Time-dependent multimode transport through quantum wires with spin-orbit interaction: Floquet scattering matrix approach

We investigate the ballistic spin transport in a multimode quantum wire with strong Rashba spin-orbit coupling and an oscillating potential. A spin-resolved Floquet scattering matrix and Floquet spin density matrix formalism is employed to study the transport properties, such as conductance and spin polarizations, of the device. Due to the strong Rashba coupling-induced drastic change of the dispersion relation, there are quasibound states formed beneath the bottom of each transverse mode. Interference between electrons through propagating modes and via the quasibound states will give a complex structure in the transport properties. By using an oscillating potential, the incident electrons can be trapped by or escape from the quasibound states via photon emission or absorption. As a result, asymmetric Fano line shapes can be found in our numerical results due to this photon-assisted interference. The properties of the mesoscopic device to retain the spin coherence of the injected spin states are also analyzed. When only one propagating mode is permitted, the spin coherence is well retained at static transport. However, when an oscillating potential is turned on, we found that the spin coherence, even when only one propagating mode is permitted, will be reduced. This is due to the existence of infinite Floquet states when the ac field is turned on. This mechanism of losing spin coherence should be taken into account in the design and operation of the mesoscopic spintronics devices with an ac field.