Physicists discover a new way to use light to shape the atomic structure of materials

"This is very important for this class of materials," said Yijing Huang, a graduate student at Stanford University who played an important role in the experiments conducted at the U.S. Department of Energy's SLAC National Accelerator Laboratory. Because their functional properties are related to their structure. By changing the properties of the light you put in, you can tailor the properties of the materials you create." The experiment is X-ray free at SLAC. Electron laser, namely Linear Coherent Light Source (LCLS). These results were reported in Physical Review X on February 14, 2022 and will be highlighted in a special issue dedicated to ultrafast science.

　Heat vs. Light

Since thermoelectric technology converts waste heat into electricity, it is considered a green energy source . Thermoelectric generators powered the Apollo moon landings, and researchers have been studying how to use them to convert body heat into electricity to charge gadgets and other things. When run in reverse, they create a thermal gradient that can be used to chill wine in a refrigerator with no moving parts.

Tin selenide is considered one of the most promising thermoelectric materials. It grows as a single crystal and is relatively cheap and easy to manufacture. Huang said that unlike many other thermoelectric materials, tin selenide is lead-free and is a more efficient heat converter. Since it is composed of regular cubic crystals, similar to those of rock salt, it is also relatively easy to make and repair.

To explore how these crystals respond to light, the team irradiated tin selenide with intense pulses of near-infrared laser light to change its structure. The light excites electrons in the sample's atoms and moves some of them, distorting their arrangement.

The researchers then used X-ray laser pulses from the LCLS to track and measure the motion of these atoms and the resulting changes in the crystal structure, fast enough to capture what was happening over hundreds of seconds. Changes within a ten thousandth of a second.

"You need the ultrafast pulses and the atomic resolution that LCLS provides to reconstruct where the atoms are moving," said co-author David Reis, a professor at SLAC and Stanford University and director of the Stanford PULSE Institute. , we will get the story wrong.”

　A surprising result

This result is quite unexpected. Of course, the rest of the team had a hard time believing Huang when she told her what she'd seen in her experiments.

A tried and tested method of changing the atomic structure of tin selenide is to apply heat, which changes the material in a predictable way, essentially making this particular material perform better. Conventional wisdom is that applying lasers will produce essentially the same results as heating.

　“This is what we initially thought would happen,” said Mariano Trigo, a staff scientist at SLAC. “But after nearly two years of discussions, Yijing finally convinced the rest of the team that "No, we're pushing the material toward a completely different structure. I think this result goes against most people's intuition about what happens when you excite electrons to higher energy levels," said SLAC's Stanford Materials Associate Professor. Investigator at the Institute of Energy Sciences (SIMES).

Theoretical calculations by Shan Yang, a graduate student at Duke University, confirmed that this interpretation of the experimental data is correct. "This material and its class are certainly very interesting because it's a system where small changes can lead to very different results. But the ability to make entirely new structures with light -- structures that we don't know how to make any other way -- Probably more common than that."

In addition, Yang adds, one area where it could be useful is the decades-old effort to make superconductors -- materials that conduct electricity without loss -- that are getting close. An exploration of working at room temperature.

Original title: Physicists discover a new way to use light to shape the atomic structure of materials