Optimizing interfaces between topological insulators and superconductors: Towards a local detection of Majorana states

该项目旨在制备拓扑绝缘体(TIS)和S波超导体(SC)之间的界面，该超导体在涡旋内存在可检测的、强大的马约拉纳激发(MES)。这一目标面临三大挑战。首先，TI的狄拉克点能量ED必须相当精确地位于费米能级EF，以降低涡旋内平凡受限态的密度，从而增加保护MES的小能隙。在第一个项目阶段，我们已经建立了两种方法来调整ED到EF，要么通过调整化学计量比，要么通过生长可变厚度的p-n结。其次，ME信号必须从SC的Caroli-de Gennes-Matricon态中分离出来。我们的方法是去除涡旋内的SC，或者更准确地说，将涡旋钉扎在S波SC的孔内。这具有一个额外的优点，即多个磁通量子可以被空穴捕获，从而在偶数个磁通量子的情况下ME特征消失。为了实现这一目标，我们建立了一种超高真空(UHV)掩模对准器，它可以将掩模结构转移到精度为100 nm的衬底上。第三，必须利用扫描隧道谱(STS)来检测ME特征。为此，我们在特高压和B场建立了低至370mK的STS，能量分辨率为0.1 meV，电压噪声低于0.02 mV，从而为MES的检测提供了所需的性质。除了这些成果外，我们还进行了一个优先计划内的讨论所产生的项目，即我们通过探测Bi14Rh3I9(第一个实验实现的弱TI)的拓扑保护的边缘态来确认其预测的弱TI性质，同时确定BIT是第二个弱TI。在第二个资助期内，我们首先着眼于从FeSe开始沉积足够的Sc，如果以单分子膜的形式沉积在SrTiO_3(110K)或BaTiO_3(70K)上，或者在充分掺杂(50K)后作为多层沉积，已经被证明具有特殊的临界温度。然而，由于先验不清楚，如果FeSe在BiSbTeSe合金上获得超导性，我们可能会回到更传统的超导体NbSe2、Nb或Pb.其次，我们将独立于SC的优化，在超高压中使用掩模对准来制备孔结构。最后，通过改变B场、温度、局部化学势、相邻空穴之间的距离，以及可能的材料组合，对所得到的结构进行详细的探测。注意，掩模对准器技术也可以转移到弱TIS，其应该在邻近诱导的边缘状态结束时主存MES。

The project aims at the preparation of interfaces between topological insulators (TIs) and s-wave superconductors (SCs) hosting detectable, robust Majorana excitations (MEs) within vortices. This aim faces three major challenges. Firstly, the Dirac point energy ED of the TI has to be rather exactly at the Fermi level EF in order to reduce the density of trivially confined states within the vortex and, thus, to increase the minigap which protects the MEs. In the first project phase, we have established two methods to tune ED to EF, either by adapting the stoichiometry or by growing a p-n junction of variable thickness. Secondly, the ME signal must be disentangled from the Caroli-de Gennes-Matricon states of the SC. Our approach is to remove the SC within the vortices or, more precisely, to pin the vortices within holes of the s-wave SC. This has the additional ad¬vantage that multiple flux quanta can be caught by the holes such that the ME signature disappears in case of an even number of flux quanta. Towards that goal, we have established an ultrahigh-vacuum (UHV) mask aligner, which transfers mask structures to a substrate with 100 nm precision. Thirdly, the ME signatures have to be detected by scanning tunneling spectroscopy (STS). To this end, we have established STS down to 370 mK in UHV and B-field with an energy resolution of 0.1 meV and a voltage noise below 0.02 mV, thus, providing the required properties for the detection of MEs. Besides these achievements, we pursued a project resulting from discussions within the priority programme, namely we confirmed the predicted weak TI properties of Bi14Rh3I9 (first experimental realization of a weak TI) by probing its topologically protected edge states and, meanwhile, identified BiTe as a second weak TI. Within the second funding period, we firstly aim at the deposition of an adequate SC starting with FeSe, which has been shown to exhibit exceptional critical temperatures, if deposited as a monolayer on SrTiO3 (110 K) or BaTiO3 (70 K) or as a multilayer after adequate doping (50 K). However, since it is not clear a priori, if FeSe gets superconducting on BiSbTeSe alloys, we might get back to more conventional superconductors as NbSe2, Nb or Pb. Secondly, we will prepare the hole structure using mask alignment in UHV independently from the optimization of the SC. Finally the resulting structures will be probed in detail by STS varying B-field, temperature, the local chemical potential, the distance between adjacent holes, and, possibly, the materials combination. Note that the mask aligner technique can also be transferred to weak TIs, which should host MEs at the end of a proximity induced edge state.