Reply to comment on “Bose–Einstein condensation and spin superfluidity of magnons in a perpendicularly magnetized yttrium iron garnet film” (JETP Letters 112, 299 (2020))

The unity of the laws of physics makes it possible to apply a unified approach to various physical systems. Bose–Einstein condensation of magnons (mBEC) and related magnon superfluidity were discovered almost four decades ago in antiferromagnetic superfluid $^3$He. Bose–Einstein condensation was then detected in a gas of ultracold atoms, a gas of exciton-polaritons, etc. A magnon BEC was then detected in antiferromagnets with coupled nuclear–electron precession and in yttrium iron garnet (YIG) films. Moreover, the properties of the magnon BEC in films are fundamentally different when the magnetic field is oriented along and across the film plane. In this short article, the main properties of mBEC and, in particular, the difference between the “classical” mBEC for magnons with zero momentum and “exotic” mBEC for magnons with nonzero momentum are highlighted.