负带隙HgCdTe和HgTe半导体能带的应力调控研究

Consisting of the negative band gap HgTe and the wide-band-gap CdTe, HgCdTe is a kind of solid solution with continuously adjustable gap. Characterized by small electron effective mass, large electron effective g-factors and strong spin-orbit coupling strength, it is easy to realize the preparation of magnetic semiconductors by the doping of magnetic elements in HgTe. In this case, HgCdTe becomes an ideal material for the research of semiconductor spintronics. Recent theory study has shown that with biaxial stress in the plane, HgTe has three phase transitions, i.e., ideal Weyl semimetal, Type II Weyl semimetal and topological insulator. Whether the existence of Weyl fermions in HgTe can be verified or not has drawn wide attention of scientists all over the world. In this project, with the HgTe single crystal films and nano-structure films and the HgCdTe bulk materials, we will use magnetic-Raman and magnetotransport measurements to conduct research on how the stress provided by the piezo can be used to tune the bandgap and the spin-orbit coupling of the samples by thinning the samples and gluing them to the face of the piezo. Some spin-dependent physical phenomena such as Weyl Fermi arc, which can provide physical foundation for building spinning electronic devices, are expected to be observed experimentally.

HgCdTe是由负带隙HgTe和宽带隙CdTe构成的固溶体，带隙连续可调，其中窄禁带、负带隙结构具有较小的电子有效质量、较大的电子有效g因子和较强的自旋轨道耦合强度等优点，且易于通过磁性元素掺杂实现磁性半导体，是研究半导体自旋电子学的理想材料。近期理论研究表明：在平面双轴应力作用下，HgTe具有三种相变，即理想的Weyl半金属、II型Weyl半金属和拓扑绝缘体。能否在HgTe中观测到Weyl费米子成为各国科学家关注的焦点。本项目拟以负带隙HgCdTe体材料和HgTe单晶薄膜、纳米结构薄膜为研究对象，通过减薄样品和粘贴在压电陶瓷片上等技术手段，采用磁-拉曼和磁输运测试方法研究应力对能带及自旋轨道耦合等物理参数的调控规律，预期在实验上观测到Weyl费米弧等一些自旋相关的物理现象，为构造自旋电子学器件提供物理基础。

近年来，关于拓扑Weyl半金属的研究越来越成为了凝聚态物理领域的热点问题。Ruan等人通过第一性原理计算分析发现，通过对HgTe施加面内应力可以使其在多种拓扑绝缘体相和拓扑Weyl半金属相之间转变，当施加的压缩应力足够大时，HgTe将显示出具有4对Weyl点的Weyl半金属；当应力较小时，HgTe将处于II型Weyl半金属相；而当面内拉伸应力足够大时，HgTe将变为拓扑绝缘体相。若是能实现这一理论研究所预测的在平面内应力调控下HgTe物相的连续转变，这将为进一步研究拓扑Weyl半金属中的新奇现象提供一个优异的研究平台。本项目首先研究了具有负禁带的HgCdTe单晶体材料在压电陶瓷施加的单轴应力作用的磁输运行为，在低温（1.5k）、高磁场（8.9T）条件下，电子自旋受应力作用产生分裂，因此朗道能级在应力调控下出现SdH振荡，此结果发表在物理学报上被评为优秀论文。其次研究了MBE生长的HgTe薄膜，通过减薄GaAs衬底至10微米以下并把它粘贴在压电陶瓷断面这样就实现了双轴应力调控。磁输运测量表明，此样品在低温（1.5k）下具有Weyl半金属的负磁阻特性，当温度升至80K时首次发现样品在某临界拉伸应力作用下电阻急剧增加转变成拓扑绝缘体。通过本项目的实施，掌握了利用压电陶瓷来调控材料应力的有效手段，在今后的研究中将此方法扩展至更多半导体材料，如InGaAs量子阱、石墨烯等。研究它们在应力作用下的带隙变化导致的新奇的物理现象。