On the thermodynamics of classical spins with isotrop Heisenberg interaction in one-dimensional quasi-periodic structures

An exact expression for the free energy of a system of classical magnetic moments, which occupy sites on a Fibonacci or Thue-Morse quasi-crystal lattice and interact via an isotropic Heisenberg interaction, is obtained. The temperature dependence of the specific heat for certain classes of one-dimensional quasi-crystal lattices is discussed. In the case, when spin rotations are confined in a plane (model of plane rotators), some Fibonacci crystals show an extra peak in the specific heat at low temperatures.