We study spectra of Toeplitz operators $T_a $ with periodic symbols in Bergman spaces $A^2(\Pi)$ on unbounded periodic planar domains $\Pi$, which are defined as the union of infinitely many copies of the translated, bounded periodic cell $\varpi$. We introduce Floquet-transform techniques and prove a version of the band-gap-spectrum formula, which is well-known in the framework of periodic elliptic spectral problems and which describes the essential spectrum of $T_a$ in terms of the spectra of a family of Toepliz-type operators $T_{ a,\eta}$ in the cell $\varpi$, where $\eta$ is the so-called Floquet variable. As an application, we consider periodic domains $\Pi_h$ containing thin geometric structures and show how to construct a Toeplitz operator $T_{ \sf a}: A^2(\Pi_h) \to A^2(\Pi_h)$ such that the essential spectrum of $T_{ \sf a}$ contains disjoint components which approximatively coincide with any given finite set of real numbers. Moreover, our method provides a systematic and illustrative way how to construct such examples by using Toeplitz operators on the unit disc $\mathbb{ D}$ e.g. with radial symbols. Using a Riemann mapping one can then find a Toeplitz operator $T_a : A^2(\mathbb{ D}) \to A^2(\mathbb{ D})$ with a bounded symbol and with the same spectral properties as $T_{ \sf a}$.
Categories: math.FA