High-Temperature Superconductivity in Graphite-Sulfur Composites: Theoretical Analysis
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HIGH-TEMPERATURE SUPERCONDUCTIVITY IN GRAPHITE-SULFUR COMPOSITES: THEORETICAL ANALYSIS D. S. Galvão, B. Laks, R. R. da Silva, J. H. S. Torres, and . Y. Kopelevich Instituto de Física, Universidade Estadual de Campinas – UNICAMP, Campinas, São Paulo, CEP 13083-970, CP 6165, Brazil. ABSTRACT Recently superconductivity in graphite-sulfur composites was experimentally observed. In this work we have analyzed the electronic structure changes associated with the presence of sulfur atoms in one and two dimensional graphite layers. We have considered ordered and disordered sulfur atoms distributions in many configurations. The density of states (DOS) of these structures were obtained using the negative factor counting (NFC) technique coupled to a tight-binding hamiltonian (Hückel type). Our results indicate that the incorporation of sulfur atoms at edge graphite layers (changing their global geometric curvature and increasing the DOS at the Fermi level) might be in the origin of the graphite superconductivity. INTRODUTION Superconductivity in graphite-sulfur composites occurring below the critical temperature of ~ 35 K has been recently reported1. The Meissner-Ochsenfeld effect, screening supercurrents, and magnetization hysteresis loops characteristic of type-II superconductors were observed. The results indicated that superconductivity occurred in a small sample fraction, possibly related to sample surface. It has been speculated2 that topological disorder in graphene (graphite) sheets could trigger superconducting instabilities. It is interesting to notice that superconductor MgB2, that has been recently the focus of much attention3, is very similar to graphite both electronically and crystalographically. In this work we have analyzed the electronic structure changes associated with the presence of sulfur atoms in one-dimension (1D) and two-dimensions (2D) graphite layers. The calculated electronic changes are addressed in terms of the experimentally observed high temperature superconductivy.
METHODOLOGY We have considered ordered and disordered sulfur atom distributions in many configurations (Fig. 1). We have analyzed the limit situations of sulfur incorporation at the edge layers (Figs. 1a and 1b), and also as connecting ‘percolative’ islands (Fig. 1c). We have considered structures up to 25,000 atoms where the number of occupied X-sites (sulfur atoms) is randomly occupied depending on their concentration (varying from 0 up to 100% of available X-sites). The corresponding density of states (DOS) of these structures are calculated using the negative factor counting (NFC) technique4,5 coupled to a tight-binding hamiltonian (Hückel type)5.
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Figure 1 – Schematic representation of the analyzed structures: (a) 1D polyacene; (b) 2D polyacene; (c) connected polyacene ‘islands’. X represents the possible sites for sulfur atom incorporation. The NFC technique allows us to obtain the eigenvalues of ver
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