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The dynamical properties at T = 0 of the one-dimensional (1D) s = 1/2 nearest-neighbor (NN) 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 Szz(q,v) and SDD(q,v), which describe (for q = π) the fluctuations of the Néel 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 Szz(p,v) and SDD(π,v), 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 Szz(q,v) but not to SDD(q,v).

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Copyright 1996 The American Physical Society.