Superconductivity in Ca-doped graphene laminates

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.

Flat-band superconductivity in strained Dirac materials

“…Another possible realization for this model is in graphene with a strain field, created either artificially or at an interface inside graphite [15]. The latter suggestion also builds on the recent experimental evidence that graphene can become superconducting under heavy doping [1,2]. Our proposal could potentially be used to increase the superconducting critical temperature much higher in the absence of external doping.…”

“…So far, graphene has been shown to be superconducting when doped with calcium and possibly also with other elements [1,2,26]. In that case doping of the pure Dirac material is required to increase the density of states so that the quantum critical point disappears.…”

“…Thus, when engineering materials for higher critical temperatures, it is natural to aim to increase the density of states. In twodimensional systems such as graphene, a traditional approach for this is to utilize doping [1], which recently lead to a breakthrough as strongly doped graphene was found to be superconducting with T c of a few Kelvin [2]. An extreme case of increased density of states is the flat-band state, where the electrons within some momentum regime are completely dispersionless, leading to diverging density of states at the corresponding energy.…”

“…We can estimate the value of the critical temperature in periodically strained graphene using the coupling constant obtained from experiments [2,26]. As shown in Ref.…”

Entanglement spectra of superconductivity ground states on the honeycomb lattice

“…Evidence of superconductivity has been experimentally observed on Caintercalated bilayer graphene and graphene laminates at 4 [13] and 6.4 K [14], respectively. Furthermore, additional experimental progress has been made regarding evidence of superconductivity in Li-decorated monolayer graphene with a transition temperature of approximately 5.9 K [15].…”