In the revolutionary experiment, researchers from the University of Groningen collaborated with colleagues from the universities of Nijmegen and Twente in the Netherlands and the Harbin Institute of Technology in China. Together, they confirmed the existence of the superconducting state, which was first predicted in 2017.

Their findings, which show evidence of a unique form of the FFLO superconducting state, were recently published in the journalism. Nature This breakthrough could be particularly important in the field of superconducting electronics.

The lead author of the paper is Professor Justin Yeh, who heads the Complex Materials Device Physics Group at the University of Groningen. Ye and his team worked on the superconducting state of Ising. This is a special case that can resist magnetic fields that normally destroy superconductivity and was described by the team in 2015.

In 2019, they created a device containing a double layer of molybdenum disulfide e that can interconnect the Ising superconducting states located in two layers. Interestingly, the device created by Ye and his team allows this shield to be switched on and off using an electric field, resulting in a superconducting transistor.

Hard to understand

Ising’s compound superconductor sheds light on a longstanding problem in the field of superconductivity. In 1964, four scientists (Fulde, Ferrell, Larkin, and Ovchinnikov) predicted a special superconducting state called the FFLO state, which can exist under conditions of low temperature and strong magnetic field.

In standard superconductivity, electrons move in opposite directions as Cooper pairs. Since they are moving at the same speed, the total kinetic momenta of these electrons is zero. However, in the case of FFLO, there is a small velocity difference between the electrons in the Cooper pairs, which means there is a net angular momentum.

“This situation is very difficult and there are only a few papers claiming it exists in normal superconductors,” says E. “But none of it is certain.”

This phase diagram shows the existence of the six-fold anisotropic FFLO orbital state, which occupies most of the phase diagram. In the upper right corner, the schematic drawings show the spatial modulation of the superconductor ordering parameter. Credit: P. Wang / University of Groningen

A strong magnetic field is required to create an FFLO state in a conventional superconductor. But the role played by the magnetic field needs to be carefully tuned. Simply put, we need to use the Zeeman effect so that the magnetic field plays two roles. This separates the electrons in the Cooper pairs according to the direction of their spin (magnetic moment), but not according to the orbital effect – another role that often destroys superconductivity.

“This is a delicate interaction between superconductivity and an external magnetic field,” Ye explains.

Finger print

Ising superconductivity, presented by Ye et al. and published in the journal Science In 2015, Zeeman suppresses the effect. “By filtering out the fundamental component that makes traditional FFLO possible, we provided enough room for the magnetic field to have another role, the orbital effect,” says Ye.

“What we show in our paper is a clear trace of an orbital effect-based FFLO state in our Ising superconductor,” Ye explains. “This is an unusual FFLO situation that was first theoretically described in 2017.” The FFLO state in conventional superconductors requires extremely low temperatures and a very strong magnetic field, making it difficult to create. However, in Ye Ying’s superconductor, the state is reached with a weaker magnetic field and at higher temperatures.

transistors

In fact, Ye first observed signs of FFLO state in his molybdenum disulfide superconductor device in 2019. “We couldn’t prove it at the time because the samples weren’t good enough,” says Ye. But doctoral student Puhua Wang has since managed to construct samples of materials that meet all the requirements to demonstrate that there is indeed a finite momentum in Cooper pairs. The first author of the article, Ye. “The actual experiments took six months, but the analysis of the results added another year,” says Wang. Nature .

This new superconducting case needs further investigation. There: “You can learn a lot about it. For example, how does kinetic momentum affect physical parameters? Studying this situation will provide new insights into superconductivity. This could allow us to control this state in devices like transistors. This is our next challenge.” Source