Ability to turn superconductor turning it off and on with a literal flip of a switch in the so-called “magic angle twist graphene” has allowed engineers at Caltech to observe an unusual phenomenon that could shed more light on the phenomenon. general superconductivity.
The study, led by Stevan Nadj-Perge, assistant professor of applied physics and materials science, was published in the journal Nature. Nature.
Magic-angle twisted graphene, first discovered in 2018, is made of two or three sheets of graphene (a form of carbon consisting of a layer of atoms in a honeycomb-like lattice pattern) layered on top of each other, with each plate twisted at exactly 1.05 degrees below it. The resulting double-layer or triple-layer has unusual electronic properties: for example, it can be made an insulator or a superconductor depending on how many electrons are added.
Superconductors are materials that exhibit a special electronic state in which electrons can flow freely through the material without resistance – that is, an electric current flows through them without loss of heat energy. Such super-efficient power transmission has a multitude of potential applications in computing, electronics and other fields.
However, the drawback of superconductivity is that in most materials it occurs at extremely low temperatures, usually only a few degrees above absolute zero (−273.15 degrees Celsius). At such temperatures, the electrons forming pairs behave in a fundamentally different way than the individual electrons and condense into a quantum mechanical state that allows the electron pairs to move without scattering.
Superconductivity was first discovered more than a century ago, but scientists still don’t fully understand the exact mechanism behind electron pair formation for some materials. In conventional superconductors, such as metallic aluminum, it is well understood that the attraction between electrons leading to the formation of electron pairs is due to the interaction of electrons with the material’s 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 share the 1972 Nobel Prize in Physics for the development of theory.
While studying graphene’s magical angled triple layers, Nadj-Perge and his colleagues discovered that the superconductivity in the material exhibits some very unusual properties that cannot be described. by BSC theory, making it an unusual superconductor.
They measured the evolution of the so-called superconducting gap when electrons are separated from the three-layer layer by flipping a switch to turn the electric field on or off. Superconducting gap is a property that describes how difficult it is to add or remove individual electrons to a superconductor. Since electrons in a superconductor want to be paired, a certain amount of energy is required to break the pairs. However, the amount of energy can be different for pairs moving in different directions relative to the lattice. As a result, “gaps” have a specific shape determined by the likelihood that the pairs will be broken by a particular amount of energy.
“Although superconductors have been around for a long time, a notable new feature in twisted graphene double and triple layers is the ability to superconductivity in these materials,” says Nadj-Perge. up simply by applying a voltage to a nearby electrode. The author of Nature paper. “An electric field effectively adds or removes excess electrons. It works in a way that is very similar to how current is controlled in conventional transistors, and this allows us to explore superconductivity in ways that are not possible in other materials.” .
The team determined that in the twisted three-layer layers, there are two modes of superconductivity with different shaped superconducting gap profiles. While one of the modes can be explained by a theory to some extent similar to that of BCS, the presence of two modes suggests that during the superconducting phase there is a possibility of a process additional conversion. This observation, along with measurements made at different temperatures and magnetic fields, points to the unusual nature of superconductivity in triple layers.
The Nadj-Perge team’s new insights provide essential clues for future theories of superconductivity in twisted graphene layers. Nadj-Perge notes that it appears that more layers make superconductivity stronger while remaining highly tunable, a property that opens up many possibilities for using three twisted layers for superconducting devices that could one day be used. could be used in quantum science and perhaps quantum information processing.
“Besides its fundamental implications for our understanding of superconductivity, it is remarkable that adding an extra layer of graphene makes it easier to study superconducting properties. Ultimately this is what triggered our findings,” said Nadj-Perge.
Written by Robert Perkins
Source: Caltech
