New Superconducting Lead Cuprates Prepared by Molecular Beam Epitaxy

Shin-ichi Karimoto and Michio Naito
Materials Science Laboratory

Since the discovery of high-T_c superconductors, most of the initial efforts made in search of new superconductors focused on conventional solid-state reactions. In the early 1990's, a new technique was introduced, namely high-pressure or high-oxygen-pressure synthesis, and this produced a number of new materials. Recently, however, there have been fewer and fewer discoveries of new superconductors by bulk synthesis. This points to the need for routes for the next strategic material search. In 1996, we began to use molecular beam epitaxy (MBE) to search for new materials [1]. In the course of this research, we have succeeded in the synthesis of new superconducting lead cuprates [2].
The crystal structure of superconducting copper oxides can be simply viewed as the stacking of two-dimensional CuO₂ planes and a charge reservoir block (Fig. 1). Empirically, charge reservoir blocks containing heavy cation elements give high T_c 's. However, Hg- and Tl-based superconductors contain toxic elements, and Bi-based superconductors have strong two-dimensionality, which is an obstacle to magnetic wire applications. This has led to the desire to develop Pb-based superconductors, but there has been no success. There are two important keys to the successful synthesis of lead cuprates. (1) Reduction of the synthesis temperature: Pb and PbO_x are known to have a high vapor pressure above 600��C . We grew thin films of PbSr₂CuO_5+d (Pb1201) at 500��C to avoid this problem. (2) An appropriate choice of lattice-matched substrates: Figure 2 shows X-ray diffraction patterns for films grown on SrTiO₃ and LaAlO₃ substrate. The film grown on SrTiO₃ is perovskite SrPbO₃, which is a thermodynamically stable phase. By contrast, the film grown on LaAlO₃ is c-axis oriented Pb1201. The use of a lattice-matched LaAlO₃ substrate makes the quasi-stable Pb1201 phase very easy to obtain.
Pb1201 contains one CuO₂ layer, and hence has T_c as low as 40 K. According to the empirical role, if we can synthesize this lead cuprate series with three CuO₂ layers, T_c higher than 120 K will be attained.
[1] H. Yamamoto et al., Jpn. J. Appl. Phys. 36 (1997) L341.
[2] S. Karimoto and M. Naito, Jpn. J. Appl. Phys. 38 (1999) L283.



Fig. 1. Crystal structures of high-T_c superconductors.




Fig. 2. X-ray diffraction patterns for films grown on different substrates.


Back