We present a first-principles computational approach to calculate thermoelectric transport coefficients via the exact solution of the linearized Boltzmann transport equation, also including the effect of nonequilibrium phonon populations induced by a temperature gradient. We use density functional theory and density functional perturbation theory for an accurate description of the electronic and vibrational properties of a system, including electron-phonon interactions; carriers' scattering rates are computed using standard perturbation theory. We exploit Wannier interpolation (both for electronic bands and electron-phonon matrix elements) for an efficient sampling of the Brillouin zone, and the solution of the Boltzmann equation is achieved via a fast and stable conjugate gradient scheme. We discuss the application of this approach to $n$-doped silicon. In particular, we discuss a number of thermoelectric properties such as the thermal and electrical conductivities of electrons, the Lorenz number and the Seebeck coefficient, including the phonon drag effect, in a range of temperatures and carrier concentrations. This approach gives results in good agreement with experimental data and provides a detailed characterization of the nature and the relative importance of the individual scattering mechanisms. Moreover, the access to the exact solution of the Boltzmann equation for a realistic system provides a direct way to assess the accuracy of different flavors of relaxation time approximation, as well as of models that are popular in the thermoelectric community to estimate transport coefficients.

We study the influence of a step defect on surface states in three-dimensional topological insulators subject to a perpendicular magnetic field. By calculating the energy spectrum of the surface states, we find that Landau levels (LLs) can form on flat regions of the surface and are distant from the step defect, and several subbands emerge at the side surface of the step defect. The subband which connects to the two zeroth LLs is spin polarized and chiral. In particular, when the electron transports along the side surface, the electron spin direction can be manipulated arbitrarily by gate voltage. Also, no reflection occurs even if the electron spin direction is changed. This provides a fascinating avenue to control the electron spin easily and coherently. In addition, regarding the subbands with a high LL index, there exist spin-momentum locking helical states and the quantum spin Hall effect can appear.

The paper reports optical orientation experiments performed in the narrow GaAs/AlGaAs quantum wells doped with Mn. We experimentally demonstrate a control over the spin polarization by means of the optical orientation via the impurity-to-band excitation and observe a sign inversion of the luminescence polarization depending on the pump power. The $g$ factor of a hole localized on the Mn acceptor in the quantum well was also found to be considerably modified from its bulk value due to the quantum confinement effect. This finding shows the importance of the local environment on magnetic properties of the dopants in semiconductor nanostructures.

We study the effects of a low concentration of adatoms or single vacancies in the linear–response transport properties of otherwise clean graphene. These impurities were treated as localized orbitals, and for each type two cases with distinct coupling symmetries were studied. For adatoms, we considered top- and hollow-site adsorbates (TOP and HS). For vacancies, we studied impurity formation by soft bond reconstruction (REC), as well as the more symmetric case of charge accumulation in unreconstructed vacancies (VAC). Our results indicate that the transport is determined by usual impurity scattering when the graphene-impurity coupling does not possess $C_{3v}$ symmetry (TOP and REC). In contrast, VAC impurities decouple from the electronic states at the Dirac points, and yield no contribution to the resistivity for a sample in charge neutrality. Furthermore, the inversion-symmetry-conserving HS impurities also decouple from entire sets of momenta throughout the Brillouin zone, and do not contribute to the resistivity within a broad range of parameters. These behaviors are protected by $C_{3v}$ and inversion symmetry, respectively, and persist for more general impurity models.

We investigate the structure and energetics of interstitial hydrogen and hydrogen molecules in layered $2H\text{−}{\mathrm{MoS}}_2$, an issue of interest both for hydrogen storage applications and for the use of ${\mathrm{MoS}}_2$ as an (opto)electronic material. Using first-principles density functional theory we find that hydrogen interstitials are deep donors. ${\mathrm{H}}_2$ molecules are electrically inactive and energetically more stable than hydrogen interstitials. Their equilibrium position is the hollow site of the ${\mathrm{MoS}}_2$ layers. The migration barrier of a hydrogen molecule is calculated to be smaller than 0.6 eV. We have also explored the insertion energies of hydrogen molecules as a function of hydrogen concentration in ${\mathrm{MoS}}_2$. For low concentrations, additional inserted ${\mathrm{H}}_2$ molecules prefer to be located in hollow sites (on top of the center of a hexagon) in the vicinity of an occupied site. Once two molecules have been inserted, the energy cost for inserting additional ${\mathrm{H}}_2$ molecules becomes much lower. Once all hollow sites are filled, the energy cost increases, but only by a modest amount. We find that up to $13{\mathrm{H}}_2$ molecules can be accommodated within the same interlayer spacing of an areal $3\times 3$ supercell.

The electronic properties and surface structures of K-doped graphene supported on Ir(111) are characterized as a function of temperature and coverage by combining low-energy electron diffraction, angle-resolved photoemission spectroscopy, and density functional theory (DFT) calculations. Deposition of K on graphene at room temperature (RT) yields a stable $\left(\surd 3\times \surd 3\right)$ R30° surface structure having an intrinsic electron doping that shifts the graphene Dirac point by $E_D=1.30\mathrm{eV}$ below the Fermi level. Keeping the graphene substrate at 80 K during deposition generates instead a $\left(2\times 2\right)$ phase, which is stable until full monolayer coverage. Further deposition of K followed by RT annealing develops a double-layer K-doped graphene that effectively doubles the K coverage and the related charge transfer, as well as maximizing the doping level $\left(E_D=1.61\mathrm{eV}\right)$. The measured electron doping and the surface reconstructions are rationalized by DFT calculations. These indicate a large thermodynamic driving force for K intercalation below the graphene layer. The electron doping and Dirac point shifts calculated for the different structures are in agreement with the experimental measurements. In particular, the ${\mathrm{K}}_{4s}$ bands are shown to be sensitive to both the K intercalation and periodicity and are therefore suggested as a fingerprint for the location and ordering of the K dopants.

We study the magnetic response of a ferromagnetic bilayer with antiferromagnetic coupling, where the layers experience magnetostrictive strains and epitaxial misfit strains. These strains cause the layers to stretch and bend as the magnetic spins of the layers rotate, resulting in elastic energy that adds to the magnetic energy of the system. The magnetic and elastic energies are computed as a function of spin direction in each layer for a given set of material and geometric parameters. By finding the rotations that minimize the total energy, we compute magnetic hysteresis loops for different combinations of magnetic and elastic parameters. The elastic contribution is reflected in the transitions at the corners of the hysteresis curves as well as in the coercive field of the main loop. The details of the elastic contribution depend in a complicated way on the magnetostriction of the layers, the epitaxial strain, the magnetic anisotropies, and the system geometry.

The discovery of superconductivity in hafnium pentatelluride $\mathrm{HfT}{\mathrm{e}}_5$ under high pressure is reported. Two structural phase transitions and metallization with superconductivity developing at around 5 GPa are observed. A maximal critical temperature of 4.8 K is attained at a pressure of 20 GPa, and superconductivity persists up to the maximum pressure of the study (42 GPa). The combination of electrical transport and crystal structure measurements as well as theoretical electronic structure calculations enables the construction of a phase diagram of $\mathrm{HfT}{\mathrm{e}}_5$ under high pressure.

An electromagnetic force density expression that is consistent with a development attributed to Einstein and Laub appears to be able to describe optical force experiments done to date with homogenized media. However, a major question that has persisted for about one century relates to the apparent discrepancy with the usual interpretation of the force description due to Lorentz in magnetized media. Specifically, it had appeared that the Einstein and Laub force density incorporated only the free-space permeability in relation to the force on the electric current density. It is shown here that the Einstein and Laub force density is consistent with the Lorentz picture in the static limit. This resolves a key impediment in establishing a unified force density description for electromagnetic waves interacting with matter.

The ballistic magnetoresistance (MR) of a domain wall constricted in a nanocontact between two $p$-type semiconducting magnetic nanowires is studied theoretically using the Landauer-Büttiker approach. Our analysis is based on coherent scattering of the carriers by the spin-dependent potential associated with the wall structure. The transmission properties of coherent states are obtained by introducing an algorithm to solve the coupled spin channels Schrödinger equation with mixed Dirichlet-Neumann boundary conditions applied far from the domain wall. Then, the local accumulated spin densities along the nanowire produced by electrical spin injection at the nanocontact are numerically calculated. It is demonstrated that the induced voltage drop due to the longitudinal spin accumulation considerably increases in the case of the narrow domain walls. Furthermore, it is shown that two spin accumulation and mistracking effects give approximately equal contributions to the wall MR ratio in the limit of the sharp domain walls. However, the MR ratio is dominantly determined by the spin accumulation effect as the domain wall width increases.

We have performed first-principles calculations of the electronic and magnetic properties of insulating double perovskite compound ${\mathrm{La}}_2{\mathrm{CuRuO}}_6$ (LCRO) which has recently been reported to exhibit intriguing magnetic properties. We derived a tight-binding Hamiltonian for LCRO based on the $N\mathrm{th}$-order muffin-tin orbital (NMTO) downfolding technique. The computed on-site energies and hopping integrals are used to estimate the dominant exchange interactions employing an extended Kugel-Khomskii model. This way the dominant exchange paths were identified and a low-energy spin model was proposed. The Green function method based on the magnetic force theorem has also been used to extract the exchange interactions to provide a more accurate estimation and to justify the model calculations. Our results show that the nearest neighbor (NN) Cu-Ru magnetic interactions are very much direction dependent and a strong antiferromagnetic next nearest neighbor Ru-Ru interaction along the crystallographic $b$ axis is responsible for the magnetic frustration observed experimentally in this system. We argue that due to the broken symmetry, NN Cu-Ru interaction becomes stronger along one direction than the other, which essentially reduces the amount of frustration and helps the system to achieve an antiferromagnetic ground state at low temperature. A detailed microscopic explanation of the exchange mechanism is discussed. We also find that spin-orbit coupling effect is significant and causes a canting of the Ru spin with respect to the Cu moments.

In this paper we consider the modeling and simulation of open quantum systems from a device engineering perspective. We derive master equations at different levels of approximation for a superconducting quantum interference device (SQUID) ring coupled to an ohmic bath. We demonstrate that the master equations we consider produce decoherences that are qualitatively and quantitatively dependent on both the level of approximation and the ring's external flux bias. We discuss the issues raised when seeking to obtain Lindbladian dissipation and show, in this case, that the external flux (which may be considered to be a control variable in some applications) is not confined to the Hamiltonian, as often assumed in quantum control, but also appears in the Lindblad terms.

We present a joint theoretical and experimental study on core-level excitations from the oxygen $K$ edge of $\beta {\text{-Ga}}_2{\text{O}}_3$. A detailed analysis of the electronic structure reveals the importance of O-Ga hybridization effects in the conduction region. The spectrum from O 1 $s$ core electrons is dominated by excitonic effects, which overall redshift the absorption onset by 0.5 eV, and significantly redistribute the intensity to lower energies. Analysis of the spectra obtained within many-body perturbation theory reveals atomic fingerprints of the inequivalent O atoms. From the comparison of energy-loss near-edge fine-structure (ELNES) spectra computed with respect to different crystal planes, with measurements recorded under the corresponding diffraction conditions, we show how the spectral contributions of specific O atoms can be enhanced while quenching others. These results suggest ELNES, combined with ab initio many-body theory, as a very powerful technique to characterize complex systems, with sensitivity to individual atomic species and to their local environment.

The currently accepted magnetic ground state of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ (the $-++-$ state) preserves inversion symmetry. This is at odds, though, with recent experiments that indicate a magnetoelectric ground state, leading to the speculation that orbital currents or more exotic magnetic multipoles might exist in this material. Here, we analyze various magnetic configurations and demonstrate that two of them, the magnetoelectric $-+-+$ state and the nonmagnetoelectric $++++$ state, can explain these recent second-harmonic generation (SHG) experiments, obviating the need to invoke orbital currents. The SHG-probed magnetic order parameter has the symmetry of a parity-breaking multipole in the $-+-+$ state and of a parity-preserving multipole in the $++++$ state. We speculate that either might have been created by the laser pump used in the experiments. An alternative is that the observed magnetic SHG signal is a surface effect. We suggest experiments that could be performed to test these various possibilities and also address the important issue of the suppression of the RXS intensity at the $L_2$ edge.

Ferromagnetic transitions between quantum Hall states with different polarization at a fixed filling factor can be studied by varying the ratio of cyclotron and Zeeman energies in tilted magnetic field experiments. However, an ability to locally control such transitions at a fixed magnetic field would open a range of attractive applications, e.g., formation of a reconfigurable network of one-dimensional helical domain walls in a two-dimensional plane. Coupled to a superconductor, such domain walls can support non-Abelian excitations. In this paper we report development of heterostructures where quantum Hall ferromagnetic (QHFm) transition can be controlled locally by electrostatic gating. A high mobility two-dimensional electron gas is formed in CdTe quantum wells with engineered placement of paramagnetic Mn impurities. A gate-induced electrostatic field shifts the electron wave function in the growth direction and changes an overlap between electrons in the quantum well and $d\text{−}\mathrm{shell}$ electrons on Mn, thus controlling the $s\text{−}d$ exchange interaction and the field of the QHFm transition. The demonstrated shift of the QHFm transition at a filling factor $\nu =2$ is large enough to allow full control of spin polarization at a fixed magnetic field.

It is well known that the magnitude of band offset (BO) at any semiconductor heterojunction is directly derivable from the distribution of charge at that interface and that the latter is decided by a minimization of total energy. However, the fact that BO formation is governed by energy minimization has not been explicitly used in theoretical BO models, likely because the equilibrium charge densities at heterojunction interfaces appear difficult to predict, except via explicit calculation. In this paper, electron densities at a large number of (100), (110), and (111) oriented heterojunctions between lattice-matched, isovalent semiconductors with the zinc blende (ZB) structure have been calculated by first-principles methods and analyzed in detail for possible common characteristics among energy-minimized densities. Remarkably, the heterojunction electron density was found to largely depend only on the immediate, local atomic arrangement. In fact, it is so much so that a juxtaposition of local electron-densities generated in oligo-cells (LEGOs) accurately reproduced the charge densities that minimize the energy for the heterojunctions. Furthermore, the charge distribution for each bulk semiconductor was found to display a striking separability of its electrostatic effect into two neutral parts, associated with the cation and the anion, which are approximately transferrable among semiconductors. These discoveries form the basis of a neutral polyhedra theory (NPT) that approximately predicts the equilibrium charge density and BO of relaxed heterojunctions from the energy minimization requirement. Well-known experimentally observed characteristics of heterojunctions, such as the insensitivity of BO to heterojunction orientation and the identity of interface bonds, the transitivity rule, etc., are all in good agreement with the NPT. Therefore, energy minimization, which essentially decides the electronic properties of all other solid and molecular systems, also governs the formation of the charge density at these heterojunction interfaces. In particular, the approach presented here eliminates the need to invoke mechanisms that are specific to semiconductor interfaces.

Using first-principles density-functional theory (DFT) calculations with/without including the spin-orbit coupling (SOC), we systematically investigate the (4/3)-monolayer structure of Pb on the Si(111) or Ge(111) surface within the two competing structural models termed the $H_3$ and $T_4$ structures. We find that the SOC influences the relative stability of the two structures in both the Pb/Si(111) and the Pb/Ge(111) systems, i.e., our DFT calculation without including the SOC predicts that the $T_4$ structure is energetically favored over the $H_3$ structure by $\mathrm{\Delta }E=25\mathrm{meV}$ for Pb/Si(111) and 22 meV for Pb/Ge(111), but the inclusion of SOC reverses their relative stability as $\mathrm{\Delta }E=-12$ and $-7\mathrm{meV}$, respectively. Our analysis shows that the SOC-induced switching of the ground state is attributed to a more asymmetric surface charge distribution in the $H_3$ structure compared to the $T_4$ structure, which is associated with the hybridization of the Pb $p_x,p_y$, and $p_z$ orbitals. This asymmetry of surface charge distribution gives rise to a relatively larger Rashba spin splitting of surface states as well as a relatively larger pseudogap opening in the $H_3$ structure. By the nudged elastic-band calculation, we obtain a sizable energy barrier from the $H_3$ to the $T_4$ structure as $\sim 0.59$ and $\sim 0.27\mathrm{eV}$ for Pb/Si(111) and Pb/Ge(111), respectively. Based on the predicted thermodynamics and kinetics of Pb/Si(111) and Pb/Ge(111), we suggest not only the coexistence of the two energetically competing structures at low temperatures, but also the order-disorder transition at high temperatures.

${\mathrm{SrTiO}}_3$ (STO) is the substrate of choice to grow oxide thin films and oxide heterojunctions, which can form quasi-two-dimensional electronic phases that exhibit a wealth of phenomena, and thus a workhorse in the emerging field of metal-oxide electronics. Hence, it is of great importance to know the exact character of the STO surface itself under various oxygen environments. Using density functional theory within the spin generalized gradient approximation we have investigated the structural, electronic, and magnetic properties of the oxygen-deficient STO surface. We find that the surface oxygen vacancies order in periodic arrays giving rise to surface magnetic moments and a quasi-two-dimensional electron gas in the occupied Ti $3d$ orbitals. The surface confinement, the oxygen-vacancy ordering, and the octahedra distortions give rise to spin-polarized ${\mathit{t}}_{2g}$ dispersive subbands; their energy split near the Brillouin zone center acts as an effective Zeeman term, which, when we turn on a Rashba interaction, produces bands with momentum-spin correlations similar to those recently discovered on oxygen-deficient STO surface.