Fe${}_{1+y}$Te${}_x$Se${}_{1-x}$: a delicate and tunable Majorana material

We report the observation for the p${}_z$ electron band and the band inversion in Fe${}_{1+y}$Te${}_x$Se${}_{1-x}$ with angle-resolved photoemission spectroscopy. Furthermore, we found that excess Fe (y>0) inhibits the topological band inversion in Fe${}_{1+y}$Te${}_x$Se${}_{1-x}$, which explains the absence of Majorana zero modes in previous reports for Fe${}_{1+y}$Te${}_x$Se${}_{1-x}$ 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.