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Abstract:

We report high-field magnetotransport studies on quasi-two dimensional β″-(BEDT-TTF) 4[(H 3O)M(C 2 O 4) 3±Õ·³Û where Y is a solvent in the anionic layer. By changing the size of the solvent the low temperatures electronic behaviour varies from superconducting (for larger solvents, Y = C 6H 5NO 2 and C 6 H 5CN) to metallic (for smaller solvents, Y = C 5 H 5N and CH 2Cl 2). These changes in the ground state are connected with modications of the Fermi surface, which varies from having one or two pockets for the superconducting charge- transfer salts to at least four pockets in the case of metallic ones. When superconducting, the materials have very large in-plane critical fields (up to 32 T) and enhanced effective masses compared with the metallic compounds. The role of the charge-order fluctuations in stabilizing the superconducting ground state and the effects of intrinsic local disorder is discussed. © 2007 Springer Science+Business Media, Inc.