Effects of Vortex Pinning and Thermal Fluctuations on the Josephson Plasma Resonance in Tl2Ba2CaCu2O8 and YBa2Cu3O6.5

Infrared spectroscopy has been used to investigate the temperature dependence and c-axis magnetic field dependence of the Josephson plasma resonance in optimally doped Tl2Ba2CaCu2O8 thin films, and underdoped YBa2Cu3O6.5 ortho - II single crystals. The resonances are directly related to the c-axis penetration depths, yielding low temperature, zero fields values of about 20 micrometer, and 7 micrometer respectively. While the temperature dependencies of the resonances in the two compounds are very similar, the magnetic field dependence in YBCO is much weaker. We attribute this weak magnetic field dependence to the lower anisotropy of YBCO, and discuss the observed behaviours in terms of thermal fluctuations and pinning of pancake vortices, valid for highly anisotropic layered superconductors.