# Observation of a gel of quantum vortices in a superconductor at very low magnetic fields

## TL;DR

This paper reports the discovery of a gel-like phase of quantum vortices in a superconductor at very low magnetic fields, characterized by dense clusters and sparse regions, with implications for vortex control.

## Contribution

It introduces the observation of a vortex gel phase in a superconductor, a novel distribution differing from known vortex states, linked to structural defects.

## Key findings

- Vortex clusters form dense regions separated by vortex-free zones.
- Vortex lattice exhibits multifractal behavior.
- Vortex distribution influenced by structural defects.

## Abstract

A gel consists of a network of particles or molecules formed for example using the sol-gel process, by which a solution transforms into a porous solid. Particles or molecules in a gel are mainly organized on a scaffold that makes up a porous system. Quantized vortices in type II superconductors mostly form spatially homogeneous ordered or amorphous solids. Here we present high-resolution imaging of the vortex lattice displaying dense vortex clusters separated by sparse or entirely vortex-free regions in $\beta$-Bi$_2$Pd superconductor. We find that the intervortex distance diverges upon decreasing the magnetic field and that vortex lattice images follow a multifractal behavior. These properties, characteristic of gels, establish the presence of a novel vortex distribution, distinctly different from the well-studied disordered and glassy phases observed in high-temperature and conventional superconductors. The observed behavior is caused by a scaffold of one-dimensional structural defects with enhanced stress close to the defects. The vortex gel might often occur in type-II superconductors at low magnetic fields. Such vortex distributions should allow to considerably simplify control over vortex positions and manipulation of quantum vortex states.