The effect of the superconducting phase transition on the near-infrared absorption of YBa sub(2)Cu sub(3)O sub(7- delta )

The optical absorption spectrum of YBa sub(2)Cu sub(3)O sub(7- delta ) in the range 2000 to 10,000 cm super(-1) shows a broad absorption band near 5500 cm super(-1) (0.7 eV). In the normal state, the intensity of this absorption band increases with decreasing temperature. When the superconducting phase transition occurs on cooling, we find that the absorption intensity decreases to a constant value at low temperature. It is suggested that this peak may be due to an excitation of normal state charge carriers. The change in optical absorption at T sub(c) could then be due to the Bose condensation of a fraction of the pre-existing bipolarons.