Superconductivity of K‐Intercalated Epitaxial Bilayer Graphene

Abstract

In particular, graphene-based materials attracted considerable scientific interest due to the recent discovery of superconductivity in twisted bilayer graphene. [3][4][5] Superconductivity has also been observed in layered, graphene-based systems upon chemical doping via metal intercalation, with transition temperatures up to 11.5 K for calcium-intercalated bulk graphite. [6,7] However, for metal-intercalated, graphenebased systems the reported critical temperatures strongly depend on i) the choice of the atomic species used for metal intercalation and ii) the number of adjacent graphene layers. [6][7][8][9][10][11][12][13] Superconductivity in metal-intercalated bulk graphite is believed to originate from electron-phonon coupling (EPC) of electrons located in the energetically shifted π* bands of graphene and parabolic interlayer states, which are occupied upon chemical doping. There, the electrons are coupled to vibrations of the graphene lattice and the metal intercalants, where low-energy modes are particularly important. [14][15][16][17][18][19][20] Accordingly, the deviating properties observed for finite numbers of graphene layers are attributed to effects of low dimensionality that might alter the EPC strength and the interlayer state. [11,13,21] However, the mechanisms leading to superconductivity in graphene thin-films are not entirely understood yet. Both the existence of multiple energy gaps that originate from pairing on different Fermi surfaces [22] and various pairing symmetries [23][24][25][26][27] have been predicted for monolayer and few-layer graphene.Detailed structural and electronic properties of our K-intercalated epitaxial bilayer graphene (BLG) on SiC(0001) are characterized in a separate publication. [28] The K atoms form a highly ordered (2 × 2) registry with respect to the graphene lattice below the topmost graphene layer, see Figure 1. However, potassium also intercalates between the SiC substrate and the buffer layer, which causes electronic decoupling of the latter from the underlying substrate. Consequently, epitaxial monolayer graphene (MLG) undergoes a transition to K-intercalated epitaxial BLG as sketched in Figure 1c, where the lateral structure of the lower-lying potassium atoms remains unknown. This configuration promotes a rigid shift of the energetically non-separated Dirac bands of both layers to higher binding energies, accompanied by a filling of two interlayer bands, which were determined on both local and area-averaged scales via Fourier-transform scanning tunneling Graphene-based materials are among the most promising candidates for studying superconductivity arising from reduced dimensionality. Apart from doping by twisted stacking, superconductivity can also be achieved by metalintercalation of stacked graphene sheets, where the properties depend on the choice of the metal atoms and the number of graphene layers. Many different and even unconventional pairing mechanisms and symmetries are predicted in the literature for graphene monolayers and few-layers. However, t...

Superconductivity in Ca-intercalated bilayer graphene: C2CaC2

“…[11][12][13][14][15][16][17][18][19][20] For the fabrication of 2D superconducting graphene, the process involves the deposition of alkali metal or alkaline-earth metal onto monolayer graphene, as well as the intercalation of alkali metal atoms into bilayer graphene. 21 For example, superconductivity has been first detected in alkali metal deposited graphene KC 8 with T c of 0.55 K. 22 Moreover, alkaline-earth metal Ca-deposited graphene CaC 6 and alkali metal Li-deposited graphene LiC 6 were theoretically predicted to be phonon-mediated superconductors with superconducting T c of 1.4 and 8.1 K, 23 respectively. Moreover, after applying 10% biaxial tensile strain, it was predicted that the T c of LiC 6 increases from 8.1 K to about 28.7 K. 24 Subsequent experiments have confirmed the superconductivity of LiC 6 at a temperature of 5.9 K. 25 It has been predicted that metals-deposited graphene, such as AlC 8 , could exhibit superconductivity with T c exceeding 22 K at the experimentally accessible hole doping and tensile strain levels.…”

“…Subsequent theoretical calculations have revealed that C 6 CaC 6 possesses the capability to sustain phonon-mediated superconductivity, with a T c ranging from 6.8 to 8.1 K. 28 Recently, a research group successfully introduced Ca atoms into the top bilayer of a trilayer graphene film on the SiC substrate to form C 6 CaC 6 , and observed T c of 8.83 K. 29 The above experimental results 27,29 agree excellently with the theoretical results. Nowadays, an experimental study has successfully prepared K-intercalated bilayer graphene, and the resulting T c was determined to be 3.6 K. 21 However, the T c of them are comparatively low. The open question in the current study is whether the T c of bilayer graphene superconductors can be increased.…”

Surface Properties of Graphene and Graphene Oxide Aerogels for Energy Storage Applications

“…It can conduct thermal and electrical conductivity even higher than copper. Since it has a high ability to conduct thermal conductivity, it will reduce the battery's temperature by dissipating the resulting heat, thus increasing the battery's efficiency and allowing it to be used at high temperatures [17]. It also has a high degree of strength and flexibility in addition to its low weight, which is about 5 times less than the weight of aluminum.…”

Van Hove Singularity and Enhanced Superconductivity in Ca-Intercalated Bilayer Graphene Induced by Confinement Epitaxy

“…Graphene-based superconductors are promising for diverse applications owing to their optical transparency, mechanical strength, and flexibility. − Among these materials, C 6 CaC 6 , a bilayer graphene intercalated with calcium, exhibits the highest critical temperature ( T c ). − It was synthesized via molecular beam epitaxy (MBE) in an ultrahigh vacuum from trilayer, bilayer, , and monolayer graphene.…”