Thermal expansion study of Bi-2223 superconductor: An evidence of polaronic mechanism of high T < inf > c < /inf > superconductivity with strong two-dimensional fluctuations

Abstract

Thermal expansion measurements have been performed on a Bi-2223 superconductor both in its normal and superconducting phases. It is shown that besides a change of slope observed at Tc, there is an additional slope change in the temperature dependence of the thermal expansion of this system, which occurs around a temperature that is somewhat higher than Tc. We attribute this anomaly to a lattice instability induced by polaron formation, which gives rise to large enhancement in the phonon anharmonicities. We argue that superconductivity in this system may be induced by dynamical Cooper-pairing of anharmonic polarons with strong two-dimensional (2D) Gaussian fluctuations.