Knowledge Bank

Optical and infrared absorption and photoinduced absorption (PA) spectroscopics have been applied to the high-$Tc$ families of materials. These techniques give insight into the types of charged excitations existing in these materials, as well as the nature of their ground states and mechanisms for superconductivity. Near-steady-state optical PA spectra on thin films and pressed pellets of $YBa2Cu3O7-6$ for $\delta \sim 0.7$ show PA peaks around 1 eV and a crossover to photoinduced bleaching (PB) at higher energies. Variability of the data from sample to sample may indicate the glassy nature of these materials. The infrared PA spectra reveal several photoinduced infrared active vibrational (IRAV) modes and a broad electronic transition centered at $\sim 1000cm-1$. The presence of the photoinduced IRAV modes and the higher-lying absorptions and bleachings shows that charged defect states lying within the energy gap are created upon photoexcitation. Several of the photoinduced IRAV modes indicate a loss of symmetry and a tetragonal-to-orthorhombic transition that occurs upon photocxcitation, involving a possible charge transfer from the $CuO2$ plane layers to the CuO chain layers. The variation of the IR and photoinduced IRAV modes with oxygen isotope substitution is in accord with expectations of the usual mass effects for vibrational modes.