Electronic Spectra of Iron Pnictide Superconductors: Influence of Multi-orbitals Hopping and Hund’s Coupling

Abstract

We present a theoretical study of the electronic spectral function of iron-pnictide superconductors like LaFeAs(O, F) material. We have attempted two-band tight-binding model Hamiltonian containing various orbitals hopping energies, intra- and inter-band electronic correlations and Hund’s coupling energy in Fe 3d orbitals. The expression of a single particle spectral function within BCS-mean-field Green’s function approach for superconducting state of iron pnictides is obtained. The spectral function at different points of the Brillouin zone is numerically calculated for extended s-wave pairing symmetry as a function of various model parameters applicable for these systems. It is pointed out through numerical analysis that on increasing nearest-neighbor hopping (t1) between (dxz/dyz) σ-orbitals, the spectral function A(k,ω) shifts towards the Fermi level and provides favorable conditions for band splitting effects in the form of two well separated peaks in the electronic spectral as observed in iron pnictides angle-resolved photoemission spectroscopic (ARPES) measurements. On increasing t2 (nearest-neighbor hopping between dxz/dyzπ-orbitals), the spectral function shows prominent peak with increase in spectral weight close to the Fermi level at the (π,0) and (0,0) points of the Brillouin zone, while the quasi-particle peak shifts away from the Fermi level at the (π,π) point with decreased spectral weight. The bonding peak of spectral function get suppressed while the antibonding spectral peak becomes prominent with increasing t3 (next nearest-neighbor hopping between same orbitals of dxz/dyz) at (π,π) of the Brillouin zone. Further, intra-band Coulomb correlations shift the spectral peak downward with respect to Fermi level along with suppression of the spectral weight. The Hund’s coupling term try to pile up spectral weight close to Fermi level and support to stabilize superconducting state in these systems. The variation of the spectral function within the two-band model is in qualitative accordance with existing ARPES measurements and theoretical investigations in iron-pnictide superconductors.