Insight into Obscure Transition Uncovered by X-rays
August 14, 2015 | Argonne National LaboratoryEstimated reading time: 4 minutes
The list of potential mechanisms that underlie an unusual metal-insulator transition has been narrowed by a team of scientists using a combination of X-ray techniques. This transition has ramifications for material design for electronics and sensors.
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.
This research was supported by grants from the U.S. Department of Energy and Department of Defense, and the University of Illinois at Chicago and Argonne. The use of the APS was supported by the DOE. The work was done at the following APS beamlines: 4-ID-D, 6-ID-B, and the Sector 27 RIXS beamline, which recently incorporated 9-ID-B.
About Argonne National Laboratory
Argonne National Laboratory seeks solutions to pressing national problems in science and technology. The nation's first national laboratory, Argonne conducts leading-edge basic and applied scientific research in virtually every scientific discipline. Argonne researchers work closely with researchers from hundreds of companies, universities, and federal, state and municipal agencies to help them solve their specific problems, advance America's scientific leadership and prepare the nation for a better future. With employees from more than 60 nations, Argonne is managed by UChicago Argonne, LLC for the U.S. Department of Energy's Office of Science.
Suggested Items
Big Win for Defense Production Act Budget Allocation in FY24 Budget
04/23/2024 | I-Connect007 Editorial TeamOne year ago, President Biden issued a determination that chips and packaging are critical for national security. Since that time, much work has been done to continue the conversation in Washington, elevating the importance of the entire chips value chain, and including printed circuit boards and substrates, without which chips cannot operate.
IDTechEx Examines the Opportunities for Wearables in Digital Health
04/19/2024 | IDTechExIDTechEx’s report, “Digital Health and Artificial Intelligence 2024-2034: Trends, Opportunities, and Outlook”, covers this ongoing trend in the consumer health wearables market and includes analysis of the opportunities and roadmap for biometric monitoring.
NCAB Appoints Tim Benjamin as New CFO
04/15/2024 | NCAB GroupNCAB Group has appointed Tim Benjamin as its new CFO. Tim has a long and broad experience of managerial business finance roles, including M&A and IT.
Plastronics and the New IPC Guidelines for In-mold Electronics (IME)
04/11/2024 | Barry Matties, I-Connect007In the ever-evolving world of electronics, the roads for the integration of electronics assemblies into 3D structures continue to grow into a set of technologies used to produce 3D plastronics parts and revolutionize mainly the automotive industry. IPC has been developing standards for this set of technologies, led by Francisco Fourcade, IPC electronics technology standards manager.
IEC USA Congratulates Bernie Consitt for 25 Years of Service
04/09/2024 | IECJoin us in celebrating Bernie Consitt's 25 years of service and camaraderie at IEC (International Electronic Components). Bernie has been instrumental in his role as Accounting Manager, helping to supervise, track, and evaluate day-to-day activities within the accounting team.