强关联拓扑材料的扫描隧道显微学探索研究

Rare-earth monopnictides CeSb and CeBi are characterized as low carrier and strongly correlated semimetals. They exhibit extremely large magnetoresistance, Kondo-type behavior and complex M-T phase diagrams. Recent band structure calculations and measurements indicate they even have nontrivial band topology. However, there are some debates about their topological properties, and some researchers claim that a topological phase transition from trivial to nontrivial occurs at the border between CeSb and CeBi. As contrast, LaX (X=Sb, Bi), which have the same lattice structure with CeX but don’t have f electron, are nonmagnetic semimetals. Their topological properties also need to be further studied. In this project, we intend to grow high quality single crystals of RX (R = Ce, La; X = Sb, Bi), perform transport measurements and then systematically study the cleaved single crystals by scanning tunneling microscope/spectroscopy (STM/STS). Although various experimental techniques have been applied to investigate RX, STM studies haven’t been reported until now. We plan to obtain the topographic images of cleaved single crystals with atomic resolution, detect the differential conductance spectra, perform quasiparticle interference (QPI) measurements to study the electronic structure under various temperatures and magnetic fields and also study the effects of surface atomic defects. We believe this project can help us to clarify the topological properties of RX, to thoroughly understand the interactions among strong electron correlation, magnetic ordered phase and topological properties, and to reveal the origin of the nontrivial topology in the family of rare-earth monopnictides.

稀土单氮族化合物CeSb、CeBi是低载流子强关联半金属材料，具有复杂的M-T相图和极大的磁阻效应，更有实验和理论计算表明它们可能是少有的兼具电子强关联效应和非平庸拓扑性质的体系，但其拓扑性质尚存争议，更有学者提出它们正好处在平庸到非平庸的拓扑相变转折点上。LaX(X=Sb、Bi)作为CeX缺少f电子的对照体，是无磁性的半金属，但拓扑性质也尚存争议。本项目拟开展对CeSb、CeBi、LaSb、LaBi的单晶制备、物性测量和扫描隧道显微学对比研究，填补它们的STM研究空白，获取单晶表面原子分辨的形貌像和扫描隧道谱，利用准粒子相干散射技术测量不同温度和磁场下的能带结构、探测它们的拓扑性质，并进行表面缺陷的研究。拟通过本项目帮助深入理解电子强关联、磁有序相、拓扑性质等效应之间的相互影响，澄清RX(R=Ce、La，X=Sb、Bi)的拓扑性质，揭示其中非平庸拓扑性质的起源。

通过本项目的实施，我们成功的制备出高质量的CeX、LaX (X=Sb、Bi)单晶，并通过X射线衍射测量和输运测量确认了它们的品质。我们成功的在超高真空环境对CeX、LaX进行了解理，并开展了一系列的超高真空扫描隧道显微学（STM/STS）研究。我们获得了CeX解理表面的形貌像并确认了它们的原子结构，发现在低温下CeX的dI/dV谱在费米能级附近会出现非对称的谷型特征，通过Fano拟合、变温实验、与理论计算结果对比，我们确认了该特征是由Ce的f电子与巡游电子杂化引起的，并且其随温度演变的行为可以近似用单杂质的近藤模型进行描述。而LaX的STM/STS对照实验提供了强有力的证据进一步证实CeX中的非对称谷型特征是由f电子与巡游电子杂化形成的近藤共振态引起的。值得注意的是，在液氦温度附近CeSb、CeBi已经处于反铁磁相的基态，因此低温下谷型特征的存在意味着CeX中的f电子既参与了磁有序结构的构建又与巡游电子发生了杂化，与此同时，我们在CeBi的一种解理面上直接探测到了反铁磁序结构、反铁磁态与近藤共振态的共存。加磁场的情况下，CeX发生系列磁相变、伴随着能带劈裂并且在STS谱上呈现出对应特征，但是不同磁性相的准粒子相干散射（QPI）测量却表明进入铁磁相的CeSb、CeBi中没有额外的非平庸拓扑态的相关特征产生，不支持CeX中存在强关联效应与非平庸的拓扑性质耦合的观点。此外，通过对不同缺陷的对比研究，我们认识了包括近藤空穴在内的几种缺陷对CeX中近藤效应的影响。