B1g-Phonon Anomaly Driven by Fermi Surface Instability at Intermediate Temperature in YBa2Cu3O7-δ

We performed temperature-and doping-dependent high-resolution Raman spectroscopy experiments on YBa2Cu3O7-delta to study B-1g phonons. The temperature dependence of the real part of the phonon self energy shows a distinct kink at T = T-B1g above T-c due to softening, in addition to the one due to the onset of the superconductivity. T-B1g is clearly different from the pseudogap temperature with a maximum in the underdoped region and resembles charge density wave onset temperature, T-CDW. We attribute the B-1g-phonon softening to an energy gap on the Fermi surface induced by a charge density wave order, which is consistent with the results of a recent electronic Raman scattering study. Our work demonstrates a way to investigate Fermi surface instabilities above T-c via phonon Raman studies.