Bose-Einstein condensation of magnons in magnetic films
Final Report Abstract
The proposed studies have been successfully accomplished, the observation of the Josephson-like effect is still to be done. Instead, rather unexpected effects such as extremely stable coupling between two components of the condensate persisting up to several days, formation of quantized vortices and unusual properties of the sound-like density waves in the condensate have been discovered. Particular achievements of the second funding period can be listed as follows: a setup for Brillouin light scattering experiments with simultaneous space-, time-, frequency-, and wavevector-resolution is developed; using this technique spatio-temporal investigation of the process of Bose-Einstein condensation of magnons driven by a microwave pumping field of different spatial configurations has been performed. Among others two spatially separated condensates of magnons with controllable sizes and spatial separation have been formed; semi-phenomenological one-dimensional theory describing the spatio-temporal dynamics of the condensate is built. Two coupled equations combining the essential features of the Gross-Pitaevskii and the complex-Ginzburg-Landau models include localized source terms, which represent the microwave field pumping magnons into the condensate; theory correctly predicts the spatio-temporal evolution of the magnon condensate. In agreement with experimental findings the theory predicts that the spatial size of the condensate increases with the growth of the pumping power, while temporal development shows a saturation; investigating the magnon condensate with sub-micrometer spatial resolution reveals a nonuniform ground state of the condensate with a real-space standing wave of the total condensate density. The spatial scale and direction of the oscillation are determined by the non-zero wave vectors kBEC corresponding to two energetically degenerate lowest-energy quantum states of the magnon gas. Thus, the observed density oscillation originate from the interference of the two condensate components corresponding to ±kBEC; by following the oscillation over a long scale a coherence length of the condensate is determined directly, and the obtained value of 30 μm is in agreement with previous data based on experiments with wavevector resolution; the observed oscillation patterns are found to be very stable in time: they have been observed over several days while the microwave pumping continuously kept the overall condensate density constant - this finding documents a high degree of coherence in the condensate; additionally, stable topological defects identified as stable quantized vortices pinned on the lattice defects were observed; applying a non-uniform, localized in real space, pulsed magnetic field with the duration of 100-300 ns we have observed quasi-adiabatical changes of the condensate density. The motion of the condensate takes place due to gradients of chemical potential determined by the non-uniform magnetic field; applying a non-uniform, localized in real space magnetic field oscillating in time with a frequency of 1-20 MHz we have observed density waves propagating in the condensate. These waves correspond to Bogoljubov excitations in BEC systems. Reflecting the magnetic nature of the condensate, the speed of sound of the observed waves demonstrates a strong field dependence.
Publications
- "Excitation of two spatially separated Bose-Einstein condensates of magnons", Phys. Rev. B 80 (2009) 060401(R)
O. Dzyapko, V.E. Demidov, M. Buchmeier, T. Stockhoff, G. Schmitz, G.A. Melkov, and S.O. Demokritov
- “Quantum coherence due to Bose-Einstein condensation of parametrically driven magnons” Proceedings of the International school of physics Enrico Fermi “Quantum coherence in solid state systems”, Varenna, IOS Press 2009, p. 327
S.O. Demokritov
- ”Ginzburg-Landau model of Bose-Einstein condensation of magnons”, Phys. Rev. B 81 (2010) 024418
B. A. Malomed, O. Dzyapko, V. E. Demidov,, and S.O. Demokritov
- „Kinetics and Bose-Einstein condensation of parametrically driven magnons at room temperature “ Physics-Uspekhi, 53 (2010) 853
O. Dzyapko, V.E. Demidov, and S.O. Demokritov
- “Bose-Einstein condensation of spin wave quanta at room temperature” Phil. Trans. R. Soc. A, 369 (2011) 3575
O. Dzyapko, V. E. Demidov, G. A. Melkov, and S. O. Demokritov
- “Spatially non-uniform ground state and quantized vortices in a two-component Bose-Einstein condensate of magnons” Sci. Rep.(Nature) 2 (2012) 482
P. Nowik-Boltyk, O. Dzyapko, V. E. Demidov, N. G. Berloff, and S. O. Demokritov
(See online at https://doi.org/10.1038/srep00482)
