Decoupling excitations in the far-infrared spectra of ac-axisYBa2Cu3O7−δsingle crystal

The infrared reflection spectra of a ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ single-crystal 5×7-${\mathrm{mm}}^2$ surface containing the c axis have been recorded from 4 K to room temperature. The spectra obtained for different values of δ with the same sample show unambiguously that an extra band, coupled with phonons, appears at low frequencies in the superconducting phase. By decoupling this excitation from the other contributions to the dielectric function, we show that the thermal evolution of the extra band follows 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 state is discussed. © 1996 The American Physical Society.