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

Microwave conductivity and dielectric constant at 6.5 GHz are reported for three tetramethyltetrathiafulvalene salts: (TMTTF)2X, X=SCN, ReO4, and SbF6. A divergence of the dielectric constant is observed at the structureless transition temperature for SbF6 (160 K) and ReO4 (220 K), contrasting with a stepwise change at the anion ordering transition of the latter salt (160 K). The dielectric constant of (TMTTF)2SCN shows a broad maximum above the anion ordering transition and a sharp decrease at Tc. Moreover the microwave conductivity exceeds dc conductivity above the structureless transition in (TMTTF)2SbF6. Also, conductivity becomes frequency dependent below the anion ordering of (TMTTF)2SCN. 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. © 1988 The American Physical Society.