Density Functional Study of Fullerene-Based Solids: Crystal Structure, Doping, and Electron -Phonon Interaction
Two decades after the discovery of C60, fullerenes continue to be intensely studied for their diverse properties, not the least of which is their unusually high superconducting transition temperature T c (up to 40 K in Cs3C60). Ab initio electronic structure methods are able to contribute to our understanding of these materials by providing electron densities, band structures, density-of-states, binding energies, and even the electron-phonon coupling. Because of the increasing computational feasibility of these large-scale calculations, these methods now play a prominent role in verifying and guiding experimental investigation of new materials. This dissertation presents results of a theoretical investigation of several scenarios where Tc enhancement exceeding those found in the alkali-doped fullerides could be exhibited: (1) field-effect doping of C60 layers, (2) C28-derived molecular solids and (3) covalently bonded C28 solids. The method employed is the Kohn-Sham formulation of density functional theory. Simpler tight-binding calculations are also used when appropriate. The study of field-effect doping was stimulated by the reports of large Tc enhancements by Schon et al., which were later retracted and found to be falsified. Even before the legitimacy of these reports came into question, we concluded that our calculations did not substantiate their claims. We present our electronic structure calculations and conclusions which are independent and potentially useful for future work on field-effect devices. The main part of this dissertation is a separate study on C28-based solids motivated by theoretical arguments suggesting that solids based on smaller fullerenes could exhibit a Tc enhancement. Among many molecular solids formed from closed-shell C28-derived molecules, we have found that solid C28H4 binds weakly and exhibit many of the salient features of solid C60, with an estimated Tc of 58 K. In this same spirit, we also study covalent solids based on endohedrally doped C28 molecules which have been produced experimentally. A qualitative estimate of Tc in these latter materials is difficult due to the role of disorder.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign