Low-temperature studies of the NMR frequency shift in superfluidA3

We report the first measurements of the longitudinal resonance frequency [${\nu }_{\mathrm{long}}$(A)] in ${}_{}{}^3{}_{}{}^{}\mathit{A}$ in the low-temperature limit, as well as new measurements near ${\mathit{T}}_{\mathit{c}}$. In the low-temperature limit, our data agree well with theoretically predicted behavior. The data allow us to make the first estimates of the BCS cutoff energy as a function of pressure and of the zero-temperature A-phase energy gap.