An illustration shows holes interacting with electrons between sheets of Graphene to form a pair. Dean is a professor at Columbia University.
When electrons flow through power lines and computers, they lose some of their energy, which cools down as heat. That's why the server farms that power the cloud need so much air conditioning to keep the machines from overheating, and why laptops get hot after being used for too long. When particles flow in a typical environment, they lose their energy. When particles form pairs called quantum condensates, there are a few exceptions. This leads to superconductivity with vanishing electrical resistance in some metals, and superfluidity in liquified helium, which can flow without dissipation.
Superconducting materials have been used to develop applications such as quantum computation and dissipationless power transmission. Superconducting materials need to be kept cold. To raise the temperature of energy-loss-free devices, researchers need to better understand what drives the formation of quantum condensates.
Superconductivity is the result of pairs of electrons. The strength of the pair is fixed in most materials, and two negatively charged particles don't normally want to pair with each other. In a new article in Science, a group of people describe a platform that uses a type of Graphene. The strength of that pair can now be varied along a continuum, which will allow the team to test theoretical predictions about the origins of quantum condensates and how they might increase the temperature limits of superconductivity.
A platform that is tunable.
The underlying theory is easy to understand. Dean said that if you can get electrons to pair, they can superconduct. The Bardeen-Cooper-Schrieffer theory states that an attractive force between electrons will cause them to form a new particle called a "Cooper pair." These particles behave like particles called bosons and can enter into a collective state and move through a material unimpeded by disorder.
There has been a problem. Dean said that electronns don't want to pair. As the saying goes, like repels like. The team has been looking at how opposites can attract to yield an equivalent 'paired' boson, rather than trying to force a bond between two negatively charged electrons.
The general idea, which was first proposed by theoretical physicists, is now being realized by the team in atom-thin sheets of Graphene, a material with unique properties that they have been working to leverage for several years. Depending on the magnetic field applied, a sheet of Graphene can be made with either negatively charged electrons or positively charged holes. When two sheets are put together, the electrons on one sheet will want to pair with the holes on the other sheet.
A second illustration shows holes interacting with electrons between sheets of Graphene to form a pair. Dean is a professor at Columbia University.
There is still some distance to be traveled. The attraction between electrons and holes in a single material is too strong, according to the lead author of the paper. They'll combine if they get too close. The team used a technique developed at Columbia to create stacks of different materials and added layers of boron nitride between the Graphene. The strength of the interaction was influenced by the distance between electrons on one sheet and holes on the other, as well as the number of layers. Li, one of the lead authors of this work, said that by varying the thickness of the separation layer, we have control over the interaction strength.
The interaction between holes and electrons isn't the only thing that needs to happen to reach a quantum state. The team could change the binding strength between the electrons and holes by tweaking the number of layers.
To raise the temperature.
Superconducting materials can only be found at extremely cold temperatures. The pair state is able to survive at temperatures as high as 200K. The existence of high temperature superconductors suggests that quantum condensate could occur at room temperature. The progress to realize even higher temperature quantum condensates using either electron-electron pairs or electron-hole pairs has been slow.
One theory is that high temperature superconductors result from electron pairs that are neither weak nor strong, but at the same time. The Bose-Einstein Condensate Theory is a challenge in high temperature superconductors since electrons repel one another. The team can now map for the first time how conductivity changes when the BEC and BCS extremes are used.
The experiments were done at negative 450 degrees Fahrenheit and under a strong, 10Tesla magnet, but neither condition is practical to build real devices that might operate on a chip inside a computer. Dean said that the work opens up new avenues of investigation. He said that the platform made it possible to test theoretical predictions in ways that had not previously been accessible.
It is possible to lose the magnet, which is needed to get the electrons moving, with different materials. Semiconductor can be manipulated to be full of holes or electrons. Getting such sheets to form stable electron-hole pairs will be dependent on a number of factors, including how clean and defect-free the materials are and whether you can make appropriate contact between them. Halperin, a physicist from Harvard, said that stabilizing electron-hole pair condensates at high temperatures and without a magnetic field could lead to practical uses.
Dean said that the underlying concept is absolutely sound. "It's no longer a fantasy; it's reality." It becomes an engineering challenge now.
The Crossover between Strongly-coupled and Weakly-coupled Exciton Superfluids was reported in Science. www.science.org/doi/10.1126/science.abg1110
Science journal information.
The news about tuning the bonds of quantum particles was retrieved fromphys.org on January 13, 2022.
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