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Register a new userNon-local correlations in Iron Pnictides and Chalcogenides
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Shinibali Bhattacharyya
Publication date
2020
fields
Physics
and research's language is
English
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Deviations of low-energy electronic structure of iron-based superconductors from density functional theory predictions have been parametrized in terms of band- and orbital-dependent mass renormalizations and energy shifts. The former have typically been described in terms of a local self-energy within the framework of dynamical mean field theory, while the latter appears to require non-local effects due to interband scattering. By calculating the renormalized bandstructure in both random phase approximation (RPA) and the two-particle self-consistent approximation (TPSC), we show that correlations in pnictide systems like LaFeAsO and LiFeAs can be described rather well by a non-local self-energy. In particular, Fermi pocket shrinkage as seen in experiment occurs due to repulsive interband finite-energy scattering. For the canonical iron chalcogenide system FeSe in its bulk tetragonal phase, the situation is however more complex since even including momentum-dependent band renormalizations cannot explain experimental findings. We propose that the long-range Coulomb interaction may play an important role in band-structure renormalization in FeSe. We further compare our evaluations of non-local quasiparticle scattering lifetime within RPA and TPSC with experimental data for LiFeAs.
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In correlated metals derived from Mott insulators, the motion of an electron is impeded by Coulomb repulsion due to other electrons. This phenomenon causes a substantial reduction in the electrons kinetic energy leading to remarkable experimental manifestations in optical spectroscopy. The high-Tc superconducting cuprates are perhaps the most studied examples of such correlated metals. The occurrence of high-Tc superconductivity in the iron pnictides puts a spotlight on the relevance of correlation effects in these materials. Here we present an infrared and optical study on single crystals of the iron pnictide superconductor LaFePO. We find clear evidence of electronic correlations in metallic LaFePO with the kinetic energy of the electrons reduced to half of that predicted by band theory of nearly free electrons. Hallmarks of strong electronic many-body effects reported here are important because the iron pnictides expose a new pathway towards a correlated electron state that does not explicitly involve the Mott transition.
Undoped iron superconductors accommodate $n=6$ electrons in five d-orbitals. Experimental and theoretical evidence shows that the strength of correlations increases with hole-doping, as the electronic filling approaches half-filling with $n=5$ electrons. This evidence delineates a scenario in which the parent compound of iron superconductors is the half-filled system, in analogy to cuprate superconductors. In cuprates the superconductivity can be induced upon electron or hole doping. In this work we propose to search for high-Tc superconductivity and strong correlations in chromium pnictides and chalcogenides with $n<5$ electrons. By means of ab-initio, slave spin and multi-orbital RPA calculations we analyse the strength of the correlations and the superconducting and magnetic instabilities in these systems with main focus on LaCrAsO. We find that electron-doped LaCrAsO is a strongly correlated system with competing magnetic interactions, being $(pi,pi)$ antiferromagnetism and nodal d-wave pairing the most plausible magnetic and superconducting instabilities, respectively.
We apply the quasi-particle self-consistent GW (QSGW) approximation to some of the iron pnictide and chalcogenide superconductors. We compute Fermi surfaces and density of states, and find excellent agreement with experiment, substantially improving over standard band-structure methods. Analyzing the QSGW self-energy we discuss non-local and dynamic contributions to effective masses. We present evidence that the two contributions are mostly separable, since the quasi-particle weight is found to be essentially independent of momentum. The main effect of non locality is captured by the static but non-local QSGW effective potential. Moreover, these non-local self-energy corrections, absent in e.g. dynamical mean field theory (DMFT), can be relatively large. We show, on the other hand, that QSGW only partially accounts for dynamic renormalizations at low energies. These findings suggest that QSGW combined with DMFT will capture most of the many-body physics in the iron pnictides and chalcogenides.
Orbital-ordering instability arising due to the intrapocket nesting is investigated for the tight-binding models of pnictides in the presence of orbital-lattice coupling. The incommensurate instabilities with small momentum, which may play an important role in the nematic-ordering transition, vary from model to model besides being more favorable in comparison to the spin-density wave instability in the absence of good interpocket nesting. We also examine the doping dependence of such instabilities. The electron-phonon coupling parameter required to induce them are compared with the first-principle calculations.
We calculate the expected finite frequency neutron scattering intensity based on the two-sublattice collinear antiferromagnet found by recent neutron scattering experiments as well as by theoretical analysis on the iron oxypnictide LaOFeAs. We consider two types of superexchange couplings between Fe atoms: nearest-neighbor coupling J1 and next-nearest-neighbor coupling J2. We show how to distinguish experimentally between ferromagnetic and antiferromagnetic J1. Whereas magnetic excitations in the cuprates display a so-called resonance peak at (pi,pi) (corresponding to a saddlepoint in the magnetic spectrum) which is at a wavevector that is at least close to nesting Fermi-surface-like structures, no such corresponding excitations exist in the iron pnictides. Rather, we find saddlepoints near (pi,pi/2) and (0,pi/2)(and symmetry related points). Unlike in the cuprates, none of these vectors are close to nesting the Fermi surfaces.
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