Charge-gap formation upon the Verwey transition inFe3O4

Abstract

The temperature-dependent electronic structure has been investigated for a single crystal of magnetite (Fe 3 O 4 ) by measurements of the optical-conductivity spectrum. Upon the charge-ordering ͑Verwey͒ transition (T V ϭ121 K), a clear opening of the optical gap ͑0.14 eV͒ is observed. Above T V , the optical conductivity spectra shows notable spectral weight transfer in the Fe 3d intersite transition region (0Ϫ2 eV), while preserving the pseudogap or polaronic feature, indicating highly diffuse charge dynamics.

Speed limit of the insulator–metal transition in magnetite

“…We used optical reflectivity to detect its existence. In thermal equilibrium, the transition from insulating to metallic behaviour is accompanied by the appearance of a spectral feature centered at 725 nm [19]. We found the same feature to appear in time-resolved, pump-probe experiments with a 1.5 ± 0.2 ps time constant 6 (see supplementary information).…”

“…The optical signature of conductivity was probed in a 700 to 750 nm wavelength interval following Ref. [19]. The magnetite crystal is shown as the blue (red) block in the monoclinic (cubic) structure.…”

“…We utilize time-resolved x-ray diffraction [18] and optical reflectivity [19,20] to disentangle the lattice and electronic degrees of freedom and study the role of phase separation in the Verwey transition. Starting from the insulating phase at 80 K, energy is rapidly injected into the electronic system via femtosecond (fs) laser excitation.…”

Terahertz conductivity at the Verwey transition in magnetite

“…The monoclinic phase is insulating and is characterized by a gap of the order of 100 meV in the excitation spectrum, which is seen in photoemission data, [9][10][11] in thermoelectric properties, 12 and by optical spectroscopy. 13,14 The charge transport in magnetite is usually explained within a polaronic picture. [13][14][15][16] However, there is no general agreement about the energy of the polaronic absorption in the conductivity spectra.…”

“…13,14 The charge transport in magnetite is usually explained within a polaronic picture. [13][14][15][16] However, there is no general agreement about the energy of the polaronic absorption in the conductivity spectra. [13][14][15][16] Recent photoemission results, 11 obtained in the same magnetite crystals as in the present paper, have been self-consistently explained using a small-polaron model.…”