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В настоящее время основные исследования в Институте проводятся в области физики
низких температур: сверхтекучесть, сверхпроводимость, квантовые кристаллы, магнетизм, взаимодействие излучения с веществом. В Институте
проходит также обучение студентов базовых кафедр МФТИ и ВШЭ.
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Семинар по магнетизму 26.11.2019 17:00
Пространственно-временная динамика магнонного
Бозе-Эйнштейновского конденсата
С.О. Демокритов, University of Muenster, Germany
In my talk I will address Bose-Einstein condensation of magnons in a quasi-equilibrium gas of magnons parametrically pumped in a high-quality yttrium-iron garnet (YIG) films. The energy exchange of pumped magnons with different subsystems of YIG leads to the formation of the room-temperature magnon Bose-Einstein condensate (mBEC). mBEC is an excellent model system for investigation of macroscopic quantum phenomena at room temperature. On one hand, using locally applied magnetic field, we can disturb and govern the condensate. On the other hand, by means of Brillouin light scattering spectroscopy we are able to record the local density of the condensate with sub-micrometer spatial- and 5-ns temporal resolutions.
In particular, I will present two recently discovered phenomena:
a) Second sound
Second sound waves were coherently excited by a spatially non-uniform radio-frequency magnetic field of the frequency 0.5-10 MHz, created by a long, stripe-shaped conductor. The resulting modulation of the total magnetic field acts as a time-dependent potential, causing an oscillation of the magnon density in the vicinity of the conductor, which, in turn, excites waves of magnon density (magnonic second sound). We have investigated the dependence of the sound velocity on its wavevector and have found a linear dispersion law, with the speed of sound being dependent on the applied magnetic field
b) Lateral transport in inhomogeneous potentials
Since magnons are believed to have attractive interaction with each other, mBEC should be fundamentally unstable with respect to collapse in the real space, which is in an apparent contradiction with experimental findings. We investigate mBEC placed in inhomogeneous potential well provoking such a collapse. We recorded spatio-temporal lateral dynamics of the condensate and compare experimental findings with the developed theoretical model based on Gross-Pitaevskiy equation. We directly demonstrate an attractive interaction of magnons and found that the effective time, describing the relaxation of linear momentum of the condensate is surprisingly long - about 10 microseconds), which is much longer that the magnon lifetime in YIG due to spin-lattice relaxation, (<1 microseconds), which might also indicate the onset of magnon superfluidity.
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