Regensburg 2004 – wissenschaftliches Programm
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TT: Tiefe Temperaturen
TT 6: Niederdimensionale Systeme (incl. Peierls-Übergang, organische Leiter, ...)
TT 6.7: Vortrag
Montag, 8. März 2004, 15:45–16:00, H18
TiOCl and TiOBr: a new inorganic spin-Peierls systems? — •R. Rückamp1, J. Baier1, M. Kriener1, M. Grüninger1, T. Lorenz1, A. Freimuth1, L. Jongen2, A. Möller2, and G. Meyer2 — 1II. Physikalisches Institut, Universität zu Köln — 2Institut für anorganische Chemie, Universität zu Köln
In TiOCl and TiOBr antiferromagnetic spin-1/2 chains are formed along the a axis. Currently, TiOCl attracts much interest due to a transition to a non-magnetic phase at 67 K which is discussed as the second inorganic example of a spin-Peierls transition [1]. However, the associated structural distortions have not been observed so far. Moreover, there are clear indications for a second phase transition at 92 K.
We present optical data in the frequency range of 6 meV - 2 eV and measurements of the magnetic susceptibility, thermal expansion and specific heat. In the optical data of TiOCl we observe new phonon modes in the low-temperature phase, in agreement with a spin-Peierls scenario. An additional new phonon mode occurs below 92 K, which disappears again in the low-temperature phase. In the literature, the importance of orbital fluctuations has been discussed. We observe d-d transitions at 0.7 and 1.6 eV, in agreement with the results of a point-charge model. Such a large splitting of the d-levels shows that orbital fluctuations are not important in these systems. Qualitatively, the data of TiOBr and TiOCl are very similar. Again, two transitions are observed, namely at 28 K and 50 K.
[1] A. Seidel et al., Phys. Rev. B 67, 020405 (2003).