Evidence for strong Coulomb interactions in alkali-TCNQ (tetracyanoquinodimethan) salts

Abstract

Measurements are reported of the dielectric function at optical and microwave frequencies in pure single crystals of potassium tetracyanoquinodimethan [${\mathrm{K}}^{+}$ ${\mathrm{}\left(\mathrm{TCNQ}\right)\mathrm{}}^{-}$], as well as measurements of the microwave conductivity and electron spin resonance. The results indicate that ${\mathrm{K}}^{+}$ ${\mathrm{}\left(\mathrm{TCNQ}\right))}^{-}$ is in the intermediate- to strong-coupling regime with electron-electron Coulomb repulsion and electron-energy bandwidth of the same magnitude. The spin-resonance studies show a single narrow line with no evidence of extra dipolar-split sidebands.