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Direct Observation of Vortex Liquid Droplets in the Iron Pnictide Superconductor CaKFe 4 As 4 at 0.5T c

Bou Marqués, Óscar [Univ. Autonoma de Madrid (Spain)] (ORCID:0009000088978449)·Moreno, Jose A. [Univ. Autonoma de Madrid (Spain)] (ORCID:0000000244175115)·García Talavera, Pablo [Univ. Autonoma de Madrid (Spain)] (ORCID:0000000339612576)·Xu, Mingyu [Ames Laboratory (AMES), Ames, IA (United States); Iowa State Univ., Ames, IA (United States)] (ORCID:0000000171386009)·Schmidt, Juan [Ames Laboratory (AMES), Ames, IA (United States); Iowa State Univ., Ames, IA (United States); Univ. of Buenos Aires (Argentina)] (ORCID:0000000176948576)·Bud'ko, Sergey L. [Ames Laboratory (AMES), Ames, IA (United States); Iowa State Univ., Ames, IA (United States)] (ORCID:0000000236035585)·Canfield, Paul C. [Ames Laboratory (AMES), Ames, IA (United States); Iowa State Univ., Ames, IA (United States)] (ORCID:0000000277150643)·Guillamón, Isabel [Univ. Autonoma de Madrid (Spain)] (ORCID:0000000226063355)·Herrera, Edwin [Univ. Autonoma de Madrid (Spain)] (ORCID:0000000273197428)·Suderow, Hermann [Univ. Autonoma de Madrid (Spain)] (ORCID:0000000259021880)
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Abstract

Type-II superconductors under magnetic fields remain in a quantum-coherent, non-dissipative state as long as vortices are pinned. Dissipation emerges when vortices depin, a process often driven by thermal fluctuations and commonly associated with a melting transition from a vortex solid to a vortex liquid. Macroscopic experiments almost always observe this transition close to the superconducting critical temperature 𝑇 𝑐 . However, how the vortex solid responds to thermal fluctuations at the scale of individual vortices, far below the melting transition, remains largely unexplored. Here, we use scanning tunneling microscopy (STM) to directly visualize vortices in the iron-based superconductor CaKFe 4 ⁢As 4 (𝑇 𝑐 ≈35 K ). We observe the formation of vortex liquid droplets—spatially localized regions where vortices exhibit strong thermal fluctuations—at temperatures as low as 0.5 𝑇 𝑐 . These results demonstrate that the onset of dissipation at the local scale occurs at temperatures significantly below 𝑇𝑐 in type-II superconductors, revealing a previously unrecognized regime of vortex dynamics.

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75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITYcondensed matter physicsdissipationmagnetic fieldscanning tunneling microscopesuperconductivitythermal fluctuationsvortex

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Bou Marqués, Óscar [Univ. Autonoma de Madrid (Spain)] (ORCID:0009000088978449), Moreno, Jose A. [Univ. Autonoma de Madrid (Spain)] (ORCID:0000000244175115), García Talavera, Pablo [Univ. Autonoma de Madrid (Spain)] (ORCID:0000000339612576), Xu, Mingyu [Ames Laboratory (AMES), Ames, IA (United States); Iowa State Univ., Ames, IA (United States)] (ORCID:0000000171386009), Schmidt, Juan [Ames Laboratory (AMES), Ames, IA (United States); Iowa State Univ., Ames, IA (United States); Univ. of Buenos Aires (Argentina)] (ORCID:0000000176948576), Bud'ko, Sergey L. [Ames Laboratory (AMES), Ames, IA (United States); Iowa State Univ., Ames, IA (United States)] (ORCID:0000000236035585), Canfield, Paul C. [Ames Laboratory (AMES), Ames, IA (United States); Iowa State Univ., Ames, IA (United States)] (ORCID:0000000277150643), Guillamón, Isabel [Univ. Autonoma de Madrid (Spain)] (ORCID:0000000226063355), Herrera, Edwin [Univ. Autonoma de Madrid (Spain)] (ORCID:0000000273197428), Suderow, Hermann [Univ. Autonoma de Madrid (Spain)] (ORCID:0000000259021880). 2026-07-03. Direct Observation of Vortex Liquid Droplets in the Iron Pnictide Superconductor CaKFe 4 As 4 at 0.5T c. https://doi.org/10.1002/advs.76387

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75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY subjectCondensed Matter Physics subjectDissipation subjectmagnetic field subjectscanning tunneling microscope subjectsuperconductivity subjectthermal fluctuations subjectvortex subject

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