IQSE AMO-CM LUNCH SEMINAR SERIES
"Bose-Einstein condensation and superfluidity of magnons in ferromagnetic films under pumping"
Dr. Valery Pokrovsky
Department of Physics,Texas A&M University
We present a brief review of the theory and experiment of quasi-equilibrium Bose-Einstein condensation
and superfluidity of magnons in a film of Yttrium Iron Garnet (YIG). The Bose-Einstein condensation of
magnons in YIG film at room temperature under the RF pumping was discovered in 2006 by the Münster
experimental team led by S. Demokritov. We describe their experiments [1,2] and explanation of their results
by theory [3,5].
There are two equal minima of energy in the magnon spectrum of YIG film at non-zero momenta and
therefore two condensates. The same experimental group has discovered the interference of the two
condensates  thereby proving their coherence.
Theory  attracted attention to the strong degeneration of the ground state of non-interacting magnons.
with respect to distribution of them between two minima. This degeneration is lifted by the magnon.
interaction. Theory predicts that interaction leads to spontaneous violation of the reflection symmetry and
non-equal number of magnons in two condensates for thick films. In thin films the condensate is symmetric
at low magnetic field and transits to to non-symmetric state at higher field. Dipolar interaction depends on
the phase of the condensate wave function. In quasi-equilibrium it traps the phase.
Two obstacles for the magnon superfluidity are the dominance of the normal magnon density over the
condensate approximately 100 times and the phase trapping. We show that the velocity of the superfluid part
of magnon gas is by 5-7 decimal orders larger than the velocity of normal part at typical values of magnetic field and its gradient. Thus, the spin current is mainly superfluid. The phase trapping is a consequence of
non-conservation of the spin angular moment that turns into the orbital moment by dipolar interaction.
Therefore, the number of magnons is not conserved locally, stationary spin current becomes inhomogeneous.
But it conserves globally due to the remaining discrete symmetry .
We propose to observe magnon superfluidity creating a soliton bouncing between two reflecting
boundaries. We discuss theory of superfluid solitons and prospects to reproduce all these phenomena in
nano-scale samples .
 S.O. Demokritov et al., Nature 443, 406 (2006);  P. Nowik-Boltyk et al., Sci. Rep. 2, 482 (2012);  F. Li et al., Sci. Rep. 3, 1372 (2013);  C. Sun et al., Phys. Rev. Lett. 116, 257205 (2016);  C. Sun et al., J. Phys. D, (2017).
Tuesday, March 7, 2017
IQSE Seminar Room (578 MPHY), 12:00 Noon
Mitchell Physics Building
Institute for Quantum Science and Engineering
Texas A&M University
(Sandwiches, salad, and soda to be served at 11:15 a.m.)
THE INSTITUTE FOR QUANTUM
SCIENCE AND ENGINEERING