DECOUPLING EXCITATIONS IN THE FAR-INFRARED SPECTRA OF A C-AXIS YBA2CU3O7-DELTA SINGLE-CRYSTAL

The infrared reflection spectra of a YBa2Cu3O7-delta single-crystal 5 X 7-mm(2) surface containing the c axis have been recorded from 4 K to room temperature. The spectra obtained for different values of delta with the same sample show unambiguously that an extra band, coupled wi th phonons, appears at low frequencies in the superconducting phase. B y decoupling this excitation from the other contributions to the diele ctric function, we show that the thermal evolution of the extra band f ollows the expected behavior for the superconducting order parameter. The excitation energy is characteristic of Cooper pairs formation. The residual low-frequency optical conductivity in the superconducting st ate is discussed.