Spin Hall Effects Due to Phonon Skew Scattering.

A diversity of spin Hall effects in metallic systems is known to rely on Mott skew scattering. In this work its high-temperature counterpart, phonon skew scattering, which is expected to be of foremost experimental relevance, is investigated. In particular, the phonon skew scattering spin Hall conductivity is found to be practically T independent for temperatures above the Debye temperature T_{D}. As a consequence, in Rashba-like systems a high-T linear behavior of the spin Hall angle demonstrates the dominance of extrinsic spin-orbit scattering only if the intrinsic spin splitting is smaller than the temperature.

(LaCrO3) m /SrCrO3 superlattices as transparent p-type semiconductors with finite magnetization.

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.

Spin-charge conversion in NiMnSb Heusler alloy films.

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.

Implementation of transmission functions for an optimized three-terminal quantum dot heat engine.

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.

Unravelling the interplay of geometrical, magnetic and electronic properties of metal-doped graphene nanomeshes.

Graphene nanomeshes (GNMs), formed by creating a superlattice of pores in graphene, possess rich physical and chemical properties. Many of these properties are determined by the pore geometry. In this work, we use first principles calculations to study the magnetic and electronic properties of metal-doped nitrogen-passivated GNMs. We find that the magnetic behaviour is dependent on the pore shape (trigonal versus hexagonal) as dictated by the number of covalent bonds formed between the 3d metal and the passivating N atoms. We also find that Cr and V doped trigonal-pore GNMs, and Ti doped GNMs are the most favourable for spintronic applications. The calculated magnetic properties of Fe-doped GNMs compare well with recent experimental observations. The studied systems are useful as spin filters and chemical sensors.

Defect engineering of the electronic transport through cuprous oxide interlayers.

The electronic transport through Au-(Cu2O)n-Au junctions is investigated using first-principles calculations and the nonequilibrium Green's function method. The effect of varying the thickness (i.e., n) is studied as well as that of point defects and anion substitution. For all Cu2O thicknesses the conductance is more enhanced by bulk-like (in contrast to near-interface) defects, with the exception of O vacancies and Cl substitutional defects. A similar transmission behavior results from Cu deficiency and N substitution, as well as from Cl substitution and N interstitials for thick Cu2O junctions. In agreement with recent experimental observations, it is found that N and Cl doping enhances the conductance. A Frenkel defect, i.e., a superposition of an O interstitial and O substitutional defect, leads to a remarkably high conductance. From the analysis of the defect formation energies, Cu vacancies are found to be particularly stable, in agreement with earlier experimental and theoretical work.

Effect of Gd doping and O deficiency on the Curie temperature of EuO.

The effect of Gd doping and O deficiency on the electronic structure, exchange interaction, and Curie temperature of EuO in the cubic and tetragonal phases is studied by means of density functional theory. For both defects, the Curie temperature is found to exhibit a distinct maximum as a function of the defect concentration. The existence of optimal defect concentrations is explained by the interplay of the on-site, RKKY, and superexchange contributions to the magnetism.

Fate of half-metallicity near interfaces: the case of NiMnSb/MgO and NiMnSi/MgO.

The electronic and magnetic properties of the interfaces between the half-metallic Heusler alloys NiMnSb, NiMnSi, and MgO have been investigated using first-principles density-functional calculations with projector augmented wave potentials generated in the generalized gradient approximation. In the case of the NiMnSb/MgO (100) interface, the half-metallicity is lost, whereas the MnSb/MgO contact in the NiMnSb/MgO (100) interface maintains a substantial degree of spin polarization at the Fermi level (∼60%). Remarkably, the NiMnSi/MgO (111) interface shows 100% spin polarization at the Fermi level, despite considerable distortions at the interface, as well as rather short Si/O bonds after full structural optimization. This behavior markedly distinguishes NiMnSi/MgO (111) from the corresponding NiMnSb/CdS and NiMnSb/InP interfaces.

Modified Li chains as atomic switches.

We present electronic structure and transport calculations for hydrogen and lithium chains, using density functional theory and scattering theory on the Green's function level, to systematically study impurity effects on the transmission coefficient. To this end we address various impurity configurations. Tight-binding results allow us to interpret our the findings. We analyze under which circumstances impurities lead to level splitting and/or can be used to switch between metallic and insulating states. We also address the effects of strongly electronegative impurities.