Dresden 2009 – wissenschaftliches Programm
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TT: Fachverband Tiefe Temperaturen
TT 19: Correlated Electrons: Metal-Insulator Transition 3
TT 19.1: Vortrag
Dienstag, 24. März 2009, 14:00–14:15, HSZ 301
On the spin-state and metal-insulator transition in RBaCo2O5.5 — •T.C. Koethe1, Z. Hu1, Hua Wu1, C. Schüssler-Langeheine1, J.C. Cezar2, F. Venturini2, N.B. Brookes2, H.H. Hsieh3, H.-J. Lin3, C.T. Chen3, S.N. Barilo4, S.V. Shiryaev4, G.L. Bychkov4, and L.H. Tjeng1 — 1II.PhysikalischesInstitut Universität zu Köln — 2ESRF, Grenoble, France — 3NSRRC, Hsinchu, Taiwan — 4Belarus Academy of Sciences,Minsk, Belarus
The novel layered perovskite RBaCo2O5.5 (R = rare earth) has attracted considerable interest in the last decade. It shows an intriguing mix of properties, including giant magneto-resistance, metal-insulator and antiferro-ferromagnetic transitions, and a sign change of the thermoelectric power across these transitions. Explanation of these properties is subject of on-going debate. The so-called spin-blockade mechanism together with the occurrence of a spin-state transition of the octahedral Co3+ ions was proposed [Maignan et al., PRL 93 026401 (2004)]. Other scenarios invoke, for example, order-disorder effects, involving essentially all possible spin state configurations for the octahedral and pyramidal Co3+ ions. Using high quality single crystals and bulk sensitive photoelectron and x-ray absorption spectroscopy, we were able to identify the spin-state of the Co ions, thereby arriving at a very different scenario than proposed so far in the literature. We also find that the transfer of spectral weight near the Fermi level across the metal-insulator transition is very modest, in contrast to existing assumptions but in agreement with our observation on the evolution of the spin-state as a function of temperature.