  Kitaev quantum spin liquid, massively quantum entangled states, is so scarce
in nature that searching for new candidate systems remains a great challenge.
Honeycomb heterostructure could be a promising route to realize and utilize
such an exotic quantum phase by providing additional controllability of
Hamiltonian and device compatibility, respectively. Here, we provide epitaxial
honeycomb oxide thin film Na3Co2SbO6, a candidate of Kitaev quantum spin liquid
proposed recently. We found a spin glass and antiferromagnetic ground states
depending on Na stoichiometry, signifying not only the importance of Na vacancy
control but also strong frustration in Na3Co2SbO6. Despite its classical ground
state, the field-dependent magnetic susceptibility shows remarkable scaling
collapse with a single critical exponent, which can be interpreted as evidence
of quantum criticality. Its electronic ground state and derived spin
Hamiltonian from spectroscopies are consistent with the predicted Kitaev model.
Our work provides a unique route to the realization and utilization of Kitaev
quantum spin liquid.
