| Popis: |
The temperature and frequency dependence of the $^{11}\mathrm{B}$ nuclear spin-lattice relaxation time in amorphous and crystalline ${\mathrm{B}}_{2}$${\mathrm{O}}_{3}$ has been investigated by pulsed and continuous-wave NMR. A ${T}_{1}$ minimum with unusual properties has been discovered at \ensuremath{\sim} 300 K. A theory is developed, based on the existence of low-frequency phonons in amorphous materials whose lifetimes are limited by structural relaxation of two-level defects, which explains many of the experimental properties of the relaxation process. An average activation energy $\frac{E}{k}=3150$ K for the two-level defects in the glass is obtained. Portis's theory of rf saturation in inhomogeneously broadened resonance lines is extended, and applied to the saturation curves of ${\mathrm{B}}_{2}$${\mathrm{O}}_{3}$. |
