Insights into obscure transition uncovered by X-rays

The transition between being electrically conductive (metallic) at high temperatures and electrically insulating at lower temperatures is known as a metal-insulator transition (MIT). Pinpointing the activation mechanism that allows crystals used in devices such as transistors in electronics and temperature-based sensor control systems used in manufacturing to change electrical state is key to developing new devices that are smaller and more efficient than those in use today.

For example, transistors, and most electronics, function by tuning conductivity, which is essentially using the level of electrical resistance as an on-off switch. Designing new electronic devices has been largely driven by trial and error. Understanding what causes large changes in electrical conductivity, as in an MIT, can allow us to design new materials that are cheaper or have higher-performance properties.

"If we understand how the transition occurs, we can exploit that knowledge to design new materials to customize transistor properties, such as selecting levels of conductivity to make transistors more efficient or to make sensors operate in customized ranges," said Mary Upton, a scientist at the Advanced Photon Source (APS), a U.S. Department of Energy User Facility at Argonne National Laboratory.

The team of researchers from Argonne and Lawrence Berkeley national laboratories and the University of Arkansas made inroads in understanding this transition by using the APS to study rare-earth crystal family perovskites, including the rare-earth atom compounds nickelates. Nickelates are compounds that contain a central nickel atom bonded to oxygen or oxygen-containing groups and are considered an ideal model for the study of this transition.

The team studied thin films of neodymium nickel oxide (NdNiO3), a nickelate, using three different beamlines at APS, which allowed an in-depth exploration of the samples. Nickelates are considered an ideal model for studying the transition because they display strongly correlated electronic behavior that gives rise to unique electronic and magnetic properties.

Many different theories exist to explain the mechanics that drive metal-insular transition. The team was able to rule out those theories based on the charge order of the particles in the material, including the widely held theory that an electronic checkerboard pattern, which has been observed in bulk, triggers the transition. This checkerboard pattern is also called charge order and charge disproportionation and is observed or not observed by using the X-ray analysis technique of resonant diffraction. The results were published in July in the journal Physical Review Letters in a paper titled "Novel electronic behavior facilitating the NdNiO3 metal-insulator transition."

"APS beamlines provide the high photon flux and energy that are critical when dealing with subtle electronic effects," said Upton, lead author on the paper. "State-of-the-art optics and collaboration between beamlines allows unparalleled detail in the study of materials. Measurements from Resonant X-ray Diffraction, X-ray Absorption Spectroscopy and Resonant Inelastic X-ray Scattering (RIXS) combined to draw a new picture of the material. The application of the RIXS technique to a long-standing problem was made possible by improved capabilities in thin film measurements at the beamline."

To study the transition, chemically identical film samples were grown with small structural distortions induced by epitaxial strain induced when growing a single crystal film on top of a crystalline substrate. Slight difference in lattice constants brought about substantial changes in electronic behavior. The effect of strain has been known for years, but never explained. A film with a tensile distortion, where all the atoms are more distant from each other, exhibits a state transition. A film grown with a slightly compressive distortion, where all atoms are closer together than in bulk, is electrically conductive at all temperatures. Neither film, however, exhibited an electronic checkerboard so it cannot be a pre-requisite for a MIT.

The measurements also suggest that tensile strain facilitates the transfer of electrons between two elementally different atoms. This observation was a surprise because the atoms in question had been assumed to be isolated from each other. These results strongly suggest a need to re-examine other, similar state transitions in perovskites.

"The state transition is neither what is called a pure Mott-Hubbard transition, despite electron localization, nor a simple charge-transfer transition," said Philip Ryan, a scientist working at the APS and co-author on the paper.

This new insight into the state transition in nickelates will help guide the design of new electronic devices.