Fig. 7-5 Fermi surface sheets obtained by relativistic band calculations and f-electron configurations in Pu and Ce ions (a) Fermi-surface sheets of PuCoGa5 in which the degree of admixture of 5f electrons is classified by color. Red shift indicates a larger admixture of the 5f component. Except for band 15, quasi-two-dimensional (Q2D), cylindrical Fermi-surface sheets are observed. (b) Fermi-surface sheets of CeIrIn5 in which the degree of admixture of 4f electrons is classified by color. Bands 14 and 15 form Q2D cylindrical sheets. On the left side of each panel, we show f-electron configurations for Ce3+ and Pu3+ ions based on a j-j coupling scheme. The band number is shifted by two due to the difference in numbers of f-electrons.

Recently, a superconducting compound containing Pu, PuCoGa5, has been discovered. The superconducting transition temperature Tc =18.5K is amazingly high. Since Tc for f-electron superconductors has been at most about 2K, this compound can be regarded as a "high-temperature" f-electron superconductor. Because of this discovery, it is necessary to reassess the previous conventional wisdom that Tc for f-electron superconductors is low. PuCoGa5 belongs to the group of compounds with the HoCoGa5-type tetragonal crystal structure, which are frequently referred to as "115" systems. As a first step toward understanding high temperature superconductivity in PuCoGa5, we have analyzed the electronic structure of a series of 115 systems by means of a relativistic band calculation method. In Fig. 7-5(a) we show Fermi-surface sheets of PuCoGa5 obtained theoretically. Significant amounts of 5f electron character are found to exist on the Fermi surface, which characterizes the electronic properties of solids. In Fig. 7-5(b), we exhibit the Fermi-surface sheets of CeIrIn5, a typical superconducting material among Ce-115 systems. We note here that in spite of different numbers of f-electrons per ion, PuCoGa5 and CeIrIn5 have quite similar Fermi-surface structures with multi quasi-two-dimensional (Q2D) sheets. This point can be understood in terms of electron-hole symmetry, which means that the carriers determining the properties of a material behave as "electrons" or "holes." In Figs. 7-5(a) and (b), we show the electronic configurations of Ce3+ and Pu3+ ions, clearly indicating that apart from the difference in character, i.e., electron vs. hole, in both cases one carrier exists per ion. Thus, the Fermi-surface structures for these two compounds become quite similar. Why is PuCoGa5 a high temperature superconductor? If we assume the same magnetic mechanism as that in Ce-115, Tc should be scaled by the electronic energy. Since the itineracy of 5f electrons is greater than that of 4f electrons, Tc for 5f-electron systems is naturally higher than that of 4f-electron materials. Unfortunately, this argument cannot explain why U-115 and Np-115 are not superconducting. However, we note that superconductivity occurs in systems with multi Q2D Fermi-surface sheets. In non-superconducting U-115 and Np-115, the number of Q2D Fermi-surface sheets is unity or zero. The relation between the occurrence of superconductivity and the number of Q2D Fermi-surface sheets has not yet been clarified. However, considering the key issue of multi Q2D Fermi-surface sheets, we predict that superconductivity will occur in Pr-115, in which the foregoing Fermi-surface condition is satisfied. We look forward to progress in future experiments to test our prediction.

Reference T. Maehira et al., Electronic Structure and the Fermi Surface of PuCoGa5 and NpCoGa5, Phys. Rev. Lett., 90, 207007 (2003).

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