Strontium titanate, some secrets revealed

A team led by University of Minnesota Twin Cities researchers has discovered how subtle structural changes in strontium titanate, a metal oxide semiconductor, can alter the material's electrical resistance and affect its superconducting properties.

An oxide of strontium and titanium, strontium titanate or fabulite, has been on the radar of scientists for the past 60 years, mostly because it displays unusual properties. For one, it can become a superconductor at low temperatures and low concentrations of electrons, and secondly, its atomic structure changes at 110 degrees Kelvin (negative 262 degrees Fahrenheit).

However, researchers are still debating what exactly causes superconductivity in this material on the microscopic level or what happens when its structure changes.

The University of Minnesota team was able to shine some light on these questions.

Using a combination of materials synthesis, analysis, and theoretical modeling, the fellows found that the structural change within strontium titanate directly affects how electric current flows through the material.

Furthermore, they revealed how small changes in the concentrations of electrons in the material affect its superconductivity.

It was these observational insights that the team hopes will ultimately inform future research on this material, including research on its unique superconducting properties.

"The backbone of human life relies on the discovery of new properties in materials, and scientists and engineers can use those properties to make new devices and technology," said Bharat Jalan, lead author and associate professor at University of Minnesota Twin Cities department of chemical engineering and materials science. "What this study shows is a connection between superconductivity and the material structure in strontium titanate. But perhaps even more importantly, it shows that a collaborative approach is essential to tackling complex problems in science and engineering."

A key reason the team was able to make this discovery was the fact that they were able to synthesize a strontium titanate material that was extremely "clean," meaning it contained very few impurities.

To do this, they used a technique called hybrid molecular beam epitaxy (MBE), an approach that Jalan's lab pioneered.

Because the material was so clean, the researchers were able to make previously unseen observations in strontium titanate, and through theoretical modeling, they were able to connect the experimentally observed macroscopic properties with the microscopic behavior of the electrons.

"The observed response of the superconducting properties to small changes in the density of electrons provides new pieces in the ongoing puzzle of superconductivity of strontium titanate," said University of Minnesota School of Physics and Astronomy Professor and contributing author Rafael Fernandes.

The research was made possible by a collaboration between three University of Minnesota Twin Cities faculty members: Jalan, whose lab spearheaded the effort and handled the material synthesis and transport measurements; Fernandes, whose group performed the theoretical calculations; and School of Physics and Astronomy Associate Professor Vlad Pribiag, who specializes in the advanced measurement of properties in thin films.

"A lot of questions in modern science and engineering are so complex that they exceed a single discipline," Pribiag said. "Having these collaborations available within the same college is extremely useful. You need all of these ingredients to solve a lot of problems."

With another step taken toward the uses and mechanisms of strontium titanate, perhaps this recent discovery will help unlock the true potential of this beguiling material, ushering in a new direction for future technologies.