In a major advance toward next-generation electronics, scientists at the University of Minnesota Twin Cities have found a way to steer electrical currents in atom-thin metals using light, at room temperature.

By applying strain to ultra-thin layers of ruthenium dioxide grown on titanium dioxide, the researchers triggered directional, light-sensitive behavior in the metal. Their discovery, published in Science Advances, could reshape how future optical sensors, memory devices, and quantum systems are designed.

Using Strain to Break Symmetry in Metals

Metals typically conduct electricity in all directions equally, but this research flips that assumption on its head. The team engineered tiny layers of ruthenium dioxide (RuO2), only a few nanometers thick, to exhibit different electrical behavior depending on direction—an effect known as anisotropy. They achieved this by growing the metal films on a TiO2 (110) surface, introducing built-in strain that distorts atomic arrangements.

“We solved this problem by carefully designing ultra-thin metal layers that interact with light in new ways—something you don’t see in the thicker version of this material,” said Bharat Jalan, senior author and Shell Chair Professor of Chemical Engineering and Materials Science at the University of Minnesota.

Fast, Tunable, and Room Temperature

Using femtosecond laser pulses, the researchers measured how quickly excited electrons relaxed back to equilibrium. The speed of this process—called carrier relaxation—turned out to be direction-dependent, especially in films thicker than 4 nanometers. This directional control of ultrafast electronic behavior is rarely observed in metals, let alone at room temperature.

“This is the first time anyone has demonstrated tunable, directional ultrafast carrier relaxation in a metal at room temperature,” said Seunggyo Jeong, the study’s lead author and a postdoctoral researcher at the University of Minnesota. “It challenges long-held assumptions in condensed matter physics.”

Why It Matters for Future Tech

The ability to tune how electricity moves through a material with light—and to do it selectively in one direction—could change how we build:

  • Polarization-sensitive optical detectors
  • Low-power memory and logic devices
  • Quantum communication components
  • Ultrafast signal processors

“The findings provide deep insight into how subtle structural distortions—like strain relaxation—can reshape the electronic landscape of metals,” said Tony Low, a co-author and professor of electrical and computer engineering at the University of Minnesota.

The Physics Behind the Phenomenon

At the atomic scale, electrons in the RuO2 films occupy specific energy bands. When strain is applied asymmetrically—more in one direction than another—it shifts these bands and affects how electrons transition between them when exposed to light. The team used density functional theory (DFT), x-ray absorption spectroscopy (XAS), and optical ellipsometry to confirm that band nesting—a kind of electron crowding at specific energy levels—drives the directional behavior.

This effect only kicked in after the RuO2 films reached about 4 nanometers in thickness, suggesting a critical threshold where strain-induced relaxation starts to dominate. Beyond that point, carrier relaxation became increasingly anisotropic and persisted even as the material returned to room temperature equilibrium.

From Lab Films to Real Devices

Next steps include integrating these engineered oxide films into working devices. The researchers also plan to test similar phenomena in other complex oxides, which may offer even stronger or more tunable responses.

Alongside the University of Minnesota team, collaborators from the Gwangju Institute of Science and Technology, Sungkyunkwan University, and the University of Kentucky contributed to the study. Funding came from the U.S. Department of Energy, the Air Force Office of Scientific Research, and the University of Minnesota’s Materials Research Science and Engineering Center.

The study redefines what’s possible in metals—a class of materials long assumed to lack this kind of finesse. For now, it opens the door to more agile, energy-efficient devices that respond to light like never before.

Journal Reference

Journal: Science Advances
DOI: 10.1126/sciadv.adw7125
Title: Anisotropic strain relaxation-induced directional ultrafast carrier dynamics in RuO2 films
Publication Date: June 27, 2025