Spinon-magnon interaction in an antiferromagnet with alternating antiferromagnetic and ferromagnetic quantum spin chains

The concepts of quasiparticles and collective modes have been successfully applied to describe low-energy excitations in physics. In conventional magnets low-energy excitations are carried by spin waves, represented by massless bosons called magnons with S = 1. However, in one-dimensional antiferromagnetic quantum spin (S = 1/2) systems, quantum fluctuations destroy LRO in the ground state and magnons do not exist. Instead, the low-energy excitations are known as spinons, with S = 1/2 . Although both magnons and two-spinon continuum have been observed in several quasi-one-dimensional antiferromagnets, magnons and spinons do not coexist in the same energy range. Here, we report the observation of coexistence of magnon and spinon excitations in Cu₂(OH)₃Br, a quantum antiferromagnet consisting of nearly decoupled, alternating ferromagnetic and antiferromagnetic S=1/2 chains. Importantly, the excitation spectra of both the magnetic chains emerge in close energy range and cross over each other, enabling strong spinon-magnon interaction.