PROVIDENCE, RI [Brown University] – In 2018, physics showed that something interesting happens when two sheets of the nanomaterial graphene are placed on top of each other. When one fold is turned to a “magic angle” of about 1.1 degrees relative to the other, the system becomes a superconductor – meaning it carries electricity with zero resistance. Even more interestingly, there was evidence that it was an unconventional form of superconductivity – a type that can occur at temperatures well above absolute zero, where most superconducting materials work.
Since its first discovery, researchers have been working to understand this fascinating situation. Now, a Brown University physics-led research team has found a new way to study in detail the nature of the superconducting state in angle-graphene graphene. This technique allows researchers to manipulate the reciprocating force between elections – the Coulomb interaction – in the system. In a study published in the journal Science, the researchers show that magical angle superconductivity becomes stronger when Coulomb interactions are reduced, an important piece of information in understanding how this superconductor works.
“This is the first time anyone has proven that you can handle Coulomb’s interaction strength directly in a strongly connected electronic system,” said Jia Li, a physics professor at Brown and a corresponding author of the research. “Superconductivity is driven by the interactions between electrons, so when we can handle that interaction, it tells us something very important about that system. In this case , demonstrating that a weaker Coulomb interaction enhances superconductivity provides an important new theoretical constraint for this system. “
The original 2018 discovery of potentially uncontroversial superconductivity in graphene-square graphene sparked great interest in the physics community. Graphene – single-atom-thick carbon sheets – is a very simple material. If it really did support supernatural wonder, the simplicity of graphene would make it a great place to study how miracle works, Li says.
“Unconventional superconductors are exciting because of the high motion temperature and their potential applications in quantum computers, lossless power grids and elsewhere,” Li said. “But we still don’t have a microscopic theory for how they work. That’s why everyone was excited when something that looked like unusual superconductivity was happening in graphene magic. Its simple chemical shape and tunability at a twisted angle promise a clearer picture. “
Conventional superconductivity was first described in the 1950s by a group of physicists that included long-time professor Brown and Nobel Prize-winning Leon Cooper. They showed that electrons in a superconductor remove the atomic surface of a material in a way that causes electrons to form quantum duos called Cooper pairs, which are able to move through that material without control. In atypical superconductors, electron pairs form in a way that is thought to be slightly different from Cooper’s mechanism, but scientists still don’t know which way that is.
For this new study, Li and his colleagues came up with a method to use Coulomb interaction to study electron pairs in magic-square graphene. Cooper pair locks electricity together at a certain distance from each other. That pair competes with Coulomb’s interaction, which is trying to push the electricity apart. If it were possible to weaken Coulomb’s interaction, the Cooper pairs should come together stronger, making the superconducting state stronger. That would give us information on whether Cooper’s equipment was happening in the system.
To manipulate the Coulomb interaction for this study, the researchers built a device that delivers a magic-square graphene sheet very close to another type of graphene sheet called the Bernal bilayer. Because the two series are so thin and so close to each other, electrons in the magic angle sample are becoming increasingly attractive to positively charged regions in the Bernal series. That pull between rows effectively weakens Coulomb’s perceived interaction between electrons within the magic-angle sample, what the researchers call Coulomb’s screening.
One feature of the Bernal series made it particularly useful in this research. The Bernal cover can be transferred from a conductor to an insulator by changing a voltage applied directly to the filler. The effect of Coulomb screening will only occur when the Bernal cover is at the behavioral stage. Thus by alternating between conduction and insulation and monitoring corresponding changes in superconductivity, the researchers were able to ensure that what they observed was due to Coulomb screening.
The work showed that the superconducting rate became stronger when Coulomb interaction was weakened. The temperature at which the rate broke was higher, and stronger to magnetic fields, which disturbs superconductors.
“I was surprised to see this Coulomb influence in this material,” Li said. “We would expect this to happen in a conventional superconductor, but there is a lot of evidence to show that magic-square graphene is an unconventional superconductor. So any microscopic theory of the level this high behavior to pay attention to this information. “
Li said the findings are a credit to Xiaoxue Liu, Brown’s postdoctoral researcher and lead author of the study, who built the tool that made the possible decisions.
“No one has ever built anything like this before,” Li said. “Everything had to be very precise down to the nanometer scale, from the twisting angle of the graphene to the spacing between layers. Xiaoxue did a really amazing job. We also benefited from Oskar Vafek’s theoretical guidance, theoretical physics from Florida State University. “
While this study provides a crucial new piece of information about square-graphene graphene, there is much more that this approach could reveal. For example, this first study looked at only one part of the level space for right-angle superconductivity. It is possible, Li says, that the behavior of the superconducting phase varies in different parts of the phase space, and further research will reveal it.
“The ability to screen Coulomb interactions gives us a new experimental niche to turn in to help understand these quantum onions,” Li said. “This method can be used with material any two-sided, so I think this approach will be useful in helping to invent new types of materials. “
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Other authors of the study were Zhi Wang, K. Watanabe and T. Taniguchi. The research was supported by Brown University and the Institute for Molecular and Nanoscale Innovation.
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