Microwave conductivity and dielectric constant of tetramethyltetrathiafulvalene salts [(TMTTF)2X,X=SCN, ReO4, SbF6]

Abstract

Microwave conductivity and dielectric constant at 6.5 GHz are reported for three tetramethyltetrathiafulvalene salts: (TMTTF${)}_2$X, X=SCN, ${\mathrm{ReO}}_4$, and ${\mathrm{SbF}}_6$. A divergence of the dielectric constant ε is observed at the structureless transition temperature for ${\mathrm{SbF}}_6$ (160 K) and ${\mathrm{ReO}}_4$ (220 K), contrasting with a stepwise change at the anion ordering transition of the latter salt (160 K). The dielectric constant of (TMTTF${)}_2$SCN shows a broad maximum above the anion ordering transition and a sharp decrease at $T_c$. Moreover the microwave conductivity exceeds dc conductivity above the structureless transition in (TMTTF${)}_2$ ${\mathrm{SbF}}_6$. Also, conductivity becomes frequency dependent below the anion ordering of (TMTTF${)}_2$SCN. These data support the pseudogap picture describing the structureless transition, a complete gap for anion ordering transition and the absence of charge-density waves in these materials.