本征磁性拓扑绝缘体MnBi4Te7的低温输运性质研究

近年来，拓扑绝缘体作为一种新的物态引起了人们极大的研究兴趣。将磁性 引入拓扑绝缘体可以在其拓扑表面态处打开带隙，并产生丰富的拓扑量子态，而 其中最受关注的是量子反常霍尔绝缘体相。人们预测将量子反常霍尔绝缘体与超 导结合，可能形成拓扑超导态，从而观察到马约拉纳费米子的出现，为拓扑量子 计算提供理想的实验平台。以磁性掺杂或磁性邻近效应的方法进行磁性的引入会 不可避免地带来磁性无序等问题，不利于量子反常霍尔效应的实现。MnBi4Te7 作 为最近发现的具有本征磁性与拓扑性质共存的新材料体系，被认为具有丰富的拓 扑量子相，能在较高温度下实现量子反常霍尔效应，因而成为了近期的研究热点。
本论文中，我们将以 MnBi4Te7 为研究对象，结合了电子束光刻技术及脉冲 激光沉积技术，制备六端霍尔器件及 Nb/MnBi4Te7/Nb 超导约瑟夫森结器件，在 不同温度及磁场条件下，对器件的输运特性开展了细致的测量与系统的研究。
我们测量了 MnBi4Te7 霍尔器件在低温下的纵向电阻与霍尔电阻，发现了其 纵向电阻在高磁场下的不饱和线性正磁阻现象，且该现象可从低温一直持续到室 温。该现象的出现指向了 MnBi4Te7 二维拓扑表面态的存在，但其产生的机制并 不能用常用的 Parish-Littlewood 模型与量子线性磁阻模型加以解释，还有待进一 步的理论研究。此外，我们还发现施加偏置电流可对 MnBi4Te7 的磁输运性质进 行有效的调控。随着偏置电流的加大，MnBi4Te7 的反常霍尔效应竟然可发生反 号。基于铁磁态 MBT 为外尔半金属相具有零带隙外尔锥这一认识，我们认为该 新奇现象很大可能与外尔半金属体系中电流激发的俄歇散射过程相关。
我们还在 Nb/MnBi4Te7/Nb 约瑟夫森结器件上进行了微分电阻谱的测量，在 Nb 的超导相变温度之下发现微分电阻谱会出现明显的微分电阻峰。通过分析， 我们认为其来自于安德列夫反射，意味着可通过超导邻近效应在磁性 MnBi4Te7 中诱导产生超导态。我们仔细研究了这一超导态随温度及磁场的变化，并利用 BCS 理论公式拟合实验数据，得出其超导转变温度为 8.4K，超导能隙为 0.503meV。
以上研究揭示了磁性拓扑绝缘体由于其自身的能带结构特点蕴含着丰富的 电输运性质。同时，该研究也为利用磁性拓扑绝缘体与超导的耦合实现可能的拓 扑超导态或马约拉纳费米子打下了基础。

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