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Strontium titanate was once used as a diamond substitute in jewelry before less fragile alternatives emerged in the 1970s. Now, researchers have explored some of its more unusual properties, which might someday be useful in quantum materials and microelectronics applications.

Writing in the journal Nature Communications, the team how they built an extremely thin, flexible strontium titanate membrane and stretched it, in the process turning on what's known as a ferroelectric state. In that state, the material generates its own electric field, somewhat similar to how a permanent magnet generates its own magnetic field.

"We applied strain to tune the membrane to a ferroelectric or non-ferroelectric state reversibly and repeatedly," said Wei-Sheng Lee, a lead scientist at the Department of Energy's SLAC National Accelerator Laboratory and a principal investigator at the Stanford Institute for Materials and Energy Sciences (SIMES), a joint SLAC-Stanford institute. "This allowed quantitative characterizations of this transition in strontium titanate with unprecedented details."

Stretching a material changes the distances between its atoms, which can alter its physical properties, including electrical ones. In the quantum material strontium titanate, this separates negatively charged oxygen and positively charged titanium ions in the material, creating an electric field and putting it in a ferroelectric state.

The ability to turn on ferroelectricity in this material—as well as superconductivity through the addition of impurities and its extensive employment in quantum material heterostructures—makes strontium titanate promising for applications in next generation computing, data storage and superconducting devices.

However, the nature of this ferroelectric transition is not well understood, so the team used X-rays to track the arrangement of ions and the electric field in strontium titanate as it was stretched into a ferroelectric state. Even then, they faced a challenge: Strontium titanate is a brittle crystal at room temperature—one of the reasons it didn't work out as a diamond substitute. In previous work, strontium titanate samples could only endure a limited amount of stretch before they snapped, which hindered their study.

Fortunately, a method developed in the lab of Harold Hwang, SIMES director and a professor at Stanford and SLAC, produces thin, flexible membranes of quantum materials. These membranes, which are only a few nanometers thick, can be peeled from the surface they were originally grown on and stretched without breaking. The research team took advantage of this technique to fabricate a stretchable strontium titanate membrane.

"Our goal was to try to implement these membranes in an X-ray setting and apply strain," said Yonghun Lee, a Ph.D. student at Stanford. Lee and SLAC postdoctoral fellow Jiarui Li developed an experimental protocol for transferring these membranes to pliable plastic sheets and attaching them to devices used to administer and measure strain at the Advanced Photon Source (APS) at Argonne National Laboratory. X-rays from the APS revealed how the membrane's electronic structure changed as it was stretched under a range of temperatures.

At temperatures closer to room temperature, the transition to a ferroelectric state in strontium titanate exhibits thermal fluctuations, the hallmark of a classical phase transition. But at cryogenic temperatures, over a couple hundred degrees below zero degrees Fahrenheit, the thermal fluctuations become negligible, suggesting the transition is shifting to quantum territory.

This quantum crossover may be the reason why strontium titanate doesn't become ferroelectric at cryogenic temperatures without being stretched. When a system enters the quantum regime, erratic switching between energetically similar states—known as quantum fluctuations—arises.

These quantum wiggles in electronic structure prevent strontium titanate from rearranging itself into ferroelectric order. Stretching the material suppresses the quantum fluctuations, allowing the material to become ferroelectric, but in a way that is different from the classical phase transition.

"Our results hint that these quantum fluctuations are playing a role at lower temperatures when the quantum effect is more predominant than the classical effect in the transition," Li said.

Next, the researchers will use this experimental protocol to study strained transitions in other quantum materials. A better understanding of this transition could help tailor strontium titanate and other quantum materials for different applications, such as microelectromechanical switches.