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**Chapter 0**

**Chapter 9**

**Eilenberger Approach to the Vortex State**

The SC gap, which characterizes the energy cost for breaking a Cooper pair, is an important quantity when clarifying the SC mechanism. The gap size and its momentum dependence reflect the strength and anisotropy of the pairing interactions, respectively. Some experiment executed by Li *et al*. [1] in response to a suggestion by Klemm [2] tested the phase of the wave function in Bi2Sr2CaCu2O8 and revived the *s*-wave viewpoint [3, 4]], which, although championed by Dynes's group [4], had been out of favor even for Bi2Sr2CaCu2O8, although not disproven. This experiment once more created uncertainty over whether the superconducting pairs are consistent with *s*-wave or *d*-wave superconductivity (Van

The discovery of Fe-based superconductors [8] generated intensive debate on the superconducting (SC) mechanism. Motivated by high-*Tc* values up to 56 K [9], the possibility of unconventional superconductivity has been intensively discussed. A plausible candidate is the SC pairing mediated by antiferromagnetic (AFM) interactions. Two different approaches, based on the itinerant spin fluctuations promoted by Fermi-surface (FS) nesting [10, 11], and the local AFM exchange couplings [12], predict the so-called *s*±-wave pairing state, in which the gap shows a *s*-wave symmetry that changes sign between different FSs. Owing to the multiorbital nature and the characteristic crystal symmetry of Fe-based superconductors, *s*++-wave pairing without sign reversal originating from novel orbital fluctuations has also been proposed [13, 14]. The unconventional nature of the superconductivity is supported by experimental observations such as strongly FS-dependent anomalously large SC gaps [15–17] and the possible sign change in the gap function [18, 19] on moderately doped BaFe2As2, NdFeAsO and FeTe1−*x*Se*x*. However, a resonance like peak structure, observed by neutron scattering measurements [18], is reproduced by considering the strong correlation effect via quasiparticle damping, without the necessity of sign reversal in the SC gap [20]. Although the *s*±-wave state is expected to be very fragile as regards impurities due to the interband scattering [21], the superconducting state is remarkably robust regarding impurities and

> ©2012 Belova et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0),which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly

©2012 Belova et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**in Iron Pnictide Superconductors**

I. Zakharchuk, P. Belova, K. B. Traito and E. Lähderanta

Additional information is available at the end of the chapter

Harlingen [5], Ginsberg [6], Tsuei and Kirtley [7]).

cited.

http://dx.doi.org/10.5772/48571

**1. Introduction**

*α*-particle irradiation [22].
