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（SCs）之间的界面，该界面在涡流中具有可检测的，强大的马约拉纳激励（MEs）。这一目标面临三大挑战。首先，TI的狄拉克点能量ED必须相当精确地处于费米能级EF，以便减少涡旋内平凡受限态的密度，从而增加保护MEs的小间隙。在第一个项目阶段，我们建立了两种方法来调整ED到EF，要么通过调整化学计量，要么通过生长变厚度的pn结。其次，ME信号必须从SC的Caroli-de genes - matricon状态中解纠缠。我们的方法是去除涡旋中的SC，或者更准确地说，将涡旋固定在s波SC的空穴内。这还有一个额外的好处，即多个通量量子可以被空穴捕获，这样在偶数个通量量子的情况下，ME信号就会消失。为了实现这一目标，我们已经建立了一个超高真空（UHV）掩模对准器，它以100纳米的精度将掩模结构转移到基板上。第三，必须通过扫描隧道光谱（STS）来检测ME特征。为此，我们在特高压和b场中建立了低至370 mK的STS，能量分辨率为0.1 meV，电压噪声低于0.02 mV，从而提供了检测MEs所需的性能。除了这些成就之外，我们还进行了一个优先计划讨论的项目，即我们通过探测其拓扑保护边缘状态来确认Bi14Rh3I9的预测弱TI性质（弱TI的第一个实验实现），同时将BiTe确定为第二个弱TI。在第二个资助期内，我们首先致力于从FeSe开始沉积足够的SC，如果在SrTiO3 （110 K）或BaTiO3 （70 K）上作为单层沉积，或在适当掺杂（50 K）后作为多层沉积，FeSe已被证明具有特殊的临界温度。然而，由于尚不清楚先验，如果FeSe在bistese合金上获得超导性，我们可能会回到更传统的超导体，如NbSe2， Nb或Pb。其次，我们将在超高压下独立于SC的优化，使用掩模对准来制备孔结构。最后，我们将通过改变b场、温度、局部化学势、相邻孔之间的距离以及可能的材料组合来详细探测所得到的结构。请注意，掩模对准器技术也可以转移到弱ti，它应该在邻近诱导边缘状态结束时承载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.