In a breakthrough that could expand the capabilities of next-generation electronics, a University of Wisconsin-Madison engineering team has discovered a new bilayer 2D crystalline material that is both superconducting and ferroelectric.
An electric current can exist indefinitely in a superconducting material, which has no electrical resistance. A ferroelectric material has a spontaneous electric polarization, and applying an electric field to the material can flip that polarization. Ordinarily, these two unique properties cannot coexist: the electrons that pair up and enable superconductivity preclude any potential polarization.
Synthesized and tested by a team led by Daniel Rhodes, an assistant professor of materials science and engineering at UW-Madison, the material is the first to demonstrate these two properties occurring together. He and his colleagues describe the advance in the January 2023 issue of the journal Nature.
In 2004, researchers synthesized the first 2D material, graphene, by pulling tape off graphite, a material best known as pencil lead. Since then, researchers—including Rhodes, who is a world expert on crystal synthesis of this unique family of materials—have developed many other 2D crystals, which are often arranged in a one-atom-thick lattice. Many of these ultrathin materials have novel properties that include superconductivity or ferroelectricity.
In his research, Rhodes investigated the 2D superconductor molybdenum ditelluride (MoTe2). During his time as a postdoctoral researcher at Columbia University, he investigated single layers of molybdenum ditelluride, and discovered that it was a 2D superconductor. However, this time, when he stacked two layers on top of each other, he found that the bilayer molybdenum ditelluride took on new properties. “Not only did it end up being a two-dimensional superconductor, it also ended up being ferroelectric,” he says. “Those two properties have never been seen coupled together in a singular material in the same ground state ever before.”
The two traits interact with one another as well: The ferroelectricity can be a “tuning knob” to control the superconductivity or switch it on and off. “We see the behavior from the ferroelectricity translates to behavior in the superconductivity,” says Rhodes. “And that’s an entirely new concept.”
He says the basis for superconductivity in the material also appears to be unique. Most superconductivity arises when electrons couple with energy-carrying particles known as phonons. That doesn’t seem to be what’s happening with the bilayer molybdenum ditelluride—but the team is working on a hypothesis to fully explain it. “As far as we can tell, there has to be some pairing interaction between electrons and holes,” says Rhodes, “which is very, very special.”
Rhodes and his team plan to continue work on bilayer molybdenum ditelluride and hope to further explore the interplay between ferroelectricity and superconductivity and how it evolves through various other methods. “This is the first time that we can really see the interplay between ferroelectricity and superconductivity,” Rhodes says. “We’re just really interested in exploring all the consequences that come out of that.”
Zizhong Li, a graduate student in materials science and engineering at UW-Madison, is a co-author on the Nature paper. Other authors include Apoorv Jindal, James C. Hone, Cory R. Dean and Abhay N. Pasupathy at Columbia University; Amartyajyoti Saha, Turan Birol and Rafael Fernandes at the University of Minnesota; and Takashi Taniguchi and Kenji Watanabe at the National Institute for Materials Science, Tsukuba, Japan.