Orbital Dimerization and Dynamic Jahn-Teller Effect in NaTiSi$_2$O$_6$

M. J. Konstantinović; J. van den Brink; Z. V. Popović; V. V. Moshchalkov; M. Isobe; Y. Ueda

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Orbital Dimerization and Dynamic Jahn-Teller Effect in NaTiSi$_2$O$_6$

M. J. Konstantinović·J. van den Brink·Z. V. Popović·V. V. Moshchalkov·M. Isobe·Y. Ueda

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Abstract

We study with the Raman scattering technique two types of phase transitions in the pyroxene compounds NaMSi$_2$O$_6$ (with M=Ti, V, and Cr). In the quasi one-dimensional S=1/2 system NaTiSi$_2$O$_6$ we observe anomalous high-temperature phonon broadening and large changes of the phonon energies and line-widths across the phase transition at 210 K. The phonon anomalies originate from an orbital order-disorder phase transition and these results --combined with theoretical considerations-- indicate that the high temperature dynamical Jahn-Teller phase of NaTiSi$_2$O$_6$ exhibits a spontaneous breaking of translational symmetry into a dimerized, Jahn-Teller distorted, orbital ordered state under the formation of spin valence bonds. In S=1 NaVSi$_2$O$_6$ orbital degrees of freedom are strongly suppressed and the magnetic excitations are well described within a Heisenberg model, indicating that at T$_N$=19K this system orders antiferromagnetically.

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M. J. Konstantinović, J. van den Brink, Z. V. Popović, V. V. Moshchalkov, M. Isobe, Y. Ueda. 2003-01-21. Orbital Dimerization and Dynamic Jahn-Teller Effect in NaTiSi$_2$O$_6$. https://arxiv.org/abs/cond-mat/0210191

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