Intermediate valence in Pu and Pu-based superconductors

Actinide materials are the backbone of nuclear energy. In order to realize the full potential of nuclear power, and to avoid the risks for society and environment, there is a need in solid understanding of the physical and chemical properties of actinide materials. Theoretical modeling of the actinides and their 5f states is a challenging problem due to the interplay between the localized and the itinerant nature of the 5f-electrons. In this presentation I will address electron correlation effects in Pu-115 family (PuCoGa₅, PuCoIn₅) of heavy-fermion superconductors, and δ-Pu making use of a combination of the density functional theory (DFT) with the exact diagonalization (ED) of a single impurity Anderson model.

PuCoGa₅ has the highest critical temperature of 18.5 K among heavy-fermion superconductors, that is one order of magnitude higher than for typical heavy-fermion compounds. Spin and charge 5f-electron valence fluctuations may have an important role in mediation of the superconducting phase in the Pu-115 family of superconductors. Specific character of the coupling mechanism remains uncovered. Very recent point-contact spectroscopy experiments [2] show that the paired superconducting electrons have wavefunction with the d-wave symmetry.

We show that the unconventional character of superconductivity in the Pu-115 compounds and the unexpected absence of magnetism in δ-Pu may have a common origin in the intermediate-valence nature of the Pu 5f-electron ground state [3]. In all three compounds the Pu atoms exhibit a 5f⁵-5f⁶ intermediate-valence ground state, with a partial delocalization of the 5f⁵ multiplet. The local 5f magnetic moment is compensated by a magnetic moment in the surrounding cloud of conduction electrons. In the case of PuCoGa₅ and δ-Pu the compensation is complete and the grounds state is a non-magnetic singlet. On the basis of these results, we discuss the role of spin and charge fluctuations for Cooper pairing, and the nature of the unconventional d-wave superconducting state in PuCoGa₅ and PuCoIn₅.

References
[1] J. L. Sarrao, L. A. Morales, J. D. Thompson, B. L. Scott, G. R. Stewart, F. Wastin, J. Rebizant, P. Boulet, E. Colineau, and G. H. Lander, Nature 420, 297 (2002).
[2] D. Daghero, M. Tortello, G. Ummarino, J.-C. Griveau, E. Colineau, R. Eloirdi, A. B. Shick, J. Kolorenc, A. I. Lichtenstein, and R. Caciuffo, Nature Comm. 3, 1785 (2012).
[3] A. B. Shick, J. Kolorenc, J. Rusz, P. M. Oppeneer, A. I. Lichtenstein, M. I. Katsnelson, and R. Caciuffo, Phys. Rev. B 87, 020505 (2013).