  Recently a nonsymmorphic topological insulator was predicted, where the
characteristic feature is the emergence of a "hourglass fermion" surface state
protected by the nonsymmorphic symmetry. Such a state has already been observed
experimentally. We propose a simple model possessing the hourglass fermion
surface state. The model is constructing by stacking the quantum-spin-Hall
insulators with the interlayer coupling introduced so as to preserve the
nonsymmorphic symmetry and the time reversal symmetry. The Dirac theory is also
derived, whose analytical results reproduce the hourglass fermion surface state
remarkably well. Furthermore, we discuss how the hourglass state is destroyed
by introducing perturbations based on the symmetry analysis. Our results show
that the hourglass fermion surface state is universal in the helical edge
system with the nonsymmorphic symmetry.
