Optical study ofMgTi2O4: Evidence for an orbital-Peierls state

Abstract

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 MgTi2O4 which undergoes a sharp metalinsulator 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 dyz and dzx orbitals. We further elaborate that the orbital-driven instability picture applies also very well to the optical data of another spinel CuIr2S4 reported earlier.

Orbital-spin order and the origin of structural distortion inMgTi2O4

“…1,2 The best studied example of TMO spinels, and historically the first one, is magnetite Fe 3 O 4 , which shows a high Curie temperature and undergoes the Verwey transition at T V Ӎ 120 K. 2 Recently, B-spinel MgTi 2 O 4 , which is characterized by a pyrochlore lattice of Ti 3+ magnetic ions with one single electron in the t 2g manifold, has attracted much attention due to a very peculiar phase transition from a metallic to a spinsinglet insulating phase near T C Ϸ 260 K. 3 The signature of the insulating state is the optical gap ͑Ϸ0.25 eV at T =10 K͒ observed in optical conductivity spectra. 4 Taken together, dc resistivity 3,5 and optical 4,6 measurements consistently indicate that MgTi 2 O 4 undergoes a sharp metalinsulator transition ͑MIT͒ with no sign of Drude weight at low frequencies below T C . The MIT is accompanied by a structural transition from cubic to tetragonal symmetry, with a concomitant drop of the magnetic susceptibility and a resistivity jump below T C .…”

Two-orbital degeneracy lifted local precursor to a metal-insulator transition in MgTi2O4

“…They both exhibit a global symmetry lowering, from cubic to tetragonal, at the MIT on cooling. They both have anomalous electrical resistivity behavior in the hightemperature metallic phase 14,23 . The symmetry lowering at the MIT is accompanied by a dimerization of transition metal ions that results in alternating short and long metalmetal bonds, and a resulting tetramerization 24 , along linear chains of ions on the pyrochlore sublattice 21 .…”

Magnetic, thermal, and transport properties of the mixed-valent vanadium oxidesLuV4O8andYV4Das1, Niazi2, Mudryk3 et al. 2010Phys. Rev. B4010View full textAdd to dashboardCite

“…Such a Peierls-type transition has been observed in the tetragonal rutile VO 2 at 340 K ͑Refs. 31 and 32͒ and in the spinel MgTi 2 O 4 at 260 K. 33,34 In both VO 2 and MgTi 2 O 4 , a complete structural transition occurs at the temperature of the metal to spin-singlet transition, 33,35 unlike YV 4 O 8 , where only the lattice parameters change without a lowering of the crystal symmetry.…”