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Advances in Condensed Matter Physics
Volume 2010 (2010), Article ID 627452, 6 pages
http://dx.doi.org/10.1155/2010/627452
Research Article

Phonons in A 3 C 6 0 Lattice and Structural Dynamics

Department of Chemistry, Chalmers University of Technology, 412 96 Göteborg, Sweden

Received 2 June 2009; Accepted 14 September 2009

Academic Editor: Dragan Mihailovic

Copyright © 2010 Sven Larsson. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The critical temperature ( 𝑇 𝐶 ) of superconductivity in A 3 C 6 0 compounds is generally lower smaller with alkali atoms (A). Furthermore 𝑇 𝐶 decreases with applied pressure. In the BCS model, these trends are explained by the lower density of states at the Fermi level for a decreased lattice constant (R). There is more than one counterexample, however, suggesting that BCS does not give the whole truth. The most important one is that the compound with the largest lattice constant, C s 3 C 6 0 , is not superconducting at all at ambient pressure. In this paper we derive a novel model where a negative lattice contribution to Hubbard U, proportional to 1/R, is taken into account. It is possible to explain why A 3 C 6 0 compounds with A = Li, and Na have a low 𝑇 𝐶 or are not superconducting at all, and why C s 3 C 6 0 is superconducting only at applied pressure and then with the highest 𝑇 𝐶 of all C 6 0 alkali fullerides. It is concluded that the density of states mechanism derived in the BCS model is in doubt. Nevertheless superconductivity in A 3 C 6 0 depends on electron-phonon coupling. The dominating phonon is the bond stretching A g phonon, a breathing phonon for the whole fullerene molecular ion.