We develop a theory for spin transported by coherent Néel dynamics through an antiferromagnetic insulator coupled to a ferromagnetic insulator on one side and a current-carrying normal metal with strong spin-orbit coupling on the other. The ferromagnet is considered within the monodomain limit and we assume its coupling to the local antiferromagnet Néel order at the ferromagnet|antiferromagnet interface through exchange coupling. Coupling between the charge current and the local Néel order at the other interface is described using spin Hall phenomenology. Spin transport through the antiferromagnet, assumed to possess an easy-axis magnetic anisotropy, is solved within the adiabatic approximation and the effect of spin current flowing into the ferromagnet on its resonance linewidth is evaluated. Onsager reciprocity is used to evaluate the inverse spin Hall voltage generated across the metal by a dynamic ferromagnet as a function the antiferromagnet thickness.

We derive the thermomagnonic torque associated with smooth magnetic textures subjected to a temperature gradient in the framework of the stochastic Landau-Lifshitz-Gilbert equation. Our approach captures on equal footing two distinct contributions: (i) a local entropic torque that is caused by a temperature dependence of the effective exchange field, the existence of which had been previously suggested based on numerics, and (ii) the well-known spin-transfer torque induced by thermally induced magnon flow. The dissipative components of two torques have the same structure, following a common phenomenology, but opposite signs, with the twice as large entropic torque leading to a domain-wall motion toward the hotter region. We compare the efficiency of the torque-driven domain-wall motion with the recently proposed Brownian thermophoresis.

We devise a quantum register based on superconducting flux qubits that circumvents the impediments posed by the presence of fixed interactions. We describe a coupling scheme wherein two physical qubits are coupled to a third which acts as a coupler via their longitudinal degree of freedom (i.e., ${\sigma }_z$). This approach provides a solution to several issues such as residual interactions between physical qubits, deteriorations of the rotating wave approximation (RWA), and correlated errors, thereby expanding the opportunities for capitalizing on the large coupling strengths achievable with these systems.

Spinel-type vanadium oxides ${\mathrm{FeV}}_2{\mathrm{O}}_4$ and ${\mathrm{MnV}}_2{\mathrm{O}}_4$ are considered to exhibit different types of ${\mathrm{V}}^{3+}$ orbital order from each other. In order to tackle this issue, the temperature dependence of x-ray absorption spectrum (XAS) and x-ray magnetic circular dichroism (XMCD) spectrum are investigated in both spinel compounds. The application of the sum rule to the obtained V edge XMCD spectra and the comparison with crystal-field multiplet calculations suggest a real orbital ordered state with quenched orbital angular momenta in both ${\mathrm{FeV}}_2{\mathrm{O}}_4$ and ${\mathrm{MnV}}_2{\mathrm{O}}_4$. A dissimilarity in XAS spectra between these compounds would originate from a quantitative difference of trigonal distortion.

Using quasiparticle self-consistent $GW$ calculations, we show that DyN has an unusual nearly zero indirect gap semimetallic band structure in which the states near the valence band maximum are fully minority spin polarized at $\mathrm{\Gamma }$ while the states near the conduction band minimum (at $X$) have fully majority spin character. This arises due to a strong hybridization of one of the minority spin $f$ states of dysprosium with the N-$2p$ bands. The reason why only one of the $f$ bands hybridizes is explained using symmetry arguments. We show that in HoN, this hybridization is already strongly reduced because of the deeper Ho-$4f$ minority spin states.

We investigate the electronic structure of the monolayer black phosphorus (BP) using density-functional methods both with and without an applied electric field. We find that a simple one-band tight-binding Hamiltonian based on the $p_z$ orbitals and nearest-neighbor hopping is sufficient to describe the band structure in the gap region rather well and justification for this is given from symmetry arguments. The anisotropic nature of the band structure leads in turn to an anisotropic Rashba effect, where the magnitude of the spin splitting caused by an applied electric field is not only momentum dependent, but also depends on the direction of $\vec{k}$. The Rashba Hamiltonian is generalized for the anisotropic case, which reads: $H_R={\alpha }_R(\vec{\sigma }\times {\vec{k}}^{\prime })·\widehat{z}$, where the scaled momentum ${\vec{k}}^{\prime }$ contains the anisotropy effect. The Rashba effect is studied quantitatively for BP from ab initio density-functional calculations in the presence of an applied electric field. A byproduct of this work is the demonstration that the strength of the spin-orbit coupling for the outermost electrons in the atoms, which are relevant for the solids, increases only as the Landau-Lifshitz $Z^2$ scaling with the atomic number $Z$, rather than the higher power $Z^4$ scaling, as sometimes thought.

We propose a $\mathbb{U}\left(1\right)\times {\mathbb{Z}}_2$ effective gauge theory for vortices in a $p_x+i{p}_y$ superfluid in two dimensions. The combined gauge transformation binds $\mathbb{U}\left(1\right)$ and ${\mathbb{Z}}_2$ defects so that the total transformation remains single-valued and manifestly preserves the particle-hole symmetry of the action. The ${\mathbb{Z}}_2$ gauge field introduces a complete Chern-Simons term in addition to a partial one associated with the $\mathbb{U}\left(1\right)$ gauge field. The theory reproduces the known physics of vortex dynamics such as a Magnus force proportional to the superfluid density. More importantly, it predicts a universal Abelian phase, $exp(i\pi /8)$, upon the exchange of two vortices. This phase is modified by nonuniversal corrections due to the partial Chern-Simon term, which are nevertheless screened in a charged superfluid at distances that are larger than the penetration depth.

We construct and discuss the field theory for tensorial nematic order parameter coupled to gapless four-component fermions at the quadratic band touching point in three (spatial) dimensions. Within a properly formulated epsilon-expansion this theory is found to have a quantum critical point, which describes the (presumably continuous) transition from the semimetal into a (nematic) Mott insulator. The latter phase breaks the rotational, but not the time-reversal, symmetry and may be relevant to materials such as gray tin or mercury telluride at low temperatures. The critical point represents a simple quantum analog of the familiar classical isotropic-to-nematic transition in liquid crystals. The properties and the consequences of this quantum critical point are discussed. Its existence supports the scenario of the “fixed-point collision,” according to which three-dimensional Fermi systems with quadratic band touching and long-range Coulomb interactions are unstable towards the gapped nematic ground state at low temperatures.

We study the dynamics of a quantum critical boson coupled to a Fermi surface in intermediate energy regimes where the Landau damping of the boson can be parametrically controlled, either via large Fermi velocity or by large-$N$ techniques. We develop a systematic approach to the BCS instability of such systems, including careful treatment of the enhanced ${\text{log}}^2$ and ${\text{log}}^3$ singularities which appear already at 1-loop. These singularities arise due to the exchange of a critical boson in the Cooper channel and are absent in Fermi liquid theory. We also treat possible instabilities to charge density wave (CDW) formation, and compare the scales ${\mathrm{\Lambda }}_{\mathrm{BCS}}$ and ${\mathrm{\Lambda }}_{\mathrm{CDW}}$ of the onset of the instabilities in different parametric regimes. We address the question of whether the dressing of the fermions into a non-Fermi liquid via interactions with the order parameter field can happen at energies $>{\mathrm{\Lambda }}_{\mathrm{BCS}},{\mathrm{\Lambda }}_{\mathrm{CDW}}$.

We present ab initio calculations of the electronic structure and the optical properties of wurtzite Si (Si-IV). We find an indirect band gap of 0.95 eV $\left({\mathrm{\Gamma }}_5\to M_1\right)$ and an optically forbidden direct gap of 1.63 eV $\left({\mathrm{\Gamma }}_5\to {\mathrm{\Gamma }}_{10}\right)$, which is due to a backfolding of the $L_1$ state of Si in the diamond structure (Si-I). Optical absorption spectra including excitonic and local-field effects are calculated. Further, the effects of hydrostatic pressure, uniaxial strain, and biaxial strain on the absorption properties are investigated. Biaxial tensile strains enhance the optical absorption of Si-IV in the spectral range which is relevant for photovoltaic applications. High biaxial tensile strains $\left(>4\%\right)$ even transform Si-IV into a direct semiconductor.

We employ hybrid density functional calculations to search for defects in different polytypes of SiC that may serve as qubits for quantum computing. We explore the divacancy in $4H\text{−}$ and $3C\text{−}\mathrm{SiC}$, consisting of a carbon vacancy adjacent to a silicon vacancy, and the nitrogen-vacancy (NV) center in $3C\text{−}\mathrm{SiC}$, in which the substitutional ${\mathrm{N}}_{\mathrm{C}}$ sits next to a Si vacancy (${\mathrm{N}}_{\mathrm{C}}-V_{\mathrm{Si}}$). The calculated excitation and emission energies of the divacancy in $4H\text{−}\mathrm{SiC}$ are in excellent agreement with experimental data, and aid in identifying the four unique configurations of the divacancy with the four distinct zero-phonon lines observed experimentally. For $3C\text{−}\mathrm{SiC}$, we identify the paramagnetic defect that was recently shown to maintain a coherent quantum state up to room temperature as the spin-1 neutral divacancy. Finally, we show that the $({\mathrm{N}}_{\mathrm{C}}-V_{\mathrm{Si}}{)}^{-}$ center in $3C\text{−}\mathrm{SiC}$ is highly promising for quantum information science, and we provide guidance for identifying this defect.

Linear optical properties can be accurately calculated using the Bethe-Salpeter equation. After introducing a suitable product basis for the electron-hole pairs, the Bethe-Salpeter equation is usually recast into a complex non-Hermitian eigenvalue problem that is difficult to solve using standard eigenvalue solvers. In solid-state physics, it is therefore common practice to neglect the problematic coupling between the positive- and negative-frequency branches, reducing the problem to a Hermitian eigenvalue problem [Tamm-Dancoff approximation (TDA)]. We use time-inversion symmetry to recast the full problem into a quadratic Hermitian eigenvalue problem, which can be solved routinely using standard eigenvalue solvers even at a finite wave vector $\mathbf{q}$. This allows us to access the importance of the coupling between the positive- and negative-frequency branch for prototypical solids. As a starting point for the Bethe-Salpeter calculations, we use self-consistent Green's-function methods ($\mathit{GW}$), making the present scheme entirely ab initio. We calculate the optical spectra of carbon (C), silicon (Si), lithium fluoride (LiF), and the cyclic dimer ${\mathrm{Li}}_2{\mathrm{F}}_2$ and discuss why the differences between the TDA and the full solution are tiny. However, at finite momentum transfer $\mathbf{q}$, significant differences between the TDA and our exact treatment are found. The origin of these differences is explained.

We study the electronic band structure and optical conductivity of a tunnel-coupled array of graphene nanoribbons. We show that due to the coupling of electronic edge states for the zigzag nanoribbon structure, the Fermi surface can become a hyperbola similarly to the case of the layered metal-dielectric structures, where the hyperbolic isofrequency contours originate from the coupling of localized surface plasmon polaritons. Moreover, we show that for both types of the ribbon edge, the optical response of the structure can be characterized by a uniaxial conductivity tensor, having principal components of the different signs. Therefore, the tunnel-coupled nanoribbon array can be regarded as a tunable hyperbolic metasurface.

Propagation of an electron wave packet through a quantum point contact (QPC) defined by electrostatic gates in bilayer graphene is investigated. The gates provide a bias between the layers, in order to produce an energy gap. If the gates on both sides of the contact produce the same bias, steps in the electron transmission probability are observed, as in the usual QPC. However, if the bias is inverted on one of the sides of the QPC, only electrons belonging to one of the Dirac valleys are allowed to pass, which provides a very efficient valley filtering.

Studies of the spin-Seebeck effect (SSE) are very important for the development of fundamental science and novel low-power-consumption technologies. The spin-Seebeck diode (SSD), in which the spin current can be driven by a forward temperature gradient but not by a reverse temperature gradient, is a key unit in spin caloritronic devices. Here, we propose a SSD design using two-dimensional (2D) materials such as silicene and phosphorene nanoribbons as the source and drain. Due to their unique band structures and magnetic states, thermally driven spin-up and spin-down currents flow in opposite directions. This mechanism is different from that of the previous one, which uses two permalloy circular disks [Phys. Rev. Lett. 112, 047203 (2014)], and the SSD in our design can be easily integrated with gate voltage control. Since the concept of this design is rather general and applicable to many 2D materials, it is promising for the realization and exploitation of SSDs in nanodevices.