狄拉克半金属Cd3As2纳米片中拓扑量子态的调控研究

Due to their potential impact on information technology, topological quantum materials have inspired intense research interests recently. The topological properties of topological semimetals are usually manifested by the chiral anomaly in collinear electromagnetic fields as well as the Fermi-arcs on surfaces. Experimentally, the negative longitudinal magnetoresistance (LMR) was usually labeled as the transport signature of chiral anomaly. While this negative LMR can also be induced by the axial anomaly , inhomogeneous current distribution (or the “current jetting” effect) as well as conductivity fluctuations in topological semimetals. Moreover, this controversial phenomenon can also be observed in topological insulators without chiral anomaly. Thus, the negative LMR itself becomes an ambiguous transport evidence for the chiral anomaly. Recent theories indicate that the nontrivial Berry curvatures and chiral anomaly can induce another key effect-the Planar Hall effect (PHE), and the PHE, combined with negative LMR in non-magnetic Weyl semimetals, can provide a key transport signature of the chiral anomaly. Moreover, the disjointed Fermi-arcs on opposite surfaces can participate in a closed magnetic orbit connecting the bulk chiral modes and can in principle be detected via quantum oscillations in a thin topological semimetal slab. If the Fermi energy is approaching to the Weyl nodes, quantum Hall effect can be developed in such an unusual Weyl magnetic orbit. Though those exotic quantum phenomena have been observed experimentally, how to effectively control those quantum states is still elusive at the moment. In this project, we intend to use Chemical Vapor Deposition (CVD) technique to grow high quality Cd3As2 nanoplates, manipulate those exotic quantum states in Dirac semimetal Cd3As2 nanoplates. By tuning the bulk Fermi energy via different methods, we can systematically investigate the evolution of Fermi-arc surface states, planar Hall effect as well as quantum Hall effect as the function of bulk Fermi energy. Our exploration may pave the way for the application of topological quantum materials in future low-energy electronic devices.

拓扑量子材料由于其在未来低功耗电子器件上的潜在应用而备受关注。Dirac半金属Cd3As2的拓扑特性主要体现在平行电磁场下的手性异常以及表面不连续的费米弧。尽管手性异常导致的负磁阻已经被报道, 但该证据仍然有较大的争议。随后理论研究表明手性异常以及非零的Berry曲率可导致平面Hall效应，该效应与负磁阻一起能为手性异常提供确凿的输运证据。此外, 拓扑半金属中由表面费米弧形成的Weyl磁轨道可形成量子霍尔效应。尽管这些现象在近期的实验中被观测到了，然而相关的调控研究尚未报道。为此，本项目旨在利用CVD方法，生长出高质量的Cd3As2纳米片。利用热处理调控和离子液体调控的办法调控系统的费米能级高度，并深入研究Cd3As2中费米弧表面态、手性异常效应等随体态费米能级的演化过程。我们也将利用现有的强磁场设备探索并调控Cd3As2中的量子霍尔效应，为该材料进一步在低功耗电子器件中的应用研究做铺垫。

在凝聚态物理领域，拓扑半金属以及磁性拓扑绝缘体是目前研究的一个热点方向。近年来人们在拓扑量子材料领域的研究取得了较大的进展，如拓扑半金属材料Cd3As2，ZrTe5，磁性拓扑绝缘体MnBi2Te4家族等等。然而，这些研究大多数针对的是量子材料块体的生长以及输运特性。拓扑材料未来在低功耗电子学领域的应用探索往往离不开小量子器件的物性研究。本课题在过去的三年时间里利用不同的探测手段，包括电学测量，热处理以及电子顺磁共振测量等等，详细研究了几种具有代表性量子材料与器件的量子物态，如Cd3As2 纳米条带， 磁性拓扑绝缘体MnBi2Te4纳米片以及ZrTe5纳米器件。我们首先围绕Cd3As2纳米片的载流子浓度和迁移率展开调控，构筑了高质量的纳米片器件，并对其输运性能进行了研究。研究发现在纳米器件经过热处理后，其载流子浓度和迁移率表现出明显的变化，且随着循环次数的增加，表现出非单调的变化趋势。其次，我们利用电子顺磁共振技术，首次在三维拓扑半金属ZrTe5中观察到非常明显的量子振荡。由于电子顺磁共振不需要对样品进行加工，因此拓扑材料本身保持了非常高的迁移率，且由于电子顺磁共振对高迁移率半金属量子材料在磁场下费米面处的朗道能级非常敏感，因此电子顺磁技术可以探测出更小磁场下的量子震荡。通过研究发现，ZrTe5存在三维拓扑相，这与输运研究的结论相一致。该研究同时给出了另外一种三维拓扑相的实验证据；随后，我们基于反铁磁拓扑绝缘体MnBi2Te4，构筑了纳米器件，并首次在实验上观察到具有π/2周期的平面霍尔效应。与此同时，我们利用磁性测量技术，对狄拉克半金属立方PtBi2进行了深入研究，获得了其能带结构的详细信息。这些实验手段（电学测量，热处理，电子顺次共振）从不同角度深入剖析了这几种拓扑量子材料新的量子特性，加深了其量子物态的理解，为进一步构筑低功耗量子器件做了一定的铺垫作用。