In 1964, four scientists predicted a special superconducting state known as the FFLO state. In the FFLO state, there is a small speed difference between the electrons in the Cooper pairs, meaning there is a net kinetic momentum. A powerful magnetic field is required in a normal superconductor to induce the FFLO state.

However, the function of the magnetic field must be carefully adjusted. We need to exploit the Zeeman effect to make the magnetic field do two orbits. This does not affect orbital impact, the other factor that typically breaks down superconductivity; rather, it separates electrons into Cooper pairs based on the direction of their spins (a magnetic moment).

Ising superconductivity suppresses the Zeeman effect.

In a groundbreaking experiment, scientists discovered the existence of a particular variant of the FFLO superconducting state. They reported the discovery of such an orbital FFLO state in the multilayer Ising superconductor 2H-NbSe2.

The work was carried out by scientists from the University of Groningen in collaboration with the Dutch universities of Nijmegen and Twente and the Harbin Institute of Technology (China).

Scientists have been working on the Ising superconducting state, a special state that can resist magnetic fields that generally destroy superconductivity. This effect was first explained in 2015. Later, in 2019, scientists developed a device consisting of a double layer of molybdenum disulfide that could couple the Ising superconductivity states in the two layers.

Interestingly, the device makes it possible to turn this protection on or off using an electric field, resulting in a superconducting transistor. However, the device sheds light on a long-standing superconductivity challenge.

The newly discovered superconducting state is an unconventional FFLO state. It was first described in theory in 2017.

Unlike conventional superconductors, which require a powerful magnetic field to induce the FFLO state, the Ising superconductor requires a weaker magnetic field and reaches the state at higher temperatures.

In fact, Ye first discovered the FFLO state in his superconducting molybdenum disulfide device in 2019.

Professor Justin Ye, head of the Device Physics of Complex Materials group at the University of Groningen, said: “At that point we couldn’t prove this because the samples weren’t good enough.”

His Ph.D. student, Puhua Wan, has since produced material samples that met all the requirements to show that there is indeed finite momentum in the Cooper pairs.

He said, “The actual experiments lasted half a year, but the analysis of the results took another year.”

“This new superconducting state needs further investigation. There is much to learn about it. For example, how does kinetic momentum affect physical parameters? Studying this state will yield new insights into superconductivity. And this may allow us to monitor this condition in devices such as transistors. That is our next challenge.”

Magazine reference:

  1. Wan, P., Zheliuk, O., Yuan, NFQ et al. Orbital Fulde-Ferrell-Larkin-Ovchinnikov state in an Ising superconductor. Nature (2023). DOI: 10.1038/s41586-023-05967-z