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

We have recently discovered that highly doped GICs can superconduct at much higher temperatures than we previously thought possible. This project was motivated by the work of Monthoux and Lonzarich [1] which posits that superconductivity is robust in 2-D antiferromagnetic systems. For this reason we decided to explore marginally magnetic graphite intercalation compounds. These compounds are formed through the insertion of the metal species into the galleries between the graphene sheets. To prepare these compounds we decided to use ytterbium (Yb) as the magnetic species. This has been found to exhibit intermediate valence compounds which show antiferromagnetic coupling [2]. In order to explore the importance of magnetism we also used calcium (Ca) which is iso-electronic with ytterbium. The ytterbium compound, C 6 Yb, was found to superconduct at 6.5 kelvin. This was an exciting result. We then studied C 6 Ca and found this compound to be superconducting at 11.5 kelvin. These values are some 40 and 80 times higher than that found in the other principle superconducting graphite compound C 8 K. However, this discovery of superconductivity does not fit neatly into magnetically mediated picture. The work of the theory group at Cambridge (TCM) suggests that it points to another exotic mechanism of exciton / plasmon enhanced phonon coupling [5].


In the figure right we see the layered nature of these compounds. This structure was first identified [6] as P6/3 mmc with the Yb atoms sited on a triangular lattice. The structure of C6Ca was first found [7] to be hexagonal, but has recently the space group was identified as rhombohedral R-3m[8]. 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 [5].

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 [9] 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.


[1] P. Monthoux & G.G. Lonzarich, Magnetically mediated superconductivity in quasi-two and three dimensions, Phys. Rev B, 63 054529 (2001).

[2] E. Bauer, Non-Fermi-liquid behaviour of ytterbium compounds, Journal of Magnetism and Magnetic Materials, 196, 873 (1999)

[3] J. Plessel, M.M. Abd-Elmeguid, J.P. Sanchez, G. Knebel, C. Geibel, O. Trovarelli & F. Steglich, Unusual behavior of the low-moment magnetic ground state of YbRh2Si2 under high pressure, Physical Review B, 67, 180403/1-4 (2003).

[4] T.E. Weller, M. Ellerby, S.S. Saxena, R.P. Smith & N.T. Skipper, Superconductivity in the Intercalated Graphite Compounds C6Yb and C6Ca, March 2005.

[5] Cs´anyi G., Littlewood P. B., Nevidomskyy A. H., Pickard C. J. and Simons B. D., Electronic Structure of the Superconducting Graphite Intercalates, March 2005.

[6] M. El Makrini, D. Guérard, P. Lagrange & A. Hérold, Intercalation of rare earth metals in graphite. Physica, 99B, 481 (1980).

[7] D. Guérard, M. Chaabouni, P. Lagrange, M. El Makrini & A. Hérold, Insertion de metaux alcalino-terraux dans le graphite. Carbon 18, 257 (1980); S. Pruvost, C. Hérold, A Hérold & P. Lagrange, Co-intercalation into graphite of lithium and sodium with an alkaline earth. Carbon 42, 1825 (2004).

[8] N. Emery, C. Hérold, M. d’Astuto,V. Garcia,Ch. Bellin, J. F. Marêché,1 P. Lagrange, & G. Loupias, Superconductivity of Bulk CaC6, Phys. Rev. Letters, 95, 087003 (2005).

[9] T. Enoki, S. Masatsugu and E. Morinobu, Graphite Intercalation Compounds and Applications. OUP: Oxford (2003).

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