Interplay of growth mode and thermally induced spin accumulation in epitaxial Al/Co2_2TiSi/Al and Al/Co2_2TiGe/Al contacts

The feasibility of thermally driven spin injectors built from half-metallic Heusler alloys inserted between aluminum leads was investigated by means of {\em ab initio} calculations of the thermodynamic equilibrium and electronic transport. We have focused on two main issues and found that: (i) the interface between Al and the closely lattice-matched Heusler alloys of type Co$_2$Ti$Z$ ($Z=$ Si or Ge) is stable under various growth conditions; and (ii) the conventional and spin-dependent Seebeck coefficients in such heterojunctions exhibit a strong dependence on both the spacer and the atomic composition of the Al/Heusler interface. The latter quantity gives a measure of the spin accumulation and varies between $+8$~$\mu$V/K and $-3$~$\mu$V/K near $300$~K, depending on whether a Ti-Ge or a Co-Co plane makes the contact between Al and Co$_2$TiGe in the trilayer. Our results show that it is in principle possible to tailor the spin-caloric effects by a targeted growth control of the samples.Comment: 16 pages, 13 figure

Electron-hole spectra created by adsorption on metals from density-functional theory

Non-adiabaticity in adsorption on metal surfaces gives rise to a number of measurable effects, such as chemicurrents and exo-electron emission. Here we present a quantitative theory of chemicurrents on the basis of ground-state density-functional theory (DFT) calculations of the effective electronic potential and the Kohn-Sham band structure. Excitation probabilities are calculated both for electron-hole pairs and for electrons and holes separately from first-order time-dependent perturbation theory. This is accomplished by evaluating the matrix elements (between Kohn-Sham states) of the rate of change of the effective electronic potential between subsequent (static) DFT calculations. Our approach is related to the theory of electronic friction, but allows for direct access to the excitation spectra. The method is applied to adsorption of atomic hydrogen isotopes on the Al(111) surface. The results are compatible with the available experimental data (for noble metal surfaces); in particular, the observed isotope effect in H versus D adsorption is described by the present theory. Moreover, the results are in qualitative agreement with computationally elaborate calculations of the full dynamics within time-dependent density-functional theory, with the notable exception of effects due to the spin dynamics. Being a perturbational approach, the method proposed here is simple enough to be applied to a wide class of adsorbates and surfaces, while at the same time allowing us to extract system-specific information.Comment: 23 pages, 9 figures, accepted for publication in Phys. Rev. B, http://prb.aps.org/, v2: some major improvements, plus correction of minor error

Native defects in the Co2_2TiZZ (Z=Z= Si, Ge, Sn) full Heusler alloys: formation and influence on the thermoelectric properties

We have performed first-principles investigations on the native defects in the full Heusler alloys Co$_2$Ti$Z$ ($Z$ one of the group IV elements Si, Ge, Sn), determining their formation energies and how they influence the transport properties. We find that Co vacancies (Vc) in all compounds and the Ti$_\text{Sn}$ anti-site exhibit negative formation energies. The smallest positive values occur for Co in excess on anti-sites (Co$_Z$ or Co$_\text{Ti}$) and for Ti$_Z$. The most abundant native defects were modeled as dilute alloys, treated with the coherent potential approximation in combination with the multiple-scattering theory Green function approach. The self-consistent potentials determined this way were used to calculate the residual resistivity via the Kubo-Greenwood formula and, based on its energy dependence, the Seebeck coefficient of the systems. The latter is shown to depend significantly on the type of defect, leading to variations that are related to subtle, spin-orbit coupling induced, changes in the electronic structure above the half-metallic gap. Two of the systems, Vc$_\text{Co}$ and Co$_Z$, are found to exhibit a negative Seebeck coefficient. This observation, together with their low formation energy, offers an explanation for the experimentally observed negative Seebeck coefficient of the Co$_2$Ti$Z$ compounds as being due to unintentionally created native defects