Charge density waves in the graphene sheets of the superconductor CaC(6).

Graphitic systems have an electronic structure that can be readily manipulated through electrostatic or chemical doping, resulting in a rich variety of electronic ground states. Here we report the first observation and characterization of electronic stripes in the highly electron-doped graphitic superconductor, CaC(6), by scanning tunnelling microscopy and spectroscopy. The stripes correspond to a charge density wave with a period three times that of the Ca superlattice. Although the positions of the Ca intercalants are modulated, no displacements of the carbon lattice are detected, indicating that the graphene sheets host the ideal charge density wave. This provides an exceptionally simple material-graphene-as a starting point for understanding the relation between stripes and superconductivity. Furthermore, our experiments suggest a strategy to search for superconductivity in graphene, namely in the vicinity of striped 'Wigner crystal' phases, where some of the electrons crystallize to form a superlattice.

Electronic structure of superconducting KC8 and nonsuperconducting LiC6 graphite intercalation compounds: evidence for a graphene-sheet-driven superconducting state.

We have performed photoemission studies of the electronic structure in LiC(6) and KC(8), a nonsuperconducting and a superconducting graphite intercalation compound, respectively. We have found that the charge transfer from the intercalant layers to graphene layers is larger in KC(8) than in LiC(6), opposite of what might be expected from their chemical composition. We have also measured the strength of the electron-phonon interaction on the graphene-derived Fermi surface to carbon derived phonons in both materials and found that it follows a universal trend where the coupling strength and superconductivity monotonically increase with the filling of graphene π(*) states. This correlation suggests that both graphene-derived electrons and graphene-derived phonons are crucial for superconductivity in graphite intercalation compounds.

Magnetic behaviour in Dy(1-x)Mm(x)Co2 compounds.

The magnetic behaviour in Dy(1-x)Mm(x)Co(2) (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5; Mm = mischmetal) compounds is reported using temperature and field dependence of magnetization (M-T and M-H respectively) measurements. A strong composition dependent irreversibility is observed in both the M-T and M-H scans below the magnetic ordering temperature (T(C)). A clear change of the first-order magnetic transition of DyCo(2) to a second-order one in Dy(0.5)Mm(0.5)Co(2) is evidenced by M-T and a series of Arrott (M(2) versus H/M) plots, obtained from the M-H isotherms around T(C). The variation in induced moments of the Co sublattice is estimated. It is found that the Mm substitution can only lead to a considerable reduction in the T(C), saturation magnetization, and Co moment. The observed behaviour of M-T and M-H plots with increasing Mm content is discussed in detail.

Anisotropic electron-phonon coupling and dynamical nesting on the graphene sheets in superconducting CaC6 using angle-resolved photoemission spectroscopy.

We present the first angle-resolved photoemission studies of electronic structure in CaC6, a superconducting graphite intercalation compound with T_{c}=11.6 K. We find that, contrary to theoretical models, the electron-phonon coupling on the graphene-derived Fermi sheets with high-frequency graphene-derived phonons is surprisingly strong and anisotropic. The shape of the Fermi surface is found to favor a dynamical intervalley nesting via exchange of high-frequency phonons. Our results suggest that graphene sheets play a crucial role in superconductivity in graphite intercalation compounds.