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

Felix Friedrich,Ioan M Pop,Wolfgang Wernsdorfer,Christoph Sürgers,Hannes Seeger,Kiril Borisov,Patrick Winkel

doi:10.1088/1361-6668/ab4918

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 ρn = 250–5550 μΩ 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.

Highlights

Superconductivity in granular aluminum films was first reported by Cohen and Abeles in 1968 [1]
For standard Al SQUIDs, ssw increases with temperature, as expected [17], while for high impedance granular aluminum (grAl) SQUIDs ssw decreases with temperature
SQUID modulation separates the metastable states of the phase particle decreases with increasing bias current, thermal activation (TA) over the barrier or macroscopic quantum tunneling (MQT) through it will trigger switching events at bias currents smaller than Ic

Summary

Present address

A detailed study on the origin of the increased critical temperature of grAl was recently presented by Pracht et al [4]. The switching dynamics of the measured SQUIDs can indicate changes at the high end of the spectrum close to the superconducting gap [16], where it is difficult to perform accurate rf experiments. For standard Al SQUIDs, ssw increases with temperature, as expected [17], while for high impedance grAl SQUIDs ssw decreases with temperature. This indicates the onset of phase diffusion [18,19,20], which could be linked to additional damping at frequencies comparable to the plasma frequency ωp [16]

Sample fabrication

Switching current measurements

SQUID modulation

Switching current distributions

Phase diffusion

Conclusion