Last year, physicists reported that, when chilled to 1.7°C above absolute zero (–273°C), sheets of carbon atoms two layers thick can conduct electricity without resistance, allowing electrons to whiz through the material without losing any energy. The double sheets of carbons, known as bilayer graphene, have captivated researchers because their structural simplicity offered a platform to explore the complex physics of superconductivity, which is also exhibited in copper-oxide materials at much higher temperatures. Now, researchers have discovered signs of superconductivity in easy-to-make three-layer sheets of graphene, renewing hope that layered graphene will soon help researchers understand how superconductivity occurs in copper-oxides. That could lead to higher temperature superconductors—or even room temperature ones—which could produce massive energy savings in electrical grids and devices.
“It’s definitely an exciting development,” says Cory Dean, a physicist at Columbia University. Dean notes that bilayer graphene superconducts only when the atomic lattices of the two graphene layers are twisted with respect to one another by a “magic” angle of 1.1°—a difficult maneuver to perform on the thinnest material known. “If you’re off by a little bit it doesn’t work,” Dean says. The trilayer graphene, by contrast, doesn’t have to be twisted. Rather, the atomic lattice of each layer aligns with those above and below, which happens naturally when multilayer graphene is produced.
David Goldhaber-Gordon, a physicist at Stanford University in Palo Alto, California, and Feng Wang, a physicist at the University of California, Berkeley, and their colleagues followed a now-standard approach for isolating flakes of graphene. It starts by sticking a piece of Scotch tape on a bulk piece of graphite—the “lead” in most pencils—and peeling it off. Repeating the process leaves flakes of graphene sticking to the tape, some just a single sheet thick, but others with two and three layers. Wang’s team previously pioneered a technique to spot unique optical signatures in trilayer graphene.
The team then used these trilayer flakes as a starting material to make electrical devices. They sandwiched trilayer flakes between layers of boron nitride, which isolate the graphene from contaminants and prevent it from buckling. In some places, the atoms in the boron nitride layers line up precisely with the carbon atoms in the graphene layers, but a few nanometers away they are offset. After about 10 nanometers the atoms in the layers align once again, creating a “moiré” repeating pattern that is also apparent in the twisted bilayer graphene. Each repeated moiré cell can hold up to four extra electrons, in addition to those in the material, altering the material’s conductivity.
Next, the researchers patterned metals on top of the flakes, building transistors with “gates” that control the addition of electrons to the material. By manipulating the electric field on their gates, the researchers were able to control exactly how many electrons were present in each repeated moiré cell. When they added three electrons to each cell and dropped the temperature below 2 kelvins, they noticed a sharp drop in electrical resistance, a sign of superconductivity, which they report today in Nature. They also noticed that when they applied an external magnetic field to their sample, the near-zero electrical resistance vanished, another sign of superconductivity. “All these things check the boxes [of superconductivity],” Goldhaber-Gordon says. But he adds the signals are not yet definitive. For one, the electrical resistance does not drop completely to zero, which is required for a superconductor. However, he points out, this could be due to impurities in the graphene flakes. “It may not be superconducting everywhere within the device,” he says.
Still, Goldhaber-Gordon notes that the apparent superconductivity from the three extra electrons is similar to what is seen in conventional high-temperature superconductors, the copper-based materials that were discovered in 1986. For Dean, that raises hopes that trilayer graphene will be a good model system for solving that long-standing mystery. Trilayer graphene, he says, “is such a clean system it provides a simple way to explore complex physics.”