Nematic order and the superconducting gap in FeSe

Abstract

The intense scrutiny of the electronic structure of FeSe over the last few years has been motivated by the opportunity to distil the essential physics of both the nematic and superconducting phases, and their interplay. Here I will address the evolution of the electronic structure in the nematic phase, below Ts = 90 K using high-resolution ARPES. The hole pocket undergoes elliptical distortions, but most dramatically, our `detwinned’ ARPES results show spectral weight on only one peanutshaped electron pocket. This unexpected result is also observed in the nematic phase of NaFeAs, and I will argue that this effect, rather than the 10-20 meV band shifts and distortions, is the critical ingredient of the electronic structure in the nematic phase. I will also present measurements of the highly anisotropic superconducting gap on both the hole and electron pockets of FeSe. The results are consistent with results from quasiparticle interference, but by considering the matrix element effects in ARPES we are explicitly able to show a scaling of the superconducting gap with the dyz orbital character. Furthermore we show that such a gap structure arises naturally from the solution to the linearized gap equation, starting from a tight-binding model with accuracy on both the band dispersions and their orbital characters, if we also take into account the one-peanut effect as observed in the nematic phase.