LiquidHe4andHe3at negative pressure

The critical pressures ${\mathit{P}}_{\mathit{c}}$ at which liquid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ and ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ each become macroscopically unstable are determined from two kinds of microscopic calculations that reproduce the equation of state. The behavior of the sound velocity c around this critical pressure is analyzed, and the critical exponent in c∝(P-${\mathit{P}}_{\mathit{c}}$ ${)}^{\nu }$ is found to be ν=1/4. A comparison with empirical analysis is also done.