Quasiparticle Interference of Monolayer FeSe_xTe_1-x on Bi₂Te₃

Monolayer FeSe_xTe_1-x is a high-temperature, unconventional, possibly topological superconductor thought to possesses a sign-changing s-wave order parameter. There is significant experimental evidence that the order parameter is of the s_+- type, but so far no experiments have been able to differentiate between the many variants of s_+- pairing, such as conventional s_+-, odd-parity s_+-, and orbital-antiphase s_+-. To address this shortcoming, we perform superconducting quasiparticle interference (QPI) measurements on FeSe_xTe_1-x grown on Bi₂Te₃, finding a striking antisymmetrized QPI response consisting of two peaks of opposite signs at the wavevector connecting the electron and hole pockets. According to the Hirschfeld-Altenfeld-Eremin-Mazin theory of QPI, a single peak of this kind indicates a sign change in the order parameter between the connected pockets and is an established experimental signature of s_+- pairing in bulk FeSe. We perform extensive QPI simulations using a detailed tight-binding model of FeSe_xTe_1-x to determine the origin of the dual peak structure and whether it is related to the sign structure of the order. By comparing simulated QPI responses for FeSe_xTe_1-x with s-wave, s_+-, odd-parity s_+-, and orbital-antiphase s_+- order parameters, we determine that a dual peak QPI response can occur for any order parameter with a sign change between the hole and electron pockets and that the additional sign changes of the odd-parity and orbital-antiphase order parameters are not necessary. Furthermore, we find that for the two peaks to be of opposite signs requires novel orbital-selective scattering, likely induced by substrate impurities. This indicates that consideration of the scattering mechanism will be key to determining the order parameters of iron-based superconductors via QPI experiments.