In a recent study published in the journal Science, researchers from Stanford University have discovered that electron pairing, a crucial property of a superconductor, happens in an unexpected material an antiferromagnetic insulator, and at temperatures far higher than previously believed. This finding implies that researchers might be able to build similar materials into superconductors that function at greater temperatures, even if the material did not exhibit zero resistance.
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Since their discovery a century ago, superconductors and their enigmatic atomic features have fascinated scientists. These unique materials do not lose any energy when electricity passes through them. Trains could levitate, thanks to them.
However, superconductors normally perform only at extremely cold temperatures. When heated, these materials either become conventional conductors, which allow electricity to flow but lose energy, or insulators, which do not conduct electricity at all.
Researchers have been working hard to find superconductor materials that can function at greater temperatures, maybe even room temperature in the future. Finding or creating such a material might transform current technology, from cellphones and computers to the power grid and transportation. Furthermore, the peculiar quantum state of superconductors makes them ideal building blocks for quantum computers.
The electron pairs are telling us that they are ready to be superconducting, but something is stopping them. If we can find a new method to synchronize the pairs, we could apply that to possibly building higher temperature superconductors.
Ke-Jun Xu, Study Co-Author and Graduate Student, Applied Physics, Stanford University
Out-of-Sync Electrons
Over the last century, researchers have learned a great deal about how superconductors function. For example, it is now known that for a material to superconduct, electrons must pair off, and these pairs must be coherent, which means that their motions must be coordinated. The material might develop into an insulator if electrons are coupled yet incoherent.
In superconductors, the electrons behave like two shy persons at a dance party. Initially, neither participant wants to dance with the other. But suddenly the DJ plays a song that they both enjoy, allowing them to unwind. They observe one another enjoying the song and feel attracted from a distance; they have paired but are not yet coherent.
Then the DJ plays a new tune that both people adore. Suddenly, the two people team up and begin to dance. Soon, everyone at the dance party follows suit: they all gather and begin dancing to the same new song. At this stage, the party becomes coherent and is in a superconducting state.
The electrons in the current study were observed by the researchers in the middle stage when they had locked eyes but were not getting up to dance.
Cuprates Acting Oddly
Not shortly after superconductors were discovered, researchers noticed that the vibrations in the underlying material caused electrons to couple up and dance.
This type of electron pairing occurs in a class of materials known as conventional superconductors, which are well understood, according to Zhi-Xun Shen, a Stanford professor and researcher at the Stanford Institute for Materials and Energy Sciences (SIMES) at SLAC who oversaw the research. Conventional superconductors generally operate at temperatures near absolute zero, or below 25 Kelvin, in ambient pressure.
Unconventional superconductors, such as the copper oxide material, or cuprate, in the current study, can function at substantially higher temperatures, up to 130 Kelvin. It is commonly assumed that something other than lattice vibrations contributes to electron pairing in cuprates. Although researchers are unsure about what is causing it, the leading option is fluctuating electron spins, which drive electrons to couple and dance with increased angular momentum.
This phenomenon is known as a wave channel, and the first signs of such a new condition were observed in an experiment at SSRL nearly three decades ago. Understanding what causes electron pairing in cuprates could help in the development of superconductors capable of operating at higher temperatures.
In this research, scientists picked a cuprate family that had not previously been extensively examined since its maximum superconducting temperature was rather low - 25 Kelvin - when compared to other cuprates. Even worse, most members of this family are excellent insulators. Researchers used ultraviolet light to expel electrons from the material to observe the atomic features of the cuprate.
An “energy gap” is created when electrons are bound together because they are somewhat more resistant to being ejected. The energy gap remains constant until 150 Kelvin, indicating that electrons are coupled at temperatures far higher than the zero resistance condition, which occurs at around 25 Kelvin. The coupling is strongest in the most insulating samples, which is the study’s most unexpected conclusion.
According to Shen, the cuprate used in the study might not be able to achieve superconductivity at ambient temperature, or about 300 Kelvin.
But maybe in another superconductor material family, we can use this knowledge for hints to get closer to room temperature.
Zhi-Xun Shen, Professor and Scientist, Materials and Energy Sciences, Stanford University
Shen further added, “Our findings open a potentially rich new path forward. We plan to study this pairing gap in the future to help engineer superconductors using new methods. On the one hand, we plan to use similar experimental approaches at SSRL to gain further insight into this incoherent pairing state. On the other hand, we want to find ways to manipulate these materials to perhaps coerce these incoherent pairs into synchronization.”
This study was funded in part by the DOE’s Office of Science. SSRL is a DOE Office of Science user facility.
Journal Reference:
Xu, K.-J., et al. (2024) Anomalous normal-state gap in an electron-doped cuprate. Science. doi.org/10.1126/science.adk4792.