How to make graphene superconducting
Graphene [1] is the physical realization of many funda- mental concepts and phenomena in solid state-physics[2], but in the long list of graphene remarkable proper- ties [3-6], a fundamental block is missing: supercon- ductivity. Making graphene superconducting is relevant as the easy manipulation of this material by nanolyto- graphic techniques paves the way to nanosquids, one- electron superconductor-quantum dot devices[7, 8], su- perconducting transistors at the nano-scale[9] and cryo- genic solid-state coolers[10]. Here we explore the doping of graphene by adatoms coverage. We show that the occurrence of superconduc- tivity depends on the adatom in analogy with graphite intercalated compounds (GICs). However, most surpris- ingly, and contrary to the GIC case[11, 12], Li covered graphene is superconducting at much higher temperature with respect to Ca covered graphene. As graphene itself is not superconducting, phonon- mediated superconductivity must be induced by an en- hancement of the electron-phonon coupling ( ), = N(0)D2 M!2 ph (1) In Eq. 1 N(0) is the electronic density of states (DOS) at the Fermi level, D is the deformation potential, while M and !ph are effective atom mass and phonon frequency that in metallic alloys reflect the role of the different atomic species and phonon vibrations involved in su- perconductivity. In undoped graphene is small and phonon-mediated superconductivity does not occur as the small number of carriers, intrinsic in a semimetal, leads to a vanishingly small N(0). In this respect the sit- uation is similar to the bulk graphite case, where, with- out intercalation of foreign atoms superconductivity is not stabilized.