We develop a new implementation of the Gutzwiller approximation in combination with the local density approximation, which enables us to study complex 4f and 5f systems beyond the reach of previous approaches. We calculate from first principles the zero-temperature phase diagram and electronic structure of Pr and Pu, finding good agreement with the experiments. Our study of Pr indicates that its pressure-induced volume-collapse transition would not occur without change of lattice structure-contrarily to Ce. Our study of Pu shows that the most important effect originating the differentiation between the equilibrium densities of its allotropes is the competition between the Peierls effect and the Madelung interaction and not the dependence of the electron correlations on the lattice structure.

We show that electron correlations lead to a bad metallic state in chalcogenides FeSe and FeTe despite the intermediate value of the Hubbard repulsion U and Hund's rule coupling J. The evolution of the quasi particle weight Z as a function of the interaction terms reveals a clear crossover at U ≃ 2.5 eV. In the weak coupling limit Z decreases for all correlated d orbitals as a function of U and beyond the crossover coupling they become weakly dependent on U while strongly depend on J. A marked orbital dependence of the Z's emerges even if in general the orbital-selective Mott transition only occurs for relatively large values of U . This two-stage reduction of the quasi particle coherence due to the combined effect of Hubbard U and the Hund's J, suggests that the iron-based superconductors can be referred to as Hund's correlated metals. The role of electron correlations in the iron-based superconductors is still a debated issue, naturally intertwined with the search for the origin of high critical temperatures. We present results that improve the qualitative understanding of how electron correlation influences fundamental electron properties of these compounds, such as the metallicity, which in turn might be important also for the understanding of the pairing mechanism. We choose two candidates of the chalogenides, FeSe and FeTe and employ f irst principles electron structure calculations combined with advanced many-body methods taking into account the local electron correlation. The chalcognides have in contrast to the pnictides a simpler atomic structure, thus easier to synthesize and also to study theoretically. In addition they are non toxic in contrast to the pnictides containing arsenic.In previously known superconductors we can identify either weakly correlated materials, like elemental superconductors or binary alloys, including MgB 2 , or highlycorrelated compound like the copper oxides and heavy fermion materials. In the first set of compounds superconductivity is explained within the Bardeen-CooperSchrieffer framework and its extensions, and it occurs as a pairing instability of a normal metal. In the second set it is widely believed that correlations revolutionize the electronic properties and that both the metallic state and the pairing mechanism deviate from standard paradigms.The iron-based pnictides and chalcognides superconductors do not fit this simple classification. The common labeling "intermediate correlation", referring to properties such as Fermi surface topology or absence of Hubbard bands [1], suggests modest effects of correlations. Conversely, the metallic state appears much less coherent than what these observations imply [2,3]. A magnetic counterpart of this dualism is the localized an itinerant nature of the spin-density-wave state of the parent compound.The characteristic property of the band structure is that several of the five d-bands cross the Fermi level. The multi-orbital nature leads to several exotic electronic properties such as orbital-selectivity [4][5][6][7][8][9] and ...

We present a self-consistent numerical approach to solve the Gutzwiller variational problem for general multi-band models with arbitrary on-site interaction. The proposed method generalizes and improves the procedure derived by Deng et al., Phys. Rev. B. 79 075114 (2009), overcoming the restriction to density-density interaction without increasing the complexity of the computational algorithm. Our approach drastically reduces the problem of the high-dimensional Gutzwiller minimization by mapping it to a minimization only in the variational density matrix, in the spirit of the Levy and Lieb formulation of DFT. For fixed density the Gutzwiller renormalization matrix is determined as a fixpoint of a proper functional, whose evaluation only requires ground-state calculations of matrices defined in the Gutzwiller variational space. Furthermore, the proposed method is able to account for the symmetries of the variational function in a controlled way, reducing the number of variational parameters. After a detailed description of the method we present calculations for multi-band Hubbard models with full (rotationally invariant) Hund's rule on-site interaction. Our analysis shows that the numerical algorithm is very efficient, stable and easy to implement. For these reasons this method is particularly suitable for first principle studies -- e.g., in combination with DFT -- of many complex real materials, where the full intra-atomic interaction is important to obtain correct results.Comment: 19 pages, 7 figure

We derive an exact operatorial reformulation of the rotational invariant slave boson method and we apply it to describe the orbital differentiation in strongly correlated electron systems starting from first principles. The approach enables us to treat strong electron correlations, spin-orbit coupling and crystal field splittings on the same footing by exploiting the gauge invariance of the mean-field equations. We apply our theory to the archetypical nuclear fuel UO2, and show that the ground state of this system displays a pronounced orbital differention within the 5f manifold, with Mott localized Γ8 and extended Γ7 electrons.

Below the critical temperature Tc ≃ 600K, an iso-structural transition, named γ-α transition, can be induced in Cerium by applying pressure. This transition is first-order, and is accompanied by a sizable volume collapse. A conclusive theoretical explanation of this intriguing phenomenon has still not been achieved, and the physical pictures proposed so far are still under debate. In this work, we illustrate zero-temperature first-principle calculations which clearly demonstrate that the γ-α transition is induced by the interplay between the electron-electron Coulomb interaction and the spin-orbit coupling. We address the still unresolved problem on the existence of a second low-T critical point, i.e., whether the energetic effects alone are sufficient or not to induce the γ-α transition at zero temperature.The γ-α iso-structural transition in Cerium [1] was discovered in 1949 [2]. Since then, a lot of theoretical and experimental work has been devoted to its understanding. The great interest in this phenomenon arises from the fact that the transition is isostructural, i.e., the lattice structure of the system is equal in the two phases. Furthermore, the possibility that the underlying mechanism lies in the electronic structure only -i.e., without it being necessary to involve other effects -makes Cerium a potential theoretical testing ground for basic concepts of correlated electron systems. Two main theoretical pictures are still under debate to explain the volume collapse: the Kondo volume collapse (KVC) [3,4] and the orbital-selective Mott transition within the Hubbard model (HM) [5]. According to the KVC the transition is induced by the rapid change of the coherence temperature across the transition boundaries, which affect dramatically the structure of the conduction spd electrons through Kondo effect. According to the HM, instead, it is the hopping between f orbitals that changes drastically across the transition between the α phase (with delocalized f electrons) and the γ phase (with localized f electrons), as for the Mott transition in the Hubbard model.Consistently with both the HM and the KVC pictures, the f -electrons are strongly correlated both in the α and in the γ phase. This fact is clearly indicated, e.g., by the photoemission spectra, which is known experimentally [6][7][8] and theoretically [9][10][11]. Despite this similarity, there is a key difference between these two models: while the KVC attributes a very important role to the interplay between the localized 4f orbitals and the itinerant spd conduction bands, the itinerant electrons are "spectators" in the HM picture.The development of LDA+DMFT (Local Density Approximation plus Dynamical Mean Field Theory) [12] results [13][14][15] has successfully reproduced many aspects of this transition, and different aspects of these studies can be understood in both physical pictures. There are still fundamental questions which have not been answered. (1) What is the role of the spin-orbit interaction (SOC) for the volume-collapse? (2) What is the ...