THE EXACT ENERGY-SPECTRUM OF THE HEISENBERG HAMILTONIAN WITH 2ND NEIGHBORS FOR A LINEAR CLOSED RING OF 16 SPINS

The exact eigenstates and eigenvalues of a linear ring of 16 spins des cribed by a Heisenberg Hamiltonian with first- and second-neighbor int eractions have been calculated. The method used is based on a hierarch y of algebras. In the present paper the eigenvalues of the ground stat e and the density of states for various values of the relative strengt h of the second-neighbor interaction are presented. Some interesting f eatures for certain values of the parameters appear.