Quantum disordered ground state in the triangular-lattice magnet NaRuO₂

Spin liquids are predicted to emerge in materials that combine strong electronic correlations with geometric frustration. Evidence has now been found for a spin liquid state in the triangular-lattice material NaRuO2. It has long been hoped that spin liquid states might be observed in materials that realize the triangular-lattice Hubbard model. However, weak spin-orbit coupling and other small perturbations often induce conventional spin freezing or magnetic ordering. Sufficiently strong spin-orbit coupling, however, can renormalize the electronic wavefunction and induce anisotropic exchange interactions that promote magnetic frustration. Here we show that the cooperative interplay of spin-orbit coupling and correlation effects in the triangular-lattice magnet NaRuO2 produces an inherently fluctuating magnetic ground state. Despite the presence of a charge gap, we find that low-temperature spin excitations generate a metal-like term in the specific heat and a continuum of excitations in neutron scattering, reminiscent of spin liquid states previously found in triangular-lattice organic magnets. Further cooling produces a crossover into a different, highly disordered spin state whose dynamic spin autocorrelation function reflects persistent fluctuations. These findings establish NaRuO2 as a cousin to organic, Heisenberg spin liquid compounds with a low-temperature crossover in quantum disorder.