Mesoscopic current patterns and orbital magnetism induced by dc-voltages

• Applicant: Professor Dr. Ferdinand Evers
• Subject Area: Theoretical Condensed Matter Physics ; Theoretical Chemistry: Molecules, Materials, Surfaces
• Term: from 2014 to 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 257888954

• Final Report Year: 2019

Final Report Abstract

Patterns in the local current density j(r) that form upon applying a bias to a disordered and phase-coherent metal have been investigated numerically. The distribution function P (|j|) of the current densities’ magnitude was found to be logarithmically broad in the case of graphene with hydrogen (H) or OH-adsorbants. This implies the formation of very strong circulating currents. It was found that the strongest eddies typically circulate around a few plaquettes. Perturbative calculations indicate that the resulting induced magnetic field is not strong enough to give significant effects; therefore, self-consistent calculations on the level of current density-functional-theory are not required for quantitative results. A transport code AITRANSS has been implemented that allows for the efficient calculation of local current densities on HPC architectures. Motivated by experiments on the spin-relaxation time in graphene, the code has been extended to also allow for computations of spin-dependent transport. In our transport simulation we have found that hydrogen adsorbants form local moments that interact with each other. The resulting spin-network acts as magnetic scatterer with a preferred direction of magnetization that leads to a significant dependency of the conductance on the spin of the injected current. To achieve a qualitative understanding, the formation of current eddies has been studied within a drastically simplified model system consisting of a ring of four sites. Spinless fermions with nearest-neighbor interaction have been considered in this setup. As it turned out, very strongly oscillating circulating currents appear as a transient phenomenon after forming the contacts to source and drain. Very surprisingly, the lifetime of the oscillations can exceed the level broadening of the molecular states by many orders of magnitude. The effect has been explained in a recent PhD work at the University of Utrecht based on arguments borrowed from perturbation theory.

Publications

• Ab-inito quantum transport through armchair graphene nanoribbons: streamlines in the current density, Phys. Rev. B 89, 195406 (2014)
  J. Wilhelm, M. Walz and F. Evers
  
• Current Patterns and Orbital Magnetism in Mesoscopic dc-Transport, Phys. Rev. Lett. 113, 136602 (2014)
  M. Walz, J.Wilhelm and F. Evers
  
• Ab initio simulations of local current densities in mesoscopic films: Current vortices in functionalized graphene nanoribbons., Doctoral thesis, Karlsruhe Institute of Technology (2015)
  M. Walz
  
• Ab initio spin-flip conductance of hydrogenated graphene nanoribbons: spin-orbit interaction and scattering with local impurity sites, Phys. Rev. B 92, 14405 (2015)
  J. Wilhelm, M. Walz and F. Evers
  
• Ab Initio Transport Calculations for Functionalized Graphene Flakes on a Supercomputer, in High Performance Computing in Science and Engineering ’15, ed. W. Nagel, D. Kröner and M. Resch, Springer pp. 139-154 (2016)
  M. Walz, A. Bagrets, F. Evers and I. Kondov
  
• Long-lived circulating currents in strongly correlated nanorings. Phys. Rev. B
  B. M. Schönauer, N. M. Gergs, P. Schmitteckert, F. Evers and D. Schuricht
  
• On non-equilibrium dynamics in low-dimensional a strongly correlated quantum systems, Doctoral thesis, Universität Utrecht (2019)
  B. Schönauer