The magnetization direction in heavy-metal (HM)/ferromagnet bilayers can be electrically controlled by spin-orbit torque (SOT); however, the efficiency of the SOT which depends on the spin-orbit coupling of the HM layer or its spin-Hall angle has to be improved further for actual applications. In this study, we report a significant enhancement of the spin-Hall effect of Pt and resultant SOT in $\mathrm{Pt}/\mathrm{Co}/\mathrm{Al}{\mathrm{O}}_x$ structures by controlling the Pt resistivity. We observed that the effective spin-Hall angle increases about three times as the resistivity of Pt layer is increased 1.6 times by changing the Ar deposition pressure from 3 to 50 mTorr. This enhancement in effective spin-Hall angle is confirmed by the reduction in the critical current for SOT-induced magnetization switching. Furthermore, x-ray absorption spectroscopy analysis reveals a non-negligible contribution of the interfacial spin-orbit coupling to the effective spin-Hall angle. Our result, the efficient control of effective spin Hall angle by controlling the HM resistivity, paves the way to improved switching efficiency in SOT-active devices.

The sensitivity of superconducting qubits allows for spectroscopy and coherence measurements on individual two-level systems present in the disordered tunnel barrier of an $\mathrm{Al}/{\mathrm{AlO}}_x$/Al Josephson junction. We report experimental evidence for the decoherence of two-level systems by Bogoliubov quasiparticles leaking into the insulating ${\mathrm{AlO}}_x$ barrier. We control the density of quasiparticles in the junction electrodes either by the sample temperature or by injecting them using an on-chip dc superconducting quantum interference device driven to its resistive state. The decoherence rates were measured by observing the two-level system's quantum state evolving under application of resonant microwave pulses and were found to increase linearly with quasiparticle density, in agreement with theory. This interaction with electronic states provides a noise and decoherence mechanism that is relevant for various microfabricated devices such as qubits, single-electron transistors, and field-effect transistors. The presented experiments also offer a possibility to determine the location of the probed two-level systems across the tunnel barrier, providing clues about the fabrication step in which they emerge.

Condensed matter systems with flat bands close to the Fermi level generally exhibit, due to their very large density of states, extraordinarily high critical ordering temperatures of symmetry-breaking orders, such as superconductivity and magnetism. Here we show that the critical temperatures follow one of two universal curves with doping away from a flat band depending on the ordering channel, which completely dictates both the general order competition and the phase diagram. Notably, we find that orders in the particle-particle channel (superconducting orders) survive decisively farther than orders in the particle-hole channel (magnetic or charge orders) because the channels have fundamentally different polarizabilities. Thus, even if a magnetic or charge order initially dominates, superconducting domes are still likely to exist on the flanks of flat bands. We apply these general results to both the topological surface flat bands of rhombohedral ABC-stacked graphite and to the Van Hove singularity of graphene.

We present density functional theory (DFT) calculations on phonon dispersions, phonon density of states, and thermodynamic quantities for the three copper oxide phases ${\mathrm{Cu}}_2\mathrm{O},{\mathrm{Cu}}_4{\mathrm{O}}_3$, and CuO. For monoclinic CuO we consider the correct antiferromagnetic ground state. Sound velocities for the acoustic phonon branches and Debye temperatures are calculated and are found to be in good agreement with experiment. We further show how the method for the treatment of dipole-dipole interactions in dynamical matrices of Gonze and Lee [Phys. Rev. B 55, 10355 (1997)] may be incorporated in the real-space (direct) method for interatomic force constants (FCs). The role of the long-ranged dipole-dipole interactions in the phonon dispersion is discussed. Based on this method, we outline a perturbationlike scheme to compute first-order derivatives of the phonon mode frequencies with respect to the wave vector which can be used to compute velocities of sound.

Suppressing phonon propagation in nanowires is an essential goal towards achieving efficient thermoelectric devices. Recent experiments have shown unambiguously that surface roughness is a key factor that can reduce the thermal conductivity well below the Casimir limit in thin crystalline silicon nanowires. We use insights gained from the experimental studies to construct a simple analytically tractable model of the phonon–surface-roughness interaction that provides a better theoretical understanding of the effects of surface roughness on the thermal conductivity, which could potentially help in designing better thermoelectric devices.

By making use of circularly polarized light and electrostatic gating, monolayer molybdenum disulfide (${\mathrm{ML}-\mathrm{MoS}}_2$) can form a platform supporting multiple types of charge carriers. They can be discriminated by their spin, valley index, or whether they are electrons or holes. We investigate the collective properties of those charge carriers and are able to identify distinct plasmon modes. We analyze the corresponding dispersion relation, lifetime, and oscillator strength, and calculate the phase relation between the oscillations in the different components of the plasmon modes. All platforms in ML-${\mathrm{MoS}}_2$ support a long-wavelength $\sqrt{q}$ plasmon branch at zero kelvins. In addition to this, for an $n$-component system, $n-1$ distinct plasmon modes appear as acoustic modes with linear dispersion in the long-wavelength limit. These modes correspond to out-of-phase oscillations in the different fermion liquids and have, although being damped, a relatively long lifetime. Additionally, we also find distinct modes at large wave vectors that are more strongly damped by intraband processes.

We study the atomic contributions to the nanoscale friction in layered $M{X}_2$ $(M=\text{Mo}$, W; $X=\text{S}$, Se, Te) transition metal dichalcogenides by combining ab initio techniques with group-theoretical analysis. Starting from stable atomic configurations, we propose a computational method, named normal-modes transition approximation (NMTA), to individuate possible sliding paths from only the analysis of the phonon modes of the stable geometry. The method provides a way to decompose the atomic displacements realizing the layer sliding in terms of phonon modes of the stable structure, so as to guide the selection and tuning of specific atomic motions promoting $M{X}_2$ sheets gliding, and to adjust the corresponding energy barrier. The present results show that main contributions to the nanoscale friction are due to few low frequency phonon modes, corresponding to rigid shifts of $M{X}_2$ layers. We also provide further evidences that a previously reported Ti-doped ${\mathrm{MoS}}_2$ phase is a promising candidate as new material with enhanced tribologic properties. The NMTA approach can be exploited to tune the energetic and the structural features of specific phonon modes, and, thanks to its general formulation, can also be applied to any solid state system, irrespective of the chemical composition and structural topology.

In this study, we investigate the structural and chemical changes of monatomic ${\mathrm{CoO}}_2$ chains grown self-organized on the Ir(100) surface [P. Ferstl et al., Phys. Rev. Lett. 117, 046101 (2016)] and on Pt(100) under reducing and oxidizing conditions. By a combination of quantitative low-energy electron diffraction, scanning tunneling microscopy, and density functional theory we show that the cobalt oxide wires are completely reduced by ${\mathrm{H}}_2$ at temperatures above 320 K and a $3\times 1$ ordered ${\mathrm{Ir}}_2\mathrm{Co}$ or ${\mathrm{Pt}}_2\mathrm{Co}$ surface alloy is formed. Depending on temperature, the surface alloy on Ir(100) is either hydrogen covered $(T<400$ K) or clean and eventually undergoes an irreversible order-disorder transition at about 570 K. The ${\mathrm{Pt}}_2\mathrm{Co}$ surface alloy disorders with the desorption of hydrogen, whereby Co submerges into subsurface sites. Vice versa, applying stronger oxidants than ${\mathrm{O}}_2$ such as ${\mathrm{NO}}_2$ leads to the formation of ${\mathrm{CoO}}_3$ chains on Ir(100) in a $3\times 1$ superstructure. On Pt(100), such a ${\mathrm{CoO}}_3$ phase could not be prepared so far, which, however, is due to the ultrahigh vacuum conditions of our experiments. As revealed by theory, this phase will become stable in a regime of higher pressure. In general, the structures can be reversibly switched on both surfaces using the respective agents ${\mathrm{O}}_2,{\mathrm{NO}}_2$, and ${\mathrm{H}}_2$.

The rotation of octahedra (octahedral tilting) is common in $AB{\mathrm{O}}_3$ perovskites and relevant to many physical phenomena, ranging from electronic and magnetic properties, metal-insulator transitions to improper ferroelectricity. Hydrostatic pressure is an efficient way to tune and control octahedral tiltings. However, the pressure behavior of such tiltings can dramatically differ from one material to another, with the origins of such differences remaining controversial. In this paper, we discover several new mechanisms and formulate a set of simple rules that allow us to understand how pressure affects oxygen octahedral tiltings via the use and analysis of first-principles results for a variety of compounds. Besides the known $A\text{−}\mathrm{O}$ interactions, we reveal that the interactions between specific $B$ ions and oxygen ions contribute to the tilting instability. We explain the previously reported trend that the derivative of the oxygen octahedral tilting with respect to pressure ($\mathit{dR}/\mathit{dP}$) usually decreases with both the tolerance factor and the ionization state of the $A$ ion by illustrating the key role of $A\text{−}\mathrm{O}$ interactions and their change under pressure. Furthermore, three new mechanisms/rules are discovered, namely that (i) the octahedral rotations in $AB{\mathrm{O}}_3$ perovskites with empty low-lying $d$ states on the $B$ site are greatly enhanced by pressure, in order to lower the electronic kinetic energy; (ii) $\mathit{dR}/\mathit{dP}$ is enhanced when the system possesses weak tilt instabilities, and (iii) for the most common phase exhibited by perovskites—the orthorhombic Pbnm state—the in-phase and antiphase octahedral rotations are not automatically both suppressed or both enhanced by the application of pressure because of a trilinear coupling between these two rotation types and an antipolar mode involving the $A$ ions. We further predict that the polarization associated with the so-called hybrid improper ferroelectricity could be manipulated by hydrostatic pressure by indirectly controlling the amplitude of octahedral rotations.

The current-carrying steady state that arises in the middle of a metallic wire connected to macroscopic leads is characterized regarding its response functions, correlations, and entanglement entropy. The spectral function and the dynamical structure factor show clear nonequilibrium signatures accessible by state-of-the-art techniques. In contrast with the equilibrium case, the entanglement entropy is extensive with logarithmic corrections at zero temperature that depend on the lead-wire coupling and, in a nonanalytic way, on voltage. This shows that some robust universal quantities found in gapless equilibrium phases do not persist away from equilibrium.

Starting from the general Berry phase theory of the Dzyaloshinskii-Moriya interaction (DMI) we derive an expression for the linear contribution of the spin-orbit interaction (SOI). Thereby, we show analytically that at the first order in SOI DMI is given by the ground-state spin current. We verify this finding numerically by ab initio calculations in Mn/W(001) and Co/Pt(111) magnetic bilayers. We show that despite the strong SOI from the $5d$ heavy metals, DMI is well-approximated by the first order in SOI, while the ground-state spin current is not. We decompose the SOI-linear contribution to DMI into two parts. One part has a simple interpretation in terms of the Zeeman interaction between the spin-orbit field and the spin misalignment that electrons acquire in magnetically noncollinear textures. This interpretation provides also an intuitive understanding of the symmetry of DMI on the basis of the spin-orbit field and it explains in a simple way why DMI and ground-state spin currents are related. Moreover, we show that energy currents driven by magnetization dynamics and associated to DMI can be explained by counter-propagating spin currents that carry energy due to their Zeeman interaction with the spin-orbit field. Finally, we discuss options to modify DMI by nonequilibrium spin currents excited by electric fields or light.

The longitudinal spin Seebeck effect (LSSE) with a vertical temperature gradient is one of the most important mechanisms to generate pure spin current. Previous studies of the LSSE excited spin current focus mainly on the magnetic insulators, a little on ferromagnetic metals, and rarely on ferromagnetic half metals. In this work, we demonstrate a significant spin current injected from the highly spin polarized ferromagnetic half metal $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ by the LSSE. The sign of the thermal voltage can be reversed by using the spin current detector Cr with a large negative spin Hall angle. The ratio of the inverse spin Hall voltage to the total thermal signal in $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ is much larger than that in ferromagnetic metals, such as permalloy and CoFeB. The nontrivial temperature-dependent voltage suggests that the thermal transport in $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ is carried by magnons. This study provides insight into the mechanism of thermally excited spin current in ferromagnetic half metals and recommends the highly spin polarized $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ as a promising candidate for metal-based spin caloritronics devices.

The Usadel equation is the standard theoretical tool for the description of superconducting structures in the diffusive limit. Here, I derive the Usadel equation for gyrotropic materials with a generic linear in momentum spin-orbit coupling. It accounts for the spin-charge/singlet-triplet coupling and in the normal state reduces to the system of spin-charge diffusion equations describing various magnetoelectric effects, such as the spin Hall effect (SHE), the spin-galvanic effect (SGE), and their inverses. Therefore the derived Usadel equation establishes a direct connection of these effects to their superconducting counterparts. The working power of the present formalism is illustrated on the example of the bulk SGE.

We solve the Bogoliubov-de Gennes equations self-consistently for the $d$-wave order parameter in nanoscale $d$-wave systems with [110] surfaces and show that spontaneous time-reversal symmetry (TRS) breaking occurs at low temperatures due to a spontaneously induced complex order parameter of extended $s$-wave symmetry. The Andreev surface bound states, which are protected by a one-dimensional (1D) topological invariant in the presence of TRS, are gapped by the emergence of this new order parameter. The extended $s$-wave order parameter is localized within a narrow region near the surfaces, which is consistent with the fact that topological protection of the gapless Andreev surface states is characterized by the 1D topological invariant. In this TRS-breaking phase, not only is the complex order parameter induced, but also the $d$-wave order parameter itself becomes complex. Furthermore, the disappearance of topological protection brings about novel vortex phenomena near the surfaces. We show that vortex-antivortex pairs are formed in the extended $s$-wave order parameter along the surfaces if the side length of a nanoisland or the width of an infinitely long nanoribbon is relatively large.

Controlling the conductance and current flow through nanostructured magnetic point contacts is a key challenge for future spintronic devices. This could be achieved by exploiting the Rashba spin-orbit coupling effect induced by an external gate in the middle of two pinned domain walls at the point contacts. Here, I investigate the electrical conductance of a half-metallic diluted magnetic semiconductor nanowire with a double point contact exploitable in switching devices controlled by lateral gate voltage. The coherent quantum interference between forward- and backward-scattered waves in the spin quantum well formed by the double point contact leads to quasibound states with finite lifetimes. The energetic position of these quasibound states could be adjusted by the lateral gate voltage so that the incident energy coincides with one of the quasibound energy levels in the spin quantum well. Conductance calculations in the presence of an applied electric field perpendicular to the nanowire surface exhibit typical resonant tunneling behavior, where the nanostructure switches to the low-resistance ON state by tuning the Rashba coupling strength in the range of a few tens of meV nm. This study paves the way for utilizing the gate-controlled Rashba spin-orbit coupling effect to design and develop practical spintronic devices.

For $N$ hard-core bosons on an arbitrary lattice with $d$ sites and independent of additional interaction terms we prove that the hard-core constraint itself already enforces a universal upper bound on the Bose-Einstein condensate given by $N_{\text{max}}=(N/d)(d-N+1)$. This bound can only be attained for one-particle states $|\phi \rangle$ with equal amplitudes with respect to the hard-core basis (sites) and when the corresponding $N$-particle state $|\mathrm{\Psi }\rangle$ is maximally delocalized. This result is generalized to the maximum condensate possible within a given sublattice. We observe that such maximal local condensation is only possible if the mode entanglement between the sublattice and its complement is minimal. We also show that the maximizing state $|\mathrm{\Psi }\rangle$ is related to the ground state of a bosonic “Hubbard star” showing Bose-Einstein condensation.

Two-dimensional flow past an infinitely long cylinder of nanoscopic radius in superfluid ${}^4\mathrm{He}$ at zero temperature is studied using time-dependent density-functional theory. The calculations reveal two distinct critical phenomena for the onset of dissipation: (i) vortex-antivortex pair shedding from the periphery of the moving cylinder, and (ii) the appearance of cavitation in the wake, which possesses similar geometry to that observed experimentally for fast-moving micrometer-scale particles in superfluid ${}^4\mathrm{He}$. The formation of cavitation bubbles behind the cylinder is accompanied by a sudden jump in the drag exerted on the moving cylinder by the fluid. Vortex pairs with the same circulation are occasionally emitted in the form of dimers, which constitute the building blocks for the Benard–von Karman vortex street structure observed in classical turbulent fluids and Bose-Einstein condensates. The cavitation-induced dissipation mechanism should be common to all superfluids that are self-bound and have a finite surface tension, which include the recently discovered self-bound droplets in ultracold Bose gases. These systems would provide an ideal testing ground for further exploration of this mechanism experimentally.

We study thermal and electrical transport in metals and superconductors near a quantum phase transition where antiferromagnetic order disappears. The same theory can also be applied to quantum phase transitions involving the loss of certain classes of intrinsic topological order. For a clean superconductor, we recover and extend well-known universal results. The heat conductivity for commensurate and incommensurate antiferromagnetism coexisting with superconductivity shows a markedly different doping dependence near the quantum critical point, thus allowing us to distinguish between these states. In the dirty limit, the results for the conductivities are qualitatively similar for the metal and the superconductor. In this regime, the geometric properties of the Fermi surface allow for a very good phenomenological understanding of the numerical results on the conductivities. In the simplest model, we find that the conductivities do not track the doping evolution of the Hall coefficient, in contrast to recent experimental findings. We propose a doping dependent scattering rate, possibly due to quenched short-range charge fluctuations below optimal doping, to consistently describe both the Hall data and the longitudinal conductivities.