Numerical method for detecting incommensurate correlations in the Heisenberg zigzag ladder

We study two Heisenberg spin-1/2 chains coupled by a frustrating ''zigzag'' interaction. We are particularly interested in the regime of weak intercha in coupling, which is difficult to analyze by either numerical or analytica l methods. Previous density matrix renormalization-group studies of the iso tropic model with open boundary conditions and sizable interchain coupling have established the presence of incommensurate correlations and of a spect ral gap. By using twisted boundary conditions with an arbitrary twist angle , we are able to determine the incommensurabilities both in the isotropic c ase and in the presence of an exchange anisotropy by means of exact diagona lization of relatively short finite chains of up to 24 sites. Using twisted boundary conditions results in a very smooth dependence of the incommensur abilities on system size, which makes the extrapolation to infinite systems significantly easier than for open or periodic chains.