We report on a novel gapless quantum spin liquid in the $S=1$ honeycomb system ${\mathrm{Cu}}_3{\mathrm{LiRu}}_2{\mathrm{O}}_6$ with ${\mathrm{Ru}}^{4+}$ ($4d^4$) where moments remain dynamic down to 50 mK. Heat capacity measurements show no sign of magnetic ordering down to 60 mK inspite of a Curie-Weiss temperature, ${\theta }_{\mathrm{CW}}=-222$ K, indicating a strong antiferromagnetic interaction. In zero field, the magnetic heat capacity $C_m$ shows a linear $T$ dependence with the Sommerfeld coefficient $\gamma =107$ mJ/mol ${\mathrm{K}}^2$, which is much larger than that found in typical Fermi liquids. Our local probe ${}^7\mathrm{Li}$ nuclear magnetic resonance (NMR) measurements find a significant temperature-independent ${}^7\mathrm{Li}$ NMR shift (and hence a nonzero spin susceptibility) at low temperature ($T$) and a linear $T$ variation of the ${}^7\mathrm{Li}$ NMR spin-lattice relaxation rate 1/$T_1$ at low $T$ reminiscent of fermionic excitations. Muon spin relaxation ($\mu \mathrm{SR}$) measurements detect neither long-range ordering nor spin freezing down to 50 mK, and the temperature variation of the muon depolarization rate $\lambda$ shows a gradual increase with decreasing temperature and a leveling off below about 1 K, evincing a persistent spin dynamics common to several spin liquid candidates. Our results provide strong signatures of a quantum spin liquid in the titled honeycomb material.

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• Quantum spin liquid
• Specific heat
• Muon spin relaxation & rotation
• Nuclear magnetic resonance