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Graphene with time-dependent spin-orbit coupling: Truncated Magnus expansion approach
We analyze the role of ac-driven Rashba spin-orbit coupling in monolayer
graphene including a spin-dependent mass term. Using the Magnus expansion as a
semi-analytical approximation scheme a full account of the quasienergie
spectrum of spin states is given. We discuss the subtleties arising in
correctly applying the Magnus expansion technique in order to determine the
quasienergy spectrum. Comparison to the exact numerical solution gives
appropriate boundaries to the validity of the Magnus expansion solution.Comment: 8 pages, 4 figure
Floquet spin states in graphene under ac driven spin-orbit interaction
We study the role of periodically driven time-dependent Rashba spin-orbit
coupling (RSOC) on a monolayer graphene sample. After recasting the originally
system of dynamical equations as two time-reversal related
two-level problems, the quasi-energy spectrum and the related dynamics are
investigated via various techniques and approximations. In the static case the
system is a gapped at the Dirac point. The rotating wave approximation (RWA)
applied to the driven system unphysically preserves this feature, while the
Magnus-Floquet approach as well as a numerically exact evaluation of the
Floquet equation show that this gap is dynamically closed. In addition, a
sizable oscillating pattern of the out-of-plane spin polarization is found in
the driven case for states which completely unpolarized in the static limit.
Evaluation of the autocorrelation function shows that the original uniform
interference pattern corresponding to time-independent RSOC gets distorted. The
resulting structure can be qualitatively explained as a consequence of the
transitions induced by the ac driving among the static eigenstates, i.e., these
transitions modulate the relative phases that add up to give the quantum
revivals of the autocorrelation function. Contrary to the static case, in the
driven scenario, quantum revivals (suppresions) are correlated to spin up
(down) phases.Comment: 10 pages, 8 figures. Typos corrected. Accepted for publication in PR
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