Stripes, spin resonance and $d_{x^2-y^2}-$pairing symmetry in FeSe-based layered superconductors

TL;DR

This paper models the spin resonance spectra of FeSe-based superconductors, revealing a nodeless d-wave pairing symmetry, incommensurate spin resonance, and temperature-dependent behavior consistent with experimental data.

Contribution

It introduces a two-orbital TB model to explain spin resonance and pairing symmetry in FeSe superconductors, aligning theoretical predictions with experimental observations.

Findings

Nodeless, isotropic superconducting gap consistent with measurements

Incommensurate spin resonance with hour-glass dispersion

Temperature evolution of spin resonance matches gap behavior

Abstract

We calculate RPA-BCS based spin resonance spectra of newly discovered iron-selenide superconductor using two orbitals tight-binding (TB) model. The slightly squarish electron pocket Fermi surfaces (FSs) at $(\pi,0)/(0,\pi)-$momenta produce leading interpocket nesting instability at incommensurate vector $q\sim(\pi,0.5\pi)$ in the normal state static susceptibility, pinning a strong stripe-like spin-density wave (SDW) or antiferromagnetic (AFM) order at some critical value of $U$. The same nesting also induces $d_{x^2-y^2}-$pairing. The superconducting (SC) gap is nodeless and isotropic on the FSs as they are concentric to the four-fold symmetry point of the $d-$wave gap maxima, in agreement with various measurements. This induces an slightly incommensurate spin resonance with `hour-glass'-like dispersion feature, in close agreement with neutron data of chalcogenides. We also calculate $T$ pendence of the SC gap solving BCS gap equations and find that the spin resonance follows the same $T$ evolution of $\Delta(T)$ both in energy and intensity, suggesting that an itinerant weak or intermediate pair coupling theory is relevant in this system.