Structure determination of Ca-intercalated bilayer graphene by positron
diffraction

Y. Fukaya1), Y. Endo2), A. Takayama3), I. Mochizuki4), T. Hyodo4) and S. Hasegawa2)
1) : Nanoscale Gr., JAEA 2): Univ. Tokyo 3): Waseda Univ.
4): KEK

Recently, bilayer graphene, which is stacked by two graphene
layers, was found to undergo superconducting transition ataround 4 K due to Ca intercalation [1]. Because of being
regarded as thinnest superconductor, bilayer graphene
superconductor has attracted increasing attention. In Caintercalated bilayer graphene, charge transfer from Ca atoms is
the key to possess the superconducting transition. In the analogyof graphite intercalation compounds (GICs), Ca atoms wereexpected to be intercalated between two graphene layers.
However, the atomic arrangement, in particular, Ca sites,remained unresolved. In this study, we investigated the atomic
arrangements of Ca-intercalated bilayer graphene synthesizedon a SiC(0001) substrate using total-reflection high-energy
positron diffraction (TRHEPD) technique [2].
TRHEPD is a surface sensitive tool owing to the positive
charge of positrons, opposite to electrons [3,4]. When striking acrystal surface, the positron beam feels a repulsive force due tothe electrostatic potential composed of a nucleus andsurrounding electron clouds. Thus, the penetration depth of
positron beam into crystal becomes considerably limited.
Especially, total reflection occurs at grazing angles, where thepenetration depth corresponds to the thickness of one atomic
layer. Therefore, TRHEPD is suitable to determine the atomic
arrangements of two-dimensional materials such as graphene.
As a result, we found that Ca atoms are intercalated in theinterface between bilayer graphene and SiC(0001) substrate, notin the bilayer graphene. We synthesized the bilayer graphene on
a SiC(0001) substrate through a standard thermal decomposition
method, followed by the intercalation of Ca atoms in a ultrahigh vacuum chamber. We measured rocking curves, diffraction
spot intensity versus the glancing angle of incident positron
beam, for the Ca-intercalated bilayer graphene (Fig. 1(a)).
Three prominent peaks (2.8°, 3.7° and 4.6°) in the curve areidentified. In order to seek the optimum structure, we performed
intensity calculations on the basis of dynamical diffraction
theory. We assumed three different atomic arrangements forplausible structure models; Ca atoms are intercalated (#1)
between the bilayer graphene layers, (#2) between the bilayer
graphene and the SiC(0001) substrate, and (#3) in the both.
Only the curve calculated using the structure model #2 (denoted
as red line in Fig. 1(a)) is in good agreement with the measured
one. The spacing between the bilayer graphene and theSiC(0001) substrate is expanded by approximately 1 Å due to
Ca intercalation (Fig. 1(b)). Whereas, there is no change in theinterlayer spacing in the bilayer graphene layers, which is nearly
the same as that for graphite. Our result differs from the priorprediction based on GICs studies, in which the Ca atoms areintercalated between the bilayer graphene layers.
We also confirmed using four-point probe measurements thatthe superconducting transition occurs at around 4 K for oursample of Ca-intercalated bilayer graphene (Fig. 1(c)).
Our findings will stimulate the reinterpretation of thesuperconducting transition mechanism for graphene.
Furthermore, it might lead to the realization of single-layer
graphene superconductor.

References[1] S. Ichinokura et al., ACS Nano 10, 2761 (2016).
[2] Y. Endo et al., Carbon 157, 857 (2020).
[3] Y. Fukaya et al., J. Phys. D Appl. Phys. 52, 013002 (2019).
[4] Y. Fukaya, in Monatomic Two-Dimensional Layers: Modern
Experimental Approaches for Structure, Properties, andIndustrial Use, Elsevier, 2018.

Fig. 1 (a) TRHEPD rocking curves for Ca-intercalated bilayer
graphene on a SiC(0001) substrate. The beam energy is set at
10 keV. Open circles indicate the measured curve and solidlines are the calculations using various structure models (#1#3). (b) Best-fitted structure model obtained in this study. (c)Temperature dependence of the resistivity for Ca-intercalated
bilayer graphene.