Regensburg 2004 – wissenschaftliches Programm

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TT: Tiefe Temperaturen

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

TT 24.41: Poster

Mittwoch, 10. März 2004, 14:30–19:00, Poster A

High energy photoemission study on the Verwey transition in Fe₃O₄ — •David Schrupp¹, Michael Sing^1,2, Sigemasa Suga², and Ralph Claessen¹ — ¹Institut 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 (T_V), 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 < T_V of the fractured samples are identified as those of a correlated insulator with a gap of at least 150 meV, which at T_V closes into a pseudogap situation. The absence of clear quasiparticle signatures characterizes the T > T_V phase as an incoherent metal, consistent with transport and optical data.