Superconductivity, Structure and Staging of Graphite Intercalates
Tom Weller1, Mark Ellerby1, Neal Skipper1, Siddharth Saxena2, Robert Smith2
1London Centre for Nanotechnology, University College London, Gower Street, London WC1E 6BT, UK
2Cavendish Laboratory,Madingley Road, Cambridge CB3 0HE
In the figure right we see the layered nature of these compounds. This structure was first identified  as P6/3 mmc with the Yb atoms sited on a triangular lattice. The structure of C6Ca was first found  to be hexagonal, but has recently the space group was identified as rhombohedral R-3m. This is also a layered compound, with the Ca sited on a triangular lattice but the unit cell is somewhat larger. In this image left fhe flux expulsion is seen clearly in both figures. The measurements were made using a SQUID magnetometer with an applied field of 50 Oe.
Using measurements of the temperature and field dependence for each of these compounds (left) we have identified the magnetic phase diagrams. From these figure we see that that there is an anisotropy in the upper critical field (HC2) for each field geometry of approximately 2. This anisotropy is determined the the Ginzburg theory by the effective masses. If we calculate a value of anisotropy based on pure graphite we find a value of 7. This suggests a move toward a more 3-D and is consistent with the band-structure calculations .
In this figure (right) we plot the transition temperature as a function of charge transferred per carbon by the metal species. These two new superconductors when plotted alongside those of earlier work  show a strong departure from any trend. There are two compounds C6Li and C3Li are missing from this picture. Most importantly C3Li should be superconducting if a simple charge transfer picture were relevant. Note that with exception of C8K, C8KHg and C4KHg all the other phases are formed under extreme conditions and exhibit metastability.
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