Optical cooling of nuclear spins in GaAs/(Al,Ga)As quantum wells at subkelvin temperatures: Evidence of the dynamic self-polarization of nuclear spins

Abstract

We investigate the dynamic polarization of nuclear spins in a nominally undoped GaAs/Al$_{0.35}$Ga$_{0.65}$As quantum well using two complementary experimental approaches: time-resolved Kerr rotation and optical orientation measurements of photoluminescence. Using the first technique, we measure a remarkably large Overhauser field of 3.1 T in a geometry close to the Faraday configuration for a 19.7 nm wide quantum well at a temperature of 1.6 K. A nuclear spin temperature of 6.4 $\mu$K is measured at an external magnetic field of 0.006 T following an adiabatic sweep from 0.6 T. Despite the quadrupole-induced nuclear spin splitting inherent to nanostructures, the nuclear spin system is found to follow the predictions of spin temperature theory. Using the optical orientation of the photoluminescence, we investigated nuclear spin dynamics at millikelvin temperatures down to 300 mK. At a temperature of 500 mK, an Overhauser field of 160 mT is generated in an oblique but nearly Voigt magnetic field using low optical power to avoid heating. The nuclear polarization build-up time is of 150 s, consistent with earlier reports at higher temperatures, where hyperfine scattering on free photoexcited electrons governs relaxation. At 500 mK, the onset of dynamic self-polarization of nuclear spins is observed, which becomes more pronounced as the lattice temperature is further reduced to 300 mK. The estimated nuclear spin temperature in the dynamic self-polarization regime can be as low as 200 nK.