Berlin 2005 – wissenschaftliches Programm

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TT: Tiefe Temperaturen

TT 12: Correlated Electrons - Spin Systems and Itinerant Magnets: Experiment

TT 12.7: Vortrag

Samstag, 5. März 2005, 12:15–12:30, TU H2053

Temperature dependent bulk sensitive valence band XPSmeasurements on LaCoO₃ single crystals — •T. Koethe¹, M.W. Haverkort¹, Z. Hu¹, M. Reuther¹, T. Lorenz¹, N.B. Brookes², J.C. Cezar², and L.H. Tjeng¹ — ¹II. Physikalisches Institut der Universität zuKöln, Germany — ²ESRF, Grenoble, France

The transition metal oxide compound LaCoO₃ displays a series of gradual spin state transitions from a diamagnetic state to a paramagnetic state with a maximum in the susceptibility at 100K and from semiconductor to metal above ≈500K. The Co³⁺ ions have a 3d⁶ configuration and can take a low spin state (LS) S = 0, high spin state (HS) S = 2 or even an intermediate spin state (IS) S = 1. There is general agreement about the LS nature of the low temperature phase, whereas the spin state in the intermediate and high temperature phases is much debated in the literature concerning the amount of contribution from IS or HS. We address this question with photoelectron spectroscopy using high photon energies on high quality single crystals in order to enhance bulk sensitivity. Our spectra show for temperatures below 300K clearly more LS contribution than previous experimental results. However, we observe a considerable HS component even at 100K. We infer that this HS spectral weight originates from the surface since the crystal field at the surface is strongly reduced due to the smaller coordination number.