NMR RELAXATION RATES IN SUPERCONDUCTORS WITH (IN)COMMENSURATE MAGNETIC ORDER - A MODEL INSPIRED BY HIGH-T-C SUPERCONDUCTORS

We propose a model for s wave superconductors (the antiferromagnetic s uperconductor (AFS) model) in the presence of nonuniform (in)commensur ate magnetic order and analyse the NMR properties of this model. It is found that the superconducting quasiparticle spectrum has nodes in th e gap when the exchange energy h is larger than the superconducting or der parameter Delta. This leads to a power-law dependence of various q uantities on temperature and frequency. By using the RPA susceptibilit y chi(ij)(k,omega) the longitudinal and transverse NMR relaxation rate s 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' q uasiparticle spectrum. The results obtained are compared with experime ntal 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 c oherence Hebel-Slichter peak is absent, (2) the slope at T-c fits rath er well by the quasiparticle damping rate Gamma = 0.8T(c)(T/T-c)(3). T he 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 applicabl e to systems where the coexistence of (in)commensurate magnetic order and superconductivity takes place: magnetic superconductors, heavy-fer mion superconductors, and rare-earth nickel boride-carbides RNi(2)B(2) C.