Thursday, July 4, 2013

1305.2887 (Lucas J. Fernández-Alcázar et al.)

Rabi oscillations in the quantum transmittance through modulated
magnetic fields: application to domain walls in magnetic nano-wires

Lucas J. Fernández-Alcázar, Horacio M. Pastawski
We address the spin-dependent electronic transport through regions where there is a spatially modulated magnetic field. We model and solve an illustrative example: a ferromagnetic nano-wire with a single domain wall (DW) whose finite width where the local field rotates while it modifies its intensity. There is a trivial case where the electrons which traverse this region may also rotate their spin projection following the field in an adiabatic transition if the slow velocities or wide enough DW. Alternatively, in a diabatic DW crossing the spin projection tends to be conserved as it can not follow the field. Adopting the quantization direction imposed by the bulk field, the electron's Zeeman energies are degenerated when they cross the DW center. However the perpendicular component of the field may be seen as a perturbation that breaks this degeneracy inducing an avoided crossing. We identify two non-trivial diabatic regimes as function of the intensity of the perturbation. On one hand, if it is weak enough, we can identify this model with a standard Landau-Zener transition problem. On the other hand, for strong transverse fields, the spin projection may have oscillations identified with the Rabi oscillation in a two level system. In order to demonstrate this statement we calculate the dynamics of the electron while it crosses the domain wall. We also study the stability of this phenomenon against decoherence by solving a spatially continuous dephasing model, as developed by D'Amato and Pastawski, which shows that Rabi oscillations are not particularly sensitive to perturbations.
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