Revealing "hidden" phases of matter through the power of light

Most people think that water only exists in one of three phases: solid ice, liquid water or gas vapor. But matter can exist in many different phases – ice, for example, has more than ten known phases, or the means by which its atoms can be arranged in space. The widespread use of piezoelectric materials, such as microphones and ultrasound, is possible thanks to a fundamental understanding of how an external force, such as pressure, temperature, or electricity, can lead to phase transitions that give materials new properties.

A new study has revealed that a metal oxide has a "hidden" phase that gives the material new ferroelectric properties, the ability to separate positive and negative charges, when it is activated by extremely bright pulses fast. The research was conducted by MIT researchers Keith A. Nelson, Xian Li and Edoardo Baldini, in collaboration with Andrew M. Rappe and Penn's graduate students, Tian Qiu and Jiahao Zhang. The results were published in Science.

Their work opens the door to creating materials to enable and disable properties in a billion seconds by simply pressing a switch, with much better control. In addition to modifying the electrical potential, this approach could be used to modify other aspects of existing materials, for example transform an insulator into a metal or reverse its magnetic polarity.

"This opens a new horizon for the rapid reconfiguration of functional materials," Rappe said.

The group studied strontium titanate, a paraelectric material used in optical instruments, capacitors and resistors. Strontium titanate has a symmetrical, nonpolar crystalline structure that can be "pushed" into a phase with a tetragonal polar structure with a pair of ions charged in opposite directions along its major axis.

The previous collaboration between Nelson and Rappe provided the theoretical basis for this new study, which relies on Nelson's experiment to use light to induce phase transitions in solid materials, as well as on Rappe's knowledge of the development of computer models at the atomic level.

"[Nelson is] the experimenter, and we are the theorists, "says Rappe. He can report what he thinks is happening on the basis of spectra, but the interpretation is speculative until we provide a solid physical understanding of what has happened. "

With the recent technological improvements and additional knowledge gained from working with the terahertz frequencies, the two chemists have sought to verify whether their theory, which is more than ten years old, is still in the news. Rappe's challenge was to complement Nelson's experiments with a precise version of computer-generated strontium titanate, with each atom tracked and represented, which reacted to light in the same way as the material tested in the laboratory.

They found that when strontium titanate is excited by light, the ions are driven in different directions, with positively charged ions moving in one direction and negatively charged ions in the other. Then, instead of the ions immediately falling back into place, as would a pendulum after pushing it, the vibratory motions induced in the other atoms prevent the ions from coming back immediately.

It is as if the pendulum, at the moment when it reached the maximum height of its oscillation, was slightly deviated from its trajectory by a small cut keeping it in place far from its original position.

Thanks to their long tradition of collaboration, Nelson and Rappe were able to shuttle between theoretical simulations and experiments, and vice versa, until they found experimental evidence that their theory was still true. .

"The collaboration was really impressive," says Nelson. "And that illustrates how ideas can simmer and come back in strength after more than 10 years."

The two chemists will collaborate with engineers on future application-oriented research, such as creating new materials with hidden phases, modifying light pulse protocols to create more durable phases, and visualizing how this works. approach for nanomaterials. For now, both researchers are excited about their results and the potential results of this breakthrough.

"It is the dream of any scientist: to make an idea emerge with a friend, to map the consequences of this idea, then to have the opportunity to translate it into a laboratory, it is extremely gratifying. We are on the right track for the future, "Rappe said.