Orbital selectivity and emergent superconducting state from quasi-degenerate $s-$ and $d-$wave pairing channels in iron-based superconductors
A major puzzle about the nature of the iron-based superconductivity appears in the case of the alkaline iron selenides. Compared to the iron pnictides, these systems have only electron Fermi pockets (i.e. no hole Fermi pockets) but comparable superconducting transition temperatures. The challenge lies in reconciling the two basic experimental features of their superconducting state: a node-less gap and the existence of a resonance in the spin excitation spectrum. We propose a mechanism based on reconstructing two quasi-degenerate pairing states, one in an $s$-wave $A_{1g}$ channel that is fully gapped, and the other in a $d$-wave $B_{1g}$ channel whose pairing function changes sign across the electron Fermi pockets at the Brillouin-zone boundary. The resulting intermediate pairing state, which we call an orbital-selective $s \times \tau_3$ state, incorporates both of the above two properties. When the leading spin-singlet pairing is in the $d_{xz}, d_{yz}$ orbital subspace, this state retains the $s$-wave form factor but has a $B_{1g}$ symmetry due to an internal $\tau_3$ structure in the orbital space. Within a five-orbital $t-J_{1}-J_{2}$ model with orbital-selective exchange couplings, we show that the proposed pairing state is energetically competitive over a finite range of control parameters. We calculate the dynamical spin susceptibility in the orbital-selective $s \times \tau_3$ superconducting state and show that a spin resonance arises and has the characteristics of observed by inelastic neutron experiments in the alkaline iron selenides. More generally, the formation of the orbital-selective $s \times \tau_3$ state represents a novel means of relieving the quasi-degeneracy between $s-$ and $d-$wave pairing states, which is a hitherto unsuspected alternative to the conventional route of linearly superposing the two into a time-reversal symmetry breaking $s+id$ state.