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

The physics of interacting nuclear spins in solids is well interpreted within the nuclear spin temperature concept. A common approach to cooling the nuclear spin system is adiabatic demagnetization of the initial, optically created, nuclear spin polarization. Here, the selective cooling of As-75 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 lowest nuclear spin temperature achieved is 0.54 mu K. The rotation of 6 kG strong Overhauser field at the As-75 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 As-75 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 manipulation of spin degree of freedom allows the exploration of novel phenomena at the nanoscale. The authors achieved unprecedented submicroKelvin temperatures in the spin system of nuclei embedded in low dimensional semiconductors. A result that opens the way to the investigation of spin-spin interactions and exotic spin-ordered phases.

Automated summary

The physics of interacting nuclear spins in solids is well understood through the concept of nuclear spin temperature. By using adiabatic demagnetization, the nuclear spin system can be cooled, as demonstrated in a nominally undoped GaAs/(Al,Ga)As quantum well where selective cooling of As-75 spins was achieved. This study opens up new possibilities for exploring spin-spin interactions and exotic spin-ordered phases at unprecedented submicroKelvin temperatures in low-dimensional semiconductors.