Determination of plutonium’s fluctuating electronic and magnetic ground state resolves long-standing “missing” magnetism puzzle.

The nature of the elemental cerium phases, undergoing an isostructural volume collapse transition, cannot be understood using conventional solid state concepts. Using the combination of density functional theory and dynamical mean-field theory, we study the magnetic properties of both the α and the γ phases. We compute the magnetic form factor, and show that it is very close to free ion behavior in both the local moment γ phase as well as the more itinerant α phase, in agreement with neutron scattering experiments. In sharp contrast, the dynamic local magnetic susceptibility of the two phases is strikingly different. In the γ phase, the sharp low energy peak due to local moment formation and consequently low Kondo temperature dominates the spectra. In the α phase two broad peaks can be identified, the first is due to Kondo screening, and the second is due to Hund's coupling. This shows that hybridization plays a central role in the α − γ transition in cerium, and that from the point of view of magnetic properties, the 4f electrons are strongly correlated in both phases.

The spin state transition in LaCoO3 has eluded description for decades despite concerted theoretical and experimental effort. In this study, we approach this problem using fully charge self-consistent Density Functional Theory + Embedded Dynamical Mean Field Theory (DFT+DMFT). We show from first principles that LaCoO3 cannot be described by a single, pure spin state at any temperature. Instead, we observe a gradual change in the population of higher spin multiplets with increasing temperature, with the high spin multiplets being excited at the onset of the spin state transition followed by the intermediate spin multiplets being excited at the metal insulator transition temperature. We explicitly elucidate the critical role of lattice expansion and oxygen octahedral rotations in the spin state transition. We also reproduce, from first principles, that the spin state transition and the metal-insulator transition in LaCoO3 occur at different temperature scales. In addition, our results shed light on the importance of electronic entropy in driving the spin state transition, which has so far been ignored in all first principles studies of this material.

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