Scientists discover an elusive superconducting state first predicted in 2017

In a groundbreaking experiment, researchers from the University of Groningen collaborated with peers from the Universities of Nijmegen and Twente in the Netherlands, and the Harbin Institute of Technology in China. Together, they confirmed the existence of a superconducting state that was first predicted in 2017.

Their findings, which establish evidence for a unique form of the superconducting FFLO state, were recently published in the journal nature. This breakthrough has the potential to be influential, particularly in the field of superconducting electronics.

This is Prof. Dr. Justin Yee, Chair of the Device Physics Group for Complex Materials at the University of Groningen in the Netherlands, and lead author of the Nature paper on the superconducting state of FFLO. Credit: Sylvia Germes

The lead author of the paper is Professor Justin Yee, who heads the Device Physics for Complex Materials group at the University of Groningen. Ye and his team have been working on the Ising superconducting case. This is a special case that can resist the magnetic fields that destroy superconductivity in general, and that was it The team described it in 2015.

In 2019, they created Device comprising a double layer of molybdenum disulfidee can be associated with the Ising superconducting states present in the two layers. Interestingly, the device that Ye and his team created makes it possible to turn this protection on or off using an electric field, resulting in a superconducting transistor.

Elusive

Ising’s double superconducting device sheds light on a long-standing challenge in the field of superconductivity. In 1964, four scientists (Fulde, Ferrell, Larkin, and Ovchinnikov) predicted a special superconducting state that can exist under conditions of low temperature and strong magnetic field, referred to as the FFLO state.

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

“This case is very elusive and there are only a few materials that claim to be ordinary superconductors,” says Ye. However, none of this is conclusive.

This phase diagram shows the existence of an anisotropic six-fold orbital state, which occupies a large part of the phase diagram. In the upper right corner, the schematic illustrations show the spatial modulation of the superconducting order parameter. Credit: P. Wan/University of Groningen

To create the FFLO state in a conventional superconductor, a strong magnetic field is needed. But the role played by the magnetic field needs to be finely tuned. Simply put, for the magnetic field to play two roles, we need to use the Zeeman effect. This separates the electrons into Cooper pairs based on their direction of spin (magnetic moment), but not on the orbital effect—the other role that usually destroys superconductivity.

“It’s a delicate negotiation between superconductivity and the external magnetic field,” Yi explains.

fingerprint

First author Buhua Wan produced samples that met all the requirements to demonstrate that there is indeed finite momentum in Cooper pairs. Credit: P. Wan/University of Groningen

is superconductivity, which were presented by Ye and his collaborators and published in the journal Sciences In 2015, he suppressed the Zeeman effect. “By filtering out the key component that makes conventional FFLO possible, we freed up ample space for the magnetic field to play its other role, which is the orbital effect,” Ye says.

“What we have shown in our paper is a clear imprint of the orbital effect-driven FFLO state in the Ising superconductor,” Yi explains. “This is an atypical FFLO case, first described theoretically in 2017.” The FFLO state in conventional superconductors requires very low temperatures and very strong magnetic fields, which makes it difficult to form. However, in Ye’s Ising superconductor, the state is reached with a weaker magnetic field and at higher temperatures.

transistors

In fact, Yi first noticed signs of the FFLO state in his superconducting device for molybdenum disulfide in 2019. “At that time, we couldn’t prove it, because the samples weren’t good enough,” says Yi. However, he has earned his Ph.D. Student Puhua Wan has since succeeded in producing samples of material that met all the requirements to show that there is indeed finite momentum in Cooper pairs. “The actual trials took half a year, but analyzing the results added another year,” says Ye. Wan is the first author of the nature paper.

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

Reference: “Orbital Fulde-Ferrell-Larkin-Ovchinnikov state in Ising superconductor” by Puhua Wan, Oleksandr Zheliuk, Noah FQ Yuan, Xiaoli Peng, Le Zhang, Minpeng Liang, Uli Zeitler, Steffen Wiedmann, Nigel E. nature.
DOI: 10.1038/s41586-023-05967-z