Discovery of non-doped superconductors
in lanthanum based copper oxide

Akio Tsukada, Hideki Yamamoto, and Michio Naito*
Materials Science Laboratory
*Tokyo University of Agriculture and Technology

　It has been believed that the non-doped parent materials of high-T_c superconductors are Mott insulators, and superconductivity emerges only after n-type or p-type doping. Against this belief, we found that superconductivity is achieved in non-doped La₂CuO₄ with Nd₂CuO₄ (T’) structure by substitution of La³⁺ by “isovalent” rare earth ions (RE³⁺) having a small ionic radius. This substitution is intended mainly to stabilized the T’ phase of La₂CuO₄ instead of T (K₂NiF₄) phase [1], so it is presumed not to cause effective doping. Therefore, our observation appears to contradict the general belief.
　 The new superconductors T’-La³⁺_2-xRE³⁺_xCuO₄ (RE³⁺ = Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Lu³⁺, Y³⁺) were prepared by molecular beam epitaxy (MBE) and showed superconducting transition at 〜 21 K [2]. Figure 1 shows the temperature (T) dependence of resistivity (ρ) for the films of La_2-xY_xCuO₄ with various x. Superconductivity appears at x = 0.09 and 0.15 with T_c 〜 20 K. Superconductivity was also observed in other RE substitution except for Pr and Nd substitution. Figure 2 summarizes the RE concentration dependence of T_c in T’-La_2-xRE_xCuO₄. Regarding the origin of superconducting carriers in T’-La_2-xRE_xCuO₄, one can think of two possible scenarios. One is that the oxygen deficiencies are a source of effective electron carriers. The other is that the T’-La₂CuO₄ is not a Mott insulator, and has intrinsic carriers, as predicted by the band calculation for Nd₂CuO₄. If the latter scenario is well-established, it will urge a reexamination of the early controversy on the “doped Mott-insulator scenario”, which is now widely accepted as a starting viewpoint for high-T_c superconductivity.

　

[1]	A. Tsukada, et al., Phys. Rev. B 66 (2002) 184545.
[2]	A. Tsukada, et al., Physica C, in press.

Fig. 1. ρ-T curves for La2-xYxCuO4.	Fig. 2. Tc for La2-xRExCuO4.

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