  Elementary quasiparticles in solids such as phonons and magnons occasionally
have nontrivial interactions between them, as well as among themselves. As a
result, their energy eigenvalues are renormalized, the quasiparticles
spontaneously decay into a multi-particle continuum state, or they are
hybridized with each other when their energies are close. As discussed in this
review, such anomalous features can appear dominantly in quantum magnets but
are not, a priori, negligible for magnetic systems with larger spin values and
noncollinear magnetic structures. We review the unconventional magnetic
excitations in two-dimensional triangular lattice antiferromagnets and discuss
their implications on related issues.
