Dielectric response in the metallic states of λ-(BEDT-TSF)2FeCl4

The microwave conductivity σ1c and dielectric constant ε1c along the c-axis in λ-(BEDT-TSF)2FeCl4 have been obtained by a cavity perturbation technique with TE0 1 1 resonant mode at 16.3 GHz. Even though the d.c. resistivity indicates a highly metallic state, we have found a novel dielectric response in ε1c amounting to the order of 103 from the metal-to-insulator transition temperature TMI=8.3 K to TFM=70 K, at which the noticeable specific-heat jump and 1H NMR spectral splitting have been recently observed. The dielectric anomaly is attributed to the dynamical response of π-electron’s charge degrees of freedom and could be interrelated to the π–d interaction.