Fig. 2
From: Bogoliubov waves and distant transport of magnon condensate at room temperature
Magnon spectrum. a Magnon spectrum of a 5.6-μm-thick YIG film magnetized in plane by a bias magnetic field H = 1690 Oe is presented for the wavevectors q perpendicular (±q⊥H) and parallel (±q||H) to the applied field. For both wavevector orientations the first 35 thickness modes are shown. The red arrows illustrate the process of injection of magnon pairs at ωp/2 frequency by means of parallel parametric pumping. Thermalization of the parametrically overpopulated magnon gas leads to the formation of Bose–Einstein condensates in two symmetric minima of the frequency spectrum ω(q) at q = ±qmin (with qmin||H). Inset b shows the 3-dimensional view of a bottom part of the spectrum (the lowest fundamental mode n = 0 within the shaded violet area in a) calculated in the range up to 200 MHz above the ωmin. The red curve shows the spectrum of magnons with ±q⊥H and the blue curve relates to the magnons with ±q||H. Due to the much higher (about 21 times) steepness of the red curve near (ωmin, qmin), and, thus, proportionally smaller effective masses of the corresponding magnons, the magnon supercurrents predominantly propagate in a film plane across H and along the ±qx directions
