Weird Energy: Physicists Uncover Unusual Properties In Ferroelectric Superlattices

FAYETTEVILLE, Ark. — University of Arkansas researchers have used computer models to study the properties of a potential material for use in ultrasound and sonar and found that it exhibits some unusual properties that might be used to enhance current technologies.

Research associate Igor Kornev and associate professor of physics Laurent Bellaiche report their findings in an upcoming issue of Physical Review Letters.

"We can design new materials based on our predictions," Kornev said.

Kornev and Bellaiche study ferroelectric materials, which convert small changes in mechanical energy into electrical energy, called a piezoelectric response, and are used in military sonar and medical ultrasound. Using a computer model first described in 2000, the researchers looked at the hypothetical ferroelectric superlattices made of different layers of lead zirconate titanate (PZT) with different titanium compositions.

More precisely, they studied superlattices consisting of between one and six layers of PZT with a titanium composition of 44 percent alternating with layers of PZT with a composition of 52 percent titanium. The average titanium composition was equal to 48 percent, which is exactly the composition for which the corresponding disordered PZT exhibits a huge piezoelectric response, making it possible for a small electrical pulse to produce a large change in shape. Kornev and Bellaiche wanted to see how the properties of this particular PZT compound are affected when growing it in superlattice form instead of in its usual disordered form.

They found that the superlattices produced unique features. For instance, when they put down six layers of each, they found that the superlattice exhibited a new phase transition never previously seen in any piezoelectric material. This transition created an even larger piezoelectric response than the regular phase transition occurring in the disordered material.

In addition, they found that the superlattice can be "stuck" in a higher energy state than than the ground state, which implies that the commonly used and assumed statistical laws are not always valid.

"These superlattices are thus important not only for technological applications but also from a fundamental physics point of view," Kornev said.

Predicting the properties of novel compounds using computer models is a must to determine what materials might be of interest to efficiently guide the discovery of compounds with new or phenomenal properties.

"In our department, we are able to grow such systems," using molecular beam epitaxy, Kornev said. The University of Arkansas is one of the few places in the world where design and realization of new materials can be accomplished side by side.