We have developed a new method to measure superconducting stiffness ρ_s  without subjecting the sample to magnetic field or attaching leads [1]. The method is based on the London equation J=-ρ_sA, where J is the current density and A is the vector potential. Using rotor free A and measuring J via the magnetic moment of a superconducting ring, we determine ρ_s. The technique is sensitive to very small stiffness, which translates to penetration depth on the order of 1 mm. Naturally, the method does not suffer from demagnetization factor complications or the presence of vortices. Therefore, the absolute value of the stiffness is obtained. We apply this method to two different La_1.875Sr_0.125CuO₄ rings: one with the current running only in the CuO₂ planes, and another where the current must cross between planes. We find different T_c for the two rings. This leads to new conclusions regarding cuprates phase transition.

References:

[1] arxiv.org/abs/1705.00624