Abstract
Despite graphene’s long list of exceptional electronic properties and many theoretical predictions regarding the possibility of superconductivity in graphene, its direct and unambiguous experimental observation has not been achieved. We searched for superconductivity in weakly interacting, metal decorated graphene crystals assembled into so-called graphene laminates, consisting of well separated and electronically decoupled graphene crystallites. We report robust superconductivity in all Ca-doped graphene laminates. They become superconducting at temperatures (Tc) between ≈4 and ≈6 K, with Tc’s strongly dependent on the confinement of the Ca layer and the induced charge carrier concentration in graphene. We find that Ca is the only dopant that induces superconductivity in graphene laminates above 1.8 K among several dopants used in our experiments, such as potassium, caesium and lithium. By revealing the tunability of the superconducting response through doping and confinement of the metal layer, our work shows that achieving superconductivity in free-standing, metal decorated monolayer graphene is conditional on an optimum confinement of the metal layer and sufficient doping, thereby bringing its experimental realization within grasp.
Highlights
Despite graphene’s long list of exceptional electronic properties and many theoretical predictions regarding the possibility of superconductivity in graphene, its direct and unambiguous experimental observation has not been achieved
Some aspects of its superconductivity remain under debate[14,15,16,17,18], main contributing factors have been identified[14,15,17] as (i) doping via metal adatoms to reach sufficiently high electron concentrations in graphite, (ii) importance of an interlayer (IL) electronic band that arises from the intercalant superlattice formed between graphene layers, and (iii) the overall electron-phonon coupling that is related to coupling involving graphene phonons and intercalant vibrations
According to recent DFT calculations[4], similar conditions are required to induce superconductivity in metal decorated graphene, i.e. adatoms are required to achieve sufficiently high electron concentrations, and to create an electronic band arising from the adatom superlattice and ensure its overlap with the graphene π * band[4]
Summary
High purity HOPG crystals were exfoliated in N-Methyl-2-pyrrolidone (NMP) in an ultrasonic bath and the resulting dispersions centrifuged at 12,000 rpm to obtain a stable suspension These were filtered through porous alumina filters to obtain several μm (3–8 μm) thick free standing laminates of graphene. Metal intercalation was done in either high vacuum or an argon-filled glove box to avoid exposure of the highly reactive alkali-/alkali-earth metals and the intercalated samples to ambient moisture and oxygen. We have used both pure-metal vapour transport[10,11] and alloy-intercalation techniques[12] to insert K, Cs, Li and Ca into GLs (Supplementary information). The spectra were taken at 345–1040 nm wavelengths, using a silver mirror as a reference
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
