Nuclear Spin Effects in Self-assembled Quantum Dots

Abstract

Th is work present original results of investigation of nuclear spin dynamics in nanostructure with negatively charged InGaAs/GaAs quantum dots characterized by strong quadrupole splitting of nuclear spin sublevels. Th e main method of the investigation is the experimental measurement and theoretical analysis of the photoluminescence polarization as the function of the transverse magnetic fi eld (eff ect Hanle). Th e dependence of the Hanle curve on temporal protocol of excitation is examined. Experimental data are analyzed using an original approach based on separate consideration of behavior of the longitudinal and transverse components of nuclear polarization. Th e rise and decay times of each component of nuclear polarization and their dependence on transverse magnetic fi eld strength is determined. To study the role of the Knight fi eld in the dynamic of nuclear polarization a weak additional magnetic fi eld parallel to the optical axis is used. Th is allows us to control the effi ciency of nuclear spin cooling and the sign of nuclear spin temperature. Th e standard nuclear spin cooling theory fails to describe the experimental Hanle curves in a certain range of control fi elds. Th is controversy is resolved by taking into account the nuclear spin fl uctuations. Th e model allows us to accurately describe the measured Hanle curves and to evaluate the parameters of the electron-nuclear spin system of the studied quantum dots. New eff ect of resonant optical pumping of nuclear spin polarization in an ensemble of singly charged (In, Ga)As/GaAs quantum dots subject to a transverse magnetic fi eld is observed. Electron spin orientation by circularly polarized light with the polarization modulated at the nuclear spin transition frequency is found to create a signifi cant nuclear spin polarization, precessing about the magnetic fi eld. Nuclear spin resonances for all isotopes in the quantum dots are found in that way. In particular, transitions between states splitted off from the 〈±1/2| doublets by the nuclear quadrupole interaction are identifi ed.