Interaction in a flat band is magnified due to the divergence in the density of states, which gives rise to a variety of many-body phenomena such as ferromagnetism and Wigner crystallization. Until now, however, most studies of the flat band physics are based on model systems, making their experimental realization a distant future. Here, we propose a class of systems made of real atoms, namely, carbon atoms with realistic physical interactions (dubbed here as Kagome graphene/graphyne). Density functional theory calculations reveal that these Kagome lattices offer a controllable way to realize robust flat bands sufficiently close to the Fermi level. Upon hole doping, they split into spin-polarized bands at different energies to result in a flat-band ferromagnetism. At a half filling, this splitting reaches its highest level of 768 meV. At smaller fillings, e.g., when ν=1/6, on the other hand, a Wigner crystal spontaneously forms, where the electrons form closed loops localized on the grid points of a regular triangular lattice. It breaks the translational symmetry of the original Kagome lattice. We further show that the Kagome lattices exhibit good mechanical stabilities, based on which a possible route for experimental realization of the Kagome graphene is also proposed.
Charge order through crystallization of Frenkel excitons: realization in kagome metals
School of Physics and Astronomy and Tsung-Dao Lee Institute, Shanghai Jiao Tong University（上海交通大学物理与天文学院及李政道研究所）；Department of Materials Science and Metallurgy, University of Cambridge（剑桥大学材料科学与工程系）；Key Laboratory of Artificial Structures and Quantum Control (Ministry of Education)（人工结构与量子控制教育部重点实验室）；Shanghai Branch, Hefei National Laboratory（合肥国家实验室上海分部）
Microscopic origins and stability of the ferromagnetism in Co$_3$Sn$_2$S$_2$
基于密度泛函理论，我们研究了Weyl半金属Co $_3$ Sn $_2$ S $_2$的铁磁性起源.我们认为Co $_3$ Sn $_2$ S $_2$的磁性具有双重性质，具有一定的巡回性和局部性。一方面，磁性是软的，其中局部磁矩强烈地依赖于温度和由这些磁矩在可果美晶格的不同Co位置处形成的角度，如对于巡回磁体所预期的。另一方面，局部自旋的图像仍然足以描述铁磁（FM）秩序相对于横向自旋涨落的局部稳定性。对于后者的目的，我们采用两种方法，这提供了相当不同的图片原子间的交换相互作用：经常磁力定理和一个正式的精确理论的基础上计算的逆响应函数。精确理论预测Co $_3$ Sn $_2$ S $_2$是一种三维铁磁体，在相邻的可归平面中次近邻之间具有最强的相互作用。配体状态被发现发挥了非常重要的作用，另外稳定FM秩序。当局部矩减小时，面间相互作用急剧减小，首先导致FM阶变为准二维，然后使其相对于垂直于kagome平面传播的自旋螺旋阶不稳定。后者的不稳定性部分是由费米面的状态贡献的，并且可能与Co $_3$ Sn $_2$ S $_2$在居里温度附近的磁性行为有关。讨论了Co $_3$ Sn $_2$ S $_2$的半金属铁磁性的特点。
YCr6Ge6 as a Candidate Compound for a Kagome Metal
我们表明，YCr 6 Ge 6，包括一个由Cr原子组成的kagome晶格，是一个合理的候选化合物的kagome金属，预计表现出异常现象，如平带铁磁性。测量了YCr 6 Ge 6单晶的电阻率、磁化强度和热容，并进行了能带结构计算以研究其电子结构。居里-外斯的磁化率，T2电阻率的依赖性，和索末菲系数从计算值增加一倍的行为表明一个中等强的电子相关。有趣的是，面内电阻率是面间电阻率的两倍，这与分层结构的简单预期相反。能带结构的计算表明，在Γ点附近，在费米能级以下存在部分平坦带，属于Cr 3d 3 z2-r2带，可能决定了该化合物的性质。
Dynamic Moiré Potentials and Robust Wigner Crystallization in Large-Scale Twisted Transition Metal Dichalcogenides
Hefei National Research Center for Physical Sciences at the Microscale, and Hefei National Laboratory, University of Science and Technology of China（中国科学技术大学合肥国家物理科学（微观）研究中心和合肥国家实验室）；Graduate School of Science and Technology, Keio University（庆应义塾大学理工学部研究生院）；Keio University Sustainable Quantum Artificial Intelligence Center (KSQAIC), Keio University（庆应义塾大学可持续量子人工智能中心）；State Key Laboratory of Precision and Intelligent Chemistry, Department of Chemical Physics, and Anhui Center for Applied Mathematics, University of Science and Technology of China（中国科学技术大学精密与智能化学国家重点实验室、化学物理系和安徽应用数学中心）
Evolution of electronic and magnetic properties in Mn- and Co-alloyed ferromagnetic kagome metal Fe3Sn2
Drexel University（德雷塞尔大学）；University of Pennsylvania（宾夕法尼亚大学）；Temple University（天普大学）；Brown University（布朗大学）；Lawrence Berkeley National Laboratory（劳伦斯伯克利国家实验室）；Oak Ridge National Laboratory（橡树岭国家实验室）；Diamond Light Source Ltd.（钻石光源有限公司）
