From Ground States to Excitations

Abstract

The focus of previous chapters has been primarily on the ground states of the Heisenberg model. Knowledge of the exact ground state—or in lack of one, of a good variational state—enables us to calculate the equal time spin correlations at T = 0. Experiments, however, can measure dynamical responses at finite frequencies and at finite temperatures. It is clear from linear response theory (see Appendix B) that dynamical correlations depend on the excited states and energies.