Kurt Stokbro

Affiliations

• QuantumWise A/S, Fruebjergvej 3, box 4, DK-2100 Copenhagen, Denmark

Title

Atomic-scale modelling of electron transport in topological insulators and magnetic memory devices

Abstract

The Atomistix ToolKit [1] enables the simulation of electron transport in spin systems using noncollinear Density Functional Theory including spin-orbit interactions. I will present the theoretical background, important features, implementation details, as well as a number of applications. One area of applications is Magnetic Tunneling Junctions, for these systems we have investigated magnetic anisotropy, exchange couplings, tunnel magnetoresistance (TMR) and Spin Transfer Torque(STT). To obtain converged results for the latter we have implement an adaptive sampling of the reciprocal space. Another area is the simulation of electron transport in topological insultator (TI)-based devices. We have predicted [2] that unpolarized charge current injected into a ballistic thin film of prototypical topological insulator (TI) Bi2Se3 will generate a noncollinear spin texture on its surface. Furthermore, the nonequilibrium spin texture will extend into 2 nm thick layer below the TI surfaces due to penetration of evanescent wavefunctions from the metallic surfaces into the bulk of TI. Averaging the texture over few Å along the longitudinal direction defined by the current flow reveals large component pointing in the transverse direction. In addition, we find an order of magnitude smaller out-of-plane component when the direction of injected current with respect to Bi and Se atoms probes the largest hexagonal warping of the Dirac-cone dispersion on TI surface. We also demonstrate how DFT calculations with properly optimized local orbital basis set can precisely match putatively more accurate calculations with plane-wave basis set for the supercell of Bi2Se3.

References

• [1] http://www.quantumwise.com/products/features.

• [2] P.-H. Chang, T. Markussen, S. Smidstrup, K. Stokbro, and B. K. Nikolić, Nonequilibrium spin texture within a thin layer below the surface of current-carrying topological insulator Bi₂Se₃: A first-principles quantum transport study, arXiv:1503.08046.