Optical study of MgTi$_2$O$_4$: Evidence for an orbital-Peierls state
  Dimension reduction due to the orbital ordering has recently been proposed to explain the exotic charge, magnetic and structural transitions in some three-dimensional (3D) transitional metal oxides. We present optical measurement on a spinel compound MgTi$_2$O$_4$ which undergoes a sharp metal-insulator transition at 240 K, and show that the spectral change across the transition can be well understood from the proposed picture of 1D Peierls transition driven by the ordering of $d_{yz}$ and $d_{zx}$ orbitals. We further elaborate that the orbital-driven instability picture applies also very well to the optical data of another spinel CuIr$_2$S$_4$ reported earlier. 
J. Zhou, G. Li, J. L. Luo, Y. C. Ma, Dan Wu, B. P. Zhu, Z. Tang, J.   Shi, and N. L. Wang