X-rays illuminate an exotic material transformation

A dry material makes a great fire starter, and a soft material lends itself to a sweater. Batteries require materials that can store lots of energy, and microchips need components that can turn the flow of electricity on and off.

Each material’s properties are a result of what’s happening internally. The structure of a material’s atomic scaffolding can take many forms and is often a complex combination of competing patterns. This atomic and electronic landscape determines how a material will interact with the rest of the world, including other materials, electric and magnetic fields, and light.

Scientists at the U.S. Department of Energy’s (DOE) Argonne National Laboratory, as part of a multi-institutional team of universities and national laboratories, are investigating a material with a highly unusual structure—one that changes dramatically when exposed to an ultrafast pulse of light from a laser.

A paper on the study was published in Nature Materials.

After the pulse, the material is caught in an exotic state outside of equilibrium, or stability. Called metastable, these states are an exciting and largely unexplored phenomenon in materials science, and they could find application in information storage and processing.

The team of scientists created the metastable state in 2019 and characterized the material before and after its transition. Using a combination of advanced X-ray and ultrafast laser capabilities, their recent experiments reveal the evolution of the material’s structure during the transition.

The researchers captured the entire process in detail across several orders of magnitude in time, ranging from the picosecond to microsecond scales (trillionths to millionths of a second).

In particular, the team is investigating metastability in a class of materials called ferroelectrics, which play an important role in sensing and memory applications. Understanding these transitions in ferroelectrics could eventually inform the design of materials for next-generation microelectronics.

Metastable states

“Most of the materials used in technology are in equilibrium—or their lowest energy state—so that a technology can work reliably without wild variations in performance,” said Venkatraman Gopalan, professor at Pennsylvania State University and an author on the study. “However, this is very restrictive, since amazing properties may lurk just beyond equilibrium.”

The challenge is that nonequilibrium states are generally short-lived. Metastable states, however, are nonequilibrium states that persist for a very long time. Diamond, for example, is a metastable state of carbon. We say they’re forever, but over the course of billions of years, diamonds decay into graphite, a more stable state of carbon.

“It’s sort of like throwing a ball up a cliff, and instead of it returning to the ground, the ball gets stuck on a ledge on the cliff wall,” Gopalan said. If the pathway to the ground is blocked by the ledge, the ball will rest there in a metastable state.

The scientists created the starting phase in this experiment by combining alternating layers of two materials—a ferroelectric and a nonferroelectric.

The configurations of the electrons within the different layers compete with each other, resulting in a swirling pattern of vortices in the electronic structure across the material. This internal frustration blocks pathways that the material might otherwise take to return to equilibrium after being excited by the laser pulse.

In other words, the competing phases create the “ledges on the cliff” that allow the material to access and remain in states beyond equilibrium.

The experiments

To induce the transformation, the scientists exposed their layered material to laser pulses less than 100 femtoseconds in duration.

“That’s very, very fast,” said Argonne Physicist Haidan Wen. “The difference between one second and one femtosecond is comparable to the difference between 30,000 years and the blink of an eye.”

To detect the evolution of the material during the transition, the team used two X-ray free-electron lasers: the Linac Coherent Light Source (LCLS), a DOE Office of Science user facility at the DOE’s SLAC National Accelerator Laboratory, and the SPring-8 Angstrom Compact free electron Laser (known as SACLA) in Japan.

These cutting-edge instruments allow scientists to probe states of matter at unprecedentedly small length and time scales. That’s because they produce ultrafast X-ray pulses with extremely high brightness, which act like a camera for capturing atomic motion.

The team conducted what are called single-shot pump-probe experiments, where they pump (or excite) a portion of the material with a laser pulse and probe the process with rapid flashes of X-rays, which take snapshots of the material’s evolution. They performed thousands of these experiments, moving around to different locations on the sample to excite them into the metastable states and record their transitions.

The data generated by the X-rays captures the movement of different features and structures in the material. To ensure these features were tracked as closely and accurately as possible, the scientists also used beamlines 33-ID-D and 7-ID-C at Argonne’s Advanced Photon Source (APS) to create highly detailed three-dimensional maps of the sample before and after transition. The APS is a DOE Office of Science user facility.

From soup to supercrystal

When photons, or light particles, from the laser pulse hit the atoms in the layered material, a slew of electrons emerge, freed by this newfound energy. Called photocarriers, these free charges are what enable the system’s transformation.

At the Center for Nanoscale Materials, another DOE Office of Science user facility at Argonne, the scientists used a technique called transient absorption spectroscopy to detect photocarrier activity within the material. This approach helped them determine how much charge gets released and how quickly the charge decays.

“This study involved a really nice combination of … capabilities,” Wen said. “Together, these complementary facilities are accelerating our understanding of metastable state creation.”

Within a trillionth of a second after the laser pulse, the excitation of the photocarriers causes the sample to enter what the researchers call the soup phase. “The order is sort of melting at this point,” Wen said. The original pattern of vortices starts to weaken, giving way to a hot and charged chaotic slush.

About a billionth of a second later, the soup begins to cool and the final structure starts to form, similar to how sugar crystals can form out of a sugar solution. The final state is an even more ordered structure called a supercrystal, a crystal made of many smaller crystals.

“The vortices still exist in the final state, but they’re twisted up in a very different way,” said John Freeland, a physicist at Argonne and author on the study. “What’s unexpected is that the system ends up more ordered than when you started, which is not common in these experiments.”

The team’s findings will help validate computational models of beyond-equilibrium states. Better understanding of the formation and behavior of metastable states could lead to the invention of new materials and devices with impressive capabilities down the line.

For example, metastable phase transitions can result in unusual electronic landscapes within materials. Using these extraordinary states to represent information in new and complex ways might one day help improve efficiency in information storage and processing.

“Length scales for microelectronics are reaching a certain limit,” said Wen. “There’s an urgent need to search for new building blocks to process information faster and represent it with higher density.”

Most immediately, the results prompt more research into the role of the soup phase and internal frustration in metastable phase transitions.

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