Huang Silu, Kim Jisun, Shelton W A, Plummer E W, Jin Rongying
Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803.
Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA 70803.
Proc Natl Acad Sci U S A. 2017 Jun 13;114(24):6256-6261. doi: 10.1073/pnas.1706657114. Epub 2017 May 24.
The subject of topological materials has attracted immense attention in condensed-matter physics because they host new quantum states of matter containing Dirac, Majorana, or Weyl fermions. Although Majorana fermions can only exist on the surface of topological superconductors, Dirac and Weyl fermions can be realized in both 2D and 3D materials. The latter are semimetals with Dirac/Weyl cones either not tilted (type I) or tilted (type II). Although both Dirac and Weyl fermions have massless nature with the nontrivial Berry phase, the formation of Weyl fermions in 3D semimetals require either time-reversal or inversion symmetry breaking to lift degeneracy at Dirac points. Here we demonstrate experimentally that canted antiferromagnetic BaMnSb is a 3D Weyl semimetal with a 2D electronic structure. The Shubnikov-de Hass oscillations of the magnetoresistance give nearly zero effective mass with high mobility and the nontrivial Berry phase. The ordered magnetic arrangement (ferromagnetic ordering in the plane and antiferromagnetic ordering along the axis below 286 K) breaks the time-reversal symmetry, thus offering us an ideal platform to study magnetic Weyl fermions in a centrosymmetric material.
拓扑材料这一主题在凝聚态物理领域引起了极大关注，因为它们拥有包含狄拉克费米子、马约拉纳费米子或外尔费米子的新型量子态物质。尽管马约拉纳费米子仅能存在于拓扑超导体表面，但狄拉克费米子和外尔费米子可在二维和三维材料中实现。后者是具有狄拉克/外尔锥的半金属，狄拉克/外尔锥要么不倾斜（I型），要么倾斜（II型）。尽管狄拉克费米子和外尔费米子都具有无质量性质且具有非平凡的贝里相位，但三维半金属中外尔费米子的形成需要时间反演或空间反演对称性破缺，以解除狄拉克点处的简并。在此，我们通过实验证明，倾斜反铁磁体BaMnSb是一种具有二维电子结构的三维外尔半金属。磁阻的舒布尼科夫 - 德哈斯振荡给出了具有高迁移率和非平凡贝里相位的近乎零的有效质量。有序的磁排列（在平面内为铁磁有序，在286 K以下沿轴为反铁磁有序）打破了时间反演对称性，从而为我们提供了一个在中心对称材料中研究磁性外尔费米子的理想平台。
