Manipulation of magnetization and spin transport in hydrogenated graphene with THz pulses

Terahertz (THz) field pulses can now be applied in scanning tunneling microscopy (THz-STM) junction experiments to study time-resolved dynamics. The relatively slow pulse compared to the typical electronic time-scale calls for approximations based on a time-scale separation. Here, we contrast three methods based on non-equilibrium Green's functions: i) the steady-state, adiabatic results, ii) the lowest-order dynamic expansion in the time variation, and iii) the auxiliary mode propagation method without approximations in the time variation. We consider a concrete THz-STM junction setup involving a hydrogen adsorbate on graphene where the localized spin polarization can be manipulated on/off by a local field from the tip electrode and/or a back-gate affecting the in-plane transport. We use steady-state non-equilibrium Green's function theory combined with density functional theory to obtain a Hubbard model for the study of the junction dynamics. Solving the Hubbard model in a mean-field approximation, we find that the near-adiabatic first-order dynamic expansion in the time variation provides a good description for STM voltage pulses up to the 1 V range.