Acoustic plasmon velocity from Raman scattering lineshapes

Abstract

The temperature dependence of the 120 and 340 cm −1 phonons and the low frequency Raman background continuum in YBa 2Cu 3O 7 are interpreted in terms of an acoustic plasmon with temperature-dependent sound velocity. We argue that the observed phonon lineshapes and positions together with the intensity redistribution in the Raman background cannot be fully explained in terms of an energy gap. We find the gap interpretation unsatisfactory, because even though the energy gap can explain some Raman results below T c, the normal state Raman properties, which are very similar to the ones in the superconducting state, remain unexplained. We propose that the observed T-dependence of the Raman background continuum, the Raman shifts and linewidths of the optical phonons are due to the high-velocity acoustic plasmon predicted by the SFS-model of high- T c superconductivity. The T-dependence of the acoustic plasmon velocity is deduced from both the Raman background and from the peak position of the 340 cm −1 phonon measured by several groups. We find the sound velocities deduced from the two different experiments to be in good qualitative agreement. We show also that the linewidths of the 120 and 340 cm −1 phonons can be explained with the same T-dependent acoustic plasmon velocity. This temperature dependence can be explained with boson localization above a temperature T L > T c.