Multiterminal Josephson junctions with tunable topological properties

Since the discovery of the Andreev reflection process at normal-metal/superconductor junctions and the corresponding Andreev bound states in superconductor/normal-metal/superconductor junctions, various multiterminal Josephson junctions have been studied to explore many exotic phases of quantum matter, where the formation of Andreev bound states in the normal region account for dissipationless supercurrent and play a central role in determining exotic properties. Recently, an intriguing aspect of the multiterminal Josephson junctions has been proposed to study the topological properties, wherein the Andreev bound states acquire topological characteristics upon tuning the phase differences of superconducting terminals. In this work, we investigate topologically non-trivial phases in four-terminal Josephson junctions based on square and graphene lattices. Additionally, we apply a gating potential that smoothly drives the Andreev bound states from a topologically non-trivial state to a topologically trivial state. Furthermore, we observe that the gating potential in our setup produces the similar physics of the topological Andreev bound states of the double (single) quantum-dot multiterminal Josephson junctions when the gating potential is small (large) compared to the superconducting energy gap.自正常金属/超导体结处的安德烈夫反射（Andreev reflection）过程，以及超导体/正常金属/超导体结中对应安德烈夫束缚态（Andreev bound states）被发现以来，学界已对各类多端约瑟夫森结（multiterminal Josephson junctions）开展广泛研究，以探索量子物质的诸多新奇物相。其中，正常区域内安德烈夫束缚态的形成是无耗散超电流产生的核心原因，并在决定体系新奇物性方面发挥核心作用。近期，多端约瑟夫森结的一个引人关注的研究方向被提出，用于拓扑性质探究：当调控超导电极的相位差时，安德烈夫束缚态将获得拓扑特性。本研究针对基于方形晶格与石墨烯晶格的四端约瑟夫森结（four-terminal Josephson junctions）中的拓扑非平庸物相展开系统探究。此外，我们施加可平滑调控的栅极电势（gating potential），使安德烈夫束缚态从拓扑非平庸态逐步演变为拓扑平庸态。我们进一步观测发现，当栅极电势相较于超导能隙（superconducting energy gap）较小时（较大时），本实验装置中的栅极电势可诱导出与双（单）量子点多端约瑟夫森结的拓扑安德烈夫束缚态相似的物理效应。