Dynamical charge density fluctuations in superconducting cuprate YBa₂Cu₃O_6+x

Charge density wave (CDW) correlations are ubiquitous in hole-doped high-temperature cuprate superconductors. The role played by dynamical CDW fluctuations on the unconventional superconductivity and anomalous normal state properties is under heated debate. Here, we have studied the charge fluctuations in both normal and superconducting states of nearly optimally-doped Yba₂Cu₃O_6+x using high-resolution resonant inelastic x-ray scattering (RIXS). In the normal state, we find that the charge response near Q_CDW can be described in the framework of fluctuations due the proximity of a quantum critical point. In addition, we have revealed strong momentum-dependent modification in the charge fluctuations as temperature is varied across T_c. While an abrupt reduction in the charge excitations is observed near Q = (0 0.16), there is a prominent enhancement of the dynamical charge density fluctuations near Q_CDW at 30 meV upon entering the superconducting state, in analogy to the magnetic "resonance" in the spin susceptibility measured by inelastic neutron scattering. Our results point to a close relationship between dynamical charge density correlations and superconductivity in high-T_c cuprates.