Infrared reflectivity spectra of underdoped and optimally doped Bi2Sr2CaCu2O8+delta thin films

We investigated the ab-plane reflectivity spectra of an optimally doped and a strongly underdoped Bi2Sr2CaCu2O8+delta (Bi-2212) thin film as a functio n of the temperature. The far-infrared (FIR, omega <1000 cm(-1)) spectra ex hibit strong doping dependencies. When the temperature decreases, both samp les show a reflectivity upturn at <omega>-800 cm(-1). This feature starts t o develop at Te for the optimally doped film, whereas it is already observe d at T similar to 6T(C) for the underdoped film. Below T-C, the FIR reflect ivity keeps increasing for the optimally doped film, whereas it is temperat ure independent for the underdoped sample. The structure of these FIR spect ra is analysed, taking into account the measured substrate response. The op ening of a pseudo-gap is discussed.