Superconductivity and antiferromagnetism in heavy-electron systems.

Abstract

Superconductivity and antiferromagnetism in heavy-electron systems are investigated from a general point of view. First we classify superconducting states in a simple cubic lattice, a body-centered tetragonal lattice, and a hexagonal close-packed lattice, having ${\mathrm{URu}}_{2}$${\mathrm{Si}}_{2}$ and ${\mathrm{UPt}}_{3}$ in mind. For that purpose we take an approach to treat the effective couplings in real space. The approach is convenient to discuss the relation between the nature of fluctuations in the system and the superconducting states. When we assume that the antiferromagnetic fluctuations reported by neutron experiments are dominant, the most promising are some of the anisotropic singlet states and there remains the possibility for some triplet states too. Then we discuss the coupling between the two order parameters based on a Ginzburg-Landau theory. We derive a general expression of the coupling term. It is pointed out that the coupling constant can be large in heavy-electron systems. The general trend of the coexistence of the superconductivity and antiferromagnetism is discussed, and it is shown that the anisotropic states are generally more favorable to the coexistence than the conventional isotropic singlet. Experimental data of ${\mathrm{URu}}_{2}$${\mathrm{Si}}_{2}$ and ${\mathrm{UPt}}_{3}$ are analyzed by the Ginzburg-Landau theory. According to the analysis ${\mathrm{URu}}_{2}$${\mathrm{Si}}_{2}$ has a small coupling constant and a large condensation energy of the antiferromagnetism. On the other hand, ${\mathrm{UPt}}_{3}$ has a large coupling constant and a small condensation energy. It means that the specific-heat anomaly at ${T}_{N}$ should be small in ${\mathrm{UPt}}_{3}$ and its superconductivity is easily destroyed when a large moment is formed.