Professor Thorsten Hesjedal FInstP

Abstract:

We report on the electronic structure of the elemental topological semimetal $\alpha$-Sn on InSb(001). High-resolution angle-resolved photoemission data allow to observe the topological surface state (TSS) that is degenerate with the bulk band structure and show that the former is unaffected by different surface reconstructions. An unintentional $p$-type doping of the as-grown films was compensated by deposition of potassium or tellurium after the growth, thereby shifting the Dirac point of the surface state below the Fermi level. We show that, while having the potential to break time-reversal symmetry, iron impurities with a coverage of up to 0.25 monolayers do not have any further impact on the surface state beyond that of K or Te. Furthermore, we have measured the spin-momentum locking of electrons from the TSS by means of spin-resolved photoemission. Our results show that the spin vector lies fully in-plane, but it also has a finite radial component. Finally, we analyze the decay of photoholes introduced in the photoemission process, and by this gain insight into the many-body interactions in the system. Surprisingly, we extract quasiparticle lifetimes comparable to other topological materials where the TSS is located within a bulk band gap. We argue that the main decay of photoholes is caused by intraband scattering, while scattering into bulk states is suppressed due to different orbital symmetries of bulk and surface states.

Abstract:

The micromagnetic domain structure of MnAs films on GaAs(001) has been systematically investigated by micromagnetic imaging and simulations. The magnetic force microscopy (MFM) contrast resulting from the stray field of the simulated three-dimensional domain patterns was calculated and found to be in excellent agreement with MFM experiments. By combining three-dimensional stray-field imaging by MFM with surface sensitive probing and micromagnetic simulations, we were able to derive a consistent picture of the micromagnetic structure of MnAs. For example, the origin of the comblike contrast observed through MFM was identified as a metastable domain configuration exhibiting a cross-tie wall.

Abstract:

We present the results of transverse-field muon-spin rotation measurements on an epitaxially grown 40 nm-thick film of MnSi on Si(111) in the region of the field-temperature phase diagram where a skyrmion phase has been observed in the bulk. We identify changes in the quasistatic magnetic field distribution sampled by the muon, along with evidence for magnetic transitions around T ≈ 40 K and 30 K. Our results suggest that the cone phase is not the only magnetic texture realized in film samples for out-of-plane fields.