THE EFFECT OF THE SUPERCONDUCTING PHASE-TRANSITION ON THE NEAR-INFRARED ABSORPTION OF YBA2CU3O7-DELTA

The optical absorption spectrum of YBa2Cu3O7-delta in the range 2000 to 10 000 cm-1 shows a broad absorption band near 5500 cm-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. The change in intensity relative to the normal state clearly demonstrates that this excitation is coupled with the superconducting order parameter. Similar behaviour has also been observed in both zinc-doped and iron-doped YBa2Cu3O7-delta. It is suggested that this peak may be due to an excitation of normal state charge carriers (e.g. a Franck-Condon transition of bipolarons). The change in optical absorption at T(c) could then be due to the Bose condensation of a fraction of the pre-existing bipolarons.