By using the diffusion Monte Carlo method, we obtained the full phase diagram of ${}^3\mathrm{He}$ on top of graphite preplated with a solid layer of ${}^4\mathrm{He}$. All the ${}^4\mathrm{He}$ atoms of the substrate were explicitly considered and allowed to move during the simulation. We found that the ground state is a liquid of density $0.007\pm 0.001$ ${\AA }^{-2}$, in good agreement with available experimental data. This is significantly different from the case of ${}^3\mathrm{He}$ on clean graphite, in which both theory and experiment agree on the existence of a gas-liquid transition at low densities. Upon an increase in ${}^3\mathrm{He}$ density, we predict a first-order phase transition between a dense liquid and a registered 7/12 phase, the 4/7 phase being found metastable in our calculations. At larger second-layer densities, a final transition is produced to an incommensurate triangular phase.

We investigate the strain-mediated control of individual single quantum dots (QDs) embedded in a multi-degree-of-freedom optomechanical resonator. The mechanical eigenfrequencies and their mode degeneracy of resonator are tailored in the MHz regime for a geometrical aspect ratio of conventional wire resonator. The stress-induced modification in the QD exciton dynamics is evaluated at the multitude of mechanical resonances including the genuine cases of mode degeneracy. We show that the multi-degree-of-freedom stress fields allow an individual control of QD emission energies with a rate of up to several meV/nm. Such an optomechanical tuning technique can pave the way for entangling distant QDs for quantum information processing or potentially generating a superradiant light pulse.

Three extensive density functional/molecular dynamics simulations of the crystallization of amorphous ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ (460 atoms) [Phys. Rev. B 90, 184109 (2014)] have been completed with simulation times of up to 8.2 ns. Together with the results of earlier simulations with and without a crystallite seed, the results clarify essential features of a complicated process. They emphasize, in particular, the stochastic nature of crystallization, the effect of bond orientations and percolation, and the importance of extended simulations of sufficiently large samples. This is particularly evident in describing the role of crystallites that can merge to form larger units or hinder complete crystallization by the formation of grain boundaries. The total pair distribution functions for the final structures are compared with available neutron and x-ray diffraction data.

We describe a mechanism to control the energy and magnetization currents in an artificial spin chain, consisting of an array of permalloy nanodisks coupled through a magnetodipolar interaction. The chain is kept out of equilibrium by two thermal baths with different temperatures connected to its ends, which control the current propagation. Transport is enhanced by applying a uniform radio-frequency pump field resonating with some of the spin-wave modes of the chain. Moreover, the two currents can be controlled independently by tuning the static field applied on the chain. Thus we describe two effective means for the independent control of coupled currents and the enhancement of thermal and spin-wave conductivity in a realistic magnonics device, suggesting that similar effects could be observed in a large class of nonlinear oscillating systems.

We show how spin-liquid (SL) states can be stabilized in a realistic three-dimensional model as a result of frustration. $\text{SU}\left(n\right)$-symmetric generalization of the Heisenberg model for quantum spin $S$ operators is used to investigate the frustrated body-centered tetragonal (BCT) lattice with antiferromagnetic interlayer coupling $J_1$ and intralayer first and second-neighbor couplings $J_2$ and $J_3$. By using complementary representations of the spin operators, we study the phase diagram characterizing the ground state of this system. For small $n$, we find that the most stable solutions correspond to four different families of long-range magnetic orders that are governed by $J_1,J_2$, and $J_3$. First, some possible instabilities of these phases are identified for $n=2$, in large $S$ expansions, up to the linear spin-wave corrections. Then, using a fermionic representation of the $\text{SU}\left(n\right)$ spin operators for $S=1/2$, we find that purely magnetic orders occur for $n\le 3$ while SL solutions are stabilized for $n\ge 10$. The SL solution governed by $J_1$ breaks the lattice translation symmetry. The modulated SL is associated with a commensurate ordering wave vector $(1,1,1)$. For $4\le n\le 9$, we show how the competition between $J_1,J_2$, and $J_3$ can turn the magnetically ordered ground state into a SL state. Finally, we discuss the relevance of this scenario for correlated systems with BCT crystal structure.

The spatial symmetry of spin pumping, i.e., the voltage amplitude of the inverse spin Hall effect varying with the orientation of static magnetization and the polarization direction of a dynamic magnetic field, has been systematically studied both theoretically and experimentally. This symmetry is quite important because it is the unique foundation and criterion for identifying the spin pumping signal from other contributions to the voltage and for unveiling the underlying physics of spin pumping and pure spin current. As a straightforward proof, we found the spin mixing conductance to be out-of-plane anisotropic, which comes from an elaborate study of the symmetry of a Pt/YIG bilayer after examining every effect that may contribute to the angular dependence of spin pumping; the spin Hall angle, however, is a constant that is independent of the polarization direction of the spin current.

We report low temperature muon spin relaxation ($\mu \mathrm{SR}$) measurements of the high-transition-temperature ($T_c$) cuprate superconductors ${\mathrm{Bi}}_{2+x}{\mathrm{Sr}}_{2-x}{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ and ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.57}$, aimed at detecting the mysterious intra-unit cell (IUC) magnetic order that has been observed by spin-polarized neutron scattering in the pseudogap phase of four different cuprate families. A lack of confirmation by local magnetic probe methods has raised the possibility that the magnetic order fluctuates slowly enough to appear static on the time scale of neutron scattering, but too fast to affect $\mu \mathrm{SR}$ or nuclear magnetic resonance signals. The IUC magnetic order has been linked to a theoretical model for the cuprates, which predicts a long-range ordered phase of electron-current loop order that terminates at a quantum crictical point (QCP). Our study suggests that lowering the temperature to $T\sim 25$ mK and moving far below the purported QCP does not cause enough of a slowing down of fluctuations for the IUC magnetic order to become detectable on the time scale of $\mu \mathrm{SR}$. Our measurements place narrow limits on the fluctuation rate of this unidentified magnetic order.

Recent experiments have revealed nonlinear features of the magnetoresistance in metallic bilayers consisting of a heavy metal (HM) and a ferromagnetic metal (FM). A small change in the longitudinal resistance of the bilayer has been observed when reversing the direction of either the applied in-plane current or the magnetization. We attribute such nonlinear transport behavior to the spin-polarization dependence of the electron mobility in the FM layer acting in concert with the spin accumulation induced in that layer by the spin Hall current originating in the bulk of the HM layer. An explicit expression for the nonlinear magnetoresistance is derived based on a simple drift-diffusion model, which shows that the nonlinear magnetoresistance appears at the first order of the spin Hall angle, and changes sign when the current is reversed, in agreement with the experimental observations. We also discuss possible ways to control sign of the nonlinear magnetoresistance and to enhance the magnitude of the effect.

The thermomechanical properties and electronic structure of vitreloy ($\mathrm{Z}{\mathrm{r}}_{41.2}\mathrm{T}{\mathrm{i}}_{13.8}\mathrm{C}{\mathrm{u}}_{12.5}\mathrm{N}{\mathrm{i}}_{10}\mathrm{B}{\mathrm{e}}_{22.5}$) are investigated using accurate ab initio molecular dynamic (AIMD) simulations and ab initio calculations. The structure of the model with 512 atoms is validated by comparison to the experimental data with calculated thermomechanical properties in good agreement with the existing measurements. Detailed calculation of the electronic structure and bonding at the density functional level is obtained. It is revealed that the traditional definition of bond length in metallic glasses has a limited interpretation, and any theory based on geometrical consideration of their values for discussion on the structural units in metallic glasses has similarly limited applications. On the other hand, we advocate the use of a quantum mechanical based metric, the total bond order density (TBOD), and their partial components or PBOD as valuable parameters to characterize the interatomic bonding in multicomponent glasses such as vitreloy.

We report the coexistence and properties of stable compact localized states (CLSs) and discrete solitons (DSs) for nonlinear spinor waves on a flat-band network with spin-orbit coupling (SOC). The system can be implemented by means of a binary Bose-Einstein condensate loaded in the corresponding optical lattice. In the linear limit, the SOC opens a minigap between flat and dispersive bands in the system's band-gap structure, and preserves the existence of CLSs at the flat-band frequency, simultaneously lowering their symmetry. Adding on-site cubic nonlinearity, the CLSs persist and remain available in an exact analytical form, with frequencies that are smoothly tuned into the minigap. Inside of the minigap, the CLS and DS families are stable in narrow areas adjacent to the FB. Deep inside the semi-infinite gap, both the CLSs and DSs are stable too.

Starting from the Dirac-Kohn-Sham equation, we derive the relativistic equation of motion of spin angular momentum in a magnetic solid under an external electromagnetic field. This equation of motion can be rewritten in the form of the well-known Landau-Lifshitz-Gilbert equation for a harmonic external magnetic field and leads to a more general magnetization dynamics equation for a general time-dependent magnetic field. In both cases there is an electronic spin-relaxation term which stems from the spin-orbit interaction. We thus rigorously derive, from fundamental principles, a general expression for the anisotropic damping tensor which is shown to contain an isotropic Gilbert contribution as well as an anisotropic Ising-like and a chiral, Dzyaloshinskii-Moriya-like contribution. The expression for the spin relaxation tensor comprises furthermore both electronic interband and intraband transitions. We also show that when the externally applied electromagnetic field possesses spin angular momentum, this will lead to an optical spin torque exerted on the spin moment.

We recently introduced the idea of $s$-sourcery [B. Swingle and J. McGreevy, Phys. Rev. B 93, 045127 (2016)], a general formalism for building many-body quantum ground states using renormalization-group-inspired quantum circuits. Here we define a generalized notion of $s$-sourcery that applies to mixed states and study its properties and applicability. For our examples we focus on thermal states of local Hamiltonians. We prove a number of theorems establishing the prevalence of mixed $s$-source fixed points, giving results for free fermion models, conformal field theories, holographic models, and topological phases. Thermal double states (also called thermofield double states) and the machinery of approximate conditional independence are used heavily in the constructions. For a large class of models we provide an information theoretic argument for the existence of a local Hamiltonian whose ground state is the thermal double state, and in some cases we construct such a Hamiltonian.

In contrast to bulk FeSe, which exhibits nematic order and low temperature superconductivity, highly doped FeSe reverses the situation, having high temperature superconductivity appearing alongside a suppression of nematic order. To investigate this phenomenon, we study a minimal electronic model of FeSe, with interactions that enhance nematic fluctuations. This model is sign problem free, and is simulated using determinant quantum Monte Carlo (DQMC). We developed a DQMC algorithm with parallel tempering, which proves to be an efficient source of global updates and allows us to access the region of strong interactions. Over a wide range of intermediate couplings, we observe superconductivity with an extended $s$-wave order parameter, along with enhanced, but short-ranged, $q=(0,0)$ ferro-orbital (nematic) order. These results are consistent with approximate weak-coupling treatments that predict that nematic fluctuations lead to superconducting pairing. Surprisingly, in the parameter range under study, we do not observe nematic long-range order. Instead, at stronger coupling an unusual insulating phase with $q=(\pi ,\pi )$ antiferro-orbital order appears, which is missed by weak-coupling approximations.

Gauging is a ubiquitous tool in many-body physics. It allows one to construct highly entangled topological phases of matter from relatively simple phases and to relate certain characteristics of the two. Here we develop a gauging procedure for general submanifold symmetries of Pauli Hamiltonians, including symmetries of fractal type. We show a relation between the pre- and postgauging models and use this to construct short-range entangled phases with fractal-like symmetries, one of which is mapped to the cubic code by the gauging.

Pump-probe optical spectroscopy was used to investigate proposed charge-carrier multiplication via impact ionization in the $M_1$ insulating phase of ${\mathrm{VO}}_2$. By comparing the transient reflectivities of the film when pumped at less than and then more than twice the band-gap energy, we observed a larger ultrafast response with the higher energy pump color while the film was still transiently in the insulating phase. We additionally identified multiple time scales within the charge dynamics and analyzed how these changed when the pump and probe wavelengths were varied. This experiment provided evidence that a fast carrier multiplication process, i.e., impact ionization, acts efficiently in this prototypical strongly correlated insulator, as was recently predicted by theoretical calculations.

We consider a two-orbital impurity system with intra- and interlevel Coulomb repulsion that is coupled to a single conduction channel. This situation can generically occur in multilevel quantum dots or in systems of coupled quantum dots. For finite energy spacing between spin-degenerate orbitals, an in-plane magnetic field drives the system from a local-singlet ground state to a “mixed-level” Kondo regime, where the Zeeman-split levels are degenerate for opposite-spin states. We use the numerical renormalization group approach to fully characterize this mixed-level Kondo state and discuss its properties in terms of the applied Zeeman field, temperature, and system parameters. Under suitable conditions, the total spectral function is shown to develop a Fermi-level resonance, so that the linear conductance of the system peaks at a finite Zeeman field while it decreases as a function of temperature. These features, as well as the local moment and entropy contribution of the impurity system, are commensurate with Kondo physics, which can be studied in suitably tuned quantum dot systems.

Scrutinizing distinct solid/liquid ($s/l$) and solid/solid ($s/s$) phase transitions (passive transitions) for large change in bulk (and homogenous) thermal conductivity, we find the $s/l$ semiconductor/metal (S/M) transition produces the largest dimensionless thermal conductivity switch (TCS) figure of merit $Z_{\text{TCS}}$ (change in thermal conductivity divided by smaller conductivity). At melting temperature, the solid phonon and liquid molecular thermal conductivities are comparable and generally small, so the TCS requires localized electron solid and delocalized electron liquid states. For cyclic phase reversibility, the congruent phase transition (no change in composition) is as important as the thermal transport. We identify $X\mathrm{Sb}$ and $X\mathrm{As}$ ($X=\text{Al}$, Cd, Ga, In, Zn) and describe atomic-structural metrics for large $Z_{\text{TCS}}$, then show the superiority of S/M phonon- to electron-dominated transport melting transition. We use existing experimental results and theoretical and ab initio calculations of the related properties for both phases (including the Kubo-Greenwood and Bridgman formulations of liquid conductivities). The $5p$ orbital of Sb contributes to the semiconductor behavior in the solid-phase band gap and upon disorder and bond-length changes in the liquid phase this changes to metallic, creating the large contrast in thermal conductivity. The charge density distribution, electronic localization function, and electron density of states are used to mark this S/M transition. For optimal TCS, we examine the elemental selection from the transition, basic, and semimetals and semiconductor groups. For CdSb, addition of residual Ag suppresses the bipolar conductivity and its $Z_{\text{TCS}}$ is over 7, and for ${\mathrm{Zn}}_3{\mathrm{Sb}}_2$ it is expected to be over 14, based on the structure and transport properties of the better-known $\beta -{\mathrm{Zn}}_4{\mathrm{Sb}}_3$. This is the highest $Z_{\text{TCS}}$ identified. In addition to the metallic melting, the high $Z_{\text{TCS}}$ is due to the electron-poor nature of II-V semiconductors, leading to the significantly low phonon conductivity.

Using atomically resolved imaging and spectroscopy, we reveal a remarkably deep polarization in nonferroelectric $\mathrm{SrTi}{\mathrm{O}}_3$ near its interface with an ultrathin nonmetallic film of $\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_3$. Electron holography shows an electric field near the interface in $\mathrm{SrTi}{\mathrm{O}}_3$, yielding a surprising spontaneous polarization density of $\sim 21\mu \mathrm{C}/\mathrm{c}{\mathrm{m}}^2$. Combining the experimental results with first-principles calculations, we propose that the observed deep polarization is induced by the electric field originating from oxygen vacancies that extend beyond a dozen unit cells from the interface, thus providing important evidence of the role of defects in the emergent interface properties of transition metal oxides.

We investigate the 3.45-eV luminescence band of spontaneously formed GaN nanowires on Si(111) by photoluminescence and cathodoluminescence spectroscopy. This band is found to be particularly prominent for samples synthesized at comparatively low temperatures. At the same time, these samples exhibit a peculiar morphology, namely, isolated long nanowires are interspersed within a dense matrix of short ones. Cathodoluminescence intensity maps reveal the 3.45-eV band to originate primarily from the long nanowires. Transmission electron microscopy shows that these long nanowires are either Ga polar and are joined by an inversion domain boundary with their short N-polar neighbors, or exhibit a Ga-polar core surrounded by a N-polar shell with a tubular inversion domain boundary at the core/shell interface. For samples grown at high temperatures, which exhibit a uniform nanowire morphology, the 3.45-eV band is also found to originate from particular nanowires in the ensemble and thus presumably from inversion domain boundaries stemming from the coexistence of N- and Ga-polar nanowires. For several of the investigated samples, the 3.45-eV band splits into a doublet. We demonstrate that the higher-energy component of this doublet arises from the recombination of two-dimensional excitons free to move in the plane of the inversion domain boundary. In contrast, the lower-energy component of the doublet originates from excitons localized in the plane of the inversion domain boundary. We propose that this in-plane localization is due to shallow donors in the vicinity of the inversion domain boundaries.