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. We present a multifrequency electron spin resonance (ESR) study down to millikelvin temperatures, revealing a rapid drop of the spin susceptibility at 6 kelvin. 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.

几何上的挫折，量子纠缠和混乱可能会阻止具有强烈交换相互作用的局部自旋的长距离有序化，从而导致一种奇特的物质状态。κ-（BEDT-TTF）₂铜₂（CN）₃被认为是这种难以捉摸的量子自旋液态的主要候选者，但其基态性质仍然令人费解。我们提出了一个多频率电子自旋共振（ESR）的研究，一直到毫微克尔文温度，揭示了在6开尔文温度下自旋磁化率迅速下降。自旋间隙的这种打开，伴随着结构的改变，与价键固态基态的形成是一致的。我们确定了杂质对ESR响应的贡献，当内在自旋形成单重态时，该贡献将占主导地位。探测电子直接表明了缺陷的关键作用，这些缺陷对于没有磁序的量子自旋系统的低能特性是至关重要的。