Giwa Rauf, Hosur Pavan
Department of Physics, University of Houston, Houston, Texas 77204, USA.
Phys Rev Lett. 2021 Oct 29;127(18):187002. doi: 10.1103/PhysRevLett.127.187002.
Many clever routes to Majorana fermions have been discovered by exploiting the interplay between superconductivity and band topology in metals and insulators. However, realizations in semimetals remain less explored. We ask, "Under what conditions do superconductor vortices in time-reversal symmetric Weyl semimetals-three-dimensional semimetals with only time-reversal symmetry-trap Majorana fermions on the surface?" If each constant-k_{z} plane, where z is the vortex axis, contains equal numbers of Weyl nodes of each chirality, we predict a generically gapped vortex and derive a topological invariant ν=±1 in terms of the Fermi arc structure that signals the presence or absence of surface Majorana fermions. In contrast, if certain constant-k_{z} planes contain a net chirality of Weyl nodes, the vortex is gapless. We analytically calculate ν within a perturbative scheme and provide numerical support with a lattice model. The criteria survive the presence of other bulk and surface bands and yield phase transitions between trivial, gapless, and topological vortices upon tilting the vortex. We propose Li(Fe_{0.91}Co_{0.09})As and Fe_{1+y}Se_{0.45}Te_{0.55} with broken inversion symmetry as candidates for realizing our proposals.
通过利用金属和绝缘体中超导电性与能带拓扑结构之间的相互作用，人们发现了许多通往马约拉纳费米子的巧妙途径。然而，半金属中的实现方式仍较少被探索。我们提出问题：“在时间反演对称的外尔半金属（仅具有时间反演对称性的三维半金属）中的超导涡旋在何种条件下会在其表面捕获马约拉纳费米子？” 如果每个常数(k_z)平面（其中(z)为涡旋轴）包含每种手性的外尔节点数量相等，我们预测会出现一个一般有能隙的涡旋，并根据费米弧结构推导出一个拓扑不变量(\nu = \pm1)，该不变量表明表面马约拉纳费米子的存在与否。相反，如果某些常数(k_z)平面包含外尔节点的净手性，则涡旋是无能隙的。我们在微扰方案内解析计算(\nu)，并通过晶格模型提供数值支持。这些标准在存在其他体带和表面带的情况下仍然成立，并且在倾斜涡旋时会产生平凡、无能隙和拓扑涡旋之间的相变。我们提出具有破缺空间反演对称性的Li(Fe_{0.91}Co_{0.09})As和Fe_{1 + y}Se_{0.45}Te_{0.55}作为实现我们提议的候选材料。
