Qu Jinfeng, Luo Chaobo, Peng Xiangyang, Chen Guoxiang, Guo Shaoli, Zhong Jianxin
School of Science, Xi'an Shiyou University, Xi'an, 710065, Shaanxi, People's Republic of China.
Hunan Key Laboratory for Micro-Nano Energy Materials and Devices, School of Physics and Optoelectronics, Xiangtan University, Xiangtan, 411105, Hunan, People's Republic of China.
J Phys Condens Matter. 2020 Mar 3;33(9):095502. doi: 10.1088/1361-648X/abcdb1.
Germanene, though with Dirac valleys, is not deemed as a good valleytronic material due to its minute band gap, negligible spin-orbit coupling and spatial inversion symmetry. In comparison of interfacing germanene with MoS, we proposed that forming heterostructure with TlS, an anti-MoS material with two outer heavy metal layers, could be more effective in raising spin-orbit coupling and band gap in germanene due to the direct Ge-metal contact. By carrying out first-principles calculations, we studied the valleytronic properties of germanene enhanced by monolayer TlS. It is found that the Ge-Tl direct interaction is strong to a proper extent so that the valleys of germanene still persist and simultaneously the valley gap is drastically increased from 23 to 370 meV. The valley spin splitting, being zero in pristine germanene, become 45 meV, which is opposite at inequivalent valleys owing to the time reversal symmetry. The inversion symmetry of germanene is broken by TlS, resulting in large Berry curvature near the valleys and hence laying the ground for Berry phase physics in germanene, e.g., valley spin Hall effect and valley-spin locking, as revealed in our study. The calculations found a perfect valley-selective circular dichroism, by which the valley and spin degrees of freedom can be manipulated selectively and correlatively.
尽管锗烯具有狄拉克谷，但由于其微小的带隙、可忽略不计的自旋轨道耦合和空间反演对称性，它并不被认为是一种良好的谷电子材料。在将锗烯与二硫化钼（MoS）进行界面比较时，我们提出，与TlS（一种具有两个外层重金属层的反二硫化钼材料）形成异质结构，由于锗与金属的直接接触，在提高锗烯的自旋轨道耦合和带隙方面可能更有效。通过进行第一性原理计算，我们研究了单层TlS增强的锗烯的谷电子特性。研究发现，锗与铊（Tl）的直接相互作用在适当程度上很强，以至于锗烯的谷仍然存在，同时谷隙从23毫电子伏特急剧增加到370毫电子伏特。在原始锗烯中为零的谷自旋分裂变为45毫电子伏特，由于时间反演对称性，在不等价谷处情况相反。锗烯的反演对称性被TlS打破，导致在谷附近出现大的贝里曲率，从而为锗烯中的贝里相物理奠定了基础，例如我们的研究中揭示的谷自旋霍尔效应和谷自旋锁定。计算发现了完美的谷选择性圆二色性，通过它可以选择性地和相关地操纵谷和自旋自由度。
