The Third Generation of the Dirac Cone as a Proof of Stacked 2D Electron Systems in Iron Pnictides
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THE THIRD GENERATION OF THE DIRAC CONE AS A PROOF OF STACKED 2D ELECTRON SYSTEMS IN IRON PNICTIDES Koichi Kusakabe1 1 Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama-cho, Toyonaka, Osaka, 560-8531, Japan ABSTRACT The nodal SDW order parameter on a cylindrical Fermi surface is thought to create the Dirac cone in the metallic ground state of iron pnictides and iron chalcogenides. Confirming appearance of the Dirac cone in DFT-GGA solutions of FeSe, we discuss origin of the bulk SDW order parameter in the stacked two-dimensional electronic system. In a layered system with vanishingly small inter-layer single-particle hopping processes, the exchange channels derived for the inter-plane magnetic interaction is the super-exchange counterpart of the twoparticle Coulomb scattering for the pair-hopping channel in the layered superconductivity. The fluctuation reference method of the multi-reference density functional theory concludes existence of the inter-layer super-exchange interaction by the Coulomb off-diagonal elements among orbitals in the semi-metallic band structure. Thus a proof of 2D nature of the third generation Dirac cone in iron pnictides induced by SDW also promotes understanding of the hightemperature superconductivity. INTRODUCTION The confirmation of the Dirac cone in the SDW state of Ba(FeAs)2[1] is attracting much attention, because this curious electronic state may have relevance for understanding of the hightemperature superconductivity. The transport experiment by Huynh et al.[2] has revealed existence of the electron- and hole-Dirac cones. Specialty of the iron pnictides has been discussed by two- or five-band models for the two-dimensional (2D) electron system in the FeAs plane, where formation of the Dirac cone is caused by the nodal SDW order.[3] This concepts suggests controlling methods of the Dirac cone by perturbation via the SDW order parameter, which is not possible for graphene and α-(BEDT-TTF)2I3. If we count Dirac cones in the latter two materials as the first and second generations, the nodal-SDW-based Dirac cone is the third generation. As for the DFT-based band structure calculations, the finding of electronic dispersion consistent with the nodal-SDW scenario was motivated researchers to improve description of the metallic anti-ferromagnetic state.[4] Therefore, construction of an electronic structure calculation method is demanded for the application to the SDW-generated Dirac cone states. We have discussed the electronic structure of the iron-pnictide superconductors based on a generalization of the ordinal DFT.[5] In this theory for the pair-hopping mechanism of the layered superconductors, we provided a framework starting from DFT-LDA[6,7] or DFT-GGA calculations incorporating electron correlation effects explicitly. This multi-reference DFT is now given as a method to find a converging series of models for the electronic state. The convergence criterion is given by a theory of the model space, where some basic theorems were provided.[8] For the case of
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