Optical detection of bond-dependent and frustrated spin in the two-dimensional cobalt-based honeycomb antiferromagnet Cu3Co2SbO6

Two-dimensional honeycomb antiferromagnets are promising materials class for realizing Kitaev quantum spin liquids. The signature of these materials includes anisotropic bond-dependent magnetic responses and persistent fluctuations in paramagnetic regime. Here, we propose Cu3Co2SbO6 heterostructures as an intriguing candidate, wherein bond-dependent and frustrated spins interact with optical excitons. First-principles spin Hamiltonian calculations and in-plane anisotropic critical fields suggest strong frustration and dominant Kitaev exchange interactions. Optical spectroscopy reveals exciton coupled to frustrated magnetism, enabling optical detection of spin states. Spin-exciton coupling displays anisotropic responses to light polarization along the bond-parallel and the bond-perpendicular directions, highlighting Kitaev interactions and persistent short-range spin correlations above twice the Néel temperatures. The robustness of short-range spin fluctuations under magnetic fields underscores the stability of the spin-fluctuation region. Our results establish Cu3Co2SbO6 as an attractive candidate for exploring quantum spin liquid, where the spin Hamiltonian and quasiparticle excitations can be probed and potentially controlled by light. Recent work proposed to search for quantum spin liquid in honeycomb cobaltates. Here the authors identify Cu3Co2SbO6 as a promising candidate for quantum spin liquid research, revealing bond-dependent frustrated magnetism and robust spin fluctuations in the unconventional paramagnetic region via optical spectroscopy.