Probing single-crystalline YBa2Cu3O7 across the superconducting transitiontemperature by positron annihilation measurements

Shape parameter S determined from the Doppler broadening of the positron an nihilation radiation line-shape measurement (DBPARL), gives the fraction of suitably defined low momentum electrons among the annihilating electrons. Since BCS pairing in momentum space is accepted for the so-called conventio nal superconductors, such studies of possible changes in the electron momen tum distribution as a function of temperature across the superconducting tr ansition in high-temperature superconductors (HTSC) may help clarify the ye t unknown mechanism of superconducting pairing in HTSC. Our S vs T data at close intervals (down to I K) of temperature revealed one minimum at T-c in Bi-2212 pellets and two minima in the T-c region in (Bi,Pb)-2223 pellets, in addition to the somewhat better known steplike change at T-c. Similar DB PARL measurements presently on Y-123 single crystals show the S vs T minima in these as well as in previous (Bi,Pb)-2223 pellets to be at T-c and 0.95 T(c). These observations can imply a redistribution of electron momentum si milar to the better discussed charge redistribution.