  The notion of a quasiparticle, such as a phonon, a roton, or a magnon, is
used in modern condensed matter physics to describe an elementary collective
excitation. The intrinsic zero-temperature magnon damping in quantum spin
systems can be driven by the interaction of the one-magnon states and
multi-magnon continuum. However, detailed experimental studies on this quantum
many-body effect induced by an applied magnetic field are rare. Here we present
a high-resolution neutron scattering study in high fields on an S=1/2
antiferromagnet C9H18N2CuBr4. Compared with the non-interacting linear
spin-wave theory, our results demonstrate a variety of phenomena including
field-induced renormalization of one-magnon dispersion, spontaneous magnon
decay observed via intrinsic linewidth broadening, unusual non-Lorentzian
two-peak structure in the excitation spectra, and a dramatic shift of spectral
weight from one-magnon state to the two-magnon continuum.
