铁基超导体的分子束外延和物性研究

We propose the molecular beam epitaxy of the high quality Fe-based superconductor single crystal thin film and focus on the synthesis of the three elements compound, such as SrFe2As2, CaFe2As2 and so on. The dynamics of the growth process will be studied to obtain the critical parameters for the epitaxy and the universal method of the molecular beam epitaxy of the multi-element compound. By using in situ scanning tunneling microscopy (STM) and angle-resolved photoelectron spectroscope (ARPES), we’ll study crystal structure, defects, electronic band structure, crystal structure phase transition, magnetic order phase transition and superconductivity transition of the epitaxial thin film. We’ll also investigate the electron-bosonic mode, the spin density wave and the vortex state in iron superconductor to reveal the pairing symmetry and mechanism. The well controlled thickness of the iron superconducting thin film by molecular beam epitaxy will be used to study the vertical quantum confinement effect along c axis of thin film and the interface effect due to the interaction between thin film and the substrate. Transport measurement will investigate the basic properties of the superconducting thin film, such as superconducting transition temperature, critical current density, vortex pinning effect and transport property under high magnetic field. The field effect device will be fabricated to study the transport property of the multi-band superconductor under the tuning of the carrier density.

本项目主要利用分子束外延技术探索高质量铁基超导体单晶薄膜制备方法，主要集中研究三元化合物生长，如“122”体系，（ SrFe2As2,CaFe2As2等,通过掺杂可以实现超导）。研究薄膜外延生长动力学过程，并能够掌握控制单晶薄膜生长的重要物理参数，总结出三元化合物分子束外延的普适规律。结合原位的低温强磁场扫描隧道显微镜，原位的角分辨光电子能谱，研究分子束外延薄膜的晶体结构，缺陷类型，能带结构，结构相变，磁相变，超导相变，并观察可能存在的电子-波色子耦合模，自旋密度波能隙等，探索非常规超导体的配对对称性和配对机制等。利用分子束外延对超导薄膜的精确控制，研究超导薄膜中由于厚度变化引起的量子尺寸效应，以及薄膜和衬底相互作用引起的界面效应。通过非原位的输运特性测量，研究超导薄膜的基本物理特性，如超导转变温度、临界电流密度、磁场下的输运特性、场效应特性和磁通钉扎效应等。

本项目主要利用分子束外延技术制备高质量铁基等超导体单晶薄膜，并结合原位的低温强磁场扫描隧道显微镜，研究其的超导特性。（1）实现了在STO(001)表面和Graphene/SiC(0001)表面四方相FeS薄膜的分子束外延生长（Chin. Phys. Lett. 34， 087401（2017）; Nano Research 11, 4722(2018)）。（2）钾掺杂FeSe薄膜中发现双dome的相图。Phys. Rev. Lett. 116, 157001(2016)）。（3）单原胞NbSe2薄膜中Ising超导态的输运和扫描隧道显微镜研究，发现由无序诱导的超导增强效应和多重分形超导态（Nano Letter 17, 6802(2017)，Nat. Phys. 15, 904 (2019)）。