Study Illuminates ‘Hidden’ Phases of Matter

Researchers from the Massachusetts Institute of Technology (MIT) and the University of Pennsylvania (Penn) demonstrated a metal oxide with a “hidden” ferroelectric phase, which can be activated with extremely fast pulses of light.

The work opens the door to creating materials in which one can turn on and off properties in a trillionth of a second with the flick of a switch.

A new study reveals a 'hidden' phase of strontium titanate. On the left, extremely fast pulses of light excite atoms within the crystal structure (red arrows), which shifts the material into a new, ferroelectric phase. Vibrations of other atoms then work to stabilize the hidden phase (right panels). Courtesy of Felice Macera.

The group, led by MIT researchers Keith A. Nelson, Xian Li, and Edoardo Baldini, in collaboration with MIT's Andrew Rappe and Penn graduate students Tian Qiu and Jiahao Zhang, studied strontium titanate, a paraelectric material used in optical instruments, capacitors, and resistors. The material has a symmetric and nonpolar crystal structure that can be “pushed” into a phase with a polar, tetragonal structure with a pair of oppositely charged ions along its long axis.

Previous collaborations by MIT researchers Nelson and Rappe paved the way for this study, which relied on Nelson’s experience using light to induce phase transitions in solid materials, along with Rappe’s knowledge in developing atomic-level computer models.

“[Nelson is] the experimentalist, and we’re the theorists,” said Rappe. “He can report what he thinks is happening based on spectra, but the interpretation is speculative until we provide strong physical understanding of what happened.”

Armed with recent improvements in technology and additional knowledge gained from working with terahertz frequencies, the two chemists set out to see if their theory, now more than a decade old, held true. Rappe’s challenge was to re-create Nelson’s experiments with a computer simulation of strontium titanate, with each atom tracked and represented, that responds to light in the same manner as the real thing.

The team found that when strontium titanate is excited with light, the ions are pulled in different directions, with positively charged ions moving in one direction, and negatively charged ions moving in the other. Then, instead of the ions immediately falling back into a place, the way a pendulum would after its been pushed, vibrational movements induced in the other atoms prevented the ions from swinging back immediately.

It was as if the pendulum, at the moment it reached the maximum height of its oscillations, was diverted slightly off course to where a small notch holds it in place away from its initial position.

“It’s been a really awesome collaboration,” Nelson said. “And it illustrates how ideas can simmer and then return in full force after more than 10 years.”

The two chemists will collaborate with engineers on future applications-driven research, such as creating new materials with hidden phases and changing light-pulse protocols to create longer-lasting phases, and seeing how the approach works for nanomaterials. For now, the researchers are excited about their results and where the breakthrough could lead.

“It’s the dream of every scientist: to hatch an idea with a friend, to map out the consequence of that idea, then to have a chance to translate it into something in the lab,” Rappe said. "It’s extremely gratifying. It makes us think we’re on the right track towards the future."