Graphene micro-substrate-induced π gap expansion in MgB2

The lattice effect induced by tensile strain on the superconductivity of graphene-MgB2 composites was studied systematically to deduce the electron-phonon coupling (EPC) and the multiple superconducting gap behavior. Compared with nano-carbon doped MgB2, graphene-MgB2 composites show larger lattice parameters and higher critical superconducting transition temperatures (Tc). The EPC strength of MgB2 with ∼2 wt.% graphene addition is even higher than that of the pure reference sample, as estimated from the Sommerfeld constant. The π gap was found to be expanded by graphene addition through the analysis of heat capacity data, and it is responsible for both the enhanced EPC strength and the weak dependence of Tc on the graphene content.