Kulic M. L., Goreatchkovski E., Lichtenstein A. I., Mehring M.
We propose a model for s wave superconductors (the antiferromagnetic superconductor (AFS) model) in the presence of nonuniform (in)commensurate magnetic order and analyse the NMR properties of this model. It is found that the superconducting quasiparticle spectrum has nodes in the gap when the exchange energy h is larger than the superconducting order parameter Delta. This leads to a power-law dependence of various quantities on temperature and frequency. By using the RPA susceptibility chi(ij)(k,omega) the longitudinal and transverse NMR relaxation rates 1/T-1 and 1/T-2, as well as the Knight shift K(T) are calculated in the AFS model. As an example we study an HTS-like system with a t-t' quasiparticle spectrum. The results obtained are compared with experimental results on YBa2Cu3O6+x. A reasonable fit for K(T) is obtained for the ratio 2 Delta/T-c = 6-8 and h approximate to (0.1-0.2)t. At low T one has 1/T-1 similar to T-3 for both O-17 and Cu-63, while (1) the coherence Hebel-Slichter peak is absent, (2) the slope at T-c fits rather well by the quasiparticle damping rate Gamma = 0.8T(c)(T/T-c)(3). The temperature dependence of the rate 1/T-2 is similar to case of the d wave pairing. The proposed AFS theory is general and it is applicable to systems where the coexistence of (in)commensurate magnetic order and superconductivity takes place: magnetic superconductors, heavy-fermion superconductors, and rare-earth nickel boride-carbides RNi(2)B(2)C.
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