Doping-dependent phonon anomaly and charge-order phenomena in the HgBa2CuO4+δ and HgBa2CaCu2 O6+δ superconductors

Using resonant X-ray diffraction and Raman spectroscopy, we study charge correlations and lattice dynamics in two model cuprates, HgBa2CuO4+δ and HgBa2CaCu2O6+δ. We observe a maximum of the characteristic charge order temperature around the same hole concentration (p≈0.09) in both compounds, and concomitant pronounced anomalies in the lattice dynamics that involve the motion of atoms in and/or adjacent to the CuO2 layers. These anomalies are already present at room temperature, and therefore precede the formation of the static charge correlations, and we attribute them to an instability of the CuO2 layers. Our finding implies that the charge order in the cuprates is an emergent phenomenon, driven by a fundamental variation in both lattice and electronic properties as a function of doping.