In this letter we report the discovery of superconductivity in the isostructural graphite intercalation compounds C 6 Yb and C 6 Ca, with transition temperatures of 6.5K and 11.5K respectively. A structural characterisation of these compounds shows them to be hexagonal layered systems in the same class as other graphite intercalates. If we assume that all the outer s-electrons are transferred from the intercalant to the graphite sheets, then the charge transfer in these compounds is comparable to other superconducting graphite intercalants such as C 8 K 1,2 . However, the superconducting transition temperatures of C 6 Yb and C 6 Ca are up to two orders of magnitude greater. Interestingly, superconducting upper critical field studies and resistivity measurements suggest that these compounds are significantly more isotropic than pure graphite. This is unexpected as the effect of

We have studied the evolution, with hydrostatic pressure, of the recently discovered superconductivity in the graphite intercalation compounds C6Yb and C6Ca. We present pressuretemperature phase diagrams, for both superconductors, established by electrical transport and magnetization measurements. In the range 0 − 1.2GPa the superconducting transition temperature increases linearly with pressure in both materials with dTc/dP = +0.39 ± 0.01K/GPa and dTc/dP = +0.50 ± 0.05K/GPa for C6Yb and C6Ca respectively. The transition temperature in C6Yb, which has been measured up to 2.3 GPa, reaches a peak at around 1.8 GPa and then starts to drop. We also discuss how this pressure dependence may be explained within a plasmon pairing mechanism.

The field of superconductivity in the class of materials known as graphite intercalation compounds has a history dating back to the 1960s [1,2]. This paper recontextualizes the field in light of the discovery of superconductivity in CaC6 and YbC6 in 2005. In what follows, we outline the crystal structure and electronic structure of these and related compounds. We go on to experiments addressing the superconducting energy gap, lattice dynamics, pressure dependence, and how this relates to theoretical studies. The bulk of the evidence strongly supports a BCS superconducting state. However, important questions remain regarding which electronic states and phonon modes are most important for superconductivity and whether current theoretical techniques can fully describe the dependence of the superconducting transition temperature on pressure and chemical composition.Corresponding author. Tel: +44 (0)7792954220 fax: +44 (0)20 7679 7145 e-mail address: mark.ellerby@ucl.ac.uk (Mark Ellerby).

We present the results of a neutron scattering study of the high energy phonons in the superconducting graphite intercalation compound CaC6. The study was designed to address hitherto unexplored aspects of the lattice dynamics in CaC6, and in particular any renormalization of the out-of-plane and in-plane graphitic phonon modes. We present a detailed comparison between the data and the results of density functional theory (DFT). A description is given of the analysis methods developed to account for the highly-textured nature of the samples. The DFT calculations are shown to provide a good description of the general features of the experimental data. This is significant in light of a number of striking disagreements in the literature between other experiments and DFT on CaC6. The results presented here demonstrate that the disagreements are not due to any large inaccuracies in the calculated phonon frequencies.

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...