|M.Sc Student||Mangel Itay|
|Subject||Doping Dependent Tc Anisotropy and Coherence Length|
Measurements in LSCO
|Department||Department of Physics||Supervisor||Professor Amit Keren|
|Full Thesis text|
The superconducting stiffness rho?s is the relation between a vector potential A and the current density Js inside superconducting (SC) materials as described by the London equation Js = -rho?s A. The coherence length xi is a measure of how large can Js be. A new way of measuring the superconducting stiffness and coherence length using a Stiffnessometer was developed in our group. The measurement is done by applying current in a thin and long excitation coil that pierces a SC ring-shaped sample, and creates a rotor-free vector potential A inside the sample. According to London's equation, SC currents emerge leading to a magnetic moment, which is measured using a superconducting quantum interference device (SQUID). The new method does not suffer from demagnetization factors complications or the presence of vortices. The method was applied to La2-xSrxCuO4 (LSCO), a member of the cuprates. The crystalline structure of LSCO is roughly tetragonal, with two symmetric directions (a and b) parallel to the CuO2 planes and the c direction perpendicular to the planes. Consequently, the stiffness is anisotropic, and one might expect different response to a vector-potential A parallel or perpendicular to the planes. Upon warming, the stiffness signal diminishes and disappears at Tc. Stiffness measurements for two different rings, one with the CuO2 planes parallel to the ring (c-ring) and another with the planes perpendicular to the ring (a-ring) were done for doping of x=12.5%. It seems as if the phase transition of the c-ring is taking place at a temperature 0.7 K higher than the a-ring.
It is not clear if this result is unique to x=12.5% or a general property of LSCO in all doping. If the transition temperature anisotropy is found only in x=12.5% and its vicinity, it means that the phenomena is related to charge ordering found in this doping. If the Tc anisotropy is found in other doping, it must be a general property related to the two dimensional nature of the CuO2 planes.
In this work I will present stiffness measurements of LSCO a-rings and c-rings with different doping and determine the dependence of this strange anisotropy in Tc on doping. Additionally, by driving the current in the coil until the linearity between A and Js breaks, the critical-current of the SC can be measured in the absence of vortices, and with no leads or out-of-equilibrium conditions. This critical-current places an upper limit on xi. We performed such a measurement using a NbTi SC coil and overdoped LSCO. We found that xi < 4 nm . Future improvements in the experimental setup will allow us to place a tighter bound on xi.