| We have investigated the superconducting transition of YBa2Cu 3O7−δ films using DC nano-volt level measurements and AC microwave measurements from 45 MHz up to 45 GHz.; In applied fields of about 4 tesla, we find that DC I-V measurements on our films can be collapsed onto scaling functions proposed for continuous-superconducting transitions, as is widely reported in the literature. We find, however, that good data collapse is achieved for a wide range of critical exponents and temperatures. These results strongly suggest that agreement with scaling alone does not prove the existence of a phase transition. To determine if such a data collapse is valid and, thus, if a phase transition occurs, we propose the opposite concavity criterion. To our knowledge, none of the high-field data reported in the literature meet our criterion.; At low magnetic fields, between 0.3 mG and 10.0 G, we also find that good data collapse is achieved for DC I-V measurements in a low-current regime where the opposite concavity criterion is not satisfied. However, at low fields, we find that a high-current regime exists where the criterion is satisfied. This high-current regime also scales but with a different Tc and exponents to those of the low-current region. We compare these two DC results with AC microwave measurements made on the same sample. We find good agreement between the Tc and exponents of the AC measurements with those of the high-current regime, whereas the low-current region shows large deviations. This supports the exponents we obtain in the high-current regime (z = 2.05 ± 0.05, ν = 1.4 ± 0.2) and indicates the importance of satisfying the opposite concavity criterion.; We finish by analyzing DC I-V curves of an ultra-thin 12Å thick YBa2Cu3O7−δ film in the context of dynamic scaling theory and the Kosterlitz-Thouless-Berezinskii (KTB) transition. As with thicker films, the analysis in the two dimensional case is found to be too flexible and not conclusive. We propose a criterion which would give conclusive evidence for a phase transition in two-dimensional superconductors. We conclude by discussing the relevance of this criterion to measurements on high-Tc superconductors. |
