A team of researchers led by Yale University has made a groundbreaking discovery that provides the strongest evidence yet of a novel type of superconducting material. This fundamental science breakthrough has the potential to revolutionize our understanding of superconductivity—the phenomenon in which electrical current flows without resistance or energy loss—and suggests new ways to achieve this behavior. The study also lends substantial support to a long-held theory in the field of condensed matter physics, proposing that superconductivity could arise from a phase of matter called electronic nematicity, in which particles lose their rotational symmetry.
The team’s discovery centers on iron selenide crystals that have been doped with sulfur. These materials are unique because they exhibit both superconductivity and nematic order, a state where electrons in a material prefer to move in one direction over another. At room temperature, the electrons in iron selenide are indifferent to direction, meaning they can move horizontally or vertically without any preference. However, as the material is cooled to lower temperatures, the electrons transition into a nematic phase, where they exhibit a directional preference, aligning their movement along one axis. In some cases, the electrons fluctuate between these directions, a behavior known as nematic fluctuation.
For decades, physicists have theorized that superconductivity might be linked to these nematic fluctuations, but proving this connection has been a difficult challenge. In their study, published in Nature Physics, the Yale-led team provides experimental evidence supporting this theory, marking a significant step forward in the field of superconductivity.
“We started on a hunch that there was something interesting happening in certain iron selenide materials mixed with sulfur, relating to the relationship between superconductivity and nematic fluctuations,” said Eduardo H. da Silva Neto, assistant professor of physics at Yale and the lead researcher. The materials the team studied were particularly well-suited for this investigation because they displayed both nematic order and superconductivity, but without the complications posed by magnetism, which can make the study of superconductivity more difficult.
To investigate these materials, the researchers subjected the iron selenide crystals to extremely low temperatures—less than 500 millikelvins—and used a scanning tunneling microscope (STM) to observe the quantum states of the electrons at the atomic level. The STM is a highly specialized tool that allows scientists to image and measure electron behavior with unparalleled precision. By focusing their study on the iron selenides with the highest levels of nematic fluctuations, the researchers were able to identify a “superconducting gap”—a key indicator that signals the presence and strength of superconductivity. The gap they observed matched exactly what was expected for superconductivity induced by electronic nematicity.
“This has been elusive to prove, because you have to do the challenging STM measurements at very low temperatures to be able to measure the gap accurately,” da Silva Neto explained. The gap observed in the STM images was a strong indicator that the superconductivity observed in these materials was indeed linked to the nematic fluctuations, providing the first solid evidence of this long-speculated connection.
The discovery opens several new avenues for future research. One of the next steps for the team is to investigate how increasing the sulfur content in the iron selenide materials affects superconductivity. They aim to determine whether the superconducting properties persist or if they diminish, and whether other factors, such as spin fluctuations, begin to emerge as the sulfur content increases.
The study was a collaborative effort involving researchers from several institutions. In addition to da Silva Neto, co-lead authors of the study were Yale graduate students Pranab Kumar Nag and Kirsty Scott. Other contributors included Xinze Yang and Aaron Greenberg from Yale, as well as researchers from the University of California, Davis; the University of Minnesota; Universidade Federal de Goiás in Brazil; the University of Campinas in Brazil; and Fairfield University.
This discovery has profound implications for both basic physics and the potential technological applications of superconductivity. Superconducting materials are already central to technologies like magnetic resonance imaging (MRI) and quantum computing, but they typically require extremely low temperatures to function, making them expensive and challenging to work with. By better understanding how superconductivity might arise from electronic nematicity, researchers may eventually be able to develop materials that exhibit superconductivity at higher temperatures, making these technologies more accessible and practical.
Source: Yale University