Superconductivity can be switched on and off in “magic-angle” graphene using a short electrical pulse, according to new work by researchers at Massachusetts Institute of Technology (MIT). Until now, such switching could only be achieved by sweeping a continuous electric field across the material. The new finding could help in the development of novel superconducting electronics such as memory elements for use in two-dimensional (2D) materials-based circuits.
Graphene is a 2D crystal of carbon atoms arranged in a honeycomb pattern. Even on its own, this so-called “wonder material” boasts many exceptional properties, including high electrical conductivity as charge carriers (electrons and holes) zoom through the carbon lattice at very high speeds.
In 2018, researchers led by Pablo Jarillo-Herrero of MIT found that when two such sheets are placed on top of each other with a small angle misalignment, things become even more fascinating. In this twisted bilayer configuration, the sheets form a structure known as a moiré superlattice, and when the twist angle between them reaches the (theoretically predicted) “magic angle” of 1.08°, the material begins to show properties such as superconductivity at low temperatures – that is, it conducts electricity without any resistance.
At this angle, the way in which electrons move in the two coupled sheets changes because they are forced to organize themselves at the same energy. This leads to “flat” electronic bands, in which electron states have exactly the same energy despite having different velocities. This flat band structure makes electrons dispersionless – that is, their kinetic energy becomes completely suppressed and they cannot move in the moiré lattice. The result is that the particles slow almost to a halt and become localized at specific positions along the coupled sheets. This enables them to interact strongly with one another, forming the pairs that are a hallmark of superconductivity.
The MIT team has now discovered a new way to control magic-angle graphene by paying attention to its alignment when sandwiched between two layers of hexagonal boron nitride (hBN, a 2D insulator). The researchers aligned the first layer of hBN exactly with the top graphene sheet, while the second layer was offset by an angle of 30° with respect to the bottom graphene sheet. With this arrangement, they could engineer bistable behaviour in which the material can sit in one of two stable electronic states, allowing its superconductivity to be switched on or off with a short electrical pulse.
“Surprisingly, this bistability coexists without disrupting the behaviour of the magic-angle graphene,” explains lead author Dahlia Klein. “This system is a rare example of a discrete switch to turn superconductivity on and off with just an electrical pulse – something that could allow it to be used as a non-volatile superconducting memory device.”
Magic-angle graphene switches from superconductor to ferromagnet
Such a memory element could be incorporated into future 2D material-based circuits, she adds.
While the researchers are unsure as to exactly what enables this switchable superconductivity, they suspect that it is related to the special alignment of the twisted graphene to both the hBN layers. The team has seen similar bistabilities before in untwisted bilayer graphene aligned to its sandwiching hBN layers and therefore hopes to solve this puzzle in future work. “There is an ongoing effort between both experimentalists and theorists to pinpoint exactly how these hBN–graphene alignments give rise to the unexpected behaviour we have observed,” Klein tells Physics World.
The work is detailed in Nature Nanotechnology.
