Physicists uncover two superconducting regimes in a Kagome lattice superconductor
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2025-03-18
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Superconductivity, which entails an electrical resistance of zero at very low temperatures, is a highly desirable and thus widely studied quantum phenomenon. Typically, this state is known to arise following the formation of bound electron pairs known as Cooper pairs, yet identifying the factors contributing to its emergence in quantum materials has so far proved more challenging.Researchers at Princeton University, the National High Magnetic Field Laboratory, Beijing Institute of Technology and the University of Zurich recently carried out a study aimed at better understanding the superconductivity observed in CsV₃Sb₅, a superconductor with a Kagome lattice (i.e., in which atoms form a hexagonal pattern that resembles that of Kagome woven baskets).Their paper, published in Nature Physics, identifies two distinct superconducting regimes in this material, which were found to be linked to different transport and thermodynamic properties."Back in 2021, our discovery of a chiral charge density wave in the Kagome superconductor AV₃Sb₅ (A = K, Rb, Cs) sparked major excitement in the field of quantum materials," Shafayat Hossain, first author of the paper, told Phys.org."These Kagome superconductors break multiple symmetries in the charge-ordered state before transitioning into a superconducting ground state. With the interplay of symmetry breaking, the multiband nature of AV₃Sb₅, and its topological band structure, an unconventional superconducting state seemed likely."When Hossain and his colleagues first started probing the origin of superconductivity in Kagome superconductors, there was no evidence in the literature suggesting that the superconductivity in AV₃Sb₅ was exotic in nature. Nonetheless, these materials presented many competing orders in their normal state, which the researchers expected would affect their superconductivity."This led us to investigate the superconducting state of CsV₃Sb₅ using transport and thermodynamic techniques to map out its phase diagram," said Hossain. "To our surprise, even our very first transport measurements revealed two distinct superconducting regimes—something we had not anticipated."As part of their study, the researchers measured the upper critical fields of CsV₃Sb₅ as a function of temperature and for two field orientations. This essentially means that they collected measurements along the material's conducting planes and perpendicularly to them.Interestingly, these measurements showed that there were two distinct superconducting regimes in CsV₃Sb₅. These states appeared to be separated by a step-like increase in upper critical fields."We also observed two anomalies in the heat capacity as a function of the temperature, indicating the opening of two superconducting gaps," said Luis Balicas, a senior author of the paper. "Finally, thermal conductivity revealed a finite and constant contribution as a function of the temperature prior to the second gap opening. This indicates that the parts of the Fermi surface remain ungapped deep in the superconducting state. Eventually a gap opens on these electronic states upon cooling."The researchers also found that for magnetic fields rotating in the conducting planes, the thermal conductivity of the Kagome superconductor became anisotropic when the material transitioned to a superconducting state. This suggests that the superconducting state observed in CsV₃Sb₅ has a complex gap structure and could thus potentially be unconventional in nature."The thermal conductivity should be transported by carriers excited across the gap, implying a mildly anisotropic gap function," explained Balicas. "Remarkably, this anisotropy rotates when the second superconducting gap opens, suggesting a distinct gap anisotropy, although we cannot yet pinpoint the pairing symmetry."Overall, the findings collected by this group of researchers hint at the possibility that the Kagome-lattice material CsV₃Sb₅ exhibits band-selective superconductivity. This is a type of superconductivity in which different electron bands host independent superconducting gaps."While we cannot expose the symmetry of the gap function, our study clearly indicates the multigap nature of the superconducting state in CsV₃Sb₅ and suggests the possibility of an unconventional pairing symmetry that still remains to be determined," said Balicas."The charge density wave (CDW) state from which the superconducting state condenses is itself unconventional. For example, it is claimed to display an anomalous Hall response, although this system is not magnetic. Therefore, one might expect an unconventional pairing for a superconducting state that coexists with such a chiral CDW state."The insight gathered by Hossain, Balicas and their collaborators could contribute to the understanding of superconductivity in CsV₃Sb₅, potentially extending to other superconductors with a Kagome lattice. In their next studies, the researchers plan to continue investigating superconductors with a multiband nature and symmetry-breaking normal states."Kagome superconductors like CsV₃Sb₅ belong to this broad class, building on decades of research on cuprates and iron pnictide superconductors," added Hossain. "The discovery of new superconductors in this category remains an exciting frontier—who knows what groundbreaking superconductor in this category will be found next? We are always on the lookout because these materials often reveal unexpected quantum states."Moving forward, we will continue our work on Kagome superconductors, focusing on their potential unconventional gap structures and in-gap states, which may harbor nontrivial topological properties."
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