Superconductivity represents a quantum physical condition wherein a metal can conduct electricity flawlessly without any resistance. Its most recognized application is in enabling potent magnets in MRI machines to generate the magnetic fields necessary for medical professionals to visualize the interior of our bodies. Up to now, substances have only been able to attain superconductivity at extremely low temperatures, close to absolute zero (a few tens of Kelvin or even colder). Nonetheless, physicists aspire for superconductive materials that might one day function at ambient temperature. Such materials could unlock entirely novel opportunities in fields like quantum computing, the energy sector, and healthcare technologies.
“Comprehending the processes that lead to the emergence of superconductivity and uncovering exotic new superconducting phases is not only one of the most thrilling endeavors in the fundamental examination of quantum materials but is also motivated by the ultimate aspiration of achieving superconductivity at room temperature,” states Stevan Nadj-Perge, a professor of applied physics and materials science at Caltech.
Currently, a team spearheaded by Nadj-Perge, including Lingyuan Kong, an AWS quantum postdoctoral scholar research associate, and other colleagues at Caltech, has uncovered a new superconducting state—a discovery that adds a vital piece to the enigma surrounding this remarkable but enigmatic phenomenon. A paper detailing the research was published on March 19 in the journal Nature.
In conventional metals, individual electrons collide with ions as they traverse the metal’s lattice structure composed of oppositely charged ions. Each collision results in the loss of energy for the electrons, thereby increasing electrical resistance. Conversely, in superconductors, electrons are weakly attracted to one another and can pair up, forming couples known as Cooper pairs. As long as the electrons remain within a specific relatively small range of energy levels referred to as the energy gap, they stay paired and do not expend energy during collisions. Therefore, superconductivity manifests within that relatively narrow energy gap.
Generally, a superconductor’s energy gap is uniform throughout the material. For instance, in a superconducting crystal devoid of impurities, all portions of the crystal would exhibit the same energy gap.
However, starting in the 1960s, scientists began proposing that the energy gap in certain superconducting materials could vary spatially, indicating that the gap might be stronger in some locations and weaker in others. Subsequently, in the 2000s, this concept was further elaborated with the introduction of what is known as the pair density wave (PDW) state, which posits that a superconducting state could emerge in which the energy gap varies with a long wavelength, fluctuating between larger and smaller measurements.
In the last decade, this idea has attracted considerable experimental attention, with various materials, including iron-based superconductors, being examined as potential candidates for a PDW state.
Now, utilizing exceedingly thin flakes of an iron-based superconductor, FeTe0.55Se0.45, Nadj-Perge and his team have identified a modulation of the superconducting gap with the smallest wavelength achievable, corresponding to the spacing of atoms within a crystal. They have designated it the Cooper-pair density modulation (PDM) state.
“The detected gap modulation, reaching up to 40 percent, signifies the most substantial report so far, offering the clearest experimental evidence to date that gap modulation can exist even at an atomic scale,” remarks Kong, the lead author of the new publication.
This unforeseen finding was made achievable through the inaugural successful execution of scanning tunneling microscopy experiments involving an iron-based superconductor on a specialized device designed for examining such thin flakes. Previous attempts had been hindered for almost two decades due to significant surface contamination, but the Caltech team, collaborating at the Kavli Nanoscience Institute (KNI), developed a new experimental methodology that allowed for a sufficiently clean surface for microscopic investigations.
Michał Papaj, an assistant professor of physics at the University of Houston, and Patrick A. Lee, the William & Emma Rogers Professor of Physics at MIT and a visiting associate at Caltech, both co-authors of the new paper, formulated a theoretical model to elucidate the possible origin of the observed gap modulation. “Their model proposes that this PDM state modulation results from the breaking of both sublattice symmetry and a distinctive rotational symmetry specific to thin flakes,” Nadj-Perge explains.
Additional contributors to the paper, “Cooper-pair density modulation state in an iron-based superconductor,” include Caltech graduate students Hyunjin Kim and Eli Baum; Yiran Zhang, now a postdoc at Harvard; Hui Li, a postdoc at Northwestern University; Kenji Watanabe and Takashi Taniguchi from the National Institute for Materials Science in Japan; and Genda Gu from Brookhaven National Laboratory. The research was chiefly supported by the Institute for Quantum Information and Matter (IQIM) at Caltech. Additional financial support was provided by the National Science Foundation, the Department of Energy, KNI, and the Gordon and Betty Moore Foundation.