Scanning tunneling microscopy and spectroscopy on Kondo insulators

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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.