Optical study for electronic state of electron-doped superconducting copper oxides

2020 Fiscal Year Final Research Report

• Project/Area Number: 18K03513
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Multi-year Fund
• Section: 一般
• Review Section: Basic Section 13030:Magnetism, superconductivity and strongly correlated systems-related
• Research Institution: Osaka University
• Principal Investigator: Tajima Setsuko 大阪大学, 理学研究科, 名誉教授 (70188241)
• Co-Investigator(Kenkyū-buntansha): 中島 正道 大阪大学, 理学研究科, 助教 (20724347)
• Project Period (FY): 2018-04-01 – 2021-03-31
• Keywords: 強相関電子系 / 銅酸化物超伝導体 / 赤外スペクトル / ラマン散乱スペクトル

Outline of Final Research Achievements

Recently the electronic phase diagram for electron-doped superconducting cuprates has been reconsidered, although it had been widely accepted for the last 30 years. By measuring far-infrared and Raman scattering spectra, we have examined the presence of oxygen deficiency in two samples, the as-grown crystal and the strongly reduced crystal. For both samples, we observed additional phonon modes that are not expected from the group theory. These are ascribed to the defect-induced modes. However, because these defect-induced modes are commonly observed in both samples, namely, independent of oxygen content, we judged that they are not induced by oxygen deficiency. As a conclusion, we found no evidence for oxygen deficiency in optical spectra. This indicates that the newly proposed phase diagram for the strongly reduced samples is intrinsic for this system.

Free Research Field

高温超伝導

Academic Significance and Societal Importance of the Research Achievements

モット絶縁体に電子を注入した場合と正孔を注入した場合が同じかどうか、といういわゆる「電子・正孔対称性問題」は、長らく議論されてきた。銅酸化物高温超伝導体が発見されてから約３０年、電子注入系と正孔注入系の電子相図は、ほぼ対称と理解されてきた。最近になって、強還元して正しい酸素濃度にした試料では、非常に僅かな正孔注入（場合によっては注入なし）で超伝導になるという電子相図が報告され、その正否が議論されている。本研究で強還元による有為な酸素欠損はないことが示され、僅かな正孔注入で超伝導になるという新しい電子相図が本質的なものであることが支持された。即ち、電子・正孔注入は非対称であることがわかった。