The idea of hidden worlds ruled by odd laws of physics sounds like something out of science fiction. Recently, however, scientists observed a hidden, flattened world within a real material built to perfectly conduct electricity. In the Proceedings of the National Academy of Sciences USA, researchers reported that electrons in a three-dimensional material behaved as if only two dimensions of space exist.
Our lives happen along three spatial dimensions: depth, width and height. Scientists can engineer extremely thin materials to effectively eliminate height, but this team, based at SLAC National Accelerator Laboratory, did not do so. While working on a new type of superconductor—a substance in which electrons flow without resistance, used in MRI machines, particle accelerators and some quantum computers—they found something unusual. Their sample, made from barium, lead, bismuth and oxygen, was fully three-dimensional. Yet examination with a powerful quantum microscope found that electrons within it disregarded the third dimension and formed perfectly flat stripes. “Superconducting electrons collapsed spontaneously into this two-dimensional system without any physical or chemical change or specific fabrication,” says lead author Carolina Parra, a physicist at the Federico Santa Maria Technical University in Chile.
Physicists had previously speculated that this material might host two-dimensional electron behavior that they could not measure. The new study observed it directly. With the researchers' quantum microscope, “you get information about the physical parameters of the sample, down to the atomic level,” says study co-author and SLAC physicist Hari Manoharan. The tool measured an effect called quantum tunneling: electrons from inside the microscope tried to sneak into the sample, revealing characteristics of single superconductor atoms and their electrons.
The electronic “flatlands” that these measurements uncovered are promising test beds for theories of superconductivity, says Nandini Trivedi, a physicist at the Ohio State University, who was not involved with the work. She studies instances in which electrons in extremely thin superconductors form lots of tight-knit islands of particles. Manoharan's team members observed their electrons self-organizing in the same way—and showing more of this distinctive behavior than seen in materials that scientists purposefully engineer as flat.
Pinpointing electrons' specific actions in a superconductor can also help push the development of superconducting materials forward. Most such materials currently known work only when cooled to hundreds of degrees below zero Fahrenheit. This requirement is impractical, but physicists do not yet know what alterations would make them conduct perfectly at room temperature, too, says study co-author and physicist Paula Giraldo-Gallo of the University of the Andes in Bogotá. Some of the tight-knit electron groups measured in this study appeared to superconduct when unexpectedly warm. “This material has the potential to be a higher-temperature superconductor,” Giraldo-Gallo says. “What's driving that is an open question.” The answer may lie in the new study's 2-D world.