Publications by Thorsten Hesjedal

Proximity-induced odd-frequency superconductivity in a topological insulator

arxiv (0)

JA Krieger, A Pertsova, SR Giblin, M Döbeli, T Prokscha, CW Schneider, A Suter, T Hesjedal, AV Balatsky, Z Salman

At an interface between a topological insulator (TI) and a conventional superconductor (SC), superconductivity has been predicted to change dramatically and exhibit novel correlations. In particular, the induced superconductivity by an $s$-wave SC in a TI can develop an order parameter with a $p$-wave component. Here we present experimental evidence for an unexpected proximity-induced novel superconducting state in a thin layer of the prototypical TI, Bi$_2$Se$_3$, proximity-coupled to Nb. From depth-resolved magnetic field measurements below the superconducting transition temperature of Nb, we observe a local enhancement of the magnetic field in Bi$_2$Se$_3$ that exceeds the externally applied field, thus supporting the existence of an intrinsic paramagnetic Meissner effect arising from an odd-frequency superconducting state. Our experimental results are complemented by theoretical calculations supporting the appearance of an odd-frequency component at the interface which extends into the TI. This state is topologically distinct from the conventional Bardeen-Cooper-Schrieffer (BCS) state it originates from. To the best of our knowledge, these findings present a first observation of bulk odd-frequency superconductivity in a TI. We thus reaffirm the potential of the TI/SC interface as a versatile platform to produce novel superconducting states.

Diameter-independent skyrmion Hall angle observed in chiral magnetic multilayers

Nature Communications Nature Research (part of Springer Nature) (0)

K Zeissler, S Finizio, C Barton, A Huxtable, J Massey, J Raabe, A Sadovnikov, S Nikitov, R Brearton, T Hesjedal, G van der Laan, M Rosamond, E Linfield, G Burnell, C Marrows

Direct observation of the energy gain underpinning ferromagnetic superexchange in the electronic structure of CrGeTe$_3$

arxiv (0)

I Marković, F Mazzola, A Rajan, EA Morales, DM Burn, THORSTEN Hesjedal, GVD Laan, S Mukherjee, TK Kim, C Bigi, I Vobornik, G Balakrishnan, MC Hatnean, PDC King, G Balakrishnan, S Mukherjee, MC Hatnean, I Vobornik, GVD Laan, PDC King, C Bigi, A Rajan, TK Kim, I Marković

We investigate the temperature-dependent electronic structure of the van der Waals ferromagnet, CrGeTe$_3$. Using angle-resolved photoemission spectroscopy, we identify atomic- and orbital-specific band shifts upon cooling through ${T_\mathrm{C}}$. From these, together with x-ray absorption spectroscopy and x-ray magnetic circular dichroism measurements, we identify the states created by a covalent bond between the Te ${5p}$ and the Cr ${e_g}$ orbitals as the primary driver of the ferromagnetic ordering in this system, while it is the Cr ${t_{2g}}$ states that carry the majority of the spin moment. The ${t_{2g}}$ states furthermore exhibit a marked bandwidth increase and a remarkable lifetime enhancement upon entering the ordered phase, pointing to a delicate interplay between localized and itinerant states in this family of layered ferromagnets.

The topological surface state of $α$-Sn on InSb(001) as studied by photoemission

arxiv Museu de Ciències Naturals de Barcelona (0)

MR Scholz, VA Rogalev, L Dudy, F Reis, F Adler, J Aulbach, LJ Collins-McIntyre, LB Duffy, HF Yang, YL Chen, T Hesjedal, ZK Liu, M Hoesch, S Muff, JH Dil, J Schäfer, R Claessen

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.

Transverse field muon-spin rotation measurement of the topological anomaly in a thin film of MnSi

arXiv:1511.04972v1 (0)

T Lancaster, F Xiao, Z Salman, IO Thomas, SJ Blundell, FL Pratt, SJ Clark, T Prokscha, A Suter, SL Zhang, AA Baker, T Hesjedal

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.

Three-dimensional micromagnetic domain structure of MnAs films on GaAs(001): Experimental imaging and simulations


R Engel-Herbert, T Hesjedal, DM Schaadt

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.

Epitaxial Heusler Alloys on III-V Semiconductors

John Wiley & Sons, Ltd (0)

T Hesjedal, KH Ploog

Calculation and experimental verification of the acoustic stress at GHz frequencies in SAW resonators

Proc. 33rd European Microwave Conference (0)

F Kubat, W Ruile, T Hesjedal, J Stotz, U Roesler, L Reindl