Two-orbital degeneracy lifted state as a local precursor to a metal-insulator transition

Abstract

The recent discovery of a local fluctuating t2g orbital-degeneracy-lifted (ODL) state in CuIr2S4 as a high temperature precursor to the metal-insulator transition (MIT) opens the door to a possible widespread presence of precursor states in scarcely studied high-temperature regimes of transition metal based quantum materials. Although in CuIr2S4 the ODL state comprises one orbital per Ir, there is no fundamental reason to exclude multi-orbital ODL states in general. The MgTi2O4 spinel exhibits a MIT on cooling at Ts ~250 K, accompanied by Ti t2g orbital ordering (OO) and spin dimerization with the average symmetry reducing to tetragonal. It shares with CuIr2S4 the pyrochlore transition metal sublattice with active t2g orbitals. This, together with its different orbital filling (t2g1 vs t2g5.5) make it a candidate for hosting a multi-orbital ODL precursor state. By combining x-ray and neutron pair distribution function analyses to track the evolution of the local atomic structure across the MIT we find that local tetragonality already exists in the metallic globally cubic phase at high temperature. Local distortions exist up to at least 500 K. Significantly, the high temperature local state is not continuously connected to the OO band insulator ground state, and so the transition cannot be characterized as a trivial order-disorder type. The shortest Ti-Ti spin singlet dimer bonds expand abruptly on warming across the transition but remain shorter than those seen in the cubic structure. These seemingly contradictory observations can be understood within the model of a local fluctuating two-orbital t2g ODL precursor state. The ODL state in MgTi2O4 has a correlation length of about 1 nm at high temperature. We discuss that this extended character of the local distortions is consistent with the two-orbital nature of the ODL state imposed by the charge filling and the bond charge repulsion.