Terahertz conductivity at the Verwey transition in magnetite

The complex cond. at the (Verwey) metal-insulator transition in Fe3O4 was investigated at terahertz and IR frequencies. In the insulating state, both the dynamic cond. and the dielec. const. reveal a power-law frequency dependence, the characteristic feature of hopping conduction of localized charge carriers. The hopping process is limited to low frequencies only, and a cutoff frequency n1.simeq.8 meV must be introduced for a self-consistent description. On heating through the Verwey transition the low-frequency dielec. const. abruptly decreases and becomes neg. Together with the cond. spectra this indicates the formation of a narrow Drude peak with a characteristic scattering rate of about 5 meV contg. only a small fraction of the available charge carriers. The spectra can be explained assuming the transformation of the spectral wt. from the hopping process to the free-carrier cond. These results support an interpretation of Verwey transition in magnetite as an insulator-semiconductor transition with structure-induced changes in activation energy. [on SciFinder (R)]