Limitations of the current-phase relation measurements by an asymmetric dc-SQUID

TL;DR

This paper demonstrates that the widely used asymmetric dc-SQUID method for measuring the current-phase relation in Josephson junctions can be inaccurate, especially when the reference junction's CPR asymmetry is not properly considered, leading to potential misinterpretations.

Contribution

The study reveals limitations of the current measurement technique and proposes considerations for more accurate CPR measurements using different reference junctions.

Findings

Measured CPRs showed both skewed and sinusoidal oscillations within single devices.

The accuracy of CPR measurement depends on the asymmetry in derivatives of CPRs, not just critical currents.

The method can lead to misinterpretation of the quantum transport phenomena in Josephson junctions.

Abstract

Exotic quantum transport phenomena established in Josephson junctions (JJs) are reflected by a non-sinusoidal current-phase relation (CPR). The solidified approach to measure the CPR is via an asymmetric dc-SQUID with a reference JJ that has a high critical current. We probed this method by measuring CPRs of hybrid JJs based on a 3D topological insulator (TI) Bi$_2$Te$_2$Se with a nanobridge acting as a reference JJ. We captured both highly skewed and sinusoidal critical current oscillations within single devices which contradicts the uniqueness of the CPR. This implies that the widely used method provides inaccurate CPR measurement and leads to misinterpretation. It was shown that the accuracy of the CPR measurement is mediated by the asymmetry in derivatives of the CPRs but not in critical currents as was previously thought. We provided considerations for an accurate CPR measurement that encourage future experiments with reference CPRs different from those that were used before.