FeSe基超导单晶与薄膜研究新进展:自旋向列序、电子相分离及高临界参数

FeSe基超导体的超导临界温度可大范围调控，物理现象丰富，是非常规超导机理研究的热点.由于较高的超导临界参数及易于加工等特点，FeSe基超导体在超导应用开发方面也日益受到重视.大尺寸高质量的单晶和薄膜形态的FeSe基超导材料，对于相关基础科学研究和应用开发都极为重要.作者近年来先后开发和发明了水热离子交换（ion-exchange）、离子脱插（ion-deintercalation）、基底辅助水热外延生长方法，成功解决了二元FeSe和插层（Li，Fe）OHFeSe超导体高质量单晶和薄膜的生长和物性调控难题.进而在相关物理问题的研究中取得新进展，包括发现二元FeSe中自旋向列序与超导电性密切相关，观测到（Li，Fe）OHFeSe中的电子相分离现象.此外，（Li，Fe）OHFeSe超导薄膜呈现很高的超导临界电流密度和上临界磁场，其应用前景值得关注.

New progress of FeSe-based superconducting single crystals and films: Spin nematicity,electronic phase separation,and high critical parameters

High-quality superconducting single crystals and thin films play an important role in the basic research and application of high-T_c superconductivity. In these two aspects, iron-based superconductors feature the merit of rich physical phenomena and high superconducting critical parameters (including the transition temperature T_c, the upper critical field H_c2 and the critical current density J_c). By developing ion-exchange and ion-de-intercalation method, we successfully synthesize a series of high-quality and sizable (Li,Fe)OHFeSe and FeSe single crystal samples. We observe Ising spin nematicity (below T_sn), and the universal linear relationship between T_c and T_sn in FeSe single crystals, indicating that the superconductivity is closely related to the spin nematicity driven by stripe antiferromagnetic spin fluctuations. In (Li,Fe)OHFeSe single crystals, we observe the coexistence of an AFM state (below T_afm~125 K) together with the SC state. We explain the coexistence by electronic phase separation, similar to that in high-T_c cuprates and iron arsenides, and establish a complete phase diagram for (Li,Fe)OHFeSe system. Here, we also make a brief introduction about our latest progress in growing a high-quality single-crystalline superconducting film of (Li,Fe)OHFeSe. The film is prepared by a hydrothermal epitaxial method. The high crystalline quality of the film is demonstrated by x-ray diffraction results, showing a single (001) orientation with a small crystal mosaic of 0.22° in terms of the full width at half maximum of the rocking curve, as well as an excellent in-plane orientation by the φ-scan of (101) plane. Its bulk superconducting transition temperature T_c of 42.4 K is characterized by both zero electrical resistance and diamagnetization measurements. Based on systematic magnetoresistance measurements, the values of upper critical field H_c2 are estimated at 79.5 T and 443 T for the magnetic field perpendicular and parallel to the ab plane, respectively. Moreover, a large critical current density J_c of a value over 0.5 MA/cm² is achieved at~20 K. Such a (Li,Fe)OHFeSe film is not only important for the fundamental research for understanding the high-T_c mechanism, but also promises the high-T_c superconductivity applications, especially in high-performance electronic devices and large scientific facilities such as superconducting accelerator.