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Spatially non-uniform ground state and quantized vortices in a two-component Bose-Einstein condensate of magnons
P Nowik-Boltyk1, O Dzyapko, V E Demidov
1Institute for Applied Physics and Center for Nonlinear Science, University of Muenster, Corrensstr. 2-4, 48149 Muenster, Germany.
Abstract:
A gas of magnons in magnetic films differs from all other known systems demonstrating Bose-Einstein condensation (BEC), since it possesses two energetically degenerate lowest-energy quantum states with non-zero wave vectors ±k(BEC). Therefore, BEC in this system results in a spontaneously formed two-component Bose-Einstein condensate described by a linear combination of two spatially non-uniform wave-functions ∝exp(±ik(BEC)z), while condensates found in other physical systems are characterized by spatially uniform wave-functions. Here we report a study of BEC of magnons with sub-micrometer spatial resolution. We experimentally confirm the existence of the two wave-functions and show that their interference results in a non-uniform ground state of the condensate with the density oscillating in space. Additionally, we observe stable topological defects in the condensate. By comparing the experimental results with predictions of a theoretical model based on the Ginzburg-Landau equation, we identify these defects as quantized vortices.
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