Direct observation of vortex liquid droplets in the iron pnictide superconductor CaKAs$_4$Fe$_4$ at $0.5T$_c$

TL;DR

This study uses scanning tunneling microscopy to directly observe vortex liquid droplets in CaKAs$_4$Fe$_4$, revealing local vortex fluctuations at temperatures much lower than the critical temperature, indicating early dissipation onset.

Contribution

First direct visualization of vortex liquid droplets in an iron pnictide superconductor at low temperatures using STM.

Findings

Vortex liquid droplets are observed at 0.5 T$_c$.

Vortex fluctuations occur well below the macroscopic melting transition.

Local dissipation begins at temperatures significantly below T$_c$.

Abstract

Type-II superconductors under magnetic fields are in a quantum coherent non-dissipative state as long as vortices remain pinned. Dissipation appears when vortices depin, eventually driven by thermal fluctuations. This can be associated to a melting transition between a vortex solid and a vortex liquid. This transition is almost always observed very close to T$_c$ when probed by macroscopic experiments. However, it remains unclear how the vortex solid responds to thermal fluctuations at the scale of individual vortices far from the melting transition. Here we use scanning tunneling microscopy (STM) to visualize vortices in CaKAs$_4$Fe$_4$ (T$_c \approx$ 35 K). We find vortex liquid droplets-localized regions in space where vortices strongly fluctuate due to thermal exctiation-at temperatures as low as 0.5\,T$_c$. Our results show that the onset of dissipation at the local scale occurs at temperatures considerably below T$_c$ in type-II superconductors.