Quantifying structural damage from self-irradiation in a plutonium superconductor

LBNL-61766 Effects of self-irradiation on local crystal structure and 5f localization in PuCoGa 5 C. H. Booth, 1 M. Daniel, 1 R. E. Wilson, 1 E. D. Bauer, 2 J. N. Mitchell, 3 N. O. Moreno, 2, 4 L. A. Morales, 3 J. L. Sarrao, 2 and P. G. Allen 5 Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA Departamento de F´ isica, Universidade Federal de Sergipe, S˜ o Cristov˜ o - SE CEP 49100-000, Brazil a a Materials Science and Technology Division, Lawrence Livermore National Laboratory, Livermore, California 94550, USA (Dated: as submitted Oct 20, 2006) X-ray absorption fine-structure (XAFS) measurements demonstrate the structural and electronic changes involved in destroying superconductivity in PuCoGa 5 due to self-irradiation damage. In particular, the Pu L III -edge data indicate a more localized f -orbital relative to the itinerant param- agnet UCoGa 5 , potentially increasing with radiation damage. Moreover, the local crystal structure in aged material is disordered much more strongly than expected, consistent with all atoms within a damage cascade displaced from their equilibrium positions. PACS numbers: 71.27.+a, 74.70.Tc, 61.80.-x, 61.10.Ht The discovery of superconductivity at T c =18.5 K in PuCoGa 5 [1] raises many fascinating questions about how a heavy-fermion superconductor can have a T c nearly an order of magnitude higher than all other known ex- amples. Although the layered nature of this material is thought to play a substantial role in analogy to the cuprates [2], the role of local 5f moments in forming the pairing state remains enigmatic. Moreover, lattice defor- mations are practically intrinsic to this material due to self-irradiation damage from the decay of the plutonium nuclei. Here, we report x-ray absorption fine-structure (XAFS) measurements that further demonstrate the lo- calized character of the 5f orbital, and the surprisingly large effect of self-irradiation damage on both the local crystal structure and the degree of 5f localization [3]. Recent photoemission results demonstrate strong evi- dence for both localized and itinerant 5f character [4]. Such a “dual nature” of these magnetic orbitals would be unusual for what appears to be a moderately heavy- fermion material, with an electronic linear specific heat Sommerfeld coefficient γ ≈ 100 mJ/mol K 2 and a nor- mal state Curie-Weiss-like susceptibility consistent with local moment behavior [1]. Moreover, the radioactiv- ity of plutonium generates damage cascades that are thought to be responsible for the high upper critical field H c2 ≈ 100 T and critical current density J c > 10 4 A/cm 2 for T > 0.9T c , although these values remain high even in the freshest material. Clearly, self-irradiation dam- age plays an important role in these superconductors by reducing the coherence length and creating pinning cen- ters, and may play a role in the local moment behavior as well. Most of the work describing the bulk proper- ties of PuCoGa 5 as a function of radioactive decay has been performed by Jutier and co-workers [5, 6]. They have found, for instance, that T c decreases by about 0.24 K per month, and that H c2 peaks ∼ 120 T after about a year in samples with mostly 239 Pu [5]. Curro et al. [7] have found that the impurity scattering rate inferred from nuclear magnetic resonance experiments agrees well with the reduction in T C an aged sample. Obtaining mi- croscopic information about how this radiation-induced disorder evolves and contributes to these changes is there- fore of fundamental importance toward understanding superconductivity in this exceptional material. In this paper, we utilize the XAFS technique to provide local atomic species-specific pair-distance information [8]. Complementing the structural aspects, L III absorption edge data are used to determine the degree of 4f elec- tron localization. Although this technique has been less useful in light-actinide than in rare-earth intermetallic systems due to the more extended nature of their 5f or- bitals, energy shifts and changes in line shape have been observed to correlate with the degree of 5f localization [9–12], and some of these observations have been repro- duced in ab initio calculations [13]. Such effects may be enhanced in plutonium intermetallics due to the position of Pu at the transition between local and delocalized f electron behavior between the light and the heavy ac- tinides [14]. Two PuCoGa 5 samples were synthesized [1]. At the time of the most recent x-ray measurement, one was 24.3 months old (Sample A), and the other was only 0.5 months old (Sample B). Sample B had a T C = 18.1 K when it was 3.1 months old, and Sample A had a T C = 12.4 when 27.4 months old. The isotopic con- tent is the same for both samples with the main ra- dioactivity coming from 93.93% 239 Pu, 5.85% 240 Pu, and 0.12% 241 Pu. This sample therefore has an α decay rate λ α ≈ 3.43×10 −5 per Pu per year. The accumulated dose has been shown to be a reasonable indicator of how sam- ple properties change with time [6], and we report the sample age in units of α-decays per atom, which we refer to as f α . We do not use the more common “displacements per atom” (dpa) unit, because it generally assumes the