SPLITTING OF THE BULK RESISTIVE TRANSITION IN HIGH-T-C SUPERCONDUCTORS - EVIDENCE FOR UNCONVENTIONAL PAIRING

We report on measurements of the resistive transition in several samples of polycrystalline YBa2Cu3O7-y and Bi2Sr2Ca2Cu3O10+x, an single crystal Bi2Sr2CaCu2O8+x. The results are analyzed in terms of the temperature derivative of the resistivity, d rho/dT. In the ceramic samples, we systematically find that the transition proceeds as a two-stage process. Initially, superconductivity establishes in some disconnected regions of the homogeneous phase. Long range ordering is obtained at lower temperatures as a consequence of a paracoherent-coherent transition involving the granular array. A new, reproducible and general feature is also revealed in our results. The principal maximum of d rho/dT, which denotes the intragranular transition, shows a two-peak splitting. This characteristic, which is observed in all investigated samples strongly suggests the occurrence of a double bulk superconducting transition, which is in itself a signature of unconventional order parameter symmetry.