External stimuli offer a route to realize exotic states with nontrivial excitations in highly frustrated kagome antiferromagnets. Here, 1.5% ${\mathrm{Gd}}^{3+}$ ions are substituted for ${\mathrm{Y}}^{3+}$ ions in ${\mathrm{YCu}}_3$(OD)${}_{6.5}{\mathrm{Br}}_{2.5}$ (abbreviated as ${\mathrm{YCu}}_3$), yielding ${\mathrm{Gd}}_{0.015}{\mathrm{Y}}_{0.985}{\mathrm{Cu}}_3$(OD)${}_{6.5}{\mathrm{Br}}_{2.5}$, where the ${\mathrm{Gd}}^{3+}$ ions act as a local probe to investigate fractionalized excitations in the $S=1/2$ kagome Heisenberg antiferromagnet ${\mathrm{YCu}}_3$. The Gd substitution level is confirmed through detailed x-ray diffraction analyses, while magnetization and magnetic-susceptibility measurements disclose the interaction between the substituted Gd ions and the Cu-spin kagome network. Specific-heat measurements in magnetic fields up to 14 T and down to dilution-refrigerator temperatures reveal an enhanced density of low-energy states, indicative of spinon-impurity coupling, yet higher-energy excitations are hardly affected. Furthermore, high-frequency electron spin resonance (ESR) measurements, enabled by the presence of Gd as a local probe, present the resonance associated with Gd ions coupled with the kagome spins. In particular, the temperature dependence of the ESR linewidth exhibits a near-quadratic power-law behavior, revealing the presence of Dirac spinon dynamics in the finite-temperature regime above 2 K.

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• Frustrated magnetism
• Magnetism
• Quantum spin liquid
• Specific heat
• Spin dynamics
• Kagome lattice
• Electron spin resonance