The Coherence Length Dependence on Doping of Cuprates

The superconducting stiffness is the relation between a vector-potential and the current density inside superconducting materials, as described by the London equation. The coherence length is a measure of how large the current density can be. In this work, we measure both of these defining superconducting characteristics using the Stiffnessometer technique, developed in our group. The measurement is done by the creation of a rotor-free vector potential inside the sample, as according to London’s equation, SC currents emerge, leading to a magnetic moment which is measured. This method does not suffer from demagnetization factors or the presence of vortices. Our superconducting subject is the Bi-2212 cuprate, with critical temperature of about 70K. It was doped at different levels in the over-doped regime. We succeeded in measuring the coherence length directly in low temperatures, being a significant challenge using most other methods. The results show the dependence of the stiffness and the coherence length on the doping level.