DISLOCATION MECHANISM OF ACOUSTIC ANOMALIES IN SUPERCONDUCTING NIOBIUM

The temperature and magnetic field dependences of low-frequency ultras ound absorption in niobium single crystals of different purity have be en investigated in the temperature range 2 to 12 K. It is shown for th e first time that the unusual sound absorption behaviour, which was pr eviously observed in niobium at the transition from the superconductin g (s-) to the normal (n-) state (via the mixed (m-) state), can be exp lained by the presence of an acoustic relaxation peak at these tempera tures and by an anomalous shift of this peak during the s-m-n transiti on. A theory of the relaxation based on the model of resonant interact ion of sound with dislocation kink chains, generated in screw dislocat ions under the action of thermoelastic stresses during rapid sample co oling, is suggested.