Ab-Initio Spin-Fluctuation Hamiltonian for Delta-Pu at Finite Temperatures

Spin/orbital-polarized density functional theory has been used extensively in the past years to explore materials properties of the Pu metal, from its polymorphic phase diagram to energetics of formation and migration of lattice defects and impurities in the various Pu phases. Calculations to date have been conducted using one or other static low-energy spin configuration. The effect of allowing for non-collinear spin/orbital fluctuations is yet largely unexplored. We have thus developed a novel technique that allows for efficient calculations of adiabatic non-collinear spin/orbital excitations within spin-constraint density-functional theory. We use this technique to extensively explore the potential energy landscape of adiabatic spin fluctuations in delta-Pu. Several families of low-energy non-collinear spin configurations are found. We construct an effective Hamiltonian for thermal spin fluctuations in delta-Pu and study its statistical mechanics via Monte Carlo simulations. We report the impact of spin/orbital fluctuations on the structure and magnetic susceptibility of delta-Pu at finite temperatures.<br/><br/>This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.