Dynamical mean-field theory investigation of specific heat and electronic structure of α- and δ-plutonium

We have carried out a comparative study of the electronic specific heat and electronic structure of α- and δ-plutonium using dynamical mean-field theory. We use the perturbative T-matrix and fluctuating exchange as a quantum impurity solver. We considered two different physical pictures of plutonium. In the first, 5f⁵+, the perturbative treatment of electronic correlations has been carried out around the nonmagnetic [local-density approximation (LDA)] Hamiltonian, which results in an f occupation around a bit above n_f=5. In the second, 5f⁶−, plutonium is viewed as being close to a 5f⁶ configuration, and perturbation theory is carried out around the (LDA+U) starting point bit below n_f=6. In the latter case, the electronic specific-heat coefficient γ attains a smaller value in δ-Pu than in α-Pu, in contradiction to experiment, while in the former case, our calculations reproduce the experimentally observed large increase of γ in δ-Pu as compared to the α phase. This enhancement of the electronic specific-heat coefficient in δ-Pu is due to strong electronic correlations present in this phase, which cause a substantial increase of the electronic effective mass, and high density of states at E_F. The densities of states of α- and δ-plutonium obtained starting from the open-shell configuration are also in good agreement with the experimental photoemission spectra.