In the quantum mechanical world, amazing things can happen when two particles become coupled with each other. In superconductors — materials that conduct electricity with zero resistance — electrons pair up to form “Cooper pairs,” which are able to glide through a material unimpeded. Particle pairs also create superfluids, which flow with no loss of kinetic energy.
Harnessing these particle pairs — known as quantum condensates — could enable power grids that transmit electricity with no loss, quantum computers and other revolutionary applications. The problem is that preserving quantum condensates generally requires impractically cold temperatures. Raising the temperature of energy-loss-free devices will require a better understanding of what drives quantum condensates in the first place.
Now, a team of researchers from Brown, Columbia and Harvard Universities has developed a platform capable of studying quantum condensates in a way that’s never been possible before. The new graphene-based platform enables researchers to precisely tune the strength of condensate pairings, which will allow the team to test theoretical predictions about the origins of bosonic quantum condensation and how they might push its temperature limits.
“We showed that by controlling the pairing strength, we can tune the system to behave like a strongly coupled superfluid, or we can tune it to behave like a weakly coupled superconductor,” said Jia Li, an assistant professor of physics at Brown and co-lead author of the study. We think this could open up a lot of future possibility to study or to engineer quantum systems to realize novel functionalities.”
The research, published in the journal Science, was a collaboration between Cory Dean and Jim Hone at Columbia; Xiaomeng Liu, Philip Kim, and Bert Halperin at Harvard; Li at Brown; and Kenji Watanabe and Takashi Taniguchi at the National Institute for Material Science in Japan.
Designing a tunable platform
The theory underlying superconductivity was first developed in the 1950s with the help of Nobel laureate and Brown Professor Emeritus Leon Cooper. According to the theory, Cooper pairs are formed by the interaction between electrons and quasiparticles called phonons. Under normal circumstances, electrons, which all have a negative charge, repel each other. But in a superconductor, the electron-phonon interaction cancels out the repulsion and creates just enough attractive force for electrons to form Cooper pairs. At low enough temperatures, Cooper pairs behave like particles called bosons, which can enter into a collective state and move through a material without banging into its atomic lattice — something no single electron can do on its own.
But because Cooper pairs have to overcome the normal repulsion between electrons, the pairs are quite fragile. So rather than trying to force a bond between two negatively charged electrons, the research team has been exploring how opposites can attract to yield an equivalent “paired” boson.
The general idea, which was first proposed by theoretical physicists, is now being realized by the team in one-atom-thin sheets of graphene, a material with unique properties that the team has been working to leverage for several years. Depending on the voltages and magnetic fields applied, it’s possible to make graphene sheets that are populated with either negatively charged electrons or with positively charged holes (a spot in an atomic lattice where an electron is absent). When two such sheets are put together, electrons on one sheet will want to pair with oppositely charged holes on the other, forming the bosonic pair.
The key to the setup, researchers say, is creating precise distance between the two sheets.
“People first tried to pair up electrons and holes in a single material, and yes, there is attraction between them, but in a sense the attraction is too strong,” said Liu, co-lead author of the paper. If the two get too close, he said, they’ll combine and disappear.
Using a technique developed at Columbia to create layered stacks of different ultra-thin materials, the team added layers of insulating boron nitride between the graphene in their platform. This created physical distance between electrons on one graphene sheet and holes on another, which also influenced the strength of the interaction: more insulating layers yielded a weaker bond; fewer layers yielded a stronger one.
“By varying the thickness of that separation layer, we have direct, tunable control over the interaction strength,” Li said.
Electrons and holes don’t just need to interact with each other; the bosonic pairs they form also need to interact with other pairs to reach a collective quantum condensate state. By tweaking the number of insulating layers, the team could control the binding between electron-hole pairings while changing the external magnetic field to adjust the interaction between bosonic pairs.
Crossing over to raise the temperature
Most superconducting materials can only exist at extremely cold temperatures, typically less than 10 Kelvin (or negative 441 degrees Fahrenheit). However, in materials called high temperature superconductors, the pair state can survive in temperatures as high as 200K (negative 100 degrees Fahrenheit). Although still very cold, the existence of high-temperature superconductors suggests that quantum condensate could even occur at room temperature. Despite several decades of research, however, the progress to realize even higher temperature quantum condensates using either electron pairs or electron-hole pairs has been slow.
One theory is that high-temperature superconductors result from electron pairing that is neither “weak” nor “strong,” but somewhere in between. Studying strong bosonic pairing — described by the Bose Einstein Condensate (BEC) Theory — has been a challenge in high-temperature superconductors, since electrons naturally repel one another, and controlling their interaction is difficult. With their tunable graphene platform that combines electrons with holes rather than electron-electron pairs, the team can now map for the first time how conductivity changes as pairing strength is shifted between the BEC and BCS extremes.
Here, Dean said, the experiments took place under a magnet that’s 100 times stronger than a typical refrigerator magnet and at liquid-Helium temperatures — negative 450 degrees Fahrenheit — which aren’t practical conditions for building real devices that might operate on a chip inside a computer. Still, he said, the work opens up new avenues of investigation.
“Because of the tunability of this platform, we can test theoretical predictions in ways that have not previously been accessible,” Dean said.
With different materials, he added, it may also be possible to lose the magnet, which is needed to get graphene’s normally non-interactive electrons moving. For example, it’s possible to manipulate semiconductors to be full of electrons and/or full of holes. Getting such sheets to form stable electron-hole pairs will come down to technicalities, like how clean and defect-free the materials are and whether it’s possible to make appropriate contact between them.
“Such electron-hole pair condensates, often called exciton condensates, if they can be stabilized at high temperatures and without a magnetic field, 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,” Dean said. “It’s no longer fantasy; it’s reality. Now it becomes, in a sense, an engineering challenge.”
This story was originally published by Brown University on January 13th, 2022