This is a schematic (top) and scanning electron microscope image (bottom) of the suspended double layer of molybdenum disulfide with ionic liquid gating. Image: Justin Ye group, Zernike Institute for Advanced Materials, University of Groningen.In superconducting materials, an electric current can flow without any resistance. While there are quite a few practical applications for this phenomenon, many fundamental questions remain unanswered. Now, by studying superconductivity in a double layer of molybdenum disulfide, Justin Ye, head of the Device Physics of Complex Materials group at the University of Groningen in the Netherlands, has managed to develop a superconducting transistor and discover some new superconducting states. He and his colleagues report these findings in a paper in Nature Nanotechnology.
Superconductivity has already been demonstrated in monolayer crystals of molybdenum disulphide or tungsten disulfide with a thickness of just three atoms. “In both monolayers, there is a special type of superconductivity in which an internal magnetic field protects the superconducting state from external magnetic fields,” Ye says.
Normal superconductivity disappears when a large external magnetic field is applied, but so-called Ising superconductivity is strongly protected. Even when exposed to the strongest static magnetic field in Europe, which has a strength of 37 Tesla, the superconductivity in tungsten disulfide does not show any change. Although it is great to have such strong protection, the next challenge is to find a way to control this protective effect, by applying an electric field.
Ye and his collaborators decided to study a double layer of molybdenum disulfide. “In that configuration, the interaction between the two layers creates new superconducting states,” he says.
They created a suspended double layer by sandwiching the double layer of molybdenum disulfide between an ionic liquid, thereby forming an electric field across the bilayer. “In the individual monolayer, such a field will be asymmetric, with positive ions on one side and negative charges induced on the other,” explains Ye. “However, in the bilayer, we can have the same amount of charge induced at both monolayers, creating a symmetrical system.”
This electric field can be used to switch molybdenum disulfide’s superconductivity on and off, thus creating a superconducting transistor gated through the ionic liquid.
In the double layer, the Ising protection against external magnetic fields disappears, because of changes in the interaction between the two layers, but the electric field can restore protection. “The level of protection becomes a function of how strongly you gate the device,” says Ye.
Apart from creating a superconducting transistor, Ye and his colleagues made another intriguing observation. In 1964, a special superconducting state was predicted to exist, called the FFLO state (named after the scientists who predicted it: Fulde, Ferrell, Larkin and Ovchinnikov). In superconductivity, electrons travel in pairs in opposite directions. Since they travel at the same speed, these Cooper pairs have a total kinetic momentum of zero. But in the FFLO state, there is a small speed difference and therefore the kinetic momentum is not zero. So far, this state has never been properly studied in experiments.
“We've met nearly all the prerequisites to prepare the FFLO state in our device,” says Ye. “But the state is very fragile and is significantly affected by contaminations on the surface of our material. We will, therefore, need to repeat the experiments with cleaner samples.”
With the suspended bilayer of molybdenum disulfide, Ye and his collaborators have all the ingredients needed to study some special superconducting states. “This is truly fundamental science that might bring us conceptual changes,” he says.
This story is adapted from material from the University of Groningen, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.