Materials Retain Shape but Change Properties Under Strain
Finding opens path for new materials and devices

Thursday, September 01, 2011
FAYETTEVILLE, Ark. – A University of Arkansas physicist and his colleagues have found that ultra-thin films of superconductors and related materials don’t lose their fundamental properties when built under strain when built as atomically thin layers, an important step towards achieving artificially designed room temperature superconductivity. This ability will allow researchers to create new types of materials and properties and enable exotic electronic phases in ultra-thin films.

Jak Chakhalian, University of Arkansas professor of physics, and his colleagues reported their findings in Physical Review Letters.

Room temperature superconductivity would change the world’s economy, Chakhalian contends. To start, superconductors can carry electricity without losing energy to heat during transmission the way all of today’s materials do. Today’s power grid loses almost 15 percent of its energy to heat. That may not seem like a high number, but it translates into a multi-billion dollar loss. Scientists have looked at many solutions to increase energy efficiency, but Chakhalian seeks radical energy solutions, like a material that acts as a room-temperature superconductor.

“With a superconductor, you could redistribute energy around the globe with zero loss,” he said.

Room-temperature superconductivity remains a dream, but the findings of Chakhalian’s team may bring it closer to reality. Scientists have known for years that putting together two simple metals, semiconducting or ferroelectric materials of different sizes causes a strain that makes those materials stretch or compress to adjust the positions of atoms to match each other, often introducing defects and making them lose their ability to conduct electricity. This basic principle has been routinely applied to microelectronics devices used in everything from cell phones to computers to solar cells. Until recently, many researchers believed the same principle applied to high temperature superconducting and other exotic electronic materials at the nanoscale. They believed that combining these materials under strain also would modify their metallic and superconducting properties and may turn into insulators.

Chakhalian and his colleagues, however, showed that in most cases, these previous beliefs were wrong.

“To our surprise, we found that with ultra-thin films of high-temperature superconductors and similar correlated electron oxides, you can perfectly match nanofilms to substrates without noticeable compressing or stretching the materials,” Chakhalian said. He and his colleagues conducted experiments at the synchrotron at Argonne National Laboratory on ultra-thin films of just a few atomic layers thick. With a technique Chakhalian perfected in past work published in Science magazine in 2007, scientists can “see” the stretching and compressing that takes place in most materials. However, with the novel electronic materials, the result markedly differed from past experience – the atoms “fit” perfectly and the nano-layers retained their shape.

“You can’t assume that nature behaves identically with different materials,” Chakhalian said.

Additionally, theorist James Rondinelli at Drexel University, conducted complex super-computer based calculations to determine why the material retains its atomic shape and unique properties. They found that, instead of stretching or compressing, the chemical bonds prefer to rotate to accommodate the strain.

“This opens the door to another whole class of materials and novel magnetic and superconducting phases,” Chakhalian said. Moreover, they discovered that the atomic level strain accommodation creates dramatically different properties depending on the direction of strain, in other words, whether the film is stretched or compressed.

“This gives us another degree of freedom,” Chakhalian said. “This is completely not symmetric, contrary to what everyone has anticipated for decades. And in nanofilms it may end up with absolutely different physics from the bulk crystals.”

This happens because the electrons in high-temperature superconducting and similar materials are keenly aware of one another. This awareness causes them to repel one another to the extreme, so compressing and stretching is not energetically easiest way. Instead, rotation of structural units works best.

“Nature is lazy. It does not want to expend energy,” Chakhalian said. “Now we can use these new-found properties as the foundation of the next generation of ultra-thin film technology with yet unknown functionalities.”

Chakhalian is the Charles and Clydene Scharlau Professor of Physics in the J. William Fulbright College of Arts and Sciences.