New Superconducting Lead Cuprates Prepared by Molecular Beam Epitaxy

Shin-ichi Karimoto and Michio Naito
Materials Science Laboratory

Since the discovery of high-Tc 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 Tc '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 PbOx are known to have a high vapor pressure above 600C . We grew thin films of PbSr2CuO5+d (Pb1201) at 500C 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 Tc as low as 40 K. According to the empirical role, if we can synthesize this lead cuprate series with three CuO2 layers, Tc 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-Tc superconductors.

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