Phonon-mediated superconductivity in graphene by lithium deposition

Abstract

Graphene1 is the physical realization of many fundamental concepts and phenomena in solid-state physics 2 . However, in the list of graphene's many remarkable properties 3-6 , superconductivity is notably absent. If it were possible to find a way to induce superconductivity, it could improve the performance and enable more efficient integration of a variety of promising device concepts including nanoscale superconducting quantum interference devices, single-electron superconductor-quantum dot devices 7,8 , nanometre-scale superconducting transistors 9 and cryogenic solid-state coolers 10 . To this end, we explore the possibility of inducing superconductivity in a graphene sheet by doping its surface with alkaline metal adatoms, in a manner analogous to which superconductivity is induced in graphite intercalated compounds 11,12 (GICs). As for GICs, we find that the electrical characteristics of graphene are sensitive to the species of adatom used. However, contrary to what happens in GICs, Li-covered graphene is superconducting at a much higher temperature with respect to Ca-covered graphene.As graphene itself is not superconducting, phonon-mediated superconductivity must be induced by an enhancement of the electron-phonon coupling (λ),In equation (1), N (0) is the electronic density of states per spin (DOS) at the Fermi level, D is the deformation potential, and M and ω ph are the effective atomic mass and phonon frequency that in metallic alloys reflect the role of the different atomic species and phonon vibrations involved in superconductivity. In undoped graphene λ is small and phonon-mediated superconductivity does not occur as the small number of carriers, intrinsic to a semimetal, leads to a vanishingly small N (0). In this respect the situation is similar to the bulk graphite case, where, without intercalation of foreign atoms superconductivity is not stabilized.A first guess to induce superconductivity could then be to fill by rigid-band doping the carbon π -states to have enough carriers. However, besides the fact that the π -DOS grows very slowly with doping, its impact will be hindered by two major difficulties. First, even if the deformation potential related to coupling between π -bands and in-plane phonon vibrations is large and leads to Kohn anomalies 13 , these vibrations are highly energetic (ω ph ≈ 0.16 eV) and λ is small owing to the ω 2 ph factor in the denominator. Second, symmetry forbids the coupling between π-states and the softer out-of-planes vibrations.To promote coupling to carbon out-of-plane vibrations, it is then necessary to promote new electronic states at the Fermi level as happens in GICs. Indeed, in superconducting GICs, an intercalant band (interlayer state) occurs at the Fermi

Superconductivity in graphite intercalation compounds

“…There is clearly activity [11] searching for a superconducting ground-state in a dressed graphene sheet material, and the work on GICs will act to inform both experimental and theoretical work on the graphene states.…”

“…This work has reinvigorated activity into superconductivity in GICs. Understanding the mechanism of superconductivity in GICs is relevant to the physics of graphene at high electron doping, [9] and has led to, as yet unconfirmed, predictions of superconductivity in metal decorated single layer graphene sheets [11].…”

Li Intercalation into Graphite: Direct Optical Imaging and Cahn–Hilliard Reaction Dynamics

“…Note that several theoretical calculations indicate that stage 1 LiC6 charge transfer from Li atom to graphene approaches 100%. 7,8 Theory The free energy density of each layer is modeled as a regular solution with a gradient energy penalty,…”

“…1 Lithium intercalated graphite shows a rich phase diagram and is widely used as the anode in batteries and other electrochemical energy storage systems. Understanding Li intercalation is a critical technological problem that has been studied principally using electrochemical methods: [2][3][4][5][6][7][8] potentiostatic intermittent titration (PITT), galvanostatic intermittent titration (GITT), electrochemical impedance spectroscopy (EIS), and slow scan rate cyclic voltammetry(CV). Additional information on elementary atomic hopping steps of Li transport has also been obtained from nuclear magnetic resonance.…”

Chirald-wave superconductivity in doped graphene

“…Obviously, the possibilities for superconductivity in graphene have also been explored. While the potential, and also realizations, of conventional phonon-mediated superconductivity in two-dimensional carbon materials is very actively pursued [26,27], there are also a large number of theoretical works on how unconventional superconductivity can emerge in graphene and related systems. Most of these consider the possibility of chiral d-wave superconductivity in doped graphene, which is exactly the focus of this review.…”