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T''-phase electron-doped superconductor Pr1−xLaCexCuO4−δ (PLCCO) thin films are successfully prepared on SrTiO3 (100) substrates by using the dc magnetron sputtering method. It is found that the films each have a highly oriented structure along the c-axis. For optimally doped films with x ≈ 0.10, the superconducting transition temperature Tc is 23.5 K, which is similar to that of a single crystal. The quadratic temperature dependence of the resistivity is observed when T > Tc, which can be attributed to the two-dimensional Fermi liquid behaviour. Besides, the optimal conditions for preparing the T''-phase PLCCO thin films are also discussed in detail.

Of all iron-based superconductors, FeSe possesses the simplest structure whereas its superconducting critical temperature can be remarkably enhanced. Compared with bulk sample fabrication, the film preparation process is very precise and controllable. Although FeSe monolayer films exhibit a high Tc, they are unstable in air, and ex-situ measurements are very difficult. Therefore, the stable films with~100 nm in thickness can serve as good candidates to explore the mechanisms of iron-based superconductors. There is no doubt that the fabrication of high-quality FeSe thin films is of significance. The pulsed laser deposition (PLD) technique has more advantages in the growth of FeSe thick films than any other film fabrication technology, because of its high efficiency and wide adaptability. In this work, we systematically optimize the growth conditions of FeSe thin film fabricated by PLD. The main results are as follows. 1) The optimal growth temperature is 350℃, where the film has the best crystallinity and the highest Tc. 2) High-quality -FeSe epitaxial thin films with the thickness ranging from 10 to 320 nm have been successfully prepared on twelve types of substrates:CaF2, LiF, SrTiO3, MgO, BaF2, TiO2, LaAlO3, MgF2, Nb-SrTiO3, LSAT, LaSr(AlO4) and MgAl2O4. The Tc for the films on CaF2 with the same thickness of 160 nm can be tuned from 2 K to 14 K. 3) The Tc of the FeSe thick films may be precisely tuned by the Fe/Se ratio which is affected by the proportion of the nominal components of the target, the laser energy density and the ablation off-stoichiometry of target. 4) The surface morphology measurement, cleavability and transferability experiments of films are performed. In addition, it is worth of mentioning that there is a significant positive correlation between Tc, lattice constant c and residual resistivity ratio (RRR), as evidenced through a detailed statistical analysis of the data from more than 1500 samples. Since c and RRR are usually associated with the vacancies or defects, we conclude that the superconductivity of -FeSe thin films is closely related to the ratio of Fe to Se. Moreover, the first principle simulation shows that 0.5% increase of Fe content does lead to a change of 0.05 of c. However, according to the angle-resolved photoelectron spectroscopy experiment, there is no obvious change near the point in the hole energy band, but the energy band changes significantly at the M point. This variation of electronic structures cannot be explained by electron filling which lifts up the Fermi energy. Therefore, the specific relationship among the superconductivity, lattice structure and electronic structure of FeSe thin films remains to be clarified. Such a series of high-quality -FeSe films offers a chance to further explore the nature of FeSe-based 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.