When electrons traveling through power lines and computers face resistance, some of their energy is lost, which is dissipated as heat. That’s why laptops grow hot after a lengthy period of usage, and why the server farms that run the cloud need so much cooling to keep the machines cool.
In a typical environment, any particles carrying energy tend to lose that energy as they travel. There are a few exceptions, which occur when particles form pairs termed quantum condensates at extremely low temperatures.
In some metals, such as aluminum, this results in superconductivity, vanishing electrical resistance, and superfluidity in liquified helium, which can flow without dissipation.
Many applications have been developed based on superconducting materials that reveal these quantum condensate states, ranging from dissipationless power transfer to quantum computation.
However, known superconducting materials must be kept extremely cold, which is typically impracticable. Researchers need to learn more about what causes quantum condensates to arise in the first place in order to raise the temperature of energy-loss-free devices.
Superconductivity is the product of coupled electrons in theory. However, in most materials, the pairing is weak since two negatively charged particles do not generally wish to pair with each other, and the pairing strength is fixed.
Cory Dean and James Hone of Columbia University, Xiaomeng Liu, Philip Kim, and Bert Halperin of Harvard University, Jia Li of Brown University, and Kenji Watanabe and Takashi Taniguchi of NIMS in Japan describe a tunable graphene-based platform that uses opposite charges electrons and holes to form quantum particle pairs under strong magnetic fields in a new article in Science.
The team will be able to test theoretical predictions regarding the origins of quantum condensates and how they can extend the temperature limits of superconductivity by varying the intensity of that pairing along a continuum.
If such electron-hole pair condensates also called exciton condensates can be stabilized at high temperatures and without a magnetic field, this might lead to practical uses.
Bert Halperin
Designing a Tunable Platform
The underlying theory is simple enough. “If you can get electrons to pair, they can superconduct,” said Dean.
According to the Bardeen-Cooper-Schrieffer (BCS) theory, a weak attractive interaction between electrons will drive them to pair up and produce a new type of particle known as a “Cooper pair.”
These behave like bosons, which can enter a collective state and flow through a material without being hampered by disorder at low enough temperatures, something that a single electron simply cannot achieve on its own.
But there’s been a problem. “Electrons do not want to pair,” said Dean.
As the saying goes, like repels like. Rather than attempting to create a link between two negatively charged electrons, the team is investigating how opposites can attract to produce a ‘paired’ boson.
The team is now realizing the overall notion, which was first proposed by theoretical physicists, in atom-thin sheets of graphene, a material with special features that they have been working with for several years.
Graphene sheets can be populated with either negatively charged electrons or positively charged holes, depending on the voltages and magnetic fields used. When two of these sheets are combined, electrons on one sheet will want to pair up with oppositely charged holes on the other, generating a bosonic pair. Some distance is still needed.
“People first tried to pair up electrons and holes in a single material and yes there is an attraction between them, but in a sense the attraction is too strong,” said Liu, the lead author of the paper.
If they approach too close, they’ll merge and vanish. The scientists put layers of insulating boron nitride between the graphene in their platform using a technology developed at Columbia to create layered stacks of multiple atom-thin materials.
This established a physical barrier between electrons on one graphene sheet and holes on another, which altered the strength of the link: more insulating layers meant a weaker bond; fewer layers meant a stronger bond.
“By varying the thickness of that separation layer, we have direct, tunable control over the interaction strength,” said Li, another lead author of this work.
To obtain a collective quantum condensate state, electrons and holes must interact not only with each other but also with other pairs of electrons and holes. The team was able to alter the binding strength between electrons and holes by adjusting the number of insulating layers, while changing the external magnetic field modified the interaction between bosonic pairs.
Crossing Over to Raise the Temperature
The majority of superconducting materials can only exist at extremely low temperatures, usually below 10 Kelvin (or -441 degrees Fahrenheit). However, the pair state can sustain temperatures as high as 200K (-100 degrees Fahrenheit) in particular materials known as high-temperature superconductors.
The occurrence of high-temperature superconductors shows that quantum condensate could develop at room temperature, despite the fact that it is still quite cold. Despite decades of effort, progress toward realizing even higher temperature quantum condensates using electron-electron or electron-hole pairs has been modest.
According to one idea, high-temperature superconductors are the product of electron pairing that is neither “weak” nor “strong,” but resides somewhere in the between.
In high-temperature superconductors, studying strong bosonic pairing described by the Bose-Einstein Condensate (BEC) Theory has proved problematic since electrons naturally resist one another, and managing their interaction is difficult.
The scientists can now map for the first time how conductivity changes as pairing strength is adjusted between the BEC and BCS extremes using their tunable graphene platform, which couples electrons with holes rather than electron-electron pairs.
The experiments were conducted at negative 450°F liquid-Helium temperatures and with a powerful 10 Tesla magnet (approximately 100 times stronger than a standard fridge magnet); none of these conditions is viable for building genuine devices that could work on a chip inside a computer. However, according to Dean, the work offers up new research avenues.
“Because of the tunability of this platform, we can test theoretical predictions in ways that have not previously been accessible,” he said.
It may also be able to lose the magnet that is required to move graphene’s generally non-interactive electrons using alternative materials. Semiconductors, for example, can be made to be full of electrons or full of holes.
It will come down to technicalities, such as how ‘clean’ and defect-free the materials are, and whether you can make appropriate contact between them, to get such sheets to produce stable electron-hole pairs.
“If such electron-hole pair condensates also called exciton condensates can be stabilized at high temperatures and without a magnetic field, this might lead to practical uses,” said Halperin, a physicist from Harvard.
“What we’re establishing with this graphene platform is that the underlying concept is absolutely sound,” said Dean. “It’s no longer fantasy; it’s reality. Now it becomes, in a sense, an engineering challenge.”
