Fe_1+yTe_xSe_1-x: A Delicate and Tunable Majorana Material

We report the observation for the p_z electron band and the band inversion in Fe_1+yTe_xSe_1-x with angle-resolved photoemission spectroscopy. Furthermore, we found that excess Fe (y> 0) inhibits the topological band inversion in Fe_1+yTe_xSe_1-x, which explains the absence of Majorana zero modes in previous reports for Fe_1+yTe_xSe_1-x with excess Fe. Based on our analysis of different amounts of Te doping and excess Fe, we propose a delicate topological phase in this material. Thanks to this delicate phase, one may be able to tune the topological transition via applying lattice strain or carrier doping.