Nuclear Magnetic Resonance in Single Crystals of CdS

Abstract

The ${\mathrm{Cd}}^{111}$ and the ${\mathrm{Cd}}^{113}$ NMR lines in CdS single crystals were studied at 4.2\ifmmode^\circ\else\textdegree\fi{}K. The spin-lattice relaxation time ${T}_{1}$ of high-resistivity samples is strongly affected by optical illumination. A typical value of ${T}_{1}$ for a sample cooled in the dark, 3\ifmmode\times\else\texttimes\fi{}${10}^{3}$ min, decreased to 30 min while the sample was being illuminated with white light. If the sample was kept at 4.2\ifmmode^\circ\else\textdegree\fi{}K, ${T}_{1}$ did not return to the original value but remained at about 4\ifmmode\times\else\texttimes\fi{}${10}^{2}$ min after the light was turned off. Bleaching with red light restored ${T}_{1}$ to 1.4\ifmmode\times\else\texttimes\fi{}${10}^{3}$ min. The lifetime ${T}_{1\ensuremath{\rho}}$ of the nuclear-spin polarization in the rotating frame was measured as a function of rf field strength ${H}_{1}$ under various illumination conditions. ${T}_{1\ensuremath{\rho}}$ decreases very rapidly when ${H}_{1}$ is decreased below 1 G. Light-created trapped paramagnetic centers are mainly responsible for these relaxation effects. In order to fit the data with nuclear-spin diffusion theory, it was necessary to consider at least three different types of trapped centers. The density of these centers under various illumination conditions was estimated. NMR signals of nuclei near the trapped centers were observed using a double-resonance technique in the rotary frame. A strong signal of ${\mathrm{S}}^{33}$ was also found using this technique.