Observation of Decrease of Supercurrent in the Gapless Region

The dependence of superfluid current density $J$ on superfluid velocity $v$ was measured in a hollow super-conducting cylinder of indium placed in an axial low-frequency magnetic field. In order to enter the $v$ region for which a decrease of $J$ as a function of $v$ is predicted, the interior of the cylinder was filled with a superconducting rod. The measured quantities were the flux ${\phi }_J$ arising from $J$ and the magnitude $H_e$ of the low-frequency magnetic field which is related to $v$. The experimental results are graphs showing ${\phi }_J$ as a function of $H_e$. At temperatures close to the transition temperature of the film, the ${\phi }_J-H_e$ graphs were linear for small $H_e$, whereas for larger $H_e$, a maximum in ${\phi }_J$ followed by a decrease was clearly observed. The temperature dependence and the maximum value of ${\phi }_J$ were found—within experimental accuracy—to be in agreement with the predictions of the Ginzburg-Landau theory.