Itinerant-Electron Magnetism in the Heisenberg Limit
Abstract
The Hubbard model in the Heisenberg limit is studied by Kondo-lattice theory. The Kondo temperature TK or kBTK, which is an energy scale of low-energy local quantum spin fluctuations, is enhanced by the resonating valence bond (RVB) mechanism, so that TK TMF/(2D), where TMF is the Neel temperature in the mean-field approximation of the corresponding Heisenberg model and D is the spatial dimensionality. Electrons certainly behave as localized spins at T TK, but they are still itinerant at T TK unless an antiferromagnetic complete gap opens. When the Neel temperature TN is so high that TN TMF/(2D), magnetism is prototypic local-moment magnetism. When TN is so low that TN TMF/(2D) because of low dimensionality or frustration, magnetism is itinerant-electron magnetism of an almost spin liquid, i.e., a normal Fermi liquid or a Tomonaga-Luttinger liquid in which the spectral weight of single-particle excitations is almost vanishing. The spin susceptibility has a temperature and wave-number dependence characteristic of itinerant-electron magnetism. This type of itinerant-electron magnetism must also be possible in the Heisenberg model.