12 research outputs found
Topological Superconductivity in a Phase-Controlled Josephson Junction
Topological superconductors can support localized Majorana states at their
boundaries. These quasi-particle excitations have non-Abelian statistics that
can be used to encode and manipulate quantum information in a topologically
protected manner. While signatures of Majorana bound states have been observed
in one-dimensional systems, there is an ongoing effort to find alternative
platforms that do not require fine-tuning of parameters and can be easily
scalable to large numbers of states. Here we present a novel experimental
approach towards a two-dimensional architecture. Using a Josephson junction
made of HgTe quantum well coupled to thin-film aluminum, we are able to tune
between a trivial and a topological superconducting state by controlling the
phase difference across the junction and applying an in-plane magnetic
field. We determine the topological state of the induced superconductor by
measuring the tunneling conductance at the edge of the junction. At low
magnetic fields, we observe a minimum in the tunneling spectra near zero bias,
consistent with a trivial superconductor. However, as the magnetic field
increases, the tunneling conductance develops a zero-bias peak which persists
over a range of that expands systematically with increasing magnetic
fields. Our observations are consistent with theoretical predictions for this
system and with full quantum mechanical numerical simulations performed on
model systems with similar dimensions and parameters. Our work establishes this
system as a promising platform for realizing topological superconductivity and
for creating and manipulating Majorana modes and will therefore open new
avenues for probing topological superconducting phases in two-dimensional
systems.Comment: Supplementary contains resized figures. Original files are available
upon reques
Interplay of Dirac nodes and Volkov-Pankratov surface states in compressively strained HgTe
Preceded by the discovery of topological insulators, Dirac and Weyl
semimetals have become a pivotal direction of research in contemporary
condensed matter physics. While easily accessible from a theoretical viewpoint,
these topological semimetals pose a serious challenge in terms of experimental
synthesis and analysis to allow for their unambiguous identification. In this
work, we report on detailed transport experiments on compressively strained
HgTe. Due to the superior sample quality in comparison to other topological
semimetallic materials, this enables us to resolve the interplay of topological
surface states and semimetallic bulk states to an unprecedented degree of
precision and complexity. As our gate design allows us to precisely tune the
Fermi level at the Weyl and Dirac points, we identify a magnetotransport regime
dominated by Weyl/Dirac bulk state conduction for small carrier densities and
by topological surface state conduction for larger carrier densities. As such,
similar to topological insulators, HgTe provides the archetypical reference for
the experimental investigation of topological semimetals.Comment: 8 pages, 7 figures, accepted in PRX, added kp calculation and
supplementar
Counterpropagating topological and quantum Hall edge channels
The survival of the quantum spin Hall edge channels in presence of an
external magnetic field has been a subject of experimental and theoretical
research. The inversion of Landau levels that accommodates the quantum spin
Hall effect is destroyed at a critical magnetic field, and a trivial insulating
gap appears in the spectrum for stronger fields. In this work, we report the
absence of this transport gap in disordered two dimensional topological
insulators in perpendicular magnetic fields of up to 16 T. Instead, we observe
that a topological edge channel (from band inversion) coexists with a
counterpropagating quantum Hall edge channel for magnetic fields at which the
transition to the insulating regime is expected. For larger fields, we observe
only the quantum Hall edge channel with transverse resistance close to .
By tuning the disorder using different fabrication processes, we find evidence
that this unexpected plateau originates from extended quantum Hall edge
channels along a continuous network of charge puddles at the edges of the
device.Comment: 8+3 pages, 5+2 figure
Velocity and confinement of edge plasmons in HgTe-based 2D topological insulators
High-frequency transport in the edge states of the quantum spin Hall (QSH)
effect has to date rarely been explored, though it could cast light on the
scattering mechanisms taking place therein. We here report on the measurement
of the plasmon velocity in topological HgTe quantum wells both in the QSH and
quantum Hall (QH) regimes, using harmonic GHz excitations and phase-resolved
detection. We observe low plasmon velocities corresponding to large transverse
widths, which we ascribe to the prominent influence of charge puddles forming
in the vicinity of edge channels. Together with other recent works, it suggests
that puddles play an essential role in the edge state physics and probably
constitute a main hurdle on the way to clean and robust edge transport.Comment: 8 pages, 6 figures, + supplementary materia
Topological band inversion in HgTe(001): surface and bulk signatures from photoemission
HgTe is a versatile topological material and has enabled the realization of a
variety of topological states, including two- and three-dimensional (3D)
topological insulators and topological semimetals. Nevertheless, a quantitative
understanding of its electronic structure remains challenging, in particular
due to coupling of the Te 5p-derived valence electrons to Hg 5d core states at
shallow binding energy. We present a joint experimental and theoretical study
of the electronic structure in strained HgTe(001) films in the 3D
topological-insulator regime, based on angle-resolved photoelectron
spectroscopy and density functional theory. The results establish detailed
agreement in terms of (i) electronic band dispersions and orbital symmetries,
(ii) surface and bulk contributions to the electronic structure, and (iii) the
importance of Hg 5d states in the valence-band formation. Supported by theory,
our experiments directly image the paradigmatic band inversion in HgTe,
underlying its non-trivial band topology
Residual strain in free-standing CdTe nanowires overgrown with HgTe
We investigate the crystal properties of CdTe nanowires overgrown with HgTe. Scanning electron microscopy and scanning transmission electron microscopy confirm that the growth results in a high ensemble uniformity and that the individual heterostructures are single-crystalline, respectively. We use high-resolution X-ray diffraction to investigate strain, caused by the small lattice mismatch between the two materials. We find that both CdTe and HgTe show changes in the lattice constant compared to the respective bulk lattice constants. The measurements reveal a complex strain pattern with signatures of both uniaxial and shear strains present in the overgrown nanowires
Approaching Quantization in Macroscopic Quantum Spin Hall Devices through Gate Training
Quantum spin Hall edge channels hold great promise as dissipationless one-dimensional conductors.However, the ideal quantized conductance of2e2=his only found in very short channels-in contradictionwith the expected protection against backscattering of the topological insulator state. In this Letter we showthat enhancing the band gap does not improve quantization. When we instead alter the potential landscapeby charging trap states in the gate dielectric using gate training, we approach conductance quantization formacroscopically long channels. Effectively, the scattering length increases to175μm, more than 1 order ofmagnitude longer than in previous works for HgTe-based quantum wells. Our experiments show that thedistortion of the potential landscape by impurities, leading to puddle formation in the narrow gap material,is the major obstacle for observing undisturbed quantum spin Hall edge channel transpor
Approaching quantization in macroscopic quantum spin hall devices through gate training
Quantum spin Hall edge channels hold great promise as dissipationless one-dimensional conductors. However, the ideal quantized conductance of 2e2/h is only found in very short channels-in contradiction with the expected protection against backscattering of the topological insulator state. In this Letter we show that enhancing the band gap does not improve quantization. When we instead alter the potential landscape by charging trap states in the gate dielectric using gate training, we approach conductance quantization for macroscopically long channels. Effectively, the scattering length increases to 175 μm, more than 1 order of magnitude longer than in previous works for HgTe-based quantum wells. Our experiments show that the distortion of the potential landscape by impurities, leading to puddle formation in the narrow gap material, is the major obstacle for observing undisturbed quantum spin Hall edge channel transport
Strain Engineering of the Band Gap of HgTe Quantum Wells Using Superlattice Virtual Substrates
Approaching quantization in macroscopic quantum spin hall devices through gate training
\u3cp\u3eQuantum spin Hall edge channels hold great promise as dissipationless one-dimensional conductors. However, the ideal quantized conductance of 2e2/h is only found in very short channels-in contradiction with the expected protection against backscattering of the topological insulator state. In this Letter we show that enhancing the band gap does not improve quantization. When we instead alter the potential landscape by charging trap states in the gate dielectric using gate training, we approach conductance quantization for macroscopically long channels. Effectively, the scattering length increases to 175 μm, more than 1 order of magnitude longer than in previous works for HgTe-based quantum wells. Our experiments show that the distortion of the potential landscape by impurities, leading to puddle formation in the narrow gap material, is the major obstacle for observing undisturbed quantum spin Hall edge channel transport.\u3c/p\u3