Anomalous Dielectric Response in the π-d Correlated Metallic State of λ-(BEDT-TSF)2FeCl4

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The temperature dependence of the microwave conductivity σ1 and dielectric constant ε1 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 TMI = 8.3 K, a novel ferroelectric-like response is observed in ε1c along c at high temperatures from TMI to TFM = 70 K, which is enhanced to the order of 103 just above TMI. Then a transition from a ferroelectric-like to paraelectric-like state with ε1c ∼ 45 takes part in the MI transition. In contrast to the dc conductivity, σ1c 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.
2001-09-15