ON THE LINEAR TEMPERATURE-DEPENDENCE OF THE SOUND-VELOCITY OF POLYCRYSTALLINE METALS ABOVE 1 K

We have investigated the relative change of the sound velocity of polycrystalline metals at frequencies 0.1 kHz less than or equal to nu less than or equal to 20 kHz and at temperatures 0.04 K less than or equal to T less than or equal to 10 K. For all metals investigated the relative change of the sound velocity decreases linearly with temperature above similar to 1 K. We show that the temperature coefficient beta of this linear dependence is directly proportional to the slope C obtained from the logarithmic temperature dependence below 1 K. This result is in agreement with theoretical results recently proposed by Neu and Wurger based on a new relaxation mechanism for tunnelling systems. Moreover, we show that the universal behaviour between beta and the elastic modulus recently found by Nava for amorphous solids also holds for polycrystalline metals and can be explained within the same model.