| Popis: |
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 \ensuremath{\epsilon} 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. |
