718,985 research outputs found
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
Temporal and diffraction effects in entanglement creation in an optical cavity
A practical scheme for entanglement creation between distant atoms located
inside a single-mode optical cavity is discussed. We show that the degree of
entanglement and the time it takes for the entanglement to reach its optimum
value is a sensitive function the initial conditions and the position of the
atoms inside the cavity mode. It is found that the entangled properties of the
two atoms can readily be extracted from dynamics of a simple two-level system.
Effectively, we engineer two coupled qubits whose the dynamics are analogous to
that of a driven single two-level system. It is found that spatial variations
of the coupling constants actually help to create transient entanglement which
may appear on the time scale much longer than that predicted for the case of
equal coupling constants. When the atoms are initially prepared in an entangled
state, they may remain entangled for all times. We also find that the
entanglement exhibits an interesting phenomenon of diffraction when the the
atoms are located between the nodes and antinodes of the cavity mode. The
diffraction pattern of the entanglement varies with time and we explain this
effect in terms of the quantum property of complementarity, which is manifested
as a tradeoff between the knowledge of energy of the exchanged photon versus
the evolution time of the system.Comment: Phys. Rev. A75, 042307 (2007
Hydraulic brake safety valve
Safety device, consisting of three separate fluid chambers, insures that two wheels of a brake system continue to function if a failure occurs
- …