Reinterpretation of femtosecond laser pump-probe and thermomodulation optical spectroscopy results on HTSC materials in terms of the resonant negative-Umodel

The laser pump/probe spectroscopy from Stevens et al (on YBCO 7 ) and the thermomodulation optical data and analysis from Holcomb et al (on a selection of HTSC materials) are examined in terms of the resonant negative- U mechanism for high temperature superconductivity (HTSC) in the mixed-valent cuprates. The work centres on the universal 1.5-2 eV features becoming visible below T c upon the optical `charge-transfer' edge. These it is argued are not simple p-to-d band-to-band excitations but are to be associated with the mixed-valent and negative- U states that instigate HTSC. This argument is supported both by the decay characteristics apparent in the new data recorded by Stevens et al and also in the outcome of the Eliashberg analysis developed by Holcomb et al to accommodate their own thermomodulation data. The interpretation provided is in line with that offered recently of the highly characteristic thermoelectric behaviour exhibited by all HTSC systems.