The superconductivity of magic graphene and that of high-temperature superconductors have an uncanny resemblance

The scientific community was taken aback by the finding of superconductivity in two single-atom-thick layers of graphene layered at an exact angle of 1.1 degrees (called ‘magic’-angle twisted bilayer graphene) in 2018. Since its discovery, physicists have debated whether the superconductivity of magic graphene can be explained using current theory, or if completely new techniques, such as those being developed to describe the mystery ceramic substance that superconducts at high temperatures, are necessary.

Princeton researchers have now put an end to the controversy by demonstrating an amazing likeness between the superconductivity of magic graphene and that of high-temperature superconductors, as published in the journal Nature. Magic graphene might be the key to unlocking novel superconductivity processes, such as high-temperature superconductivity.

The research was headed by Ali Yazdani, the Class of 1909 Professor of Physics and Director of Princeton University’s Center for Complex Materials. Over the years, he and his colleagues have explored a variety of superconductors, and they have recently focused on magic bilayer graphene.

“Some have argued that magic bilayer graphene is actually an ordinary superconductor disguised in an extraordinary material,” said Yazdani, “but when we examined it microscopically it has many of the characteristics of high-temperature cuprate superconductors. It is a déjà vu moment.”

One of nature’s most fascinating phenomena is superconductivity. It’s a condition in which electrons are free to move without encountering any barrier. Electrons are subatomic particles with negative electric charges that power our everyday gadgets; they are essential to our way of life.

STM is a perfect tool for doing these types of experiments. There are many different measurements that STM can do. It can access physical variables that are typically inaccessible to other experimental techniques.

Kevin Nuckolls

In normal circumstances, electrons act erratically, hopping and jostling against one another in an inefficient and energy-wasting way. However, with superconductivity, electrons couple up and begin to flow in synchrony, much like a wave.

The electrons in this form not only do not lose energy, but they also exhibit a variety of unique quantum features. These qualities have led to a variety of practical applications, such as MRI magnets and particle accelerators, as well as the development of quantum bits for use in quantum computers.

Superconductivity was initially observed in elements like aluminum and niobium at extremely low temperatures. It’s been discovered in ceramic compounds at temperatures slightly above the boiling point of liquid nitrogen (77 degrees Kelvin) and at temperatures near to room temperature under extremely high pressure in recent years. However, not all superconductors are the same.

Researchers refer to superconductors consisting of pure materials like aluminum as “conventional.” The Bardeen-Cooper-Schrieffer (BCS) hypothesis explains the superconductive condition in which the electrons couple together. Since the late 1950s, this has been the accepted definition of superconductivity.

However, beginning in the late 1980s, additional superconductors that did not suit the BCS hypothesis were identified. The ceramic copper oxides (called cuprates) that have remained a mystery for the past thirty years are the most remarkable of these “unconventional” superconductors.

The first discovery of superconductivity in magic bilayer graphene by Pablo Jarillo-Herrero and his colleagues at the Massachusetts Institute of Technology (MIT) demonstrated that the material begins as an insulator but turns superconducting with the addition of a few charge carriers. One of the features of many unusual superconductors, notably the cuprates, is the formation of superconductivity from an insulator rather than a metal.

“They suspected that superconductivity could be unconventional, like the cuprates, but they, unfortunately, did not have any specific experimental measurements of the superconducting state to support this conclusion,” said Myungchul Oh, a postdoctoral research associate and one of the lead co-authors of the paper.

Oh and his colleagues utilized a scanning tunneling microscope (STM) to see the infinitesimally small and complicated world of electrons to explore the superconductive capabilities of magic bilayer graphene. This gadget works by funneling electrons between the microscope’s sharp metallic tip and the sample, a process known as “quantum tunneling.” The microscope views the world of electrons on the atomic scale using this tunneling current rather than light.

“STM is a perfect tool for doing these types of experiments,” said Kevin Nuckolls, a graduate student in physics and one of the paper’s lead co-authors. “There are many different measurements that STM can do. It can access physical variables that are typically inaccessible to other experimental techniques.”

When the researchers examined the data, they observed two distinct features, or “signatures,” that indicated that the magic bilayer graphene sample was displaying unusual superconductivity. The first hint was that superconducting paired electrons had a limited angular momentum, similar to what was discovered in high-temperature cuprates twenty years ago.

In a normal superconductor, when pairs form, they have no net angular momentum, similar to an electron coupled to a hydrogen atom in the hydrogen’s s-orbital.

Tunneling electrons in and out of the material is how STM works. The current between the sample and the STM tip is only feasible when the superconductor’s pairs are split apart in a superconductor, where all the electrons are coupled.

“It takes energy to break the pair apart, and the energy dependence of this current depends on the nature of the pairing. In magic graphene we found the energy dependence that is expected for finite momentum pairing,” Yazdani said. “This finding strongly constrains the microscopic mechanism of pairing in magic graphene.”

The Princeton researchers also revealed how the superconducting state of magic bilayer graphene is quenched by raising the temperature or adding a magnetic field. When superconductivity is eliminated in typical superconductors, the material behavior is identical to that of regular metal.

In unusual superconductors, however, the electrons appear to retain some correlation even when they are not superconducting, a circumstance that occurs when the energy required to remove electrons from the sample is about equal. This threshold energy is referred to as a “pseudogap” by physicists, a phenomenon seen in the non-superconducting state of many unusual superconductors. For more than two decades, its genesis has remained a mystery.

“One possibility is that electrons are still somewhat paired together even though the sample is not superconducting,” said Nuckolls. “Such a pseudogap state is like a failed superconductor.”

The third hypothesis, as mentioned in the Nature publication, is that before superconductivity can occur, some other sort of collective electronic state, which is accountable for the pseudogap, must first exist.

“Either way, the resemblance of an experimental signature of a peusdogap with the cuprates as well as finite momentum pairing can’t be all a coincidence,” Yazdani said. “These problems look very much related.”

According to Oh, future studies will focus on figuring out what causes electrons to the couple in unconventional superconductivity, a phenomena that continues to perplex scientists. The BCS hypothesis is based on electrons’ weak interaction, with their pairing enabled by their mutual interaction with the ions’ underlying vibration.

However, the pairing of electrons in unconventional superconductors is frequently more stronger than in basic metals, but the reason of the “glue” that holds them together is unknown at this time.

“I hope our research will help the physics community to better understand the mechanics of unconventional superconductivity,” Oh said. “We further hope that our research will motivate experimental physicists to work together to uncover the nature of this phenomenon.”

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