2kF and 4kF charge-density waves in TTF0.4TSeF0.6-TCNQ — An X-ray study

In the quasi one-dimensional conductor TTF_0.4TSeF_0.6-TCNQ, it is found that the 2k_F CDW has three-dimensional correlations of relatively long range below the metal-insulator transition temperature ～40K, although the 4k_F CDWs are only one-dimensional in character at all temperatures. This result supports the speculation that the 2k_F CDWs and the 4k_F CDWs are present predominantly on the TCNQ and the fulvalene stacks, respectively. The 2k_F-value of the present material is similar to that of TTF-TCNQ but far from that of TSeF-TCNQ. The change of the value with the concentration of TSeF (or TTF) is nonlinear. The transverse period of the ordered 2k_F CDW along the a-axis is $\text{a}$ just below the transition temperature and becomes 4$\text{a}$ below 34.5K. A discussion is given based on the two-chain model.