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TITLE: Enhancement of the Upper Critical Field Due to a Fermi-Surface Effect in Quasi-Two-Dimensional Superconductors in Parallel Magnetic Fields |
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AUTHOR(S): Hiroshi Shimahara |
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JOURNAL: J.Phys.Soc.Jpn. 68 No.9, (1999) pp.3069-3075. |
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ABSTRACT: It is shown that the upper critical field of the superconductivity can be remarkably enhanced due to the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) effect, even if the system does not have any flat portion of the Fermi-surface (FS). As an example, we examine a two-dimensional tight binding model for various hole concentrations. The optimum electron concentration at which the ratio of the critical field to the Pauli paramagnetic limit of the uniform BCS state takes maximum is found to be far away from the half-filling. Near there, the ratio exceeds 5 times the Pauli paramagnetic limit. We explain a new mechanism by which the critical field is enhanced due to an FS structure. The FFLO state is discussed in connection with the organic superconductors. |
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KEYWORDS: Fulde-Ferrell-Larkin-Ovchinnikov state, FFLO state, nonuniform superconductivity, organic superconductor, two-dimensional system, Pauli paramagnetic limit, upper critical field |