  We propose a new mechanism for polarizing nuclear spins in quantum dots,
based on periodic modulation of the hyperfine coupling by electric driving at
the electron spin resonance frequency. Dynamical nuclear polarization results
from resonant excitation rather than hyperfine relaxation mediated by a thermal
bath, and thus is not subject to Overhauser-like detailed balance constraints.
This allows polarization in the direction opposite to that expected from the
Overhauser effect. Competition of the electrically-driven and bath assisted
mechanisms can give rise to spatial modulation and sign reversal of
polarization on a scale smaller than the electron confinement radius in the
dot. The relation to reverse Overhauser polarization observed in GaAs quantum
dots is discussed.
