Spontaneous Symmetry Breaking in the Bose-Einstein Condensate of Spin Waves
In 2006 the phenomenon of Bose-Einstein condensation at room temperature was discovered in a ferromagnet YIG (Yttrium-Iron garnet) by the group of experimenters from University of Münster (Germany). They also presented a theoretical explanation of their experiment. In my talk I will describe this experiment and its explanation. Quite recently (June 2012) the same group discovered a low-contrast interference pattern in the intensity of Brillouin light scattering. The spectrum of spin waves in a thin film of YIG has two minima at finite values of momentum + .
The interference demonstrates that the two condensates have very different densities despite complete symmetry of the system to space inversion. In our work (Fuxiang Li, W.M. Saslow and V.L. P.) we propose a theory explaining why and how this symmetry is violated. Our theory predicts that in thinner films it is possible to observe a symmetric state of condensates. The transition between these two states can be driven by magnetic field. The theory also predicts a new type of the collective oscillations. The dipolar interaction plays a special role trapping the sum of the two condensate phases at the values 0 or π. Transition between these two states is possible in nonsymmetric phase and can be observed as a cusp in the plot of contrast vs. magnetic field.