Thematic Programs

【Seminars】Special Seminars by Prof. Axel Hoffmann, Assistant Prof. Mikhail Titov and Prof. Xavier Waintal

Information

Date

November 26, 2015 14: 00 – 16: 30

Venue

TOKYO ELECTRON House of Creativity 3F, Lecture Theater, Katahira Campus, Tohoku University
MAP

Speakers

Axel Hoffmann (Argonne National Laboratory, USA)
Mikhail Titov (Radboud University Nijmegen, The Netherlands)
Xavier Waintal (CEA Grenoble, France)

Title and Abstract

14:00 –
Thierry Valet (SPICE Center, Johannes Gutenberg University, Mainz, Germany)

Title:
Manipulating Room Temperature Magnetic Skyrmions

Abstract:
Magnetic skyrmions are endowed with quasi-particle like behavior due to their distinct topology. At the same time they can be manipulated with very low electric currents, which makes them interesting for extreme low-power information technologies [A. Hoffmann and S. D. Bader, Phys. Rev. Appl. 4, 047001 (2015)], where data is envisioned to be encoded in topological charges, instead of electronic charges as in conventional semiconducting devices. Towards the realization of this goal we demonstrated how the spin Hall effect [A. Hoffmann, IEEE Trans. Magn. 49, 5172 (2013)] can be used to generate and manipulate magnetic skyrmions at room temperature [W. Jiang, et al., Science 349, 283 (2015), O. Heinonen, et al., arXiv:1511.04630]. Furthermore, I will show how the current driven motion reveals the transverse motion expected from the skyrmion Hall effect. Lastly, by optimizing our magnetic heterostructures, we show that we can stabilize skyrmions with sizes around 100 nm even in the absence of externally applied magnetic fields.

 

14:45 –
Mikhail Titov (Radboud University Nijmegen, The Netherlands)

Title:
Anomalous Hall effect due to skew scattering on rare impurity configurations

Abstract:
Anomalous Hall effect arises in systems with both spin-orbit coupling and magnetization. Generally, one may distinguish three mechanisms contributing to anomalous Hall conductivity: intrinsic, side jump, and skew scattering. The standard diagrammatic approach to the anomalous Hall effect is limited to computation of ladder diagrams that would correspond to classical diffusion in the case of longitudinal conductivity. I will demonstrate that this approach is always invalid when it concerns anomalous Hall conductivity. An important additional contribution comes from diagrams with a single pair of intersecting disorder lines. This contribution constitutes an inherent part of skew scattering on pairs of closely located defects and essentially modifies previously obtained results for anomalous Hall conductivity in all models. I illustrate the phenomenon by an explicit calculation for two-dimensional massive Dirac fermions and for Bychkov-Rashba model for 2D ferromagnet with weak disorder. In the latter case the skew scattering originating in rare impurity configurations provides the only contribution to anomalous Hall effect for Fermi-energies belonging to the upper band, i.e. in the most relevant metallic regime. Rare impurity fluctuations also affect such quantities as anti-damping-like spin-orbit torque, spin-Hall conductivity and many related quantities. At the end of my presentation I will also discuss the universality of the obtained results and their generalisation to arbitrary isotropic models in two dimensions.

 

15:30 –
Xavier Waintal (CEA Grenoble, France)

Title:
Not Russian? Teach your computer how to calculate Feynman diagrams for you.

Abstract:
In this seminar, I will discuss a technique that we recently introduce to calculate high order many-body perturbation theory corrections numerically. The technique is based on the Keldysh formalism in real time and real space and maps the original problem onto a stochastic motion in Fock space. I will apply it to an Anderson impurity and show results up to 15th order in power of the electron electron interactions. Correlated features such as the Kondo ridge can be directly recovered.