The physical properties and chemical trends of defects in GaAs under the hyperdoping situation are investigated and found to be very different from those in the impurity limit. We show that at high dopant concentrations, a defect complex denoted as the $DDX$ center becomes the dominant “killer” to limit the electron carrier concentration, whereas in the impurity limit, the electron carrier concentration is usually limited by the well-known $DX$ center. The $DDX$ center shows some opposite chemical trends compared to the $DX$ center. For example, to avoid the $DX$ center, anion site donors are preferred, but to avoid the $DDX$ center, cation site donors are better. Our proposed mechanism is able to explain some puzzling experimental observations.

The charge transport in organic semiconductors is still not completely understood. We use electroreflectance spectroscopy to investigate charge transport in organic field effect devices in order to obtain optical information on the charges within the charge accumulation layer at the organic semiconductor-insulator interface. Here, the reflectance geometry allows an analysis of devices prepared on opaque substrates. The recorded spectra were analyzed based on optical layer stack simulations to extract quantitatively the change of the dielectric function due to the charge injection of the accumulation layer in poly(3-hexylthiophene) (P3HT). For the simulation the anisotropic optical response of spin-coated P3HT layers is experimentally determined by spectroscopic ellipsometry. Using the developed theoretical approach the characteristic change of three different preparations of the organic semiconductor-insulator interface is analyzed. Laterally resolved measurements close to an injecting contact have revealed characteristic spectral changes in dependence of the preparation conditions, and significant spectral changes within the first $90\mu \mathrm{m}$ from the contact were found, which were attributed to an energetic relaxation of charges during the charge transport process.

We use angle-resolved photoemission spectroscopy to study the influence of mechanically induced uniaxial strain on the electronic structure of the oxide semiconductor SrTiO${}_3$. We observe an orbital splitting between the Ti $3d_{yz}$ and $3d_{xy}$ bands, which are degenerate when unperturbed. Using the k$·$p method, we qualitatively explain the direction and the size of the observed energy splitting. Our comprehensive understanding of band splitting explains the strain induced mobility enhancement of electron-doped SrTiO3${}_3$ in terms of band degeneracy breaking and reduced interband scattering. Our approach can be extended to differently strained oxide systems.

Doping behavior in quantum dots (QDs) differs from that in the bulk. Despite many efforts, the doping properties are still not fully understood. Using first-principles methods, we have calculated the formation energies of various group-III acceptors and group-V donors doping at all nonequivalent sites in a Si QD (Si${}_{147}$H${}_{100}$). To analyze the trend of the formation energy, we decompose it into two terms: the unrelaxed formation energy (chemical energy) and the relaxation energy. We find that the unrelaxed formation energy generally increases as the dopant moves from the center of the QD to the surface. The variation of the unrelaxed formation energy in the surface region is explained by the variation of the local potential of the QD and the size effect. The relaxation energy gain increases as the size mismatch between the dopant and Si atom increases. Generally, the relaxation effect becomes more significant as the dopant moves toward the surface of the QD. The trend of the formation energy is determined by the two terms discussed above. If the size mismatch between the dopant and the Si atom is small, the trend of the formation energy generally follows that of the unrelaxed formation energy, increasing as the dopant moves from the center to the surface; thus, these dopants have a better chance of staying in the core region. On the other hand, if the size mismatch is large, the relaxation effect dominates and the formation energy decreases, which indicates these dopants cannot enter the core region under equilibrium growth conditions.

A systematic study of ensembles of nickel nanoparticles fabricated by Ni${}^{+}$-ion implantation at a dose of $(0.5–1.0)\times {10}^{17}$ ions/cm${}^2$ in a thin near-surface layer of an amorphous SiO${}_2$ matrix by means of transmission electron microscopy (TEM), dc magnetic measurements, and magneto-optical technique is presented. TEM characterization of Ni nanoparticles proves the formation of isolated spherical nickel nanoparticles with diameters from 2 to 16 nm. The crystal structure and lattice constant of the nanoparticles correspond to face-centered-cubic Ni. The larger size nanoparticles are shown to have core-shell structure, which is unusual for the implantation conditions used. The shell of these nanoparticles consists of Ni, while the core has supposedly the composition coinciding with the matrix, i.e., SiO${}_2$. The core-shell nanoparticles in the investigated sample coexist with ordinary pure Ni nanoparticles, which strongly affects the magnetic and especially magneto-optic properties of the samples. For all three doses, the nanoparticles are in the superparamagnetic state at room temperature passing to the “frozen” state at lower temperatures. However, only the sample implanted with the lowest dose demonstrates the classic superparamagnetic behavior according to the shape of the experimental magnetization temperature dependencies for the zero-field-cooled (ZFC) and field-cooled regimes. This shape deviation from that characteristic of the pure superparamagnetic ensembles is ascribed mainly to the particle core-shell structure. The Ni nanoparticles’ anisotropy constant estimated with the help of ZFC curves appears to exceed the bulk Ni anisotropy second constant approximately by two orders of magnitude. Magnetic circular dichroism (MCD) is characterized by spectral dependence modified strongly as compared to the MCD spectra of a continuous Ni film. In the spectral range $1.1–4.2$ eV, the MCD spectrum consists of two broad maxima of opposite sign with the characteristics depending on the implantation dose and the measurement temperature. The MCD spectra analysis allows one to show that the higher-energy maximum (at $3.34–3.48$ eV depending on the dose) is related to the surface plasmon resonance (SPR) excitation in pure Ni nanoparticles, while the lower-energy maximum (at $2.19–2.73$ eV depending on the dose) should be associated with the SPR excitation in core-shell nanoparticles.

We measure the current vs voltage ($I$-$V$) characteristics of a diodelike tunnel junction consisting of a sharp metallic tip placed at a variable distance $d$ from a planar collector and emitting electrons via electric-field assisted emission. All curves collapse onto one single graph when $I$ is plotted as a function of the single scaling variable $V{d}^{-\lambda }$, $d$ being varied from a few mm to a few nm, i.e., by about six orders of magnitude. We provide an argument that finds the exponent $\lambda$ within the singular behavior inherent to the electrostatics of a sharp tip. A simulation of the tunneling barrier for a realistic tip reproduces both the scaling behavior and the small but significant deviations from scaling observed experimentally.

We have performed a detailed study of the evolution of the charge-density wave (CDW) and spin-density wave (SDW) on the surface of nanoscale Cr islands grown on W(110). Using low-temperature scanning tunneling microscopy (STM), we find a striking nonmonotonic thickness dependence of the CDW wavelength at 50 K. As the local island thickness $\Theta$ decreases from about 50 to 5.2 nm, the CDW wavelength gradually increases from the bulk value by approximately 30$\%$. We find a gap without any CDW at coverages ${\Theta }_{\mathrm{gap}}$ between 5.2 and 3.7 nm. Spin-resolved STM data reveal that within this CDW gap the SDW modulation also disappears on the Cr island surface. At $\Theta <3.7$ nm the CDW reappears. This unusual behavior of CDW and SDW in Cr nanoislands can be understood by a reorientation of the SDW wave vector Q which is potentially driven by the pinning of the SDW node at the island surface at ${\Theta }_{\mathrm{gap}}$.

We study the effects of trigonal warping and electric field on the Zitterbewegung of low-energy electrons in bilayer graphene. A time-dependent average velocity of electrons is calculated by using Gaussian wave packets. The Zitterbewegung of electric current shows a transient behavior. The anisotropy of the eigenspectrum due to the trigonal warping distortion breaks the valley symmetry to induce valley-polarized Zitterbewegungs of electric current. An external electric field, applied perpendicular to the layer, opens a band gap and makes the group velocities of electrons at conduction and valence bands slower. Using this feature, we demonstrate that the decay time of the transient Zitterbewegung can be made longer by the electric field. Combining these effects, it is expected that valley-polarized Zitterbewegungs of electric current with longer decay time, which eventually become two valley-polarized constant currents at large times, can exist in a biased bilayer graphene with trigonal warping.

Thermoelectric properties of the system La${}_2$NiO${}_{4+\delta }$ have been recently discussed [V. Pardo, A. S. Botana, and D. Baldomir, Phys. Rev. B 86, 165114 (2012)] via ab initio calculations. An optimum hole-doping value was obtained with reasonable thermopower and thermoelectric figure of merit being calculated. Here, a large increase in the thermoelectric performance through lattice strain and the corresponding atomic relaxations is predicted. This increase would be experimentally attainable via growth in thin films of the material on top of different substrates. A small tensile strain would produce large thermoelectric figures of merit at high temperatures, $zT$ $\sim$ 1 in the range of oxygen excess $\delta \sim 0.05$–0.10 and in-plane lattice parameter in the range 3.95–4.05 Å. In that relatively wide range of parameters, thermopower values close to 200 $\mu$V/K are obtained. The best performance of this compound is expected to occur in the high-temperature limit.

We describe a recent implementation of the combined $GW$ and dynamical mean field method ($GW$+DMFT) for the two-dimensional Hubbard model with onsite and nearest-neighbor repulsion. We clarify the relation of the $GW$+DMFT scheme to alternative approaches in the literature, and discuss the corresponding approximations to the free-energy functional of the model. Furthermore, we describe a numerically exact technique for the solution of the $GW$+DMFT equations, namely, the hybridization expansion continuous-time algorithm for impurity models with retarded interactions. We compute the low-temperature phase diagram of the half-filled extended Hubbard model, addressing the metal-insulator transition at small intersite interactions and the transition to a charge-ordered state for stronger intersite repulsions. $GW$+DMFT introduces a nontrivial momentum dependence into the many-body self-energy and polarization. We find that the charge fluctuations included in the present approach have a larger impact on the latter than on the former. Finally, within the $GW$+DMFT framework, as in extended DMFT, the intersite repulsion translates into a frequency dependence of the local effective interaction. We analyze this dependence and show how it affects the local spectral function.

The rare-earth nickelates ($R$NiO${}_3$) exhibit interesting phenomena such as unusual antiferromagnetic order at wave vector q $=$ (½, 0, ½) and a tunable insulator-metal transition that are subjects of active research. Here we present temperature-dependent transport measurements of the resistivity, magnetoresistance, Seebeck coefficient, and Hall coefficient ($R_{\mathrm{H}}$) of epitaxial SmNiO${}_3$ thin films with varying oxygen stoichiometry. We find that from room temperature through the high temperature insulator-metal transition, the Hall coefficient is holelike and the Seebeck coefficient is electronlike. At low temperature the Néel transition induces a crossover in the sign of $R_{\mathrm{H}}$ to electronlike, similar to the effects of spin density wave formation in metallic systems but here arising in an insulating phase ∼200 K below the insulator-metal transition. We propose that antiferromagnetism can be stabilized by band structure even in insulating phases of correlated oxides, such as $R$NiO${}_3$, that fall between the limits of strong and weak electron correlation.

We report polarization of a dense nuclear-spin ensemble in diamond and its dependence on magnetic field and temperature. The polarization method is based on the transfer of electron spin polarization of negatively charged nitrogen vacancy (NV) color centers to the nuclear spins via the excited-state level anticrossing of the center. We polarize 90$\%$ of the ${}^{14}$N nuclear spins within the NV centers, and 70$\%$ of the proximal ${}^{13}$C nuclear spins with hyperfine interaction strength of 13–14 MHz. Magnetic-field dependence of the polarization reveals a sharp decrease in polarization at specific field values corresponding to cross relaxation with substitutional nitrogen centers, while temperature dependence of the polarization reveals that high polarization persists down to 50 K. This work enables polarization of the ${}^{13}$C in bulk diamond, which is of interest in applications of nuclear magnetic resonance, in quantum memories of hybrid quantum devices, and in sensing.

The specific topology of the line-centered square lattice (known also as the Lieb lattice) induces remarkable spectral properties such as the macroscopically degenerated zero-energy flat band, the Dirac cone in the low-energy spectrum, and the peculiar Hofstadter-type spectrum in a magnetic field. We study here the properties of the finite Lieb lattice with periodic and vanishing boundary conditions. We investigate the behavior of the flat band induced by disorder and external magnetic and electric fields. We show that in the confined Lieb plaquette threaded by a perpendicular magnetic flux there are edge states with nontrivial behavior. The specific class of twisted edge states, which have alternating chirality, are sensitive to disorder and do not support integer quantum Hall effect (IQHE), but contribute to the longitudinal resistance. The symmetry of the transmittance matrix in the energy range where these states are located is revealed. The diamagnetic moments of the bulk and edge states in the Dirac-Landau domain, and also of the flat states in crossed magnetic and electric fields are shown.

We investigate adiabatic and nonadiabatic emission of single particles into an edge state using an analytically solvable dynamical scattering matrix model of an on-demand source. We compare adiabatic and nonadiabatic emissions by considering two geometries: a collider geometry where two emitters are coupled to two different edge states and a series geometry where two emitters are coupled to the same edge state. Most effects observed for adiabatic emitters also occur for nonadiabatic emitters. In particular this applies to effects arising due to the overlap of wave packets colliding at a quantum point contact. Specifically we compare the Pauli peak (the fermionic analog of the bosonic Hong-Ou-Mandel dip) for the adiabatic and nonadiabatic collider and find them to be similar. In contrast we find a striking difference between the two operating conditions in the series geometry in which particles are emitted into the same edge state. Whereas the squared average charge current can be nullified for both operating conditions, the heat current can be made to vanish only with adiabatic emitters.

We present a stress model, “the Friedel stress” model, based on the second-moment tight-binding approach, as a quantitative model of in-plane surface stress on transition metal surfaces. By comparing the results obtained with the stress model and ab initio local stress calculation on fcc (111) surfaces, we demonstrate that the in-plane surface stress originates from a surface $d$-band width change due to the reduced coordination number. This indicates that surface stress can occur without the direct effects of charge redistribution, which is widely accepted as the origin of surface stress. The variation in in-plane stress induced by the strain parallel to the surface is also reproduced correctly by the Friedel stress, which further proves the dependence of surface stress on the surface $d$-band width. We demonstrate that surface stress has a strong correlation with the surface chemical reactivity mediated by the surface $d$-band behavior.

Temperature-dependent stress-strain behavior of various soft Pb${}_{0.98}$Ba${}_{0.01}$(Zr${}_{1-x}$Ti${}_x$)${}_{0.98}$Nb${}_{0.02}$O${}_3$, $0.40\le x\le 0.60$, compositions was characterized between 25 and 400 °C to determine the influence of crystal structure on ferroelasticity. The ferroelastic response was found to depend significantly on the crystal phase as well as the spontaneous strain, both of which varied with temperature. The remanent strain for rhombohedral materials was shown to be above the theoretical maximum allowed by the crystal spontaneous strain. This observed behavior indicates the presence of hysteretic processes in addition to ferroelasticity during mechanical compression. A phenomenological free energy analysis was used to predict the effects of stress on the stable phase in ferroelectrics and indicates the susceptibility of the rhombohedral and tetragonal structure to a stress-induced phase transition. Modeling results indicate the relative importance of such phase transitions on macroscopic stress-strain behavior, giving an indirect method to observe field-induced phase transitions in polycrystalline ferroelectrics.

We study the Anderson localization in systems, in which transport channels with rather different properties are coupled together. This problem arises naturally in systems of hybrid particles, such as exciton-polaritons, where it is not obvious which transport channel dominates the coupled system. Here we address the question of whether the coupling between a strongly and a weakly disordered channel will result in localized (insulating) or delocalized (metallic) behavior. Complementing an earlier study in 1D [H. Y. Xie, V. E. Kravtsov, and M. Müller, Phys. Rev. B 86, 014205 (2012)], the problem is solved here on a bilayer Bethe lattice with parametrically different parameters. The comparison with the analytical solution in 1D shows that dimensionality plays a crucial role. In $D=1$ localization is in general dominated by the dirtier channel, which sets the backscattering rate. In contrast, on the Bethe lattice a delocalized channel remains almost always delocalized, even when coupled to strongly localized channels. We conjecture that this phenomenology holds true for finite dimensions $D>2$ as well. Possible implications for interacting many-body systems are discussed.

A direct three-dimensional minimization of the standard energy functional shows that in thin films of cubic helimagnets, chiral skyrmions are modulated along three spatial directions. The structure of such three-dimensional skyrmions can be thought of as a superposition of conical modulations along the skyrmion axis and double-twist rotation in the perpendicular plane. We show that chiral modulations across the layer thickness radically change the skyrmion energetics and provide a thermodynamical stability of a skyrmion lattice in a broad range of applied magnetic fields. These results disclose a basic physical mechanism underlying the formation of skyrmion states recently observed in nanolayers of cubic helimagnets.

The structural phase transition has been observed for the first time in the Rb${}_2$NaYF${}_6$ crystal and studied by EPR and optical spectroscopy. EPR spectra of Dy${}^{3+}$ and Yb${}^{3+}$ ions present as unintentional dopants in the nominally undoped crystal and forming tetragonal paramagnetic centers have been identified. A characteristic splitting of some optical lines has been observed in the temperature dependence of the Yb${}^{3+}$ optical spectra. It indicates the splitting of the cubic quartet energy levels of Yb${}^{3+}$ ions by the tetragonal crystal field. The empirical schemes of the energy levels for cubic and tetragonal paramagnetic centers of Yb${}^{3+}$ ions have been established and parameters of the corresponding crystal fields have been determined. The latter have been used for analyzing the crystal lattice distortions occurring in the vicinity of the Yb${}^{3+}$ ion during the phase transition. It has been established using the superposition model that the nearest octahedral environment of the Yb${}^{3+}$ ion is distorted as follows: the fluorine ions are rotated by the angle of 2.1° around the fourfold axis; the F${}^{-}$ ions located symmetrically in the plane perpendicular to the rotation axis approach the dopant by 0.0014 nm, whereas the F${}^{-}$ ions located on the rotation axis move away by 0.0028 nm. It has been concluded that the studied phase transition includes the critical rotations of the octahedral F groups and noncritical displacement of atoms in the rotated fluorine octahedra.