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Properties of the multiorbital Hubbard models for the iron-based superconductors
Elbio Dagotto, Adriana Moreo, Andrew Nicholson, Qinglong Luo, Shuhua Liang, Xiaotian Zhang
Frontiers of Physics. 2011, 6 (4 ): 379-397.
https://doi.org/10.1007/s11467-011-0222-z
A brief review of the main properties of multiorbital Hubbard models for the Fe-based superconductors is presented. The emphasis is on the results obtained by our group at the University of Tennessee and Oak Ridge National Laboratory, Tennessee, USA, but results by several other groups are also discussed. The models studied here have two, three, and five orbitals, and they are analyzed using a variety of computational and mean-field approximations. A “physical region” where the properties of the models are in qualitative agreement with neutron scattering, photoemission, and transport results is revealed. A variety of interesting open questions are briefly discussed such as: what are the dominant pairing tendencies in Hubbard models? Can pairing occur in an interorbital channel? Are nesting effects of fundamental relevance in the pnictides or approaches based on local moments are more important? What kind of magnetic states are found in the presence of iron vacancies? Can charge stripes exist in iron-based superconductors? Why is transport in the pnictides anisotropic? The discussion of results includes the description of these and other open problems in this fascinating area of research.
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Structural, magnetic and electronic properties of the iron–chalcogenide A x Fe2-y Se2 (A =K, Cs, Rb, and Tl, etc.) superconductors
Dai-xiang Mou, Lin Zhao, Xing-jiang Zhou
Frontiers of Physics. 2011, 6 (4 ): 410-428.
https://doi.org/10.1007/s11467-011-0229-5
The latest discovery of a new iron–chalcogenide superconductor A x Fe2-y Se2 (A = K, Cs, Rb, and Tl, etc.) has attracted much attention due to a number of its unique characteristics, such as the possible insulating state of the parent compound, the existence of Fe-vacancy and its ordering, a new form of magnetic structure and its interplay with superconductivity, and the peculiar electronic structures that are distinct from other Fe-based superconductors. In this paper, we present a brief review on the structural, magnetic and electronic properties of this new superconductor, with an emphasis on the electronic structure and superconducting gap. Issues and future perspectives are discussed at the end of the paper.
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Superconductivity in the cuprates: Deduction of mechanism for d-wave pairing through analysis of ARPES
Han-Yong Choi, Chandra M. Varma, Xing-jiang Zhou
Frontiers of Physics. 2011, 6 (4 ): 440-449.
https://doi.org/10.1007/s11467-011-0231-y
In the Eliashberg integral equations for d-wave superconductivity, two different functions (α 2 F )n (ω , θ ) and (α 2 F )p ,d (ω ) determine, respectively, the “normal” self-energy and the “pairing” self-energy. ω is the frequency of fluctuations scattering the fermions whose momentum is near the Fermi-surface and makes an angle θ to a chosen axis. We present a quantitative analysis of the high-resolution laser based Angle Resolved Photoemission Spectroscopy (ARPES) data on a slightly under doped cuprate compound Bi2212 and use the Eliashberg equations to deduce the ω and θ dependence of (α 2 F )n (ω , θ ) for T just above T c and below T c . Besides its detailed ω dependence, we find the remarkable result that this function is nearly independent of θ between the (π; π)-direction and 25 degrees from it, except for the dependence of the cut-off energy on θ . Assuming that the same fluctuations determine both the normal and the pairing self-energy, we ask what theories give the function (α 2 F )p ,d (ω ) required for the d-wave pairing instability at high temperatures as well as the deduced (α 2 F )n (θ , ω ). We show that the deduced (α 2 F )n (θ , ω ) can only be obtained from antiferromagnetic (AFM) fluctuations if their correlation length is smaller than a lattice constant. Using (α 2 F )p ,d (ω ) consistent with such a correlation length and the symmetry of matrix-elements scattering fermions by AFM fluctuations, we calculate T c and show that AFM fluctuations are excluded as the pairing mechanism for d-wave superconductivity in cuprates. We also consider the quantumcritical fluctuations derived microscopically as the fluctuations of the observed loop–current order discovered in the under-doped cuprates, and which lead to the marginal Fermi–liquid properties in the normal state. We show that their frequency dependence and the momentum dependence of their matrix-elements to scatter fermions are consistent with the θ and ω dependence of the deduced (α 2 F )n (ω , θ ). The pairing kernel (α 2 F )p ,d (ω ) calculated using the experimental values in the Eliashberg equation gives d-wave instability at T c comparable to the experiments.
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