Intermediate phase in the spiral antiferromagnet Ba₂CuGe₂O₇

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 σ 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 H₁ and H₂ such that H₁<H_c<H₂. Hence we predict the existence of an intermediate phase whose detailed nature is also studied to some extent.