拓扑磁电效应的实验研究

When in a quantum state where the Hall conductivity drops to zero, topological insulators are predicted to exhibit a quantized electric polarization under an applied magnetic field or a quantized magnetization in an electric field. These phenomena are called topological magnetoelectric effects (TMEs). Magnetically doped topological insulators are considered the best candidate for realizing the TMEs. This project aims at verifying the TME by measuring the polarized current induced by an AC magnetic field in a topological insulator selectively doped with two types of magnetic elements. The theoretically proposed structure that is suitable for realizing TME consists of two layers of topological materials doped with different magnetic elements. We propose to experimentally grow such a structure via precisely controlled molecular beam epitaxy and verify the existence of the quantum state with zero Hall conductivity by low temperature magnetotransport. The topological system in such a quantum state can be utilized to generate a polarization current if an AC magnetic field is applied. We plan to detect such a polarization current thus verify the TME. Furthermore, we are going to study the TME as a function of sample thicknesses, chemical potential variations, as well as the doping procedures. This study not only solves one of the key remaining issues in the field of topological insulator but also can boost the development of transport techniques

拓扑绝缘体在霍尔电导为零的量子态下会出现磁场引致的量子化的电极化或者电场引致的量子化的磁极化效应—拓扑磁电效应。掺磁性杂质的拓扑绝缘体是实验上验证该效应的极佳平台。本项目拟通过探测二元磁性掺杂拓扑绝缘体中外加交流磁场而引起的极化电流来首次在实验上探测到拓扑磁电信号。我们将利用分子束外延技术，实现理论提议的在拓扑绝缘体中分上下层掺入不同磁性元素。我们期望通过生长的精确调控进而在低温磁输运中发现霍尔电导为零的量子态。进一步的，我们将在处于该特殊量子态的拓扑绝缘体上加上交流磁场，测量理论预言的拓扑磁电信号并研究该信号对于样品厚度、化学势位置、掺杂浓度等的依赖关系，揭示新的物理性质。本研究的成功将不仅解决拓扑绝缘体领域一项关键性的问题，而且也将推动输运测量技术的新发展。

在本项目中，我们发现了一类新的超导体--锡烯。我们通过极低温电阻测量和抗磁性测量两方面验证了该材料的超导电性，并确定了该材料中仍然具有的拓扑非平庸的能带结构，这为进一步开展拓扑超导的研究打下了基础。此外，我们还发展了离子固体和离子液体调控技术，分别对空穴型和电子型的铜氧化物高温超导体进行了有效的载流子调控，并研究了调控下超导电性的演化，这为进一步理解高温超导的机理提供了新的技术手段。