Spin-dependent electron transport properties in a parallel double-dot structure with four terminals are investigated theoretically by means of the non-equilibrium Green function technique. By applying a local Rashba spin-orbit interaction on an individual quantum dot and introducing a finite bias between the longitudinal terminals, it is found that, in the presence of appropriate structure parameters, transverse pure spin currents without an accompanying charge current appear. Moreover, the flow direction of such spin currents can be inverted by altering the structure parameters, i.e. magnetic flux, bias voltage, and values of quantum-dot levels with respect to the zero point of energy.
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