Publications by Year: 2014

Bouhassoune M, Zimmermann B, Mavropoulos P, Wortmann D, Dederichs PH, Blügel S, Lounis S. Quantum well states and amplified spin-dependent Friedel oscillations in thin films. NATURE COMMUNICATIONS. 2014;5:5558.Abstract
Electrons mediate many of the interactions between atoms in a solid. Their propagation in a material determines its thermal, electrical, optical, magnetic and transport properties. Therefore, the constant energy contours characterizing the electrons, in particular the Fermi surface, have a prime impact on the behaviour of materials. If anisotropic, the contours induce strong directional dependence at the nanoscale in the Friedel oscillations surrounding impurities. Here we report on giant anisotropic charge density oscillations focused along specific directions with strong spin-filtering after scattering at an oxygen impurity embedded in the surface of a ferromagnetic thin film of Fe grown on W(001). Utilizing density functional theory, we demonstrate that by changing the thickness of the Fe films, we control quantum well states confined to two dimensions that manifest as multiple flat energy contours, impinging and tuning the strength of the induced charge oscillations which allow to detect the oxygen impurity at large distances (approximate to 50 nm).
Long NH, Mavropoulos P, Zimmermann B, Bauer DSG, Blügel S, Mokrousov Y. Spin relaxation and spin Hall transport in 5d transition-metal ultrathin films. PHYSICAL REVIEW B. 2014;90:064406.Abstract
The spin relaxation induced by the Elliott-Yafet mechanism and the extrinsic spin Hall conductivity due to the skew scattering are investigated in 5d transition-metal ultrathin films with self-adatom impurities as scatterers. The values of the Elliott-Yafet parameter and of the spin-flip relaxation rate reveal a correlation with each other that is in agreement with the Elliott approximation. At 10-layer thickness, the spin-flip relaxation time in 5d transition-metal films is quantitatively reported about few hundred nanoseconds at atomic percent. This time scale is one and two orders of magnitude shorter than the values in Au and Cu thin films, respectively. The anisotropy effect of the Elliott-Yafet parameter and of the spin-flip relaxation rate with respect to the direction of the spin-quantization axis in relation to the crystallographic axes is also analyzed. We find that the anisotropy of the spin-flip relaxation rate is enhanced due to the Rashba surface states on the Fermi surface, reaching values as high as 97% in 10-layer Hf(0001) film or 71% in 10-layer W(110) film. Finally, the spin Hall conductivity as well as the spin Hall angle due to the skew scattering off self-adatom impurities are calculated using the Boltzmann approach. Our calculations employ a relativistic version of the first-principles full-potential Korringa-Kohn-Rostoker Green function method.
Fukushima T, Katayama-Yoshida H, Sato K, Bihlmayer G, Mavropoulos P, Bauer DSG, Zeller R, Dederichs PH. Hubbard U calculations for gap states in dilute magnetic semiconductors. JOURNAL OF PHYSICS-CONDENSED MATTER. 2014;26:274202.Abstract
On the basis of constrained density functional theory, we present ab initio calculations for the Hubbard U parameter of transition metal impurities in dilute magnetic semiconductors, choosing Mn in GaN as an example. The calculations are performed by two methods: (i) the Korringa-Kohn-Rostoker (KKR) Green function method for a single Mn impurity in GaN and (ii) the full-potential linearized augmented plane-wave (FLAPW) method for a large supercell of GaN with a single Mn impurity in each cell. By changing the occupancy of the majority t(2) gap state of Mn, we determine the U parameter either from the total energy differences E(N + 1) and E(N - 1) of the (N +/- 1) -electron excited states with respect to the ground state energy E(N), or by using the single-particle energies for n(0) +/- 1/2 occupancies around the charge-neutral occupancy n(0) (Janak's transition state model). The two methods give nearly identical results. Moreover the values calculated by the supercell method agree quite well with the Green function values. We point out an important difference between the `global' U parameter calculated using Janak's theorem and the `local' U of the Hubbard model.
Kovacik R, Mavropoulos P, Wortmann D, Blügel S. Spin-caloric transport properties of cobalt nanostructures: Spin disorder effects from first principles. PHYSICAL REVIEW B. 2014;89:134417.Abstract
The fundamental aspects of spin-dependent transport processes and their interplay with temperature gradients, as given by the spin Seebeck coefficient, are still largely unexplored and a multitude of contributing factors must be considered. We used density functional theory together with a Monte-Carlo-based statistical method to simulate simple nanostructures, such as Co nanowires and films embedded in a Cu host or in vacuum, and investigated the influence of spin disorder scattering on electron transport at elevated temperatures. While we show that the spin-dependent scattering of electrons due to temperature-induced disorder of the local magnetic moments contributes significantly to the resistance, thermoelectric, and spin-caloric transport coefficients, we also conclude that the actual magnitude of these effects cannot be predicted, quantitatively or qualitatively, without such detailed calculations.
Zimmermann B, Chadova K, Koedderitzsch D, Blügel S, Ebert H, Fedorov DV, Long NH, Mavropoulos P, Mertig I, Mokrousov Y, et al. Skew scattering in dilute ferromagnetic alloys. PHYSICAL REVIEW B. 2014;90:220403.Abstract
The challenging problem of skew scattering for Hall effects in dilute ferromagnetic alloys, with intertwined effects of spin-orbit coupling, magnetism, and impurity scattering, is studied here from first principles. Our main aim is to identify chemical trends and work out simple rules for large skew scattering in terms of the impurity and host states at the Fermi surface, with particular emphasis on the interplay of the spin and anomalous Hall effects in one and the same system. The predicted trends are benchmarked by referring to three different ab initio methods based on different approximations with respect to the electronic structure and transport properties.