Electron-Doped High-T C Superconducting Thin Films Grown by Molecular Beam Epitaxy

Shin-ichi Karimoto and Michio Naito
Materials Science Laboratory

    High-T C superconductors are classified as hole-doped or electron-doped and there seems to be apparent asymmetry between the two classes. Hole-doped superconductors are major, and their highest T C is 135 K. Electron-doped superconductors are minor (only two kinds), and their highest T C is 40 K. This raises two important questions: why these T C values are so different, and whether or not superconductivity mechanism is the same. To answer these questions, it is highly desirable to find new electron-doped superconductors and obtain high-quality single-crystalline thin films. Recently, we have succeeded in the MBE-growth of single-crystalline films of the following two electron-doped superconductors that are difficult to obtain by bulk synthesis.
(1) (Sr, La)CuO2 (T C =40 K)
    Infinite-layer compounds have the simplest structure among high- T C superconductors (see Fig. 1(a)). However, despite this simple structure and the fact that they are the electron-doped superconductors with the highest T C, they have been studied very little. This is because the synthesis of these compounds requires a high pressure of above 2.5 GPa. MBE enables us to synthesize this high-pressure phase via an epitaxial effect. Figure 1(b) shows the r-T curve of our single-crystalline (Sr, La)CuO2 thin film grown on a special substrate (KTaO3). The low resistivity and metallic behavior indicate excellent crystallinity [1]. (2)T'- (La, Ce)2CuO4 (T C =30K)
    The other kind of electron-doped superconductor is represented by (Ln,Ce)2CuO4. This structure is formed with Ln=Pr, Nd, Sm, Eu, Gd, by using the conventional solid-state reaction method. The maximum T C (25 K) of this series is achieved with (Pr,Ce)2CuO4 and (Nd,Ce)2CuO4. Low-temperature MBE growth makes it possible to synthesize (La,Ce)2CuO4, which attains the highest T C in this group (30 K). Moreover, we found that there is a tendency for the T C to become higher as increasing the ionic radius of the Ln elements. This gives us a clear guiding principle in the search for new high-T C superconductors [2].

[1] S. Karimoto et al., submitted to Appl. Phys. Lett.
[2] M. Naito and M. Hepp, Jpn. J. Appl. Phys. 39 (2000) L485.

Fig. 1.(a) Crystal structure and (b) best r -T curve for (Sr, La) CuO2 thin film.

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