Nuclear spin-lattice relaxation in amorphous and crystallineB2O3

Abstract

The temperature and frequency dependence of the ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ nuclear spin-lattice relaxation time in amorphous and crystalline ${\mathrm{B}}_2$ ${\mathrm{O}}_3$ has been investigated by pulsed and continuous-wave NMR. A $T_1$ minimum with unusual properties has been discovered at ∼ 300 K. A theory is developed, based on the existence of low-frequency phonons in amorphous materials whose lifetimes are limited by structural relaxation of two-level defects, which explains many of the experimental properties of the relaxation process. An average activation energy $\frac{E}{k}=3150\mathrm{}$ K for the two-level defects in the glass is obtained. Portis's theory of rf saturation in inhomogeneously broadened resonance lines is extended, and applied to the saturation curves of ${\mathrm{B}}_2$ ${\mathrm{O}}_3$.