Relevance of Kondo physics for the temperature dependence of the bulk modulus in plutonium

Abstract

The recent PNAS paper by Migliori et al. (1) attempts to explain the unusually strong temperature dependence of the bulk modulus of fcc plutonium (δ-Pu) by use of the disordered local moment (DLM) model. It is our opinion that this approach does not correctly incorporate the dynamic magnetism of δ-Pu.First, we note that, contrary to the comments in Migliori et al. (1), the results of our recent inelastic neutron experiments (2) do not contradict the results of earlier experiments (3-5), which show that the ground state is nonmagnetic. All of these experiments are consistent with the interpretation that δ-Pu has a nonmagnetic fluctuating valence ground state. This is reinforced by dynamical mean-field theory (DFT+DMFT) calculations (2, 6). This theory quantitatively explains our data, correctly calculates the magnetic moment and its momentum-transfer dependence in the excited state, and predicts the Kondo temperature of 975 K to within 20%.Second, Söderlind et al.(1, 7) attempt to explain the small susceptibility observed in δ-Pu by assuming a complete cancellation of the spin and orbital angular momentum contributions. The spin and orbital moments in Pu do partially cancel, but we have performed a careful experimental analysis (2, 8) to show that a net magnetic moment of 0.6 ± 0.2 μ B is present in the excited state. This analysis is not based on any specific model; it uses data from two incident neutron energies, takes account of the experimental errors, and evaluates the moment through use of sum rules. The proposed complete cancellation of spin 7 that the spin and orbital moments cancel is based on an analysis of the neutron form factor that ignores the large statistical and systematic error bars associated with the data at low momentum transfer.)The DLM model was used successfully by Niklasson et al. (9) to calculate the ground-state volumes and bulk moduli of the early actinides, but the authors of that paper were quite explicit in stating that this approach inherently fails to explain the magnetism, in that it leaves out the Kondo physics. It is quite possible that Kondo physics is also responsible for an appreciable fraction of the bulk modulus anomaly in δ-Pu. This idea is based on comparison with rare earth valence fluctuation systems, such as CePd 3 and CeBe 13 , which exhibit bulk moduli anomalies as large as 10% because of the Kondo thermal excitation (10). In any case, careful examination of the results of Migliori et al. (1) shows that the DLM model, which calculates a 6% decrease in the bulk modulus in the temperature range 300-500 K, underestimates the measured effect by a factor of four.Hence, in our view, the DLM approach of Migliori et al. (1) is not in accord with the experimentally determined magnetism in δ-Pu, and the interesting large decrease of the bulk modulus in Pu remains an unsolved problem.

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Pseudogap in elemental plutonium

“…Contemporary proposals for the absence of magnetic moments in Pu include one, based on density functional theory (DFT) [14], in which the magnetic moments are disordered and exhibit a high degree of spin and orbital compensation, and another, based on dynamical mean field theory (DMFT) [6,11,12], in which the moments are strongly hybridized with conduction electrons. It may be argued that either scenario can account for the fluctuating magnetic moments at an energy equivalent to ≈ 1000 K reported in inelastic neutron scattering experiments [15,16,22,23]. However, both models significantly underestimate the unusually large Sommerfeld coefficient of 45 γ el 60 mJmol −1 K −2 of δ-Pu [24][25][26] (much larger than that of any other element) derived from calorimetry data.…”