Quantum Disordered Ground State in the Heisenberg-Kitaev Candidate NaRuO2

Abstract The realization of spin liquid states born from the near-critical regime of the triangular lattice Hubbard model in inorganic materials remains a long-standing challenge, where weak spin-orbit coupling and other small perturbations often induce conventional spin freezing or order. Strong enough spin-orbit coupling, however, can renormalize the electronic wave function and induced anisotropic exchange interactions that promote magnetic frustration. Through the cooperative interplay of spin-orbit coupling and correlation effects, here we show that the triangular lattice magnet NaRuO2 hosts an inherently fluctuating magnetic ground state with thermodynamic properties suggestive of a crossover between dynamic ground states. Despite the presence of a charge gap, we find that low-temperature spin excitations generate a metal-like term in the specific heat and continuum excitations in neutron scattering, reminiscent of spin liquid states found in triangular lattice organic magnets. Further cooling reveals that these fluctuations crossover into a state whose dynamic spin autocorrelation function reflects persistent fluctuations within a highly disordered spin state. These findings instantiate NaRuO2 as a unique, Heisenberg-Kitaev cousin to organic, Heisenberg spin liquid compounds with a low-temperature crossover in quantum disorder driven via the interplay between geometric frustration, extended hopping, and relativistic spin-orbit coupling.