Simultaneous Excitation of a Magnon plus a Phonon in CoBr2·2H2O: A New Type of Bound State?

The first observation of the excitation of a magnon plus a phonon is reported. The agreement between far-infrared measurements on Co${\mathrm{Br}}_2$·2${\mathrm{H}}_2$O and a simple theory is excellent. In addition, the transition probability is shown to be a consequence of a simple magnon-phonon interaction. We also discuss the possibility and evidence that this excitation corresponds to a bound state of a magnon plus a phonon.