Evidence for superconductivity in Li-decorated monolayer graphene

Abstract: Monolayer graphene exhibits many spectacular electronic properties, with superconductivity being arguably the most notable exception. It was theoretically proposed that superconductivity might be induced by enhancing the electron-phonon coupling through the decoration of graphene with an alkali adatom superlattice [Profeta et al. Nat. Phys. 8, 131-134 (2012)]. While experiments have indeed demonstrated an adatom-induced enhancement of the electron-phonon coupling, superconductivity has never been observed. Using angle-resolved photoemission spectroscopy (ARPES) we show that lithium deposited on graphene at low temperature strongly modifies the phonon density of states, leading to an enhancement of the electron-phonon coupling of up to $\lambda!\simeq!0.58$. On part of the graphene-derived $\pi^*$-band Fermi surface, we then observe the opening of a $\Delta!\simeq!0.9$ meV temperature-dependent pairing gap. This result suggests for the first time, to our knowledge, that Li-decorated monolayer graphene is indeed superconducting with $T_c!\simeq!5.9 K$.

• The paper reports evidence for superconductivity in Li-decorated graphene by detecting a pairing gap of 0.9 meV and a coupling constant (λ) of 0.58 via ARPES.
• The study employs ARPES to measure phonon density of states modifications, confirming a superconducting transition near 5.9 K in line with density functional theory predictions.
• Findings indicate that Li-induced modifications of lower-energy phonon modes in monolayer graphene create the conditions necessary for superconductivity, paving the way for engineered 2D superconductors.

Superconductivity in Li-Decorated Monolayer Graphene: An ARPES Investigation

The paper "Evidence for superconductivity in Li-decorated monolayer graphene" reports the observation of superconductivity in lithium (Li) decorated monolayer graphene. Using angle-resolved photoemission spectroscopy (ARPES), the authors show significant electron-phonon coupling enhancements which suggest the emergence of a superconducting state. This study is framed within the context of prior theoretical predictions and experimental observations related to graphene and graphite intercalation compounds (GICs).

The theoretical underpinning discussed in the paper is closely related to the ability to induce superconductivity in graphene by enhancing electron-phonon interactions. Despite extensive experimental efforts to observe superconductivity in GICs, such phenomena were previously unreported in decorated graphene. Prior studies indicated that superconductivity in GICs hinges on the enhanced electron-phonon coupling from intercalant layers, with calcium-intercalated graphite (CaC₆) achieving a critical temperature ($T_c$) of up to 11.5 K.

The authors employ ARPES to detect modifications in the phonon density of states caused by Li deposition at low temperatures, which results in the enhancement of electron-phonon coupling. Critical observations include a coupling constant $\lambda=\!0.58$ and a temperature-dependent pairing gap $\Delta\!=\!0.9$ meV on part of the graphene-derived $\pi^*$-band Fermi surface. The reported transition temperature $T_c\!\simeq\!5.9$ K aligns reasonably with theoretical predictions from density functional theory for monolayer LiC₆.

This work marks a significant advance in understanding superconductivity in low-dimensional systems. The authors suggest that the nearly detectable pairing gap, anisotropic nature of the gap, and its correlation to the Fermi surface provide robust evidence of a superconducting state, akin to known phase states in other superconductors such as niobium and CaC₆. Particularly, the Li-induced modification of lower-energy phonon modes, notably Li in-plane (Li$_{xy}$) and C out-of-plane (C$_z$) modes, is considered crucial for achieving the necessary coupling strength to facilitate superconductivity.

Looking forward, the implications of these observations may extend to the broader study of superconductivity in 2D materials. The ability to realize superconductivity in monolayer graphene opens opportunities for exploring unconventional superconducting mechanisms and leveraging these phenomena in scientific and technological pursuits. Furthermore, this study hints at the potential for engineering other superconducting constructs via adatom-induced modifications of graphene and similar 2D materials.

While the results are compelling, the paper carefully avoids definitive conclusions concerning the symmetry of the superconducting order parameter, suggesting room for further investigation. As research in graphene progresses, one might anticipate additional theoretical and experimental advancements to elucidate the precise mechanisms and applications of superconductivity in these promising material systems. This work thus paves the way for deeper exploration into the properties of graphene and its potential utility in future technologies.