Jnawali Giriraj, Xiang Yuan, Linser Samuel M, Shojaei Iraj Abbasian, Wang Ruoxing, Qiu Gang, Lian Chao, Wong Bryan M, Wu Wenzhuo, Ye Peide D, Leng Yongsheng, Jackson Howard E, Smith Leigh M
Department of Physics and Astronomy, University of Cincinnati, Cincinnati, OH, 45221, USA.
Department of Mechanical & Aerospace Engineering, The George Washington University, Washington, D.C., 20052, USA.
Nat Commun. 2020 Aug 10;11(1):3991. doi: 10.1038/s41467-020-17766-5.
Trigonal tellurium (Te) is a chiral semiconductor that lacks both mirror and inversion symmetries, resulting in complex band structures with Weyl crossings and unique spin textures. Detailed time-resolved polarized reflectance spectroscopy is used to investigate its band structure and carrier dynamics. The polarized transient spectra reveal optical transitions between the uppermost spin-split H and H and the degenerate H valence bands (VB) and the lowest degenerate H conduction band (CB) as well as a higher energy transition at the L-point. Surprisingly, the degeneracy of the H CB (a proposed Weyl node) is lifted and the spin-split VB gap is reduced upon photoexcitation before relaxing to equilibrium as the carriers decay. Using ab initio density functional theory (DFT) calculations, we conclude that the dynamic band structure is caused by a photoinduced shear strain in the Te film that breaks the screw symmetry of the crystal. The band-edge anisotropy is also reflected in the hot carrier decay rate, which is a factor of two slower along the c-axis than perpendicular to it. The majority of photoexcited carriers near the band-edge are seen to recombine within 30 ps while higher lying transitions observed near 1.2 eV appear to have substantially longer lifetimes, potentially due to contributions of intervalley processes in the recombination rate. These new findings shed light on the strong correlation between photoinduced carriers and electronic structure in anisotropic crystals, which opens a potential pathway for designing novel Te-based devices that take advantage of the topological structures as well as strong spin-related properties.
三角碲（Te）是一种手性半导体，既缺乏镜面对称性也缺乏反演对称性，导致其能带结构复杂，存在外尔点交叉和独特的自旋纹理。利用详细的时间分辨偏振反射光谱来研究其能带结构和载流子动力学。偏振瞬态光谱揭示了最上层自旋分裂的H和H与简并的H价带（VB）以及最低简并的H导带（CB）之间的光学跃迁，以及在L点处的高能跃迁。令人惊讶的是，在光激发后，H导带（一个假定的外尔节点）的简并性被消除，自旋分裂的价带隙减小，随后随着载流子衰减弛豫到平衡态。通过从头算密度泛函理论（DFT）计算，我们得出动态能带结构是由碲薄膜中的光致剪切应变引起的，该应变破坏了晶体的螺旋对称性。能带边缘各向异性也反映在热载流子衰减率上，沿c轴方向的衰减率比垂直于c轴方向慢两倍。能带边缘附近的大多数光激发载流子在30 ps内复合，而在1.2 eV附近观察到的较高能级跃迁似乎具有长得多的寿命，这可能是由于复合率中间谷过程的贡献。这些新发现揭示了光致载流子与各向异性晶体中电子结构之间的强相关性，为设计利用拓扑结构以及强自旋相关特性的新型碲基器件开辟了一条潜在途径。
