Condensed Matter Physics, 2005, vol. 8, No. 4(44), p. 813-824, English

Title: Entropy of fermionic models on highly frustrated lattices
  A.Honecker (Technische Universität Braunschweig, Institut für Theoretische Physik, Mendelssohnstrasse 3, 38106 Braunschweig, Germany) ,
  J.Richter (Institut für Theoretische Physik, Otto-von-Guericke Universität Magdeburg, 39016 Magdeburg, Germany) ,

Spinless fermions on highly frustrated lattices are characterized by the lowest single-particle band which is completely flat. Concrete realizations are provided by the sawtooth chain and the kagom\'e lattice. For these models a real-space picture is given in terms of localized states. Furthermore, we find a finite zero-temperature 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 as well as the large temperature variations carry over to the repulsive Hubbard model. The relation to flat-band ferromagnetism is also discussed briefly.

Key words: flat band, localized states, frustrated lattice, spinless fermions, Hubbard model, magnetocaloric effect
PACS: 71.10.Fd, 65.40.Gr, 75.30.Sg, 75.10.Jm

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