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Geometrical frustration, quantum entanglement, and disorder may prevent long-range order of localized spins with strong exchange interactions, resulting in an exotic state of matter. κ-(BEDT-TTF)₂Cu₂(CN)₃ is considered the prime candidate for this elusive quantum spin liquid state on a triangular lattice, but its ground-state properties remain puzzling. It is not clear in which way disorder affects the measured electronic ground state in one or the other way. At 6 K an enigmatic anomaly has been observed in various physical properties that triggered speculations in many directions. On the fundamental issue of the existence of a spin gap, conflicting conclusions have been drawn from magnetic torque, μSR, NMR, thermal transport and specific heat measurements. In contrast to these indirect methods, electron spin resonance (ESR) directly probes the magnetic excitation spectrum of the conduction electrons, which allows us to unambiguously identify the intrinsic response and separate it from other contributions. For that reason, we developed a broadband ultra-low-temperature ESR technique based on coplanar waveguide resonators.

Our multifrequency ESR study on κ-(BEDT-TTF)₂Cu₂(CN)₃ reveals a rapid drop of the spin susceptibility at T≈6K. This opening of a spin gap, accompanied by structural modifications, is consistent with the formation of a valence bond solid ground state. We identify an impurity contribution to the ESR response that becomes dominant when the intrinsic spins form singlets. Probing the electrons directly manifests the pivotal role of defects for the low-energy properties of frustrated quantum spin systems without magnetic order.

Having answered the longstanding question of the ground state of κ-(BEDT-TTF)₂Cu₂(CN)₃, we can now deliberately tune the effects of correlations, impurities and geometrical frustration, and eventually turn to other quantum spin liquids.