Quantum spin chains in a magnetic field

We demonstrate that the ''worm'' algorithm allows very effective and precis e 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 l ow temperature. Magnetization curves for the s=1/2 and s=1 chains are prese nted and compared with existing Bethe ansatz and exact diagonalization resu lts. 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].