In condensed matter systems with the Coulomb interaction playing an important role one expects, besides the on-site (local) Hubbard-type interaction, that also other (non-local) terms depending on the site occupancy, known as correlated or assisted hopping, exist. Even though such terms in quantum dots tunnel coupled to external electrodes may have quite an appreciable amplitude, the interpretation of experiments on these systems-usually in the linear response regime-seems not to require their presence. However, since the correlated-hopping term breaks the particle-hole symmetry of the standard Anderson model and modifies all transport characteristics of the system, the detailed knowledge of its influence on measurable characteristics, especially in the non-linear regime, is a prerequisite for its experimental detection. In this paper, the non-linear transport properties of junctions composed of a quantum dot tunnel coupled to external electrodes are studied. The system is modeled by the single-impurity Anderson Hamiltonian with Hubbard on-site interaction and with a non-local correlated-hopping interaction. Using the previously found general expression for the spin-dependent transport and spectral Green functions as well as general formulae for charge and heat transport, relevant transport characteristics are calculated in the strongly non-linear regime.

Ordinary superconductors are widely assumed insensitive to small concentrations of random nonmagnetic impurities, whereas strong disorder suppresses superconductivity, ultimately leading to a superconductor-insulator transition. In between these limiting cases, a most fascinating regime may emerge where disorder enhances superconductivity. This effect is discussed here for the $\beta$-phase of vanadium-titanium alloys. Disorder is modeled using the coherent potential approximation while local electronic interactions are treated using dynamical mean-field theory. The McMillan formula is employed to estimate the superconducting transition temperature, showing a maximum at a Ti concentration of around $0.33$ for a local Coulomb interaction $U$ in the range of $2$ to $3$ eV. Our calculations quantitatively agree with the experimentally observed concentration dependent increase of $T_c$, and its maximal value of about $20\%$.

The electronic and magnetic properties of (LaCrO3)m/SrCrO3 superlattices are investigated using first principles calculations. We show that the magnetic moments in the two CrO2 layers sandwiching the SrO layer compensate each other for even m but give rise to a finite magnetization for odd m, which is explained by charge ordering with Cr3+ and Cr4+ ions arranged in a checkerboard pattern. The Cr4+ ions induce in-gap hole states at the interface, implying that the transparent superlattices are p-type semiconductors. The availability of transparent p-type semiconductors with finite magnetization enables the fabrication of transparent magnetic diodes and transistors, for example, with a multitude of potential technological applications.

The transmission through a magnetic layer of correlated electrons sandwiched between non-interacting normal-metal leads is studied within model calculations. We consider the linear regime in the framework of the Meir-Wingreen formalism, according to which the transmission can be interpreted as the overlap of the spectral function of the surface layer of the leads with that of the central region. By analyzing these spectral functions, we show that a change of the coupling parameter between the leads and the central region significantly and non-trivially affects the conductance. The role of band structure effects for the transmission is clarified. For a strong coupling between the leads and the central layer, high-intensity localized states are formed outside the overlapping bands, while for weaker coupling this high-intensity spectral weight is formed within the leads' continuum band around the Fermi energy. A local Coulomb interaction in the central region modifies the high-intensity states, and hence the transmission. For the present setup, the major effect of the local interaction consists in shifts of the band structure, since any sharp features are weakened due to the macroscopic extension of the configuration in the directions perpendicular to the transport direction.

application of resonant X-ray and neutron re fl ectometry for the study of perovskite transition metal oxide heterostructures Both for the nondestructive study of interfacial reconstructions and interactions of spin, and orbitals at the nanoscale. into physics of oxide heterostructures.

The transport properties of junctions composed of a central region tunnel-coupled to external electrodes are frequently studied within the single-impurity Anderson model with Hubbard on-site interaction. In the present work, we supplement the model with an important ingredient, namely, the charge-bond interaction, also known as correlated or assisted hopping. Correlated hopping enters the second-quantized Hamiltonian, written in the Wannier representation, as an off-diagonal many-body term. Using the equation of motion technique, we study the effect of the correlated hopping on the spectral and transport characteristics of a two-terminal quantum dot. Two different Green functions (GFs) appear: one of them describes the spectral properties of the quantum dot, the other the transport properties of the system. The calculation of the transport GF requires the knowledge of the spectral one. We use decoupling procedures similar to those which properly describe the standard Anderson model within the Kondo regime and outside of it. For an arbitrary ratio x between the amplitudes of correlated and single-particle hopping terms, the transport GF fulfils the x <-> 2 - x symmetry of the model. The average occupation of the dot also obeys this symmetry, albeit the spectral function of the quantum dot, calculated within an analogous decoupling scheme as for the transport GF, does not. We identify the physical reason for this behavior and propose a way to cure it. Since the correlated-hopping term breaks the particle-hole symmetry of the model and modifies all transport characteristics of the system, the detailed knowledge of its influence on measurable characteristics is a prerequisite for its experimental detection. Simple, experimentally feasible methods are proposed.

The structural, electronic, and optical properties of metal (Si, Ge, Sn, and Pb) mono‐ and co‐doped anatase TiO 2 nanotubes are investigated, to elucidate their potential for photocatalytic applications. It is found that Si‐doped TiO 2 nanotubes are more stable than those doped with Ge, Sn, or Pb. All dopants lower the bandgap, except the (Ge, Sn) co‐doped structure, the decrease depending on the concentration and the type of dopant. Correspondingly, a redshift in the optical properties for all kinds of dopings is obtained. Even though a Pb mono‐ and co‐doped TiO 2 nanotube has the lowest bandgap, these systems are not suitable for water splitting, due to the location of the conduction band edges, in contrast to Si, Ge, and Sn mono‐doped TiO 2 nanotubes. On the other hand, co‐doping of TiO 2 does not improve its photocatalytic properties. The findings are consistent with recent experiments, which show an enhancement of light absorption for Si‐ and Sn‐doped TiO 2 nanotubes.

In this work, we investigate models for bulk, bi-, and multilayers containing half-metallic ferromagnets (HMFs), at zero and at finite temperature, in order to elucidate the effects of strong electronic correlations on the spectral properties (density of states). Our focus is on the evolution of the finite-temperature many-body induced tails in the half-metallic gap. To this end, the dynamical mean-field theory (DMFT) is employed. For the bulk, a Bethe lattice model is solved using a matrix product states based impurity solver at zero temperature and a continuous-time quantum Monte Carlo (CT-QMC) solver at finite temperature. We demonstrate numerically, in agreement with the analytical result, that the tails vanish at the Fermi level at zero temperature. In order to study multilayers, taken to be square lattices within the layers, we use the real-space DMFT extension with the CT-QMC impurity solver. For bilayers formed by the HMF with a band or correlated insulator, we find that charge fluctuations between the layers enhance the finite-temperature tails. In addition, in the presence of interlayer hopping, a coherent quasiparticle peak forms in the otherwise correlated insulator. In the multilayer heterostructure setup, we find that by suitably choosing the model parameters, the tails at the HMF-Mott insulator interface can be reduced significantly and that a high spin polarization is conceivable, even in the presence of long-ranged electrostatic interactions.

This paper analyses the thermoelectric power of two- and three-terminal quantum dot devices under large thermal Δ T and voltage V biases, and their performance as thermoelectric heat engines. The focus is on the interaction between electrons, far-from-equilibrium conditions, and strongly nonlinear transport, which all are important factors affecting the usefulness of the devices. To properly characterise the thermoelectric properties under such conditions, two different Seebeck coefficients are introduced, generalizing the linear response expression. In agreement with previous work, we find that the efficiency of the three-terminal thermoelectric heat engine, as measured by the delivered power, is optimal far from equilibrium. Moreover, strong Coulomb interactions between electrons on the quantum dot are found to diminish the efficiency at maximum power, and the maximal value of the delivered power, both in the Kondo regime and beyond.

Due to their characteristic geometry, TiO(2)nanotubes (TNTs), suitably doped by metal-substitution to enhance their photocatalytic properties, have a high potential for applications such as clean fuel production. In this context, we present a detailed investigation of the magnetic, electronic, and optical properties of transition-metal doped TNTs, based on hybrid density functional theory. In particular, we focus on the 3d, the 4d, as well as selected 5dtransition-metal doped TNTs. Thereby, we are able to explain the enhanced optical activity and photocatalytic sensitivity observed in various experiments. We find, for example, that Cr- and W-doped TNTs can be employed for applications like water splitting and carbon dioxide reduction, and for spintronic devices. The best candidate for water splitting is Fe-doped TNT, in agreement with experimental observations. In addition, our findings provide valuable hints for future experimental studies of the ferromagnetic/spintronic behavior of metal-doped titania nanotubes.

The electronic structure and the transport properties of copper phthalocyanine (CuPc), an organic semiconductor, attached to leads are investigated using density functional theory (DFT) the non-equilibriums Green’s function formalism. Mainly, we discuss the electronic structure of CuPc attached to gold chains – a prototypical one-dimensional molecular device, which shall serve as model system for CuPc in transistors. We determine, in particular, the charge density of the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital). For CuPc molecules attached to leads either the HOMO or LUMO is shifted to the Fermi level of the contact, thus electrons are transfered to the leads or onto the molecule, respectively, depending on the contact geometry. For a planar contact, the material and the dimensionality of the contacts does not influence the shape of the molecular orbitals. In all studied cases, electrons are injected onto the molecule. On the other hand, molecules contacted in perpendicular geometry, become hole doped. In addition, we determine the transmission coefficient and the I-V characteristic. By comparing density of states and transmission coefficient we can show which molecular levels can transmit charge. Thereby we see, that molecular orbitals localized on the Cu do not contribute to the transport in planar geometry, but provide the transmitting channel in perpendicular contact geometry. Finally we compare to contacts with two molecules in the contact region and fluorinated F16CuPc.

Half-metallic Heusler alloys are attracting considerable attention because of their unique half-metallic band structures, which exhibit high spin polarization and yield huge magnetoresistance ratios. Besides serving as ferromagnetic electrodes, Heusler alloys also have the potential to host spin-charge conversion. Here, we report on the spin-charge conversion effect in the prototypical Heusler alloy NiMnSb. An unusual charge signal was observed with a sign change at low temperature, which can be manipulated by film thickness and ordering structure. It is found that the spin-charge conversion has two contributions. First, the interfacial contribution causes a negative voltage signal, which is almost constant versus temperature. The second contribution is temperature dependent because it is dominated by minority states due to thermally excited magnons in the bulk part of the film. This work provides a pathway for the manipulation of spin-charge conversion in ferromagnetic metals by interface-bulk engineering for spintronic devices.

We measure and analyze the effective spin Hall angle of platinum in the low-residual resistivity regime by second-harmonic measurements of the spin-orbit torques for a multilayer of Pt vertical bar Co vertical bar AlOx. An angular-dependent study of the torques allows us to extract the effective spin Hall angle responsible for the damping-like torque in the system. We observe a strikingly nonmonotonic and reproducible temperature dependence of the torques. This behavior is compatible with recent theoretical predictions which include both intrinsic and extrinsic (impurities and phonons) contributions to the spin Hall effect at finite temperatures.

In ferromagnet/normal-metal bilayers, the sensitivity of the spin Hall magnetoresistance and the spin Nernst magnetothermopower to the boundary conditions at the interface is of central importance. In general, such boundary conditions can be substantially affected by current-induced spin polarizations. In order to quantify the role of the latter, we consider a Rashba two-dimensional electron gas with a ferromagnet attached to one side of the system. The geometry of such a system maximizes the effect of current-induced spin polarization on the boundary conditions, and the spin Hall magnetoresistance is shown to acquire a non-trivial and asymmetric dependence on the magnetization direction of the ferromagnet.

X iv :1 70 3. 10 48 6v 1 [ co nd -m at .m tr lsc i] 3 0 M ar 2 01 7 Thermoelectric properties of the misfit cobaltate Ca3Co4O9 Bin Amin, Ulrich Eckern, ∗ and Udo Schwingenschlögl † KAUST, PSE Division, Thuwal 23955-6900, Kingdom of Saudi Arabia Universität Augsburg, Institut für Physik, 86135 Augsburg, Germany Abstract The layered misfit cobaltate Ca3Co4O9, also known as Ca2CoO3[CoO2]1.62, is a promising p-type thermoelectric oxide. Employing density functional theory, we study its electronic structure and determine, on the basis of Boltzmann theory within the constant-relaxation-time approximation, the thermoelectric transport coefficients. The dependence on strain and temperature is determined. In particular, we find that the xx-component of the thermopower is strongly enhanced, while the yy-component is strongly reduced, when applying 2% tensile strain. A similar anisotropy is also found in the power factor. The temperature dependence of the conductivity in the a-b plane is found to be rather weak above 200 K, which clearly indicates that the experimentally observed transport properties are dominated by inhomogeneities arising during sample growth, i.e., are not intrinsic.

The (LaV3+O3)(m)/SrV4+O3 (m = 5, 6) superlattices are investigated by first principles calculations. While bulk LaVO3 is a C-type antiferromagnetic semiconductor and bulk SrVO3 is a paramagnetic metal, semiconducting A-type antiferromagnetic states for both superlattices are found due to epitaxial strain. At the interfaces, however, the V spins couple antiferromagnetically for m = 5 and ferromagnetically for m = 6 (m-dependence of the magnetization). Electronic reconstruction in form of charge ordering is predicted to occur with V3+ and V4+ states arranged in a checkerboard pattern on both sides of the SrO layer. As compared to bulk LaVO3, the presence of V4+ ions introduces in-gap states that strongly reduce the bandgap and influence the orbital occupation and ordering.

The magnetoresistance of a three-dimensional Rashba metal placed on top of a ferromagnetic insulator is theoretically investigated. In addition to the intrinsic Rashba spin–orbit interaction, we consider extrinsic spin–orbit coupling via side-jump and skew scattering, as well as Elliott–Yafet spin relaxation. The latter is anisotropic due to the mass anisotropy which reflects the noncentrosymmetric crystal structure. A quasiclassical approach is employed to derive coupled spin-diffusion equations, which are supplemented by boundary conditions that account for the spin-transfer torque at the interface of the bilayer. The magnetoresistance is fully determined by the current-induced spin polarization, i.e., it cannot in general be ascribed to a single (bulk) spin Hall angle. Our theory reproduces several features of the experiments, at least qualitatively, and contains established phenomenological results in the relevant limiting cases. In particular, the anisotropy of the Elliott–Yafet spin relaxation mechanism plays a major role for the interpretation of the observed magnetoresistance.

By means of first principles calculations, we comprehensively investigate the stability of O vacancies at the different possible sites in the (LaVO3)(6)/SrVO3 superlattice and their effect on the electronic structure. Formation energy calculations demonstrate that O vacancies are formed most easily in or close to the SrO layer. We show that O vacancies at these energetically favorable sites conserve the semiconducting character of the superlattice by reducing V4+ ions next to the SrO layer to V3+ ions, while all other sites result in a metallic character.

We consider two modifications of a recently proposed three-terminal quantum dot heat engine. First, we investigate the necessity of the thermalization assumption, namely that electrons are always thermalized by inelastic processes when traveling across the cavity where the heat is supplied. Second, we analyze various arrangements of tunneling-coupled quantum dots in order to implement a transmission function that is superior to the Lorentzian transmission function of a single quantum dot. We show that the maximum power of the heat engine can be improved by about a factor of two, even for a small number of dots, by choosing an optimal structure.