  We report the hysteresis of optically-pumped nuclear spin polarization and
the degree of circular polarization of photoluminescence on the excitation
power and electron spin polarization in single InAlAs quantum dots. By
increasing (or decreasing) the excitation power at a particular excitation
polarization, an abrupt rise (or drop) and a clear hysteretic behavior were
observed in the Overhauser shift of the photoluminescence of the exciton and
exciton complexes from the same single quantum dot under an external magnetic
field of 5 T. However, the degree of circular polarization shows different
behaviors between a positively charged exciton and a neutral exciton or
biexciton; further, only positively charged exciton exhibits the precisely
synchronized change and hysteretic behavior. It is suggested that the electron
spin distribution is affected by the flip-flop of electron-nuclear spins.
Further, the hysteresis is observed as a function of the degree of circular
polarization of the excitation light and its dependence on the excitation power
is studied. The saturation of the Overhauser shift after the abrupt rise
indicates the almost complete cancellation of the external magnetic field by
the nuclear field created within the width that is decided by the correlation
time between the electron and the nuclei spin system.
