π-d correlation in λ-(BEDT-TSF)2Fe1-xGaxCl4 by ESR measurements

Electron spin resonance experiments with 15.4 GHz microwave have been performed on λ-(BEDT-TSF)2Fe1-xGaxCl4 (x=0.0, 0.4, and 1.0) single crystals at various temperatures down to 1.5 K, where BEDT-TSF denotes bis(ethyleneditho)tetraselena-fulvalene. For λ-(BEDT-TSF)2GaCl4, the resonance signal due to π-electrons at the BEDT-TSF molecule layer is observed. For γ-(BEDT-TSF)2Fe0.6Ga0.4Cl4, signals which originate both from π-electrons at the BEDT-TSF molecule layer and from d-electrons at Fe ions are observed. For λ-(BEDT-TSF)2FeCl4, only one resonance signal is observed. The magnetic field direction dependencies of the g-value do not show the symmetry of the conducting ac-plane above 20 K. This means that the observed signals are not mainly due to the π-electrons at the BEDT-TSF molecule layer but due to the d-electrons at Fe ions. The resonance due to the π-electrons is expected to be renormalized by the strong correlation between the π- and d-electrons. The anomaly has been observed in the g-value as well as the line width both for λ-(BEDT-TSF)2FeCl4 at around 70K and for λ-(BEDT-TSF)2Fe0.6Ga0.4Cl4 at around 95K. This might be correlated to the dielectric and structural anomaly observed for λ-(BEDT-TSF)2FeCl4. (author)