Far-infrared ellipsometric study of the spectral gap in thec-axis conductivity ofY1−xCaxBa2Cu3O7−δcrystals

Abstract

The temperature (T) and doping dependence of the spectral gap in the far-infrared c-axis conductivity of ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ 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.