Unconventional features of oxide HTSC from positron annihilation measurements

Our recent measurements of positron lifetime (related to electron density a t the annihilating sites) components or tau(i) and Doppler broadening of po sitron annihilation radiation lineshape or S-parameter (denoting the fracti on of low-momentum electrons among the annihilating electrons) on Bi-2212 a nd (Bi,Pb)-2223 pellets and Y-123 single crystals firstly confirm step-like changes across T-c. Our second and new observation is that S versus T grap h in the superconducting transition region shows a double minimum for 2223 and 123 and a single minimum for 2212. These steps and minima. not seen for superconductors like Nb3Sn, are linked to such features of HTSC electronic structure as are not found in conventional superconductors. (C) 2000 Elsev ier science B.V. All rights reserved.