Ground-state interplane superconducting coherence length of La1.875Sr0.125CuO4

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 $\ensuremath{\lambda}$ is known, this experiment measures, hypothetically, the coherence length $\ensuremath{\xi}$. 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 $\ensuremath{\xi}$. We then apply the method to two ${\mathrm{La}}_1.875{\mathrm{Sr}}_0.125{\mathrm{CuO}}₄$ rings at $T\ensuremath{\rightarrow}0$. In one, the current flows in the ${\mathrm{CuO}}₂$ planes, hence it is set by ${\ensuremath{\xi}}_ab$. In the other, the current must cross planes and is determined by ${\ensuremath{\xi}}_c$. We find that ${\ensuremath{\xi}}_c=1.3\ifmmode\pm\else\textpm\fi{}0.1$ nm, and ${\ensuremath{\xi}}_ab<2.3\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$, indicating that at low temperatures the Cooper pairs are three dimensional.