Single-crystalline epitaxial TiO film: A metal and superconductor, similar to Ti metal

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Abstract
Titanium monoxide (TiO), an important member of the rock salt 3d transition-metal monoxides, has not been studied in the stoichiometric single-crystal form. It has been challenging to prepare stoichiometric TiO due to the highly reactive Ti2+. We adapt a closely lattice-matched MgO(001) substrate and report the successful growth of single-crystalline TiO(001) film using molecular beam epitaxy. This enables a first-time study of stoichiometric TiO thin films, showing that TiO is metal but in proximity to Mott insulating state. We observe a transition to the superconducting phase below 0.5 K close to that of Ti metal. Density functional theory (DFT) and a DFT-based tight-binding model demonstrate the extreme importance of direct Ti–Ti bonding in TiO, suggesting that similar superconductivity exists in TiO and Ti metal. Our work introduces the new concept that TiO behaves more similar to its metal counterpart, distinguishing it from other 3d transition-metal monoxides.
Funding Information
  • U.S. Department of Energy
  • Office of Science
  • University of Saskatchewan
  • Canadian Institutes of Health Research
  • Natural Sciences and Engineering Research Council of Canada
  • National Research Council Canada
  • Canada Foundation for Innovation
  • Canada First Research Excellence Fund
  • National Key Research and Development Program of China Stem Cell and Translational Research (2018YFA0305604)
  • National Science Foundation of China (11874403)
  • Office of Basic Energy Sciences (DE-SC0019211)
  • Government of Saskatchewan
  • Quantum Materials and Future Technologies Program
  • Max Planck-UBC-UTokyo Centre for Quantum Materials
  • Materials Science and Engineering Divisions, Brookhaven National Lab
  • Office of Basic Sciences of the U.S. Department of Energy (DESC0012704)

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