固体缺陷的准确探测是困扰科学家的经典问题，众所周知，气体中高次谐波的产生可以使我们能够以前所未有的分辨率探测原子和分子中的阿秒电子动力学。不幸的是，这些技术最初是为原子和分子气体开发，如果扩展到固体研究，则需要了解其物理学的一些重要方面，之前只从理论的角度作过少量讨论。
来自印度理工学院的Gopal Dixit教授和来自德国马克斯•普朗克物质结构与动力学研究所的Angel Rubio教授共同领导的科研团队，基于含时密度泛函理论（TD-DFT）方法，研究了空位对单层六角氮化硼高次谐波发射的影响，证明了这些真实自旋极化缺陷如何影响谐波发射以及原始固体和缺陷固体内谐波的重要区别，展示了自旋极化缺陷的高次谐波（HHG）对固体中自旋极化缺陷进行成像的可能性。人们可能认为，具有硼原子空位或具有氮原子空位的h-BN会表现出相似的HHG光谱，因其都是去除了h-BN中的单个原子。然而情况并非如此，因为从带隙状态可明显看出硼和氮的空位会导致质子的电子结构不同。
人们曾发现一旦从h-BN中除去硼或氮原子，该系统就会在空位附近以非零磁矩旋转自旋极化。结果，他们发现对于每个自旋通道和每个空位，由缺陷引起的带隙状态是不同的。根据缺陷状态的顺序和占据率，这些结果很大程度上取决于自旋。可见，通过分析自旋极化谱可理解缺陷状态，其结果与自旋极化能带结构一致。该研究建立了缺陷状态在固体强场动力学中的作用。这一结果为进一步研究二维和扩展系统中的强场电子动力学，特别是涉及孤立缺陷的强场电子动力学开辟了有趣的前景。
该文近期发表于npj Computational Materials 6: 10 (2020)，英文标题与摘要如下，点击https://www.nature.com/articles/s41524-020-0275-z可以自由获取论文PDF。
High-harmonic generation from spin-polarised defects in solids
M. S. Mrudul, Nicolas Tancogne-Dejean , Angel Rubio and Gopal Dixit
The generation of high-order harmonics in gases enabled to probe the attosecond electron dynamics in atoms and molecules with unprecedented resolution. Extending these techniques to solids, which were originally developed for atomic and molecular gases, requires a fundamental understanding of the physics that has been partially addressed theoretically. Here, we employ time- dependent density-functional theory to investigate how the electron dynamics resulting in high-harmonic emission in monolayer hexagonal boron nitride is affected by the presence of vacancies. We show how these realistic spin-polarised defects modify the harmonic emission and demonstrate that important differences exist between harmonics from a pristine solid and a defected solid. In particular, we found that the different spin channels are affected differently by the presence of the spin-polarised point defect. Moreover, the localisation of the wavefunction, the geometry of the defect, and the electron–electron interaction are all crucial ingredients to describe high-harmonic generation in defected solids.
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