Spectroscopy of a fractional Josephson vortex molecule

TL;DR

This paper experimentally investigates the eigenfrequencies of fractional Josephson vortex molecules in long Josephson junctions, revealing how their collective oscillatory modes depend on vortex separation, topological charge, and bias current.

Contribution

It provides the first detailed experimental analysis of eigenfrequency dependence in fractional vortex molecules, confirming theoretical models through microwave spectroscopy.

Findings

Eigenfrequency splitting occurs as vortex separation decreases.

Collective oscillatory modes emerge with vortex proximity.

Experimental results agree with theoretical predictions.

Abstract

In long Josephson junctions with multiple discontinuities of the Josephson phase, fractional vortex molecules are spontaneously formed. At each discontinuity point a fractional Josephson vortex carrying a magnetic flux $|\Phi|<\Phi_0$, $\Phi_0\approx 2.07\times 10^{-15}$ Wb being the magnetic flux quantum, is pinned. Each vortex has an oscillatory eigenmode with a frequency that depends on $\Phi/\Phi_0$ and lies inside the plasma gap. We experimentally investigate the dependence of the eigenfrequencies of a two-vortex molecule on the distance between the vortices, on their topological charge $\wp=2\pi\Phi/\Phi_0$ and on the bias current $\gamma$ applied to the Josephson junction. We find that with decreasing distance between vortices, a splitting of the eigenfrequencies occurs, that corresponds to the emergence of collective oscillatory modes of both vortices. We use a resonant microwave spectroscopy technique and find good agreement between experimental results and theoretical predictions.