The discovery may enhance efforts to create superconductors that operate at elevated temperatures.
Since their emergence a century ago, superconductors and their enigmatic atomic behaviors have fascinated scientists. These remarkable materials enable electricity to pass through them without any energy loss and can even make trains float.
However, superconductors generally function only at extremely low temperatures. When these substances are warmed up, they become regular conductors, which allow the flow of electricity but with some energy loss, or insulators, which do not conduct electricity at all.
Scientists have been diligently searching for superconductor materials that can perform their unique functions at elevated temperatures—potentially even at room temperature in the future. Discovering or creating such materials could revolutionize modern technology, impacting everything from computers and mobile devices to the power grid and transportation. Additionally, the extraordinary quantum state of superconductors makes them ideal candidates for quantum computing.
Recently, researchers have found that a critical trait of a superconductor—electron pairing—occurs at much higher temperatures than previously assumed, and in a material that is typically unexpected: an antiferromagnetic insulator. Although this material doesn’t exhibit zero resistance, this discovery implies that scientists might be able to engineer similar materials into superconductors functioning at elevated temperatures. The research group from SLAC National Accelerator Laboratory, Stanford University, and other institutions shared their findings on August 15 in Science.
“The electron pairs indicate that they are ready to be superconducting, but something is holding them back,” explained Ke-Jun Xu, a graduate student at Stanford in applied physics and a co-author of the paper. “If we can find a novel method to synchronize the pairs, we could potentially apply that to develop superconductors that operate at higher temperatures.”
Electrons Out of Sync
Throughout the past century, scientists have gained extensive insights into the functioning of superconductors. It is known that for a material to become superconducting, electrons must pair up, and these pairs have to be coherent, meaning their movements should be synchronized. If electrons are paired but lack coherence, the material might behave like an insulator.
In superconductors, the interactions between electrons resemble two shy individuals at a dance. Initially, neither person is inclined to dance with the other. Then, a song starts playing that they both enjoy, easing their tension. They begin to notice each other from a distance and form a connection—they have paired but haven’t yet synchronized.
Once the DJ plays a song that both find irresistible, they pair up and begin to dance. Soon everyone at the event follows their lead, coming together and grooving to the same beat. This moment marks the transition to a synchronized state, signifying superconductivity.
In the latest study, researchers identified a phase where the electrons had engaged but weren’t yet ready to dance.
Unusual Behavior in Cuprates
Shortly after the initial discovery of superconductors, scientists discovered that the vibrations within the material contributed to the electron pairing necessary for superconductivity. This type of electron pairing is typical in conventional superconductors, which are well understood, according to Zhi-Xun Shen, a Stanford professor involved in the research, who leads the Stanford Institute for Materials and Energy Sciences (SIMES) at SLAC. Conventional superconductors usually operate close to absolute zero, around 25 Kelvin, under normal pressure conditions.
On the other hand, unconventional superconductors—like cuprates, the focus of this study—function at significantly higher temperatures, sometimes reaching up to 130 Kelvin. In cuprates, it is believed that factors beyond lattice vibrations contribute to the electron pairing. While the exact processes remain unclear, a leading theory involves fluctuating electron spins, allowing the electrons to form pairs with enhanced angular momentum. This occurrence is referred to as a wave channel, and early evidence of such a distinct state was observed about thirty years ago in experiments at SSRL. Gaining more insight into electron pairing in cuprates could aid in fabricating superconductors that operate at elevated temperatures.
In this research, scientists examined a cuprate family that had not previously been thoroughly investigated, as its maximum superconducting temperature was relatively low—only 25 Kelvin—compared to other cuprates. Additionally, many members of this group act as effective insulators. To analyze the atomic characteristics of the cuprate, researchers illuminated material samples with ultraviolet light, which ejected electrons from the samples. When electrons are bound, they exhibit slightly greater resistance to ejection, leading to the formation of an “energy gap.” This energy gap remains evident up to 150 Kelvin, indicating that electrons are pairing at significantly higher temperatures than the zero-resistance condition around 25 Kelvin. Interestingly, this study found that electron pairing is most pronounced in the most insulating samples.
While the cuprate in this study may not directly lead to room temperature superconductivity (approximately 300 Kelvin), Shen remarked, “Perhaps we can leverage this knowledge with another family of superconductors to inch closer to room temperature.”
“Our results pave the way for a promising future,” Shen added. “We intend to investigate this pairing gap further to help engineer superconductors through new techniques. We will continue utilizing similar experimental methods at SSRL to deepen our understanding of this incoherent pairing state. Moreover, we aim to explore ways to manipulate these materials to encourage synchronization among these incoherent pairs.”
This research was partially funded by the DOE’s Office of Science. SSRL functions as a user facility for the DOE Office of Science.
