  High Tc superconductors show a rich variety of phases associated with their
charge degrees of freedom. Valence charges can give rise to charge ordering or
acoustic plasmons in these layered cuprate superconductors. While charge
ordering has been observed for both hole- and electron-doped cuprates, acoustic
plasmons have only been found in electron-doped materials. Here, we use
resonant inelastic X-ray scattering (RIXS) to observe the presence of acoustic
plasmons in two families of hole-doped cuprate superconductors [La2-xSrxCuO4
(LSCO) and Bi2Sr1.6La0.4CuO6+d (Bi2201)], crucially completing the picture.
Interestingly, in contrast to the quasi-static charge ordering which manifests
at both Cu and O sites, the observed acoustic plasmons are predominantly
associated with the O sites, revealing a unique dichotomy in the behaviour of
valence charges in hole-doped cuprates.
