The electronic structure of the first Pu-based superconductor PuCoGa 5 is explored using photoelectron spectroscopy and a novel theoretical scheme. Exceptional agreement between calculation and experiment defines a path forward for understanding electronic structure aspects of Pu-based materials. The photoemission results show two separate regions of 5f electron spectral intensity, one at the Fermi energy and another centered 1.2 eV below the Fermi level. The results for PuCoGa 5 clearly indicate 5f electron behavior on the threshold between localized and itinerant. Comparisons to delta phase Pu metal show a broader framework for understanding the fundamental electronic properties of the Pu 5f levels in general within two configurations, one localized and one itinerant. PACS: 71.27.+a,71.28.+d, The recent discovery of PuCoGa 5 , the first Pu-based superconductor, with a T C =18.5 K and unconventional superconductivity demonstrates the rich and complex nature of Pu-based materials [1]. The role of the 5f electrons in bonding and hybridization is intimately intertwined with the wide range of ground state properties found in actinide materials including enhanced mass, magnetism, superconductivity, as well as spin and charge density waves. Within the actinide series, Pu occupies the position between the clearly hybridized 5f states of uranium [2] and clearly localized 5f states of americium[3] which due to its J=0 ground state is a superconductor. Pu defines the localized/itinerant boundary for the 5f electrons in the actinides [4,5]. There have been several recent papers reporting photoemission [6,8,7,9] and electronic structure calculations [4,5,10,11,12,13,14] for the fcc (δ ) phase of Pu metal. A classic failure of density functional theory (DFT) within the local density approximation (LDA) or generalized gradient approximation (GGA) is observed in the case of δ -Pu with the volume from LDA falling over 25% short (GGA only slightly better) of the experimental volume in the largest discrepancy to date in DFT for a crystal.To accommodate the large volume of the delta phase of Pu metal, there have been magnetically ordered electronic structures proposed which, to some degree, effectively localize 5f electrons, reducing the contribution to the chemical bonding and increasing the volume[10], however there is no experimental evidence for magnetism in δ -Pu [15]. Another approach used for δ -Pu is the dynamical mean field theory (DMFT) which may offer promise with further development [11]. Currently the agreement between DMFT and experiment is weak with the PES shown in Ref.[11] being more of an idealized representation than actual data. The link between δ -Pu metal and PuCoGa 5 is strong and follows the same path as the family of Ce-based heavy fermion-superconductors (CeMIn 5 , M=Co, Rh, Ir) [16] having the cubic CeIn 3 as the root crystal structure. Here we have δ -Pu as the parent cubic phase structure with Ga substituting on the face centers to form PuGa 3 and insertion of the CoGa 2 layer into the cubic s...
