Mechanism of nuclear spin-lattice relaxation in insulators at very low temperatures.

Abstract

Nuclear spin-lattice relaxation by paramagnetic impurities in insulating crystals is expected in conventional theory to be frozen out in the millikelvin temperature region owing to the freezing of the electron-spin orientation into the lowest-energy spin state in the presence of a static field. Experimental relaxation times are much shorter than the conventional theory predicts. The authors show that by extending the normal relaxation theory to include the slight change in electron-spin quantization arising from a nuclear spin flip, the electron effectively gains a new degree of freedom, which we call wobble, which takes much less energy to excite than a complete electron-spin flip. The electron-spin orientation is, therefore, effectively unfrozen even at millikelvin temperatures. This new mechanism is expected to dominate in high field at temperatures of tens of millikelvin and below.