Scanning tunneling microscopy and spectroscopy on Kondo insulators

理论上预测在所谓的近藤绝缘体中存在拓扑保护表面状态，仍有待明确的实验验证。即使是被大量研究的材料SmB6，也有相互矛盾的实验结果。为了进一步研究，我们将利用扫描隧道显微镜和光谱学在低温和磁场下研究SmB6隙内态的确切性质。到目前为止，STM研究只存在于（001）表面。然而，由于这个表面的极地性质，出现了一些复杂情况。因此，我们将把我们的研究扩展到（110）和（111）表面，在这些表面上，预计会存在受晶体对称保护的狄拉克点。通过使用La和Gd替代材料，以及使用电子束蒸发器在SmB6的切割表面上原位沉积磁性和非磁性杂质，探索磁性和非磁性杂质对表面状态的影响。这些研究的结果将与理论预测进行比较。我们的研究还将包括其他候选近藤绝缘体，例如Ce3Bi4Pt3， CeNiSn和CeOs4Sb12。如果可以确定表面态的拓扑非平凡性质，则可以设想几种器件应用。通过SPP1666内部的合作，互补光谱方法，如角分辨光电子能谱（ARPES）、拉曼光谱、其他光谱学，以及使用多探针STM测量表面输运，将被用于研究隙内态。此外，理论输入将通过与SPP1666参与者的合作获得。这些将包括基于密度泛函理论和动态平均场理论（DFT + DMFT）的计算，以及隧道光谱和准粒子干涉模式的计算。通过这样的共同努力，我们希望能够深入了解近藤绝缘体的物理性质及其表面状态。

The theoretically predicted existence of topologically protected surface states in the so-called Kondo insulators still awaits clear-cut experimental verification. Even in case of the heavily studied material SmB6, there are conflicting experimental results. In order to advance, we will focus our efforts on investigating the exact nature of the in-gap states in SmB6 using scanning tunneling microscopy and spectroscopy to lower temperatures and in magnetic field. STM investigations up to now exist only for the (001) surface. However, due to the polar nature of this surface several complications arise. We will, therefore, extend our investigations also to (110) and (111) surfaces, on which Dirac points protected by crystal symmetry are predicted to be present. The effects of magnetic and non-magnetic impurities on the surface states will be explored by using La and Gd substituted materials as well as by in situ deposition of magnetic and non-magnetic impurities using an e-beam evaporator on cleaved surfaces of SmB6. The outcome of these investigations will be compared to theoretical predictions. Our studies will also include other candidate Kondo insulators, e.g. Ce3Bi4Pt3, CeNiSn and CeOs4Sb12. If the topologically non-trivial nature of the surface states can be established several device applications are conceivable. Through collaborations within the SPP1666 complementary spectroscopic methods, such as angle-resolved photoelectron spectroscopy (ARPES), Raman spectroscopy, other optical spectroscopies, and measurements of surface transport using a multi-probe STM will be utilized to study the in-gap states. In addition, theoretical input will be obtained via collaborations with SPP1666 participants. These will include calculations based on density functional theory and dynamical mean field theory (DFT + DMFT) as well as calculations of tunneling spectra and quasiparticle interference patterns. Through such concerted efforts we hope to achieve deepened insight into the physics of Kondo insulators and the nature of its surface states.