Spinons, solitons and random-singlet-like behaviours in the spin-chain compound copper benzoate

The S = 1/2 Heisenberg antiferromagnetic chain is a paradigmatic quantum system hosting exotic excitations. Using nuclear magnetic resonance (NMR) on copper benzoate, we characterise three distinct types of excitations by tuning the magnetic field at ultra-low temperatures. At 0.2 T, a nearly constant spin-lattice relaxation rate (1/T1) and a divergence-like spin-spin relaxation rate (1/T2G) upon cooling provide experimental evidence for spinon excitations. Below 0.4 K, an additional relaxation channel, characterised by 1/T1 ∝ T with spectral weight growing as -ln (T/T₀) and a divergence of the NMR Knight shift (Kn), reveals random-singlet-like behaviours. Above 1 T, a field-induced spin gap Δ ∝ H2/3 signifies the onset of soliton excitations. This work offers NMR identification of spinons, solitons, and the low-energy behaviours of a random-singlet-like state in copper benzoate, and establishes this material as a unique platform for studying the interplay of quantum integrability and disorder. A detailed low-temperature NMR study on copper benzoate reveals spinon excitations, low-temperature random-singlet-like behaviours and field-induced solitons, highlighting the interplay between crystal structure and weak disorder in a spin-chain system.