Doping and interface effects in topological materials

Abstract

Topological materials are characterized by their exotic electronic bulk and boundary states. This rich phenomenology can be further extended by introducing novel properties into these materials via doping or via proximity effects at interfaces. This thesis presents a detailed investigation of these systems, establishing low energy muon spin spectroscopy (LE-μSR) and soft X-ray angle resolved photoemission spectroscopy (SX-ARPES) as complementary probes of their magnetic and electronic properties. We show that these techniques are suited both to further our fundamental understanding of the physics in topological materials and to reveal tuning parameters that can be subsequently used in designing devices. First, we applied this approach to investigate the role of magnetic and superconducting doping in topological insulators. We were able to show that Cr and V doping in the topological insulator (Bi,Sb)2Te3 can generate long range magnetic order, but we found that this approach inherently suffers from a broad magnetic transition where the magnetism appears gradually in parts of the sample. We identified the critical dopant concentration, below which the topological insulator remains only partially magnetically ordered even at the lowest temperature. Furthermore, we discovered the presence of a non-dispersive V impurity band in the vicinity of the Fermi level. We identify these factors as main contributors to the low temperature limits on establishing a quantum anomalous Hall effect in these systems. We then studied proximity induced magnetism and superconductivity in topological insulators. We compared the magnetic proximity between EuS and the topological insulator Bi2Se3 with proximity of EuS to the topologically trivial metal titanium and show that the local magnetic fields behave similarly in both systems, implying that their origin is mostly independent of the topological properties of the interface electronic states. In addition, we revealed the band structure of the buried topological insulator. At the interface between the Bi2Se3 and the conventional superconductor Nb, we discovered an odd-frequency superconducting state that is proximity induced into the topological insulator. This manifests itself as an intrinsic paramagnetic Meissner effect in Bi2Se3, which we detected in depth-resolved measurements of the local magnetic field. Finally we presented first attempts at introducing magnetism into Weyl semimetals. For this we have identified 2H-MoTe2 as an intrinsic semiconducting antiferromagnet. Such a component could be invaluable for various spintronics and topotronics devices, especially due to its layered structure which allows it to be easily integrated into heterostructures. The results of this thesis emphasize the importance of a thorough microscopic understanding of doping and interface effects and demonstrate that using a combination of LE-μSR and SX-ARPES offers a more generally applicable experimental route towards this goal.