Microwave dielectric echoes in Na β -alumina

Microwave dielectric echoes have been observed in Na β-alumina at temperatures between 18 and 75 mK. The results provide further evidence for the existence of low-energy two-level tunneling systems (TLS). The TLS electric dipole moment p and density of states P̄ were determined to be p=4.8±0.8 D and P̄=1.5×10³² erg^-1 cm^-3, respectively. The nonexponential time-dependent echo decay provides evidence for spectral diffusion. Previously proposed models incorporating spectral diffusion processes in echo decay are shown to approximate the observed time-decay rates at a given temperature. However, the observed dependence of the echo decay on temperature was much weaker than predicted by spectral diffusion models.