Anomalous temperature dependence ofD andT 2 for dilute solutions of3He in solid4He

Abstract

Results of measurements of the spin diffusion coefficientD and NMR relaxation timesT1,T2, and T1p are presented for a range of fractional3He concentrations 1 × 10−4≤x3≤2.5 × 10−3 in solid4He at molar volumes 19.85≤Vm≤21.0 cm3 and temperatures 0.4<T<2 K in both hcp and bcc phases. We observe a minimumD(T) atx3=5×10−4, which is interpreted in terms of a transition from coherent impuriton motion to thermally activated diffusion. ForT<0.8 K, ∂(lnD)/∂(lnVm)=60±8. TheT2 measurements show a minimum as a function of temperature forx3≤10−3. TheT2(T) andT1(T) results yield values for activation energy and tunneling frequency of vacancies in these dilute solutions. Forx3=5×10−4 andT ∼ 0.5 K,T2(Vm) is anomalous.T1ρ measurements at the same concentration indicate there is an important contribution to the spectral density of dipole field fluctuations in the kHz region.