Using fully self-consistent methods, we study spin transport in fabricable spin valve systems consisting of two magnetic layers, a superconducting layer, and a spacer normal layer between the ferromagnets. Our methods ensure that the proper relations between spin current gradients and spin transfer torques are satisfied. We present results as a function of geometrical parameters, interfacial barrier values, misalignment angle between the ferromagnets, and bias voltage. Our main results are for the spin current and spin accumulation as functions of position within the spin valve structure. We see precession of the spin current about the exchange fields within the ferromagnets, and penetration of the spin current into the superconductor for biases greater than the critical bias, defined in the text. The spin accumulation exhibits oscillating behavior in the normal metal, with a strong dependence on the physical parameters both as to the structure and formation of the peaks. We also study the bias dependence of the spatially averaged spin transfer torque and spin accumulation. We examine the critical-bias effect of these quantities, and their dependence on the physical parameters. Our results are predictive of the outcome of future experiments, as they take into account imperfect interfaces and a realistic geometry.

We investigated the topological property of magnon bands in the collinear magnetic orders of zigzag and stripe phases for the antiferromagnetic honeycomb lattice and identified Berry curvature and symmetry constraints on the magnon band structure. Different symmetries of both zigzag and stripe phases lead to different topological properties, in particular, the magnon bands of the stripe phase being disentangled with a finite Dzyaloshinskii-Moriya (DM) term with nonzero spin Chern number. This is corroborated by calculating the spin Nernst effect. Our study establishes the existence of a nontrivial magnon band topology for all observed collinear antiferromagnetic honeycomb lattices in the presence of the DM term.

The local electronic and magnetic properties of superconducting FeSe have been investigated by $K\beta$ x-ray emission and simultaneous x-ray absorption spectroscopy (XAS) at the Fe $K$ edge at high pressure and low temperature. Our results indicate a sluggish decrease of the local Fe spin moment under pressure up to 7 GPa, in line with previous reports, followed by a sudden increase at higher pressure. The magnetic surge is preceded by an abrupt change of the Fe local structure as observed by the decrease of the XAS preedge region intensity and corroborated by ab initio simulations. This pressure corresponds to a structural transition from the $Cmma$ form to the denser $Pbnm$ form with octahedral coordination of iron. Finally, the near-edge region of the XAS spectra shows a change before this transition at 5 GPa, corresponding well with the onset pressure of the sudden enhancement of $T_c$. Our results emphasize the delicate interplay between structural, magnetic, and superconducting properties in FeSe under pressure.

The aim of this paper is to investigate the consequences of finite-size effects on the thermodynamics of nanoparticle assemblies and isolated particles. We consider a binary phase-separating alloy with a negligible atomic size mismatch, and equilibrium states are computed using off-lattice Monte Carlo simulations in several thermodynamic ensembles. First, a semi-grand-canonical ensemble is used to describe infinite assemblies of particles with the same size. When decreasing the particle size, we obtain a significant decrease of the solid/liquid transition temperatures as well as a growing asymmetry of the solid-state miscibility gap related to surface segregation effects. Second, a canonical ensemble is used to analyze the thermodynamic equilibrium of finite monodisperse particle assemblies. Using a general thermodynamic formulation, we show that a particle assembly may split into two subassemblies of identical particles. Moreover, if the overall average canonical concentration belongs to a discrete spectrum, the subassembly concentrations are equal to the semi-grand-canonical equilibrium ones. We also show that the equilibrium of a particle assembly with a prescribed size distribution combines a size effect and the fact that a given particle size assembly can adopt two configurations. Finally, we have considered the thermodynamics of an isolated particle to analyze whether a phase separation can be defined within a particle. When studying rather large nanoparticles, we found that the region in which a two-phase domain can be identified inside a particle is well below the bulk phase diagram, but the concentration of the homogeneous core remains very close to the bulk solubility limit.

In spin-polarized surface positronium annihilation measurements, the spin polarizations of graphene and $h$-BN on Co(0001) were higher than those on Ni(111), while no significant differences were seen between graphene and $h$-BN on the same metal. The obtained spin polarizations agreed with those expected from first-principles calculations considering the positron wave function and the electron density of states from the first surface layer to the vacuum region. The higher spin polarizations of graphene and $h$-BN on Co(0001) as compared to Ni(111) simply reflect the spin polarizations of these metals. The comparable spin polarizations of graphene and $h$-BN on the same metal are attributed to the creation of similar electronic states due to the strong influence of the metals: the Dirac cone of graphene and the band gap of $h$-BN disappear as a consequence of $d-\pi$ hybridization.

We present a computational study of the adhesive and structural properties of the ${\mathrm{Al}/\mathrm{Al}}_2{\text{O}}_3$ interfaces as building blocks of the metal-insulator-metal (MIM) tunnel devices, where electron transport is accomplished via tunneling mechanism through the sandwiched insulating barrier. The main goal of this paper is to understand, on the atomic scale, the role of the geometrical details in the formation of the tunnel barrier profiles. Initially, we concentrate on the adhesive properties of the interfaces. To provide reliable results, we carefully assess the accuracy of the traditional methods used to examine ${\mathrm{Al}/\mathrm{Al}}_2{\text{O}}_3$ systems. These are the most widely employed exchange-correlation functionals—local-density approximation and two different generalized gradient approximations; the universal binding-energy relation for predicting equilibrium interfacial distances and adhesion energies; and the ideal work of separation as a measure of junction stability. In addition, we show that the established interpretation of the computed ideal work of separation might be misleading in predicting the optimal interface structures. Finally, we perform a detailed analysis of the atomic and interplanar relaxations in each junction, and identify their contributions to the tunnel barrier parameters. Our results imply that the structural irregularities on the surface of the Al film have a significant contribution to lowering the tunnel barrier height, while atomic relaxations at the interface and interplanar relaxations in ${\mathrm{Al}}_2{\text{O}}_3$ may considerably change the width of the barrier and, thus, distort its uniformity. Both the effects may critically influence the performance of the MIM tunnel devices.

A possible connection between extremely large magnetoresistance and the presence of Weyl points has garnered much attention in the study of topological semimetals. Exploration of these concepts in transition-metal diphosphides ${\mathrm{WP}}_2$ has been complicated by conflicting experimental reports. Here we combine angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations to disentangle surface and bulk contributions to the ARPES intensity, the superposition of which has plagued the determination of the band structure in ${\mathrm{WP}}_2$. Our results show that while the hole- and electronlike Fermi surface sheets originating from surface states have different areas, the bulk-band structure of ${\mathrm{WP}}_2$ is electron-hole compensated in agreement with DFT. Furthermore, the ARPES band structure is compatible with the presence of at least four temperature-independent Weyl points, confirming the topological nature of ${\mathrm{WP}}_2$ and its stability against lattice distortions.

Graphene exhibits extremely strong optical nonlinearity in a perpendicular magnetic field, the optical conductivities show complicated field dependence at a moderate light intensity, and the perturbation theory fails. The full optical currents induced by a periodic field are nonperturbatively investigated in an equation-of-motion framework based on the Floquet theorem, with the scattering described phenomenologically. The nonlinear responses are understood in terms of the dressed electronic states, or Floquet states, which could be characterized by a weak probe light field. The method is illustrated for a magnetic field at 5 T and a driving field with photon energy 0.05 eV. Our results show that the perturbation theory works for weak fields $<3$ kV/cm, confirming the unusual strong light-matter interaction for Landau levels of graphene. Our approach can be easily extended to other systems.

We study the effects of Rashba spin-orbit coupling on two-dimensional Rydberg exciton systems. Using analytical and numerical arguments we demonstrate that this coupling considerably modifies the wave functions and leads to a level repulsion that results in a deviation from the Poissonian statistics of the adjacent level distance distribution. This signifies the crossover to nonintegrability of the system and hints at the possibility of quantum chaos emerging. Such behavior strongly differs from the classical realization, where spin-orbit coupling produces highly entangled, chaotic electron trajectories in an exciton. We also calculate the oscillator strengths and show that randomization appears in the transitions between states with different total momenta.

We construct the finite-temperature dynamical phase diagram of the fully connected transverse-field Ising model from the vantage point of two disparate concepts of dynamical criticality. An analytical derivation of the classical dynamics and exact diagonalization simulations are used to study the dynamics after a quantum quench in the system prepared in a thermal equilibrium state. The different dynamical phases characterized by the type of nonanalyticities that emerge in an appropriately defined Loschmidt-echo return rate directly correspond to the dynamical phases determined by the spontaneous breaking of ${\mathbb{Z}}_2$ symmetry in the long-time steady state. The dynamical phase diagram is qualitatively different depending on whether the initial thermal state is ferromagnetic or paramagnetic. Whereas the former leads to a dynamical phase diagram that can be directly related to its equilibrium counterpart, the latter gives rise to a divergent dynamical critical temperature at vanishing final transverse-field strength.

Tunneling spin transport characteristics of a magnetic skyrmion are described theoretically in magnetic scanning tunneling microscopy (STM). The spin-polarized charge current in STM (SP-STM) and tunneling spin transport vector quantities, the longitudinal spin current and the spin transfer torque, are calculated in high spatial resolution within the same theoretical framework. A connection between the conventional charge current SP-STM image contrasts and the magnitudes of the spin transport vectors is demonstrated that enables the estimation of tunneling spin transport properties based on experimentally measured SP-STM images. A considerable tunability of the spin transport vectors by the involved spin polarizations is also highlighted. These possibilities and the combined theory of tunneling charge and vector spin transport pave the way for gaining deep insight into electric-current-induced tunneling spin transport properties in SP-STM and to the related dynamics of complex magnetic textures at surfaces.

One-dimensional systems obtained as low-energy limits of hybrid superconductor-topological insulator devices provide means of production, transport, and destruction of Majorana bound states (MBSs) by variations of the magnetic flux. When two or more pairs of MBSs are present in the intermediate state, there is a possibility of a Landau-Zener transition wherein even a slow variation of the flux leads to production of a quasiparticle pair. We study numerically a version of this process with four MBSs produced and subsequently destroyed and find that, quite universally, the probability of quasiparticle production in it is 50%. This implies that the effect may be a limiting factor in applications requiring a high degree of quantum coherence.

The understanding of the atomistic origin of the morphotropic phase boundary (MPB) occurring in composition-temperature phase diagrams of ferroelectric solid solutions is a key topic in material science because materials often exhibit anomalous properties at the MPB. Here we reveal mesoscopic-scale structural correlations for a leading Pb-free ferroelectric system, $\left(1\text{−}x\right){\mathrm{Na}}_{0.5}{\mathrm{Bi}}_{0.5}{\mathrm{TiO}}_3\text{−}x{\mathrm{BaTiO}}_3$ (NBT-$x\mathrm{BT}$), by examining atomic pair distribution functions and Raman scattering data at ambient conditions. We demonstrate that the amplification of the piezoelectric properties of NBT-$x\mathrm{BT}$ at the MPB are predominantly driven by an easy switchability resulting from a progressive decoupling between strain and polarization as the Ba content increases from zero to the critical MPB composition. It was observed that as Ba content increases towards MPB, competing local correlations, such as A-site chemical order, antiferrodistortive correlations of correlated ${\mathrm{BO}}_6$ tilts, and antipolar Bi shifts, are reduced, which in turn renders favorable conditions for easy switching of local dipoles under external fields. In addition, the evolving characteristics of the atomic dynamics as a function of composition suggest that the local potential functions of the cations are not completely flat at the MPB. Altogether, our results reveal atomistic mechanisms responsible for the observed elevated MPB properties in the case of NBT-$x\mathrm{BT}$ which imply that the so-called MPB of NBT-$x\mathrm{BT}$ should not be categorized as originally introduced for Pb-containing solid solutions.

Density functional theory augmented with Hubbard-$U$ corrections (DFT+$U$) is currently one of the most widely used methods for first-principles electronic structure modeling of insulating transition-metal oxides (TMOs). Since $U$ is relatively large compared to bandwidths, the magnetic excitations in TMOs are expected to be well described by a Heisenberg model. However, in practice the calculated exchange parameters $J_{ij}$ depend on the magnetic configuration from which they are extracted and on the functional used to compute them. In this work we investigate how the spin polarization dependence of the underlying exchange-correlation functional influences the calculated magnetic exchange constants of TMOs. We perform a systematic study of the predictions of calculations based on the local density approximation plus $U$ (LDA+$U$) and the local spin density approximation plus $U$ (LSDA+$U$) for the electronic structures, total energies, and magnetic exchange interactions $J_{ij}$ extracted from ferromagnetic (FM) and antiferromagnetic (AFM) configurations of several transition-metal oxide materials. We report that for realistic choices of Hubbard $U$ and Hund's $J$ parameters, LSDA+$U$ and LDA+$U$ calculations result in different values of the magnetic exchange constants and band gap. The dependence of the band gap on the magnetic configuration is stronger in LDA+$U$ than in LSDA+$U$ and we argue that this is the main reason why the configuration dependence of $J_{ij}$ is found to be systematically more pronounced in LDA+$U$ than in LSDA+$U$ calculations. We report a very good correspondence between the computed total energies and the parametrized Heisenberg model for LDA+$U$ calculations, but not for LSDA+$U$, suggesting that LDA+$U$ is a more appropriate method for estimating exchange interactions.

We present a combination of first-principles and experimental results regarding the structural and magnetic properties of olivine-type ${\mathrm{LiFePO}}_4$ under pressure. Our investigations indicate that the starting $Pbnm$ phase of ${\mathrm{LiFePO}}_4$ persists up to 70 GPa. Further compression leads to an isostructural transition in the pressure range of 70–75 GPa, inconsistent with a former theoretical study. Considering our first-principles prediction for a high-spin to low-spin transition of ${\mathrm{Fe}}^{2+}$ close to 72 GPa, we attribute the experimentally observed isostructural transition to a change in the spin state of ${\mathrm{Fe}}^{2+}$ in ${\mathrm{LiFePO}}_4$. Compared to relevant Fe-bearing minerals, ${\mathrm{LiFePO}}_4$ exhibits the largest onset pressure for a pressure-induced spin state transition.

The coupling between spin waves and optical Mie resonances inside a dielectric magnetic spherical particle, which acts simultaneously as a photonic and magnonic (photomagnonic) cavity, is investigated by means of numerical calculations accurate to arbitrary order in the magnetooptical coupling coefficient. Isolated dielectric magnetic particles with diameters of just a few microns support high-$Q$ optical Mie resonances at near-infrared frequencies and localized spin waves, providing an ultrasmall and compact platform in the emerging field of cavity optomagnonics. Our results predict the occurrence of strong interaction effects, beyond the linear-response approximation, which lead to enhanced modulation of near-infrared light by spin waves through multimagnon absorption and emission mechanisms.

We consider an external feedback control loop implementing the action of a Maxwell demon. Applying control actions that are conditioned on measurement outcomes, the demon may transport electrons against a bias voltage and thereby effectively converts information into electric power. While the underlying model—a feedback-controlled quantum dot that is coupled to two electronic leads—is well explored in the limit of small tunnel couplings, we can address the strong-coupling regime with a fermionic reaction-coordinate mapping. This exact mapping transforms the setup into a serial triple quantum dot coupled to two leads. We find that a continuous projective measurement of the central dot occupation would lead to a complete suppression of electronic transport due to the quantum Zeno effect. In contrast, by using a microscopic detector model we can implement a weak measurement, which allows for closure of the control loop without transport blockade. Then, in the weak-coupling regime, the energy flows associated with the feedback loop are negligible, and dominantly the information gained in the measurement induces a bound for the generated electric power. In the strong coupling limit, the protocol may require more energy for operating the control loop than electric power produced, such that the whole device is no longer information dominated and can thus not be interpreted as a Maxwell demon.

${\mathrm{Cu}}_2{\mathrm{GeTe}}_3$ (CGT) phase-change material, a promising candidate for advanced fast nonvolatile random-access-memory devices, has a chalcopyritelike structure with $s{p}^3$ bonding in the crystalline phase; thus, the phase-change (PC) mechanism is considered to be essentially different from that of the standard PC materials (e.g., Ge-Sb-Te) with threefold to sixfold $p$-like bonding. In order to reveal the PC mechanism of CGT, the electronic structure change due to PC has been investigated by laboratory hard x-ray photoelectron spectroscopy and combined first-principles density-functional theory molecular-dynamics simulations. The valence-band spectra, in both crystalline and amorphous phases, are well simulated by the calculations. An inherent tendency of Te $5s$ lone-pair formation and an enhanced participation of Cu $3d$ orbitals in the bonding are found to play dominant roles in the PC mechanism. The electrical conductivity of as-deposited films and its change during the PC process is investigated in connection with valence-band spectral changes near the Fermi level. The results are successfully analyzed, based on a model proposed by Davis and Mott for chalcogenide amorphous semiconductors. The results suggest that robustness of the defect-band states against thermal stress is a key to the practical application of this material for memory devices.

Excited conduction electrons, conduction holes, and valence holes in monolayer electron-doped germanene exhibit unusual Coulomb decay rates. The deexcitation processes are studied using the screened exchange energy. They might utilize the intraband single-particle excitations (SPEs), the interband SPEs, and the plasmon modes, depending on the quasiparticle states and the Fermi energies. The low-lying valence holes can decay through the undamped acoustic plasmon, so that they present very fast Coulomb deexcitations, nonmonotonous energy dependence, and anisotropic behavior. However, the low-energy conduction electrons and holes are similar to those in a two-dimensional electron gas. The higher-energy conduction states and the deeper-energy valence ones behave similarly in the available deexcitation channels and have a similar dependence of decay rate on the wave vector $\mathbf{k}$.