[论文解读] LK-99_ab_initio_studies

R. Kurleto, Stephan Lany+4 more|arXiv (Cornell University)|Aug 1, 2023

Crystal Structures and Properties被引用 15

一句话总结

DFT研究CuPb9(PO4)6O（Cu掺杂的铅磷灰石）在对称约束的计算得到金属平坦Cu-O带在E_F，而对称性破缺的松弛打开电荷转移带隙；Cu仍处于d9态，局部磁矩约0.7 μB，提示通向平带物理和可能的高Tc行为的潜在路径。

ABSTRACT

Based on DFT calculations, we present the basic electronic structure of CuPb9(PO4)6O (Cu-doped lead apatite, LK-99), in two scenarios: (1) where the structure is constrained to the P3 symmetry and (2) where no symmetry is imposed. At the DFT level, the former is predicted to be metallic while the latter is found to be a charge-transfer insulator. In both cases the filling of these states is nominally d9, consistent with the Cu2+ valence state, and Cu with a local magnetic moment ~0.7mB. In the metallic case we find these states to be unusually flat (0.2 eV dispersion), giving high DOS at EF that we argue can be a host for novel electronic physics, including potentially high temperature superconductivity. The flatness of the bands is the likely origin of symmetry-lowering gapping possibilities that would remove the spectral weight from EF. Since some experimental observations show metallic/semiconducting behavior, we propose that disorder is responsible for closing the gap. We consider a variety of possibilities that could possibly close the gap, but limit consideration to kinds of disorder that preserve electron count. For all possibilities we considered (spin disorder, O on vacancy sites, Cu on different Pb sites), the local Cu moment, and consequently the gap remains robust. We conclude that disorder responsible for metallic behavior entails some kind of doping where the electron count changes. We claim that the emergence of the flat bands should be due to weak wave function overlap between the Cu and O orbitals, owing to the directional character of the constituent orbitals. So, finding an appropriate host structure for minimizing hybridization between Cu and O while allowing them to still weakly interact should be a promising route for generating flat bands at EF which can lead to interesting electronic phenomena, regardless of whether LK-99 is a room-temperature superconductor.

研究动机与目标

• 在约束与非约束对称性情形下，研究CuPb9(PO4)6O（LK-99）的电子结构。
• 确定Cu掺杂如何影响带结构、轨道特性和磁矩。
• 评估在不同结构松弛下，Cu-O平坦带在E_F是否能维持金属性或表现为绝缘行为。
• 探讨无序和掺杂位置如何影响带隙的形成及电子活动。

提出的方法

• 在VASP中使用GGA、对Cu-d的U=5 eV的GGA+U，以及带PAW的SCAN进行DFT计算。
• 在41原子、82原子和328原子晶胞中建模Cu替代Pb(4f)和Pb(6h)位点。
• 比较对称性约束（P3）与完全放松、对称性破缺的结构，以识别 Jahn-Teller 式的畸变。
• 计算带结构和投影态密度，以识别Cu-d和O-p在E_F附近的贡献。
• 分析局部磁矩和自旋组态（FM/AFM）以评估带隙形成机制。

实验结果

研究问题

• RQ1 在对称性受限条件下，CuPb9(PO4)6O的Cu替代是否会在费米能级形成平带？
• RQ2 放宽对称性并允许结构畸变如何影响带隙形成和Cu价态（d9）在CuPb9(PO4)6O中的表现？
• RQ3 E_F附近带的轨道特征是什么，且它如何依赖于对称性约束和晶胞大小？
• RQ4 局部Cu磁矩是否对无序和自旋涨落具有鲁棒性，以及它们与潜在的 Mott/电荷转移带隙的关系？

主要发现

• 在P3对称性约束下，CuPb9(PO4)6O显示金属性/半金属性行为，Cu-O带极为平坦（色散约0.2 eV）且每个Cu的净磁矩为1 μB。
• 当对称性不受约束且结构发生弛豫时，带隙打开，Cu呈d9构型，局部磁矩约0.7 μB，体系转变为绝缘体。
• GGA+U和SCAN在对称性破缺情形下预测的带隙比GGA更大，带隙约1.12–1.46 eV（取决于配置和泛函）。
• 超晶胞计算（82原子和328原子）表明局部Cu磁矩和带隙的形成对无序和自旋组态具有鲁棒性；反铁磁排列略有降低能量。
• 态密度和带投影显示E_F附近态主要来自Cu-d和O-p，在对称性约束下的贡献不同；在约束情形，d(xy)、d(xz)、d(yz)在E_F附近贡献，而在无约束情形Cu特征离开E_F。