Spin Gap and Resonance at the Nesting Wave Vector in SuperconductingFeSe0.4Te0.6

Abstract

Neutron scattering is used to probe magnetic excitations in ${\mathrm{FeSe}}_{0.4}{\mathrm{Te}}_{0.6}$ ($T_c=14\mathrm{K}$). Low energy spin fluctuations are found with a characteristic wave vector ($\frac{1}{2}\frac{1}{2}L$) that corresponds to Fermi surface nesting and differs from ${\mathbf{Q}}_m=(\delta 0\frac{1}{2})$ for magnetic ordering in ${\mathrm{Fe}}_{1+y}\mathrm{Te}$. A spin resonance with $\hslash {\Omega }_0=6.51(4)\mathrm{meV}\approx 5.3k_B{T}_c$ and $\hslash \Gamma =1.25(5)\mathrm{meV}$ develops in the superconducting state from a normal state continuum. We show that the resonance is consistent with a bound state associated with $s_{\pm }$ superconductivity and imperfect quasi-2D Fermi surface nesting.