The S=1/2 antiferromagnetic Heisenberg chain is a paradigmatic quantum system hosting exotic excitations such as spinons and solitons, and forming random singlet state in the presence of quenched disorder. Realizing and distinguishing these excitations in a single material remains a significant challenge. Using nuclear magnetic resonance (NMR) on a high-quality single crystal of copper benzoate, we identify and characterize all three excitation types by tuning the magnetic field at ultra-low temperatures. At a low field of 0.2 T, a temperature-independent spin-lattice relaxation rate (1/T1) over more than a decade confirms the presence of spinons. Below 0.4 K, an additional relaxation channel emerges, characterized by 1/T1∝T and a spectral weight growing as −ln(T/T0), signaling a random-singlet ground state induced by weak quenched disorder. At fields above 0.5 T, a field-induced spin gap Δ∝H2/3 observed in both 1/T1 and the Knight shift signifies soliton excitations. Our results establish copper benzoate as a unique experimental platform for studying one-dimensional quantum integrability and the interplay of disorder and correlations.
@article{arxiv.2510.11551,
title = {Spinons, solitons and random singlets in the spin-chain compound copper benzoate},
author = {Ying Chen and Guijing Duan and Yuejiu Zhao and Ning Xi and Bingying Pan and Xiaoyu Xu and Zhanlong Wu and Kefan Du and Shuo Li and Ze Hu and Rui Bian and Xiaoqun Wang and Wei Li and Long Zhang and Yi Cui and Shiyan Li and Rong Yu and Weiqiang Yu},
journal= {arXiv preprint arXiv:2510.11551},
year = {2025}
}