X-ray diffuse-scattering study of the pinned charge-density waves in tetramethyltetraselenafulvalene dimethyltetracyanoquinodimethane (TMTSF-DMTCNQ) disordered by irradiation

We present an x-ray diffuse-scattering study of the quasi-one-dimensional conductor tetramethyltetraselenafulvalene dimethyltetracyanoquinodimethane (TMTSF-DMTCNQ) containing different amounts of irradiation-induced disorder. The pinning of the charge-density waves (CDW) to the defects destroys the three-dimensional long-range order even at 10 K, and anisotropically broadened reflections appear in the irradiated samples instead of the superstructure reflections in the pure sample. The values of the longitudinal as well as transverse correlation lengths have been measured as a function of the temperature and the irradiation dose. They have been interpreted within the framework of a model where each defect pins the charge-density wave in a given volume of the crystal. This volume should contain three chain segments of ten molecules. At small defect concentrations, the intensity of the thermal 4kF anomaly observed in the pure TMTSF-DMTCNQ decreases and becomes almost undetectable at 0.3 mol% defect concentration. At higher doses, of 0.7 and 2.8 mol%, the 4kF anomaly is resurrected by the defects and becomes static in the whole temperature range. Strong similarities in the temperature behavior of 2kF and 4kF scatterings in the TMTSF salt containing 1 or 2 mol% of radiation defects and in other unirradiated organic conductors such as Qn(TCNQ)₂ (quinolinium tetracyanoquinodimethane), have been observed.