Emergent charge order from correlated electron-phonon physics in cuprates

Charge-density wave order is now understood to be a widespread feature of underdoped cuprate high-temperature superconductors, although its origins remain unclear. While experiments suggest that the charge-ordering wavevector is determined by Fermi-surface nesting, the relevant sections of the Fermi surface are featureless and provide no clue as to the underlying mechanism. Here, focusing on underdoped YBa2Cu3O6+x, we propose that charge-density waves form from the incipient softening of a bond-buckling phonon. The momentum dependence of its coupling to itinerant electrons favourably selects the wavevector found in experiments. But, it requires quasiparticle renormalization by strong electronic correlations to enable a unique enhancement of the charge susceptibility near the B-1g-phonon selected wavevector. The B-1g phonon frequency softens by a few percent, and finite-range charge-density wave correlations will form locally, if nucleated by defects or dopant disorder. These results suggest that underdoped cuprates cannot be understood in the context of strong electronic correlations alone. The origin of charge density waves in high-temperature cuprates and its connection with the superconducting state is a longstanding topic of debate. Here, the authors investigate the underlying mechanisms of charge density waves in an underdoped cuprate and describe the relationship with the electron-phonon coupling of bond-buckling phonons.