Soft x-ray photoemission on Fe₃O₄: Small polaron physics and the Verwey transition

Ralph Claessen

Experimentelle Physik IV, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany

Conventional photoemission spectroscopy (PES) is extremely surface-sensitive due to the small photoelectron mean free path. This is particularly prohibitive for the study of transition metal oxides (or other strongly correlated electron systems) with polar surfaces and large unit cells. The probing depth of PES can however be considerably enhanced by employing soft-x-ray excitation. Using magnetite (Fe3O4) as an example I will demonstrate the potential of soft x-ray photoemission, which allows us to distinguish surface and volume effects in the electronic structure. The pseudogap behavior and temperature dependence of the thus identified intrinsic volume spectra give evidence for the existence of strongly bound small polarons in this material. With respect to the recently reheated debate on the nature of the Verwey transition in magnetite our findings (together with other recent experimental and theoretical results) support a picture in which the low-temperature phase is better characterized by a cooperative Jahn-Teller effect than by a charge-ordered state.