Intermediate phase in the spiral antiferromagnet Ba2CuGe2O7 -: art. no. 064433

The magnetic compound Ba2CuGe2O7 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.