Probing electronic order via coupling to phonons in Bi2Sr2CuO6 (Bi2201)

Recent work on several members of the cuprate family of high-temperature superconductors has revealed the occurrence of broken lattice translational, rotational, and time reversal symmetries in various materials at low temperatures. We report on measurements of acoustic phonons on Bi$_2$(La,Sr)$_2$CuO$_{6+\delta}$, which reveal a coupling to an underlying electronic density-wave state. Longitudinal acoustic phonons are found to be anomalously broadened at a wavevector of q $\approx$ 1/4 rlu along the Cu-O direction. At low temperatures a disparity between the scattered intensity at positive and negative energy transfer is seen. These measurements indicates a breaking of time reversal and inversion symmetry in the bulk.