Tetramer Orbital Ordering And Lattice Chirality In Mnti2o4

The presence of orbital degree of freedom in strongly correlated systems leads to unusual orderings. In this paper we use a combination of density-functional theory calculations and various experimental investigations to reveal a unique ground state for a Ti3+ containing spinel oxide, MnTi2O4, which hosts an exotic combination of a rare tetramer orbital (associated with Ti3+ 3d(1) electron) ordering along equivalent < 111 > directions involving all three t(2g) orbitals, a ferrimagnetic Mn-Ti, and a ferromagnetic Ti-lattice spin ordering. A combination of spin-orbital superexchange and Jahn-Teller-effect related strain-energy optimization provides a microscopic understanding for the stabilization of the unique ground state, which is found to be also electrically polar. The tetramer orbital ordering induces Ti-Ti bond length modulations and the short and long Ti-Ti bonds form helices around the crystallographic c axis with a particular winding direction, causing the ground state structure to become spatial chiral.