Spin-Lattice Relaxation of Kramers Doublets in Zero External Magnetic Field

We have measured, for the first time, the spin-lattice relaxation rate between hyperfine levels of Kramers-doublet ions dilutely dispersed in crystals in zero external magnetic field. The expected amount of reduction in the rate, at temperatures well below 2°K, relative to the rates at higher fields, is not realized. The measurements have been made for the ${}_{}{}^{143}{}_{}{}^{}\mathrm{Nd}_{}^{+3\mathrm{}}{}_{}{}^{}$ ion substituted dilutely for ${\mathrm{La}}^{+3\mathrm{}}$ in La${\mathrm{Cl}}_3$ crystals.