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Enhancement of magnon flux toward a Bose-Einstein condensate

Franziska Kühn1, Matthias R. Schweizer1, Tamara Azevedo1, Vitaliy I. Vasyuchka1, Georg von Freymann1,2, Victor S. L'vov3,*, Burkard Hillebrands1, and Alexander A. Serga1,†

  • *Contact author: victor.lvov@gmail.com
  • †Contact author: serha@rptu.de
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Phys. Rev. B 113, 014409 – Published 8 January, 2026

DOI: https://doi.org/10.1103/nxsh-tfy8

Abstract

We present a combined theoretical and experimental study of angle-dependent parametric pumping of magnons in yttrium iron garnet films, with a focus on the mechanisms that transfer parametrically injected magnons toward the spectral minimum where Bose-Einstein condensation occurs. Using a classical Hamiltonian formalism, we analyze the threshold conditions for parametric instability as a function of the angle αp between the microwave pumping field hp(t) and the external magnetic field Hext, continuously tracing the transition between parallel (for αp=0) and transverse pumping (for αp=π/2). We also describe two competing four-magnon scattering mechanisms that transfer parametric magnons toward the bottom of their frequency spectrum: The step-by-step Kolmogorov-Zakharov cascade, which is allowed for all values of Hext, and the kinetic instability mechanisms that provide a much more efficient single-step channel in transferring magnons directly to the lowest-energy states but occurs within specific regions of αp and Hext where the conservation laws permit it. In the experimental part, we employ microfocused Brillouin light scattering spectroscopy in combination with a vector magnet, allowing for angle-resolved mapping of the magnon population spectrum under controlled pumping angle αp. We observe that transverse pumping, although characterized by a higher instability threshold, yields a markedly stronger population at the spectral minimum compared to parallel pumping. These observations demonstrate that the kinetic instability channel plays a dominant role in transferring magnons to the spectral minimum under such conditions. These results reveal the crucial role of pumping geometry in shaping the magnon distribution and provide guidelines for optimizing the flux of magnons into the condensate, thereby advancing the control of magnon Bose-Einstein condensation in magnetic insulators.

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Phys. Rev. B 113, 014409– Published 8 January, 2026
  • Received 11 November 2025
  • Revised 12 December 2025
  • Accepted 17 December 2025
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