Analytic Properties of Bloch Waves and Wannier Functions

The one-dimensional Schrödinger equation with a periodic and symmetric potential is considered, under the assumption that the energy bands do not intersect. The Bloch waves, ${\varphi }_{n,k}$, and energy bands, $E_{n,k}$, are studied as functions of the complex variable, $k$. In the complex plane, they are branches of multivalued analytic and periodic functions, ${\varphi }_k$, and $E_k$, with branch points, $k^{\prime }$, off the real axis. A simple procedure is described for locating the branch points. Application is made to the power series and Fourier series developments of these functions. The analyticity and periodicity of ${\varphi }_{n,k}$ has some consequences for the form of the Wannier functions. In particular, it is shown that for each band there exists one and only one Wannier function which is real, symmetric or antisymmetric under an appropriate reflection, and falling off exponentially with distance. The rate of falloff is determined by the distance of the branch points $k^{\prime }$ from the real axis.