Itinerant Spin-Dynamics in Structures of Reduced Dimensionality

• Applicant: Professor Dr. Stefan Kettemann
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2010 to 2014
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 163472456

• Final Report Year: 2014

Final Report Abstract

In this project we studied the spin dynamics of itinerant electrons in structure of reduced dimensions, such as quantum wires, disordered graphene and topological insulator thin films. In particular, we studied the effect of nonmagnetic resonant impurity states on the Spin Hall Effect (SHE). While it was known before that nonmagnetic disorder diminishes the spin-Hall conductance of noninteracting electrons we showed in this project that also resonant impurities diminish the SHE. While we found a maximum in the SHE in the middle of an impurity band formed by the hybridised resonant impurity levels, its magnitude is found to be diminished continously as the impurity concentration is increased. We found furthermore a minimum of the Spin Hall conductance and a change of sign when the Fermi energy is moved between the impurity band and the conduction band. We studied in detail the origin of sign changes in the SHC, by considering different lattice structures, in particular the SHC on a triangular lattice. We could understand its origin geometrically in terms of Berry phases and the changes of sign and amplitude of the SHC have been associated to the topology of the Fermi surface, and to the sign of the velocity in the absence of SO coupling. Since it is known that the spin relaxation rate is minimal close to metal-insulator transitions, we studied the spin relaxation and its effect on the MIT. We particularly studied the effect of magnetic moments on the Anderson metal-insulator transition, finding a new finite temperature transition, driven by the interplay between Kondo effect and Andersonlocalisation. We also studied the dynamics of magnetic moments in graphene, motivated by the collaboration with the experimental group of Prof. Hu-Jong Lee at Postech, which showed that magnetic moments are important to understand the spin relaxation in graphene. In collaboration with visiting professor Georges Bouzerar and Post Doc Akash Chakraborty, we also studied the spin dynamics in dilute magnetic semiconductors. Paul Wenk moved to the University of Regensburg, and he pursues the study of the Spin Hall Effect in dilute magnetic semiconductors.

Publications

• Spin Hall Conductivity on the Anisotropic Triangular Lattice, Phys. Rev. B 86, 075441 (2012)
  Paul Wenk, Stefan Kettemann, Georges Bouzerar
  
• Spontaneous magnetization in presence of nanoscale inhomogeneities in diluted magnetic systems, Phys. Rev. B 86, 214402 (2012)
  Akash Chakraborty, Paul Wenk, Richard Bouzerar, Georges Bouzerar
  
• Spin-wave excitations in presence of nanoclusters of magnetic impurities. New Journal of Phys., Volume 16, 033004, March 2014
  Akash Chakraborty, Paul Wenk, Stefan Kettemann, Richard Bouzerar, Georges Bouzerar