Superconducting Phases in Lithium Decorated Graphene LiC 6

Abstract

A study of possible superconducting phases of graphene has been constructed in detail. A realistic tight binding model, fit to ab initio calculations, accounts for the Li-decoration of graphene with broken lattice symmetry, and includes $s$ and $d$ symmetry Bloch character that influences the gap symmetries that can arise. The resulting seven hybridized Li-C orbitals that support nine possible bond pairing amplitudes. The gap equation is solved for all possible gap symmetries. One band is weakly dispersive near the Fermi energy along $\Gamma\rightarrow M$ where its Bloch wave function has linear combination of $d_{x^2-y^2}$ and $d_{xy}$ character, and is responsible for $ d_{x^{2}-y^{2}}$ and $d_{xy}$ pairing with lowest pairing energy in our model. These symmetries almost preserve properties from a two band model of pristine graphene. Another part of this band, along $K\rightarrow \Gamma$, is nearly degenerate with upper $s$ band that favors extended $s$ wave pairing which is not found in two band model. Upon electron doping to a critical chemical potential $\mu_1=0.22 eV$ the pairing potential decreases, then increases until a second critical value $\mu_2$=1.3 eV at which a phase transition to a new phase which is not appear in two band model. This phase in the pristine graphene converts to usual extended s-wave pairing.