基于拓扑绝缘体纳米线约瑟夫森结的反常临界超流增强和半整数夏皮洛台阶

Anomalous critical supercurrent and half-integer Shapiro steps based on Josephson junction of topological insulator nanowires

基于拓扑绝缘体材料的约瑟夫森结是寻找马约拉纳零能模的候选器件,因而受到拓扑量子计算研究领域的关注.这方面实验的关键之一,是制备具有优质结区的约瑟夫森器件.本工作在三维拓扑绝缘体Bi_(2)Te_(3)和Bi_(2)(Se_(x)Te_(1-x))_(3)纳米线上制作了约瑟夫森结器件,研究了其结区的超导邻近效应、多重安德列夫反射和超流-相位关系,观测到了约瑟夫森结的临界超流随磁场增大而反常地增大、其交流约瑟夫森效应出现半整数的夏皮洛台阶的实验结果.本文还讨论了这些反常现象的可能来源,特别是与结区界面处超导电极的Ti缓冲层和拓扑绝缘体纳米线中的Te元素形成TiTe铁磁性合金层的关系.

Topological insulator-based Josephson junction,as a candidate device for searching for Major ana zero energy modes,has attracted much attention.One of the key issues along this direction is to fabricate Josephson junctions with high-quality interfaces,hoping to searching for 4π-period current-phase relation in topologically non-trivial Josephson junction.In this work,the Josephson junctions based on three-dimensional topological insulator nano wires Bi_(2)Te_(3) and Bi_(2)(Se_(x)Te_(1-x))_(3) are fabricated to study their superconducting proximity effects,multiple Andreev reflections,and current-phase relations.A number of interesting phenomena are observed,including the anomalous enhancement in junctions’critical super current with magnetic field,and the appearance of half-integer Shapiro steps in the ac Josephson effect.And,we discuss the possible origins of the observed anomalous behaviors in general,and their relation with the ferromagnetic layer of TiTe alloy formed at the interface between the topological insulator nano wires and the Ti buffer layer of the metallic electrodes,in particular.We provide the experimental evidence for the formation of a ferromagnetic TiTe alloy layer at the interface of our device.And,we believe that the formation of such a layer in our Josephson device breaks the time reversal symmetry,leading to the observed anomalous enhancement of the critical supercurrent with magnetic field,as well as the appearance of half-integer Shapiro steps.Our results suggest that to study the topologically non-trivial behaviors such as 4π-period current-phase relation,one still needs to improve the interface quality of the superconductor-normal metal-superconductor type of Josephson junction devices.