Professor Thorsten Hesjedal FInstP

Abstract:

Topological insulators (TIs) have drawn considerable attention for next generation high sensitivity, high efficiency devices thanks to their unique metallic surface conductivity via their topologically-protected surface states (TSSs). In this study, we utilized near-field nanoimaging and nanoscale spectroscopy in the Terahertz (THz) frequency range to explore the optical response of the surface layer from Bi2Te3 TI flakes. In all cases, we observe a higher near-field optical response from the TI flakes compared to a Si substrate. This contrast increases for more surface sensitive probing, which indicates the presence of a conductive surface state. In addition, we have performed temperature dependent THz nanoscopy and demonstrate a significantly enhanced near-field contrast for the TI flake at a sample temperature of 8.9 K compared to the Si substrate. This temperature-dependent behavior provides vital insight into the underlying mechanisms behind the observed conductive surface state in TI materials.

Abstract:

In this perspective article, we explore some of the promising spin and topology material platforms (e.g., spins in semiconductors and superconductors, skyrmionic, topological, and two-dimensional materials) being developed for such quantum components as qubits, superconducting memories, sensing, and metrological standards, and discuss their figures of merit. Spin- and topology-related quantum phenomena have several advantages, including high coherence time, topological protection and stability, low error rate, relative ease of engineering and control, and simple initiation and readout. However, the relevant technologies are at different stages of research and development, and here, we discuss their state-of-the-art, potential applications, challenges, and solutions.

Abstract:

Disordered two-dimensional (2D) lattices, including hexatic and various glassy states, are observed in a wide range of 2D systems including colloidal nanoparticle assemblies and fluxon lattices. Their disordered nature determines the stability and mobility of these systems, as well as their response to the external stimuli. Here we report on the controlled creation and characterization of a disordered 2D lattice of nontopological magnetic bubbles in the noncentrosymmetric ferrimagnetic alloy Mn1.4PtSn. By analyzing the type and frequency of fundamental lattice defects, such as dislocations, the orientational correlation, as well as the induced motion of the lattice in an external field, a nonergodic glassy state, stabilized by directional application of an external field, is revealed.

Abstract:

Magnetron sputtering offers a scalable route to magnetic topological insulators (MTIs) based on Cr-doped Sb2Te3. We combine a range of X-ray diffraction (XRD), reciprocal-space mapping (RSM), scanning transmission electron microscopy (STEM), scanning TEM-energy-dispersive X-ray spectroscopy (STEM-EDS), and X-ray absorption spectroscopy, and X-ray magnetic circular dichroism (XAS/XMCD) techniques to study the structure and magnetism of Cr-doped Sb2Te3 films. Symmetric θ-2θ XRD and RSM establish a solubility window. Layered tetradymite order persists up to ∼10 at.-% Cr, while higher doping yields CrTe/Cr2Te3 secondary phases. STEM reveals nanocrystalline layered stacking at low Cr and loss of long-range layering at higher Cr concentrations, consistent with XRD/RSM. Magnetometry on a 6% film shows soft ferromagnetism at 5 K. XAS and XMCD at the Cr L2,3 edges exhibits a depth dependence: total electron yield (TE; surface sensitive) shows both nominal Cr2+ and Cr3+, whereas fluorescence yield (FY; bulk sensitive) shows a much higher Cr2+ weight. Sum rules applied to TEY give mL=(0.20±0.04) μB/Cr, and mS=(1.6±0.2) μB/Cr, whereby we note that the applied maximum field (3 T) likely underestimates mS. These results define a practical growth window and outline key parameters for MTI films.