Effects of cortex pinning and thermal fluctuations on the Josephson plasmaresonance in Tl2Ba2CaCu2O8 and YBa2Cu3O6.5

We investigated the temperature dependence and c-axis magnetic field depend ence of the Josephson plasma resonance in optimally doped Tl2Ba2CaCu2O8 thi n films and underdoped YBa2Cu3O6.5 (YBCO) ortho-II single crystals using in frared spectroscopy. We obtained the c-axis penetration depths, at low temp erature, in zero fields of about 20 and 7 mum, respectively. While the temp erature dependencies of the resonances in the two compounds are very simila r, the magnetic field dependence in YBCO is much weaker. We attribute this weak magnetic field dependence to the lower anisotropy of YBCO and interpre t the observed behaviors in terms of thermal fluctuations and pinning of pa ncake vortices.