Complex variation of electron momentum distribution in (Bi,Pb)-2223 oxides across the superconducting transition

High Tc superconductors like (Bi,Pb)-2223 show an abrupt change, across Tc, in the S-parameter, calculated from Doppler broadened positron annihilation radiation lineshape (DBPARL) data. The S-parameter gives the fraction of low momentum electrons. Here we report the new feature of a sharp fall in the S-parameter on cooling the sample to its Tc (R=0) and a subsequent complex but partial rise on further cooling. The detailed observations and possible implications are discussed. (orig.)