Mechanism of 3He-mediated nuclear spin-lattice relaxation at surfaces

Abstract

We measured the nuclear spin-lattice relaxation time T 1, of several surface-bound nuclei, 1H, 19F, 11B, 13C, 29Si, and 2H, immersed in liquid 3He over the temperature range 0.01 K ⩽ T < 1 K. The Larmor frequencies of these nuclei in a 3.39 T field extended from 22 to 144 MHz. All T 1 values were temperature-independent and ranged from a few seconds to several hours, depending on the particular nucleus and the surface geometry of the sample. The results indicate that the coupled relaxation of surface spins is a phenomenon occurring in all solids immersed in 3He and thus provides a general mechanism for obtaining high nuclear polarization in solids, that the relaxation is controlled by direct dipole-dipole interactions between the surface spins and 3He in the first surface layer, that the 3He motion dynamics do not change appreciably from one surface to another, and that measurements of T 1 may thus be useful for determining the structure of surfaces.