Pairing Symmetry in a Two-Orbital Exchange Coupling Model of Oxypnictides

Abstract

We study the pairing symmetry of a two orbital J1 − J2 model for FeAs layers in oxypnictides. We vary the doping and the value of J1 and J2 to compare all possible pairing symmetries in a mean-field calculation. We show that the mixture of an intra-orbital unconventional s x 2 y 2 ∼ cos(kx) cos(ky) pairing symmetry and a d x 2 −y 2 ∼ cos(kx) − cos(ky) pairing symmetry is favored in a large part of J1 − J2 phase diagram. A pure s x 2 y 2 pairing state is favored for J2 >> J1. The signs of the d x 2 −y 2 order parameters in two different orbitals are opposite. While a small dxy ∼ sin(kx) sin(ky) inter-orbital pairing order coexists in the above phases, the intra-orbital dxy pairing symmetry is not favored even for large values of J2. PACS numbers:High temperature superconductivity (at 56K) has been recently reported in the rare-earth electron and hole-doped oxypnictide compounds 1,2,3,4,5,6 . Preliminary evidence 7,8,9 suggests that the superconducting state in the electron-doped oxypnictides, like that in the Cuprates, has gapless nodal quasiparticle excitations and hence an unconventional pairing symmetry. A number of theoretical studies have predicted or conjectured different possible pairing symmetries, anywhere from p-wave to a π-shifted s − wave 10 .A natural approach to the physics of oxypnictides is by drawing comparisons with that of the Cuprates. In the case of Cuprates, superconductivity is produced by doping a half-filled antiferromagnetic insulator. The antiferromagnetic exchange naturally provides for a singlet pairing amplitude 11 , with equal mean-field critical temperatures for both d−wave and extended s−wave 11,12 in the ultra-Mott limit. Upon doping, due to the Fermi surface shape, the d-wave condensate has higher mean-field transition temperature than the extended s-wave one 12 . The nearest neighbor hoping in the square lattice of Cu atoms is dominant and provides for a large Fermi surface with large effective mass in the Mott limit.The electronic properties of oxypnictides differ from those in Cuprates in several important ways. Most importantly, the undoped oxypnictides are metallic but their resistivity is strikingly high. They can hence be interpreted both as a bad metal or as a poor insulator, leaving open the question of whether a weak or strong coupling fixed point governs their physics. From the band structure point of view, barring the existence of un-physically strong crystal fields, it seems likely that all 3d orbitals of the Fe atoms are involved in the low energy electronic properties. Numerical results based on first principle calculations show the presence of small Fermi surfaces 13 . In the unfolded Brillouin zone consisting of one Fe per unit cell, electron and hole pockets exist around the M and Γ, (π, π) points respectively. The magnetic properties of the oxypnictides are also different from those of the Cuprates. Neutron experiments have shown that the magnetic structure in undoped oxypnictides is not a simple antiferromagnetic order 14 but rather a stripe spin-density wave...

Penetration depth, lower critical fields, and quasiparticle conductivity in Fe-arsenide superconductors

“…Various theories have been proposed for the pairing symmetry in the doped Fe-based oxypnictides [11,12,13,14,15,16,57,58,59,60]. As confirmed by recent ARPES measurements [52], doped Fe-based oxypnictides have hole pockets in the Brillouin zone center and electron pockets in the zone edges [51].…”

“…Unconventional superconducting pairings, most notably the sign-reversing s ± state, have been suggested by several theories [11,12,13,14,15,16] featuring the importance of the nesting between the hole and electron bands. This is also in sharp contrast to other multiband superconductors such as MgB 2 , where the coupling between the different bands is very weak.…”

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