OPTICAL-PROPERTIES ALONG THE C-AXIS OF YBA2CU3O6+X, FOR X=0.50-]0.95 - EVOLUTION OF THE PSEUDOGAP

The reflectance of high-quality, unpolished single crystals of YBa2Cu3O6+x, for the doping range x = 0.50 --> 0.95, has been measured with radiation polarized along the c-axis from approximate to 50 cm(-1) to 5000 cm(-1) between 10 K and 300 K. In highly doped (x = 0.95) material, the normal-state conductivity shows a metallic response. For intermediate dopings (x = 0.85 --> 0.90) the conductivity is no longer metallic, increasing with temperature, and for low dopings (x = 0.50 --> 0.80) this behavior is clearly seen to be caused by a broad pseudogap at approximate to 290 cm(-1), which develops well above T-c. In the superconducting state, in the optimally doped material, a gap-like depression develops below T-c but there is residual conductivity to very low frequency indicating either an anisotropic gap, or a gap with nodes. In the underdoped materials the superconductivity-induced changes to the conductivity are harder to see, but it appears that the weight of the condensate originates at frequencies much higher than the conventional weak-coupling BCS value of 3.5k(B)T(c).