Magnetic Field Dependence of the London Penetration Depth of Bi2Sr2CaCu2Oy

Abstract

London penetration depth ${\lambda }_L$ was investigated as a function of dc magnetic field $H$ in the Meissner state of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_y$. At low temperatures, ${\lambda }_L$ changes linearly in $H$, which is in contrast to the $H^2$ behavior observed in conventional superconductors. With increasing temperature, systematic rounding around zero field was observed. Together with the comparative study for a conventional superconductor ${\mathrm{V}}_3$Si, it is suggested that the superconducting state of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_y$ is unconventional. These findings are the first confirmation of a recent new theoretical prediction for $d$-wave superconductors.