Superconducting phase transitions in frustrated Josephson-junction arrays on a dice lattice

TL;DR

This study investigates superconducting phase transitions in frustrated dice Josephson-junction arrays at specific frustration levels, revealing high-temperature vortex ordering and insights into degeneracy removal mechanisms, with experimental results aligning with Monte Carlo simulations.

Contribution

It provides the first experimental evidence of high-temperature vortex ordering in dice Josephson arrays with frustration, highlighting the role of zero-energy domain walls and the order-from-disorder mechanism.

Findings

Both $f=1/3$ and $f=1/2$ arrays undergo high-temperature vortex ordering.

Experimental results for $f=1/3$ challenge existing theoretical expectations.

Zero-energy domain walls lead to slower critical relaxation in dice arrays.

Abstract

Transport measurements are carried out on dice Josephson-junction arrays with the frustration index $f=1/3$ and 1/2 which possess, within the limit of the $XY$ model, an accidental degeneracy of the ground states as a consequence of the formation of zero-energy domain walls. The measurements demonstrate that both the systems undergo a phase transition to a superconducting vortex-ordered state at considerably high temperatures. The experimental findings are in apparent contradiction with the theoretical expectation that frustration effects in the $f=1/3$ system are particularly strong enough to suppress a vortex-ordering transition down to near zero temperature. The data for $f=1/2$ are more consistent with theoretical evaluations. The agreement between the experiments and the Monte Carlo simulations of a $XY$ model for $f=1/3$ suggests that the order-from-disorder mechanism for the removal of an accidental degeneracy may still be effective in the $f=1/3$ system. The transport data also reveal that the dice arrays with zero-energy domain walls experience a much slower critical relaxation than other frustrated arrays only with finite-energy walls.