Nuclear spin lattice relaxation rate in the excitonic state

Abstract

The nuclear spin lattice relaxation rate in the excitonic state is studied. Use is made of the two-band model, where the Fermi radii of the electron band and the hole band are assumed to be of the same size. It is important to distinguish two cases; the spin singlet case (i.e., the nonmagnetic excitonic state) and the spin triplet case (i.e., the antiferromagnetic state). In the case of the spin singlet excitonic state the nuclear spin lattice relaxation rate first increases in the excitonic state just below the transition temperature and then decreases rapidly as the temperature decreases. In the case of the spin triplet excitonic state the relaxation rate depends on whether the nuclear spin is polarized parallel or perpendicular to the spin-density wave describing the excitonic state. In the parallel case the relaxation rate decreases monotonically in the excitonic state as the temperature decreases, while in the transverse case it has a small peak just below the transition temperature.