Spin dynamics of Heisenberg spin-1/2 chain in incommensurate field

Based on Bethe ansatz, we develop a non-perturbative analytical method to study the Heisenberg spin-1/2 chain in incommensurate (IC) field. The obtained dynamical spectra exhibit a series of zero-energy peaks which can be attributed to the scattering between Bethe states connected by the IC field. With IC field tuning, a gap gradually develops in the dynamical spectrum of Bethe strings and psinon-antipsinon pairs. Our results reveal a novel behavior of those quasiparticles under the IC field tuning, and the non-perturbative approach also allows us to analytically investigate physics beyond integrability.