Orbitally-driven Peierls state in correlated oxides

In studying superstructures in transition metal oxides such as charge or orbital ordering, one usually considers site-centered superstructures. Hovewer there exist another possibility: bond-centered superstructures, such as e.g. the Peierls state in low-dimensional systems. In this talk we will consider the possibility of existence of site-centered and bond-centered structures on a few examples. We will show that in spinels and in some other frustrated systems a site-centered orbital ordering (ODW-Orbital Density Wave) may lead to the formation of bond-centered singlet Peierls-like states [1]. This picture gives a simple explanation of extremely strange superstructures observed recently below metal-insulator transitions in MgTi$_2$O$_4$ [2] and CuIr$_2$S$_4$ [3], and may be relevant for several other materials, such as NaTiO$_2$, La$_4$Ru$_2$O$_{10}$ [4] and some others. We will also give a general discussion in which cases bond-centered structures may be favourable, and discuss the role of orbital degrees of freedom for the insulator-metal transitions. [1] D.Khomskii and T.Mizokawa, cond-mat/0407458 [2] M.Schmidt et al., Phys.Rev.Lett. \textbf{92}, 056402 (2004) [3] P.G.Radaelli et al., Nature \textbf{416}, 155 (2002) [4] P.Khalifah et al., Science \textbf{297}, 2237 (2002)