Anomalous dielectric response in the π-d correlated metallic state of λ-(BEDT-TSF)₂FeCl₄

The temperature dependence of the microwave conductivity σ₁ and dielectric constant ∈₁ has been measured on λ-(BEDT-TSF)₂FeCl₄ by a cavity perturbation method at 16.3 GHz, employing the TE₀₁₁ resonant mode. In spite of the highly metallic state; above the metal-to-insulator (MI) transition at T_MI = 8.3 K, a novel ferroelectric-like response is observed in ∈_l^c along c at high temperatures from T_MI to T_FM = 70 K, which is enhanced to the order of 10³ just above T_MI. Then a transition from a ferroelectric-like to paraelectric-like state with ∈_l^c ∼ 45 takes part in the MI transition. In contrast to the dc conductivity, σ_l^c becomes much more conductive as considered by a collective mode. The ferroelectric-like response is associated with the dynamical response of the charge degrees of freedom in the π electronic system. An absence of these dielectric and thermodynamical anomalies in the nonmagnetic Ga analogue indicates the importance of the π-d interaction for the ferroelectric-like state as well as the MI transition.