This is a schematic representation of (a) Ti4O7 and (b) ?-Ti3O5. Image: Scientific Reports.Titanium dioxide (TiO2) is a well-known whitener commonly used in sunscreens and paints such as the white lines seen on tennis courts. Less well known are other, higher titanium oxides – those with a higher number of titanium and oxygen atoms – that are the subject of intensifying research due to their potential for use in next-generation electronic devices.
Now, researchers at Tokyo Institute of Technology in Japan have reported superconductivity in two kinds of higher titanium oxides prepared in the form of ultrathin films. With a thickness of around 120nm, these materials display properties that are only just beginning to be explored.
"We succeeded in growing thin films of Ti4O7 and γ-Ti3O5 for the first time," says Kohei Yoshimatsu, lead author of a paper on the work in Scientific Reports.
Until now, these two materials had only been studied in bulk form, where they behave as insulators. The formation of electrically conductive thin films is therefore seen as a big advance for fundamental physics.
The researchers found that the superconducting transition temperature was 3.0K for Ti4O7 and 7.1K for γ-Ti3O5. Achieving superconductivity at 7.1K in simple metal oxides is "an amazing result", says Yoshimatsu, as "it represents one of the highest known among these oxides."
The thin films are epitaxial, meaning that they have a well-aligned crystalline structure. "They are extremely difficult to grow," says Yoshimatsu. "In our study, instead of using conventional TiO2 as the starting material, we chose to begin with the slightly more reduced Ti2O3." Then, under precisely controlled atmospheric conditions, the Ti4O7 and γ-Ti3O5 films were grown layer-by-layer upon sapphire substrates in a process called pulsed-laser deposition.
To verify the crystalline structures of the films, the team collaborated with researchers at the National Institute for Materials Science (NIMS). They employed characterization techniques such as X-ray diffraction (XRD) using synchrotron radiation at SPring-8, one of the world's largest facilities of its kind situated in Hyogo Prefecture, western Japan.
As yet, no one understands exactly how superconductivity arises in these titanium oxides. The irregular (or non-stoichiometric) arrangement of oxygen atoms is thought to play an important role. This arrangement introduces oxygen vacancies that are not stable in bulk form; by creating just enough conductive electrons, these oxygen vacancies may help induce superconductivity.
Yoshimatsu says that more work will be needed to examine the underlying mechanisms. As titanium oxides are cheap and relatively simple compounds made of only two kinds of elements, they are attractive for further research, he adds.
The study may also advance the development of Josephson junctions – quantum mechanical devices consisting of a sandwich of two superconducting layers, one above and one below an ultrathin non-superconducting layer. These junctions could be used to build new kinds of electronic circuits and, ultimately, faster computers.
This story is adapted from material from Tokyo Institute of Technology, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.