  In conventional quasi-one-dimensional antiferromagnets with quantum spins,
magnetic excitations are carried by either magnons or spinons in different
energy regimes: they do not coexist independently, nor could they interact with
each other. In this Letter, by combining inelastic neutron scattering, quantum
Monte Carlo simulations and Random Phase Approximation calculations, we report
the discovery and discuss the physics of the coexistence of magnons and spinons
and their interactions in Botallackite-Cu2(OH)3Br. This is a unique quantum
antiferromagnet consisting of alternating ferromagnetic and antiferromagnetic
Spin-1/2 chains with weak inter-chain couplings. Our study presents a new
paradigm where one can study the interaction between two different types of
magnetic quasiparticles, magnons and spinons.
