The field of topological materials science has recently been focussing on three-dimensional Dirac semimetals, which exhibit robust Dirac phases in the bulk. However, the absence of characteristic surface states in accidental Dirac semimetals (DSM) makes it difficult to experimentally verify claims about the topological nature using commonly used surface-sensitive techniques. The chiral magnetic effect (CME), which originates from the Weyl nodes, causes an E · B-dependent chiral charge polarization, which manifests itself as negative magnetoresistance. We exploit the extended lifetime of the chirally polarized charge and study the CME through both local and non-local measurements in Hall bar structures fabricated from single crystalline flakes of the DSM Bi0.97Sb0.03. From the non-local measurement results we find a chiral charge relaxation time which is over one order of magnitude larger than the Drude transport lifetime, underlining the topological nature of Bi0.97Sb0.03.

Transition metal dichalcogenides of type VX2 (X = S, Se, Te) have recently attracted great interest as it has been predicted that they host ferromagnetism at room temperature. Whether ferromagnetism is indeed present is an open experimental question. An in-depth study of the structural and magnetoelectric properties of VTe2 thin films is presented in this work. The VTe2 thin films were grown through molecular beam epitaxy, which allows for precise control of thicknesses, ranging from several nanometers down to monolayers. The low-temperature magnetoelectric transport studies reveal no sign of intrinsic ferromagnetism. However, a transition from positive to negative magnetoresistance is present upon decreasing film thickness.

Fractional charges can be induced by magnetic fluxes at the interface between a topological insulator (TI) and a type-II superconductor due to axion electrodynamics. In a Josephson junction array with a hole in the middle, these electronic states can have phase interference in an applied magnetic field with ṕ 4 2 period, in addition to the 2π interference of the Cooper pairs. Here, we test an experimental configuration for probing the fractional charge and report the observation of phase interference effect in superconducting arrays with a hole in the middle in both Au-and TI-based devices. Our numerical simulations based on resistive shunted capacitive junction model are in good agreement with the experimental results. However, no clear sign of an axion charge-related interference effect was observed. We will discuss possible reasons and perspectives for future experiments.

Members:p prof. dr. J. van den Brink Leibniz Institute for Solid State and Materials Research Dresden dr. E van Heumen University of Amsterdam prof. dr. ir. J.W.M. Hilgenkamp University of Twente dr. ir. M.P. de Jong University of Twente prof. dr. A.V. Kimel Radboud University prof. dr. T.T.M. Palstra University of TwenteThe field of axion electrodynamics first emerged in high energy physics and then found its way to the field of condensed matter physics when it was found that modifications to Maxwell's equations can be present near the boundaries of topological materials.In this chapter, we review the derivation of the equations governing axion electrodynamics, and then discuss the experimental proposals that form the basis of the upcoming Chapters 5-7. While the equations are well established already for a long time, their implications and the requirements for the observation of the topological magnetoelectric effects in topological matter are still under debate. We aim to give an overview of the recent advances and open questions within the field.

Ultrathin films of the ternary topological insulator (Bi0.4Sb0.6)2Te3 are fabricated by molecular beam epitaxy. Although it is generally assumed that the ternary topological insulator tellurides grow by van der Waals epitaxy, our results show that the influence of the substrate is substantial and governs the formation of defects, mosaicity, and twin domains. For this comparative study, InP (111)A, Al2O3 (001), and SrTiO3 (111) substrates were selected. While the films deposited on lattice-matched InP (111)A show van der Waals epitaxial relations, our results point to a quasi-van der Waals epitaxy for the films grown on substrates with a larger lattice mismatch.