A Triplet Resonance in Superconducting Fe1.03Se0.4Te0.6☆☆Supported by the National Basic Research Program of China under Grant Nos 2012CB921700 and 2011CBA00112, the National Natural Science Foundation of China under Grant Nos 11034012 and 11190024, the National Science Foundation under Grant No DMR-0645305, the US DOE under Grant No DE-FG02-07ER46358, and the US Department of Energy, Office of Science, Office of Basic

Abstract

From heavy fermion compounds and cuprates to iron pnictides and chalcogenides, a spin resonance at is a staple of nearly magnetic superconductors. Possible explanations include a two-particle bound state or loss of magnon damping in the superconducting state. While both scenarios suggest a central role for magnetic fluctuations, distinguishing them is important to identify the right theoretical framework to understand these types of unconventional superconductors. Using an inelastic neutron scattering technique, we show that the spin resonance in the optimally doped Fe1.03Se0.4Te0.6 superconductor splits into three peaks in a high magnetic field, a signature of a two-particle S = 1 triplet bound state.