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Regensburg 2004 – scientific programme

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TT: Tiefe Temperaturen

TT 24: Postersitzung III: Korrelierte Elektronen, ”Orbital Physics”

TT 24.41: Poster

Wednesday, March 10, 2004, 14:30–19:00, Poster A

High energy photoemission study on the Verwey transition in Fe3O4 — •David Schrupp1, Michael Sing1,2, Sigemasa Suga2, and Ralph Claessen11Institut für Physik, Universität Augsburg, 86135 Augsburg, Germany — 2Department of Material Physics, Osaka University, Osaka 560-8531, Japan

Magnetite (Fe3O4) undergoes a first-order phase transition called the Verwey transition (VWT). Although known for a long time the VWT is currently a matter of intensive debate questioning fundamental issues such as the charge ordering of Fe2+ and Fe3+ ions which is widely believed to be the driving mechanism of the VWT.

We present a study of the VWT by valence band high energy photoemission. Compared to VUV PES this enhances the probing depth by at least a factor of 2 with surface effects greatly reduced. Nonetheless, we still observe subtle differences in the valence band spectra between differently prepared surfaces. While the spectra of sputtered, annealed, and post-oxidized (111) and (100) single crystal surfaces display a continuous temperature evolution through the Verwey temperature (TV), a first order VWT is observed on surfaces obtained by fracturing a crystal at 40 K. The non-equilibrium atomic configuration of these surfaces is thus apparently rather bulk-like, whereas the annealed samples display strong surface relaxations. The spectra at T < TV of the fractured samples are identified as those of a correlated insulator with a gap of at least 150 meV, which at TV closes into a pseudogap situation. The absence of clear quasiparticle signatures characterizes the T > TV phase as an incoherent metal, consistent with transport and optical data.

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