The leader of the collaboration is Professor Justin Ye from Groningen. Ye and his team have been working on the Ising superconducting state, a state they discovered at HFML-FELIX in 2015. This is a special state that can resist magnetic fields that generally destroy superconductivity. In 2019, they created a device comprising a double layer of molybdenum disulfide that could couple the Ising superconductivity states residing 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.
The coupled Ising superconductor 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 could 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 travel at the same speed, these electrons have a total kinetic momentum of zero. However, 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.
To create the FFLO state in a conventional superconductor, a strong magnetic field is needed. But the role played by the magnetic field needs careful tweaking. Simply put, for two roles to be played by the magnetic field, we need to use the Zeeman effect. This separates electrons in Cooper pairs based on the direction of their spins (a magnetic moment), but not on the orbital effect—the other role that normally destroys superconductivity. Ising superconductivity suppresses the Zeeman effect. ‘By filtering out the key ingredient that makes conventional FFLO possible,’ says Ye, ‘ we provided ample space for the magnetic field to play its other role, namely the orbital effect.’
This paper provides the first clear fingerprint of the orbital effect-driven FFLO state in an Ising superconductor. The FFLO state in conventional superconductors requires extremely low temperatures and a very strong magnetic field, which makes it difficult to create. However, in Ye's Ising superconductor, the state is reached with a weaker magnetic field and at higher temperatures. The high magnetic fields at HFML-FELIX were nevertheless important to enabling the researchers to establish the full phase diagram of this novel phenomenon.
This new superconducting state still needs further investigation, however. Ye: ‘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.’