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Register a new userCoherence-incoherence crossover in the normal state of iron-oxypnictides and importance of the Hunds rule coupling
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A new class of high temperature superconductors based on iron and arsenic was recently discovered, with superconducting transition temperature as high as 55 K. Here we show, using microscopic theory, that the normal state of the iron pnictides at high temperatures is highly anomalous, displaying a Curie Weiss susceptibility and a linear temperature dependence of the resistivity. Below a coherence scale T*, the resistivity sharply drops and susceptibility crosses over to Pauli-like temperature dependence. Remarkably, the coherence-incoherence crossover temperature is a very strong function of the strength of the Hunds rule coupling J_Hund. On the basis of the normal state properties, we estimate J_Hund to be 0.35-0.4 eV. In the atomic limit, this value of J_Hund leads to the critical ratio of the exchange constants J_1/J_2~2. While normal state incoherence is in common to all strongly correlated superconductors, the mechanism for emergence of the incoherent state in iron-oxypnictides, is unique due to its multiorbital electronic structure.
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In most magnetically-ordered iron pnictides, the magnetic moments lie in the FeAs planes, parallel to the modulation direction of the spin stripes. However, recent experiments in hole-doped iron pnictides have observed a reorientation of the magnetic moments from in-plane to out-of-plane. Interestingly, this reorientation is accompanied by a change in the magnetic ground state from a stripe antiferromagnet to a tetragonal non-uniform magnetic configuration. Motivated by these recent observations, here we investigate the origin of the spin anisotropy in iron pnictides using an itinerant microscopic electronic model that respects all the symmetry properties of a single FeAs plane. We find that the interplay between the spin-orbit coupling and the Hunds rule coupling can account for the observed spin anisotropies, including the spin reorientation in hole-doped pnictides, without the need to invoke orbital or nematic order. Our calculations also reveal an asymmetry between the magnetic ground states of electron- and hole-doped compounds, with only the latter displaying tetragonal magnetic states.
The theoretical understanding of the nematic state of iron-based superconductors and especially of FeSe is still a puzzling problem. Although a number of experiments calls for a prominent role of local correlations and place iron superconductors at the entrance of a Hund metal state, the effect of the electronic correlations on the nematic state has been theoretically poorly investigated. In this work we study the nematic phase of iron superconductors accounting for local correlations, including the effect of the Hunds coupling. We show that Hunds physics strongly affects the nematic properties of the system. It severely constraints the precise nature of the feasible orbital-ordered state and induces a differentiation in the effective masses of the zx=yz orbitals in the nematic phase. The latter effect leads to distinctive signatures in different experimental probes, so far overlooked in the interpretation of experiments. As notable examples the splittings between zx and yz bands at Gamma and M points are modified, with important consequences for ARPES measurements.
The electronic structure in the normal state of CeFeAsO0.89F0.11 oxypnictide superconductors has been investigated with x-ray absorption and photoemission spectroscopy. All the data exhibit signatures of Fe d-electron itinerancy. Exchange multiplets appearing in the Fe 3s core level indicate the presence of itinerant spin fluctuations. These findings suggest that the underlying physics and the origin of superconductivity in these materials are likely to be quite different from those of the cuprate high-temperature superconductors. These materials provide opportunities for elucidating the role of magnetic fluctuations in high-temperature superconductivity.
The ground-state properties of CuFeAs were investigated by applying density functional theory calculations within generalized gradient approximation (GGA) and GGA+U. We find that the bicollinear antiferromagnetic state with antiparallel orbital magnetic moments on each iron which violates the Hunds rule is favored by the on-site Coulomb interaction, which is further stabilized by Cu vacancy. The magnetic ground state can be used to understand weak antiferromagnetism in CuFeAs observed experimentally. We argue that breakdown of the Hunds rule may be the possible origin for reduced magnetism in iron pnictides, rather than magnetic fluctuations induced by electronic correlations.
Angle-resolved photoemission spectroscopy (ARPES) is used to study the scattering rates of charge carriers from the hole pockets near Gamma in the iron-based high-Tc hole doped superconductors KxBa1-xFe2As2 x=0.4 and KxEu1-xFe2As2 x=0.55$ and the electron doped compound Ba(Fe1-xCox)2As2 x=0.075. The scattering rate for any given band is found to depend linearly on energy, indicating a non-Fermi liquid regime. The scattering rates in the hole-doped compound are considerably larger than those in the electron-doped compounds. In the hole-doped systems the scattering rate of the charge carriers of the inner hole pocket is about three times bigger than the binding energy indicating that the spectral weight is heavily incoherent. The strength of the scattering rates and the difference between electron and hole doped compounds signals the importance of Hunds exchange coupling for correlation effects in these iron-based high-Tc superconductors. The experimental results are in qualitative agreement with theoretical calculations in the framework of combined density functional dynamical mean-field theory.
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