Electronic structure and magnetism in UGa2 : DFT+DMFT approach

The debate whether uranium $5f$ electrons are closer to being localized or itinerant in the ferromagnetic compound $\mathrm{U}{\mathrm{Ga}}₂$ is not yet fully settled. The experimentally determined magnetic moments are large, approximately $3\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$, suggesting the localized character of the $5f$ electrons. In the same time, one can identify signs of itinerant as well as localized behavior in various spectroscopic observations. The band theory, employing local exchange-correlation functionals, is biased toward itinerant $5f$ states and severely underestimates the moments. Using material-specific dynamical mean-field theory (DMFT), we probe how a less approximate description of electron-electron correlations improves the picture. We present two variants of the theory: starting either from spin-restricted (LDA) or spin-polarized (LSDA) band structure. We show that the L(S)$\mathrm{DA}+\mathrm{DMFT}$ method can accurately describe the magnetic moments in $\mathrm{U}{\mathrm{Ga}}₂$ as long as the exchange interaction between the uranium $6d$ and $5f$ electrons is preserved by a judicious choice of the spin-polarized double-counting correction. We discuss the computed electronic structure in relation to photoemission experiments and show how the correlations reduce the Sommerfeld coefficient of the electronic specific heat by shifting the $5f$ states slightly away from the Fermi level.