Breathing Stars and the Most Beautiful Scalpel

Look at any material on an atomic level and you see a dynamic world of interconnected atoms and electrons. Negatively-charged electrons throughout the material swarm around the positively-charged ions, and the electrostatic force between them holds the material together.

At a nonzero temperature, the ions in the material vibrate around their equilibrium positions. Those collective vibrations are called phonons. As the ions move, the electron cloud—as well as its quantum properties—sways accordingly, and vice versa. We call this electron-phonon coupling.

For many quantum materials, the electronic properties, such as whether it will conduct electricity or not, depends on how phonons and electrons are coupled—in other words, how they interact with each other. Understanding those interactions—and manipulating them—is crucial to understanding the world around us.

“How the electrons talk to phonons is a very fundamental physical problem. It determines the properties of many materials, including superconductivity,” said Xun Shi, a postdoctoral researcher in the Kapteyn-Murnane Group at JILA. “People always want to learn how electrons connect to phonons, and it’s a challenge to measure or calculate.”

To study those interactions, you need a very fine scalpel to peel through the jumble of phonons and electrons in a material, and isolate the important ones that determine how the material behaves. And Xun Shi, Yingchao Zhang and Wenjing You at the Kapteyn-Murnane Group have found that scalpel.

In a study published on April 2, 2020 in the Proceedings of the National Academy of Sciences, the team found that by using ultrafast laser pulses, they can precisely pinpoint how electrons and phonons interact, transforming nearly 50 years of theory and understanding.

“We had simple ways of understanding materials since the 1970s, and now we can see that the charges and the lattice are coupled in very intriguing ways,” said JILA Fellow Margaret Murnane. “The ultrafast laser had previously been seen as a hammer, but it’s actually the most beautiful scalpel.”

The tangled phonon-electron web

Materials have a jumble of phonons—vibrations of different periods and wavelengths. And when it comes to understanding the properties of a material—say whether an exotic material will act as an insulator or a conductor—not all of those phonons matter, Murnane pointed out.

Isolating the right electron-phonon couplings has been tricky to date. Think of it this way, Shi said: if we heat something up, everything inside the material goes from low temperature to high temperature. All of the phonons and electrons heat up at the same rate, at the same time. You can’t distinguish the ones that are important from the spectators.

For these investigations, ultrafast lasers were usually seen as big hammers, Murnane added—a big burst of energy which violently excites all the electrons, ions, and phonons in a material. Physicists thought ultrafast lasers were great for creating out-of-equilibrium physics, but not for delicately manipulating individual electron-phonon couplings.

But the Kapteyn-Murnane Group found exactly the opposite.

“This is a dream that people have been struggling with for a long time. It turns out that at a high-level, ultrafast pulses were always a scalpel,” Murnane said. “We just could not see how they were changing and manipulating the material.”

Breathing Stars of David and the beautiful scalpel

Shi, Zhang, and You looked at tantalum diselenide, where the ions are held together in six-pointed, Star of David-shaped formations. It’s a very unique material, and makes sorting the important phonons from the unimportant ones easier. That’s where the ultrafast lasers come in.

“If we use an ultrafast laser pulse, we can selectively excite electrons, not the atoms,” Shi said.

Rather than smashing energy into the entire electron-ion web, ultrafast lasers put energy into just the electrons, he explained. The electrons are smaller and faster than the ions, and they spread out, moving away from the ions in the Star of David.

With the electrons spread out, the atom formation began expanding and contracting; the star starts to “breathe,” Murnane explained. When the star expands, it became more metallic, and when it shrinks, it became more insulating. As it breathes, Shi, You, and Zhang could see that the electron temperature oscillates—swings back and forth—which had never before been seen experimentally. Moreover, the electron-phonon coupling also oscillates.

By preciously tuning the laser power, they can manipulate the material to change it from an insulator to a metal, and finally into a new metastable state never observed previously, Murnane said. That metastable state lasts for a nanosecond—100 million times faster than you can blink—and that’s a long time in non-equilibrium physics, Shi pointed out, giving scientists the opportunity to study how they can control those electron-phonon interactions.

A gentle nudge means big changes

Finding this new transitional regime led the team to a new discovery about how electrons and phonons are coupled—and how they can manipulate the material.

As the star breathes, the temperature of the electrons oscillates at the same frequency as the atoms, Zhang explained, like a piston in a can of compressed gas.

“When you compress the piston, the gas in the piston will create a higher temperature and heat it up. When the piston expands it will go to a lower temperature,” Zhang explained. “The lattice is also like this.”

Theoretically, the stronger the laser pulse, the cooler the star would be when it expanded, Zhang went on. But in this new transition mode, they noticed something that made their jaws drop. With the pump laser power around a critical value, a gentle nudge from the laser power could change whether stars were hot when they expanded or cold. In other words, the electron temperature oscillation exhibits a 180-degree phase change relative to the breathing vibration, when the material enters the new transitional regime.

“That is not predicted by any theory,” Zhang said. “We are still looking for how to explain this.”

“Normally when you heat up electrons, they lose energy to the lattice - to the phonons - and it’s unidirectional or monotonic. You heat them up and then they cool down by heating up the lattice,” Murnane explained. “But in this exotic quantum material, they are so coupled that in this weird transition state, you can switch how the electrons talk to the phonons.”

Which means that scientists can control electron-phonon couplings in a material by changing the laser power.

“We still have a lot of work to do to control this interaction,” Shi said, but this transitional regime opens a world of possibilities. Changing a material’s properties could be useful in new technologies, especially ones that need to quickly change its conductivity or become superconductors.

The study is published in the Proceedings of the National Academies of Science, and is supported by the National Science Foundation Physics Frontiers Center grant, and a Gordon and Betty Moore Foundation EPiQS Award.

Written by Rebecca Jacobson