A brief analysis of annealing process for electron-doped cuprate superconductors

The high-Tc copper-oxide superconductors (cuprates) break the limit of superconducting transition temperature predicted by the BCS theory based on electron-phonon coupling, and thus it opens a new chapter in the superconductivity field. According to the valence of substitutents, the cuprates could be categorized into electron-and hole-doped types. So far, an enormous number of high-Tc cuprate superconductors have been intensively studied, most of them are hole-doped. In comparison with the hole-doped cuprates, the advantages of electron-doped cuprates (e.g. lower upper critical field, less-debated origin of “pseudogap”, etc.) make this family of compounds more suitable for unveiling the ground states. However, the difficulties in sample syntheses prevent a profound research in last several decades, in which the role of annealing process during sample preparation has been a big challenge. In this review article, a brief comparison between the electron-doped cuprates and the hole-doped counterparts is made from the aspect of electronic phase diagram, so as to point out the necessity of intensive work on the electron-doped cuprates. Since the electronic properties are highly sensitive to the oxygen content of the sample, the annealing process in sample preparation, which varies the oxygen content, turns out to be a key issue in constructing the phase diagram. Meanwhile, the distinction between electron-and hole-doped cuprates is also manifested in their lattice structures. It has been approved that the stability of the superconducting phase of electron-doped cuprates depends on the tolerance factor t (affected by dopants) doping concentration, temperature, and oxygen position. Yet it is known that the annealing process can vary the oxygen content as well as its position, the details how to adjust oxygen remain unclear. Recently, the experiment on Pr2-xCexCuO4-δ suggests that the oxygen position can be tuned by pressure. And, our new results on [La1.9Ce0.1CuO4-δ/SrCoO3-δ]N superlattices indicate that more factors, like strain, should be taken into account. In addition, the superconductivity in the parent compounds of electron-doped cuprates has emerged by employing a so-called “protective annealing” process. Compared to the traditional one-step annealing process, this new procedure contains an extra annealing step at higher temperature at partial oxygen pressure. In consideration of the new discoveries, as well as the Tc enhancement observed in multilayered structures of electron-doped cuprates by traditional annealing, a promising explanation based on the idea of repairing the oxygen defects in copper oxide planes is proposed for the superconductivity in parent compounds. Finally, we expect a comprehensive understanding of the annealing process, especially the factors such as atmosphere, temperature, and strain, which are not only related to the sample quality, but also to a precise phase diagram of the electron-doped cuprates.