NMR Studies of 3He Impurities in 4He in the Proposed Supersolid Phase

Abstract

Preliminary results are reported for measurements of the NMR relaxation times of very dilute 3He in samples of solid 4He at low temperature, 0.25 K <T< 1.3 K. The results were obtained for carefully prepared samples with different 3He concentrations. The measurements of the spin-spin relaxation time, T2, show several interesting features. A temperature independent plateau attributed to the exchange motional narrowing is observed down to the lowest temperature studied, and the observed variation of T2 with 3He concentration favors the nonlinear theory suggested by Landesman. The best fit to the data is given by T2∝x3−1.89±0.1 rather than x3−1. No evidence of an exchange-phonon bottleneck for the spin-lattice relaxation is seen down to 25 mK. The vacancy activation energy is determined to be 13.5±0.3 K for a sample with x3=5×10−4 and molar volume 20.9 cm3.