Intermediate phase in the spiral antiferromagnet
Ba2CuGe2O7
J. Chovan and N. Papanicolaou
Department of Physics, University of Crete, and
Research Center of Crete, Heraklion, Greece
S. Komineas
Physikalisches Institut, Universitaet Bayreuth D-95440 Bayreuth, Germany
Abstract
The magnetic compound Ba$_{2}$CuGe$_{2}$O$_{7}$ 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