Sometimes a twist might be as good as a jiggle. Or at least, a new study suggests, twisting electrons appear to take the place of jiggling ions in an exotic kind of superconductor.
It’s the first experiment to show that a certain kind of twisting fluctuations among the electrons in the material could explain its superconductivity, scientists report in the Nov. 20 Nature. The research also supports a 20-year-old theory about how such twists in the spin axes of electrons in general could enable superconductivity for some materials.
In superconductors, electric current flows with zero resistance, so a current in a loop of superconducting wire would keep flowing forever even without a power source. Conventional superconductors must be kept extremely cold to work, requiring expensive liquid helium to chill them to near absolute zero (–273° Celsius). But some materials, such as certain cuprates, become superconducting when cooled to transition temperatures as high as about –110° C, which makes these unconventional superconductors cheaper and more practical to use.
“This work can probably be a foundation for better understanding high-temperature superconductors and can lead to new forms of unconventional superconductivity,” says Tuson Park, coauthor of the study and a condensed matter physicist at Los Alamos National Laboratory in New Mexico.
The mechanism that allows charge-carrying electrons to move unfettered through all conventional superconductors, regardless of their transition temperatures, is well understood. These resistance-free wires power MRI machines in hospitals and the world’s largest particle accelerators.
But unconventional superconductors, only some of which have high transition temperatures, work by different mechanisms that are still poorly understood. Discovering how such materials work is “the holy grail of superconducting research because only unconventional superconductors can have high transition temperatures,” comments Qimiao Si, a condensed matter theorist at Rice University in Houston. “That’s why we’re interested in them.”
To explore the physics of unconventional superconductors, the researchers measured the electrical resistance of an exotic material containing the elements cerium, rhodium and indium. Park and his colleagues cooled the material to near absolute zero so that it would pass through a transition proposed by theorists called the local quantum critical point. Around that point, the spin axes of the electrons in the material should begin twisting and fluctuating wildly.
These fluctuations could help electrons to pair up, Park and his colleagues believe. Only pairs of electrons can move through a superconductor with zero inhibition, but electrons naturally repel each other because they have the same negative electric charge. In conventional superconductors, the jiggling of the atoms’ nuclei in response to the movement of one electron helps another electron to follow closely behind, providing a kind of quantum glue.
“The idea is similar to the conventional except that the glue is different,” Park explains. In the experiments, fluctuations in the electrons’ spin axes appeared to provide the glue.
While the test material was cooled to near absolute zero to make this phenomenon more visible, understanding the mechanism involved could help scientists better understand unconventional superconductors at higher temperatures as well. “There are not that many prototype materials in which you can study that mechanism for superconductivity in a systematic fashion, and this is one of them,” Si says. “Perhaps the mechanism is more broadly relevant.”