Undamped Second-Sound Waves in aHe3-He4Mixture Heated from Below

By using a thermodynamic analysis, it is shown that a horizontal layer of the /sup 3/He-/sup 4/He superfluid mixture heated from below is unstable with respect to oscillatory convection. The neutral oscillations of this overstability are undamped standing second-sound waves. Estimates show the possibility to observe the predicted effect experimentally in the vicinity of the tricritical point of /sup 3/He-/sup 4/He mixture.