Newswise — In a pioneering test, researchers from the University of Groningen, in collaboration with peers from the Dutch universities of Nijmegen and Twente and the Harbin Institute of Technology (China), have detected a previously foreseen superconductive state. On 24 May, they unveil proof of a unique form of the FFLO superconductive state in the Nature journal. This finding holds promising implications, especially in the realm of superconducting electronics.
The main author of the article is Professor Justin Ye, who leads the Device Physics of Complex Materials group at the University of Groningen. Ye and his team have been investigating the Ising superconducting state, which is a unique state capable of withstanding magnetic fields that typically disrupt superconductivity. The team initially described this state in 2015. In 2019, they devised a device composed of a dual layer of molybdenum disulfide that could interconnect the Ising superconductivity states present in both layers. Notably, the device created by Ye and his team enables the activation or deactivation of this protection through the use of an electric field, resulting in a superconducting transistor.
Elusive
The linked Ising superconductor tool illuminates an enduring issue in superconductivity research. In 1964, four scientists (Fulde, Ferrell, Larkin, and Ovchinnikov) anticipated a unique superconducting condition that could emerge under low temperature and intense magnetic field, denoted as the FFLO state. In conventional superconductivity, electron motion occurs in opposing directions within Cooper pairs. As these electrons move at equal speeds, their combined kinetic momentum amounts to zero. Nonetheless, in the FFLO state, a slight disparity in electron velocities within the Cooper pairs leads to the presence of a residual kinetic momentum.
‘This condition is highly elusive, and only a few articles assert its presence in regular superconductors,’ states Ye. ‘However, none of these articles provide definitive proof.’ To induce the FFLO state in a conventional superconductor, a powerful magnetic field is required. Yet, the magnetic field's function necessitates precise adjustment. In simple terms, to achieve the desired effect of the magnetic field, we rely on the Zeeman effect. This effect segregates electrons within Cooper pairs based on their spin direction (a magnetic moment), without affecting their orbital motion—the other factor that typically disrupts superconductivity. ‘It involves a delicate interplay between superconductivity and the external magnetic field,’ clarifies Ye.
Fingerprint
Ye and his colleagues introduced Ising superconductivity, which was published in the journal Science in 2015, as a means to suppress the Zeeman effect. 'By eliminating the crucial factor that enables conventional FFLO states, we have created a favorable environment for the magnetic field to fulfill its alternate role, known as the orbital effect,' explains Ye.
‘In our research paper, we have provided compelling evidence for the presence of the FFLO state driven by the orbital effect in our Ising superconductor,' clarifies Ye. 'This particular FFLO state is unorthodox and was initially proposed in theoretical work in 2017.' Achieving the FFLO state in conventional superconductors demands extremely low temperatures and an intense magnetic field, making it challenging to create. However, in Ye's Ising superconductor, the state can be achieved with a comparatively weaker magnetic field and at higher temperatures.
Transistors
In 2019, Ye initially noticed indications of an FFLO state in his superconducting device made of molybdenum disulfide. 'However, at that time, we were unable to provide conclusive evidence due to the subpar quality of the samples,' explains Ye. Fortunately, Ye's PhD student, Puhua Wan, has since succeeded in producing samples of the material that met all the necessary criteria to demonstrate the existence of a finite momentum within the Cooper pairs. 'The experimental work itself took around six months, but the subsequent analysis of the results added another year,' reveals Ye. Wan serves as the first author of the Nature paper presenting these findings.
This novel superconducting state requires extensive further investigation, according to Ye. 'There is still much to uncover about it. For instance, we need to understand how the kinetic momentum influences various physical parameters,' he explains. Exploring this state in-depth will offer fresh perspectives on superconductivity, potentially leading to the ability to manipulate and control this state in devices like transistors. Ye emphasizes that tackling this challenge constitutes their next objective.
Reference: Puhua Wan, Oleksandr Zheliuk, Noah F.Q. Yuan, Xiaoli Peng, Le Zhang, Minpeng Liang, Uli Zeitler, Steffen Wiedmann, Nigel E. Hussey, Thomas T.M. Palstra & Jianting Ye: Orbital Fulde–Ferrell–Larkin–Ovchinnikov state in an Ising superconductor. Nature, 24 May 2023.
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