Caltech engineers have been able to observe an unusual phenomenon which may shed light on superconductivity as a whole.
Stevan Nadj Perge, assistant professor in applied physics, conducted the research and it was published in Nature on June 15.
The 2018 discovery of magic-angle twisted graphene is composed of two or three sheets each of graphene. This graphene form, which is a type of carbon that consists of one layer of atoms in an intricate honeycomb-like lattice structure, is layered on top of one another with each sheet being twisted exactly 1.05 degrees relative to the one below. The resultant bilayer or trilayer can have unusual electronic properties. It can be made to an insulator, or superconductor, depending on how many electrons were added.
Materials that are superconductors have an electronic state that allows electrons to flow freely through them without resistance. This means that electricity flows through them with no loss of heat energy. This super-efficient transmission of electricity can be used in many fields, including electronics and computing.
Superconducting occurs at very low temperatures. Usually, they are only a few degrees below absolute zero (-273.15 Celsius). These temperatures cause electrons to form pairs which behave fundamentally differently to individual electrons. They condense to a quantum mechanical state that allows electron pairs to flow freely without being scattered.
Although superconductivity was discovered over a century ago, scientists are still unsure of the exact mechanisms that lead to the formation of electron-pairs in some materials. It is known that superconductors such as metal aluminium, attract electrons, which leads to formation of electron pairs. This is because electrons interact with the crystal lattice. The behavior of these materials is described using the Bardeen-Cooper-Schrieffer (BCS) theory, named after John Bardeen, Leon Cooper, and John Robert Schrieffer, who shared the Nobel Prize in Physics in 1972 for the theory's development.
Nadj Perge and his colleagues discovered superconductivity in graphene. They studied trilayers of graphene that were magic-angle twisted, which was a very rare property that could not be explained using BSC theory. This makes it likely that this superconductor is also unusual.
The evolution of the superconducting gap was measured as electrons were removed from the trilayer using a flip switch to turn on or off an electric field. The superconducting gaps is a property that indicates how difficult it can be to add or subtract individual electrons from a superconductor. A superconductor's electrons want to be paired so it requires a certain amount energy to separate those pairs. The amount of energy required to break pairs can vary depending on how they move relative to the crystallized lattice. The likelihood of pairs being broken by a certain amount of energy determines the shape of the "gap".
Nadj Perge, the Nature paper's corresponding author, says that although superconductors are known for many years, one of the most striking new features in twisted graphene trilayers and bilayers is the ability to turn on superconductivity by simply applying a voltage to a nearby electrode. An electric field adds or subtracts electrons. It functions in the same way that conventional transistors control current. This allowed us to study superconductivity in ways other materials cannot.
Two superconductivity regimes exist in twisted trilayers. Each has a different superconducting gap profile. Although one of these regimes could be explained by a theory similar to BCS to some degree, the existence of both regimes suggests that there is likely to be an additional transition within the superconducting phase. These observations, along with measurements at different temperatures and magnetic fields, point to the unusual nature of superconductivity within the trilayers.
These new insights from Nadj Perge's team provide crucial clues to the future theories about superconductivity within twisted graphene multilayers. Nadj Perge points out that superconductivity becomes more robust with more layers. This makes it more tunable. This property opens up many possibilities for using twisted trilayers in superconducting devices, which could one day be used in quantum science or quantum information processing.
"In addition to its fundamental implications for our understanding of superconductivity it is amazing that an additional graphene layer made superconducting properties more accessible. This is ultimately what allowed our findings," Nadj Perge states.
"Evidence for unconventional superconductivity within twisted trilayer graphene" is the title of the paper.