Study of interfacial chiral superconductivity in high Tc SQUID architecture

Recent advances in van der Waals fabrication techniques have revived interest in twisted cuprate systems. Many facets of the twisted interfacial Josephson junction (JJ) are still unexplored in cuprate systems. The d-wave symmetry of the superconducting order parameter of the top and bottom layers determines the Cooper-pair transport mechanism and nature of the interfacial order parameter. Theoretical studies suggest that for a 45° twisted interface, Cooper pair tunneling will be suppressed, and Cooper pair co-tunneling or simultaneous tunneling of two Cooper pairs will be the dominant charge transport mechanism. Additionally, it suggests that the interfacial order parameter is time-reversal symmetry broken for a 45°-twisted interface.

So far, no experimental studies have shed light on whether this time-reversal symmetry-broken order parameter is chiral or not. To probe the properties of the twisted interface of Bi₂Sr₂CaCu₂O_8+δ (BSCCO), we have fabricated a c-axis asymmetric dc SQUID. We have employed the cryogenic exfoliation technique to fabricate a pristine twisted interface for SQUID devices and create the SQUID geometry by making an asymmetric cut across the junction. In our study, we observe a clear signature of quantum interference as the magnetic field threading the SQUID loop is varied for SQUIDs of different twist angles. Crucially, we directly observe an anomalous phase difference that results from superconducting phases of opposite chirality in the two JJs in the SQUID loop. We can also infer a prominent second-harmonic contribution in the CPR and the breaking of time-reversal symmetry. Moreover, we found that as the twist angle approaches 45°, the second harmonic contribution becomes increasingly dominant. From a practical application perspective, we demonstrate that our device performs as a well-suited flux sensor with state-of-the-art specifications as a SQUID with flux noise sensitivity ~ 1.5 μΦ₀/√Hz.