SLAC and Stanford scientists observed splashes of 2D superconducting behavior resulting from a 3D unconventional superconductor, which conducts electricity with 100% efficiency at extremely high temperatures. Their study suggests that this so-called ’emerging’ behavior may be how 3D superconductors reorganize themselves just before they suddenly transition to an insulating state, where electrons are confined to their atoms and cannot move at all. Credit: Greg Stewart / SLAC National Accelerator Laboratory
This is an example of how surprising properties can arise spontaneously in complex materials – a phenomenon that scientists hope to use for new technologies.
Creating a two-dimensional material, only a few atoms thick, is often a difficult process that requires advanced equipment. Scientists were therefore surprised to see 2D splashes emerge in a three-dimensional superconductor – a material that moves electrons with 100% efficiency and zero resistance – without asking.
Within the splashes, superconducting electrons acted as if they were in an incredibly thin, skin-like plane, a situation that required them to somehow transition to another dimension, where different rules of quantum physics apply.
“This is a tantalizing example of emerging behavior, which is often difficult or impossible to repeat by trying to construct it from scratch,” said Hari Manoharan, a professor at Stanford University and researcher at the Stanford Institute for Materials and Energy Sciences (SIMES), said. at the SLAC National Accelerator Laboratory of the Department of Energy, which led the research.
“It’s as if the power is given to superconductivity,” he said, “the 3D electrons themselves choose to live in a 2D world.”
The research team calls this new phenomenon ‘inter-dimensional superconductivity’, and in a report in the Proceedings of the National Academy of Sciences On April 12, 2021, they propose that this is how 3D superconductors reorganize themselves just before undergoing a sudden shift in an insulating state, where electrons are confined to their atoms and cannot move at all.
‘What we found was a system where electrons act unexpectedly. That’s the beauty of physics, ”said Carolina Parra, a postdoctoral researcher at SLAC and Stanford, when the experiments were performed that led to the visualization of this interesting result. “We were very lucky to find this behavior.”
Electrons that act strangely
Although superconductivity was discovered more than a century ago, its usefulness is limited by the fact that materials have only become conductive at temperatures close to those in deep space.
The announcement in 1986 that scientists had discovered a new and unexpected class of superconducting materials that were much higher – though still very cold – caused a tsunami of research to continue, with the aim of finding out how the new material used and develops versions that work closer to room temperature for applications such as perfectly efficient power lines and maglev trains.
This study started with a high temperature superconductor called BPBO for its four atomic constituents – barium, lead, bismuth and oxygen. It was synthesized in the laboratory of Stanford Professor and SIMES researcher Ian Fisher by Paula Giraldo-Gallo, then a PhD student.
As researchers have done this through routine tests, including determining the transition temperature at which it rotates between a superconducting and an insulating phase – such as water that turns into steam or ice – they have realized that their data show that electrons act as if they to ultra-thin limited, 2D layers or stripes within the material. This was a mystery because BPBO is a 3D superconductor whose electrons are normally free to move in any direction.
Manoharan’s team was intrigued up close with a scanning tunnel microscope, or STM – a tool that can identify and even move individual atoms in the top pair of atomic layers of a material.
Interaction places
The stripes, they discovered, apparently had nothing to do with the way the atoms of the material were organized, or with small bumps and droplets on the surface.
“Instead, the stripes were low where electrons act as if they were confined to 2D, puddle-like areas in the material,” Parra said. “The distance between pools is short enough for the electrons to ‘see’ and communicate with each other in a way that enables them to move without resistance, which is the hallmark of superconductivity.”
Carolina Parra (middle), who as Stanford postdoctoral fellow conducted the experiments that led to the visualization of these interesting results, now heads a laboratory at the Federico Santa María Technical University in Valparaíso, Chile, with the focus on interdisciplinary studies of biological materials at nanoscale. She recently received an award to acquire and operate the first low-temperature scanning tunnel microscope in South America, which she plans to use to continue this line of research. Credit: Photo courtesy of Carolina Parra
The 2D puddles appeared when the scientists carefully adjusted the temperature and other conditions to the transition point where the superconductor would become an insulator.
Their observations are consistent with a theory of ’emerging electronic graininess’ in superconductors, developed by Nandini Trivedi of Ohio State University and colleagues.
“The predictions we made were contrary to the standard paradigm for superconductors,” Trivedi said. “Usually, the stronger a superconductor is, the more energy it needs to break the bond between its superconducting electron pairs – a factor we call the energy gap. But my group predicted that in this particular type of disordered superconductor the opposite would apply: the system would form emerging ponds where superconductivity was strong, but the pairs could be broken with much less energy than expected.
“It was very exciting to see the predictions confirmed by the Stanford Group’s STM measurements!”
The spread of science
The results have practical implications for the production of 2D materials, Parra said.
“Most of the methods of making 2D materials are engineering approaches, such as growing films of a few atomic layers thick or creating a sharp interface between two materials and limiting a 2D state there,” he said. she said. ‘It provides an extra way to get to these 2D superconducting states. It’s cheaper, you do not need luxury equipment that requires very low temperatures and it does not last days and weeks. The only tricky part is to get the composition of the material just right. ”
Parra now heads a laboratory at the Federico Santa María Technical University in Valparaíso, Chile, focusing on interdisciplinary studies of biological materials on nanoscale. She recently received an award to acquire and operate the first low-temperature scanning tunnel microscope in South America, which she plans to use to continue this line of research.
‘If I had this equipment in the lab,’ she said, ‘I would connect it to all the things I learned in Hari’s lab, and use it to teach a new generation of researchers that we work in nanoscience and nanotechnology. in Chile. ”
Reference: April 12, 2021, Proceedings of the National Academy of Sciences.
DOI: 10.1073 / pnas.201781011
The research was funded by the DOE Office of Science.