新界面超导体系的探索

The discovery of high-temperature superconductivity with a superconducting gap of ∼20 meV in single unit cell FeSe on SrTiO3 substrates has stimulated tremendous interest in the superconductivity community, for it opens a new avenue for both raising superconducting transition temperature and understanding the pairing mechanism of unconventional high temperature superconductivity. In this project, we plan to prepare ultrathin superconducting films on semiconductor/oxide substrates by molecular beam epitaxial growth and study the superconductivity properties therein by combining in-situ scanning tunneling spectroscopy and transport measurement. This aims to find new interface superconductivity. We will also modulate the doping level by surface doping, substitution and electric field effect to figure out the doping level dependence of superconductivity. Furthermore, we will study the impurity states induced by magnetic and non-magnetic atoms and bound states at the center of a vortex to check the pairing symmetry. Based on the above experimental study and the comparison with bulk superconductivity, we expect to deepen our understanding on interface superconductivity and figure out the key factors for enhancing superconductivity. This project may shed light on understanding the unconventional superconductivity in cuprate and iron-based superconductors, both of which exhibit sandwiched structures consisting of alternative superconducting layers and charge reservoir layers, and searching for new high temperature superconductors.

外延于SrTiO3衬底上单层FeSe薄膜具有十倍于体态值的超导能隙这一实验发现引发了国内外对界面超导的研究热情。本项目将利用分子束外延生长技术制备原子级厚度超导薄膜，构建导电/半导体(氧化物)异质界面；结合低温强磁场扫描隧道谱与输运测量，研究尺寸效应、界面效应对超导性质的影响，探索新的界面增强超导体系。进一步，利用表面掺杂、替位掺杂以及场效应调节等手段实现载流子浓度调控，获得超导转变温度对掺杂浓度的相图；研究各类超导薄膜中磁性原子和非磁性原子杂质态和磁通中心的准粒子态，探索杂质对超导的影响，揭示超导电子配对的对称性和配对机制。对比不同界面体系及体相超导性质的异同，探索导致界面增强超导的关键因素。这一研究项目旨在发现新的界面超导体系，积累更多界面超导实例及更深入理解增强超导特性的关键因素。这一项目的顺利开展将对理解高温超导机理以及发现更高转变温度的超导材料都具有重要意义。

在本项目执行过程中，按照申请书中研究计划进行了超导层/氧化物界面超导探索工作。发现了FeSe/MgO和FeSe/NdGaO3界面增强超导体系。我们还利用SrTiO3衬底模板作用获得了四方相CoSe和CoSb薄膜，结合谱学和抗磁性测试表明CoSb/SrTiO3可能是新的超导体系。进一步研究了FeSe/SrTiO3界面增强超导机制，发现了超导能隙与界面氧空位浓度正相关关系。发展了SrTiO3双晶衬底处理技术和利用放电等离子烧结技术快速制备硒化铁/氧化物混合多晶材料的方法。以上相关工作为理解界面高温超导机制和探索更多界面超导体系提供了丰富的实验依据。