We report a neutron-scattering investigation of the spin-crossover compound $\left[\mathrm{Fe}{\left(\mathrm{ptz}\right)}_6\right]{\left({\mathrm{BF}}_4\right)}_2$, which undergoes an abrupt thermal spin transition from high spin (HS), $S=2$, to low spin (LS), $S=0$, around 135 K. The HS magnetic state can be restored at low temperature under blue/green light irradiation. We have developed a specially designed optical setup for neutron scattering to address the magnetic properties of the light-induced HS state. By using neutron diffraction, we demonstrate that significant HS/LS ratios (of up to 60%) can be obtained with this experimental setup on a sample volume considered large (400 mg), while a complete recovery of the LS state is achieved using near-infrared light. Finally, with inelastic neutron scattering (INS) we have observed magnetic transitions arising from the photo-induced metastable HS $S=2$ state split by crystal-field and spin-orbit coupling. We interpret the INS data assuming a spin-only model with a zero-field splitting of the $S=2$ ground state. The obtained parameters are $D\approx -1.28\pm 0.03$ meV and $\left|E\right|\approx 0.08\pm 0.03$ meV. The present results show that in situ magnetic inelastic neutron-scattering investigations on a broad range of photomagnetic materials are now possible.

The ground state of the frustrated $A$-site magnetic spinel $\mathrm{CoA}{\mathrm{l}}_2{\mathrm{O}}_4$ has been a controversial issue whether it is a collinear antiferromagnetic ordering or a spiral spin-liquid state, as the ratio of the two competing interactions $J_2/J_1$ lies close to the boundary between these two ground states. Here we address the magnetic ground state in $\mathrm{CoA}{\mathrm{l}}_2{\mathrm{O}}_4$ with different amounts of $\mathrm{C}{\mathrm{o}}^{2+}/\mathrm{A}{\mathrm{l}}^{3+}$ site disorder from the study of magnetoelectric effect and Monte Carlo simulations. $\mathrm{CoA}{\mathrm{l}}_2{\mathrm{O}}_4$ with low site disorder exhibits a linear magnetoelectric effect below the magnetic ordering temperature. With increasing disorder, the magnetoelectric effect is suppressed and the sample with $14\%$ disorder exhibits a spin glass behavior without the magnetoelectric effect. Monte Carlo simulations support the experimental findings and suggest that the site disorder suppresses long-range antiferromagnetic order and induces a spin glass state. Since the linear magnetoelectric effect requires a long-range magnetic ordering, we suggest that the ground state of $\mathrm{CoA}{\mathrm{l}}_2{\mathrm{O}}_4$ with low site disorder is a collinear antiferromagnet.

Searching for ultrahard materials is of great interest in scientific research and for industrial applications. Boron carbide $\left({\mathrm{B}}_4\mathrm{C}\right)$ is one of the hardest known materials, but its Vickers hardness (30 GPa) is much less than diamond (115 GPa) and cubic boron nitride (48 GPa). A new ${\mathrm{B}}_4\mathrm{C}$ phase with twinlike character, denoted as $\tau -{\mathrm{B}}_4\mathrm{C}$, is characterized using density functional theory. This $\tau -{\mathrm{B}}_4\mathrm{C}$ is based on the Cmcm orthorhombic space group and is slightly less stable than the known rhombohedral ${\mathrm{B}}_4\mathrm{C}$ ($\mathrm{R}-{\mathrm{B}}_4\mathrm{C}$) by $3.87\mathrm{meV}/{\mathrm{B}}_4\mathrm{C}$. Resulting shear deformation along the least stress slip system shows that $\tau -{\mathrm{B}}_4\mathrm{C}$ has an ideal shear strength of 45.1 GPa, which is 15.6% higher than $\mathrm{R}-{\mathrm{B}}_4\mathrm{C}$, suggesting that $\tau -{\mathrm{B}}_4\mathrm{C}$ is much stronger than $\mathrm{R}-{\mathrm{B}}_4\mathrm{C}$. However, under biaxial shear deformation to mimic indentation stress conditions, the critical shear stress for $\tau -{\mathrm{B}}_4\mathrm{C}$ is 28.8 GPa, which is similar to that of $\mathrm{R}-{\mathrm{B}}_4\mathrm{C}$ (28.5 GPa), indicating that the intrinsic hardness is similar for these two phases. The failure mechanism of the $\tau -{\mathrm{B}}_4\mathrm{C}$ is the deconstruction of the icosahedra arising from the B-C bond breaking within the icosahedron. It may be worth exploring how to synthesize $\tau -{\mathrm{B}}_4\mathrm{C}$ experimentally because of its high strength.

The existence of inverted hysteresis loops (IHLs) in magnetic materials is still in debate due to the lack of direct evidence and convincing theoretical explanations. Here we report the direct observation and physical interpretation of complete IHL in $\mathrm{N}{\mathrm{i}}_{45}\mathrm{F}{\mathrm{e}}_{55}$ films with 1 to 2 nm thin $\mathrm{N}{\mathrm{i}}_3\mathrm{Fe}$ secondary phases at the grain boundaries. The origin of the inverted loop, however, is shown to be due to the exchange bias coupling between $\mathrm{N}{\mathrm{i}}_{45}\mathrm{F}{\mathrm{e}}_{55}$ and $\mathrm{N}{\mathrm{i}}_3\mathrm{Fe}$, which can be broken by the application of a high magnetic field. A large positive exchange bias $(H_{EB}=14\times H_C)$ is observed in the NiFe composite material giving novel insight into the formation of a noninverted hysteresis loop (non-IHL) and IHL, which depend on the loop tracing field range ($H_R$). The crossover from non-IHL to IHL is found to be at 688 Oe.

Here we address two nonequilibrium Green's-function approaches for a resonant tunneling structure under a sudden switch of a bias. Our aim is to stress that the time-dependent Keldysh formulation of Jauho, Wingreen, and Meir and the partition-free scheme of Stefanucci and Almbladh are formally equivalent in the ubiquitous case of wide-band limit and noninteracting electrons, if leads and dot are in equilibrium before the time-dependent perturbation. We develop explicit closed formulas of the lesser Green's function and time-dependent current, reminding that the different integration limits preclude a face-to-face comparison of two approaches. This study sheds light on both practices, which are of great interest to the mesoscopic transport community.

We show that ac driven skyrmions interacting with an asymmetric substrate provide a realization of a class of ratchet system which we call a vector ratchet that arises due to the effect of the Magnus term on the skyrmion dynamics. In a vector ratchet, the dc motion induced by the ac drive can be described as a vector that can be rotated clockwise or counterclockwise relative to the substrate asymmetry direction. Up to a full ${360}^{\circ }$ rotation is possible for varied ac amplitudes or skyrmion densities. In contrast to overdamped systems, in which ratchet motion is always parallel to the substrate asymmetry direction, vector ratchets allow the ratchet motion to be in any direction relative to the substrate asymmetry. It is also possible to obtain a reversal in the direction of rotation of the vector ratchet, permitting the creation of a reversible vector ratchet. We examine vector ratchets for ac drives applied parallel or perpendicular to the substrate asymmetry direction, and show that reverse ratchet motion can be produced by collective effects. No reversals occur for an isolated skyrmion on an asymmetric substrate. Since a vector ratchet can produce motion in any direction, it could represent a method for controlling skyrmion motion for spintronic applications.

We experimentally study temperature-dependent spin relaxation including lateral spin diffusion in heavily doped $n$-type silicon ($n^{+}-\mathrm{Si}$) layers by measuring nonlocal magnetoresistance in small-sized CoFe/MgO/Si lateral spin-valve (LSV) devices. Even at room temperature, we observe large spin signals, 50-fold the magnitude of those in previous works on $n^{+}-\mathrm{Si}$. By measuring spin signals in LSVs with various center-to-center distances between contacts, we reliably evaluate the temperature-dependent spin diffusion length (${\lambda }_{\mathrm{Si}}$) and spin lifetime (${\tau }_{\mathrm{Si}}$). We find that the temperature dependence of ${\tau }_{\mathrm{Si}}$ is affected by that of the diffusion constant in the $n^{+}-\mathrm{Si}$ layers, meaning that it is important to understand the temperature dependence of the channel mobility. A possible origin of the temperature dependence of ${\tau }_{\mathrm{Si}}$ is discussed in terms of the recent theories by Dery and co-workers.

We extend the model of exciton-plasmon materials to include a rovibrational structure of molecules using wave-packet propagations on electronic potential energy surfaces. Our model replaces conventional two-level emitters with more complex molecules, allowing us to examine the influence of alignment and vibrational dynamics on strong coupling with surface plasmon-polaritons. We apply the model to a hybrid system comprising a thin layer of molecules placed on top of a periodic array of slits. Rigorous simulations are performed for two types of molecular systems described by vibrational bound-bound and bound-continuum electronic transitions. Calculations reveal new features in transmission, reflection, and absorption spectra, including the observation of significantly higher values of the Rabi splitting and vibrational patterns clearly seen in the corresponding spectra. We also examine the influence of anisotropic initial conditions on optical properties of hybrid materials, demonstrating that the optical response of the system is significantly affected by an initial prealignment of the molecules. Our work demonstrates that prealigned molecules could serve as an efficient probe for the subdiffraction characterization of the near-field near metal interfaces.

The quest for high-efficiency heat-to-electricity conversion has been one of the major driving forces toward renewable energy production for the future. Efficient thermoelectric devices require high voltage generation from a temperature gradient and a large electrical conductivity while maintaining a low thermal conductivity. For a given thermal conductivity and temperature, the thermoelectric power factor is determined by the electronic structure of the material. Low dimensionality (1D and 2D) opens new routes to a high power factor due to the unique density of states (DOS) of confined electrons and holes. The 2D transition metal dichalcogenide (TMDC) semiconductors represent a new class of thermoelectric materials not only due to such confinement effects but especially due to their large effective masses and valley degeneracies. Here, we report a power factor of $\mathrm{Mo}{\mathrm{S}}_2$ as large as $8.5\mathrm{mW}{\mathrm{m}}^{-1}{\mathrm{K}}^{-2}$ at room temperature, which is among the highest measured in traditional, gapped thermoelectric materials. To obtain these high power factors, we perform thermoelectric measurements on few-layer $\mathrm{Mo}{\mathrm{S}}_2$ in the metallic regime, which allows us to access the 2D DOS near the conduction band edge and exploit the effect of 2D confinement on electron scattering rates, resulting in a large Seebeck coefficient. The demonstrated high, electronically modulated power factor in 2D TMDCs holds promise for efficient thermoelectric energy conversion.

We investigated magnetotransport properties and charge dynamics of strain-free perovskite ${\mathrm{SrIrO}}_3$. Both the longitudinal and transverse magnetoresistivity (MR) are significantly enhanced with decreasing temperature, in accord with the evolution of the Dirac semimetallic state. The electron correlation effect in the Dirac state shows up as a dramatic change in charge dynamics with temperature and as an enhanced paramagnetic susceptibility. We propose that the field-induced topological transition of the Dirac node coupled to the enhanced paramagnetism causes the unique MR of correlated Dirac electrons.

The spin-orbit interaction of two-dimensional electrons in quantum wells grown from the III-V semiconductors consists of two parts with different symmetry: the Bychkov-Rashba and the Dresselhaus terms. The last term is usually attributed to the bulk spin-orbit Hamiltonian which reflects the $T_d$ symmetry of the zincblende lattice. While it is known that the quantum well interfaces may also contribute to the Dresselhaus term, the exact structure and relative importance of the interface and bulk contributions are not well understood. To deal with this problem, we perform tight-binding calculations of the spin splittings of the electron levels in [100] GaAs/AlGaAs quantum wells. We show that the obtained spin splittings can be adequately described within the one-band electron Hamiltonian containing, together with the bulk contribution, the two interface contributions to the Dresselhaus term. The magnitude of the interface contribution to the spin-orbit interaction for sufficiently narrow quantum wells is of the same order as the bulk contribution.

We theoretically investigate coherent optical excitations of collective modes in two-band BCS superconductors, which accommodate two Higgs modes and one Leggett mode corresponding, respectively, to the amplitude and relative-phase oscillations of the superconducting order parameters associated with the two bands. We find, based on a mean-field analysis, that each collective mode can be resonantly excited through a nonlinear light-matter coupling when the doubled frequency of the driving field coincides with the frequency of the corresponding mode. Among the two Higgs modes, the higher-energy one exhibits a sharp resonance with light, while the lower-energy mode has a broadened resonance width. The Leggett mode is found to be resonantly induced by a homogeneous ac electric field because the leading nonlinear effect generates a potential offset between the two bands that couples to the relative phase of the order parameters. The resonance for the Leggett mode becomes sharper with increasing temperature. All of these light-induced collective modes along with density fluctuations contribute to the third-harmonic generation. We also predict an experimental possibility of optical detection of the Leggett mode.

We use the Bose-Hubbard model with an effective infinite-range interaction to describe the correlated lattice bosons in an optical cavity. We study both static and spectral properties of such system within the bosonic dynamical mean-field theory, which is the state-of-the-art method for strongly correlated bosonic systems. Both similarities and differences are found and discussed between our results and those obtained within different theoretical methods and experiment.

The spin-charge coupled dynamics in a thin, magnetized metallic system are investigated. The effective driving force acting on the charge carriers is generated by a dynamical magnetic texture, which can be induced, e.g., by a magnetic material in contact with a normal-metal system. We consider a general inversion-asymmetric substrate/normal-metal/magnet structure, which, by specifying the precise nature of each layer, can mimic various experimentally employed setups. Inversion symmetry breaking gives rise to an effective Rashba spin-orbit interaction. We derive general spin-charge kinetic equations which show that such spin-orbit interaction, together with anisotropic Elliott-Yafet spin relaxation, yields significant corrections to the magnetization-induced dynamics. In particular, we present a consistent treatment of the spin density and spin current contributions to the equations of motion, inter alia, identifying a term in the effective force which appears due to a spin current polarized parallel to the magnetization. This “inverse-spin-filter” contribution depends markedly on the parameter which describes the anisotropy in spin relaxation. To further highlight the physical meaning of the different contributions, the spin-pumping configuration of typical experimental setups is analyzed in detail. In the two-dimensional limit the buildup of dc voltage is dominated by the spin-galvanic (inverse Edelstein) effect. A measuring scheme that could isolate this contribution is discussed.

We theoretically study a tunneling effect in a two-dimensional Dirac semimetal with two Dirac points protected by nonsymmorphic symmetries. The tunnel barrier can be arranged by a magnetic exchange potential which opens a gap at the Dirac points which can be induced by a magnetic proximity effect of a ferromagnetic insulator. We found that the tunnel decay length increases with a decrease in the strength of the spin-orbit coupling, and moreover the dependence is attributed to the correlation of sublattice and spin degrees of freedom which lead to symmetry-protected Dirac points. The tunnel probability is quite different in two Dirac points, and thus the tunnel effect can be applied to the highly selective valley filter.

Periodically driven quantum systems can exhibit topologically nontrivial behavior, even when their quasienergy bands have zero Chern numbers. Much work has been conducted on noninteracting quantum-mechanical models where this kind of behavior is present. However, the inclusion of interactions in out-of-equilibrium quantum systems can prove to be quite challenging. On the other hand, the classical counterpart of hard-core interactions can be simulated efficiently via constrained random walks. The noninteracting model, proposed by Rudner et al. [Phys. Rev. X 3, 031005 (2013)], has a special point for which the system is equivalent to a classical random walk. We consider the classical counterpart of this model, which is exact at a special point even when hard-core interactions are present, and show how these quantitatively affect the edge currents in a strip geometry. We find that the interacting classical system is well described by a mean-field theory. Using this we simulate the dynamics of the classical system, which show that the interactions play the role of Markovian, or time-dependent disorder. By comparing the evolution of classical and quantum edge currents in small lattices, we find regimes where the classical limit considered gives good insight into the quantum problem.

We investigate the structural, electronic, transport, and thermoelectric properties of ${\mathrm{LaNiO}}_3/{\mathrm{SrTiO}}_3\left(001\right)$ superlattices containing either exclusively $n$- or $p$-type interfaces or coupled interfaces of opposite polarity by using density functional theory calculations with an on-site Coulomb repulsion term. The results show that significant octahedral tilts are induced in the ${\mathrm{SrTiO}}_3$ part of the superlattice. Moreover, the La-Sr distances and Ni-O out-of-plane bond lengths at the interfaces exhibit a distinct variation by about $7\%$ with the sign of the electrostatic doping. In contrast to the much studied ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ system, the charge mismatch at the interfaces is exclusively accommodated within the ${\mathrm{LaNiO}}_3$ layers, whereas the interface polarity leads to a band offset and to the formation of an electric field within the coupled superlattice. Features of the electronic structure indicate an orbital-selective quantization of quantum well states. The potential- and confinement-induced multiband splitting results in complex cylindrical Fermi surfaces with a tendency towards nesting that depends on the interface polarity. The analysis of the thermoelectric response reveals a particularly large positive Seebeck coefficient $(135\mu \mathrm{V}/\text{K})$ and a high figure of merit (0.35) for room-temperature cross-plane transport in the $p$-type superlattice that is attributed to the participation of the ${\mathrm{SrTiO}}_3$ valence band. Superlattices with either $n$- or $p$-type interfaces show cross-plane Seebeck coefficients of opposite sign and thus emerge as a platform to construct an oxide-based thermoelectric generator with structurally and electronically compatible $n$- and $p$-type oxide thermoelectrics.

The fractional quantum Hall effect at $\nu =2+3/8$, which has been definitively observed, is one of the last fractions for which no viable explanation has so far been demonstrated. Our detailed study suggests that it belongs to a new class of exotic states described by the Bonderson-Slingerland wave function. Its excitations are non-Abelian anyons similar to those of the well studied Pfaffian state at 5/2, but its wave function has a more complex structure. Using the effective edge theory, we make predictions for various measurable quantities that should enable a confirmation of the underlying topological order of this state.

Linear and nonlinear optical properties of low-dimensional nanostructures have attracted great interest from the scientific community as tools to probe the strong confinement of electrons and for possible applications in optoelectronic devices. In particular it has been shown that the linear optical response of carbon nanotubes [F. Wang et al., Science 308, 838 (2005)] and graphene nanoribbons [Nat. Commun. 5 4253 (2014)] is dominated by bounded electron-hole pairs, excitons. The role of excitons in linear response has been widely studied, but still, little is known about their effect on nonlinear susceptibilities. Using a recently developed methodology [Phys. Rev. B 88, 235113 (2013)] based on well-established ab initio many-body perturbation theory approaches, we find that quasiparticle shifts and excitonic effects significantly modify the third-harmonic generation in carbon nanotubes and graphene nanoribbons. For both systems the net effect of many-body effects is to reduce the intensity of the main peak in the independent-particle spectrum and redistribute the spectral weight among several excitonic resonances.