Topological Characters in Fe(Te$_{1 x}$Se$_x$ thin films
We investigate topological properties in the Fe(Te,Se) thin films. We find that the single layer FeTe$_{1 x}$Se$_x$ has nontrivial $Z_2$ topological invariance which originates from the parity exchange at \Gamma$ point of Brillouin zone. The nontrivial topology is mainly controlled by the Te(Se) height. Adjusting the height, which can be realized as function of $x$ in FeTe$_{1 x}$Se$_x$ can drive a topological phase transition. In a bulk material, the two dimensional $Z_2$ topology invariance is extended to a strong three dimensional one. In a thin film, we predict that the topological invariance oscillates with the number of layers. The results can also be applied to iron pnictides. Our research establishes FeTe$_{1 x}$Se$_x$ as a unique system to integrate high T$_c$ superconductivity and topological properties in a single electronic structure.