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Sr₂CuO₃ is a model strongly correlated cuprate chain antiferromagnet with nearly ideal one-dimensional (1D) Heisenberg Luttinger-spin-liquid behavior [1]. While fractional spinon excitations in this material have bandwidth of up to 0.7 eV [2], it only orders three-dimensionally (3D) below T_N ≈ 5.5K, indicating extremely weak interchain coupling, J'/J < 5·10^-4. In the weakly 3D ordered antiferromagnetic phase, at T N, recent ESR experiments [3] have found an unusual spin excitation mode, which was conjectured to be a hybrid Higgs-Goldstone mode because it develops together with the field-dependent gapped transverse pseudo-Goldstone magnons, intrinsic to a collinear antiferromagnet with weak two-axial anisotropy. Additionally, ultrasound measurements indicated field-induced phase transitions to novel phases whose origin is unclear and which are unexpected in a system of coupled Luttinger liquids [4]. Here, we report neutron scattering measurements of the low-energy magnetic excitations in Sr₂CuO₃ covering the extent of the intra-chain dispersion in magnetic field up to 14T aiming to verify the ESR results. While magnetic field dependence of spin gaps observed in our measurements is consistent with the behavior expected for pseudo Goldstone magnons, the excitation spectrum seems to have substantial energy width which appears to depend on magnetic field. The observed behavior suggests that freely propagating unbound spinons might be present at low energies in the ordered phase, which would disagree with a standard chain mean field theory. 

References:

1. I. A. Zaliznyak, Nat. Mater. 4, 273 (2005).

2. A. C. Walters, T. G. Perring, J.-S. Caux, A. T. Savici, G. D. Gu, C.-C. Lee, W. Ku, and I. A. Zaliznyak, Nat. Phys. 5, 867 (2009).

3. E. G. Sergeicheva, S. S. Sosin, L. A. Prozorova, G. D. Gu, and I. A. Zaliznyak. Phys. Rev. B 95, 020411(R) (2017)

4. E. G. Sergeicheva, S. S. Sosin, D. I. Gorbunov, S. Zherlitsyn, G. D. Gu, and I. A. Zaliznyak. Phys. Rev. B 101, 201107(R) (2020)