Hysteretic response of the electron-nuclear spin system in single In0.75Al0.25As quantum dots:: Dependences on excitation power and polarization

Optically pumped nuclear spin polarization in single InAlAs quantum dots was investigated in detail with regards to the dependences on the excitation power and electron spin polarization. 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 under an external magnetic field of 5 T. Those features appear equivalently for the exciton and exciton complexes of the same quantum dot since the created nuclear field is equally effective. However, the degree of circular polarization shows different behaviors between a positively charged exciton and a neutral exciton or biexciton; further, only the 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 indicates the almost complete cancellation of the external magnetic field by the nuclear field.