Charge neutral fermionic excitations in Kondo insulators

2020 Fiscal Year Final Research Report

• Project/Area Number: 18H01180
• Research Category: Grant-in-Aid for Scientific Research (B)
• Allocation Type: Single-year Grants
• Section: 一般
• Review Section: Basic Section 13030:Magnetism, superconductivity and strongly correlated systems-related
• Research Institution: Kyoto University
• Principal Investigator: Kasahara Yuichi 京都大学, 理学研究科, 准教授 (10511941)
• Co-Investigator(Kenkyū-buntansha): 伊賀 文俊 茨城大学, 理工学研究科(理学野), 教授 (60192473)
• Project Period (FY): 2018-04-01 – 2021-03-31
• Keywords: 近藤絶縁体 / 中性フェルミオン / トポロジカル表面状態

Outline of Final Research Achievements

We have investigated low-energy excitations in YbB12 by thermal transport and specific heat measurements. A finite linear-in-temperature specific heat coefficient and the residual thermal conductivity is observed, providing evidence for the presence of charge-neutral fermionic quasiparticles. We observed quantum oscillations in both insulating state and field-induced metallic state. From a detailed analyses of oscillation amplitude and periods, we found that the neutral fermions give rise to quantum oscillations in both insulating and metallic state. Moreover, a signature of topological surface metallic state is detected by transport measurements in microstructured crystals.
We also observed signatures of charge-neutral fermionic excitations in a new Kondo insulator YbIr3Si7 that exhibits antiferromagnetic order. The neutral fermions change its properties across the field-induced magnetic transition, indicating the strong coupling between the neutral fermions and electronic spins.

Free Research Field

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Academic Significance and Societal Importance of the Research Achievements

近藤絶縁体は発見から半世紀近くにもなる古い物質系であるが、本研究で観測された量子振動や中性フェルミオンはこれまでの常識では予測されてこなかった新しい現象であり、近藤絶縁体において特異な量子凝縮状態が実現していることが明らかとなった。本研究成果に続いて、他の絶縁体物質においてこれらの現象が報告されるなど、物性研究の新展開となりつつある。中性フェルミオンとして量子コンピューターの基本粒子となるマヨラナ粒子も提案されており、中性フェルミオンの起源の解明は物性物理だけでなく関係応用分野においても極めて重要である。