Entropy of fermionic models on highly frustrated lattices

Spinless fermions on highly frustrated lattices are characterized by a lowest single-particle band which is completely flat. Concrete realizations are provided by the sawtooth chain and the kagomé lattice. For these models a real-space picture is given in terms of localized states. Furthermore, we find a finite zerotemperature entropy for a suitable choice of the chemical potential. The entropy is computed numerically at finite temperature and one observes a strong cooling effect during adiabatic changes of the chemical potential. We argue that the localized states, the associated zero-temperature entropy and thus also the large temperature variations carry over to the repulsive Hubbard model. The relation to flat-band ferromagnetism is also discussed briefly.