Berlin 2015 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 100: Correlated Electrons: Other Materials

TT 100.3: Talk

Thursday, March 19, 2015, 15:30–15:45, H 3005

On the absence of an orbital-ordering transition in KCuF₃ — •Hunter Sims¹, Eva Pavarini², and Erik Koch¹ — ¹German Research School for Simulation Sciences, 52428 Jülich, Germany — ²Institute for Advanced Simulation, Forschungszentrum Jülich, 52428 Jülich, Germany

The Mott insulating perovskite KCuF₃ is considered the paradigm of a long-ranged orbitally-ordered material with cooperative Jahn-Teller distortion. We have found, however, that neither the Kugel-Khomskii superexchange mechanism nor the Jahn-Teller effect can, even qualitatively, account for the observed absence of a transition to a symmetric phase at high temperature. Instead, we show that the distortion of the F-octahedra is only limited by the shortest distance between Cu and F, leading to an increased distortion for the thermally expanded material. To quantitatively understand this we calculate the temperature-dependent Born-Oppenheimer surfaces in LDA+U. Based on this we determine the temperature-dependence of the distortion using a generalization of the Halperin-Englman mean-field model.