A previous experiment in this laboratory found that the addition of a few tens of parts per ${10}^6$ of ${}^{\mathrm{3}}\mathrm{H}\mathrm{e}$ impurities to solid ${}^{\mathrm{4}}\mathrm{H}\mathrm{e}$ crystals resulted in a phase transition below 200 mK. In the experiment described here the interaction of acoustic waves with waves generated by heat pulses is measured and shows that there is an energy gap in the excitations of the high temperature phase and that coherent waves are carried by nonphonon excitations at the lowest temperatures.

• Received 31 January 2002

DOI:https://doi.org/10.1103/PhysRevLett.89.095301