It is the Kronig-Penny Model.
The particle in one-dimensional lattice is a problem that occurs in the model of periodic crystal lattice.The potential is caused by periodic arrangement of ions in the crystal structure. The graph presents the real part of transmission matrix element P11 plotted versus energy. The allowed energy bands correspond to abs(Re(P11)) smaller than 1, otherwise , the forbidden bands.
Researchers should cite this work as follows:
Heng Li (2007), "Energy Bands In Periodic Potentials," https://nanohub.org/resources/2263.