Deep optical cooling of coupled nuclear spin-spin and quadrupole reservoirs in a GaAs/(Al,Ga)As quantum well

TL;DR

This paper demonstrates the optical cooling of $^{75}$As nuclear spins in a GaAs/(Al,Ga)As quantum well to record low temperatures by exploiting coupled spin reservoirs and adiabatic demagnetization.

Contribution

It introduces a method for selective optical cooling of nuclear spins in a semiconductor quantum well, accounting for quadrupole effects and spin reservoir coupling.

Findings

Achieved nuclear spin temperature of 0.54 μK, the lowest in semiconductors.

Demonstrated rotation of Overhauser field at 5.5 MHz.

Showed dependence of heat capacity on magnetic field orientation.

Abstract

The selective cooling of $^{75}$As spins by optical pumping followed by adiabatic demagnetization in the rotating frame is realized in a nominally undoped GaAs/(Al,Ga)As quantum well. The rotation of 6 kG strong Overhauser field at the $^{75}$As Larmor frequency of 5.5 MHz is evidenced by the dynamic Hanle effect. Despite the presence of the quadrupole induced nuclear spin splitting, it is shown that the rotating $^{75}$As magnetization is uniquely determined by the spin temperature of coupled spin-spin and quadrupole reservoirs. The dependence of heat capacity of these reservoirs on the external magnetic field direction with respect to crystal and structure axes is investigated. The lowest nuclear spin temperature achieved is 0.54 $\mu$K, which is the record low value for semiconductors and semiconductor nanostructures.