X-Ray Observation of Crossover of 2kF to 4kF Scattering in (N−Methylphenazinium)x(Phenazine)1−x(Tetracyanoquinodimethane)[(NMP)x(Phen)1−x(TCNQ)], 0.5<~x<~1.0

Abstract

We report the temperature-dependent x-ray diffuse scattering of ${\left(\mathrm{NMP}\right)}_x{\left(\mathrm{Phen}\right)}_{1-x}$-(TCNQ) as a function of conduction-electron density ($x$). With decreasing $x$ three different unique electronic instability regimes are successively observed on the TCNQ chains: (i) the $2k_{\mathrm{F}}$ instability for $\frac{2}{3}<~x<~1$, (ii) both $2k_{\mathrm{F}}$ and $4k_{\mathrm{F}}$ instabilities for $0.57<~x<~\frac{2}{3}$, and (iii) the $4k_{\mathrm{F}}$ instability alone for $x$ close to 0.5. These results provide direct evidence for the critical role of interchain screening in determining the magnitude of the effective Coulomb interaction.