The proximity effect refers to the phenomenon whereby superconducting properties are induced in a normal conductor that is in contact with an intrinsically superconducting material. In particular, the combination of nano-structured semiconductors with bulk superconductors is of interest because these systems can host unconventional electronic excitations such as Majorana fermions when the semiconductor's charge carriers are subject to a large spin-orbit coupling. The latter requirement generally favors the use of hole-doped semiconductors. On the other hand, basic symmetry considerations imply that states from typical simple-metal superconductors will predominantly couple to a semiconductor's conduction-band states and, therefore, in the first instance generate a proximity effect for band electrons rather than holes. In this article, we show how the superconducting correlations in the conduction band are transferred also to hole states in the valence band by virtue of inter-band coupling. A general theory of the superconducting proximity effect for bulk and low-dimensional hole systems is presented. The interplay of inter-band coupling and quantum confinement is found to result in unusual wave-vector dependencies of the induced superconducting gap parameters. One particularly appealing consequence is the density tunability of the proximity effect in hole quantum wells and nanowires, which creates new possibilities for manipulating the transition to nontrivial topological phases in these systems.
Magneto-optical conductivity of monolayer transition metal dichalcogenides in the presence of proximity-induced exchange interaction and external electrical field
Weak localization as probe of spin-orbit-induced spin-split bands in bilayer graphene proximity coupled to WSe$_2$
RWTH Aachen University（亚琛工业大学）；Forschungszentrum Jülich（于利希研究中心）；Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST（加泰罗尼亚纳米科学与技术研究所（ICN2，西班牙高等科研理事会与巴塞罗那自治大学联合））；National Institute for Materials Science（国立材料研究所）
Electrically tunable exchange splitting in bilayer graphene on monolayer Cr$_2$X$_2$Te$_6$ with X=Ge, Si, and Sn
采用第一性原理方法研究了双层石墨烯在铁磁多层膜Cr $_2$ X $_2$ Te $_6$（X=Ge，Si，Sn）上的电子能带结构和邻近交换效应.在每种情况下，异质结构的本征电场在石墨烯双层中诱导约10 meV的轨道间隙。邻近交换是强烈的波段依赖性。例如，在Cr $_2$ Ge $_2$ Te $_6$的情况下，双层石墨烯的低能价带具有8 meV的交换分裂，而低能导带的分裂小30倍（0.3meV）。这种显著的差异源于与铁磁基底的层依赖性杂交。值得注意的是，施加几V/nm的垂直电场可以逆转交换，使我们能够有效地打开和关闭双层石墨烯中的邻近磁性。这样的场效应对于铁磁绝缘体上的范德华双层来说应该是通用的，为基于自旋的设备开辟了新的可能性。
Single and bilayer graphene on the topological insulator Bi$_2$Se$_3$: Electronic and spin-orbit properties from first principles
我们详细研究了单层和双层石墨烯在拓扑绝缘体Bi $_2$ Se $_3$附近的电子和自旋轨道性质。我们的方法是基于第一性原理计算，结合对称导出的模型哈密顿量，捕捉低能带特性。我们考虑单层和双层石墨烯上的1- 3五重层的Bi $_2$ Se $_3$和提取轨道和邻近诱导的自旋轨道耦合（SOC）参数。我们发现，石墨烯得到显着的空穴掺杂（350毫电子伏），但线性色散被保留。邻近感生SOC参数的幅度约为1 meV，是谷塞曼型。诱导的SOC弱依赖于Bi $_2$ Se $_3$的五重层数。我们还研究了横向电场的效果，即施加在单层和双层石墨烯的异质结构上方的1个五重层的Bi $_2$ Se $_3$。我们的研究结果表明，能带偏移，以及邻近诱导SOC参数可以调整的字段。最有趣的是双层石墨烯的情况下，其中的带隙，起源于异质结构的本征偶极子，可以关闭和重新打开，与反转带字符。强邻近SOC从传导到价带的切换实现了自旋轨道阀。此外，我们发现当我们将石墨烯和Bi $_2$ Se $_3$之间的层间距离仅减小10%时，邻近诱导的SOC的巨大增加约为200%。最后，对于不同的衬底材料Bi $_2$ Te $_2$ Se，带偏移显著不同，其中石墨烯狄拉克点位于费米能级处，而与Bi $_2$ Se $_3$衬底相比，感生的SOC强度在量值上保持相似。
