Dynamical Correlations and Screened Exchange on the Experimental Bench: Spectral Properties of the Cobalt PnictideBaCo2As2

Abstract

Understanding the Fermi surface and low-energy excitations of iron or cobalt pnictides is crucial for assessing electronic instabilities such as magnetic or superconducting states. Here, we propose and implement a new approach to compute the low-energy properties of correlated electron materials, taking into account both screened exchange beyond the local density approximation and local dynamical correlations. The scheme allows us to resolve the puzzle of BaCo2As2, for which standard electronic structure techniques predict a ferromagnetic instability not observed in nature.PACS numbers: 71.27.+a, 71.45.Gm, 74.70.Xa, The discovery of unconventional superconductivity in iron pnictides and chalcogenides in 2008 has aroused strong interest into the Fermi surfaces and low-energy excitations of transition metal pnictides and related compounds. Angle-resolved photoemission spectroscopy (ARPES) has been used to systematically map out quasiparticle dispersions, and to identify electron and hole pockets potentially relevant for low-energy instabilities [1][2][3][4][5][6][7]. Density functional theory (DFT) calculations have complemented the picture, yielding information about orbital characters [8], or the dependence of the topology of the Fermi surface on structural parameters or element substitution [9,10]. DFT within the local density approximation (LDA) or generalized gradient schemes has also served as a starting point for refined many-body calculations addressing band renormalizations and quasiparticle dispersions directly from a theoretical perspective (see e.g. Ref. [11][12][13][14][15][16][17][18]), and its combination with dynamical mean field theory (LDA+DMFT) [19][20][21][22][23][24][25] is nowadays the state-of-the-art ab initio many-body approach to low-energy properties of transition metal pnictides. Despite tremendous successes, however, limitations have also been pointed out e.g. in the description of the Fermi surfaces. Prominent examples include Ba(Fe,Co) 2 As 2 [26,27] or LiFeAs [14,26]. Interestingly, many-body perturbation theory approximating the selfenergy by its first order term in the screened Coulomb interaction W (so-called "GW approximation") results in a substantially improved description: calculations using the quasi-particle self-consistent (QS)GW method [28] have pinpointed non-local self-energy corrections to the LDA Fermi surfaces not captured in LDA+DMFT as pivotal [26]. Yet, as a perturbative method, GW cannot be expected to describe materials away from the weak coupling limit [29], and the description of incoherent regimes [13,17] including coherence-incoherence crossovers [30], local moment behavior [15] or the subtle effects of doping or temperature changes [17] are still reserved for DMFT.In this Letter, we propose and implement a new approach to the spectral properties of correlated electron materials taking into account screened exchange beyond the local density approximation and correlations as described by dynamical mean field theory with frequency-dependent local Hubbard inter...

Influence of Fock exchange in combined many-body perturbation and dynamical mean field theory

“…This suggests the possibility of studying complex multiband materials, where a full GW +EDMFT computation would be too costly, using techniques in the spirit of the recent screened exchange + dynamical DMFT (SEx+DDMFT) method [26,27,59]. In realistic materials, the simple single-band description is not sufficient, and substantial screening effects resulting from the presence of higher-energy degrees of freedom must be taken into account [60][61][62].…”

“…Therefore, what is relevant here is indeed the exchange term calculated using the effective bare interaction of the low-energy Hamiltonian. For realistic electronic structure calculations, this interaction should correspond to a partially screened interaction, where screening by high-energy degrees of freedom is taken into account (as done, e.g., in the screened exchange + DMFT scheme [27,28]). We refer the interested reader to Refs.…”

Magnetic interactions in iron superconductors: A review

“…The dependence of interactions on frequency [186], non-local correlations beyond single site approximations [214], and deviations from the nominal filling not captured by five-orbital models [188,215] have been claimed to have an effect on the correlations in iron superconductors, but they are beyond the scope of this paper. Hund's coupling and the orbital degree of freedom have a very significant effect on the magnetic state of iron superconductors.…”

Electronic polarons, cumulants and doubly dynamical mean field theory: Theoretical spectroscopy for correlated and less correlated materials

“…of "LDA++" [7] or screened exchange [48,49,39] form), a local interaction term H V that is of Hubbard form but with the local interactions given by the unscreened local matrix elements of the bare Coulomb interactions V and the Hund's exchange coupling J (assumed not to be screened by the bosons and thus frequency-independent)…”

“…In the first attempts putting up a "LDA+U(ω)+DMFT" scheme [50,51], H 0 is given by the Kohn-Sham Hamiltonian of DFT, suitably corrected for double counting terms. More recently, in the so-called "screened exchange dynamical mean field theory" a screened exchange Hamiltonian is used as a starting point [49,48,39].…”