Direct observation of bulk charge modulations in optimally doped Bi1.5Pb0.6Sr1.54CaCu2O8+delta

Bulk charge density modulations, recently observed in high critical-temperature (T-c) cuprate superconductors, coexist with the so-called pseudogap and compete with superconductivity. However, its direct observation has been limited to a narrow doping region in the underdoped regime. Using energy-resolved resonant x-ray scattering we have found evidence for such bulk charge modulations, or soft collective charge modes (soft CCMs), in optimally doped Bi1.5Pb0.6Sr1.54CaCu2O8+delta (Pb-Bi2212) around the summit of the superconducting dome with momentum transfer q vertical bar vertical bar similar to 0.28 reciprocal lattice units (r. l. u.) along the Cu-O bond direction. The signal is stronger at T similar or equal to T-c than at lower temperatures, thereby confirming a competition between soft CCMs and superconductivity. These results demonstrate that soft CCMs are not constrained to the underdoped regime, suggesting that soft CCMs appear across a large part of the phase diagram of cuprates and are intimately entangled with high-T-c superconductivity.