T h e optical absorption spectrum of YBa,Cu,O,., in the range 2000 to 10000 cm-' shows a broad absorption band near 5500 cm-' (0.7 eV). In the normal state. the intensity of this absorption band increases with decreasing temperature. When t h e 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 YBa,Cu,O,.,.It is suggested that this peak may be due to an excitation of normal State charge carriers (e.g. a FranckkCondon transition of bipolarons). The change in optical absorption at T, could then be due to the Bose condensation of a fraction of the pre-existing bipolarons.
The E//c far-infrared reflection spectra of a YBa2Cu3O7 single crystal were fitted for three different temperatures (10 K, 60 K and 100 K) using phononic oscillators, a mid-infrared excitation and a low-energy plasmon. The plasmon frequency changes from 600 cm-1 in the normal state to 365 cm-1 in the superconducting state. Some consequences of strong plasmon-phonon interaction for the pairing mechanism are discussed.
AbslmcLWe have measured the optical absorption speclrum of YBa2Cb01-6 crystalline thin films and powders in the range 2013) 10 10000 ."I.A broad absorption band near 5500 cm-' (0.7 eV) b obsen'ed, which has the same absorption profile in both cases. Our work on lhin-film samples shows lhal th$ excitation causing this absorption is polarized in the ab plane. Funher work on doped powder samples provides evidence lhat the excitation is localized in the CuO2 planes of the slruclure. which are known 10
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