Nuclear spin-lattice relaxation by optically bistable defects inCdF2:In

Nuclear spin-lattice relaxation rates have been measured for ${}^{113}\mathrm{Cd}$ and ${}^{19}\mathrm{F}$ in ${\mathrm{CdF}}_{2}$ containing optically bistable In impurities. In the present work, relaxation rates were measured from 25 K to 500 K for ${}^{113}\mathrm{Cd}$ and from 200 K to 400 K for ${}^{19}\mathrm{F}.$ The rates for both species are strongly enhanced in semiconducting ${\mathrm{CdF}}_{2}:\mathrm{In}$ compared with undoped materials. Except below 100 K, nuclear relaxation is attributed to relaxation by thermally excited conduction electrons for ${}^{113}\mathrm{Cd}$ and thermally excited hydrogenic shallow donor states ${\mathrm{for}}^{19}$F. From the temperature dependences of the relaxation rates, activation energies of $185\ifmmode\pm\else\textpm\fi{}10 \mathrm{meV}$ and $70\ifmmode\pm\else\textpm\fi{}10 \mathrm{meV}$ are inferred for the concentrations of conduction electrons and hydrogenic states, respectively. These results are consistent with the $\mathrm{DX}$ model of the bistable impurities and with binding energies determined previously. Following in situ white light illumination below 100 K, persistent photoenhancement of the ${}^{113}\mathrm{Cd}$ relaxation rates was observed and attributed to metastable photogenerated shallow donor states.