Abstract
Pair formation occurs when the free energy for charge disproportionation is negative, i.e., U < 0. The Mott-Hubbard energy U is here approximated by U = I − A−1/R, where I − A > 0 is the difference between ionization energy and electron affinity, and R is the distance between the lattice sites. −1/R (atomic units) is the particle-hole attraction after charge separation in the case of two C60 ions, the much needed negative term. In the case of alkali C60 salts of type A3C60, the attractive term 1 /R is the smallest for the largest ion A+, i.e., Cs+. Hence, U is increasing with the lattice constant R, i.e., with the size of the alkali ion. Lack of superconductivity in Cs3C60 can be interpreted as U >0 for this salt, while U < 0 for the other A3C60. This is also consistent with the antiferromagnetic ground state for Cs3C60 at ambient pressure, and superconductivity for pressures >4 kbar.
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Larsson, S. T C Dependence on Hubbard U in the Fullerenes. J Supercond Nov Magn 28, 315–317 (2015). https://doi.org/10.1007/s10948-014-2881-9
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DOI: https://doi.org/10.1007/s10948-014-2881-9