Microscopic approach to magnetism and superconductivity of f-electron systems with filled skutterudite structure

In order to gain a deep insight into f-electron properties of filled skutterudite compounds from a microscopic viewpoint, we investigate the multiorbital Anderson model including Coulomb interactions, spin-orbit coupling, and crystalline electric field effect. First we examine the local f-electron state in detail in comparison with the results of LS and j-j coupling schemes. For each case of n = 1-13, where a is the number of f electrons per rare-earth ion, the model is analyzed by using the numerical renormalization group (NRG) method to evaluate magnetic susceptibility and entropy off electron. In particular, for the f(2)-electron system corresponding to the Pr-based filled skutterudite, it is found that magnetic fluctuations significantly remain at low temperatures, even when the ground state is F, singlet, if F(2) 4 triplet is the excited state with small excitation energy. In order to make further step to construct a simplified model which can be treated even in a periodic system, we also analyze the Anderson model constructed based on the j-j coupling scheme by using the NRG method. It is clearly observed that the magnetic properties are quite similar to those of the original Anderson model. Then, we construct an orbital degenerate Hubbard model based on the j-j coupling scheme to investigate the mechanism of superconductivity of filled skutterudites. In the 2-site model, we carefully evaluate the superconducting pair susceptibility for the case of n = 2 and find that the susceptibility for off-site Cooper pair is clearly enhanced only in a transition region in which the singlet and triplet ground states are interchanged. We envision a scenario that unconventional superconductivity induced by magnetic fluctuations may occur in the f(2)-electron system with Gamma(1) ground state such as Pr-based filled skutterudite compounds.