Self-consistentGWdetermination of the interaction strength: Application to the iron arsenide superconductors

Abstract

We introduce a first principles approach to determine the strength of the electronic correlations based on the fully self consistent GW approximation. The approach provides a seamless interface with dynamical mean field theory, and gives good results for well studied correlated materials such as NiO. Applied to the recently discovered iron arsenide materials, it accounts for the noticeable correlation features observed in optics and photoemission while explaining the absence of visible satellites in X-ray absorption experiments and other high energy spectroscopies.Many metals, semiconductors, and insulators are well described by the "standard model" of solid state physics. In this picture the excitations are band electrons, and their dispersion can be computed quantitatively in perturbation theory starting from the density functional theory using the GW method [1]. When this standard model fails, we talk about strongly correlated electron systems. The presence of strong correlations is debated with each new material discovery, as for example in the context of the iron pnictide superconductors. On the experimental side, controversies arose because optical experiments revealed significant mass renormalizations [2-4] while Xray absorption, core level spectroscopies and resonant inelastic Xray scattering indicated the absence of satellite peaks [5,6], which are standard fingerprints of strong correlations. Photoemission studies indicate that the overall bandwidth is narrowed by a factor of two [7,8] but substantially larger mass renormalizations are present near the Fermi level [9]. Similar controversies arose within the first principles approaches to the treatment of correlations with some theoretical studies supporting the notion of weak correlations [10][11][12][13][14], while others advocate a more correlated picture [15][16][17][18]. To make progress on this issues one needs to develop fully ab initio tools for addressing the problem of determining the strength of correlations and test their predictions against experiments.In this letter we introduce a new first principles methodology for evaluating the strength of the correlations based on the self-consistent GW method. This approach has been shown to predict accurate total energy [19,20], and we expect to obtain reliable estimates for the interaction strength since this quantity can be thought as a second derivative of the total energy with respect to the occupation of the correlated orbitals. We test successfully the method on the well studied example of a correlated material NiO, and then we apply it to a prototypical iron pnictide BaFe 2 As 2 . We find that the correlations in iron pnictides are strong, as pointed out in Refs. [15][16][17][18] but unlike earlier studies our ab-initio method accounts for the absence of well defined Hubbard bands in the spectral functions. Our results are thus in excellent agreement with experiment and reconcile the results of apparently conflicting spectroscopies.We start with the one-particle electron Green's function in t...

Nature of the Insulating Ground State of the5dPostperovskiteCaIrO3

“…The double-counting functional is expressed as Φ dc (n d ) = n d E dc , where E dc = U(n 0 d − 1/2) − J/2(n 0 d − 1) and the nominal occupancy n 0 d of Ir ion in CaIrO 3 is 5. Due to the large energy window, the local Coulomb interaction parameters are relatively system-independent and estimated [27] as U ≈ 4.5 eV and J ≈ 0.8 eV [28]. We set different local coordinates on each Ir atom in order to simulate the antiferromagnetic structure.…”

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“…On the other hand, the experimentally observed large electrical resistivity ("bad metal" with k F l ∼ 1), a strong suppression of Drude weight 4 , and the temperatureinduced spectral weight transfer [5][6][7] have suggested sizable strength of electronic correlations. These experimental evidences have suggested the placement of the iron pnictides in close proximity of a putative Mott transition [8][9][10] . In a metallic system close to a Mott transition, quasi-local moments are expected to arise, which for the iron pnictides are described in terms of J 1 − J 2 couplings 8,9 ; here J 1 and J 2 refer to nearest-neighbor and next-nearest-neighbor spin exchange interactions on the iron square lattice.…”

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