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



Most people think that water exists only in one of three phases: solid ice, liquid water or gas vapor. However, matter can exist in many different phases ̵

1; for example, ice has more than ten known phases or ways in which its atoms can be spatially arranged. The widespread use of piezoelectric materials such as microphones and ultrasound is possible because of the fundamental knowledge about how an external force such as pressure, temperature or electricity can lead to phase transitions that fulfill materials with new properties.

A recent study 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 activated by extremely fast light pulses. The research was led by MIT researchers Keith A. Nelson, Xian Li and Edoardo Baldini in collaboration with Andrew M. Rappe and Penn graduates Tian Qiu and Jiahao Zhang. The results were published in Science .

Their work opens the door to creating materials that allow you to turn features on and off in a trillionth of a second with a touch of a button, now with much better control. This approach can be used not only to change the electrical potential, but also to change other aspects of existing materials, e.g. As the conversion of an insulator into a metal or the reversal of its magnetic polarity.

says Rappe.

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

The theoretical basis for this was provided by the earlier collaboration of Nelson and Rappe's new study, which relied on Nelson's experience with light to induce phase transitions in solid materials and on Rape's knowledge in the development of computer models at the atomic level.

"[Nelson is] The experimentalist, and we are the theorists," says Rappe. "He can tell from spectra what he thinks is happening, but the interpretation is speculative until we have an accurate physical understanding of what happened." The chemists wanted to find out if their more than a decade old theory was correct. Rape's challenge was to supplement Nelson's experiments with a precise computer-generated version of strontium titanate, in which each atom reacts to light in the same way as the material tested in the laboratory.

Strontium titanate is excited by light, the ions are pulled in different directions, with positively charged ions moving in one direction and negatively charged ions in the other direction. Instead of the ions immediately falling back into place as a pendulum would do after being pushed, the vibrational motions induced in the other atoms prevent the ions from swinging back immediately.

It is as if the pendulum at the moment it reaches the maximum height of its vibration, is easily kept off course, where a small notch keeps it from its original position.

Thanks to their many years of collaboration, Nelson and Rappe were able to go back and forth from the theoretical simulations to the experiments, and vice versa, until they found experimental evidence that showed their theory was true.

"It was a really great collaboration," says Nelson. "And it shows how ideas can simmer after more than 10 years and then return in full."

The two chemists will work with engineers on future application-oriented research, such as the development of new hidden-phase materials and changing light-level Pulse protocols to create longer-lasting phases and see how this approach works for nanomaterials. For the time being, both researchers are curious about their results and where this fundamental breakthrough could lead in the future.

"It is the dream of any scientist to work with a friend to come up with an idea to pinpoint the consequences of this idea." Then, having the opportunity to translate it into something in the lab is extremely gratifying. We believe we are on the right path to the future, "says Rappe.


Intelligent materials used in ultrasound behave in a similar way to water, chemists report


Further information:
Xian Li et al., Terahertz field-induced ferroelectricity in quantum paraelectric SrTiO3, Science (2019). DOI: 10.1126 / science.aaw4913

Provided by
University of Pennsylvania




Quote :
Uncover "Hidden" Phases of Matter Through the Force of Light (2019, June 14)
retrieved on June 14, 2019
from https://phys.org/news/2019-06-revealing-hidden-phases-power.html

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