Bulk Evidence for Single-Gaps-Wave Superconductivity in the Intercalated Graphite SuperconductorC6Yb

Abstract

We report measurements of the in-plane electrical resistivity and thermal conductivity of the intercalated graphite superconductor C 6 Yb down to temperatures as low as T c =100. When a field is applied along the c axis, the residual electronic linear term 0 =T evolves in an exponential manner for H c1 < H < H c2 =2. This activated behavior is compelling evidence for an s-wave order parameter, and is a strong argument against the possible existence of multigap superconductivity. DOI: 10.1103/PhysRevLett.98.067003 PACS numbers: 74.70.Wz, 74.25.Fy, 74.25.Op Carbon is a remarkably versatile element -in its pure form it may exist as an electronic insulator, semiconductor, or semimetal depending on its bonding arrangement. When dopant atoms are introduced, superconductivity may be added to this list, observed in graphite [1,2], fullerenes [3], and even diamond [4]. Superconductivity in doped carbon was first discovered in the graphite intercalate compounds (GICs), materials composed of sheets of carbon separated by layers of intercalant atoms. The first of these compounds contained alkali atoms, and had modest transition temperatures of 0.13-0.5 K [1]. The recent discovery of T c 's two orders of magnitude higher than this in C 6 Yb [5] and C 6 Ca [5,6] has, however, refocused attention on this intriguing family of compounds.The effects of the intercalant atoms in the GICs are twofold: they dramatically change the electronic properties of the host graphite lattice by both increasing the separation of the carbon sheets, as well as contributing charge carriers. This causes the two-dimensional graphite bands to dip below the Fermi level. The graphite interlayer band, previously unoccupied, also crosses the new Fermi level, contributing three-dimensional, free-electron-like states located between the carbon sheets. This new interlayer band hybridizes strongly with the bands, and its occupation appears to be linked with the occurrence of superconductivity in the GICs [7].There are still several fundamental questions remaining about superconductivity in the GICs, especially in C 6 Yb and C 6 Ca, where little experimental data exist. The pairing mechanism is unresolved, with speculation ranging from a conventional route involving the intercalant phonons [8][9][10] to superconductivity via acoustic plasmons [7].Early theoretical studies motivated by the alkali-metal GICs [11,12] emphasized a two-gap model for the superconducting state, where gaps of different magnitudes exist on different sheets of the Fermi surface. Such a scenario is plausible, as there are notable similarities between the GICs and MgB 2 [7,13], a known multigap superconductor. Indeed, some aspects of graphite intercalate superconductivity can be understood by this two-gap phenomenology; however, there is little direct evidence to support this picture, and recent band structure calculations suggest this scenario is unlikely [14].A necessary starting point is to establish the superconducting order parameter, but in C 6 Yb, this task is complicated as th...

Origin of superconductivity of CaC6 and of other intercalated graphites

“…Similar conclusions have been obtained from the specific heat jump at superconducting transition [14]. Thermal conductivity data in the presence of a magnetic field [15] indicate that in YbC 6 the order parameter has s-wave symmetry too and exclude the occurrence of multiple gaps. Isotope effect measurements [16] show a huge Ca isotope coefficient, (Ca) 0 5 α = .…”

Superconductivity at 5 K in alkali-metal-doped phenanthrene

“…14), respectively. Such high transition temperatures are unprecedented although it is characterized by single-gap s-wave superconductivity 15 and the coupling with intercalant phonon is believed to be main force for superconductivity 16 . Therefore, it indicates that superconductivity for the superconductors with π-electron networks is not simply explained by BCS theory.…”

Nonadiabatic phonons within the doped graphene layers ofXC6compounds

“…The question of the electron-phonon interaction is also interesting in light of the discovery of superconductivity at 11.5 K in CaC 6 . 6,7 Measurements of a finite Ca isotope effect 8 and of an s-wave symmetry superconducting gap 9,10 suggests that the superconductivity is electron-phonon mediated. However, several experimental and theoretical techniques implicate different groups of phonons and electrons are responsible for the superconductivity.…”