NONADIABATIC SUPERCONDUCTIVITY .2. GENERALIZED ELIASHBERG EQUATIONS BEYOND MIGDALS THEOREM

被引:157
作者
GRIMALDI, C
PIETRONERO, L
STRASSLER, S
机构
[1] UNIV ROMA LA SAPIENZA,DIPARTIMENTO FIS,I-00185 ROME,ITALY
[2] HUBA CONTROL AG,CH-8116 WURENLOS,SWITZERLAND
来源
PHYSICAL REVIEW B | 1995年 / 52卷 / 14期
关键词
D O I
10.1103/PhysRevB.52.10530
中图分类号
T [工业技术];
学科分类号
08 ;
摘要
We generalize the Eliashberg equations to include the first nonadiabatic effects beyond Migdal's theorem. The resulting theory is nonperturbative with respect to lambda and perturbative with respect to ((lambda omega(D)/E(F)). The main effects are due to the vertex corrections and the cross diagram that show a complex behavior with respect to the exchanged momentum (q) and frequency (omega). Positive corrections and a corresponding enhancement of T-c arise naturally if the electron phonon scattering is characterized mainly by small q values. For this reason we discuss our results in terms of an upper cutoff q(c) for the scattering. The generalized Eliashberg equations are solved numerically and analytically and we also provide a generalization of the McMillan equation that includes the nonadiabatic effects. For relatively small values of q(c), normal values of the coupling (lambda similar or equal to 0.5-1.0) can lead to strong enhancements of T-c in the range of the observed values. This situation leads also to a complex behavior of the isotope effect that can be anomalously large (alpha > 1/2) in some region of parameters but it can also vanish for omega(D) > E(F). It is therefore important to identify which features of more realistic models can lead to a situation in which the nonadiabatic effects are mainly positive. One way to achieve this is to have an upper cutoff q(c) for the electron-phonon scattering and electronic correlations appear to be a natural candidate to produce this effect. The nonadiabatic effects are expected to play an important role also for the normal properties of the system that can deviate appreciably from a normal Fermi liquid. It is important to study these effects in the future because they should lead to specific predictions of new effects that can be tested experimentally.
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页码:10530 / 10546
页数:17
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