  Hyperfine coupling of electron spins to nuclear spins is studied for a
GaAs-based double quantum dot in the spin blockade regime where the electron
conduction is mostly blocked by Pauli effect unless the electron spin state in
the double dot is changed. A current flowing through the double dot shows
time-dependent oscillations with a period of as long as 200 sec in a certain DC
magnetic field range. The oscillatory behavior is significantly diminished by
application of an AC magnetic field whose frequency can induce nuclear magnetic
resonance for 71Ga and 69Ga, respectively. A possible nuclear spin polarization
mechanism due to hyperfine flip-flop scattering is proposed.
