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Superconductivity is an intriguing property observed in some materials, which entails the ability to conduct electric current combined with an electrical resistance of zero at low temperatures. Â鶹ÒùÔºicists have observed this property in various solid materials with different characteristics and atomic thicknesses.

A team of researchers at Nanjing University in China recently carried out a study aimed at further exploring the behavior of niobium diselenide (NbSeâ‚‚), a layered material that has been found to be a superconductor when it is atomically thin. Their paper, in Â鶹ÒùÔºical Review Letters, unveils resilient superconducting fluctuations in atomically thin NbSeâ‚‚, which could play a part in the anomalous metallic state previously observed in this material.

"Our study was inspired by a long-standing puzzle in condensed matter physics, which can be summarized by the question: can metals truly exist in two dimensions as the ground state?" Xiaoxiang Xi, senior author of the paper, told Â鶹ÒùÔº. "While we understand the behavior of everyday metals and insulators, ultrathin materials—like sheets just one atom thick—challenge these conventional rules."

Previous studies observed a physical state known as the "anomalous metallic state" in some atomically thin materials, including in NbSeâ‚‚ when a small magnetic field disrupts its superconducting state. When in this state, the material exhibits metal-like behavior, along with unusual properties that are not quite aligned with the conventional behavior of metals.

"Over the years, various theories have been proposed, but the exact mechanism behind this state remains unclear," said Xi. "Our study was aimed at revisiting this phenomenon in NbSeâ‚‚ and deepening our understanding of its origins."

To further explore the anomalous metallic state previously observed in atomically thin NbSeâ‚‚, Xi and his colleagues collected electrical transport measurements, which provide insight into how electrical charge flows through materials. They also used optical techniques to examine how the superconductivity in the material adapts when a magnetic field is applied to its surface perpendicularly.

Using these methods, the researchers observed a superconducting Higgs mode in the atomically thin NbSeâ‚‚ sample. This is a collective oscillation of particles in a superconductor, producing a "synchronized dance" of sorts that mirrors the organization of electrons in a material.

"To observe the Higgs mode, we used Raman scattering—shining light on the material and analyzing how the light's color shifts as it interacts with these particle 'dances,'" explained Xi. "While this mode was observed decades ago in much thicker NbSe₂ crystals, detecting it in atomically thin layers was more challenging. To capture its subtle signals, we had to overcome difficulties related to the small, delicate samples."

Previous works had collected evidence suggesting that the anomalous metallic state in NbSeâ‚‚ could be an experimental artifact, caused by external electrical noise coupling to the sample analyzed. The findings gathered by Xi and his colleagues, on the other hand, suggest that even if it is influenced by electrical noise, the metallic state still defies the standard rules of metals.

"We found that its properties closely resemble the 'anomalous metallic state' observed in other systems where noise had been eliminated," said Xi. "Using Raman scattering and Hall resistance measurements, we gathered evidence that the material's electrons still form Cooper pairs (the pairs of electrons typically responsible for superconductivity), but these pairs do not condense into a fully coherent superconducting state. Surprisingly, these incoherent pairs are more resilient to magnetic fields than expected."

The recent work by this team of researchers sheds new light on the superconductivity of atomically thin NbSeâ‚‚, challenging the present understanding of the anomalous metallic state emerging from this prototypical two-dimensional superconductor. In the future, it could pave the way for further studies focusing on this state, potentially leading to more discoveries about this state and its underlying physics.

"In our next studies, we plan to explore the microscopic features of this anomalous metallic state using other experimental techniques," added Xi. "We are also interested in finding ways to control this state, so we can better understand the conditions and mechanisms for its formation."

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