In order to investigate electronic properties of recently discovered heavy fermion superconductors CeTIn5 (T=Ir and Co), we employ the relativistic linear augmented-plane-wave (RLAPW) method to clarify the energy band structures and Fermi surfaces of those materials. The obtained energy bands mainly due to the large hybridization between Ce 4f and In 5p states well reproduce the Fermi surfaces consistent with the de Haas-van Alphen experimental results. However, when we attempt to understand magnetism and superconductivity in CeTIn5 from the microscopic viewpoint, the energy bands obtained in the RLAPW method are too complicated to analyze the system by further including electron correlations. Thus, it is necessary to prepare a more simplified model, keeping correctly the essential characters of the energy bands obtained in the band-structure calculation. For the purpose, we construct a tight-binding model for CeTIn5 by including f -f and p-p hoppings as well as f -p hybridization, which are expressed by the Slater-Koster integrals, determined by the direct comparison with the band-calculation result. Similarity and difference between CeIrIn5 and CeCoIn5 are discussed based on the obtained tight-binding model, suggesting a significant importance of the effect of crystalline electric field to understand the difference in electronic properties among CeTIn5.

By using a relativistic linear augmented-plane-wave method, we clarify energy band structures and Fermi surfaces of recently discovered plutonium-based superconductor PuCoGa5. We find several cylindrical sheets of Fermi surfaces with large volume, very similar to CeMIn5 (M=Ir and Co) isostructural with PuCoGa5, in spite of different f -electron numbers between Ce 3+ and Pu 3+ ions. The similarity is understood by a concept of electron-hole conversion in a tight binding model constructed based on the j-j coupling scheme. Based on the present results, we provide a possible scenario to explain why a transition temperature is so high as 18.5K in PuCoGa5. PACS numbers: 74.25.Jb, 74.70.Tx, 71.18.+y, 71.15.Rf Recently it has been discovered that PuCoGa 5 exhibits superconductivity [1]. Surprisingly its superconducting transition temperature T c is 18.5K, which is the highest among those yet observed f -electron materials and high enough even compared with other well-known intermetallic compounds. It has been also found that PuRhGa 5 becomes superconducting with T c =8.6K [2]. These plutonium intermetallic compounds PuMGa 5 have the same HoCoGa 5 -type tetragonal structure as CeMIn 5 , a family of cerium-based heavy fermion superconductors [3]. Note, however, that superconductivity occurs for M=Ir (T c =0.4K) and Co (2.3K) in CeMIn 5 , while antiferromagnetic (AFM) phase has been found for M=Rh at ambient pressure. Another isostructural material including uranium is UMGa 5 [4], but superconductivity has not been found yet. These HoCoGa 5 -type compounds are frequently referred to as "115".Regarding superconducting mechanism in the 115 compounds, first let us consider Ce-115. It has been widely considered that it is unconventional d-wave superconductor induced by AFM spin fluctuations. In fact, there are some evidences such as T 3 behavior in nuclear relaxation rate [5] and node structure measured by thermal conductivity [6]. For the phase diagram of Ce(Co,Rh,Ir)In 5 [7], AFM phase is found to exist in adjacent to the superconducting phase. Those experimental facts remind us of high-T c cuprates, but a clear difference from cuprates should be remarked. Namely, high-T c superconductivity in cuprates occurs by hole doping into AFM insulators, while in Ce-115, no hole doping is needed. To understand the appearance of superconductivity induced by AFM spin fluctuations without hole doping, a crucial role of orbital degree of freedom has been pointed out by Takimoto et al. [8].Concerning Pu-115, it is still premature to draw a definitive conclusion about the mechanism of superconductivity, but we notice that some normal-state properties in PuMGa 5 seem to be dominated by AFM spin fluctuations, e.g., the Curie-Weiss behavior in magnetic susceptibility and electric resistivity in proportion to T 1.35 [1]. Thus, it may be natural to consider that superconductivity in Pu-based compounds is also induced by AFM fluctuations. However, several problems still exist, even if d-wave superconductivity is confirmed in both Ce-115 and Pu...

A microscopic Hamiltonian reflecting the correct symmetry of f -orbitals is proposed to discuss superconductivity in heavy fermion systems. In the orbitally degenerate region in which not only spin fluctuations but also orbital fluctuations develop considerably, cancellation between spin and orbital fluctuations destabilizes d x 2 −y 2 -wave superconductivity. Entering the non-degenerate region by increasing the crystalline electric field, d x 2 −y 2 -wave superconductivity mediated by antiferromagnetic spin fluctuations emerges out of the suppression of orbital fluctuations. We argue that the present scenario can be applied to recently discovered superconductors CeTIn5 (T=Ir, Rh, and Co).

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