Superconductivity of Ca-intercalated bilayer graphene enhanced by confinement epitaxy

抄録

Graphene-based two-dimensional superconductors are promising for diverse applications owing to their inherent transparency and flexibility. Among these materials, C₆CaC₆, a bilayer graphene intercalated with calcium on a SiC substrate, exhibits the highest T_c, although the reported value has scattered from 4 to 8.8 K [1–4]. Early studies primarily accepted the free-standing C₆CaC₆ model based on the observation of its electronic structure [5]. Recent studies also imply the intercalation at the interface of C₆CaC₆ and the SiC substrate [2, 3]. Furthermore, since the formation of multiple two-dimensional layers at the SiC/graphene interface without intercalation in between the graphene layers has also been reported for some elements (confinement epitaxy) [6], a systematic investigation of the interface structure upon Ca deposition to the graphene/SiC systems is desired. In this investigation, we have successfully acquired compelling evidence of manifestation of confinement epitaxy in the superconducting C₆CaC₆/SiC system. We meticulously tracked the progression of the C₆CaC₆ growth on SiC using a sophisticated ultrahigh vacuum multi-probe system combining electron diffraction, photoemission spectroscopy, and transport measurements. We found that surplus deposition of calcium alters the electronic state of C₆CaC₆ from the band structures depicted in Fig. 1(a) to that elucidated in Fig. 1(b); a X-shaped metallic (XM) band emerges alongside theα*, β*, and interlayer (IL) band intrinsic to C₆CaC₆. Furthermore, the hybridization of the XM and β* bands leads to the emergence of flat bands. Through comprehensive electronic state analysis, we deduce that the XM band originates from the metallic calcium sublayer positioned between C₆CaC₆ and the calcium terminated SiC, as illustrated schematically in Figs. 1(c) and (d). A comparative analysis of the transport characteristics of C₆CaC₆ structures, both in the presence and absence of the Ca sublayer is presented in Fig. 1(e), which unveils the impact of the Ca sublayer in augmenting T_c. Two distinct categories of many-body phenomena are envisaged as contributing T_c: the intensified pairing potential ascribed to the flat band and the reduction of the scattering potential on the substrate surface owing to the existence of the interfacial metallic layer. This infers that the uncharacterized interface structure in prior investigations was responsible for the T_c scattering. [1] S. Ichinokura et al., ACS Nano 10, 2761 (2016). [2] Y. Endo et al. Carbon 157, 857 (2020). [3] H. Toyama et al., ACS Nano 16, 3582 (2022). [4] X. Wang et al., Nano Letters 22, 7651 (2022). [5] K. Kanetani et al., Proceedings of the National Academy of Sciences 109, 19610 (2012). [6] N. Briggs et al., Nature Materials 19, 637 (2020).