Propagating spinons and magnons in coupled quantum spin chains*

In conventional magnets with magnetic long range order (LRO), low-energy excitations are carried by spin waves, represented by massless bosons called magnons with S = 1. However, in one-dimensional (1D) antiferromagnetic quantum spin systems, quantum fluctuations destroy LRO. Their low-energy excitations are spinons, a fractionalized fermion with S = 1/2, instead of magnons. In quasi-1D 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 talk, I will present our recent neutron scattering and theoretical studies of a unique quasi-1D quantum spin system, Cu₂(OH)₃Br, which consists of weakly-coupled, ferromagnetic and antiferromagnetic alternating chains [1]. As a result, this system shows coexistence of two different magnetic quasiparticles: the ferromagnetic chains give rise to conventional magnon excitation, while the antiferromagnetic chains yield spinons. Furthermore, these magnetic quasiparticles interact via weak interchain interactions, leading to gap opening of magnetic excitations and asymmetric spectral weight.
This work was done in collaboration with H. Zhang, Z. Zhao, D. Gautreau, M. Raczkowski, A. Saha, V.O. Garlea, H. Cao, T. Hong, H. O. Jeschke, Subhendra D. Mahanti, T. Birol, and F. F. Assaad

[1] H. Zhang et al, Phys. Rev. Lett. 125, 037204 (2020).