Ca intercalated bilayer graphene as a thinnest limit of superconducting C ₆ Ca

Abstract

Success in isolating a 2D graphene sheet from bulky graphite has triggered intensive studies of its physical properties as well as its application in devices. Graphite intercalation compounds (GICs) have provided a platform of exotic quantum phenomena such as superconductivity, but it is unclear whether such intercalation is feasible in the thinnest 2D limit (i.e., bilayer graphene). Here we report a unique experimental realization of 2D GIC, by fabricating calcium-intercalated bilayer graphene C 6 CaC 6 on silicon carbide. We have investigated the structure and electronic states by scanning tunneling microscopy and angle-resolved photoemission spectroscopy. We observed a free-electron-like interlayer band at the Brillouin-zone center, which is thought to be responsible for the superconductivity in 3D GICs, in addition to a large π* Fermi surface at the zone boundary. The present success in fabricating Ca-intercalated bilayer graphene would open a promising route to search for other 2D superconductors as well as to explore its application in devices.ARPES | STM | band structure G raphene has attracted considerable attention because it exhibits a variety of unusual physical properties such as the massless charge carriers and the quantum Hall effect (1). Bipolar supercurrent (2) and spin injection at room temperature (3, 4) demonstrate its high potential for application in spintronic devices. To fabricate practical devices based on graphene, it is essential to modify and control electronic parameters such as the sign and concentration of carriers as well as the band gap at the Dirac point. In fact, several attempts have been made to achieve material engineering by introducing an external electric field (5-9) or by depositing atoms or molecules on a graphene sheet (10, 11). In graphite, however, intercalation of guest atoms and molecules into graphite layers (12-15) is known to considerably modify the electronic structure, leading to unique physical properties and technological applications such as superconductivity and rechargeable batteries. It is thus quite challenging to fabricate the thinnest limit of a graphite intercalation compound (GIC), namely, intercalated bilayer graphene, and to investigate its electronic structure to promote graphene engineering.

Observation of a universal donor-dependent vibrational mode in graphene

“…For example, Ca is disordered on monolayer graphene on SiC (ref. 24) and in the present case while it is ordered in GICs and in intercalated few-layer graphene 25 (evidenced by diffraction measurements). This in turn might enhance formation of interlayer bands, which will increase l by providing the additional electronic states for scattering.…”

“…Regarding (i) and (ii), previous results 24 , which are confirmed in our present work, state that Ca does not form an ordered phase when deposited onto monolayer graphene. However, an ordered phase of intercalated Ca atoms has been reported for bilayer graphene 25 . It therefore appears that the intercalation process is important for achieving dopant order.…”

The electronic properties of bilayer graphene

“…Like monolayer, intrinsic bilayer has no band gap between its conduction and valence bands, but the low-energy dispersion is quadratic (rather than linear as in monolayer) with massive chiral quasiparticles [8,9] rather than massless ones. As there are two layers, bilayer graphene represents the thinnest possible limit of an intercalated material [35,36]. It is possible to address each layer separately leading to entirely new functionalities in bilayer graphene including the possibility to control an energy band gap of up to about 300 meV through doping or gating [9,10,[16][17][18][19][20].…”

“…These include excellent electrical conductivity with room temperature mobility of up to 40, 000 cm 2 V −1 s −1 in air [22]; the possibility to tune electrical properties by changing the carrier density through gating or doping [1,8,16]; high thermal conductivity with room temperature thermal conductivity of about 2, 800 W m −1 K −1 [23,24]; mechanical stiffness, strength and flexibility (Young's modulus is estimated to be about 0.8 TPa [25,26]); transparency with transmittance of white light of about 95 % [27]; impermeability to gases [28]; and the ability to be chemically functionalised [29]. Thus, as with monolayer graphene, bilayer graphene has potential for future applications in many areas [21] including transparent, flexible electrodes for touch screen displays [30]; high-frequency transistors [31]; thermoelectric devices [32]; photonic devices including plasmonic devices [33] and photodetectors [34]; energy applications including batteries [35,36]; and composite materials [37,38].…”

Approaching the limits of transparency and conductivity in graphitic materials through lithium intercalation

“…Fundamental studies on intercalation in graphite have been extensively carried out 13 , and nanostructured 2D materials have recently gained much interest 14 . Reports on the intercalation of various species such as FeCl 3 (refs 15,16), Br 17 and Ca 18 in few-layer graphene (FLG) have offered a new route to designing and synthesizing graphene-based materials with novel conductive, magnetic or superconductive properties.…”