Physicists from MIT have experimentally proved that the thinnest superconductor has unique properties. It is already being tipped for applications in medical diagnostics, quantum computing and energy transportation. Writes about this journal Nature Communications
The subject of research belongs to the class of superconductors, which become superconducting at temperatures an order of magnitude higher than their conventional counterparts, which simplifies their practical application. Conventional superconductors only work at temperatures around -263.15 degrees Celsius.
However, these so-called high-temperature superconductors are still not fully understood. “Their microscopic excitations and dynamics are necessary to understand superconductivity, but after 30 years of research, many questions still remain open,” says Riccardo Comin, assistant professor of physics at MIT.
In 2015, scientists discovered a new kind of high-temperature superconductor: a sheet of iron selenide just one atomic layer thick, capable of superconducting at -208.15 degrees Celsius. In contrast, massive samples of the same material superconduct at a much lower temperature (-265.15 degrees Celsius). The discovery sparked a flurry of investigations.
In ordinary metal, electrons behave in the same way as individual people dancing in a room. In a superconducting metal, electrons move in pairs, like pairs in a dance. And all these pairs move in unison, as if they were part of quantum choreography, which ultimately led to the creation of a kind of electronic superfluid.
Scientists have known for a long time that in conventional superconductors, the “glue” that holds electrons together is formed by the movement of atoms inside the material. “If you look at a solid sitting on a table, it looks like it’s not doing anything,” Comin says. However, a lot is happening at the nanoscale. Inside this material, electrons fly in all possible directions, and atoms rattle; they vibrate. In conventional superconductors, electrons use the energy stored in the movement of an atom to form pairs.
The “glue” behind the bonding of electrons in high-temperature superconductors is different. Scientists have suggested that it is associated with a specific property of electrons – spin. “Rotation can be viewed as an elementary magnet,” says Jonathan Pelliciari, an assistant physicist at Brookhaven National Laboratory. The idea is that in a high-temperature superconductor, electrons can take some of the energy from these spins. And this energy is the “glue” that they use to create a pair.
Until now, most physicists thought that it was impossible to detect or measure spin excitations in a material that was only an atomic layer thick. But physicists not only discovered spin excitations, but, among other things, they also showed that the spin dynamics in an ultrathin sample is drastically different from the spin dynamics in a massive sample. In particular, the energy of the fluctuating spins in the ultrathin sample was much higher – four or five times – than the energy of the spins in the bulk sample. A resonant inelastic X-ray scattering (RIXS) instrument was used for the study.
“This is the first experimental evidence of spin excitations in an atomically thin material,” says Pelliziari.