Onset of phase diffusion in high kinetic inductance granular aluminum micro-SQUIDs

TL;DR

This study investigates how high kinetic inductance in granular aluminum micro-SQUIDs affects their switching current behavior, revealing a transition to phase diffusion likely caused by increased resistivity and altered electromagnetic environment.

Contribution

It provides new insights into the onset of phase diffusion in high kinetic inductance granular aluminum SQUIDs through detailed switching current measurements.

Findings

Suppressed modulation and triangular distortion in switching current versus flux.

Temperature dependence of switching current histograms changes with resistivity.

Indication of phase diffusion onset due to increased resistivity.

Abstract

Superconducting granular aluminum is attracting increasing interest due to its high kinetic inductance and low dissipation, favoring its use in kinetic inductance particle detectors, superconducting resonators or quantum bits. We perform switching current measurements on DC-SQUIDs, obtained by introducing two identical geometric constrictions in granular aluminum rings of various normal-state resistivities in the range from $\rho_\mathrm{n} = 250\,\mu\Omega\mathrm{cm}$ to $5550\,\mu\Omega\mathrm{cm}$. The relative high kinetic inductance of the SQUID loop, in the range of tens of nH, leads to a suppression of the modulation in the measured switching current versus magnetic flux, accompanied by a distortion towards a triangular shape. We observe a change in the temperature dependence of the switching current histograms with increasing normal-state film resistivity. This behavior suggests the onset of a diffusive motion of the superconducting phase across the constrictions in the two-dimensional washboard potential of the SQUIDs, which could be caused by a change of the local electromagnetic environment of films with increasing normal-state resistivities.