Study of Bose-Einstein condensate of magnons by means of fs-laser Faraday magnetometry
Final Report Abstract
The main aim of this project is to study dynamic properties of magnon Bose-Einstein condensate (BEC) such as its frequency and the spectral linewidth. To accomplish this very challenging task, we have developed a novel experimental technique – heterodyne magneto-optical Kerr-effect fsspectroscopy. Thanks to the ultimate frequency resolution of the technique of better than 100 Hz, we were able to directly determine the spectral linewidth of BEC and its dependence on the condensate density. The obtained results clearly demonstrate that the studied system fulfils the stability criteria known for atomic BEC. On the other hand, the measured linewidth is larger than that obtained from the experimentally determined lifetime of a condensate using a standard equations for real particles. This result indicates that the connection between the linewidth and the lifetime of a quasi-equilibrium BEC is not the same as for a single particle. Within the project, we also studied the density waves in BEC. We have shown that sound-like waves of density with a gapless linear dispersion exist in BEC. Although these waves are reminiscent of the second sound in superfluid He, they are strongly dissipative, which is connected with the fact that condensed magnons correspond to a small fraction of the total number of magnons existing in our system. The obtained experimental data stimulated a development of theoretical models describing different properties of magnon BEC. This is an extremely challenging task, since one has to take into account the quasi-equilibrium nature of the condensate. Last but not least, the project brought together two groups (Münster and Nijmegen) which belong to the world leaders in the field of magnetic dynamics. Exchange of know-how between the two groups will definitely result in further collaboration and new discoveries.
Publications
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“High-resolution magneto-optical Kerr effect spectroscopy of magnon Bose-Einstein condensate” IEEE Magnetics Letters, V. 7 (2016)
O. Dzyapko, P. Nowik-Boltyk, B. Koene, V.E. Demidov, J. Jersch, A. Kirilyuk, Th. Rasing, and S. O. Demokritov
