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

Abstract

The MgTi₂O₄ spinel exhibits a metal-insulator transition on cooling below Ts≈250 K, accompanied by Ti t_{1/2 g} 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 t_2g 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 themore » two-orbital nature of this state imposed by the charge filling and the bond charge repulsion. Overall, 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.« less

Authors:

Yang, Long  ^[1];

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• Search DOE PAGES for ORCID "0000-0001-8731-0172"

Koch, Robert J.  ^[2];

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• Search DOE PAGES for ORCID "0000-0003-4995-2101"

Zheng, Hong ^[3]; Mitchell, J. F.  ^[3];

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Yin, Weiguo  ^[2];

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Tucker, Matthew G.  ^[4];

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• Search DOE PAGES for ORCID "0000-0002-2891-7086"

Billinge, Simon L.  ^[5];

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Bozin, Emil S.  ^[2]

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• Search DOE PAGES for ORCID "0000-0002-6682-0365"

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+ Show Author Affiliations

1. Columbia Univ., New York, NY (United States)
2. Brookhaven National Lab. (BNL), Upton, NY (United States)
3. Argonne National Lab. (ANL), Argonne, IL (United States)
4. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
5. Columbia Univ., New York, NY (United States); Brookhaven National Lab. (BNL), Upton, NY (United States)

Publication Date:

Mon Dec 14 00:00:00 EST 2020

Research Org.:

Argonne National Lab. (ANL), Argonne, IL (United States); Brookhaven National Lab. (BNL), Upton, NY (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES)

OSTI Identifier:

1757938

Alternate Identifier(s):

OSTI ID: 1716780

Report Number(s):

BNL-220624-2020-JAAM
Journal ID: ISSN 2469-9950; 162555; TRN: US2205694

Grant/Contract Number:  

AC02-06CH11357; SC0012704

Resource Type:

Accepted Manuscript

Journal Name:

Physical Review B

Additional Journal Information:

Journal Volume: 102; Journal Issue: 23; Journal ID: ISSN 2469-9950

Publisher:

American Physical Society (APS)

Country of Publication:

United States

Language:

English

Subject:

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; metal-insulator transition; orbital order; neutron scattering; x-ray diffraction