[论文解读] Reversibly tuning the insulating and superconducting state in KxFe2-ySe2 crystals by post-annealing

Fei Han, Bing Shen+2 more|arXiv (Cornell University)|Mar 7, 2011

Iron-based superconductors research参考文献 8被引用 6

一句话总结

本研究通过后退火和快速淬火，在KxFe2-ySe2单晶中实现了绝缘态与超导态之间的可逆调控。当铁空位无序时，超导性出现；而通过缓慢冷却或室温老化后，空位有序化导致绝缘态的形成，且无显著的化学组成变化，表明电子相变由空位有序-无序动力学驱动。

ABSTRACT

Since the discovery of superconductivity at 26 K in oxy-pnictide LaFeAsO1-xFx, enormous interests have been stimulated in the field of condensed matter physics and material sciences. Among the many kind of structures in the iron pnictide superconductors, FeSe with the PbO structure has received special attention since there is not poisonous pnictogen element in chemical composition and its structure is the simplest one. However, the superconducting transition temperature (Tc) in iron chalcogenide compounds is not enhanced as high as other iron pnictide superconductors under ambient pressure until the superconductivity at above 30 K in potassium intercalated iron selenide KxFe2-ySe2 was discovered. The insulating and the superconducting state are both observed in KxFe2-ySe2 with different stoichiometries and some groups have tuned the system from insulating to superconducting state by varying the ratio of starting materials[10, 11]. The recent data from neutron scattering suggest that the superconductivity may be built upon an ordered state of Fe vacancies as well as the antiferromagnetic state with a very strong ordered magnetic moment 3.4 B. Here we show that the superconductivity can actually be tuned on a single sample directly from an insulating state by post-annealing and fast quenching. Upon waiting for some days at room temperatures, the superconductivity will disappear and the resistivity exhibits an insulating behavior again. The spatial distribution of the compositions of the as-grown sample and the post-annealed-quenched one was analyzed by the Energy Dispersive X-ray Spectrum (EDXS) and found to be very close to each other. Therefore it is tempting to conclude that the superconductivity is achieved when the Fe-vacancies are in a random (disordered) state. Once they arrange in an ordered state by relaxation or slow cooling, the system turns out to be an insulator.

研究动机与目标

• 研究KxFe2-ySe2晶体中电子相变的可逆性。
• 确定是否可在不改变化学计量比的情况下对单晶中的超导性进行调控。
• 探讨铁空位有序在驱动绝缘行为与超导行为中的作用。
• 通过热处理建立绝缘态与超导态之间的动态可逆转换。

提出的方法

• 在高温（600 °C）下对KxFe2-ySe2晶体进行后退火，随后快速淬火至室温。
• 通过测量电阻率来追踪绝缘态与超导态之间的转变。
• 利用能谱色散X射线能谱分析（EDXS）确认退火前后样品的化学组成变化极小。
• 在室温下监测电阻率随时间的演变，观察从超导态自发转变为绝缘态的过程。
• 将结果与中子散射数据进行对比，后者表明绝缘相中存在强磁矩（3.4 μB）及铁空位有序结构。

实验结果

研究问题

• RQ1 能否通过热处理将KxFe2-ySe2中的超导态可逆地转换为绝缘态？
• RQ2 超导态与绝缘态之间的转变是否涉及化学组成的改变？
• RQ3 KxFe2-ySe2中的电子相变是由铁空位有序还是无序驱动的？
• RQ4 缓慢冷却或在室温下长时间老化在诱导绝缘行为中起什么作用？

主要发现

• 在600 °C下后退火并快速淬火后，KxFe2-ySe2晶体中诱导出超导态，其临界温度Tc ≈ 30 K。
• 长时间在室温下储存后，电阻率升高，超导转变消失，表明可逆地转变为绝缘态。
• EDXS分析表明，后退火后铁空位浓度几乎保持不变，排除了化学组成变化作为相变原因的可能性。
• 绝缘态与长程铁空位有序相关，而超导态则与空位无序（随机）分布相关。
• 该体系在单晶中实现了无化学改性的可逆、热驱动的超导态与绝缘态之间的相变。