SANS Study of Vortex Lattice Structural Transition in Optimally Doped (Ba$_{1-x}$K$_{x}$)Fe$_{2}$As$_{2}$

TL;DR

This study uses small-angle neutron scattering to investigate the structural transition of vortex lattices in optimally doped Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$, revealing a transition from ordered to disordered vortex phases linked to magnetic field changes.

Contribution

It provides new insights into vortex lattice phase transitions in iron-based superconductors through neutron scattering analysis.

Findings

Vortex solid to polycrystal transition observed at high magnetic fields.

Correlation between vortex transition and second-peak feature in hysteresis loops.

Explanation of structure factor drop via vortex ensemble freezing dynamics.

Abstract

Small-angle neutron scattering on high quality single crystalline Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ reveals the transition from a low--field vortex solid phase with orientational order to a vortex polycrystal at high magnetic field. The vortex order-disorder transition is correlated with the second-peak feature in isothermal hysteresis loops, and is interpreted in terms of the generation of supplementary vortex solid dislocations. The sharp drop of the structure factor above the second peak field is explained by the dynamics of freezing of the vortex ensemble in the high field phase.