Observation of superconductivity in structure-selected Ti2O3 thin films

Abstract

The search for new superconductors capable of carrying loss-free current has been a research theme in condensed matter physics for the past decade. Among superconducting compounds, titanates have not been pursued as much as Cu2+ (3d9) (cuprate) and Fe2+ (3d6) (pnictide) compounds. Particularly, Ti3+-based compounds or electron systems with a special 3d1 filling are thought to be promising candidates as high-TC superconductors, but there has been no report on such pure Ti3+-based superconducting titanates. With the advent of thin-film growth technology, stabilizing new structural phases in single-crystalline thin films is a promising strategy to realize physical properties that are absent in the bulk counterparts. Herein, we report the discovery of unexpected superconductivity in orthorhombic-structured thin films of Ti2O3, a 3d1 electron system, which is in strong contrast to the conventional semiconducting corundum-structured Ti2O3. This is the first report of superconductivity in a titanate with a pure 3d1 electron configuration. Superconductivity at 8 K was observed in the orthorhombic Ti2O3 films. Leveraging the strong structure-property correlation in transition-metal oxides, our discovery introduces a previously unrecognized route for inducing emergent superconductivity in a newly stabilized polymorph phase in epitaxial thin films. Researchers have discovered a titanate -based single-crystal film with a rare structure that allows superconductivity to emerge at low temperatures. Titanium (III) oxide, Ti2O3, has garnered recent interest for solar and energy harvesting devices because its electrons are easily excited by electromagnetic radiation. A team led by Tom Wu from University of New South Wales in Sydney, Australia and Shuai Dong from Southeast University in Nanjing, China, report that these electrons have a very high density comparable to metals and are strongly coupled with lattice in constrained thin films. The researchers used a sapphire substrate to direct the growth of nanometer-thick Ti2O3 layers into a solid-state structure not normally found in free-grown crystals. One particular combination of growth temperature and film thickness yielded an orthorhombic crystal phase that exhibited an unusual semiconductor-to-superconductor electronic transition when cooled to 8 K. Polymorph-dependent superconductivity was discovered in orthorhombic-structured Ti2O3 thin films, which is in strong contrast to the conventional semiconducting trigonal corundum-structured Ti2O3. Leveraging on the strong structure-property correlation in transition-metal oxides, our discovery introduces a hitherto unrecognized route towards inducing superconductivity in 3d1 electron systems via polymorph engineering.