Quantum Dissipative Rashba Spin Ratchets

We predict the possibility to generate a finite stationary spin current by applying an unbiased ac driving to a quasi-one-dimensional asymmetric periodic structure with Rashba spin-orbit interaction and strong dissipation. We show that under a finite coupling strength between the orbital degrees of freedom the electron dynamics at low temperatures exhibits a pure spin ratchet behavior, i.e., a finite spin current and the absence of charge transport in spatially asymmetric structures. It is also found that the equilibrium spin currents are not destroyed by the presence of strong dissipation.

Figure 1

A schematic picture of the isolated asymmetric periodic quasi-1D structure described by the Hamiltonian (1). In the center of the quasi-1D wire the periodic potential is weaker and gets stronger closer to the edges. Thus the electron group velocity is higher in the central region and tails off away from the center.

Figure 2

Nonequilibrium spin current, ${\overline{J}}_{n-e,S}^{\infty }$, as a function of the amplitude, $F$, of the driving force for different values of the viscosity coefficient $\eta$. Temperature $k_{\mathrm{Boltz}.}T=0.5$, spin-orbit coupling strength $k_{\mathrm{so}}L=\pi /2$, orbit-orbit coupling strength $\gamma =0.1$, driving frequency $\Omega =1$. The inset displays the shape of the periodic potential.

Figure 3

Nonequilibrium spin current, ${\overline{J}}_{n-e,S}^{\infty }$, as a function of the spin-orbit coupling strength, $k_{\mathrm{so}}$, for different values of the orbit-orbit coupling strength, $\gamma$. The driving amplitude and viscosity coefficient are $F=2\hslash {\omega }_0/L$, $\eta =0.5$. The other parameters are as in Fig. 2.