Probing electronic order via coupling to low energy phonons in   superconducting Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+\delta}$
  We report high-resolution inelastic x-ray scattering measurements of the acoustic phonons in the single-layer cuprate $Bi_{2}Sr_{2-x)La_{x}CuO_{6+\delta}$. These measurements reveal anomalous broadening of the longitudinal acoustic phonon near the (1/4,1/4,0) wavevector. The observed wavevector and its doping dependence indicate the coupling of the phonons to an underlying electronic density wave state. In addition, a comparison of the scattered intensities for x-ray energy-gain and x-ray energy-loss indicates that both time-reversal and inversion symmetries are broken in the material. Upon cooling, the effects of symmetry breaking are enhanced in the pseudogap state. 
C. J. Bonnoit, D. R. Gardner, R. Chisnell, A. H. Said, Y. Okada, T.   Kondo, T. Takeuchi, H. Ikuta, D. E. Moncton, and Y. S. Lee
cond-mat.supr-con