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Center for Nonlinear Studies |
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). Quite recently (2012) the same group discovered a low-contrast interference pattern in the intensity of Brillouin light scattering in the real space. The spectrum of spin waves in a thin film of YIG has two minima at finite values of momentum + Q and -Q . The interference demonstrates that the two condensates have very different densities despite complete symmetry of the system to space inversion. In our work 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 pi. Transition between these two states is possible in non-symmetric phase and can be observed as a cusp in the plot of contrast vs. magnetic field.
Host: Nikolai Sinitsyn