Dispersive charge density wave excitations in Bi2Sr2CaCu2O8+?

Experimental evidence on high-Tc cuprates reveals ubiquitous charge density wave (CDW) modulations1-10, which coexist with superconductivity. Although the CDWhad been predicted by theory11-13, important questions remain about the extent to which the CDW influences lattice and charge degrees of freedom and its characteristics as functions of doping and temperature. These questions are intimately connected to the origin of the CDW and its relation to the mysterious cuprate pseudogap10,14. Here, we use ultrahigh-resolution resonant inelastic X-ray scattering to reveal new CDW character in underdopedBi2.2Sr1.8Ca0.8Dy0.2Cu2O8C .At lowtemperature,we observe dispersive excitations from an incommensurate CDW that induces anomalously enhanced phonon intensity, unseen using other techniques. Near the pseudogap temperature T, the CDWpersists, but the associated excitations significantly weaken with an indication of CDW wavevector shift. The dispersive CDW excitations, phonon anomaly, and analysis of the CDW wavevector provide a comprehensive momentumspace picture of complex CDWbehaviour and point to a closer relationship with the pseudogap state.