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Register a new userModel for the Magnetic Order and Pairing Channels in Fe Pnictide Superconductors
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A two-orbital model for Fe-pnictide superconductors is investigated using computational techniques on two-dimensional square clusters. The hopping amplitudes are derived from orbital overlap integrals, or by band structure fits, and the spin frustrating effect of the plaquette-diagonal Fe-Fe hopping is remarked. A spin striped state is stable in a broad range of couplings in the undoped regime, in agreement with neutron scattering. Adding two electrons to the undoped ground state of a small cluster, the dominant pairing operators are found. Depending on parameters, two pairing operators were identified: they involve inter-xz-yz orbital combinations forming spin singlets or triplets, transforming according to the B_2g and A_2g representations of the D_4h group, respectively.
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The interplay between the structural and magnetic phase transitions occurring in the Fe-based pnictide superconductors is studied within a Ginzburg-Landau approach. We show that the magnetoelastic coupling between the corresponding order parameters is behind the salient features observed in the phase diagram of these systems. This naturally explains the coincidence of transition temperatures observed in some cases as well as the character (first or second-order) of the transitions. We also show that magnetoelastic coupling is the key ingredient determining the collinearity of the magnetic ordering, and we propose an experimental criterion to distinguish between a pure elastic from a spin-nematic-driven structural transition.
The electronic origin of the huge magnetostructural effect in layered Fe-As compounds is elucidated using LiFeAs as a prototype. The crucial feature of these materials is the strong covalent bonding between Fe and As, which tends to suppress the exchange splitting. The bonding-antibonding splitting is very sensitive to the distance between Fe and As nuclei. We argue that the fragile interplay between bonding and magnetism is universal for this family of compounds. The exchange interaction is analyzed in real space, along with its correlation with covalency and doping. The range of interaction and itinerancy increase as the Fe-As distance is decreased. Superexchange makes a large antiferromagnetic contribution to the nearest-neighbor coupling, which develops large anisotropy when the local moment is not too small. This anisotropy is very sensitive to doping.
We present a comprehensive comparison of the infrared charge response of two systems, characteristic of classes of the 122 pnictide (SrFe2As2) and 11 chalcogenide (Fe_1.087Te) Fe compounds with magnetically-ordered ground states. In the 122 system, the magnetic phase shows a decreased plasma frequency and scattering, and associated appearance of strong mid-infrared features. The 11 system, with a different magnetic ordering pattern, also shows decreased scattering, but an increase in the plasma frequency, while no clear mid-infrared features appear below the ordering temperature. We suggest how this marked contrast can be understood in terms of the diverse magnetic ordering patterns of the ground state, and conclude that while the high temperature phases of these systems are similar, the magnetic ordering strongly affects the charge dynamical response. In addition, we propose an optical absorption mechanism which appears to be consistent with information gained from several different experiments.
The interplay of high and low-energy mass renormalizations with band-shifts reflected by the positions of van Hove singularities (VHS) in the normal state spectra of the highest hole-overdoped and strongly correlated AFe$_2$As$_2$ (A122) with A = K, Cs is discussed phenomenologically based on ARPES data and GGA band-structure calculations with full spin-orbit coupling. The big increase of the Sommerfeld coefficient $gamma$ from K122 to Cs122 is ascribed to an enhanced coupling to low-energy bosons in the vicinity of a quantum critical point to an unknown, yet incommensurate phase different from the commensurate Mott one. We find no sizeable increase in correlations for Cs122 in contrast to F. Eilers et al., PRL v. 116, 237003 (2016) [3]. The empirical (ARPES) VHS positions as compared with GGA-predictions point even to slightly weaker correlations in Cs122 in accord with low-$T$ magnetic susceptibility $chi(T)$ data and a decreasing Wilson ratio $propto chi(0)/gamma$.
A microscopic theory of superconductivity is formulated within an effective $p$-$d$ Hubbard model for a CuO2 plane. By applying the Mori-type projection technique, the Dyson equation is derived for the Green functions in terms of Hubbard operators. The antiferromagnetic exchange caused by interband hopping results in pairing of all carries in the conduction subband and high Tc proportional to the Fermi energy. Kinematic interaction in intraband hopping is responsible for the conventional spin-fluctuation pairing. Numerical solution of the gap equation proves the d-wave gap symmetry and defines Tc doping dependence. Oxygen isotope shift and pressure dependence of Tc are also discussed.
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