浅析电子型掺杂铜氧化物超导体的退火过程

铜氧化物高温超导体的发现, 打破了基于电声子相互作用BCS理论所预言的超导转变温度极限, 掀开了高温超导材料探索和高温超导机理研究的序幕. 根据掺杂类型的不同, 铜氧化物超导材料可以分为空穴型掺杂和电子型掺杂两类. 受限于样品, 对电子型掺杂铜氧化物的研究工作远少于空穴型掺杂体系. 本文简要回顾有关电子型掺杂铜氧化物超导体近期研究成果, 通过对比电子型掺杂和空穴型掺杂铜氧化物的相图来阐明电子型掺杂铜氧化物的研究对探索高温超导机理的必要性, 并特别针对电子型掺杂样品制备中的关键因素“退火过程”展开讨论. 结合课题组最新实验结果和相关实验报道我们发现电子型掺杂铜氧化物超导体在制备过程中除受到温度和氧分压的影响外, 退火效果还受到界面应力的强烈调制. 在综合考虑样品生长过程中温度、气氛及应力等多种因素的基础上, 探讨了“保护退火”方法导致电子型体系化学掺杂相图变化的起因.

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

The high-T_c 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-T_c 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 Pr_2-xCe_xCuO_4-δ suggests that the oxygen position can be tuned by pressure. And, our new results on La_1.9Ce_0.1CuO_4-δ/SrCoO_3-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 T_c 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.