Thermodynamic properties of the XX model in a chain with a period two and three coupling

The exact solutions for the energy spectrum of the XX model with a periodic coupling and an external transverse magnetic field h are obtained. The diagonalization procedure is discussed, and analytical and numerical solutions are given. Using the solutions for period-two coupling, the free energy, entropy, and specific heat are calculated as functions of temperature and applied transverse external magnetic field. Their expressions show that below a particular value v and above a value u of the magnetic field |h|, the entropy and the specific heat vanish exponentially in the low temperature limit. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 2007