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-Tc 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 (Tf) structure by substitution of La3+ by gisovalenth rare earth ions (RE3+) having a small ionic radius. This substitution is intended mainly to stabilized the Tf phase of La2CuO4 instead of T (K2NiF4) phase , so it is presumed not to cause effective doping. Therefore, our observation appears to contradict the general belief.
@ The new superconductors Tf-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 . Figure 1 shows the temperature (T) dependence of resistivity (Ď) for the films of La2-xYxCuO4 with various x. Superconductivity appears at x = 0.09 and 0.15 with Tc ` 20 K. Superconductivity was also observed in other RE substitution except for Pr and Nd substitution. Figure 2 summarizes the RE concentration dependence of Tc in Tf-La2-xRExCuO4. Regarding the origin of superconducting carriers in Tf-La2-xRExCuO4, 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 Tf-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 gdoped Mott-insulator scenarioh, which is now widely accepted as a starting viewpoint for high-Tc superconductivity.
 A. Tsukada, et al., Phys. Rev. B 66 (2002) 184545.  A. Tsukada, et al., Physica C, in press.
Fig. 1. Ď-T curves for La2-xYxCuO4.
Fig. 2. Tc for La2-xRExCuO4.