HIGHLIGHTS
• A two-dimensional superconductor was developed by twisting one layer at an angle of 45 degrees relative to the second layer.
• A unique cryogenic solvent-free Van der Waals bonding transfer technique was used to prepare
the two-dimensional twisted superconductor. 
• This superconductor is considered to be a new phase of matter. 

Identifying superconductors that can operate at temperatures close to ambient remains an elusive goal. From a tribology standpoint, working with superconductors represents the ultimate goal because these materials do not generate friction. Their potential use in manufacturing and transportation applications would lead to the ability to conduct and store electricity without loss. As sustainability becomes more important, superconductors, if identified, may provide an approach for saving energy and minimizing emissions. 

The potential for using superconductors was detailed in a previous TLT article¹ which discussed the concept of a superconducting highway as a means to significantly reduce fossil fuel consumption. The authors of this concept envision vehicles equipped with permanent magnets levitating above a superconducting guideway which has the function of transmitting and storing electrical power. The added bonus is that the highway can be used to transport and store liquid hydrogen which will be needed to cool superconducting cables established underneath the highway. If such a concept is commercialized, the authors predict that automobiles will be able to travel five times faster at speeds up to 640 kilometers per hour which will enable individuals to move at a rate comparable to an airplane. While travel is taking place through levitation, emissions will be reduced because internal combustion engines will not be required. 

One of the most widely studied classes of superconductors is known as cuprates, which are compounds based on copper oxides. Upon discovery decades ago, cuprates were found to exhibit superconductivity at temperatures as high as -140 °C. This temperature was much higher than theorists had predicted. 

Two-dimensional materials are undergoing intensive study over the past decade because they have been found to exhibit unique properties. One of the most studied two-dimensional materials is graphene which is showing promise as a lubricant. Jedediah Pixley, associate professor in the department of physics and astronomy at Rutgers University in Piscataway, N.J., says, “Neighboring atomic layers have been found to interact through Van der Waals bonding. This weak bonding represents an opportunity for identifying new materials with unique properties.” 

One approach for synthesizing new materials is to literally twist one layer to a particular angle with respect to an adjacent layer. Pixley says, “This potential for twisting layers of atoms was discovered in 2010² and eventually led to the field now known as twistronics. Past theoretical work³ had determined how twisting one layer of graphene versus a second layer will change the material’s properties most dramatically at a predicted ‘magic-angle.’” Pixley and collaborators generalized these ideas to a broad class of systems⁴ including superconductors.⁵ 

Superconducting cuprates are three-dimensional materials, but if atomic sheets could be prepared, then there is potential for realizing a two-dimensional superconductor and then manipulating it by twisting. Such an approach has now been accomplished on the cuprate Bi2Sr2CaCu2O8+y known colloquially as BSCCO (bismuth strontium calcium copper oxide).^6,7  

45-degree twist 
Pixley and his colleagues conducted a theoretical study to determine how the twist angle controls the nature of the Josephson junction that is formed at the interface between a twisted pair of superconducting flakes. He says, “We evaluated the critical supercurrent at the interface between the two layers of BSCCO at various angles and found that twisting one layer at an angle of 45 degrees relative to the other led to the preparation of a new superconductor which we believe is a new phase of matter.” 

Verifying the results from the theoretical study proved to be challenging. Pixley says, “BSCCO starts exhibiting superconductivity at the relatively high temperature of -177 °C which is above the boiling point of nitrogen (-196 °C). This means that liquid nitrogen can be used to keep the temperature of BSCCO low enough to maintain superconductivity in an inert environment.” 

The researchers in Professor Philip Kim’s laboratory at Harvard University created a unique cryogenic solvent-free Van der Waals bonding transfer technique at a temperature less than -90 °C in an argon atmosphere. The temperature was controlled through liquid nitrogen cooling. 

An exfoliated BSCCO crystal that was extremely thin was split into two layers between bismuth oxide planes. During this process, the cold temperature conditions prevented oxygen migration and other chemical processes at the surface of the crystal. The process did not disrupt the crystalline order at the interfacial layer with no change in the lattice structure in the direction between atomic layers. As a result, the twisted BSCCO retained superconductivity with no change in its superconductivity temperature. 

A schematic showing the stacked twisted cuprate superconductor with two layers at a 45 degree angle to each other is shown in Figure 3. The twisted structure is analogous to an ice cream sandwich with askew wafers. 


Figure 3. Graphical representation of the stacked, twisted cuprate superconductor, with accompanying data in the background. Figure courtesy of Lucy Yip, Yoshi Saito, Alex Cui and Frank Zhao—Harvard University.

The synthesis of a twisted cuprate superconductor opens up the potential for using this material in making electronic devices. Pixley says, “A potential application for this twisted superconductor is as the world’s first high-temperature superconducting diode that can act as a switch to enable current to flow in one direction without the need for applying a magnetic field. This finding leads to the opportunity to conduct more experiments to synthesize new materials at different twist angles and increasing number of layers in an effort to identify new characteristics.” 

Besides this future opportunity, future work will be done to better understand the topology of this twisted superconductor according to Pixley. Doping twisted superconductors also may be an approach for developing new structures that may provide interesting properties. 

Additional information can be found in a recent article⁸ or by contacting Pixley at jed.pixley@physics.rutgers.edu.

REFERENCES
1. Canter, N. (2023), “Superconducting highway,” TLT, 79 (8), pp. 14-15. Available here.
2. Li, G., Luican, A., Santos, J., Neto, A., Reina, A., Kong, J. and Andrei, E. (2010), “Observation of Van Hov singularities in twisted graphene layers,” Nature Physics, 6, pp. 109-113.
3. Bistritzer, R. and MacDonald, A. (2011), “Moiré bands in twisted double-layer graphene,” Proceedings of the National Academy of Sciences, 108 (30), pp. 12233-12237.
4. Fu, Y., König, E., Wilson, J., Chou, Y. and Pixley, J. (2020), “Magic-angle semimetals,” npj Quantum Materials, 5, pp. 1-8.
5. Volkov, P., Wilson, J., Lucht, K. and Pixley, J. (2023), “Current- and field-induced topology in twisted nodal superconductors,” Physical Review Letters, 130, 186001.
6. Zhao, S., Pocia, N., Panetta, M., Yu, C., Johnson, J., Yoo, H., Zhong, R., Gu, G., Watanabe, K., Tanguchi, T., Postolova, S., Vinokur, V. and Kim, P. (2019), “Sign reversing hall effect in atomically thin high temperature Bi2.1Sr1.9CaCu2.0O8+δ superconductors,” Physics Review Letters, 122, 247001.
7. Yu, Y., Ma, L., Cai, P., Zhong, R., Ye, C., Shen, J., Gu, G., Chen, X. and Zhang, Y. (2019), “Hightemperature superconductivity in monolayer Bi2Sr2CaCu2O8+δ,” Nature, 575, pp. 156-163.
8. Zhao, S., Cui, X., Volkov, P., Yoo, H., Lee, S., Gardener, J., Akey, A., Engelke, R., Ronene, Y., Zhong, R., Gu, G., Plugge, S., Tummuru, T., Kim, M. Franz, M., Pixley, J., Poccia, N. and Kim, P. (2023), “Time-reversal symmetry breaking superconductivity between twisted cuprate superconductors,” Science, 382 (6677), pp. 1422- 1427.