Researchers from California Institute of Technology (Caltech) and Japan's National Institute for Materials Science have shown that when tungsten diselenide is added to graphene, graphene's electrical properties can be enhanced.
When two or more graphene sheets are stacked on top of each other, the resulting material can exhibit vastly different electronic properties depending on the alignment of those sheets in relation to one another. For instance, when the second sheet of graphene is "twisted" by just 1.05 degrees (a value known as the "magic angle") in relation to the sheet it is laid on top of, the resulting stack can be either a superconductor that conducts electricity with absolutely no resistance whatsoever or an insulator that completely blocks the passage of electricity. Research conducted in 2022 shows that even untwisted graphene bilayers can exhibit superconductivity. Untwisted bilayers of graphene are easier to fabricate in bulk than their twisted counterparts, but the superconductive state in these untwisted bilayers is more delicate, harder to tune, and only occurs at temperatures that are about a hundred times lower than in twisted structures (such temperatures typically can only be achieved through the use of liquid helium). The recent research shows a way to significantly improve upon this fragile superconductivity with tungsten diselenide.
In the new work, Caltech's Stevan Nadj-Perge, assistant professor of applied physics and materials science, and his colleagues discovered that when tungsten diselenide is placed on top of graphene bilayers, the untwisted graphene's superconductivity is greatly improved. Notably, the superconducting critical temperature—that is, the warmest temperature at which the material can superconduct—is enhanced by a factor of 10. By being in close proximity to graphene, tungsten diselenide bestows the benefits of the "magic angle" twist to the more mass-producible untwisted graphene. This finding provides new insight into the nature of superconductivity and suggests strategies for enhancing superconductivity in other related graphene-based materials.
"These graphene bilayer devices are remarkably tunable," says Nadj-Perge. "For example, by applying electric fields, we can add or remove electrons from the bilayer as well as push them toward and away from tungsten diselenide. This allowed us to carefully study the enhancement of superconductivity in the system."
"The high level of tunability opens up possibilities for future applications," Nadj-Perge continues. "One of the main advantages of untwisted graphene superconductors compared to their twisted counterparts is that they are much cleaner in terms of disorder and defects, and technically much easier to fabricate. That implies these structures may be more suited for applications where one would need to make many identical copies of the same device architecture."