to the valley Chern number, the magnetic field and strain-induced electric polarization is related to the nontrivial Berry curvature and the derivative of the orbital magnetic moment per valley. We also show that the elastic vector field ${\mathcal{A}}^{el}$ present in these systems drives the system out of equilibrium in presence of a magnetic field, allowing for such electrical response. In contrast, we show that the strain plays a very different role in the generation of a nonequilibrium magnetization and optical activity. In this case, it is the modification of the band structure induced by strain that allows for the magnetic response to external electric perturbations.

The ferroelectric and magnetic phases of the double perovskite $\mathrm{CaMnT}{\mathrm{i}}_2{\mathrm{O}}_6$ with $A$-site order have been investigated by soft x-ray absorption and magnetic circular dichroism. All spectra point to a very ionic state of divalent Mn and tetravalent Ti atoms. The effects of the crystal field produced by O ligands around tetravalent titanium and the dissimilar Mn1 and Mn2 sites were investigated. Both the so-called square-planar and the octahedrally coordinated Mn sites spectroscopically contribute in a rather similar way, with little influence by the oxygen environment. Multiplet calculations suggest a small $O2p\text{−}\mathrm{Ti}3d$ charge-transfer component in the FE phase. Magnetic symmetry calculations were performed to determine probable configurations of Mn spins compatible with the acentric $P{4}_{2}mc$ structure and, in combination with the computational magnetic results in Inorg. Chem. 56, 11854 (2017), we have identified the $P{4}_2^{\prime }{m}^{\prime }c$ as the most likely magnetic space group keeping invariant the unit cell below $T_{\mathrm{N}}$. This symmetry forces the sign of the magnetic coupling along the Mn columns parallel to $c$ to reverse with respect to the coupling between neighboring columns. Below $T_{\mathrm{N}}$, the dichroic magnetization loops at the $\mathrm{Mn}{L}_3$ edge confirm the absence of spontaneous ferromagnetism, although a very small field-induced spin polarization was detected in the sample.

An ideal $1H$ phase monolayer ${\mathrm{MoS}}_2$ has mirror reflection symmetry, but this symmetry is broken in common experimental situations, where the monolayer is placed on a substrate. By using $\mathrm{k}·\mathrm{p}$ perturbation theory, we investigate the effect of mirror symmetry breaking on the Berry curvature of the material. We find that the symmetry breaking may modify the Berry curvature considerably, and the spin/valley Hall effect due to modified Berry curvature is in qualitative agreement with a recent experimental result [Science 344, 1489 (2014)], which cannot be explained by previous theories that ignore mirror symmetry breaking.

Magnetization and high-resolution x-ray diffraction measurements of the Kitaev-Heisenberg material $\alpha \text{−}{\mathrm{RuCl}}_3$ reveal a pressure-induced crystallographic and magnetic phase transition at a hydrostatic pressure of $p\sim 0.2$ GPa. This structural transition into a triclinic phase is characterized by a very strong dimerization of the Ru-Ru bonds, accompanied by a collapse of the magnetic susceptibility. Ab initio quantum-chemistry calculations disclose a pressure-induced enhancement of the direct $4d\text{−}4d$ bonding on particular Ru-Ru links, causing a sharp increase of the antiferromagnetic exchange interactions. These combined experimental and computational data show that the Kitaev spin-liquid phase in $\alpha \text{−}{\mathrm{RuCl}}_3$ strongly competes with the crystallization of spin singlets into a valence bond solid.

The correlation energy of an inhomogeneous electron gas is ruled by the density correlation function $L$. It obeys a Bethe-Salpeter equation (BSE), whose kernel is the functional derivative of the particle self-energy. It is solved on equal footing for two-particle and single-particle Green's functions within Hedin's GW approximation. According to the Hartree and exchange-correlation contributions to the kernel, the correlation expression decays into random-phase-approximation (RPA) -like and screened-exchange-like parts. The actual treatment of the screened potential $W$ in the BSE for $L$ leads to different second-order screened exchange corrections (SOSEX). Including correlation in the kernel even yields approximations beyond SOSEX. The progress beyond RPA is illustrated for spinless and spin-polarized homogeneous electron gases, indicating that the screening treatment is substantial.

We investigate the non-Abelian topological chiral spin-liquid phase in the two-dimensional Kitaev honeycomb model subject to a magnetic field. By combining density matrix renormalization group and exact diagonalization we study the energy spectra, entanglement, topological degeneracy, and expectation values of Wilson loop operators, allowing for a robust characterization. While the ferromagnetic Kitaev spin liquid is already destroyed by a weak magnetic field with Zeeman energy $H_{*}^{\text{FM}}\approx 0.02$, the antiferromagnetic (AFM) spin liquid remains robust up to a magnetic field that is an order of magnitude larger, $H_{*}^{\text{AFM}}\approx 0.2$. Interestingly, for larger fields $H_{*}^{\text{AFM}}<H<H_{**}^{\text{AFM}}$, an intermediate gapless phase is observed, before a second transition to the high-field partially polarized paramagnet. We attribute this rich phase diagram, and the remarkable stability of the chiral topological phase in the AFM Kitaev model, to the interplay of strong spin-orbit coupling and frustration enhanced by the magnetic field. Our findings suggest relevance to recent experiments on ${\mathrm{RuCl}}_3$ under magnetic fields.

The family of “Jack states” related to antisymmetric Jack polynomials are the exact zero-energy ground states of particular model short-range many-body repulsive interactions, defined by a few nonvanishing leading pseudopotentials. Some Jack states are known or anticipated to accurately describe many-electron incompressible ground states emergent from the two-body Coulomb repulsion in the fractional quantum Hall effect. By extensive numerical diagonalization, we demonstrate the emergence of Jack states from suitable pair interactions. We find empirically a simple formula for the optimal two-body pseudopotentials for the series of most prominent Jack states generated by contact many-body repulsion. Furthermore, we seek a realization of arbitrary Jack states in realistic quantum Hall systems with Coulomb interaction, i.e., in the partially filled lowest and excited Landau levels in quasi-two-dimensional layers of conventional semiconductors such as GaAs or in graphene.

The Sachdev-Ye-Kitaev (SYK) model is a quantum-mechanical model of fermions interacting with $q$-body random couplings. For $q=2$, it describes free particles and is nonchaotic in the many-body sense, while for $q>2$ it is strongly interacting and exhibits many-body chaos. In this work we study the entanglement entropy (EE) of the $\mathrm{SYK}q$ models for a bipartition of $N$ real or complex fermions into subsystems containing $2m$ real/$m$ complex fermions and $N-2m$/$N-m$ fermions in the remainder. For the free model $\mathrm{SYK}2$, we obtain an analytic expression for the EE, derived from the $\beta$-Jacobi random matrix ensemble. Furthermore, we use the replica trick and path-integral formalism to show that the EE is maximal when one subsystem is small, i.e., $m\ll N$, for arbitrary $q$. We also demonstrate that the EE for the SYK4 model is noticeably smaller than the Page value when the two subsystems are comparable in size, i.e., $m/N$ is $O\left(1\right)$. Finally, we explore the EE for a model with both SYK2 and SYK4 interactions and find a crossover from SYK2 (low-temperature) to SYK4 (high-temperature) behavior as we vary energy.

We use ab initio density functional theory calculations to understand the electronic, dynamical, and thermoelectric behavior of layered crystalline phase-change materials. We perform calculations on the pseudobinary compounds ${\left(\mathrm{GeTe}\right)}_x/\left({\mathrm{Sb}}_2{\mathrm{Te}}_3\right)$ (GST) with $x=1$, 2, and 3. Since the stable configuration of these compounds remains somehow unsettled, we study one stacking configuration for GST124 ($x=1$), three for GST225 ($x=2$), and two for GST326 ($x=3$). A supercell approach is used to check the dynamical stability of the systems while thermoelectric properties are obtained by solving the Boltzmann transport equation. We report that the most accepted stacking configuration of GST124, GST225, and GST326 have metallic character and for the case of $x=2$ and 3, those are the ones with the lowest energy. However, we find the metallic of GST326 configuration to be dynamically unstable. In general, our values of the Seebeck coefficient and thermal conductivity for compounds with $x=1$ and 2 agree very well with the available experimental data. The small differences that we observe with respect to experimental data are attributed to the disorder that is present experimentally and that we have not taken into account. We do not find a Dirac cone in the electronic band structure of GST225, contrarily to previous reports. We attribute this due to the theoretical strain induced by the choice of the pseudopotential.

Landauer's principle states that erasure of each bit of information in a system requires at least a unit of energy $k_{B}Tln2$ to be dissipated. In return, the blank bit may possibly be utilized to extract usable work of the amount $k_{B}Tln2$, in keeping with the second law of thermodynamics. While in principle any collection of spins can be utilized as information storage, work extraction by utilizing this resource in principle requires specialized engines that are capable of using this resource. In this work, we focus on heat and charge transport in a quantum spin Hall device in the presence of a spin bath. We show how a properly initialized nuclear spin subsystem can be used as a memory resource for a Maxwell's demon to harvest available heat energy from the reservoirs to induce charge current that can power an external electrical load. We also show how to initialize the nuclear spin subsystem using applied bias currents which necessarily dissipate energy, hence demonstrating Landauer's principle. This provides an alternative method of “energy storage” in an all-electrical device. We finally propose a realistic setup to experimentally observe a Landauer erasure/work extraction cycle.

We report an observation of coherent phonons of $E_g^1, E_u^1, A_{1g}^1$, and $E_g^2$ symmetry generated in a single-crystal film of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ by an intense single-cycle THz pulse. The atomic vibrations reveal themselves through periodic modulation of the refractive index of the film. The largest signal is detected at the frequency of 4.05 THz that corresponds to the $E_g^2$ mode. The generation of $E_g^2$ phonons is interpreted as resonant excitation of the Raman mode by the second harmonic of THz-driven nonlinear $E_u^1$ oscillator, the fundamental frequency of which (2.05 THz) is approximately half that of $E_g^2$. The origin of nonlinearity in this case is cubic lattice anharmonicity, while generation of $E_g^1$ (1.1 THz) and $A_{1g}^1$ (2.25 THz) phonons is a manifestation of quartic anharmonicity enhanced by the occasional combination relations between phonon frequencies in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$.