Double butterfly spectrum for two interacting particles in the Harper   model
  We study the effect of interparticle interaction $U$ on the spectrum of the Harper model and show that it leads to a pure-point component arising from the multifractal spectrum of non interacting problem. Our numerical studies allow to understand the global structure of the spectrum. Analytical approach developed permits to understand the origin of localized states in the limit of strong interaction $U$ and fine spectral structure for small $U$. 
Armelle Barelli, Jean Bellissard, Philippe Jacquod (1) and Dima L.   Shepelyansky (Laboratoire de Physique Quantique, Univer...