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  - Brüne, Christoph
A1  - Thienel, Cornelius
A1  - Stuiber, Michael
A1  - Böttcher, Jan
A1  - Buhmann, Hartmut
A1  - Novik, Elena G.
A1  - Liu, Chao-Xing
A1  - Hankiewicz, Ewelina M.
A1  - Molenkamp, Laurens W.
T1  - Dirac-Screening Stabilized Surface-State Transport in a Topological Insulator
JF  - Physical Review X
N2  - We report magnetotransport studies on a gated strained HgTe device. This material is a three-dimensional topological insulator and exclusively shows surface-state transport. Remarkably, the Landau-level dispersion and the accuracy of the Hall quantization remain unchanged over a wide density range (3×1011  cm−2<n<2×1012  cm−2). These observations imply that even at large carrier densities, the transport is surface-state dominated, where bulk transport would have been expected to coexist already. Moreover, the density dependence of the Dirac-type quantum Hall effect allows us to identify the contributions from the individual surfaces. A k⋅p model can describe the experiments but only when assuming a steep band bending across the regions where the topological surface states are contained. This steep potential originates from the specific screening properties of Dirac systems and causes the gate voltage to influence the position of the Dirac points rather than that of the Fermi level.
KW  - condensed matter physics
KW  - topological insulators
Y1  - 2014
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-118091
SN  - 2160-3308
VL  - 4
IS  - 4
ER  - 
TY  - JOUR
A1  - Sessi, Paolo
A1  - Biswas, Rudro R.
A1  - Bathon, Thomas
A1  - Storz, Oliver
A1  - Wilfert, Stefan
A1  - Barla, Alessandro
A1  - Kokh, Konstantin A.
A1  - Tereshchenko, Oleg E.
A1  - Fauth, Kai
A1  - Bode, Matthias
A1  - Balatsky, Alexander V.
T1  - Dual nature of magnetic dopants and competing trends in topological insulators
JF  - Nature Communications
N2  - Topological insulators interacting with magnetic impurities have been reported to host several unconventional effects. These phenomena are described within the framework of gapping Dirac quasiparticles due to broken time-reversal symmetry. However, the overwhelming majority of studies demonstrate the presence of a finite density of states near the Dirac point even once topological insulators become magnetic. Here, we map the response of topological states to magnetic impurities at the atomic scale. We demonstrate that magnetic order and gapless states can coexist. We show how this is the result of the delicate balance between two opposite trends, that is, gap opening and emergence of a Dirac node impurity band, both induced by the magnetic dopants. Our results evidence a more intricate and rich scenario with respect to the once generally assumed, showing how different electronic and magnetic states may be generated and controlled in this fascinating class of materials.
KW  - magnetic properties and materials
KW  - topological insulators
KW  - magnetic dopants
Y1  - 2016
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-172704
VL  - 7
ER  - 
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  - Vigliotti, Lucia
A1  - Calzona, Alessio
A1  - Traverso Ziani, Niccolò
A1  - Bergeret, F. Sebastian
A1  - Sassetti, Maura
A1  - Trauzettel, Björn
T1  - Effects of the spatial extension of the edge channels on the interference pattern of a helical Josephson junction
JF  - Nanomaterials
N2  - Josephson junctions (JJs) in the presence of a magnetic field exhibit qualitatively different interference patterns depending on the spatial distribution of the supercurrent through the junction. In JJs based on two-dimensional topological insulators (2DTIs), the electrons/holes forming a Cooper pair (CP) can either propagate along the same edge or be split into the two edges. The former leads to a SQUID-like interference pattern, with the superconducting flux quantum ϕ\(_0\) (where ϕ\(_0\)=h/2e) as a fundamental period. If CPs’ splitting is additionally included, the resultant periodicity doubles. Since the edge states are typically considered to be strongly localized, the critical current does not decay as a function of the magnetic field. The present paper goes beyond this approach and inspects a topological JJ in the tunneling regime featuring extended edge states. It is here considered the possibility that the two electrons of a CP propagate and explore the junction independently over length scales comparable to the superconducting coherence length. As a consequence of the spatial extension, a decaying pattern with different possible periods is obtained. In particular, it is shown that, if crossed Andreev reflections (CARs) are dominant and the edge states overlap, the resulting interference pattern features oscillations whose periodicity approaches 2ϕ\(_0\).
KW  - edge states
KW  - Josephson junctions
KW  - topological insulators
KW  - interference pattern
KW  - 2ϕ\(_0\) periodicity
Y1  - 2023
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-304846
SN  - 2079-4991
VL  - 13
IS  - 3
ER  - 
TY  - JOUR
A1  - Oostinga, Jeroen B.
A1  - Maier, Luis
A1  - Schüffelgen, Peter
A1  - Knott, Daniel
A1  - Ames, Christopher
A1  - Brüne, Christoph
A1  - Tkachov, Grigory
A1  - Buhmann, Hartmut
A1  - Molenkamp, Laurens W.
T1  - Josephson Supercurrent through the Topological Surface States of Strained Bulk HgTe
JF  - Physical Review X
N2  - Strained bulk HgTe is a three-dimensional topological insulator, whose surface electrons have a high mobility (~ 30 000 cm\(^2\)=Vs), while its bulk is effectively free of mobile charge carriers. These properties enable a study of transport through its unconventional surface states without being hindered by a parallel bulk conductance. Here, we show transport experiments on HgTe-based Josephson junctions to investigate the appearance of the predicted Majorana states at the interface between a topological insulator and a superconductor. Interestingly, we observe a dissipationless supercurrent flow through the topological surface states of HgTe. The current-voltage characteristics are hysteretic at temperatures below 1 K, with critical supercurrents of several microamperes. Moreover, we observe a magnetic-field-induced Fraunhofer pattern of the critical supercurrent, indicating a dominant \(2\pi\)-periodic Josephson effect in the unconventional surface states. Our results show that strained bulk HgTe is a promising material system to get a better understanding of the Josephson effect in topological surface states, and to search for the manifestation of zero-energy Majorana states in transport experiments.
KW  - topological insulators
KW  - mesoscopics
KW  - superconductivity
Y1  - 2013
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-129834
VL  - 3
IS  - 021007
ER  - 
TY  - JOUR
A1  - Ünzelmann, M.
A1  - Bentmann, H.
A1  - Figgemeier, T.
A1  - Eck, P.
A1  - Neu, J. N.
A1  - Geldiyev, B.
A1  - Diekmann, F.
A1  - Rohlf, S.
A1  - Buck, J.
A1  - Hoesch, M.
A1  - Kalläne, M.
A1  - Rossnagel, K.
A1  - Thomale, R.
A1  - Siegrist, T.
A1  - Sangiovanni, G.
A1  - Di Sante, D.
A1  - Reinert, F.
T1  - Momentum-space signatures of Berry flux monopoles in the Weyl semimetal TaAs
JF  - Nature Communications
N2  - Since the early days of Dirac flux quantization, magnetic monopoles have been sought after as a potential corollary of quantized electric charge. As opposed to magnetic monopoles embedded into the theory of electromagnetism, Weyl semimetals (WSM) exhibit Berry flux monopoles in reciprocal parameter space. As a function of crystal momentum, such monopoles locate at the crossing point of spin-polarized bands forming the Weyl cone. Here, we report momentum-resolved spectroscopic signatures of Berry flux monopoles in TaAs as a paradigmatic WSM. We carried out angle-resolved photoelectron spectroscopy at bulk-sensitive soft X-ray energies (SX-ARPES) combined with photoelectron spin detection and circular dichroism. The experiments reveal large spin- and orbital-angular-momentum (SAM and OAM) polarizations of the Weyl-fermion states, resulting from the broken crystalline inversion symmetry in TaAs. Supported by first-principles calculations, our measurements image signatures of a topologically non-trivial winding of the OAM at the Weyl nodes and unveil a chirality-dependent SAM of the Weyl bands. Our results provide directly bulk-sensitive spectroscopic support for the non-trivial band topology in the WSM TaAs, promising to have profound implications for the study of quantum-geometric effects in solids. Weyl semimetals exhibit Berry flux monopoles in momentum-space, but direct experimental evidence has remained elusive. Here, the authors reveal topologically non-trivial winding of the orbital-angular-momentum at the Weyl nodes and a chirality-dependent spin-angular-momentum of the Weyl bands, as a direct signature of the Berry flux monopoles in TaAs.
KW  - electronic properties and materials
KW  - topological insulators
Y1  - 2021
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-260719
VL  - 12
IS  - 1
ER  - 
TY  - JOUR
A1  - Wagner, N.
A1  - Crippa, L.
A1  - Amaricci, A.
A1  - Hansmann, P.
A1  - Klett, M.
A1  - König, E. J.
A1  - Schäfer, T.
A1  - Di Sante, D.
A1  - Cano, J.
A1  - Millis, A. J.
A1  - Georges, A.
A1  - Sangiovanni, G.
T1  - Mott insulators with boundary zeros
JF  - Nature Communications
N2  - The topological classification of electronic band structures is based on symmetry properties of Bloch eigenstates of single-particle Hamiltonians. In parallel, topological field theory has opened the doors to the formulation and characterization of non-trivial phases of matter driven by strong electron-electron interaction. Even though important examples of topological Mott insulators have been constructed, the relevance of the underlying non-interacting band topology to the physics of the Mott phase has remained unexplored. Here, we show that the momentum structure of the Green’s function zeros defining the “Luttinger surface" provides a topological characterization of the Mott phase related, in the simplest description, to the one of the single-particle electronic dispersion. Considerations on the zeros lead to the prediction of new phenomena: a topological Mott insulator with an inverted gap for the bulk zeros must possess gapless zeros at the boundary, which behave as a form of “topological antimatter” annihilating conventional edge states. Placing band and Mott topological insulators in contact produces distinctive observable signatures at the interface, revealing the otherwise spectroscopically elusive Green’s function zeros.
KW  - electronic properties and materials
KW  - topological insulators
Y1  - 2023
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-358150
VL  - 14
ER  - 
TY  - JOUR
A1  - Dziom, V.
A1  - Shuvaev, A.
A1  - Pimenov, A.
A1  - Astakhov, G.V.
A1  - Ames, C.
A1  - Bendias, K.
A1  - Böttcher, J.
A1  - Tkachov, G.
A1  - Hankiewicz, E.M.
A1  - Brüne, C.
A1  - Buhmann, H.
A1  - Molenkamp, L.W.
T1  - Observation of the universal magnetoelectric effect in a 3D topological insulator
JF  - Nature Communications
N2  - The electrodynamics of topological insulators (TIs) is described by modified Maxwell’s equations, which contain additional terms that couple an electric field to a magnetization and a magnetic field to a polarization of the medium, such that the coupling coefficient is quantized in odd multiples of α/4π per surface. Here we report on the observation of this so-called topological magnetoelectric effect. We use monochromatic terahertz (THz) spectroscopy of TI structures equipped with a semitransparent gate to selectively address surface states. In high external magnetic fields, we observe a universal Faraday rotation angle equal to the fine structure constant α=e\(^{2}\)/2E\(_{0}\)hc (in SI units) when a linearly polarized THz radiation of a certain frequency passes through the two surfaces of a strained HgTe 3D TI. These experiments give insight into axion electrodynamics of TIs and may potentially be used for a metrological definition of the three basic physical constants.
KW  - topological matter
KW  - infrared spectroscopy
KW  - topological insulators
KW  - topological magnetoelectric effect
Y1  - 2017
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-170875
VL  - 8
IS  - 15197
ER  - 
TY  - JOUR
A1  - König, Markus
A1  - Baenninger, Matthias
A1  - Garcia, Andrei G. F.
A1  - Harjee, Nahid
A1  - Pruitt, Beth L.
A1  - Ames, C.
A1  - Leubner, Philipp
A1  - Brüne, Christoph
A1  - Buhmann, Hartmut
A1  - Molenkamp, Laurens W.
A1  - Goldhaber-Gordon, David
T1  - Spatially Resolved Study of Backscattering in the Quantum Spin Hall State
JF  - Physical Review X
N2  - The discovery of the quantum spin Hall (QSH) state, and topological insulators in general, has sparked strong experimental efforts. Transport studies of the quantum spin Hall state have confirmed the presence of edge states, showed ballistic edge transport in micron-sized samples, and demonstrated the spin polarization of the helical edge states. While these experiments have confirmed the broad theoretical model, the properties of the QSH edge states have not yet been investigated on a local scale. Using scanning gate microscopy to perturb the QSH edge states on a submicron scale, we identify well-localized scattering sites which likely limit the expected nondissipative transport in the helical edge channels. In the micron-sized regions between the scattering sites, the edge states appear to propagate unperturbed, as expected for an ideal QSH system, and are found to be robust against weak induced potential fluctuations.
KW  - mesoscopics
KW  - topological insulators
KW  - transport
KW  - charge
KW  - wells
KW  - branched flow
KW  - nanostructures
Y1  - 2013
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-127225
SN  - 2160-3308
VL  - 3
IS  - 2
ER  - 
TY  - JOUR
A1  - Assaad, F. F.
A1  - Bercx, M.
A1  - Hohenadler, M.
T1  - Topological Invariant and Quantum Spin Models from Magnetic pi Fluxes in Correlated Topological Insulators
JF  - Physical Review X
N2  - The adiabatic insertion of a \(\pi\) flux into a quantum spin Hall insulator gives rise to localized spin and charge fluxon states. We demonstrate that \(\pi\) fluxes can be used in exact quantum Monte Carlo simulations to identify a correlated \(Z_2\) topological insulator using the example of the Kane-Mele-Hubbard model. In the presence of repulsive interactions, a \(\pi\) flux gives rise to a Kramers doublet of spin-fluxon states with a Curie-law signature in the magnetic susceptibility. Electronic correlations also provide a bosonic mode of magnetic excitons with tunable energy that act as exchange particles and mediate a dynamical interaction of adjustable range and strength between spin fluxons. \(\pi\) fluxes can therefore be used to build models of interacting spins. This idea is applied to a three-spin ring and to one-dimensional spin chains. Because of the freedom to create almost arbitrary spin lattices, correlated topological insulators with \(\pi\) fluxes represent a novel kind of quantum simulator, potentially useful for numerical simulations and experiments.
KW  - topological insulators
KW  - strongly correlated materials
Y1  - 2013
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-129849
VL  - 3
IS  - 1
ER  - 
TY  - JOUR
A1  - Bareille, C.
A1  - Fortuna, F.
A1  - Rödel, T. C.
A1  - Bertran, F.
A1  - Gabay, M.
A1  - Hijano Cubelos, O.
A1  - Taleb-Ibrahimi, A.
A1  - Le Fèvre, P.
A1  - Bibes, M.
A1  - Barthelemy, A.
A1  - Maroutian, T.
A1  - Lecoeur, P.
A1  - Rozenberg, M. J.
A1  - Santander-Syro, A. F.
T1  - Two-dimensional electron gas with six-fold symmetry at the (111) surface of KTaO3
JF  - Scientific Reports
N2  - Two-dimensional electron gases (2DEGs) at transition-metal oxide (TMO) interfaces, and boundary states in topological insulators, are being intensively investigated. The former system harbors superconductivity, large magneto-resistance, and ferromagnetism. In the latter, honeycomb-lattice geometry plus bulk spin-orbit interactions lead to topologically protected spin-polarized bands. 2DEGs in TMOs with a honeycomb-like structure could yield new states of matter, but they had not been experimentally realized, yet. We successfully created a 2DEG at the (111) surface of KTaO3, a strong insulator with large spin-orbit coupling. Its confined states form a network of weakly-dispersing electronic gutters with 6-fold symmetry, a topology novel to all known oxide-based 2DEGs. If those pertain to just one Ta-(111) bilayer, model calculations predict that it can be a topological metal. Our findings demonstrate that completely new electronic states, with symmetries not realized in the bulk, can be tailored in oxide surfaces, promising for TMO-based devices.
KW  - LAALO3/SRTIO3 interfaces
KW  - topological insulators
KW  - superconductivity
KW  - oxides
KW  - SRTIO3
Y1  - 2014
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-117703
SN  - 2045-2322
VL  - 4
ER  - 
TY  - JOUR
A1  - Fleszar, Andrzej
A1  - Hanke, Werner
T1  - Two-dimensional metallicity with a large spin-orbit splitting: DFT calculations of the atomic, electronic, and spin structures of the Au/Ge(111)-(√3 x √3)R30° surface
JF  - Advances in Condensed Matter Physics
N2  - Density functional theory (DFT) is applied to study the atomic, electronic, and spin structures of the Au monolayer at the Ge(111) surface. It is found that the theoretically determined most stable atomic geometry is described by the conjugated honeycomb-chained-trimer (CHCT) model, in a very good agreement with experimental data. The calculated electronic structure of the system, being in qualitatively good agreement with the photoemission measurements, shows fingerprints of the many-body effects (self-interaction corrections) beyond the LDA or GGA approximations. The most interesting property of this surface system is the large spin splitting of its metallic surface bands and the undulating spin texture along the hexagonal Fermi contours, which highly resembles the spin texture at the Dirac state of the topological insulator Bi\(_{2}\)Te\(_{3}\). These properties make this system particularly interesting from both fundamental and technological points of view.
KW  - topological insulators
KW  - gas
KW  - density functional theory
KW  - conjugated honeycomb-chained-trimer
KW  - spin structures
KW  - Au/Ge(111)
Y1  - 2015
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-149221
VL  - 2015
IS  - 531498
ER  - 
TY  - JOUR
A1  - Ma, Eric Yue
A1  - Calvo, M. Reyes
A1  - Wang, Jing
A1  - Lian, Biao
A1  - Mühlbauer, Mathias
A1  - Brüne, Christoph
A1  - Cui, Yong-Tao
A1  - Lai, Keji
A1  - Kundhikanjana, Worasom
A1  - Yang, Yongliang
A1  - Baenninger, Matthias
A1  - König, Markus
A1  - Ames, Christopher
A1  - Buhmann, Hartmut
A1  - Leubner, Philipp
A1  - Molenkamp, Laurens W.
A1  - Zhang, Shou-Cheng
A1  - Goldhaber-Gordon, David
A1  - Kelly, Michael A.
A1  - Shen, Zhi-Xun
T1  - Unexpected edge conduction in mercury telluride quantum wells under broken time-reversal symmetry
JF  - Nature Communications
N2  - The realization of quantum spin Hall effect in HgTe quantum wells is considered a milestone in the discovery of topological insulators. Quantum spin Hall states are predicted to allow current flow at the edges of an insulating bulk, as demonstrated in various experiments. A key prediction yet to be experimentally verified is the breakdown of the edge conduction under broken time-reversal symmetry. Here we first establish a systematic framework for the magnetic field dependence of electrostatically gated quantum spin Hall devices. We then study edge conduction of an inverted quantum well device under broken time-reversal symmetry using microwave impedance microscopy, and compare our findings to a noninverted device. At zero magnetic field, only the inverted device shows clear edge conduction in its local conductivity profile, consistent with theory. Surprisingly, the edge conduction persists up to 9 T with little change. This indicates physics beyond simple quantum spin Hall model, including material-specific properties and possibly many-body effects.
KW  - topological insulators
KW  - surface states
KW  - HgTe
KW  - Hg1-xCdxTe
KW  - vacancies
Y1  - 2015
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-143185
VL  - 6
IS  - 7252
ER  -