Quantum spin chains in a magnetic field

We demonstrate that the ''worm'' algorithm allows very effective and precise quantum Monte Carlo (QMC) simulations of spin systems in a magnetic field, and its autocorrelation time is rather insensitive to the value of H at low temperature. Magnetization curves for the s=1/2 and s=1 chains are presented and compared with existing Bethe ansatz and exact diagonalization results. From the Green-function analysis we deduce the magnon spectra in the s=1 system, and directly establish the "relativistic'' form E(p)=(Delta(2) + upsilon(2)p(2))(1/2) of the dispersion law. [S0163-1829(98)05845-7].