CORRELATION-FUNCTIONS IN THE XXO HEISENBERG CHAIN AND THEIR RELATIONSWITH SPECTRAL SHAPES

The spectral shape of one-dimensional systems (describing for instance the behaviour of Frenkel excitons) is approached through the exactly solvable model of the XXO Heisenberg quantum spin chain in a transvers e magnetic field. Some results for finite size chains concerning 2N-po int correlators are presented in detail. In particular, the finite lat tice, finite temperature 2-point correlators are explicitly worked our . Moreover, results in closed form are given for 2N-point correlators in the most general situation (finite lattice and thermodynamic limit, finite temperature, finite space and/or time separations). Their rela tions with frequency moments of the spectral shape are pointed out and the connection with moment expansion through continued fraction repre sentation is given.