Far-infrared ellipsometric study of the spectral gap in the c-axis conductivity of Y1-xCaxBa2Cu3O7-δ a crystals

The temperature (T) and doping dependence of the spectral gap in the far-infrared c-axis conductivity of Y1-xCaxBa2Cu3O7-delta has been studied by ellipsometry. For underdoped crystals a T-independent spectral gap is observed which persists almost unchanged into the normal state. The normal-state gap disappears at optimum doping whereas a conventional T dependence of the spectral gap is found for overdoped samples. The analogy to the results of angle-resolved photoemission suggests that the c-axis conductivity may be dominated by the carriers at the X point of the two-dimensional Brillouin zone. [S0163-1829(99)50910-7].