Nonuniform and coherent motion of superconducting vortices in the picometer-per-second regime

TL;DR

This study used scanning tunneling microscopy to observe superconducting vortices in NbSe₂ moving at picometer-per-second speeds due to magnetic field decay, revealing nonuniform, collective vortex motion and preferred configurations.

Contribution

It provides the first detailed observation of vortex dynamics at picometer-per-second speeds, highlighting nonuniform motion and energetically favored vortex arrangements in NbSe₂.

Findings

Vortices moved collectively while maintaining a triangular lattice.

Vortex motion speed and direction varied over time.

Existence of energetically favored vortex configurations despite non-uniform motion.

Abstract

We investigated vortex dynamics in a single-crystal sample of type-II superconductor NbSe$_{2}$ using scanning tunneling microscopy at 4.2 K. The decay of the magnetic field at a few nT/s in our superconducting magnet induced the corresponding motion of vortices at a few pm/s. Starting with an initial magnetic field of 0.5 T, we continued to observe motion of vortices within a field of view of 400$\times$400 nm$^2$ subject to decay of the magnetic field over a week. Vortices moved collectively, and maintained triangular lattices due to strong vortex-vortex interactions during the motion. However, we observed two peculiar characteristics of vortex dynamics in this superconductor. First, the speed and direction of the vortex lattice motion were not uniform in time. Second, despite the non-uniform motion, we also found that there exists an energetically favored configuration of the moving vortices in the single-crystal sample of NbSe$_{2}$ based on the overlaid trajectories and their suppressed speeds. We model the system with weak bulk pinning, strong bulk pinning, and edge barrier effects.