  The attenuation of longitudinal ultrasonic wave with the frequency of 30 MHz
in Cu-Nb copper-niobium (40 vol%) composite system at low temperatures from
0.35 K up to 2 K is researched. It was found that the ultrasonic attenuation
decreases in a Cu-Nb multi-filamentary composite sample at low temperatures in
distinction to the pure Cu copper or Nb niobium homogeneous bulk samples. It is
well known that the contact between the normal metal N and the superconductor S
is characterized by an appearance of superconducting properties in the thin
surface layer of normal metal N, because of the presence of proximity effect at
low temperatures T. This phenomenon is observed on the temperature dependent
distance \xi_N(T) in the normal metal N at the normal metal - superconductor NS
boundary. It is assumed that the transition from the normal state N to the
superconducting state S must be accompanied by the decrease of magnitude of
ultrasonic wave's electronic energy absorption in normal metal N. The
experimental results show that the superconducting electron coherence length in
the normal thin layer is temperature dependent \xi_N(T). The electron mean free
path l, which is dependent on the impurity scattering in volume and near Cu-Nb
interfaces, was measured by the three independent methods at low temperatures.
It is understood that the Andreev reflections are present in S-N-S structure,
making some influence on the measured ultrasonic longitudinal wave attenuation
in Cu-Nb composite samples at ultra low temperatures. The measured experimental
results of ultrasonic longitudinal wave attenuation in Cu-Nb composite samples
at low temperatures are in good agreement with the theoretical modeling data,
obtained in case of dirty local limit.
