Experiment and theoretical study of electronic correlations in carbon nanotubes and graphite from Auger spectroscopy

Cini, Michele1; Perfetto, Enrico1; Ugenti , Simona1; Castrucci, Paola1; De Crescenzi, Maurizio1; Rosei, Francesco2; El Khakani, M. A.2 1Italy; 2Canada

Carbon compounds are attracting recently much the attention after the discovery of several unusual physical properties. Among them we mention the observation of the integer quantum Hall effect in planar graphene and the measurement of superconductivity at 11.5 K in Ca intercalated graphite and intrinsic superconductivity at 15 K in ultrasmall single-wall and at 12 K in multi-wall nanotubes. Here we give a quantitative estimate of the screened on-site Coulomb repulsion U in carbon nanotubes and HOPG graphite, which is a crucial parameter entering in any theoretical interacting model and ab initio LDA+U calculations. This is obtained by measuring the Auger spectrum of these materials and performing a theoretical study based on the Green-function method proposed by Cini[1]. The experimental lineshape is very well reproduced by the theory, where only one fit parameter is employed. This allows to extract the value of the screened on-site repulsion between 2p states, which results U= 5.0 eV for nanotubes and U=2.4 eV for graphite. [1] M. Cini, Solid. St. Comm. 24, 681 (1977).

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