Inelastic X-ray scattering studies of broken symmetry in BSCCO

Abstract

Interactions between charge and lattice degrees of freedom play a critical role in determining the properties of canonical BCS superconductors where integration out of the phonon subsystem results in an effective pairing interaction between electrons. In the study of high temperature superconductors the importance of phonons is less well understood and charge ordering properties vary between the families of high-Tc cuperates. While superconductivity in these materials is not believed to originate from phonon excitations, there is evidence for strong electron-phonon coupling from significant electron dispersion renormalization and the observation of increased breadth in optical Cu-O bond modulating phonons. Here we present measurements of acoustic phonons in single and double layer BSCCO which show several effects: broadening of the longitudinal acoustic in correspondence to approximately period-four ordering tendencies and signatures of time-reversal and inversion symmetry breaking. Measurement of these anomalous properties is feasible due to renormalization of the lattice propagator by strong interactions with underlying symmetry-breaking electronic states. These symmetries are broken at room temperature for all materials in the 'strange metal' state above the pseudogap, but are enhanced, particularly around the period four intercell ordering wavevector, as the system is cooled into the pseudogap state. In-plane acoustic phonons are a probe of the electron physics localized on the Cu-O plane due to the residual eigenvector components in this plane. These phonon measurements then present a picture of BSCCO in which charge correlations stay dynamic with a pronounced tendency toward ordering at a specific wave-vector and an underlying symmetry-breaking ground state.