Direct visualization of vortex pattern transition inZrB12with Ginzburg-Landau parameter close to the dual point

Abstract

In nature, many systems exhibit modulated phases with periodic macroscopic patterns and textures mainly due to the competitive interactions of different phases. Vortex systems in superconductors, which are easy to access, offer the possibility of tuning the ratio between the competitive interactions, providing a unique tool to study the evolution and equilibrium of similar patterns. The $\ensuremath{\kappa}\ensuremath{-}T$ phase diagram of clean superconductors shows the transition from type-I to type-II superconductivity via a narrow $\ensuremath{\kappa}$ range near the dual point $\ensuremath{\kappa}=1/\sqrt{2}$ where the vortices attract each other at long distances and repel each other at short distances. This $\ensuremath{\kappa}$ range, which is termed the type-II/1 phase, becomes larger with decreasing temperature. The direct imaging at the scale of individual vortices of the vortex pattern transition would provide valuable information. Therefore, by using scanning Hall probe microscopy, we have performed direct visualization of the vortex pattern transition in a ${\mathrm{ZrB}}_{12}$ single crystal across the type-II and type-II/1 phases. By gradually lowering the temperature, and thereby tuning vortex interactions, a transition is observed from the ordered Abrikosov vortex lattice to a disordered vortex pattern with large areas of Meissner phase, vortex chains, and vortex clusters. The formation of vortex chains and clusters has been found to arise from the combined effect of quenched disorder and the attractive vortex-vortex interaction in the type-II/1 phase. The clusters and chains serve as the vortex reservoir to enable the formation of a triangular vortex lattice of the type-II phase at high temperatures.