For the first time, scientists see 2D pools of electrons emerge spontaneously in a 3D superconducting material

Making a two-dimensional material just a few atoms thick is often a laborious process requiring sophisticated equipment. So scientists were surprised to see 2D puddles appear in a three-dimensional superconductor – a material that allows electrons to travel with 100% efficiency and zero resistance – without the need for anything.

Within those pools, superconducting electrons acted as if they were trapped in an incredibly thin, sheet-like plane, a situation where they somehow had to cross over into another dimension, where different rules of quantum physics apply.

“This is a tantalizing example of emergent behavior, which is often difficult or impossible to replicate by developing it from scratch,” said Hari Manoharan, a Stanford University professor and researcher at the Stanford Institute for Materials and Energy Sciences (SIMES) at the Department of Energy’s SLAC National Accelerator Laboratory, which led the study.

“It’s like when they get the power to be superconducting,” he said, “the 3D electrons choose to live in a 2D world.”

The research team calls this new phenomenon “interdimensional superconductivity,” and in a report in the US Proceedings of the National Academy of Sciences today they suggest this is how 3D superconductors reorganize themselves just before undergoing an abrupt shift to an insulating state, where electrons are confined to their home atoms and cannot move at all.

“What we discovered was a system in which electrons behave in unexpected ways. That’s the beauty of physics,” said Carolina Parra, a postdoctoral researcher at SLAC and Stanford at the time of the study, who conducted the experiments that led to the visualization. of this. intriguing result. “We were lucky enough to discover this behavior.”

Electrons behave strangely

Although superconductivity was discovered over a century ago, its usefulness was limited by the fact that materials became superconducting only at temperatures close to those in deep space.

So the announcement in 1986 that scientists had discovered a new and unexpected class of superconducting materials operating at much higher – though still very cold – temperatures triggered a tsunami of research that continues to this day, with the aim of investigating. explore how the new materials work and develop versions that work closer to room temperature for applications such as perfectly efficient power lines and magnetic trains.

This study started with a high-temperature superconductor called BPBO because of its four atomic ingredients: barium, lead, bismuth and oxygen. It was synthesized in the laboratory of Stanford Professor and SIMES researcher Ian Fisher by Paula Giraldo-Gallo, a Ph.D. student at that time.

While researchers there routinely tested it there, including determining the transition temperature at which it switches between a superconducting and an insulating phase – such as water turning into steam or ice – they realized that their data showed that electrons behaved as if they were limited to ultra-thin, 2D layers or stripes in the material. This was a puzzle, because BPBO is a 3D superconductor whose electrons are normally free to move in any direction.

Intrigued, Manoharan’s team took a look at a scanning tunneling microscope, or STM – an instrument that can identify and even move individual atoms in the top few atomic layers of a material.

Interacting puddles

The stripes, they found, did not appear to be related to the way the material’s atoms were organized or to small bumps and dimples on the surface.

“Instead, the streaks were layers where electrons behave as if they were confined to 2D, pool-like regions in the material,” Parra said. “The distance between the pools is so small that the electrons can ‘see’ each other and interact with each other in a way that allows them to move without resistance, which is the hallmark of superconductivity.”

The 2D pools emerged as the scientists carefully adjusted the temperature and other conditions toward the transition point where the superconductor would become an insulator.

Their observations are closely related to a theory of emergent electronic granularity in superconductors developed by Nandini Trivedi of Ohio State University and colleagues.

“The predictions we made went against the standard superconductor paradigm,” said Trivedi. “Usually, the stronger a superconductor is, the more energy it takes to break the bond between the superconducting electron pairs – a factor we call the energy gap. True: The system would form emerging pools where the superconductivity was strong, but the pairs could be broken with much less energy than expected.

“It was quite exciting to see those predictions confirmed by the Stanford group’s STM measurements!”

Spreading science

The results have practical implications for creating 2D materials, Parra said.

“Most of the methods for making 2D materials are technical approaches, such as growing films a few atomic layers thick or creating a sharp interface between two materials and limiting a 2D state there,” she said. “This provides an additional way to get into these 2D superconducting states. It’s cheaper, you don’t need expensive equipment that requires very low temperatures, and it doesn’t take days and weeks. The only tricky thing is to change the composition of the material. exactly right. “

Parra now heads a laboratory at the Federico Santa María Technical University in Valparaíso, Chile, with a focus on interdisciplinary studies of nanoscale biological materials. She recently won a grant to acquire and operate the very first low-temperature scanning tunneling microscope in South America, which she plans to use to continue this line of research.

“When I have this equipment in the lab,” she said, “I’ll connect it to everything I’ve learned in Hari’s lab and use it to teach a new generation of researchers that we’re going to work in nanoscience and nanotechnology. in Chile. ”