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The exotic superconductivity beyond the well-established BCS theory has attracted substantial attention in both applied and fundamental research across various scientific disciplines. Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconductivity is known as one of the unconventional superconducting states, which can be stabilized even above the Pauli limit. In these days, the FFLO state has found a place in numerous textbooks as a non-zero center-of-mass momentum pairing state with spatial modulation in real space. However, some crucial characteristics of the FFLO state remain experimentally elusive, such as direct observation of spatial modulation and the vulnerability to non-magnetic disorders, due to the experimental difficulties.

To confirm one of the main characteristics of the FFLO state, vulnerability to non-magnetic disorders, in this study, we performed high-field ultrasound measurements of an X-ray irradiated organic superconductor κ-(BEDT-TTF)₂Cu(NCS)₂ (k-NCS, T_c = 10.2 K), where the concentration of non-magnetic disorder can be artificially controlled, and conclusively demonstrated that the FFLO state is devastated by introducing disorder. Our results indicate that the FFLO state requires as electron mean free path longer than the spatial modulation period, which agrees with the simplest theoretical prediction of the FFLO state.