Theoretical interpretation of the variety of spin gap in high-T_c cuprates

Magnetic properties of underdoped high-T_c cuprates characterized by a spin gap are interpreted in a unified scheme based on the mode-mode coupling theory. By taking one phenomenological relation for the damping rate of the collective modes, it takes account of two different types of pseudogap behavior in spin excitations and spin gap: (I) the damping rate of spin excitations decreases as the temperature decreases, but the spin correlation length does not; (II) the spin correlation length itself turns to decrease. In case (I), low-energy spectral weights shift to higher energies near the spin gap determined by the superconducting transition temperature, which reproduces the observed resonance peak.