⇦Chinese Physics Letters, 2019, Vol. 36, No. 9, Article code 090101Views & Comments Theoretical Proposal for a Planar Single-Layer Carbon That Shows a Potential in Superconductivity Yan-Ming Ma (马琰铭)** Affiliations State Key Lab of Superhard Materials, College of Physics, Jilin University, Changchun 130012 Received 12 August 2019, online 26 August 2019 **Email: mym@jlu.edu.cn; mym@calypso.cn Citation Text: Ma Y M 2019 Chin. Phys. Lett. 36 090101    Abstract DOI:10.1088/0256-307X/36/9/090101 PACS:01.10.-m © 2019 Chinese Physics Society Article Text Single-layer superconductors[1] have been the subject of considerable interests as they are ideal systems for the fundamental understanding of two-dimensional (2D) physics and for device applications. A few single-layer superconductors are experimentally achieved (e.g., FeSe, MoS$_{2}$, and NbSe$_{2}$[2–4]) in the field where either charge doping or tensile strain is often required to promote the superconductivity. An intrinsic single-layer superconductor is highly demanded since no extra controlling efforts are needed for the superconductivity. In this issue, Gu et al.[5] theoretically proposed that a planar single-layer carbon sheet consisting of 4- and 8-membered rings (named as T-graphene) has a potential to be an $intrinsic$ 2D superconductor with a predicted superconducting transition temperature ($T_{\rm c}$) at $\sim$20 K. Carbon has many different allotropes (e.g., diamond, graphite, fullerene, and graphene, etc.). None of these pure carbon phases exhibits superconducting behavior. However, by introduction of dopant, superconductivity appears in common with $T_{\rm c}$ reaching the values in the range of 1–30 K. Experimentally known carbon-based superconductors include metals intercalated graphite, alkali-metals-doped fullerenes, boron-doped diamond, and magic-angle graphene superlattices,[6] etc. Graphite intercalation superconductors (e.g., ${\rm C}_{8}$ K, ${\rm C}_{6}$Ca, and ${\rm C}_{6}$Yb, etc.) attract particular attention, since they structurally share the similarity with the well-known conventional superconductor of MgB$_{2}$ ($T_{\rm c} = 39$ K) at ambient pressure where Mg atoms are intercalated into the interspace of graphene-like boron layers.[7] Using a first-principle based crystal structure searching method,[8] Gu et al.[5] predicted a new graphite intercalation compound with a stoichiometry of ${\rm C}_{4} $K that is thermodynamically stable under an external pressure of around 11.5 gigapascals, a pressure that is achievable in current laboratories for acquiring an actual synthesis on the compound. The C$_{4}$K compound exhibits a potential to have superconductivity at a theoretical $T_{\rm c}$ of $\sim$30 K.[5] Interestingly, the single carbon layer in the C$_{4}$K compound adopts a structure of planar T-graphene. With the similar structure, the Dirac-like band properties of a "buckled" T-graphene were discussed in a previous work.[9] This T-graphene was calculated to be an intrinsic 2D superconductor with a predicted $T_{\rm c} $ at $\sim$20 K.[5] The superconducting mechanism is different from that in magic-angle graphene[6] where the coupling of two graphene layers via van der Waals interaction is essential to generate the superconductivity. The experimental synthesis of this T-graphene remains to be challenging. There is a requirement to synthesize C$_{4}$K under high pressure conditions. After quenched to ambient conditions, T-graphene can be achieved from a direct exfoliation of either bulk C$_{4}$K or bulk C$_{4}$ after evaporation of the K atoms. Though challenging, it is worthy to have it at hand for device applications, so that one may stack such a 2D superconductor on top of other 2D materials (e.g., TMDs or h-BN) to fabricate van der Waals superconductor/semiconductor heterojunction devices. The work by Gu et al.[5] provides a road map of seeking for intrinsic single-layer superconductors in carbon-base materials, where high pressure has played an important role in stabilization of the material. References Highly crystalline 2D superconductorsInterface-Induced High-Temperature Superconductivity in Single Unit-Cell FeSe Films on SrTiO 3Evidence for two-dimensional Ising superconductivity in gated MoS2Ising pairing in superconducting NbSe2 atomic layersSuperconducting Single-Layer T-Graphene and Novel Synthesis RoutesUnconventional superconductivity in magic-angle graphene superlatticesSuperconductivity at 39 K in magnesium diborideA novel superhard tungsten nitride predicted by machine-learning accelerated crystal structure searchStructural and Electronic Properties of T Graphene: A Two-Dimensional Carbon Allotrope with Tetrarings
[1] Saito Y, Nojima T and Iwasa Y 2017 Nat. Rev. Mater. 2 16094
[2] Wang Q Y, Li Z, Zhang W H, Zhang Z C, Zhang J S, Li W, Ding H, Ou Y B, Deng P, Chang K, Wen J, Song C L, He K, Jia J F, Ji S H, Wang Y Y, Wang L L, Chen X, Ma X C and Xue Q K 2012 Chin. Phys. Lett. 29 037402
[3] Lu J M, Zheliuk O, Leermakers I, Yuan N F Q, Zeitler U, Law K T and Ye J T 2015 Science 350 1353
[4] Xi X, Wang Z, Zhao W, Park J H, Law K T, Berger H, Forró L, Shan J and Mak K F 2016 Nat. Phys. 12 139
[5] Gu Q Y, Xing D Y and Sun J 2019 Chin. Phys. Lett. 36 097401
[6] Cao Y, Fatemi V, Fang S, Watanabe K, Taniguchi T, Kaxiras E and Jarillo-Herrero P 2018 Nature 556 43
[7] Nagamatsu J, Nakagawa N, Muranaka T, Zenitani Y and Akimitsu J 2001 Nature 410 63
[8] Xia K, Gao H, Liu C, Yuan J, Sun J, Wang H T and Xing D 2018 Sci. Bull. 63 817
[9] Liu Y, Wang G, Huang Q, Guo L and Chen X 2012 Phys. Rev. Lett. 108 225505