Strong off-diagonal polarizability and electron-lattice coupling in high-temperature superconductors

High-temperature superconducting (HTS) cuprates are highly anisotropic materials which exhibit metallic-like behavior in the CuO2 planes while retaining dielectric properties in the perpendicular, c-axis, direction. Experimental data show however that in HTS systems the in-plane electronic excitations are strongly coupled to c-axis polarized vibrations. This interaction is manifest in various settings, for example in the resonant Raman profile of phononic excitations, inelastic quasi-particle tunneling, as observation of notch-like features and forbidden scattering for in-plane optical conductivity, colossal c-axis photo-expansion upon in-plane illumination as well as in high-resolution electron energy-loss spectra. We propose that this anisotropic coupling is driven by strong unscreened Coulomb interactions and the preponderance of the Madelung component to the cohesion energy, in particular by the large atomic displacements in the spacer layers induced by charge redistribution within the CuO2 planes.