Control of trigonal crystal field in honeycomb cobaltate antiferromagnet towards Kitaev quantum spin liquid

Recently, layered honeycomb cobaltates have been predicted as a new promising system for realizing the Kitaev quantum spin liquid, a many-body quantum entangled ground state characterized by fractional excitations. Yet, these cobaltates exhibit classical antiferromagnetic ordering at low temperatures due to their trigonal crystal field, which impedes the formation of the expected quantum state, similar to other candidate materials. In this letter, we demonstrate the control of the trigonal crystal field of Co ions is a key to destabilizing classical antiferromagnetic ordering in layered honeycomb cobaltates. By utilizing heterostructure engineering in Cu3Co2SbO6 thin films, we adjust the trigonal distortion of CoO6 octahedra and the associated trigonal crystal field. The original N\'eel temperature of 16 K in bulk Cu3Co2SbO6 decreases (increases) to 7.8 K (22.7 K) in strained Cu3Co2SbO6 films by reducing (raising) the magnitude of the trigonal crystal fields. Our experimental finding substantiates the potential of layered honeycomb cobaltate heterostructures to accomplish the extremely elusive quantum phase of matter.