QUANTUM EFFECTS IN THE SPIN DYNAMICS OF THE LINEAR HEISENBERG ANTI-FERROMAGNET

We present an approximate analytic expression for the dynamical spin correlation function of the S=1/2 linear Heisenberg antiferromagnet at T=0. The basis for our approach is that in zero field the spectrum is dominated by a double continuum [in (q,ω) -space] of triplet spin wave excitations. The S=1/2 integrated intensity agrees very well with recent neutron scattering results on CPC, unlike the corresponding classical intensity. Moreover, the S=1/2 spectral weight function shows increasing asymmetry as q→π, a quantum effect, observable in more recent neutron scattering data. In non-zero magnetic field, there exist two, partly overlapping, double continua, each giving rise to a peak situated at the lower boundary. The (zz component of) spectral weight function therefore has a double-peaked structure, as observed experimentally. Theory and experiment are in apparent agreement concerning the energy difference between the peaks.