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.