Terahertz Conductivity at the Verwey Transition in Magnetite

TL;DR

This study investigates the THz and infrared conductivity changes in magnetite during the Verwey transition, revealing a shift from hopping conduction to free-carrier behavior and supporting an insulator-semiconductor transition model.

Contribution

It provides detailed spectroscopic evidence of the transition from hopping conduction to free carriers at the Verwey transition in magnetite.

Findings

Power-law frequency dependence of conductivity in the insulating state

Abrupt decrease and sign change of dielectric constant at transition

Formation of a narrow Drude peak indicating free carriers

Abstract

The complex conductivity at the (Verwey) metal-insulator transition in Fe_3O_4 has been investigated at THz and infrared frequencies. In the insulating state, both the dynamic conductivity and the dielectric constant 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 nu_1 ~ 8 meV must be introduced for a self-consistent description. On heating through the Verwey transition the low-frequency dielectric constant abruptly decreases and becomes negative. Together with the conductivity spectra this indicates a formation of a narrow Drude-peak with a characteristic scattering rate of about 5 meV containing only a small fraction of the available charge carriers. The spectra can be explained assuming the transformation of the spectral weight from the hopping process to the free-carrier conductivity. These results support an interpretation of Verwey transition in magnetite as an insulator-semiconductor transition with structure-induced changes in activation energy.