Electron-phonon interaction and pairing mechanism in superconducting Ca-intercalated bilayer graphene.

E R Margine,Feliciano Giustino,Henry Lambert

doi:10.1038/srep21414

E R Margine, Feliciano Giustino + Show 1 more

Open Access

https://doi.org/10.1038/srep21414

Copy DOI

Journal: Scientific Reports	Publication Date: Feb 19, 2016
Citations: 73	License type: CC BY 4.0

Affiliation: Binghamton University, University of Oxford

Abstract

Using the ab initio anisotropic Eliashberg theory including Coulomb interactions, we investigate the electron-phonon interaction and the pairing mechanism in the recently-reported superconducting Ca-intercalated bilayer graphene. We find that C6CaC6 can support phonon-mediated superconductivity with a critical temperature Tc = 6.8–8.1 K, in good agreement with experimental data. Our calculations indicate that the low-energy Caxy vibrations are critical to the pairing, and that it should be possible to resolve two distinct superconducting gaps on the electron and hole Fermi surface pockets.

Highlights

Using the ab initio anisotropic Eliashberg theory including Coulomb interactions, we investigate the electron-phonon interaction and the pairing mechanism in the recently-reported superconducting Caintercalated bilayer graphene
Two independent studies reported the observation of superconductivity in Ca-intercalated bilayer graphene at 4 K4 and in Ca-intercalated graphene laminates around 6.4 K5, while a third study presented evidence for superconductivity in Li-decorated monolayer graphene around 5.9 K6
The electron-phonon interaction is expected to play a central role in these observations, it is important to develop a detailed understanding of electron-phonon physics in these newly-discovered superconductors

Summary

Methods

The calculations are performed within the local density approximation to density-functional theory[41,42] using planewaves and norm-conserving pseudopotentials[43,44], as implemented in the Quantum-ESPRESSO suite[45]. Bulk CaC6 is described using the rhombohedral lattice, and the optimized lattice constant and rhombohedral angle are a = 5.04 Å and α = 50.23°, respectively. The electronic charge density is calculated using an unshifted Brillouin zone mesh with 242 and 83 k-points for C6CaC6 and CaC6, respectively, and a Methfessel-Paxton smearing of 0.02 Ry. The dynamical matrices and the linear variation of the self-consistent potential are calculated within density-functional perturbation theory[46] on the irreducible set of regular 62 (C6CaC6) and 43 (CaC6) q-point grids. The electronic wavefunctions required for the Wannier-Fourier interpolation[48,49] in EPW are calculated on uniform unshifted Brillouin-zone grids of size 122 (C6CaC6) and 83 (CaC6). The normal-state self-energy is calculated on fine meshes consisting of 100,000 inequivalent q-points, using a broadening parameter of 10 meV and a temperature T = 20 K. The energy cutoff of the dielectric matrix is 10 Ry (815 planewaves)

Author Contributions

Findings

Additional Information