Doping dependent Tc anisotropy and coherence length measurements in LSCO

The superconducting (SC) stiffness ρ_s is the relation between a vector potential A and the current density J inside a superconductor as described by the London equation J=ρ_sA. A new method of measuring ρ using a Stiffnessometer was recently developed in our group. The measurement is done by applying current in a thin and long coil which pierces a SC ring, thus creates a vector potential A inside the ring, and measuring the emerging current in the ring. We apply the method to LSCO where the crystalline structure is roughly tetragonal, with two equivalent directions (a and b) parallel to the CuO₂ planes, and one in-equivalent direction (c) perpendicular to the CuO₂ plane. Consequently, the stiffness is an-isotropic, and one might expect different response to a vector-potential A parallel to the CuO₂ planes or perpendicular to them. Upon warming, the stiffness signal diminishes and disappears at T_c. Stiffness measurements for two different rings, one with the CuO₂ planes parallel to the ring plane (c-ring) and another with the planes perpendicular to the ring plane (a-ring) were done for different doping. We found doping dependent Tc anisotropy.

When using the Stiffnessometer at a fixed temperature, and driving the solenoid current, the critical-current of the SC can be measured in the absence of vortices and with no leads. This critical-current places an upper limit on the Ginzburg-Landau coherence length ξ. We performed such a measurement using a NbTi SC solenoid and over doped LSCO ring. We found that ξ<4 nm.

The implications of the two findings will be discussed.