Field induced spontaneous quasiparticle decay and renormalization of   quasiparticle dispersion in a quantum antiferromagnet
  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. 
Tao Hong, Y. Qiu, M. Matsumoto, D. A. Tennant, K. Coester, K. P.   Schmidt, F. F. Awwadi, M. M. Turnbull, H. Agrawal, and A....
cond-mat.mtrl-sci cond-mat.str-el