Magnetic penetration depth in superconductingLa2−xSrxCuO4films

We have measured the magnetic penetration depth $\lambda \mathrm{}\left(T\right)\mathrm{}$ in a series of superconducting ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ films with Sr concentrations from $x=0.135\mathrm{}$ to 0.175. ${\lambda }^{-2}\mathrm{}\left(T\right)\mathrm{}-{\lambda }^{-2}\mathrm{}\left(0\right)\mathrm{}$ is quadratic in T for $0.4<T<\mathrm{}10\mathrm{}$ K, which puts an upper limit of about 4 K on a possible isotropic gap. The larger magnitude of $\lambda \mathrm{}\left(0\right),$ reduced $T_C,$ and higher resistivity of films relative to bulk samples, plus the $T^2$ behavior of ${\lambda }^{-2}\mathrm{}\left(T\right)\mathrm{}-{\lambda }^{-2}\mathrm{}\left(0\right),$ lead naturally to the conclusion that superconductivity in ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ is d wave. Near $T_C$ the real part of the conductivity and $\lambda \mathrm{}\left(T\right)\mathrm{}$ are analyzed for critical behavior. The data indicate that the critical region is no wider than 1 K.