Direct Observation of Vortex Liquid Droplets in the Iron Pnictide Superconductor CaKFe₄As₄ at 0.5T_c

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 ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$ ( $T_c\approx 35K$ ). We observe the formation of vortex liquid droplets-spatially localized regions where vortices exhibit strong thermal fluctuations-at temperatures as low as $0.5T_c$ . These results demonstrate that the onset of dissipation at the local scale occurs at temperatures significantly below $T_c$ in type-II superconductors, revealing a previously unrecognized regime of vortex dynamics.