Regensburg 2004 – wissenschaftliches Programm

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TT: Tiefe Temperaturen

TT 14: FV-internes Symposium ”Orbital Physics”

TT 14.9: Fachvortrag

Dienstag, 9. März 2004, 17:20–17:40, H18

The interplay of orbital and lattice degrees of freedom in titanates — •M. Grüninger, R. Rückamp, A. Gößling, M. Cwik, H. Roth, J. Baier, M. Kriener, T. Lorenz, M. Braden, and A. Freimuth — II. Physikalisches Institut, Universität zu Köln, Germany

Recently, several interesting phenomena have been proposed in which orbitals play a key role, e.g. orbital order, dispersing orbital waves or orbital liquids. However, the driving force in most theoretical treatments is based on electronic interactions, the possible role of the lattice is often neglected.

One interesting phenomenon which still has to be confirmed experimentally is the existence of orbital liquids. The pseudo-“cubic” d¹ antiferromagnet LaTiO₃ seemed a promising candidate. We discuss the structure, the thermal expansion and the optical conductivity of LaTiO₃ and of orbitally ordered YTiO₃. A consistent description of the data of both compounds is obtained within a Jahn-Teller scenario. Orbital excitations are observed in the optical conductivity spectra of both LaTiO₃ and YTiO₃ at about 0.25 eV, which is difficult to reconcile with an orbital liquid ground state of LaTiO₃. On contrary, the data suggest that the coupling to the lattice is of dominant importance not only for e_g systems, but also for t_2g electrons. This point of view is corroborated by the optical conductivity data of TiOCl. There, the splitting of the t_2g levels is larger than 0.5 eV.