Pumping and Focusing Pure Spin
C.M. Marcus, B.I. Halperin

With growing interest in the spin of electrons comes the need for a spin battery, analogous to a conventional (charge) battery for creating voltages in a circuit. We have used quantum coherence to produce a pump that can independently pump spins of different orientation (with respect to an externally applied magnetic field). When the two independent currents — one for spin-up and one for spin-down — exactly cancel, then no charge is pumped. However, this condition means that a spin-down current is moving to the left, say, and an equal spin-up current to the right. We have realized such a "pure" spin pump. What’s more, we have taken the spin current and made it bend around a loop, focus it, and inject it into a narrow gap.

The figure shows in the top frame total pumped current using a mesoscopic quantum pump. The bottom frame shows spin sensitive (green) and spin insensitive (black) current detection (bottom frame, black) measured at the same time as the current measured in the top frame. At zeros of total current, the spin dependent current is nonzero. At these points a pure spin current is being pumped. Top inset shows a schematic of the cyclic pumping cycle needed to make a charge or spin pump. The bottom inset shows the measurement set-up where a pumped dot is embedded in a focusing geometry. The quantum point contact on the collector can be made spin sensitive (in an external field) by adjusting the gate voltage to have partial transmission.