ABSTRACT
The large coherence length of conventional superconductors is a feature of bcs theory, and it arises through the large size of the pair wavefunction and not from the range of the forces of interaction between the pairs, which is much smaller. In conventional superconductors the excitations are usually scattered chiefly by impurities. For such elastic scattering the mean free path is the same as in the normal state and is independent of temperature: many calculations based on bcs theory. The magnetic penetration depth in the cuprates has been obtained by various techniques, such as measurement of the susceptibility of powders of known size distribution, observations of frequency shift in microwave resonators, and measurements of the spin resonance profile for trapped muons in the mixed state. In principle, tunnelling can give so much useful information that it should be the most important single diagnostic tool in assessing details of microscopic theories for the cuprates.
