Topological Josephson junction with arbitrary phase

2018 Fiscal Year Final Research Report

• Project/Area Number: 17K18756
• Research Category: Grant-in-Aid for Challenging Research (Exploratory)
• Allocation Type: Multi-year Fund
• Research Field: Condensed matter physics and related fields
• Research Institution: Osaka University
• Principal Investigator: Niimi Yasuhiro 大阪大学, 理学研究科, 准教授 (00574617)
• Project Period (FY): 2017-06-30 – 2019-03-31
• Keywords: ジョセフソン接合 / 超伝導材料・素子 / トポロジー / スピン蓄積

Outline of Final Research Achievements

In this work, we aimed 1) to verify that Bi/Ni bilayer is a topological superconductor by measuring the spin transport properties and 2) to demonstrate “Topological Josephson junction” with arbitrary phase using the Bi/Ni bilayer superconductor. We first performed spin transport measurements in Bi/Ni nanowires. The estimated spin diffusion length of Bi/Ni nanowires is 3 nm, which is shorter than that of thin Bi films. This result indicates that the Ni layer has a magnetic moment. The spin Hall signal in Bi/Ni below the superconducting transition temperature is enhanced by 1000 times compared to that above the transition temperature. In addition, it has a peculiar peak near the zero field, which has been never observed in an s-wave superconductor. We also fabricated Josephson junctions using the Bi/Ni bilayer superconductor. We are now investigating the relation between the critical current and the ferromagnetism of the Ni layer.

Free Research Field

低温スピン物性

Academic Significance and Societal Importance of the Research Achievements

現在様々な形で量子コンピュータの研究が進んでいるが、超伝導磁束量子回路で用いられている物質の多くがニオブNbをベースとしたs波超伝導体である。しかし、トポロジカル超伝導体を用いた量子デバイスを創製できれば、波動関数の位相制御という新しい機能を付与できる。これにより、超伝導量子コンピュータへの応用（コンパクト化や省エネルギー化）も期待される。さらに、量子力学の位相を積極的に利用した新しい位相物理学を切り開く可能性もある。