The magnetic compound Ba_2CuGe_2O_7 has recently been shown to be an essentially two-dimensional spiral antiferromagnet that exhibits an incommensurate-to-commensurate phase transition when a magnetic field applied along the c-axis exceeds a certain critical value H_c. The T=0 dynamics is described here in terms of a continuum field theory in the form of a nonlinear sigma model. We are thus in a position to carry out a complete calculation of the low-energy magnon spectrum for any strength of the applied field throughout the phase transition. In particular, our spin-wave analysis reveals field-induced instabilities at two distinct critical fields H_1 and H_2 such that H_1 < H_c < H_2. Hence we predict the existence of an intermediate phase whose detailed nature is also studied to some extent in the present paper.
@article{arxiv.cond-mat/0103217,
title = {Intermediate phase in the spiral antiferromagnet Ba_2CuGe_2O_7},
author = {J. Chovan and N. Papanicolaou and S. Komineas},
journal= {arXiv preprint arXiv:cond-mat/0103217},
year = {2009}
}