Supersolidity and the quantum spin liquid state in monolayer of helium

2021 Fiscal Year Final Research Report

• Project/Area Number: 18H01170
• 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: The University of Tokyo
• Principal Investigator: FUKUYAMA Hiroshi 東京大学, 低温科学研究センター, 特任研究員 (00181298)
• Co-Investigator(Kenkyū-buntansha): 村川 智 東京大学, 低温科学研究センター, 准教授 (90432004)
• Project Period (FY): 2018-04-01 – 2021-03-31
• Keywords: 量子液晶 / 超固体 / スピン液体 / 単原子層

Outline of Final Research Achievements

At low temperatures, helium atoms form a monatomic adsorbed layer on a crystalline graphite surface, where atomic motions are restricted to two dimensions (2D). In this 2D quantum system, the novel quantum liquid-crystal (QLC) phase with finite fluidity even at absolute zero is expected to emerge as an intermediate phase between the quantum liquid and solid phases. In this study, by performing simultaneous measurements of mechanical response of the adsorbed helium-4 film under substrate oscillation and specific heat of the film, we have succeeded in observing an anomalous superfluid state, which strongly supports the QLC hypothesis. In addition, we have shown that the novel quantum spin-liquid magnetism observed in the adsorbed film of helium-3, an isotope of helium-4, can be explained by assuming the QLC structure behind. A preliminary but encouraging result of synchrotron radiation X-ray diffraction experiment aiming for structural identification of the QLC phase was also obtained.

Free Research Field

低温物理学

Academic Significance and Societal Importance of the Research Achievements

物質が取り得る状態（相）には気体、液体、固体、液晶、アモルファスの5態ある。ヘリウム原子からなる物質系の場合、量子力学効果が顕在化する絶対零度では液体と固体のみが存在し（それぞれ量子固体と量子液体）、後者は粘性のない超流動状態となる。本研究では、表面吸着したヘリウム単原子層膜に「量子液晶」という物質の新たな状態とその超流動相が存在することを強く示唆する結果が得られた。これは人類の自然界の理解をより豊かなものとし、量子計算機の原理である量子もつれと深く関係する「量子スピン液体」磁性など他分野への波及効果もある。高輝度放射光を利用した極低温研究という新分野にも先鞭をつけることができた。