A type of material that allows the flow of electricity with little resistance is called a conductor. Like the copper in the electrical wiring in your house, the resistance is low, but it still exists. This resistance means that the electricity requires energy to flow through the wiring, and that energy loss heats up the wire. This is why too much current can cause electronics to overheat.
A special type of material with zero electrical resistance is called a superconductor. Without resistance, no energy is lost to heat and electric current will flow through a superconducting wire even after the power source is removed. In most cases, scientists must cool the material to extremely low temperatures to achieve superconductivity. For example, mercury is superconductive at or below a temperature of -452°F, or -269°C.
Scientists have discovered that certain nanomaterials can act like superconductors. A single layer of carbon atoms arranged in a hexagonal pattern like a honeycomb is called graphene. In recent years, scientists discovered that when two layers of graphene are stacked and slightly twisted relative to each other, they become superconductive.
This structure is referred to as magic-angle twisted bilayer graphene, or MATBG, where the “magic-angle” refers to the specific amount of twisting that leads to superconductivity. Scientists suggest the superconductivity in MATBG arises because the twisting leads to a mismatch in the stacked atoms, such that one carbon atom is no longer directly below the carbon atom in the second layer. These overlaid mismatched patterns are referred to as a moiré pattern in mathematics.
Scientists more recently explored a system of three graphene layers twisted relative to each other, or magic-angle twisted trilayer graphene (MATTG), and discovered superconductivity existed when the middle layer was twisted at specific angles. When a superconductor is placed in a magnetic field, there is a critical magnetic field strength at which the superconductor becomes a conductor again. Scientists found that the superconductivity in the three layer system persisted in magnetic fields approximately three times larger than in the two layer system. This led them to wonder if these layered graphene systems were actually part of a larger family of moiré superconductors.
Scientists from MIT recently fabricated and tested graphene devices with four and five layers, referred to as MAT4G and MAT5G respectively. Using a laser, they cut small graphene flakes approximately the same width as a grain of pollen. They then stacked the graphene flakes, alternating which layer was twisted. In the four layer structure, the bottom and third layer were aligned and the second and fourth layer were aligned. In the five layer structure, the bottom, middle, and top layers were aligned. The amount of twisting, or magic angle, was computed from a theoretical model. Using this technique, the scientists fabricated four MAT4G devices and six MAT5G devices.
The scientists placed the graphene devices into an ultra cold freezer and ran electrical current through each device. By measuring the output, they could determine when the resistance went to zero. The scientists measured superconductivity in three of the four MAT4G devices and all six MAT5G devices. In previous experiments on MATBG, only about half the devices exhibited superconductivity. The scientists explained that the two layer structure was more likely to relax back toward smaller angles and stop superconducting.
The scientists then explored the superconductive state of each device under a magnetic field. They suspected the superconductive behavior related to the atomic symmetry in each system, and therefore depended on whether the number of layers was even or odd. Following this reasoning, they expected the superconductivity in MAT5G to persist at higher magnetic fields than MAT4G, similar to what was previously observed in two and three layer devices. However, the scientists were surprised to find both devices remained superconductive under strong magnetic fields, similar to MATTG.
The scientists explained that the magnetic field essentially adds enough momentum to the atoms that the electrons can tunnel between two layers. In the two layer system, this tunneling can break up electron pairs that are essential for superconductivity. When there are more layers in the structure, these effects cancel each other out and the electron pairs persist.
The scientists also discovered that all the magic angle layered graphene devices exhibited a special electronic structure where energy is independent of momentum, called a flat band. In other words, all the electrons in the material have the same energy regardless of how fast they are moving. In this state, electrons can slow down and pair up, which is a likely explanation for how superconductivity in these materials arise. They suggest future work could further explore the relationship between flat bands and superconductivity. They also propose expanding the research to other moiré superconductors and their potential applications.