Exciton optical absorption in self-similar aperiodic lattices

Exciton optical absorption in self-similar aperiodic one-dimensional systems is considered, focusing our attention on Thue-Morse and Fibonacci lattices as canonical examples. The absorption line shape is evaluated by solving the microscopic equations of motion of the Frenkel-exciton problem on the lattice, in which on-site energies take on two values, according to the Thue-Morse or Fibonacci sequences. Results are compared to those obtained in random lattices with the same stoichiometry and size. We find that aperiodic order causes the occurrence of well-defined characteristic features in the absorption spectra, which clearly differ from the case of random systems, indicating a most peculiar exciton dynamics. The origin of all the absorption lines is assigned by considering the self-similar aperiodic lattices as composed of two-center blocks, within the same spirit of the renormalization group ideas.