Isotope Effect in Superconductors with Coexisting Interactions of Phonon and Nonphonon Mechanisms
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概要
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We examine the isotope effect of superconductivity in systems with coexisting interactions of phonon and nonphonon mechanisms in addition to the direct Coulomb interaction. The interaction mediated by the spin fluctuations is discussed as an example of the nonphonon interaction. Extended formulas for the transition temperature T_c and the isotope-effect coefficient a are derived for cases (a) ω_<np> < ω_D and (b) ω_<np> > ω_D, whereω_<np> is an effective cutoff frequency of the nonphonon interaction that corresponds to the Debye frequency ω_D in the phonon interaction. In case (a), it is found that the nonphonon interaction does not change the condition for the inverse isotope effect, i.e., μ> λ_<ph>/2, but it modifies the magnitude of a markedly. In particular, it is found that a giant isotope shift occurs when the phonon and nonphonon interactions cancel each other largely. For instance, strong critical spin fluctuations may give rise to the giant isotope effect. In case (b), it is found that the inverse isotope effect occurs only when the nonphonon interaction and the repulsive Coulomb interaction, in total effect, work as repulsive interactions against the superconductivity. We discuss the relevance of the present result to some organic superconductors, such as κ-(ET)_2Cu(NCS)_2, and Sr_2RuO_4 superconductor, in which inverse isotope effects have been observed, and briefly to high-T_c cuprates, in which giant isotope effects have been observed
- 社団法人日本物理学会の論文
- 2003-08-15
著者
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Shimahara Hiroshi
Department Of Quantum Matter Science Adsm Hiroshima University
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Shimahara Hiroshi
Department Of Quantum Matter Science Adsm
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