Thermoelectric transport in strained Si and Si/Ge heterostructures

The anisotropic thermoelectric transport properties of bulk silicon strained in [111]-direction were studied by detailed first-principles calculations focussing on a possible enhancement of the power factor. Electron as well as hole doping were examined in a broad doping and temperature range. At low temperature and low doping an enhancement of the power factor was obtained for compressive and tensile strain in the electron-doped case and for compressive strain in the hole-doped case. For the thermoelectrically more important high temperature and high doping regime a slight enhancement of the power factor was only found under small compressive strain with the power factor overall being robust against applied strain. To extend our findings the anisotropic thermoelectric transport of an [111]-oriented Si/Ge superlattice was investigated. Here, the cross-plane power factor under hole-doping was drastically suppressed due to quantum-well effects, while under electron-doping an enhanced power factor was found. With that, we state a figure of merit of ZT$=0.2$ and ZT$=1.4$ at T=\unit[300]{K} and T=\unit[900]{K} for the electron-doped [111]-oriented Si/Ge superlattice. All results are discussed in terms of band structure features

Microscopic origin of the anomalous Hall effect in noncollinear kagome magnets

The anomalous Hall effect is commonly considered a signature of ferromagnetism. However, recently, an enormous anomalous Hall conductivity was measured in the compensated kagome magnets Mn3Sn and Mn3Ge. The occurrence of this effect is allowed by the magnetic point group of these materials; however, its emergence is still lacking a microscopic explanation. Herein we show that the spin-orbit coupling and an out-of-plane tilting of the texture are equivalent for several kagome magnets. Consequently, a coplanar system with spin-orbit coupling behaves as if it were virtually noncoplanar. We show via tight-binding model calculations that the Hall effect can mainly be interpreted as a topological Hall effect generated by the opening angle of the virtually tilted texture. Furthermore, upon tilting the fixed texture out of the kagome plane, we find a critical tilting angle for which the Hall conductivity vanishes for all energies. In this case, the Hamiltonian is invariant under a combined time-reversal and mirror symmetry, because the virtual texture is coplanar

Tunneling of Bloch electrons through vacuum barrier

Tunneling of Bloch electrons through a vacuum barrier introduces new physical effects in comparison with the textbook case of free (plane wave) electrons. For the latter, the exponential decay rate in the vacuum is minimal for electrons with the parallel component of momentum ${\bf k}_\parallel=0$, and the prefactor is defined by the electron momentum component in the normal to the surface direction. However, the decay rate of Bloch electrons may be minimal at an arbitrary ${\bf k}_\parallel$ (``hot spots''), and the prefactor is determined by the electron's group velocity, rather than by its quasimomentum.Comment: 4 pages, no fig

First principles design of Ohmic spin diodes based on quaternary Heusler compounds

The Ohmic spin diode (OSD) is a recent concept in spintronics, which is based on half-metallic magnets (HMMs) and spin-gapless semiconductors (SGSs). Quaternary Heusler compounds offer a unique platform to realize the OSD for room temperature applications as these materials possess very high Curie temperatures as well as half-metallic and spin-gapless semiconducting behavior within the same family. Using state-of-the-art first-principles calculations combined with the non-equilibrium Green's function method we design four different OSDs based on half-metallic and spin-gapless semiconducting quaternary Heusler compounds. All four OSDs exhibit linear current-voltage ($I-V$) characteristics with zero threshold voltage $V_T$. We show that these OSDs possess a small leakage current, which stems from the overlap of the conduction and valence band edges of opposite spin channels around the Fermi level in the SGS electrodes. The obtained on/off current ratios vary between $30$ and $10^5$. Our results can pave the way for the experimental fabrication of the OSDs within the family of ordered quaternary Heusler compounds.Comment: 7 pages, 5 figure

On calculating the Berry curvature of Bloch electrons using the KKR method

We propose and implement a particularly effective method for calculating the Berry curvature arising from adiabatic evolution of Bloch states in wave vector k space. The method exploits a unique feature of the Korringa-Kohn-Rostoker (KKR) approach to solve the Schr\"odinger or Dirac equations. Namely, it is based on the observation that in the KKR method k enters the calculation via the structure constants which depend only on the geometry of the lattice but not the crystal potential. For both the Abelian and non-Abelian Berry curvature we derive an analytic formula whose evaluation does not require any numerical differentiation with respect to k. We present explicit calculations for Al, Cu, Au, and Pt bulk crystals.Comment: 13 pages, 5 figure

Influence of strain on anisotropic thermoelectric transport of Bi2_2Te3_3 and Sb2_2Te3_3

On the basis of detailed first-principles calculations and semi-classical Boltzmann transport, the anisotropic thermoelectric transport properties of Bi$_2$Te$_3$ and Sb$_2$Te$_3$ under strain were investigated. It was found that due to compensation effects of the strain dependent thermopower and electrical conductivity, the related powerfactor will decrease under applied in-plane strain for Bi$_2$Te$_3, while being stable for Sb$_2$Te$_3$. A clear preference for thermoelectric transport under hole-doping, as well as for the in-plane transport direction was found for both tellurides. In contrast to the electrical conductivity anisotropy, the anisotropy of the thermopower was almost robust under applied strain. The assumption of an anisotropic relaxation time for Bi$_2$Te$_3$ suggests, that already in the single crystalline system strong anisotropic scattering effects should play a role