Abstract
This Chapter lays the foundations for the interpretation of the NMR measurements in the superconducting state which will be presented in Chapter 7. The behavior of the spin shift for spin-singlet and spin-triplet superconductors will be discussed in Section 3.1. Possible other shift contributions will also be pointed out. Section 3.2 starts with a detailed deduction of the spin-lattice relaxation rate of a BCS superconductor in the superconducting state. After deducing the spin-lattice relaxation rate of a single-band BCS superconductor and comparing it to the ultrasonic attenuation, some additional remarks about other possible gap symmetries will be shortly made. As will be pointed out, the comparison between NMR and ultrasonic attenuation was a significant proof of the BCS theory. At the end of Section 3.2 it will be shown what one should expect for the spin-lattice relaxation rate in a two-band model for s++ and s± symmetries of the superconducting order parameter, since these symmetries will be important for the discussion of the spin-lattice relaxation rate measurements on pnictides later on.
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- 1.
This transformation contains the constraint, that if one wants to describe only one excited quasiparticle \( {\vec k\sigma } \) out of a superconducting Cooper pair \( {\vec k\sigma , - \vec k - \sigma } \), one has to annihilate simultaneously its partner \( { - \vec k - \sigma } \), since the excitation of one quasiparticle always entails the excitation of its Cooper pair partner.
- 2.
The general treatment of the effect of coupling strength is done by applying the Eliashberg theory, which is the generalization of the BCS theory to arbitrary coupling strengths.
- 3.
A classical BCS gap fulfills the relation \( 2\Delta = 3.52{k_B}{T_c} \).
- 4.
Some examples of superconducting energy gaps: bulk aluminum: \( \Delta = 0.16{\rm{meV}} \) meV [109],
Bi2Sr2CaCu2O8: \( \Delta = 0.16{\rm{meV}} \) [110]
LiFeAs: \( {\Delta _1} = 1.5 - 2.5{\rm{meV}} \) and \( {\Delta _2} = 2 - 3.5{\rm{meV}} \) [111],
Ba1-x K x Fe2As2: \( {\Delta _1} = 9.1{\rm{meV}} \) and \( {\Delta _2} = 1.5{\rm{meV}} \) [112].
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© 2012 Vieweg+Teubner Verlag | Springer Fachmedien Wiesbaden
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Hammerath, F. (2012). NMR in the Superconducting State. In: Magnetism and Superconductivity in Iron-based Superconductors as Probed by Nuclear Magnetic Resonance. Vieweg+Teubner Verlag, Wiesbaden. https://doi.org/10.1007/978-3-8348-2423-3_3
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DOI: https://doi.org/10.1007/978-3-8348-2423-3_3
Publisher Name: Vieweg+Teubner Verlag, Wiesbaden
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Online ISBN: 978-3-8348-2423-3
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