Observation of a universal donor-dependent vibrational mode in graphene

Abstract

Electron-phonon coupling and the emergence of superconductivity in intercalated graphite have been studied extensively. Yet, phonon-mediated superconductivity has never been observed in the 2D equivalent of these materials, doped monolayer graphene. Here we perform angle-resolved photoemission spectroscopy to try to find an electron donor for graphene that is capable of inducing strong electron-phonon coupling and superconductivity. We examine the electron donor species Cs, Rb, K, Na, Li, Ca and for each we determine the full electronic band structure, the Eliashberg function and the superconducting critical temperature T c from the spectral function. An unexpected low-energy peak appears for all dopants with an energy and intensity that depend on the dopant atom. We show that this peak is the result of a dopant-related vibration. The low energy and high intensity of this peak are crucially important for achieving superconductivity, with Ca being the most promising candidate for realizing superconductivity in graphene.

Efficient gating of epitaxial boron nitride monolayers by substrate functionalization

“…It is clear that both core levels exhibit a shift towards higher binding energy (BE) in the latter case. This behavior is similar to graphene and graphite [8,24]. However, as we will show in the following, it is not due to a charge transfer but interfacial electric fields that yield potential differences between the substrate and hBN.…”

“…Figure 1(b) depicts the angledependent XPS spectra of both the K2p and Li1s core levels. The relative intensity ratio (normalized to the N1s core-level intensity) indicates the preferred location of each species (either above or below the hBN plane) [24]. As a result, we obtain a preference of K to intercalate in between hBN and Au/Ni(111) and a preference of Li to adsorb on top of hBN.…”

“…These methods allow us to study the interface structure of the dopant/hBN system, i.e., whether the dopant preferably is above or under hBN, and to probe the unoccupied states, respectively. The crucial point in this approach is that hBN is on a Au substrate that readily ionizes the deposited alkali atoms, which is somewhat similar for doped graphene on metal substrates, where ARPES and density functional theory (DFT) have shown that a part of the electron charge is transferred to the metal [8,24]. The present case is different from the graphene case, in the sense that graphene accepts a large fraction of the alkali atom's charge while hBN does not.…”

Raman spectroscopy of graphite intercalation compounds: Charge transfer, strain, and electron–phonon coupling in graphene layers

“…In addition, Ca does not present an ordered phase when deposited onto monolayer graphene in contrast to bilayer graphene. The latter highlights the importance of intercalation to achieve ordered phases of intercalant with pairing coupling constants similar to the bulk graphitic compounds [103,101].…”

“…Using the full phonon density of states, electronic dispersion and the Eliashberg function, a very strong electron-phonon coupling constant could be determined [101]. This could induce superconductivity at elevated temperature in graphene.…”