Dimer and Néel order-parameter fluctuations in the spin-fluid phase of the s=1/2 spin chain with first- and second-neighbor couplings.

The dynamical properties at {ital T}=0 of the one-dimensional (1D) {ital s}=1/2 nearest-neighbor (NN) {ital XXZ} model with an additional isotropic next-nearest-neighbor (NNN) coupling are investigated by means of the recursion method in combination with a weak-coupling continued-fraction analysis. The focus is on the dynamic structure factors {ital S}{sub {ital zz}}({ital q},{omega}) and {ital S}{sub {ital DD}}({ital q},{omega}), which describe (for {ital q}={pi}) the fluctuations of the N{acute e}el and dimer order parameters, respectively. We calculate the dependence on the exchange constants of the infrared exponent, the renormalized bandwidth of spinon excitations, and the spectral-weight distribution in {ital S}{sub {ital zz}}({pi},{omega}) and {ital S}{sub {ital DD}}({pi},{omega}), all in the spin-fluid phase, which is realized for planar NN anisotropy and sufficiently weak NNN coupling. For some parameter values we find a discrete branch of excitations above the spinon continuum. They contribute to {ital S}{sub {ital zz}}({ital q},{omega}) but not to {ital S}{sub {ital DD}}({ital q},{omega}). {copyright} {ital 1996 The American Physical Society.}