  The elementary excitations of skyrmion crystals experience both emergent
magnetic fields and anharmonic interactions brought about by the topologically
nontrivial noncollinear texture. The resulting flat bands cause strong
spontaneous quasiparticle decay, dressing the eigenmodes of skyrmion crystals
with a finite zero-temperature quantum lifetime. Sweeping the flat bands
through the spectrum by changing the magnetic field leads to an externally
controllable energy-selective magnon breakdown. In particular, we uncover that
the three fundamental modes, i.e., the anticlockwise, breathing, and clockwise
mode, exhibit distinct decay behavior, with the clockwise (anticlockwise) mode
being the least (most) stable mode out of the three.
