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, which means that there is a net kinetic momentum. A powerful magnetic field is required in a normal superconductor to induce the FFLO state.
However, the magnetic field’s function needs to be carefully adjusted. We must harness the Zeeman effect to allow the magnetic field to fulfill two jobs. This does not affect the orbital impact, which is the other factor that typically breaks down superconductivity; rather, it separates electrons in Cooper pairs based on the direction of their spins (a magnetic moment).
Ising superconductivity suppresses the Zeeman effect.
In a ground-breaking experiment, scientists discovered the existence of a particular variant of the FFLO superconductive state. They reported the discovery of such an orbital FFLO state in the multilayer Ising superconductor 2H-NbSe2.
The work was performed 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 on, in 2019, scientists developed a device that consists 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 switch this protection on or off using an electric field, resulting in a superconducting transistor. However, the device sheds light on a long-standing challenge in superconductivity.
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, at higher temperatures, reaches the state.
In reality, Ye discovered the FFLO state in his molybdenum disulfide superconducting device for the first time in 2019.
Professor Justin Ye, who heads the Device Physics of Complex Materials group at the University of Groningen, said, “At that time, we could not prove this because the samples were not good enough.”
However, his Ph.D. student, Puhua Wan, has since produced material samples that fulfilled all the requirements to show that there is indeed a finite momentum in the Cooper pairs.
He said, “The actual experiments took half a year, but the analysis of the results added another year.”
“This new superconducting state needs further investigation. There is a lot to learn about it. For example, how does the kinetic momentum influence the physical parameters? Studying this state will provide new insights into superconductivity. And this may enable us to control this state in devices such as transistors. That is our next challenge.”
- Wan, P., Zheliuk, O., Yuan, N.F.Q. et al. Orbital Fulde–Ferrell–Larkin–Ovchinnikov state in an Ising superconductor. Nature (2023). DOI: 10.1038/s41586-023-05967-z