Two-dimensional (2D) materials with nodal-loop band crossing have been attracting great research interest. However, it remains a challenge to find 2D nodal loops that are robust against spin-orbit coupling (SOC) and realized in magnetic states. Here, based on first-principles calculations and theoretical analysis, we predict that monolayer MnN is a 2D nodal-loop half metal with fully spin polarized nodal loops. We show that monolayer MnN has a ferromagnetic ground state with out-of-plane magnetization. Its band structure shows half metallicity with three low-energy bands belonging to the same spin channel. The crossing between these bands forms two concentric nodal loops centered around the $Γ$ point near the Fermi level. Remarkably, the nodal loops and their spin polarization are robust under SOC, due to the protection of a mirror symmetry. We construct an effective model to characterize the fully polarized emergent nodal-loop fermions. We also find that a uniaxial strain can induce a loop transformation from a localized single loop circling around $Γ$ to a pair of extended loops penetrating the Brillouin zone.
Al$_2$B$_2$ and AlB$_4$ monolayers: emergence of multiple two-dimensional Dirac nodal line semimetals with novel properties
二维材料中的拓扑半金属相由于其在纳米器件开发中的潜在应用而受到广泛关注。尽管预测的狄拉克/外尔点在各种各样的二维候选人，材料具有拓扑节点线仍然是非常稀缺的。在这里，我们预测两个稳定的二硼化铝薄膜与超和亚化学计量的Al $_2$ B $_2$和AlB $_4$作为新的二维狄拉克节线半金属（NLSMs），承诺提供许多新的功能。我们详细的电子结构计算结合分析研究表明，除了多个狄拉克点，这些二维配置主机周围的费米能级，所有这些都是半金属状态的时间反演和平面内镜像对称性保护的各种类型的I封闭节线（NL）。Al $_2$ B $_2$中最有趣的NL包围了K点，并以相当大的色散穿过费米能级，从而为探索色散狄拉克节线中的奇异性质提供了一个新的场所。更引人注目的是，在具有高转变温度的2D超导体AlB $_4$的情况下，我们提供了第一个证据，证明在弱自旋-轨道耦合极限下存在一组2D开放的II型非线性激光，与费米能级附近的封闭I型非线性激光相一致。AlB $_4$中超导性与非平凡能带拓扑结构的共存不仅使其成为一种有希望展现新型拓扑超导相的材料，而且该结构中II型节线的较大能量色散，为实现二维极限下的新型拓扑结构提供了一个卓越的平台。
Higher Order Topology and Nodal Topological Superconductivity in Fe(Se,Te) Heterostructures
我们从理论上证明了单层FeTe $_{1-x}$ Se $_x$-超导量子自旋霍尔材料-与单层FeTe -双共线反铁磁体-的异质结构实现了更高阶的拓扑超导相，其特征在于钉扎到样品角落的紧急马约拉纳零模式。给出了该系统的最小有效模型，分析了高阶拓扑的来源，并对拓扑相图进行了充分的刻画。尽管传统的s波配对，我们发现相当令人惊讶的出现一个新的拓扑节点超导体的相图。其特点是边缘依赖的马约拉纳平坦带，拓扑节点相的反铁磁手征对称性的保护。我们还讨论了实验的可行性，估计现实的模型参数，和鲁棒性的马约拉纳角模式对磁无序。我们的工作为高阶拓扑和非阿贝尔马约拉纳物理提供了一个新的实验可行的高温平台。
Kagome goldene with flat bands and Dirac nodal line fermions via line-graph epitaxy
Hunan University（湖南大学）；Research Institute of Hunan University in Chongqing（重庆湖大研究院）；Institute for Research in Fundamental Sciences (IPM)（伊朗基础科学研究所）；University of Guilan（吉兰大学）
