Weak ferromagnetism with the Kondo screening effect in the Kondo lattice systems

We carefully consider the interplay between ferromagnetism and the Kondo screening effect in the conventional Kondo lattice systems at finite temperatures. Within an effective mean-field theory for small conduction electron densities, a complete phase diagram has been determined. In the ferromagnetic ordered phase, there is a characteristic temperature scale to indicate the presence of the Kondo screening effect. We further find two distinct ferromagnetic long-range ordered phases coexisting with the Kondo screening effect: spin fully polarized and partially polarized states. A continuous phase transition exists to separate the partially polarized ferromagnetic ordered phase from the paramagnetic heavy Fermi liquid phase. These results may be used to explain the weak ferromagnetism observed recently in the Kondo lattice materials.Comment: 6 pages, 6 figures; published versio

D-wave superconductivity induced by short-range antiferromagnetic correlations in the two-dimensional Kondo lattice model

The possible heavy fermion superconductivity is carefully reexamined in the two-dimensional Kondo lattice model with an antiferromagnetic Heisenberg superexchange between local magnetic moments. In order to establish an effective mean field theory in the limit of the paramagnetic heavy Fermi liquid and near the half-filling case, we find that the spinon singlet pairing from the local antiferromagnetic short-range correlations can reduce the ground state energy substantially. In the presence of the Kondo screening effect, the Cooper pairs between the conduction electrons is induced. Depending on the ratio of the Heisenberg and the Kondo exchange couplings, the resulting superconducting state is characterized by either a d-wave nodal or d-wave nodeless state, and a continuous phase transition exists between these two states. These results are related to some quasi-two dimensional heary fermion superconductors.Comment: 6 pages, 6 figure