Enhanced electron-phonon coupling for charge-density-wave formation in La_1.8-xEu_0.2Sr_xCuO₄

Charge density waves (CDW) are prevalent in all copper-oxide superconductors and their connection to high-temperature superconductivity is hotly debated. A key issue is that, although widely studied, that the microscopic origin of CDW is still unclear. For conventional CDWs in metals, electron-phonon coupling (EPC) plays an important role for the CDW formation. Resonant inelastic X-ray scattering (RIXS) has recently become an effective tool to directly determine the momentum dependence of the electron-phonon coupling strength. Here we present the first RIXS study of the CDW and Cu-O bond-stretching phonons in La_1.8-xEu_0.2Sr_xCuO₄ compounds. By varying the sample doping and temperature, we show that the EPC is surprisingly enhanced around the CDW wave vector. Our results support a picture in which lattice excitations play a central role in the formation of the CDW, and furthermore exhibit a feedback effect in which the CDW enhances the EPC.