Origin of the phonon anomalies in the C-axis optical spectrum of YBa 2Cu 3O 7−δ

Abstract

The c-axis optical spectra of YBa 2Cu 3O 7−δ crystals were investigated, mainly focusing on phonon behaviors. Using the crystals with three different oxygen contents, we observed typical phonon spectra for tetragonal (P4/mmm), orthorhombic I (Pmmm), and possibly orthorhombic II structure. In addition to the anomalous temperature dependences of phonon frequencies reported so far, another type of anomaly was found around 400 cm −1 in the oxygen reduced crystal with T c=50K, which might indicate an appearance of an electronic excitation.