Pressure-induced enhancement of electron-phonon coupling in superconducting CaC6 from first principles

The pressure effects on the electron-phonon coupling (EPC) in superconducting CaC6 have been extensively studied using first-principles calculations within the rigid muffin-tin approximation. It is found that pairing electrons are mainly attributed to the Ca 3d electrons mediated by Ca low-lying, in-plane phonon vibrations. An increase in mean square of the EPC matrix element < I-2 > and a decrease in Ca low-lying phonon frequency with increasing pressure are mainly responsible for the experimental observation of enhanced superconducting transition temperature. Furthermore, Mulliken population analysis suggests that the pressure-induced charge transfers from Ca to the graphite layers might mainly contribute to the softening behavior of Ca in-plane phonons.