Multiple Ground States Discovered in Ferroelectric Materials

An important family of ferroelectric compounds just became more complicated in a good way, thanks to University of Arkansas physicists and their colleagues.

A paper published in the Feb. 9, 2011, journal Nature shows that ferroelectric materials used in cell phones, computer memories, medical ultrasound and naval sonar may exhibit new properties as a result of so-called "geometric frustration."

"Geometric frustration gives rise to new kinds of physics," said Narayani Choudhury, a research scientist in physics working at the University of Arkansas. "It opens up a new area of research in ferroelectrics."

Narayani Choudhury, Laurent Bellaiche, professor of physics at the University of Arkansas, Laura Walizer at the Engineer Research and Development Center in Vicksburg, Mississippi, and Sergey Lisenkov in the physics department at the University of South Florida employed first-principles-based Monte Carlo simulations to understand how properties change when they subjected barium-strontium titanate compositionally-graded systems to strain.

"Epitaxial strain arising from lattice mismatch with substrate is known to give rise to novel material behaviors desirable for applications in microelectronic devices. Such studies are thus extremely important," said Choudhury.

Like couch potatoes, ferroelectrics form domain states that like to exist in the lowest energy state possible. The researchers found that the domains in these strained materials are radically different from conventional ferroelectrics. As a matter of fact, barium-strontium titanate compositionally-graded systems have multiple ground states and several domain configurations were found to have similar low energy in these materials.

Careful visualizations revealed that microstructures of these domain configurations differed greatly from one another, and some were far more complex than the others. The researchers found a correlation between a critical exponent and the complexity of the microstructures, which allowed them to measure the amount of geometric frustration taking place. The electric dipoles self-organized into complex structures and their cooperative bending give rise to curvatures, spiral states, defects and herringbone patterns in the domain microstructures.

"All these features like ground state degeneracies, exotic microstructures, bending of dipolar lines, spiral states and critical behaviors, which have been predicted to occur in compositionally graded ferroelectrics are fingerprints of the fundamental phenomenon known as geometric frustration," said Choudhury. "We find that curvature is one driving mechanism of geometric frustration in these systems." By studying the relative energy contributions from various terms, they could provide a microscopic understanding of the origins of geometric frustration.

Scientists have previously documented geometric frustration in magnetic materials, but the phenomenon has not been examined in detail in ferroelectric materials. In magnetic materials, geometric frustration has led to the discovery of new kinds of systems, including spin ice and spin liquids. These systems behave very differently from regular materials. The researchers believe that the examination of geometric frustration in ferroelectric materials, which has not been looked at before in great length, may generate a better understanding of many important materials, including high temperature superconductors, glasses and neural networks.

The researchers used the supercomputers at the Arkansas High Performance Computing Center and the Center for Piezoelectrics by Design, College of William and Mary, Va., to perform the complex calculations, which took more than 15 months to complete. Now that Choudhury and her colleagues have predicted that geometric frustration can occur in these compounds, the next step will be to synthesize these materials using molecular beam epitaxy or pulse laser deposition to create the compounds and examine their properties via experiments.

"The box has just been opened, and there are goodies inside," Choudhury said.This work was supported by the National Science Foundation, the Office of Naval Research and the Department of Energy.


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