NMR studies on HTS cuprates: Re-assessing the models

Superconducting and pseudogap correlations in a variety of high-T-c superco nductors are investigated using nuclear magnetic resonance (NMR) and accura tely modelled at all doping levels in terms of competing, independent corre lations. We demonstrate the existence of two independent low-energy scales, the complete absence of an isotope effect in the pseudogap, the absence of a well-defined T* at which the pseudogap opens and show that the correlati on length for the pseudogap is very short. The data cast serious doubts on a variety of pseudogap models. (C) 1998 Elsevier Science Ltd. All rights re served.