Decay of quasiparticles in quantum spin liquids

Magnetic excitations are studied in gapped quantum spin systems, for which spontaneous two-magnon decays are allowed by symmetry. Interaction between one- and two-particle states acquires nonanalytic frequency and momentum dependence near the boundary of two-magnon continuum. This leads to a termination point of single-particle branch for one-dimensional systems and to strong suppression of quasiparticle weight in two dimensions. The momentum dependence of the decay rate is calculated in arbitrary dimensions and effect of external magnetic field is discussed.