Scientists discover new superconducting state

Scientists from the University of Groningen, in collaboration with researchers from Dutch universities and the Harbin Institute of Technology in China, have made a groundbreaking discovery in the field of superconductivity. They have confirmed the existence of a unique variant of the FFLO superconducting state, a state that was initially proposed in 2017. The findings of this study have been published in the journal Nature and hold great potential for applications in superconducting electronics.

Professor Justin Ye, the head of the Device Physics of Complex Materials group at the University of Groningen, served as the lead author of the research paper. His team has been focused on investigating the Ising superconducting state, which possesses the remarkable ability to withstand magnetic fields that typically disrupt superconductivity. The team first described this state in 2015.

In 2019, they developed a device consisting of a double layer of molybdenum disulfide that allowed for the coupling of Ising superconductivity states present in both layers. Interestingly, this device permits the activation or deactivation of this protection 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) proposed the existence of a unique superconducting state, known as the FFLO state, which could occur under specific conditions of low temperature and a strong magnetic field.

In conventional superconductivity, electrons within Cooper pairs move in opposite directions, resulting in a total kinetic momentum of zero since they travel at the same speed. However, in the FFLO state, there is a slight speed difference between the electrons within the Cooper pairs, leading to a nonzero net kinetic momentum.

According to Professor Ye, the FFLO state has been elusive, with only a few articles suggesting its presence in normal superconductors, but none of them providing conclusive evidence. Creating the FFLO state in a traditional superconductor requires a strong magnetic field, and the role of the magnetic field needs to be delicately adjusted.

In simple terms, the Zeeman effect is employed to enable two distinct roles for the magnetic field. It separates electrons within Cooper pairs based on the direction of their spins (a magnetic moment), while preserving the orbital effect, which is the other role that typically disrupts superconductivity. Professor Ye explains that achieving the delicate balance between superconductivity and the external magnetic field is crucial.

The Ising superconductivity introduced by Professor Ye and his collaborators in 2015 suppresses the Zeeman effect. This filtering out of the key ingredient necessary for conventional FFLO enables ample room for the magnetic field to act through the orbital effect.

The recent Nature paper provides clear evidence of the orbital effect-driven FFLO state in their Ising superconductor. This unconventional FFLO state was first proposed in theory in 2017. Importantly, the Ising superconductor developed by Professor Ye allows for the attainment of the FFLO state with a weaker magnetic field and at higher temperatures compared to conventional superconductors.

Initial signs of the FFLO state were observed in the molybdenum disulfide superconducting device in 2019. However, at that time, the necessary evidence could not be fully substantiated due to the limitations of the samples. Nevertheless, Ph.D. student Puhua Wan has since produced material samples that meet all the requirements for demonstrating the existence of finite momentum within the Cooper pairs. Wan serves as the first author of the Nature paper.

Further research is required to gain a deeper understanding of this new superconducting state. Professor Ye emphasizes the need to investigate how the kinetic momentum influences various physical parameters and properties. Exploring this state will provide valuable insights into superconductivity and potentially enable the control of this state in devices such as transistors. This presents the next challenge for the researchers involved in this study. The discovery of the FFLO state in the Ising superconductor opens up new avenues for the development of advanced superconducting technologies with enhanced capabilities and potential applications in various fields. The researchers are excited about the possibilities and the opportunities for further exploration and experimentation in this fascinating area of study.

Source: University of Groningen

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