Gapless quantum spin liquid in the 𝑆=1 4𝑑4 honeycomb material Cu3⁢LiRu2⁢O6

Abstract

We report on a novel gapless quantum spin liquid in the 𝑆=1 honeycomb system Cu3⁢LiRu2⁢O6 with Ru4+ (4⁢𝑑4) 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, 𝜃CW=−222 K, indicating a strong antiferromagnetic interaction. In zero field, the magnetic heat capacity 𝐶𝑚 shows a linear 𝑇 dependence with the Sommerfeld coefficient 𝛾=107 mJ/mol K2, which is much larger than that found in typical Fermi liquids. Our local probe 7 Li nuclear magnetic resonance (NMR) measurements find a significant temperature-independent 7 Li NMR shift (and hence a nonzero spin susceptibility) at low temperature (𝑇) and a linear 𝑇 variation of the 7 Li NMR spin-lattice relaxation rate 1/𝑇1 at low 𝑇 reminiscent of fermionic excitations. Muon spin relaxation (𝜇⁢SR) measurements detect neither long-range ordering nor spin freezing down to 50 mK, and the temperature variation of the muon depolarization rate 𝜆 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.