A LDA+U and LDA+DMFT study of uranium mononitride: from nonmagnetic to paramagnetic and ferromagnetic
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A LDA+U and LDA+DMFT study of uranium mononitride: from nonmagnetic to paramagnetic and ferromagnetic Weiwei Sun1, 2 Igor Di Marco2, Pavel Korzhavyi1 1 Department of Materials Science and Engineering, KTH Royal Institute of Technology, SE10044 Stockholm, Sweden. 2 Department of Physics and Astronomy, Materials Theory, Uppsala University, Box 516 SE751 20 Uppsala, Sweden. ABSTRACT The combination of density functional theory in local density approximation and dynamical mean field theory (LDA+DMFT) was employed in a preliminary study of the strong electron correlation effects in a promising nuclear fuel—uranium mononitride (UN). For the ferromagnetic phase, the effective impurity problem arising in the LDA+DMFT [1-3] cycle is solved with the spin-polarized T-matrix fluctuation exchange (SPTF) solver, which includes spin–orbit interactions. Concerning the paramagnetic phase, the disordered local moment (DLM) approach was used, based on both standard local density approximation (LDA) and LDA+U. Basic spectral properties and material properties, such as the spin, orbital and total magnetic moments on U atom were calculated for various values of the Hubbard parameter U with a fixed exchange parameter J. Our main focus was to compare the calculated spectral functions (density of states) for different magnetic phases and different methods to the experimental XPS data [4]. On top of that, the total moments of the paramagnetic and ferromagnetic phases are compared with the measured values by neutron spectroscopy [4, 5]. INTRODUCTION The binary uranium systems UX (X = C, N, S, Se, Te, As, Sb) crystallize in the rocksalt structure. They have been extensively studied both theoretically and experimentally, owing to their interesting and puzzling physical and magnetic properties [6]. Among them, UN has received significant attention due to the potential usage as a high-temperature nuclear fuel [7] in the Generation-IV reactors [8]. In the fission reactions, the fast neutrons require a small core with a high power density and a highly efficient heat transfer. The carbide/nitride fuels have shown excellent thermal conductivity, which allowed them to become the alternative materials to oxide-based fuels, like UO2. Theoretically, most of the studies are focused on the structural properties, magnetic ordering, and electronic structure with emphasis on the comparison with photoemission experiments. Beyond density functional theory (DFT), the self-interaction corrected (SIC)-LDA is a parameter free theory of the ground state, which allows one to determine the atomic-like valence state by comparing total energies [9]. Petit et al. [10] have studied the localization of 5f electrons in UN using this approach, and suggested that the valence state is mixed f0 and f1 configurations. Other options beyond DFT include the very efficient LDA+U approach and the more sophisticated dynamical mean field theory (DMFT) scheme based on LDA [1-3]. By means of the Hubbard U correction [11], the calculated lattice constant, acal is gradually improved for bo
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