| Each week Quanta Magazine explains one of the most important ideas driving modern research. This week, physics staff writer Charlie Wood describes the ongoing effort to better understand superconductivity, both near absolute zero and closer to room temperature. | | How Researchers Are Exploring the Roots of Superconductivity By CHARLIE WOOD | | Last summer, online commentators went wild for LK-99, an alleged room-temperature superconductor that seemed to foreshadow a wondrous age of perfectly efficient power lines and magnetically levitating trains.
Many physicists, meanwhile, kept their cool. Claims of materials conducting electricity with zero resistance (or "superconducting") at everyday temperatures pop up regularly enough that the community has a name for the phenomenon: unidentified superconducting objects, or USOs. Like reports of alien spacecraft sightings, USOs tend to go unconfirmed. Sure enough, other labs failed to replicate the LK-99 findings, and the hype soon subsided. (The field is also susceptible to mistakes, misperceptions and hoaxes: Recently, for the second time in three years, the journal Nature retracted a report of room-temperature superconductivity from the University of Rochester lab of Ranga Dias, citing scientific irregularities.)
While such claims of remarkable materials often grab the spotlight, a great deal of superconductivity research seeks to dig deeper. If searching for room-temperature superconductors is akin to the high-risk, high-reward search for extraterrestrial life, this more fundamental side of the field more closely resembles biology: It seeks to better understand the key ingredients and conditions that underlie the known instances of a fragile and miraculous phenomenon.
Our understanding of superconductivity did not come easily, because superconductivity is not easy to understand. Albert Einstein and Richard Feynman — masters of space-time and quantum theory — worked on the problem in the 1910s and 1950s, respectively, but couldn't quite solve the mystery. The physicists John Bardeen, Leon Cooper and J. Robert Schrieffer finally cracked it in 1957, almost half a century after the discovery of the phenomenon.
Superconductivity arises, they theorized, when a negatively charged electron moving through a material pulls atomic nuclei toward it, creating a positively charged wave traveling with the electron. This positive ripple — the quantum version of a sound wave — pulls in a second electron. Crucially, the resulting pair of electrons, known as a Cooper pair, obeys different quantum rules than individual electrons do. In particular, Cooper pairs gain the ability to meld into a unified quantum state known as a superfluid that doesn't allow for the particle-on-particle collisions that normally generate resistance.
Thanks to Bardeen, Cooper and Schrieffer, researchers know one basic recipe for superconductivity: Glue electrons together with sound waves. But are there other, more exotic recipes — ones with different glues, or even different particles? And can experimentalists whip them up in the lab? Recently, researchers have made progress on answering these fundamental questions.
What's New and Noteworthy
For the last 40 years, one of the biggest mysteries in superconductivity has been why a family of copper-based materials, the "cuprates," can superconduct at temperatures roughly 10 times higher than those at which normal metals can. What is the glue holding those electron pairs together? In 2016, Natalie Wolchover reported on what researchers found underlying the superconductivity when they used one of the planet's strongest magnetic fields to pry the pairs of electrons apart. And in 2022, I reported on a new experimental technique that gave researchers their most direct view yet of the cuprate glue holding electrons together — confirming that it has nothing to do with sound waves.
More recently, blockbuster experiments placing one atomically thin sheet of graphene over another at just the right "magic angle," as David Freedman reported in 2019, have unearthed a new way to play with superconductivity. Some researchers believe that these twisted graphene sandwiches showcase a potentially novel electron glue even stronger than that of the cuprates, as I reported in 2021, while others suspect that sound waves might still lie at the root of it all.
Other labs are pursuing superconductivity by pairing up different ingredients. Instead of two electrons, they use an electron and the space — or "hole" — left behind when an electron goes missing. Holes are positively charged, so electrons and holes naturally attract. In that case, the challenge isn't getting them together, but rather keeping them apart. Last year I chronicled the efforts of two Cornell physicists, the married power couple Jie Shan and Kin Fai Mak, to create currents of these "exciton" pairs.
Any of these researchers would be thrilled if their work someday enabled devices that harnessed the quantum oddity of superconductivity to build new technologies, but that's not typically why they pursue their research. They study superconductivity because it's an inherently strange and fascinating phenomenon that is only partially understood. They want to fully understand it, and push it to its limits, wherever that may lead. | | | The Condensed Concepts blog of the theoretical physicist Ross H. McKenzie discussed the long-standing frustration felt by many researchers at the recurring reports of unidentified superconducting objects. | | | Nature published a news story by Dan Garisto that reconstructed how the case for LK-99 as a superconductor fell apart. | | | The PhysicsHigh channel on YouTube has a video that explains how sound waves can draw electrons together. | | | | | | |
