Coupling of order parameter and spin fluctuations in underdoped high-T-c cuprates

We calculate self-consistently the spin and d(x2-y2)-wave order-parameter fluctuations (OPFL's) above T-c within an extension of the FLEX (fluctuation exchange approximation for the two-dimensional Hubbard model. The quasiparticle interaction due to OPFL's can compete with that due to spin fluctuations for temperatures near T-c and short pairing correlation lengths like those in the cuprates. The coupling of these fluctuations via self-energy renormalization leads to a suppression of T-c and to a decrease of the dynamical spin susceptibility for decreasing T below a crossover temperature T* > T-c. The temperature T* increases while T-c decreases with increasing strength of OPFL`s or decreasing doping. The resulting neutron-scattering intensity and NMR relaxation rates 1/T-1 and 1/T-2G decrease below T* for decreasing T. This agrees qualitatively with the observed spin gap behavior in the underdoped high-T-c cuprates.