Pure spin currents are shown to be generated by an electrically controlled
quantum pump applied at the edges of a topological insulator. The electric
rather than the more conventional magnetic control offers several advantages
and avoids, in particular, the necessity of delicate control of magnetization
dynamics over tiny regions. The pump is implemented by pinching the sample at
two quantum point contacts and phase modulating two external gate voltages
between them. The spin current is generated for the full range of parameters.
On the other hand, pumping via amplitude modulation of the inter-boundary
couplings generates both charge and spin currents, with a pure charge current
appearing only for special values of the parameters for which the Bohm-Aharonov
flux takes integer values. Our setup can therefore serve to fingerprint the
helical nature of the edges states with the zeros of the pumped spin and charge
currents occurring at distinct universal locations where the Fabry-Perot or the
Aharonov-Bohm phases take integer values.Comment: 5 pages, 5figure
