Ground-state interplane superconducting coherence length of La_1.875Sr_0.125CuO₄

A long excitation coil piercing a superconducting ring is used to generate an ever increasing persistent current in the ring, until the current destroys the order parameter. Given that the penetration depth lambda is known, this experiment measures, hypothetically, the coherence length xi. We examine various aspects of this theoretically driven hypothesis by testing niobium rings with different dimensions, and by comparing the results to the known values of xi. We then apply the method to two La1.875Sr0.125CuO4 rings at T -> 0. In one, the current flows in the CuO2 planes, hence it is set by xi(ab). In the other, the current must cross planes and is determined by xi(c). We find that xi(c)=1.3 +/- 0.1 nm, and xi(ab)<2.3nm, indicating that at low temperatures the Cooper pairs are three dimensional.