Signature of charge-3e tunneling in twisted cuprate Josephson junctions

Abstract

Cooper pairing is a fundamental aspect of superconductivity and results in charge transport with a basic unit of 2e (e is the electron charge). This basic unit can manifest itself as the denominator in the formula for Shapiro steps of ac Josephson effect-nhf/(2e), where h is the Planck constant, f is the ac frequency and n is an integer. Violation of this standard formula may suggest unconventional physics, such as nhf/4e for charge-4e superconductivity. Here, we report discovery of a new sequence of nhf/3e in twisted cuprate Josephson junctions. The microwave frequency/power, temperature, and current annealing dependences of this exotic sequence consistently point to hf/3e as the unit for Shapiro steps, indicating a Josephson tunneling process involving a trio of electrons (3e). This bunching of an odd number of charges distinguishes itself from well-established superconducting processes, which are carried by composite bosons made of an even number of charges. The unusual observations highlight the important interplay between competing charge orders in twisted bilayer systems.