We present a comprehensive low-temperature ${}^1\mathrm{H}$ nuclear magnetic resonance (NMR) study of magnetic field–induced phases and spin fluctuations in an isotropic Haldane chain system $m\text{−}{\mathrm{NO}}_2\mathrm{PhBNO}$ (BoNO) in fields up to 34 T. NMR spectral and $T_1^{-1}$ relaxation rate data are used to map out the transition temperature $T_c(B)$ and we show that it defines a complete phase boundary between the field-induced Bose-Einstein condensate (BEC) and Tomonaga-Luttinger liquid (TLL) phases by reproducing its shape via quantum Monte Carlo calculations. We also observe the theoretical critical behavior $T_c(B)\propto (B-B_{\mathrm{c}2}{)}^{\nu }$ with $\nu =2/3$ and universal quasiparticle scaling in the low-temperature limit close to $B_{\mathrm{c}2}$. The extensive Bayesian inference analysis of the temperature and field dependence of $T_1^{-1}(T,B)$ reveals attractive interactions across the TLL phase of a Haldane chain. Our results are further corroborated by magnetostriction measurements in pulsed magnetic fields and density matrix renormalization group calculations.

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• Critical exponents
• Critical field
• High magnetic fields
• Phase diagrams
• Quantum criticality
• Spin dynamics
• 1-dimensional spin chains
• Antiferromagnets
• Bose-Einstein condensates
• Density matrix renormalization group
• Nuclear magnetic resonance
• Quantum Monte Carlo