TY  - JOUR
A1  - Stühler, R.
A1  - Kowalewski, A.
A1  - Reis, F.
A1  - Jungblut, D.
A1  - Dominguez, F.
A1  - Scharf, B.
A1  - Li, G.
A1  - Schäfer, J.
A1  - Hankiewicz, E. M.
A1  - Claessen, R.
T1  - Effective lifting of the topological protection of quantum spin Hall edge states by edge coupling
JF  - Nature Communications
N2  - The scientific interest in two-dimensional topological insulators (2D TIs) is currently shifting from a more fundamental perspective to the exploration and design of novel functionalities. Key concepts for the use of 2D TIs in spintronics are based on the topological protection and spin-momentum locking of their helical edge states. In this study we present experimental evidence that topological protection can be (partially) lifted by pairwise coupling of 2D TI edges in close proximity. Using direct wave function mapping via scanning tunneling microscopy/spectroscopy (STM/STS) we compare isolated and coupled topological edges in the 2D TI bismuthene. The latter situation is realized by natural lattice line defects and reveals distinct quasi-particle interference (QPI) patterns, identified as electronic Fabry-Pérot resonator modes. In contrast, free edges show no sign of any single-particle backscattering. These results pave the way for novel device concepts based on active control of topological protection through inter-edge hybridization for, e.g., electronic Fabry-Pérot interferometry.
KW  - topological insulators
KW  - two-dimensional materials
Y1  - 2022
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-300886
VL  - 13
ER  - 
TY  - JOUR
A1  - Deacon, R. S.
A1  - Wiedenmann, J.
A1  - Bocquillon, E.
A1  - Domínguez, F.
A1  - Klapwijk, T. M.
A1  - Leubner, P.
A1  - Brüne, C.
A1  - Hankiewicz, E. M.
A1  - Tarucha, S.
A1  - Ishibashi, K.
A1  - Buhmann, H.
A1  - Molenkamp, L. W.
T1  - Josephson Radiation from Gapless Andreev Bound States in HgTe-Based Topological Junctions
JF  - Physical Review X
N2  - Frequency analysis of the rf emission of oscillating Josephson supercurrent is a powerful passive way of probing properties of topological Josephson junctions. In particular, measurements of the Josephson emission enable the detection of topological gapless Andreev bound states that give rise to emission at half the Josephson frequency f\(_{J}\) rather than conventional emission at f\(_{J}\). Here, we report direct measurement of rf emission spectra on Josephson junctions made of HgTe-based gate-tunable topological weak links. The emission spectra exhibit a clear signal at half the Josephson frequency f\(_{J}\)/2. The linewidths of emission lines indicate a coherence time of 0.3–4 ns for the f\(_{J}\)/2 line, much shorter than for the f\(_{J}\) line (3–4 ns). These observations strongly point towards the presence of topological gapless Andreev bound states and pave the way for a future HgTe-based platform for topological quantum computation.
KW  - condensed matter physics
KW  - Josephson junctions
KW  - topological materials
KW  - gapless Andreev bound states
Y1  - 2017
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-170969
VL  - 7
IS  - 021011
ER  - 
TY  - JOUR
A1  - Kernreiter, T.
A1  - Governale, M.
A1  - Zülicke, U.
A1  - Hankiewicz, E. M.
T1  - Anomalous Spin Response and Virtual-Carrier-Mediated Magnetism in a Topological Insulator
JF  - Physical Review X
N2  - We present a comprehensive theoretical study of the static spin response in HgTe quantum wells, revealing distinctive behavior for the topologically nontrivial inverted structure. Most strikingly, the q=0 (long-wavelength) spin susceptibility of the undoped topological-insulator system is constant and equal to the value found for the gapless Dirac-like structure, whereas the same quantity shows the typical decrease with increasing band gap in the normal-insulator regime. We discuss ramifications for the ordering of localized magnetic moments present in the quantum well, both in the insulating and electron-doped situations. The spin response of edge states is also considered, and we extract effective Landé g factors for the bulk and edge electrons. The variety of counterintuitive spin-response properties revealed in our study arises from the system’s versatility in accessing situations where the charge-carrier dynamics can be governed by ordinary Schrödinger-type physics; it mimics the behavior of chiral Dirac fermions or reflects the material’s symmetry-protected topological order.
KW  - spin response
KW  - magnetism
KW  - nanophysics
KW  - topological insulators
Y1  - 2016
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-166582
VL  - 6
IS  - 021010
ER  - 
TY  - JOUR
A1  - Rothe, D. G.
A1  - Reinthaler, R. W.
A1  - Liu, C-X
A1  - Molenkamp, L. W.
A1  - Zhang, S-C
A1  - Hankiewicz, E. M.
T1  - Fingerprint of different spin-orbit terms for spin transport in HgTe quantum wells
N2  - Using k · p theory, we derive an effective four-band model describing the physics of the typical two-dimensional topological insulator (HgTe/CdTe quantum well (QW)) in the presence of an out-of-plane (in the z-direction) inversion breaking potential and an in-plane potential. We find that up to third order in perturbation theory, only the inversion breaking potential generates new elements to the four-band Hamiltonian that are off-diagonal in spin space. When this new effective Hamiltonian is folded into an effective twoband model for the conduction (electron) or valence (heavy hole) bands, two competing terms appear: (i) a Rashba spin–orbit interaction originating from inversion breaking potential in the z-direction and (ii) an in-plane Pauli term as a consequence of the in-plane potential. Spin transport in the conduction band is further analysed within the Landauer–Büttiker formalism. We find that for asymmetrically doped HgTe QWs, the behaviour of the spin-Hall conductance is dominated by the Rashba term.
KW  - Physik
Y1  - 2010
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-68362
ER  -