Induced Topological Superconductivity in Two Dimensional Systems

摘要：在发现超导电性近一百年后，它仍然是最吸引人的物质相之一。1957年，巴丁、库珀和施里弗（BCS）提出了他们的超导理论，用空间各向同性波函数中排列的电子对和自旋单重态来描述这种状态。此后不久，一项研究开始寻找具有非常规超导性的材料，这些材料的配对偏离了传统的BCS理论。一种特殊的非常规超导体包括以三重态而不是单线态排列的对。这种超导体可以实现自旋的无耗散输运，也可以产生不服从传统费米或玻色统计的基本激发，而是具有非阿贝尔统计，其中两个粒子的交换将系统的状态转换为新的量子力学状态。这个项目探索了非传统的超导性，这种超导性是由于传统超导体和具有强自旋轨道相互作用的半导体之间的接近效应而产生的。研究小组希望识别非阿贝尔激励的特征，并利用各种传输技术探索它们的性质。该研究涉及美国国家科学基金会提出的三个更广泛的影响标准：通过探索无耗散存储和信息传输来造福社会；通过发展本科课程促进教学、培训和教育；通过积极聘用有前途的女科学家，让代表性不足的群体更广泛地参与进来。技术摘要：提出了一个研究二维系统拓扑超导性的研究项目。这项工作遵循了最近的理论工作，提出了三个不同的实验平台，其中二维拓扑超导性出现。第一个由拓扑HgCdTe量子阱组成。在这个系统中看到的量子自旋霍尔效应的强大螺旋性质及其对外部磁场的预测依赖，特别有希望探索潜在的安德烈夫束缚态谱，作为观测马约拉纳模式特征的直接手段。第二个平台由二维Josephson结组成，其中正常介质由具有强自旋轨道相互作用的非拓扑二维半导体（如InAs量子阱和HgCdTe的窄阱）组成。这种平台最近被认为是一种很有前途的可控系统，其中拓扑超导性可以通过外部平面内磁场和两个超导触点之间的相位差来调节。第三种方法是利用HgTe中的量子霍尔边和掺钒（Bi,Sb）2Te3中的量子反常霍尔边来探索手性边的拓扑超导性。HgTe量子阱中的强自旋轨道耦合和探索异常量子霍尔效应所需的外部磁场的缺乏，为探索手性Majorana模式开辟了丰富而未被探索的可能性。提出的实验方法包括Andreev束缚态的隧道光谱和金刚石中氮空位中心的噪声磁强计，以探索这些体系中的分数电流相关系。

Non-Technical Abstract:Nearly a hundred years after its discovery, superconductivity remains one of the most intriguing phases of matter. In 1957 Bardeen, Cooper and Schrieffer (BCS) presented their theory of superconductivity describing this state in terms of pairs of electrons arranged in a spatially isotropic wave function with no net momentum and a spin singlet configuration. Immediately thereafter, a search began to find materials with unconventional superconductivity where pairing deviates from conventional BCS theory. One particular class of unconventional superconductors involves pairs arranged in triplet rather than singlet configurations. Such superconductors may enable dissipationless transport of spin and may also give rise to elementary excitations that do not obey the conventional Fermi or Bose statistics but rather have non-Abelian statistics where the exchange of two particles transforms the state of the system into a new quantum mechanical state. This project explores unconventional superconductivity that arises due to the proximity effect between a conventional superconductor and a semiconductor with strong spin-orbit interaction. The research team looks to identify signatures of non-Abelian excitations and explore their properties using a variety of transport techniques. The research addresses three of the broader impact criteria set forth by the NSF: Benefits to Society through the exploration of dissipationless storage and transfer of information; Promoting teaching, training and education through the development of undergraduate courses; and broader participation of underrepresented groups through the active hiring of promising female scientists into the program.Technical Abstract:A research project is proposed to investigate topological superconductivity in two dimensional systems. The work follows recent theoretical work suggesting three distinct experimental platforms where topological superconductivity in two dimensions emerges. The first consists of topological HgCdTe quantum wells. The robust helical nature of the quantum spin Hall Effect seen in this system and its predicted dependence on an external magnetic field is particularly promising for exploring the underlying Andreev bound states spectrum as direct means for observing signatures of Majorana modes. The second platform consists of two dimensional Josephson junctions where the normal medium consists of non-topological 2D semiconductors with strong spin-orbit interaction (such as InAs quantum wells and narrow wells of HgCdTe). Such a platform was recently proposed as a promising controllable system where topological superconductivity can be tuned using an external in-plane magnetic field and the phase difference across the two superconducting contacts. The third approach consists of using quantum Hall edges in HgTe and quantum Anomalous Hall edges in vanadium doped (Bi,Sb)2Te3 for exploring topological superconductivity in chiral edges. The strong spin orbit coupling in HgTe quantum wells and the absence of an external magnetic field required for exploring the anomalous quantum Hall effect opens up rich and unexplored possibilities for exploring chiral Majorana modes. The experimental methods proposed include tunneling spectroscopy of Andreev bound states and noise magnetometry using nitrogen vacancy centers in diamond for exploring the fractional current phase relations in these systems.