Single valley Dirac fermions in zero-gap HgTe quantum wells

Büttner, Bastian; Liu, C. X.; Tkachov, G.; Novik, E. G.; Brüne, C.; Buhmann, H.; Hankiewicz, Ewelina M.; Recher, Patrik; Trauzettel, Björn; Zhang, S. C.; Molenkamp, L. W.

doi:10.1038/nphys1914

Cited by 277 publications

(309 citation statements)

References 26 publications

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“…(10) involves only one species of regular fermion that becomes gapless at the critical point. This is a key difference between the Dirac spectrum found here and the analogous Dirac spectrum of the nodal quasiparticles in, say, d x 2 −y 2 superconductors or HgTe quantum wells 27 where there are two species of gapless fermions corresponding to the spin degeneracy. Thus, our system avoids the fermion doubling theorem consequently giving rise to Majorana fermions and topological superconductivity whereas these other systems do not.…”

Section: Scaling Of Dissipative Susceptibilitymentioning

confidence: 87%

Probing a topological quantum critical point in semiconductor-superconductor heterostructures

Tewari

Scarola

et al. 2012

Phys. Rev. B

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Quantum ground states on the nontrivial side of a topological quantum critical point (TQCP) have unique properties that make them attractive candidates for quantum information applications. A recent example is provided by s-wave superconductivity on a semiconductor platform, which is tuned through a TQCP to a topological superconducting (TS) state by an external Zeeman field. Despite many attractive features of TS states, TQCPs themselves do not break any symmetries, making it impossible to distinguish the TS state from a regular superconductor in conventional bulk measurements. Here we show that for the semiconductor TQCP this problem can be overcome by tracking suitable bulk transport properties across the topological quantum critical regime itself. The universal low-energy effective theory and the scaling form of the relevant susceptibilities also provide a useful theoretical framework in which to understand the topological transitions in semiconductor heterostructures. Based on our theory, specific bulk measurements are proposed here in order to characterize the novel TQCP in semiconductor heterostructures.

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Section: Scaling Of Dissipative Susceptibilitymentioning

confidence: 87%

Probing a topological quantum critical point in semiconductor-superconductor heterostructures

Tewari

Scarola

et al. 2012

Phys. Rev. B

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“…3a). This rules out a conventional topological phase transition, in which case the bulk gap is required to close 27 ; the transition must thus occur by breaking the spin-symmetry on which the QSH effect relies. In fact, a canted antiferromagnetic state (Fig.…”

mentioning

confidence: 99%

Tunable symmetry breaking and helical edge transport in a graphene quantum spin Hall state

Young

Sanchez-Yamagishi

Hunt

et al. 2013

Nature

268

349

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Low-dimensional electronic systems have traditionally been obtained by electrostatically confining electrons, either in heterostructures or in intrinsically nanoscale materials such as single molecules, nanowires, and graphene. Recently, a new paradigm has emerged with the advent of symmetry-protected surface states on the boundary of topological insulators, enabling the creation of electronic systems with novel properties. For example, time reversal symmetry (TRS) endows the massless charge carriers on the surface of a three-dimensional topological insulator with helicity, locking the orientation of their spin relative to their momentum 1,2 . Weakly breaking this symmetry generates a gap on the surface, 3 resulting in charge carriers with finite effective mass and exotic spin textures 4 . Analogous manipulations of the one-dimensional boundary states of a two-dimensional topological insulator are also possible, but have yet to be observed in the leading candidate materials 5,6 . Here, we demonstrate experimentally that charge neutral monolayer graphene displays a new type of quantum spin Hall (QSH) effect 7,8 , previously thought to exist only in time reversal invariant topological insulators 5,9-11 , when it is subjected to a very large magnetic field angled with respect to the graphene plane. Unlike in the TRS case 5,9,10 , the QSH presented here is protected by a spin-rotation symmetry that emerges as electron spins in a half-filled Landau level are polarized by the large in-plane magnetic field. The properties of the resulting helical edge states can be modulated by balancing the applied field against an intrinsic antiferromagnetic instability [12][13][14] , which tends to spontaneously break the spin-rotation symmetry. In the resulting canted antiferromagnetic (CAF) state, we observe transport signatures of gapped edge states, which constitute a new kind of one-dimensional electronic system with tunable band gap and associated spin-texture 15 .In the integer quantum Hall effect, the topology of the bulk Landau level (LL) energy bands 16 requires the existence of gapless edge states at any interface with the vacuum. The metrological precision of the Hall quantization can be traced to the inability of these edge states to backscatter due to the physical separation of modes with opposite momentum by the insulating sample bulk 17 . In contrast, counterpropagating boundary states in a symmetry-protected topological (SPT) insulator coexist spatially but are prevented from backscattering by a symmetry of the experimental system 1,2 . The local symmetry that protects transport in SPT surface states is unlikely to be as robust as the inherently nonlocal physical separation that protects the quantum Hall effect. However, it enables the creation of new electronic systems in which momentum and some quantum number such as spin are coupled, potentially leading to devices with new functionality. Most experimentally realized SPT phases are based on TRS, with counterpropagating states protected from intermixing by the Kram...

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“…14 In 2D TI systems, charge carriers with opposite spins move dissipationless in opposite directions on a given edge. 8,10,11,15,16 In this Article, we extend the concept of the SPS beyond the IQHE paradigm. Contrary to the SPS, we plan to use the edge states of a 2D TI, thus without external magnetic fields.…”

Section: Introductionmentioning

confidence: 99%

Proposal for an on-demand source of polarized electrons into the edges of a topological insulator

Inhofer

Bercioux

2013

Phys. Rev. B

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We propose a device that allows for the emission of pairs of spin-polarized electrons into the edge-states of a two dimensional topological insulator. Charge and spin emission is achieved using a periodically driven quantum dot weakly coupled to the edge states of the host topological insulator. We present calculations of the emitted time-dependent charge and spin currents of such a dynamical scatterer using the Floquet scattering matrix approach. Experimental signatures of spin-polarized two-particle emission can be found in noise measurements. Here a new form of noise suppression, named Z2-antibunching, is introduced. Additionally, we propose a set-up in which entanglement of the emitted electrons is generated. This entanglement is based on a post-selection procedure and becomes manifest in a violation of a Clauser-Horne-Shimony-Holt inequality.

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Single valley Dirac fermions in zero-gap HgTe quantum wells

Cited by 277 publications

References 26 publications

Probing a topological quantum critical point in semiconductor-superconductor heterostructures

Probing a topological quantum critical point in semiconductor-superconductor heterostructures

Tunable symmetry breaking and helical edge transport in a graphene quantum spin Hall state

Proposal for an on-demand source of polarized electrons into the edges of a topological insulator

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