  We investigate nematic states in both bulk FeSe and FeSe thin films. It is
found that their band structures and signature features that were observed in a
variety of experiments can be perfectly explained by introducing the $d$-wave
nematic orders that are required to have contributions from all $t_{2g}$
d-orbitals, which contradicts the conventional wisdom that the nematicity is
simply driven by the orbital degeneracy between the $d_{xz}$ and $d_{yz}$
orbitals. These orders can be generated by the Coulomb interaction between the
nearest neighbor Fe sites. In the presence of spin-orbital couplings, we
predict that the nematic order can drive a topological quantum phase transition
through a band inversion at the $M$ point of the Brillouin zone to produce
topologically protected edge states near the Fermi level. The prediction makes
FeSe as a tunable system to integrate topological properties into high
temperature superconductivity to realize Majorana related physics.
