Dissipation of Flow in SuperfluidHe3-A

Abstract

The first direct measurements of the dissipation due to flow are performed and the effects of that flow on the NMR properties of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-$A$ are studied. A temperature-independent critical velocity $v_c=0.52\mathrm{}$ mm/s is observed, above which a pressure difference proportional to $v_s-v_c$ develops across our flow orifice. Simultaneously, the NMR frequency in ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-$A$ is lowered by an amount also proportional to $v_s-v_c$, in contrast to current theoretical predictions.