Electronic Coherence inδ-Pu: A Dynamical Mean-Field Theory Study

Abstract

A combination of density functional theory and the dynamical mean-field theory (DMFT) is used to calculate the magnetic susceptibility, heat capacity, and the temperature dependence of the valence band photoemission spectra for -Pu. We predict that -Pu has a Pauli-like magnetic susceptibility near ambient temperature, as in experiment, indicating that electronic coherence causes the absence of local moments. Additionally, we show that volume expansion causes a crossover from incoherent to coherent electronic behavior at increasingly lower temperatures.Pu embodies a forefront of both technology and theoretical condensed-matter physics. Elemental Pu displays exotic physical behavior that continues to defy explanation. For example, it exhibits six allotropic phases at ambient pressure, the low-density fcc phase has a negative coefficient of thermal expansion, and the volume expands by more than 25% when the system is heated from the high-density monoclinic phase to the phase. Regarding the pure phase, a complicating factor is that it is only stable in the temperature range 580 K < T < 700 K. However, the phase can be stabilized at low temperatures by a variety of alloying elements such as Ga and Am. This allows for the experimental exploration of the phase at low temperatures, with the caveat that it is not clear what changes the alloying element may be inducing.Lashley et al.[1] have measured the magnetic susceptibility to be Pauli-like in both the and phases, and hence detect no presence of localized magnetic moments. Similarly, Heffner et al. [2 -4] have used SR and showed that there are no ordered magnetic moments in Pu nor in -stabilized Pu (i.e., 4.3% Ga) for temperatures down to 4 K. Nuclear magnetic resonance (NMR) measurements by Curro and Morales [5] also show an absence of magnetic moments.The linear coefficient of the specific heat for -stabilized Pu has been measured by various groups and the resulting values are 42 mJ mol K 2 for alloying with 2% of Ga [1], 64 mJ mol K 2 for 5% of Al [6], and 35-55 mJ mol K 2 for 8%-20% of Am [7]. The large variation among these measurements may be due to the fact that the phase has been stabilized by a different alloying element in each study.Several previous studies have applied a combination of density functional theory and the dynamical mean-field theory (DFT DMFT) [8] to Pu. DMFT requires a solution of an auxiliary quantum impurity problem, and for the corresponding impurity model of Pu, no exact method was available in the past. Savrasov et al. [9] used an interpolative solver to calculate the energy and the photoemission spectra of Pu. The approach yielded a significant improvement for the volume of the phase of Pu compared to DFT. Shick et al.[10] computed the photoemission spectra using the Hubbard I impurity solver and were successful in predicting the three-peak structure in the photoemission spectra. Pourovskii et al. [11] computed the photoemission spectra and the heat capacity using the Fluctuation Exchange Approximation (FLEX) as an impurity solver. Th...

Phase Diagram and Electronic Structure of Praseodymium and Plutonium

“…Calculations within the framework of DFT in combination with dynamical mean field theory (DFT þ DMFT) have explained several aspects of the electronic structure of Pu (see, e.g., Refs. [80][81][82][83][84]). Nevertheless, the computational complexity of this approach made it impossible to calculate the pressure-volume phase diagram of all of the phases of Pu.…”

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“…Investigation of the low temperature properties of Pu has shown that the total specific magnetic susceptibility of the defects has a Curie-Weiss temperature dependence (χ ~ 1/T) [1,13,14]. It has been argued that radiation damage is either creating or uncovering local magnetic moments in Pu which may be shielded by screening in the undamaged lattice [15].…”

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