Spin Thermometry: A Straightforward Measure of Millikelvin Deuterium Spin Temperatures Achieved by Dynamic Nuclear Polarization.

Dynamic nuclear polarization of samples at low temperatures, typically between 1.2 and 4.2 K, allows one to achieve spin temperatures of as low as 2 mK so that for many nuclear isotopes the high-temperature approximation is violated for the nuclear Zeeman interaction. This leads to characteristic asymmetries in powder spectra. We show that the line shapes due to the quadrupolar couplings of deuterium spins present in virtually all solvents used for such experiments (DNP juice) allow the quick yet accurate determination of the deuterium spin temperature or, equivalently, the deuterium polarization. The observation of quadrupolar echoes excited by small flip-angle pulses allows one to monitor the build-up and decay of the positive or negative deuterium polarization.