Elastic and Acoustic Measurements in Solid 4He

We describe results of elastic and acoustic measurements in solid helium in the range of parameters where supersolidity is observed. A variety of polycrystalline and mono-crystalline samples has been studied. We observed a decrease with temperature of the uniaxial compression modulus in a polycrystalline sample by up to 25 %. Such softening is usually explained in term of decrease of base-plane shear modulus, c (44), of hcp He-4 solid. Our results translates into a decrease of c (44) of the constituent crystallites between 77 % and 82 %, depending on the model. In the acoustic measurements, we attempted to check whether the description of softening via c (44) is appropriate by fitting frequencies of observed resonances to numerical calculations. Neither the measured positions, nor the line shapes of the resonances fit the simulations at low temperatures, indicating that an alternative approach is required.