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

TL;DR

This paper compares three non-equilibrium Green's function methods to study how terahertz pulses influence magnetization and spin transport in hydrogenated graphene, demonstrating effective modeling of dynamic spin manipulation.

Contribution

It introduces and compares three NEGF-based methods for modeling THz pulse effects on spin transport in hydrogenated graphene, highlighting the near-adiabatic expansion's accuracy.

Findings

The near-adiabatic expansion accurately models dynamics up to 1 V pulses.

Steady-state NEGF combined with DFT provides a Hubbard model for the system.

The auxiliary mode method offers exact solutions without time-variation approximations.

Abstract

Terahertz (THz) field pulses can now be applied in Scanning Tunnelling 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 (NEGF): (i) the steady-state, adiabatic results, (ii) the lowest order dynamic expansion in the time-variation (DE), and (iii) the auxiliary mode (AM) 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 NEGF combined with Density Functional Theory (DFT-NEGF) 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 dynamical expansion provides a good description for STM voltage pulses up to the 1 V range.