Edge currents in frustrated Josephson junction ladders

TL;DR

This study numerically investigates frustrated Josephson junction ladders, revealing edge currents that emerge and vanish at a critical coupling, suggesting a topological origin for these phenomena.

Contribution

It establishes a novel correspondence between Josephson ladder energy states and a tight-binding model, linking edge currents to topological band structure changes.

Findings

Edge currents appear above a critical coupling ratio t

Edge currents disappear when a band gap opens at t=1

Edge current behavior indicates a topological phase transition

Abstract

We present a numerical study of quasi-1D frustrated Josephson junction ladders with diagonal couplings and open boundary conditions, in the large capacitance limit. We derive a correspondence between the energy of this Josephson junction ladder and the expectation value of the Hamiltonian of an analogous tight-binding model, and show how the overall superconducting state of the chain is equivalent to the minimum energy state of the tight-binding model in the subspace of one-particle states with uniform density. To satisfy the constraint of uniform density, the superconducting state of the ladder is written as a linear combination of the allowed k-states of the tight-binding model with open boundaries. Above a critical value of the parameter t (ratio between the intra-rung and inter-rung Josephson couplings), the ladder spontaneously develop currents at the edges which spread to the bulk as t is increased until complete coverage is reached. Above a certain value of t, which varies with ladder size (t = 1 for an infinite-sized ladder), the edge currents are destroyed. The value t = 1 corresponds, in the tight-binding model, to the opening of a gap between two bands. We argue that the disappearance of the edge currents with this gap opening is not coincidental, and that this points to a topological origin for these edge current states.