Two Superconductivity States Coincide in Ultrathin Films
To understand superconductors, researchers explore their behavior at the limits of superconductivity, such as at high temperature or under strong magnetic field. New experiments investigate superconductivity at the limits of thickness, finding unexpected vortex behavior in ultrathin films [1]. Using a high-resolution magnetic imaging technique, Nofar Fridman from the Hebrew University of Jerusalem and colleagues measured vortex sizes in superconducting samples of various thicknesses and found larger-than predicted vortices in films made up of six or fewer atomic layers. The results suggest that thin superconductors host two superconducting states: one in the bulk of the material, the other confined to the surface layers. This behavior challenges our present understanding of how superconductors behave.
When a superconductor is exposed to an external magnetic field, it generates electrical screening currents, which generate a counter magnetic field, explains team member Yonathan Anahory from the Hebrew University of Jerusalem. The net effect is the external field lines bend around the superconductor without penetrating the material.
However, the situation changes in thin superconducting films, where the material’s ability to completely expel magnetic fields is weakened. Instead of being fully excluded, the field enters the film through narrow columns, called vortices, around which superconducting screening currents flow. Inside each vortex, there is exactly one quantum of magnetic flux.
The size of a vortex is indicative of the effectiveness of the screening currents at confining the magnetic field within the vortex. If the currents are strong, then the magnetic field around the vortex will drop abruptly to zero. But if the currents are weak, then the magnetic field will penetrate a certain distance into the material. For thin films, this penetration distance is called the Pearl length. Physicists have widely assumed that the Pearl length must increase as superconducting samples become thinner, leading to larger and larger vortex sizes. However, this assumption hasn’t been tested in few-atom-thick samples until now.
In their study, Fridman and colleagues investigated films of niobium diselenide (NbSe2), a type of layered superconductor whose thickness can be tightly controlled, even down to atomic scales. To explore the vortex behavior, the researchers used a high-resolution magnetic imaging technique called scanning SQUID microscopy, which allowed them to measure extremely subtle variations in magnetic-field strength across the surfaces of their superconducting films, Anahory says.
The SQUID magnetometer consists of a tiny pipette coated with lead—which also becomes a superconductor at low temperatures. By scanning the pipette over the NbSe2 surface, screening currents generated in the lead coating allowed the team to determine both the locations and sizes of the NbSe2 vortices. From the sizes, the researchers estimated the Pearl length.
To examine the influence of thickness on the material’s Pearl length, Fridman and colleagues applied the SQUID technique to NbSe2 samples ranging from 53 atomic layers down to just 3 layers. For the thicker samples, they found that the Pearl length increased as thickness decreased, as expected. But the situation changed for the thinnest samples, with the Pearl length plateauing at a constant value of around 0.1 mm—far higher than the team expected.
To explain this behavior, the researchers proposed that two superconducting states may be coexisting within NbSe2. For the thicker films, screening currents flow throughout the bulk of the sample—with the strength of the currents depending on the thickness of the sample. But when samples become ultrathin, another state begins to dominate, where screening currents are only present in the surface layers, Anahory says. This would mean that the material’s screening strength becomes extremely weak—but also remains constant, even as thickness varies.
The discovery raises the question of whether these same coexisting states could be found in other types of ultrathin superconductors, says Christophe Berthod, a condensed-matter physicist at the University of Geneva, who wasn’t involved in the study. “Two conditions are needed for this [coexistence] to occur,” he says. First, the electronic structure of the sample surfaces must differ from that of the bulk in some essential way. Second, the bulk superconductivity must disappear in ultrathin samples. For now, it remains unclear whether these two conditions could be met simultaneously in other ultrathin superconductors.
Yet, according to Hermann Suderow, a condensed-matter physicist at the Autonomous University of Madrid, the question could soon be answered through future experiments. “It would be good to do the same experiment in an ultrathin amorphous thin film,” he says. “There, the system is expected to be homogeneous up to the surface—unless there is another surprise!”
–Samuel Jarman
Samuel Jarman is a science writer based in the UK.
References
- N. Fridman et al., “Anomalous thickness dependence of the vortex pearl length in few-layer NbSe2,” Nat. Commun. 16, 2696 (2025).