Gapped magnetic ground state in quantum-spin-liquid candidate κ-(BEDT-TTF)_2Cu_2(CN)_3

Geometrical frustration, quantum entanglement and disorder may prevent long-range ordering 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, but its ground-state properties remain puzzling. Here we present a multi-frequency electron-spin-resonance (ESR) study down to millikelvin temperatures, revealing a rapid drop of the spin susceptibility at T* = 6 K. 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 quantum-spin systems without magnetic order.