Two-Dimensional Superconductivity of Ca-Intercalated Graphene on SiC: Vital Role of the Interface between Monolayer Graphene and the Substrate

Abstract

Ca-intercalation has enabled superconductivity in graphene on SiC. However, the atomic and electronic structures that are critical for superconductivity are still under discussion. We find an essential role of the interface between monolayer graphene and the SiC substrate for superconductivity. In the Ca-intercalation process, at the interface a carbon layer terminating SiC changes to graphene by Ca-termination of SiC (monolayer graphene becomes a bilayer), inducing more electrons than a free-standing model. Then, Ca is intercalated in between the graphene layers, which shows superconductivity with the updated critical temperature (T C ) of up to 5.7 K. In addition, the relation between T C and the normal-state conductivity is unusual, "dome-shaped". These findings are beyond the simple C 6 CaC 6 model in which s-wave BCS superconductivity is theoretically predicted. This work proposes a general picture of the intercalation-induced superconductivity in graphene on SiC and indicates the potential for superconductivity induced by other intercalants.

Strong Coupling Superconductivity in Ca-Intercalated Bilayer Graphene on SiC

“…C 6 CaC 6 was prepared on a trilayer graphene film epitaxially grown on a SiC(0001) substrate by a direct Ca intercalation. This is in sharp contrast to previous work, in which the preparation of metal-intercalated graphene needed a preintercalation of Li atoms. ,,− We succeeded in observing a superconducting energy gap and diamagnetic response of C 6 CaC 6 , from which the strong coupling superconductivity with an isotropic s -wave symmetry was revealed. Notably, the critical temperature ( T c ) found here is remarkably higher than that in previous works, , which can possibly be attributed to the suppression of charge-density waves (CDW).…”

“…This is in sharp contrast to previous work, in which the preparation of metal-intercalated graphene needed a preintercalation of Li atoms. ,,− We succeeded in observing a superconducting energy gap and diamagnetic response of C 6 CaC 6 , from which the strong coupling superconductivity with an isotropic s -wave symmetry was revealed. Notably, the critical temperature ( T c ) found here is remarkably higher than that in previous works, , which can possibly be attributed to the suppression of charge-density waves (CDW).…”

“…Moreover, the intensity of each streak in the C RHEED pattern shows only one maximum, while that of the Ca(111) islands should have multiple maxima in its transmission pattern. Therefore, the streaks present in Figure b could not originate from bulk Ca(111) islands observed in a previous report, where Ca atoms were deposited at a lower sample temperature . The length of the C unit cell is exactly times of that of the C(1 × 1) cell, as evidently observed in the atomic-scale STM image shown in Figure c.…”

“…The observation of superconductivity with unconventional behavior in magic-angle twisted bilayer graphene (MATBG) has stimulated considerable research interest in many-body effects and associated novel phenomena in this chemically pristine material. − The flat band with a high density of states near the Fermi energy ( E F ) resulted from hybridization of the Dirac cones via the moiré superlattice potential is responsible for the enhancement of a strong correlation effect and thus the emergence of superconductivity in MATBG. − Alternatively, the native flat band near the van Hove singularity far above the Dirac point ( E D ) can be shifted to E F by heavy carrier doping via intercalation of foreign atoms in single or bilayer graphene, as observed experimentally, − and thus may also intrigue fascinating phenomena, including unconventional superconductivity and a chiral spin density wave. − However, direct and unambiguous experimental evidence for intrinsic superconductivity in single-layer graphene is still missing, although a proximity-induced supercurrent and superconducting-like energy gap have been observed. − The observation of superconductivity in carrier-doped few-layer graphene is very limited, partially because it requires an in situ experimental technique to overcome the obstacle that the doped few-layer graphene is highly reactive to air. Previous reports have shown the observation of zero resistance in Ca-intercalated bilayer graphene (C 6 CaC 6 ) by using an in situ micro - 4-point-probe (M4PP) electrical transport technique, , but the nature of the superconductivity such as the pairing symmetry and coupling strength has yet to be unveiled. Moreover, the diamagnetic response, which is another prerequisite besides zero resistance for the experimental determination of superconductivity, has not yet been elucidated in C 6 CaC 6 .…”

“…The thickness of the graphene film was determined by the scanning tunneling spectroscopy (STS) described below. In contrast to the previous studies that used Li as a precursor for the intercalation of Ca into the graphene film, ,,, here Ca atoms were directly deposited on the graphene surface kept at 248 °C, followed by a postannealing at the same temperature for 10 min. The process of substrate heating, Ca deposition, and postannealing was repeated twice until the in situ reflection high-energy electron diffraction (RHEED) shows a very sharp streak pattern of with respect to the graphene C(1 × 1) surface lattice.…”