Friday, July 26, 2013

1307.6782 (P. J. Heikkinen et al.)

Relaxation of Bose-Einstein Condensates of Magnons in Magneto-Textural
Traps in Superfluid 3He-B

P. J. Heikkinen, S. Autti, V. B. Eltsov, J. J. Hosio, M. Krusius, V. V. Zavjalov
In superfluid 3He-B externally pumped quantized spin-wave excitations or magnons spontaneously form a Bose-Einstein condensate in a 3-dimensional trap created with the order-parameter texture and the polarizing field minimum. The condensation is manifested by coherent precession of the magnetization with a common frequency in a large sample volume. The trap shape is controlled by the profile of the applied magnetic field and by the condensate itself via the spin-orbit interaction. The trapping potential can be experimentally determined with the spectroscopy of the magnon levels in the trap. We have measured the decay of the ground-level condensates after switching the pumping off in the temperature range (0.14{\div}0.2)Tc at 0.5 bar pressure. Two contributions to the relaxation are identified: spin-diffusion with the diffusion coefficient proportional to the density of thermal quasiparticles and the approximately temperature-independent radiation damping caused by the losses in the NMR pick-up circuit. The measured dependence of the relaxation on the shape of the trapping potential is in a good agreement with our calculations based on the magnetic field profile and the magnon-modified texture shape. The obtained values of the spin diffusion coefficient at low temperatures agree with the theoretical prediction and earlier measurements at temperatures above 0.5Tc.
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