Synthesis of the Parent Compound Superconductors RE2CuO4 by MBE and Specially-designed Post-reduction


Hideki Yamamoto, Osamu Matsumoto*, Keitaro Yamagami, and Michio Naito*
Materials Science Laboratory, *Tokyo Univ. of Agricul. & Technol.

 It is commonly believed that the parent compounds of high-Tc cupratres are, universally, Mott insulators (charge-transfer insulators). There has been, however, accumulating evidence that indicates a series of parent compounds T’-RE2CuO4 [RE stands for rare earth element] are superconductors with Tc even exceeding 30 K when they are optimally synthesized [1]. Most of the recent progress has been achieved by using the metal-organic decomposition (MOD) method [1]. Most likely, this originates from the necessity of removing impurity interstitial oxygen while simultaneously preserving regular oxygen sites occupied, which is a prerequisite for achieving superconductivity in square-coplanar CuO2 plane. It appears that this requirement is easier to be fulfilled in the MOD films due to rather small grains in them [2], which in turn makes it difficult to reproduce the results using other synthesis methods. In this study, we systematically investigated the post-reduction process using MBE-grown T’-RE2CuO4 (RE=Pr, Nd) films on SrTiO3 (001) substrates for an easier reproduction through a better understanding of the process.
 Figure 1 shows several important parameters in the growth and post-reduction procedures. To make the parent compounds superconducting, a specially-designed 2-step reduction process was necessary, as in the case of MOD films. By optimizing oxygen partial pressure and annealing temperature with constant duration, the MBE-grown RE2CuO4 (RE=Pr, Nd) films show superconductivity (Fig. 2) [3].
 These findings are useful for synthesizing bulk specimens of the parent compound superconductors, which makes evaluation of oxygen stoichiometry feasible. They are also useful for synthesizing the thin-film specimens through an all-UHV process, with which a direct observation of electronic structure by photoemission spectroscopy becomes possible. Both will lead us toward the true electronic phase diagram for high-Tc cuprates.

[1] O. Matsumoto et al., Phys. Rev. B 79 (2009) 100508; Physica C 469 (2009) 924.
[2] O. Matsumoto, A. Tsukada, H. Yamamoto, T. Manabe, and M. Naito, Physica C (in press).
[3] H. Yamamoto, A. Tsukada, O. Matsumoto, and M. Naito, Physica C (in press).

Fig. 1. Parameters in MBE growth and post-reduction
of RE2CuO4 thin films.
Fig. 2. ρ-T characteristics of MBE-grown
Nd2CuO4 and Pr2CuO4 thin films.

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