Orbital-Selective Superconductivity and the Effect of Lattice Distortion in Iron-Based Superconductors

Abstract

The superconducting (SC) state of iron-based compounds in both tetragonal and orthorhombic phases is studied on the basis of an effective Hamiltonian composed of the kinetic energy including the five Fe 3d-orbitals, the orthorhombic crystalline electric field (CEF) energy, and the two-orbital Kugel'-Khomskiĭ-type superexchange interaction. Our basic assumption is that the antiferromagnetic (AF) state in the parent compounds can be described by the $d_{xz}$ and $d_{yz}$ orbitals, and that the electrons in these orbitals have relatively strong electron correlation in the vicinity of the AF state. In order to study the physical origin of the structure-sensitive SC transition temperature, the effect of orthorhombic distortion is taken into account as the energy-splitting, ${\Delta }_{\text{ortho.}}$, between the $d_{xz}$ and $d_{yz}$ orbitals. We find that the eigenvalue of the linearized gap equation decreases accompanied with the reduction of the partial density of states for the $d_{xz}$ and $d_{yz}$ orbitals as ${\Delta }_{\text{ortho.}}$ increases, and that the dominant pairing symmetry is an unconventional fully gapped $s_{+-}$-wave pairing. We also find large anisotropy of the SC gap function in the orthorhombic phase. We propose that the CEF energy plays an important role in controlling $T_{\text{c}}$ and the SC gap function, and that orbital-selective superconductivity is a key feature in iron-based superconductors, which causes the structure-sensitive $T_{\text{c}}$.

11 pages, To appear in J. Phys. Soc. Jpn

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• Nematicity, magnetism and superconductivity in FeSe
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• Enhanced superconductivity accompanying a Lifshitz transition in electron-doped FeSe monolayer
• Superconductivity across Lifshitz transition and anomalous insulating state in surface K-dosed (Li0.8Fe0.2OH)FeSe
• Strong pairing at iron $3d_{xz,yz}$ orbitals in hole-doped BaFe${}_2$As${}_2$
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• Orbital-dependent self-energy effects and consequences for the superconducting gap structure in multi-orbital correlated electron systems
• Effects of momentum-dependent quasiparticle renormalization on the gap structure of iron-based superconductors
• Real-space orbital tiling approach for the design of novel superconductors