  The electron spin in a semiconductor quantum dot can be coherently controlled
by an external electric field, an effect called electric-dipole spin resonance
(EDSR). Several mechanisms can give rise to the EDSR effect, among which there
is a hyperfine mechanism, where the spin-electric coupling is mediated by the
electron-nucleus hyperfine interaction. Here, we investigate the influence of
frequency modulation (FM) on the spin-flip efficiency. Our results reveal that
FM plays an important role in the hyperfine mechanism. Without FM, the electric
field almost cannot flip the electron spin; the spin-flip probability is only
about 20%. While under FM, the spin-flip probability can be improved to
approximately 70%. In particular, we find that the modulation amplitude has a
lower bound, which is related to the width of the fluctuated hyperfine field.
