Experimental Study of Classical and Quantum Internal Friction in Solid 4He

We measured the dissipation resulting from internal friction in hcp solid 4He at temperatures between 0.8 K and 2.5 K. Solid 4He is contained inside an annular metal cell forming a part of a torsional oscillator. An oscillatory motion of the cell walls applies shear stress on the solid 4He. The resulting shear strain within the solid 4He generates dissipation because of the internal friction. The experimental sensitivity was high enough to detect dissipation caused by internal friction associated with elementary excitations of the solid. At temperatures below 1.6 K, internal friction is associated with diffusion of single point defects responsible for the climb of dislocations. At higher temperatures, the main mechanism of internal friction appears to be associated with phonon exchange between parts of the solid moving relative to each other under the applied shear stress. This particular dissipative mechanism was called “quantum phonon friction” [Popov in Phys. Rev. Lett. 83:1632–1635, 1999]. The physical mechanism associated with this type of friction involves an irreversible transfer of momentum from the phonons to the lattice via an Umklapp process. Our data are in a very good agreement with this model.