  We studied the zero-field dynamic nuclear spin polarization in a single
In$_{0.75}$Al$_{0.25}$As/Al$_{0.3}$Ga$_{0.7}$As quantum dot. Even without any
external magnetic field, the positive trion excited by the circularly-polarized
light generated an Overhauser field of up to $\sim$0.8 T. From the excitation
power dependences of the Overhauser field and degree of circular polarization
of the photoluminescence spectra, the relation between the Overhauser field and
Knight field under zero external magnetic field was revealed clearly. In
addition, we found that the nuclear depolarization rate decreased as the
magnitude of the longitudinal magnetic field increased, which seemed to be
caused by the influence of the quadrupolar interaction of nuclear spins.
Further, the key parameters describing the dynamics of a coupled
electron-nuclear spin system, the electron g-factor and the fluctuation of the
Overhauser field, were evaluated in a typical single InAlAs quantum dot.

Mesoscale and Nanoscale Physics