Abstract
We consider the possibility to employ a quantum wire realized in a two-dimensional electron gas (2DEG) as a spin ratchet. We show that a net spin current without accompanying net charge transport can be induced in the nonlinear regime by an unbiased external driving via an ac voltage applied between the contacts at the ends of the quantum wire. To achieve this, we make use of the coupling of the electron spin to inhomogeneous magnetic fields created by ferromagnetic stripes patterned on the semiconductor heterostructure that harbors the 2DEG. Using recursive Green function techniques, we numerically study two different set-ups, consisting of one and two ferromagnetic stripes, respectively.
Highlights
In the context of mesoscopic physics, further concepts such as adiabatic spin pumping [14]–[16] and coherent spin ratchets [17, 18] have been proposed for generating spin-polarized currents
Using symmetry arguments and numerical calculations, we demonstrate that the two set-ups introduced in sections 3 and 4 act as spin ratchets
We have shown that the coupling of the electron spin to the magnetic fringe fields of ferromagnetic stripes via the Zeeman interaction can be used to generate a spin ratchet effect in a coherent mesoscopic conductor subject to an adiabatic ac driving with finite bias
Summary
We consider a quantum wire in the x-direction embedded in a 2DEG in the (x, y)-plane. Contrary to the charge current that obeys a continuity equation, the spin current can take different values if evaluated in the left or in the right lead This usually happens in systems where the Hamiltonian does not commute with the Pauli matrices σ giving rise to a torque inside the scattering region [37], which can change the spin state of the electrons. Here we employ heuristic models for g(x, y; U ), assuming that the voltage primarily drops in regions, where the magnetic field strongly varies spatially This model is based on the fact that the corresponding Zeeman term acts as an effective potential barrier, and takes into account that a more rapid potential variation leads to enhanced wave reflection and to a steeper local voltage drop [41] (details will be given ). This is used to calculate the elements of the scattering matrix of the system via lattice Green’s functions and a recursive Green’s function algorithm [42]
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