New class of T′-structure cuprate superconductors |
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Authors: | A Tsukada Y Krockenberger M Noda H Yamamoto L Alff |
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Institution: | a NTT Basic Research Laboratories, 3-1 Morinosato, Atsugi, Kanagawa 243, Japan b Max-Planck-Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany c Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan d Vienna University of Technology, ISAS, Applied Electronic Materials, Gusshausstr. 27-29/366, A-1040 Wien, Austria e Department of Applied Physics, Tokyo University of Agriculture and Technology (TUAT), 2-24-16 Naka-cho, Koganei, Tokyo 184-8588, Japan |
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Abstract: | High-temperature superconductivity has been discovered in La2−xBaxCuO4 J.G. Bednorz, K.A. Müller, Z. Phys. B 64 (1986) 189. 1]], a compound that derives from the undoped La2CuO4 crystallizing in the perovskite T-structure. In this structure oxygen octahedra surround the copper ions. It is common knowledge that charge carriers induced by doping in such an undoped antiferromagnetic Mott-insulator lead to high-temperature superconductivity V.J. Emery, Phys. Rev. Lett. 58 (1987) 2794; C.M. Varma, S. Schmitt-Rink, E. Abrahams, Solid State Commun. 62 (1987) 681; E. Dagotto, Rev. Mod. Phys. 66 (1994) 763. 2], 3] and 4]]. The undoped material La2CuO4 is also the basis of the electron-doped cuprate superconductors Y. Tokura, H. Takagi, S. Uchida, Nature (London) 337 (1989) 345. 5]] of the form La2−xCexCuO4+y M. Naito, M. Hepp, Jpn. J. Appl. Phys. 39 (2000) L485; A. Sawa, M. Kawasaki, H. Takagi, Y. Tokura, Phys. Rev. B 66 (2002) 014531. 6] and 7]] which, however, crystallize in the so-called T′-structure, i.e. without apical oxygen above or below the copper ions of the CuO2-plane. It is well known that for La2−xCexCuO4+y the undoped T′-structure parent compound cannot be prepared due to the structural phase transition back into the T-structure occurring around x∼0.05. Here, we report that if La is substituted by RE=Y, Lu, Sm, Eu, Gd, or Tb, which have smaller ionic radii but have the same valence as La, nominally undoped La2−xRExCuO4 can be synthesized by molecular beam epitaxy in the T′-structure. The second important result is that all these new T′-compounds are superconductors with fairly high critical temperatures up to 21 K. For this new class of cuprates La2−xRExCuO4, which forms the T′-parent compounds of the La-based electron doped cuprates, we have not been able to obtain the Mott-insulating ground state for small x before the structural phase transition into the T-structure takes place. |
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Keywords: | 74 62 Dh 74 72 &minus h 74 72 Dn 74 78 Bz |
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