Study demonstrates tailored Ising superconductivity in intercalated bulk niobium diselenide

When 2D layered materials are made thinner (i.e., at the atomic scale), their properties can change drastically, sometimes causing entirely new features to emerge and others to be lost. Although new or emerging properties can be very beneficial for the development of new technologies, it is often just as important to retain some of the original properties of the material.

Researchers from Tsinghua University, the Chinese Academy of Sciences, and the Frontier Science Center for Quantum Information were recently able to achieve tailor-made Ising superconductivity in a sample of intercalated bulk niobium diselenide (NbSe₂), a feature of bulk NbSe₂ which is generally compromised in atomically thin layers. The methods they used, described in an article published in Natural Physicscould pave the way for the fabrication of superconducting materials in 2D thin layers.

“Atomically thin 2D materials exhibit interesting properties that are often distinct from their bulk materials, which consist of hundreds and thousands of layers,” Shuyun Zhou, one of the researchers who conducted the study, told Phys.org. “However, atomically thin films/flakes are difficult to fabricate, and emerging new properties are sometimes achieved by sacrificing some other important properties.”

Zhou and his colleagues have been trying to identify experimental methods to obtain new properties comparable to atomically thin samples without losing any vital material properties for a few years now. In their recent study, they specifically evaluated the efficiency of electrochemical intercalation, a valuable strategy for tuning the electronic properties of solid layered materials.

“The bulk material is immersed in the ionic liquid, which consists of cations and anions,” Zhou explained. “Such ionic liquids have been widely used to inject electrons into samples at a few layers, while the ions remain in the liquid. We have found that by applying a higher negative voltage, large organic cations can be entrained in the van der Waals gap (the empty space between active layers, NbSe₂ layers in this case), forming hybrid materials.”

In their experiments, Zhou and co-workers found that intercalation is an effective strategy to control both the dimensionality and the carrier concentration of their NbSe₂ layered sample. Using this strategy, they were able to achieve a tailored Ising superconductivity that exceeded both that seen in bulk NbSe₂ NbSe crystals and monolayer₂ samples, but in an intercalated NbSe volume₂ to taste.

Essentially, intercalation strategies consist of the immersion of a bulk material in an ionic liquid and the subsequent application of an electrical voltage. This process causes the spacing between the active layers of a bulk layered material to increase, thereby reducing the interactions between them.

“Although the intercalated NbSe₂ the material always consists of many layers, its properties behave quite similar to those of single-layer NbSe₂ samples,” Zhou said. “Specifically, the superconductivity of the intercalated material can survive under a large in-plane magnetic field, but the superconducting transition temperature is higher than the NbSe monolayer.₂. Additionally, cations can transfer charges to active layers and act as protective layers, making the hybrid material stable in air.”

While Zhou and co-workers specifically used their intercalation-based strategy to expand the properties of a layered 2D NbSe₂ sample, the same strategy could also be applied to a wide range of layered materials to achieve properties comparable to, or even better than, single-layer versions of these materials. So far, this method has made it possible to customize the Ising superconductivity in NbSe₂enhanced superconductivity in the Weyl semimetal MoTe₂ and semiconductor-superconductor transition in SnSe₂.

“Our intercalation method is quite generic and can be easily extended to a wide variety of layered materials and a large selection of ionic liquids with different cations,” Zhou added. “Therefore, our work provides an important pathway to create hybrid materials with tunable functionality eventually exceeding bulk crystals and single-layer samples. Besides superconductors, we would like to apply this strategy to many other layered materials to obtain properties We hope that through intercalation, intriguing properties beyond both bulk crystals and monolayer samples will soon be enabled in an increasing number of layered materials.”

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