Mode-locked electron spin dynamics in many-electron quantum dots

Sergej Markmann¹, Christian Reichl², Werner Wegscheider² and Gian Salis¹¹IBM Research-Zurich, Säumerstrasse 4, 8803 Rüschlikon, Switzerland²ETH Zurich, Otto-Stern-Weg 1,8093 Zurich, Switzerland

Electron spin dynamics is investigated in an array of nominally identical GaAs quantum dots which are fabricated by dry etching, have a diameter between 300 and 1800 nm and contain many electrons each. We find the electron spin precession frequency to be locked to the repetition rate of the optical spin excitation. Spin mode-locking has been observed before in singly charged self-assembled quantum dots [1]. Explanations of this effect [2] rely on the excitation of trions where the resulting spin excitation depends on the orientation of the resident electron spin. We expect this effect to be much less pronounced for our larger many-electron dots. Indeed, by depolarizing the nuclear spins in the dots, we find the spin mode-locking to disappear, indicating that hyperfine interaction to nuclear spins focuses the electron spin precession frequency to multiples of the laser repetition rate. We develop a model which takes dynamical nuclear polarization (DNP) into account. The model shows that a saturation in the resonant spin amplification of mode-locked spins leads to a sharp decrease of DNP, thus providing a feed-back mechanism that drives all individual quantum dots into mode-locking. These results demonstrate that mode-locking by nuclear focusing is a much more general effect that appears in a wide range of dot parameters.

[1] A. Greilich, et al., Science 317, 1896-1899 (2007)
[2] I. A. Yugova, et al., PRB 85, 125304 (2012)