高质量FeSe单晶薄膜的制备及相关性能表征

在铁基超导体中，FeSe具有最简单的晶体结构和化学组成，而且其超导转变温度具有较大的调控空间，因此适合作为超导机理研究和应用的载体.高质量样品的研制是物性研究和器件应用的前提，本文系统地研究了利用激光脉冲沉积技术制备FeSe薄膜的工艺条件，在多种衬底上成功地制备出高质量的β-FeSe薄膜，并首次实现了超导临界转变温度从小于2 K到14 K的连续调控，这为FeSe超导机理研究提供了样品支持.为探究FeSe薄膜超导电性变化的起因，从β-FeSe超导电性与晶格常数c正相关出发，基于简单的费米面填充假设，第一性原理计算可以很好地解释晶格常数c的变化规律，但该假设并不能完全符合角分辨光电子能谱实验给出的电子结构演变过程.因此β-FeSe薄膜的超导电性、晶格结构和电子结构三者之间的关系还有待澄清，该问题的解决将为FeSe超导机理研究提供重要的线索，而上述系列高质量的β-FeSe薄膜样品恰好能为该问题的研究提供理想的载体.本文根据实验和已有的相关研究结果，详细介绍了FeSe薄膜的脉冲激光沉积制备及其优化，以期为后续的薄膜研究应用提供参考.

Preparation and characterization of high-quality FeSe single crystal thin films

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 T_c, 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 T_c. 2) High-quality β-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 T_c for the films on CaF₂ with the same thickness of 160 nm can be tuned from < 2 K to 14 K. 3) The T_c 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 T_c, 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 β-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 Å of c. However, according to the angle-resolved photoelectron spectroscopy experiment, there is no obvious change near the Γ 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 β-FeSe films offers a chance to further explore the nature of FeSe-based superconductors.