Zinc-blende (zb) boron arsenide (BAs) has been confirmed to have impressively high thermal conductivity. However, studies on its phase transitions under pressure have been few. Here, through a recently developed structure search method, we predicted that many polytypes with the structural features of cubic and hexagonal diamond, which are known to be unstable even up to very high pressures for carbon and boron nitride, can become stable at low pressures and might be retained to ambient conditions. Moreover, some of these BAs polytypes are calculated to have impressively high thermal conductivities at ambient conditions and the thermal conductivities for zb- and $2H$-BAs will decrease with increasing pressure, which are mainly attributed to the stronger third anharmonic interaction. The current study will open up a route to the search for high thermal conductors.

We derive exact results for the Lindblad equation for a quantum spin chain (one-dimensional quantum compass model) with dephasing noise. The system possesses doubly degenerate nonequilibrium steady states due to the presence of a conserved charge commuting with the Hamiltonian and Lindblad operators. We show that the system can be mapped to a non-Hermitian Kitaev model on a two-leg ladder, which is solvable by representing the spins in terms of Majorana fermions. This allows us to study the Liouvillian gap, the inverse of relaxation time, in detail. We find that the Liouvillian gap increases monotonically when the dissipation strength $\gamma$ is small, while it decreases monotonically for large $\gamma$, implying a kind of phase transition in the first decay mode. The Liouvillian gap and the transition point are obtained in closed form in the case where the spin chain is critical. We also obtain the explicit expression for the autocorrelator of the edge spin. The result implies the suppression of decoherence when the spin chain is in the topological regime.

The effects of rhenium doping in the range 0–10 at.% on the static and dynamic magnetic properties of ${\mathrm{Fe}}_{65}{\mathrm{Co}}_{35}$ thin films have been studied experimentally as well as with first-principles electronic structure calculations focusing on the change of the saturation magnetization $\left(M_s\right)$ and the Gilbert damping parameter $\left(\alpha \right)$. Both experimental and theoretical results show that $M_s$ decreases with increasing Re-doping level, while at the same time $\alpha$ increases. The experimental low temperature saturation magnetic induction exhibits a 29% decrease, from 2.31 to 1.64 T, in the investigated doping concentration range, which is more than predicted by the theoretical calculations. The room temperature value of the damping parameter obtained from ferromagnetic resonance measurements, correcting for extrinsic contributions to the damping, is for the undoped sample $2.1\times {10}^{-3}$, which is close to the theoretically calculated Gilbert damping parameter. With 10 at.% Re doping, the damping parameter increases to $7.8\times {10}^{-3}$, which is in good agreement with the theoretical value of $7.3\times {10}^{-3}$. The increase in damping parameter with Re doping is explained by the increase in the density of states at the Fermi level, mostly contributed by the spin-up channel of Re. Moreover, both experimental and theoretical values for the damping parameter weakly decrease with decreasing temperature.

The interaction of a tilted magnetic field and the intrinsic anisotropy of magnetic thin films leads to profound effects on the deformation, orientation, and stability of the skyrmion crystal (SkX) phase, which has yet to be explored comprehensively. Here, we study the combined effects of a tilted magnetic field and the exchange anisotropy on SkX in thin films of helimagnets. By applying an analytical framework suitable for deformed or rotated SkX, not only the strength of the magnetic field but also the polar angle and the azimuthal angle, which describe the direction of the magnetic field, are found to affect the stability of SkX. Moreover, they reorientate SkX at certain conditions. Our findings pave a way for the manipulation of SkX.

Using first principles only, we calculate the melting point of MgO, also called periclase or magnesia. The random phase approximation (RPA) is used to include the exact exchange as well as local and nonlocal many-body correlation terms, in order to provide high accuracy. Using the free energy method, we obtain the melting temperature directly from the internal energies calculated with DFT. The free energy differences between the ensembles generated by the molecular dynamics simulations are calculated with thermodynamic integration or thermodynamic perturbation theory. The predicted melting temperature is $T_m^{\text{RPA}}=3043\pm 86\mathrm{K}$ and the values obtained with the PBE and SCAN functionals are $T_m^{\text{PBE}}=2747\pm 59\mathrm{K}$ and $T_m^{\text{SCAN}}=3032\pm 53\mathrm{K}$.

We predict that cubic $M{\mathrm{N}}_4$ ($M$=Mn, Fe, Co) are all half-metals with the largest spin gap up to $\sim 5$ eV. They possess robust ferromagnetic ground states with the highest Curie temperature up to $\sim {10}^3$ K. Our calculations indicate these compounds are energetically favored, dynamically and mechanically stable. It is proposed that self-doping of these $3d$ transition metals occurs in $M{\mathrm{N}}_4$ due to the reduction in electronegativity of ${\mathrm{N}}_2$ dimers. This model can well explain the calculated integer magnetic moments, large spin gaps of $M{\mathrm{N}}_4$, and semiconducting behavior for ${\mathrm{NiN}}_4$ as well. Our results highlight the difference in electronegativity between transition metal ions and nonmetal entities in forming half-metals and the role of ${\mathrm{N}}_2$ dimer in enlarging the spin gaps for nitride half-metals.

Using the polar magneto-optical Kerr effect microscope, we have observed in a prototypical multilayer system of Ta/CoFeB/MgO a spin current-driven transformation of an initial labyrinthine domain pattern. The labyrinthine domain phase can either transform into a dense array of skyrmions or simply change in the orientational order, depending on the strengths of the applied spin current pulse and the external magnetic field. We conclude both from experimental studies and from micromagnetic simulations that the applied spin current leads to the generation/deletion of topological defects through modifying the motion of domain walls and the magnetic configurations within the domain walls, leading to the observed transformations. This is different from the thermally induced transitions where the nucleation and proliferation of topological defects are the dominant factors.

We consider a dilute gas of bosons with repulsive contact interactions, described on the mean-field level by the Gross-Pitaevskiǐ equation, and bounded by an impenetrable “hard” wall (either rigid or flexible). We solve the Bogoliubov-de Gennes equations for excitations on top of the Bose-Einstein condensate analytically, by using matrix-valued hypergeometric functions. This leads to the exact spectrum of gapless Bogoliubov excitations localized near the boundary. The dispersion relation for the surface excitations represents for small wave numbers $k$ a ripplon mode with fractional power law dispersion for a flexible wall, and a phonon mode (linear dispersion) for a rigid wall. For both types of excitation we provide, for the first time, the exact dispersion relations of the dilute quantum liquid for all $k$ along the surface, extending to $k\to \infty$. The small wavelength excitations are shown to be bound to the surface with a maximal binding energy $\mathrm{\Delta }=\frac{1}{8}{(\sqrt{17}-3)}^{2}m{c}^2\simeq 0.158m{c}^2$, which both types of excitation asymptotically approach, where $m$ is mass of bosons and $c$ bulk speed of sound. We demonstrate that this binding energy is close to the experimental value obtained for surface excitations of helium II confined in nanopores, reported in Phys. Rev. B 88, 014521 (2013).

We study the quantum many-body instabilities of interacting electrons with $\mathrm{SU}\left(2\right)\times \mathrm{SU}\left(2\right)$ symmetry in spin and orbital degrees of freedom on the triangular lattice near van-Hove filling. Our work is motivated by effective models for the flat bands in hexagonal moiré heterostructures like twisted bilayer boron nitride and trilayer graphene-boron nitride systems. We consider an extended Hubbard model including onsite Hubbard and Hund's couplings, as well as nearest-neighbor exchange interactions, and analyze the different ordering tendencies with the help of an unbiased functional renormalization group approach. We find three classes of instabilities controlled by the filling and bare interactions. For a nested Fermi surface at van-Hove filling, Hund-like couplings induce a weak instability towards spin or orbital density wave phases. An SU(4) exchange interaction moves the system towards a Chern insulator, which is robust with respect to perturbations from Hund-like interactions or deviations from perfect nesting. Further, in an extended range of fillings and interactions, we find topological $d\pm id$ and (spin-singlet)-(orbital-singlet) $f$-wave superconductivity.

We present a theoretical investigation of the ultrafast processes and dynamics of the produced excited carriers upon irradiation of silicon with femtosecond pulsed lasers in the mid-infrared (mid-IR) spectral region. The evolution of the carrier density and thermal response of the electron-hole and lattice subsystems are analyzed for various wavelengths ${\lambda }_L$ in the range between 2.2 and 3.3 μm, where the influence of two- and three-photon absorption mechanisms is explored. The role of induced Kerr effect is highlighted and it manifests a more pronounced influence at smaller wavelengths in the mid-IR range. Elaboration on the conditions that lead to surface plasmon (SP) excitation indicate the formation of weakly bound SP waves on the material surface. The lifetime of the excited SP is shown to rise upon increasing wavelength, yielding a larger one than that predicted for higher laser frequencies. The calculation of damage thresholds for various pulse durations ${\tau }_p$ shows that they rise according to a power law $(\sim {\tau }_p^{\zeta \left({\lambda }_L\right)})$ where the increasing rate is determined by the exponent $\zeta \left({\lambda }_L\right)$. Investigation of the multiphoton absorption rates and impact ionization contribution at different ${\tau }_p$ manifests a lower damage for ${\lambda }_L=2.5\mu \mathrm{m}$ compared to that for ${\lambda }_L=2.2\mu \mathrm{m}$ for long ${\tau }_p$.

In search of two level quantum systems that implement a qubit, the nitrogen-vacancy (NV) center in diamond has been intensively studied for years. Despite favorable properties such as remarkable defect spin coherence times, the addressability of NV centers raises some technical issues. The coupling of a single NV center to an external driving field is limited to short distances, since an efficient coupling requires the NV to be separated by only a few microns away from the source. As a way to overcome this problem, an enhancement of coherent coupling between NV centers and a microwave field has recently been experimentally demonstrated using spin waves propagating in an adjacent yttrium iron garnet (YIG) film [P. Andrich et al., npj Quantum Inf. 3, 28 (2017)]. In this paper we analyze the optically detected magnetic resonance spectra that arise when an NV center is placed on top of a YIG film for a geometry similar to the one in the experiment. We analytically calculate the oscillating magnetic field of the spin wave on top of the YIG surface to determine the coupling of spin waves to the NV center. We compare this coupling to the case when the spin waves are absent and the NV center is driven only with the antenna field and show that the calculated coupling enhancement is dramatic and agrees well with the one obtained in the recent experiment.

The vibrational and electronic properties of two-dimensional (2D) materials can be efficiently tuned by external strain due to their good stretchability. Resonant Raman spectroscopy is a versatile tool to study the physics of phonons, electrons, and their interactions simultaneously, which is particularly useful for the investigation of strain effect on 2D materials. We report the resonant Raman study of strained few-layer InSe ($\gamma$ phase). Under $\sim 1\%$ of uniaxial tensile strain, one order of magnitude enhancement of Raman intensity for the longitudinal optical (LO) phonon is observed, while other modes exhibit only modest change. Further analysis demonstrates that it arises from the intraband electron-phonon scattering channel for a LO phonon in resonance. The large enhancement of Raman intensity provides us with a sensitive method to characterize the strain effect, and a mapping of the strain distribution in a wrinkled sample is demonstrated. In addition, we observe sizable redshifts of first-order optical phonon modes. The shift rate exhibits phonon-mode dependence, in excellent agreement with density functional perturbation theory calculations. Our study paves the way for sensitive strain quantification in few-layer InSe and its application in flexible electronic and optoelectronic devices.

We introduce a physical model to describe the influence of a perpendicular electric field on circular polarization (CP) and valley polarization (VP) in bilayer transition metal dichalcogenides. Our results uncover that electric-field-dependent CP and VP are quite distinct from each other. The dependence of CP on the electric field harbors a W pattern and possesses the minimum when the potential energy difference between the two layers is equal to the strength of spin-orbit coupling. Such dependence of CP stems from the modulation of energy cost for interlayer hopping and spin-dependent layer polarization. In contrast, VP is strictly absent in primitive bilayers and increases monotonically with increasing strength of electric field, resulting from the continuous variation of valley magnetic moments and inversion-symmetry breaking. Our model elaborates well the recent experimental observations for which the origin is under debate. Moreover, we demonstrate that the manipulation of layer and valley pseudospin is fully tunable by perpendicular electric fields, paving the way for prospects in electrical control of exotic layer-valleytronics.

Fano resonances in metamaterials can be explained by the coupling between different modes sustained by the meta-atom. While the associated spectral features are usually quite easy to identify for top-down metamaterials, thanks to the deterministic arrangement of the meta-atom relative to the illumination, the identification of spectral features due to Fano resonances is much more challenging for bottom-up metamaterials. There, the response is spurred by the random arrangement of the meta-atoms relative to the illumination. To improve the situation, we introduce a quantity that measures the nonorthogonality of the modes sustained by the meta-atom. The measure allows us to disclose whether specific details in the spectral response emerge from Fano features or whether they are only due to the incoherent superposition of different modes. We strengthen our argumentation while discussing the multipolar decomposition of the different modes that contribute to the Fano features for a representative meta-atom.

Symmetry-breaking states of matter can transmit symmetry breaking to nearby atoms or molecular complexes, perturbing their spectra. We calculate one such effect, involving the “axion electrodynamics” relevant to topological insulators, quantitatively, and identify a signature for $T$ violating superconductivity. We provide an operator framework whereby effects of this kind can be analyzed systematically.

Imperfections in correlated materials can alter their ground state as well as finite-temperature properties in significant ways. Here, we develop a method based on numerical linked-cluster expansions for calculating exact finite-temperature properties of disordered lattice models directly in the thermodynamic limit. We show that a continuous distribution for disordered parameters can be achieved using a set of carefully chosen discrete modes in the distribution, which allows for the averaging of properties over all disorder realizations. We benchmark our results for thermodynamic properties of the square-lattice Ising and quantum Heisenberg models with bond disorder against Monte Carlo simulations and study them as the strength of disorder changes. We also apply the method to the disordered Heisenberg model on the frustrated checkerboard lattice, which is closely connected to ${\mathrm{Sr}}_2\mathrm{Cu}\left({\mathrm{Te}}_{0.5}{\mathrm{W}}_{0.5}\right){\mathrm{O}}_6$. Our method can be used to study finite-temperature properties of other disordered quantum lattice models, including those for interacting lattice fermions.

A central problem in understanding the superconductor-insulator transition in disordered superconductors is that the properties of grains and intergrain medium cannot be independently studied. Here we demonstrate an approach to the study of strongly disordered superconducting films by relying on the stochastic nature of the disorder probed by electrostatic gating in a restricted geometry. Charge tuning and magnetotransport measurements in quasihomogeneous TiN nanowires embedded in a superinductor environment allow us to classify different devices and distinguish between spontaneously formed Coulomb islands (with typical blockade voltage in the mV range) and homogeneous wires showing behavior indicative of coherent quantum phase slips (with significantly smaller blockade voltage).

We report the extremely large magnetoresistance (XMR) property and Shubnikov–de Haas (SdH) oscillations in a quasi-one-dimensional material ${\mathrm{W}}_2{\mathrm{As}}_3$ single crystal, which shows good metallic behavior with a typical residual resistivity ratio $\rho \left(300\mathrm{K}\right)/\rho \left(1.8\mathrm{K}\right)=1240$. In a magnetic field, the resistivity shows a quick upturn behavior and saturates with a plateau as the temperature cools down. At $T=1.8\mathrm{K}$, the magnetoresistance of ${\mathrm{W}}_2{\mathrm{As}}_3$ increases quadratically with the magnitude of the magnetic field and reaches up to $32000\%$ ($B=9\mathrm{T}$) without any trend of saturation, which suggests the semiclassical nature during the transport process. The result of Hall resistivity measurements further points out that the XMR property in ${\mathrm{W}}_2{\mathrm{As}}_3$ originates from the compensation of electrons and holes with high mobility (${\mu }_e=4.9\times {10}^4{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ at 1.8 K). Furthermore, SdH oscillations can be distinguished clearly from both magnetoresistivity and Hall resistivity. The quantum mobility calculated from the SdH oscillations is about 1–2 orders lower than that calculated from the transport mobility, which suggests that the small angle scattering is dominant in ${\mathrm{W}}_2{\mathrm{As}}_3$. The angle dependence of SdH oscillations implies the complex Fermi surface structure with obvious anisotropy in ${\mathrm{W}}_2{\mathrm{As}}_3$.