  In mesoscopic systems with spin-orbit coupling, spin-injection into quantum
dots at zero magnetic field is expected under a wide range of conditions.
However, up to now, a viable approach for experimentally identifying such
injection has been lacking. We show that electron spin injection into a
spin-blockaded double quantum dot is dramatically manifested in the breaking of
magnetic- field-inversion symmetry of nuclear polarization instabilities. Over
a wide range of parameters, the asymmetry between positive and negative
instability fields is extremely sensitive to the injected electron spin
polarization and allows for the detection of even very weak spin injection.
This phenomenon may be used to investigate the mechanisms of spin transport,
and may hold implications for spin-based information processing.
