Kafle Tika R, Zhang Yingchao, Wang Yi-Yan, Shi Xun, Li Na, Sapkota Richa, Thurston Jeremy, You Wenjing, Gao Shunye, Dong Qingxin, Rossnagel Kai, Chen Gen-Fu, Freericks James, Kapteyn Henry C, Murnane Margaret M
Department of Physics and JILA, University of Colorado and NIST, Boulder, Colorado 80309, USA.
Institute of Physics and Beijing National Laboratory for Condensed Matter Physics, CAS, Beijing, China.
Struct Dyn. 2025 Feb 5;12(1):014501. doi: 10.1063/4.0000273. eCollection 2025 Jan.
Topological materials are of great interest because they can support metallic edge or surface states that are robust against perturbations, with the potential for technological applications. Here, we experimentally explore the light-induced non-equilibrium properties of two distinct topological phases in NaCdAs: a topological crystalline insulator (TCI) phase and a topological insulator (TI) phase. This material has surface states that are protected by mirror symmetry in the TCI phase at room temperature, while it undergoes a structural phase transition to a TI phase below 200 K. After exciting the TI phase by an ultrafast laser pulse, we observe a leading band edge shift of >150 meV that slowly builds up and reaches a maximum after ∼0.6 ps and that persists for ∼8 ps. The slow rise time of the excited electron population and electron temperature suggests that the electronic and structural orders are strongly coupled in this TI phase. It also suggests that the directly excited electronic states and the probed electronic states are weakly coupled. Both couplings are likely due to a partial relaxation of the lattice distortion, which is known to be associated with the TI phase. In contrast, no distinct excited state is observed in the TCI phase immediately or after photoexcitation, which we attribute to the low density of states and phase space available near the Fermi level. Our results show how ultrafast laser excitation can reveal the distinct excited states and interactions in phase-rich topological materials.
拓扑材料因其能够支持对微扰具有鲁棒性的金属边缘或表面态而备受关注，具有技术应用潜力。在此，我们通过实验探索了NaCdAs中两种不同拓扑相的光致非平衡特性：一种拓扑晶体绝缘体（TCI）相和一种拓扑绝缘体（TI）相。这种材料在室温下具有在TCI相中由镜面对称性保护的表面态，而在低于200 K时会经历结构相变到TI相。在用超快激光脉冲激发TI相后，我们观察到一个大于150 meV的主导带边位移，它缓慢增加并在约0.6 ps后达到最大值，并持续约8 ps。激发电子群体和电子温度的缓慢上升时间表明，在这个TI相中电子和结构序强烈耦合。这也表明直接激发的电子态和被探测的电子态弱耦合。这两种耦合可能都归因于晶格畸变的部分弛豫，已知其与TI相相关。相比之下，在TCI相中，无论是立即还是在光激发后，都未观察到明显的激发态，我们将其归因于费米能级附近可用的低态密度和相空间。我们的结果展示了超快激光激发如何揭示富含相的拓扑材料中不同的激发态和相互作用。
