Evidence of 3D-XY critical behaviour in La_2-xSr_xCuO₄ films

We report measurements of the "zero field" ac sheet impedance Z = R + iwLk for thin, c-axis-oriented La2-xSrxCuO4 films. For sufficiently thin films of thickness d, the magnetic penetration depth λab, is given by Lk = λ2ab/d. We find that the temperature and intriguing doping dependence of Lk as well as the amplitude of the perpendicular real-space phase correlation length ξφc0 are fully consistent with the critical behaviour of the three-dimensional XY model and finite-size scaling. Moreover, invoking finite-size scaling, we determine the value of the critical amplitude of ξφc0.