Abstract
The MgTi2O4 spinel exhibits a metal-insulator transition on cooling below Ts˜250 K, accompanied by Ti t2g1 orbital ordering and spin-singlet dimerization with associated average symmetry reduction to tetragonal. By combining x-ray and neutron pair distribution function analyses to track the evolution of the local atomic structure across the transition we find that local tetragonality already exists in the metallic globally cubic phase at high temperature. Local distortions are observed up to at least 500 K, the highest temperature assessed in this study. Significantly, the high-temperature local state is not continuously connected to the orbitally ordered band insulator ground state and so the transition cannot be characterized as a trivial order-disorder type. The shortest Ti-Ti spin-singlet dimer bond lengths expand abruptly on warming across the transition, but remain shorter than those seen in the cubic average structure. These seemingly contradictory observations can be understood within the model of a local fluctuating two-orbital t2g orbital degeneracy lifted (ODL) precursor state derived from electron filling, Ti substructure topology, and point symmetry considerations. 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 this state imposed by the charge filling and the bond charge repulsion. The MgTi2O4 spinel exemplifies multiorbital ODL state and presents the possibility of a widespread presence of such precursor states in scarcely studied high-temperature regimes of transition-metal-based quantum materials.
Original language | English |
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Article number | 235128 |
Journal | Physical Review B |
Volume | 102 |
Issue number | 23 |
DOIs | |
State | Published - Dec 14 2020 |
Funding
Work at Brookhaven National Laboratory was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences under Contract No. DE-SC0012704. L.Y. and M.G.T. acknowledge support from the ORNL Graduate Opportunity program, which was funded by the Neutron Science Directorate, with support from the Scientific User Facilities Division, Office of Basic Energy Science, U.S. DOE. Work in the Materials Science Division at Argonne National Laboratory (sample synthesis and characterization) was sponsored by the U.S. DOE Office of Science, Basic Energy Sciences, Materials Science and Engineering Division. X-ray PDF measurements were conducted at 28-ID-1 and 28-ID-2 beamlines of the National Synchrotron Light Source II, a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory. Neutron diffraction experiments were carried out at the NOMAD beamline of the Spallation Neutron Source, Oak Ridge National Laboratory, which was sponsored by the Scientific User Facilities Division, Office of Basic Energy Science, U.S. DOE.