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Register a new userInfluence of Lifshitz transitions and correlation effects on the scattering rates of the charge carriers in iron-based superconductors
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J. Fink
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Minimum model calculations on the co-action of hole vanishing Lifshitz transitions and correlation effects in ferropnictides are presented. The calculations predict non-Fermi-liquid behaviour and huge mass enhancements of the charge carriers at the Fermi level. The findings are compared with recent ARPES experiments and with measurements of transport and thermal properties of ferropnictides. The results from the calculation can be also applied to other unconventional superconductors and question the traditional view of quantum critical points.
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Electronic interactions in multiorbital systems lead to non-trivial features in the optical spectrum. In iron superconductors the Drude weight is strongly suppressed with hole-doping. We discuss why the common association of the renormalization of the Drude weight with that of the kinetic energy, used in single band systems, does not hold in multi-orbital systems. This applies even in a Fermi liquid description when each orbital is renormalized differently, as it happens in iron superconductors. We estimate the contribution of interband transitions at low energies. We show that this contribution is strongly enhanced by interactions and dominates the coherent part of the spectral weight in hole-doped samples at frequencies currently used to determine the Drude weight.
We examine the relevance of magneto-elastic coupling to describe the complex magnetic and structural behaviour of the different classes of the iron superconductors. We model the system as a two-dimensional metal whose magnetic excitations interact with the distortions of the underlying square lattice. Going beyond mean field we find that quantum fluctuation effects can explain two unusual features of these materials that have attracted considerable attention. First, why iron telluride orders magnetically at a non-nesting wave-vector $(pi/2, pi/2)$ and not at the nesting wave-vector $(pi, 0)$ as in the iron arsenides, even though the nominal band structures of both these systems are similar. And second, why the $(pi, 0)$ magnetic transition in the iron arsenides is often preceded by an orthorhombic structural transition. These are robust properties of the model, independent of microscopic details, and they emphasize the importance of the magneto-elastic interaction.
We report on the influence of spin-orbit coupling (SOC) in the Fe-based superconductors (FeSCs) via application of circularly-polarized spin and angle-resolved photoemission spectroscopy. We combine this technique in representative members of both the Fe-pnictides and Fe-chalcogenides with ab initio density functional theory and tight-binding calculations to establish an ubiquitous modification of the electronic structure in these materials imbued by SOC. The influence of SOC is found to be concentrated on the hole pockets where the superconducting gap is generally found to be largest. This result contests descriptions of superconductivity in these materials in terms of pure spin-singlet eigenstates, raising questions regarding the possible pairing mechanisms and role of SOC therein.
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.
Electronic structure calculations combining the local-density approximation with an exact diagonalization of the Anderson impurity model show an intermediate 5f^5-5f^6-valence ground state and delocalization of the 5f^5 multiplet of the Pu atom 5f-shell in PuCoIn_5, PuCoGa_5, and delta-Pu. The 5f-local magnetic moment is compensated by a moment formed in the surrounding cloud of conduction electrons. For PuCoGa_5 and delta-Pu the compensation is complete and the Anderson impurity ground state is a singlet. For PuCoIn_5 the compensation is partial and the Pu ground state is magnetic. We suggest that the unconventional d-wave superconductivity is likely mediated by the 5f-states antiferromagnetic fluctuations in PuCoIn_5, and by valence fluctuations in PuCoGa_5.
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