A model for the photoinduced infrared absorption line shape in YBa2Cu3O6.25

Abstract

The lineshape of photoinduced infrared absorption in YBa 2 Cu 3 O 6.25 observed by Kim et al. is interpreted in terms of transitions to a small-polaron band from a defect level just below this band. Simplifications is the theory include 9i) use of a single effective phonon frequency appropriate for the problem, and (ii) very simple forms of broadening of the δ-function peaks in the absorption which would occur if there were no phonon dispersion and zero polaron bandwidth. After we fix the phonon energy at 0.060 eV, a value between the energies of two photoinduced phonon modes modes observed by Taliani et al., there are four parameters which occur in fitting the data. These are: (1) a parameter proportional to the strength of the absorption; (2) the small-polaron bandwidth, W; (3) the defect binding energy, B, below the bottom of the polaron band; (4) a parameter S giving the preferred number of phonons to be emitted when a photon is absorbed. The best weighted fit to thirteen points taken approximately from a curve in a manuscript by Foster et al. gives W = (0.075 ± 0.019) eV, B = (0.013 ± 0.014) eV and S = 2.57 ± 0.11.