モット転移近傍の異常金属相-銅酸化物の異常金属相と高温超伝導体-

1995 年度 実績報告書

• 研究課題/領域番号: 07237105
• 研究機関: 名古屋大学
• 研究代表者: 佐藤 正俊 名古屋大学, 理学部, 教授 (40092225)
• 研究分担者: 藤田 敏三 広島大学, 理学部, 教授 (20004369) ; 朝山 邦輔 大阪大学, 基礎工学部, 教授 (20029416) ; 内田 慎一 東京大学, 工学研究科, 教授 (10114399) ; 遠藤 康夫 東北大学, 理学研究科, 教授 (00013483) ; 伊土 政幸 北海道大学, 理学研究科, 教授 (90111145)
• キーワード: 2次元強相関電子系 / 異常金属相 / 高温超伝導 / スピンギャップ / 超伝導対称性 / モット転移

研究概要

高温超伝導体の異常金属相は温度Tとキャリア濃度pのT-p平面において,モット絶縁相近傍のT<T_0(p)にはっきりと定義できる。ここでT_0(p)は輸送特性の異常が顕著になる温度である。この異常相領域内で,Y123系等では“スピンギャップ異常"がT【less than or equal】T_<SG>〜150Kに見られる。低温電子比熱係数の異常が見られるのもこの領域内である。高温超伝導の発現機構をあいまいさなしに理解するには,パラメーターの選択に大きく依存する超伝導転移温度の計算で満足することなく,これらの極めて特徴のある複数個の異常現象を矛盾のない形で記述する枠組を構築するのが良い。T_0やT_<SG>での異常,γ異常,高温超伝導出現を全て,CuO_2面内の強相関電子系のおりなす物性現象として統一的に理解すれば,確固とした物理像となるからである。本年度はこの異常金属相の成因及びそれと超伝導出現との関連をしらべる実験を(i)銅酸化物の高温物性の精密測定や(ii)La_<1-x+α>Sr_xVS_<3+α>(α〜0.17),BaCo_<1-x>Ni_xS_2やCaV_4O_9等,銅酸化物と同様にモット絶縁相にあるか,モット転移を有する2次元電子系物質の異常物性と銅酸化物のそれとの比較を通して行った。異常金属相がモット転移近傍の2次元系に共通して存在すること,それが磁気相関の成長に伴って出現すること,さらにはそれと矛盾するかに見えるスピンギャップ異常も同時にはじまる可能性が見えはじめた事等大きな進展があった。
超伝導対称性については中性子磁気非弾性散乱,フォノン測定,STM/STSによる超伝導ギャップ研究,低温電子比熱の詳細な測定,NMRによる測定,非線型マイスナー効果,光学測定等多様な実験手段で研究され,d波それも(x^2-y^2)的対称性に収束した。これはおそらく異常金属相の特徴である磁気的相関とつながる超伝導発現機構が働いていることを示す。

研究成果

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