【时间】2-March-2023（Thursday）3:30pm (Beijing time)

【线上】Zoom Meeting  ID：662 921 9901 

【主持】Kai Chang (BAQIS）

【题目】Imaging Putative Majorana Modes and Quasiparticle Poisoning in Superconducting Vortices

【摘要】For a superconductor in the vortex state, quasiparticles can localize as bound states in the vortex cores where the superconducting order parameter vanishes, but they can also escape the cores. Understanding localized quasiparticles, such as distinguishing between Majorana and trivial vortex bound states, is one of the prime tasks in quantum condensed matter physics, since Majorana bound states are predicted as promising candidates for error-resistant qubits. On the other hand, tracing the delocalized quasiparticles is crucial in minimizing quasiparticle poisoning of Majorana bound states for applications in topological quantum computing. Local shot noise measurements have been suggested to distinguish different vortex bound states, and more generally, as a probe into vortex physics, but despite much theoretical work, no local measurement of the shot noise of a vortex core exists.

In this talk, I will show the first local shot-noise spectroscopy to study the tunneling process into the vortex cores of both a conventional superconductor NbSe₂ and the putative Majorana platform FeTe_0.55Se_0.45. We find that tunneling into vortex bound states in both cases exhibit charge transfer of a single electron charge. Our data for the zero-energy bound states in FeTe_0.55Se_0.45 excludes the possibility of Yu-Shiba-Rusinov states and is consistent with Majorana bound states. However, it is also consistent with trivial vortex bound states. In the second part of my talk, I will show the first image of delocalized quasiparticles around vortices in NbSe₂ by scanning tunneling noise microscopy. From shot-noise imaging, we extract spatial variation of the effective charge when tunneling into the vortex state, which can be understood within the Ginzburg-Landau model. Furthermore, we find that quasiparticle poisoning dominates when vortices are less than 4 times the coherence length apart. Our results call for further theoretical investigations taking into account dynamics and superconducting tips, and open a new avenue for investigating the exotic states in vortex cores and for future Majorana devices.

【报告人简介】葛剑峰，2016年获上海交通大学博士学位，2016-2019年赴美国哈佛大学从事博士后工作， 2019年至今在荷兰莱顿大学做博士后。主要研究方向为拓扑、超导量子材料以及基于扫描隧道显微镜的新兴测量技术; 在Nature Materials，Science，PRL等期刊上发表学术论文10篇。