  Nuclear spins are polarized electrically in a breakdown regime of an
odd-integer quantum Hall effect (QHE). Electron excitation to the upper Landau
subband with the opposite spin polarity flips nuclear spins through the
hyperfine interaction. The polarized nuclear spins reduce the spin-splitting
energy and accelerate the QHE breakdown. The Knight shift of the nuclear spins
is also measured by tuning electron density during the irradiation of
radio-frequency magnetic fields.
