Exploring the energy spectrum of a four-terminal Josephson junction: towards topological Andreev matter

Hybrid multiterminal Josephson junctions (JJs) are emerging platforms for manipulating Andreev bound states (ABSs). In these systems, ABSs depend on multiple superconducting phases, leading to new properties such as molecule-like states, phase-induced spin splitting, and ground-state parity transitions. In a four-terminal geometry, ABSs are predicted to hybridize, resulting in a non-trivial topological band structure characterized by Weyl nodes at zero energy [1-6]. In this talk, I present the realization of a four-terminal JJ fabricated on a hybrid Al/InAs heterostructure, where three phase differences are independently controlled via flux biasing. Using tunnelling spectroscopy, we explore the ABS band structure across the 3D phase space. Specifically, we identify spectral signatures indicative of effective hybridization among three ABSs. Our measurements are supported by theoretical simulations that reproduce the signatures of the Andreev tri-molecule experimentally observed. Moreover, our simulations predict the formation of Weyl nodes, which remain stable within an extended region of parameter space accessible by our experimental system. Our approach provides a deep understanding of the complex ABS band structure in four-terminal devices, establishing solid experimental foundations for future realizations of topological phases in multiterminal JJs.

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