116 research outputs found
Ballistic quantum spin Hall state and enhanced edge backscattering in strong magnetic fields
The quantum spin Hall (QSH) state, observed in a zero magnetic field in HgTe
quantum wells, respects the time-reversal symmetry and is distinct from quantum
Hall (QH) states. We show that the QSH state persists in strong quantizing
fields and is identified by counter-propagating (helical) edge channels with
nonlinear dispersion inside the band gap. If the Fermi level is shifted into
the Landau-quantized conduction or valence band, we find a transition between
the QSH and QH regimes. Near the transition the longitudinal conductance of the
helical channels is strongly suppressed due to the combined effect of the
spectrum nonlinearity and enhanced backscattering. It shows a power-law decay
1/B^2N with magnetic field B, determined by the number of backscatterers on the
edge, N. This suggests a rather simple and practical way to probe the quality
of recently realized quasiballistic QSH devices using magnetoresistance
measurements.Comment: 4 pages, 3 figures, minor changes, accepted for publication in PR
Anomalous Spin Response and Virtual-Carrier-Mediated Magnetism in a Topological Insulator
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-wave-length) 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 Lande g-factors for the bulk and edge electrons. The
variety of counter-intuitive 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 Schrodinger-type physics,
mimics the behavior of chiral Dirac fermions, or reflects the material's
symmetry-protected topological order.Comment: 15 pages, 8 figures, RevTex4.1; v2: extended and expanded results and
presentatio
Signatures of topology in ballistic bulk transport of HgTe quantum wells
We calculate bulk transport properties of two-dimensional topological
insulators based on HgTe quantum wells in the ballistic regime. Interestingly,
we find that the conductance and the shot noise are distinctively different for
the so-called normal regime (the topologically trivial case) and the so-called
inverted regime (the topologically non-trivial case). Thus, it is possible to
verify the topological order of a two-dimensional topological insulator not
only via observable edge properties but also via observable bulk properties.
This is important because we show that under certain conditions the bulk
contribution can dominate the edge contribution which makes it essential to
fully understand the former for the interpretation of future experiments in
clean samples.Comment: 5 pages, 4 figure
Spin-Hall effect in a [110] quantum well
A self-consistent treatment of the spin-Hall effect requires consideration of
the spin-orbit coupling and electron-impurity scattering on equal footing. This
is done here for the experimentally relevant case of a [110] GaAs quantum well
[Sih {\it et al.}, Nature Physics 1, 31 (2005)]. Working within the framework
of the exact linear response formalism we calculate the spin-Hall conductivity
including the Dresselhaus linear and cubic terms in the band structure, as well
as the electron-impurity scattering and electron-electron interaction to all
orders. We show that the spin-Hall conductivity naturally separates into two
contributions, skew-scattering and side-jump, and we propose an experiment to
distinguish between them.Comment: The connection with the recent experiment on [110] quantum wells is
emphasize
Anomalous galvanomagnetism, cyclotron resonance and microwave spectroscopy of topological insulators
The surface quantum Hall state, magneto-electric phenomena and their
connection to axion electrodynamics have been studied intensively for
topological insulators. One of the obstacles for observing such effects comes
from nonzero conductivity of the bulk. To overcome this obstacle we propose to
use an external magnetic field to suppress the conductivity of the bulk
carriers. The magnetic field dependence of galvanomagnetic and electromagnetic
responses of the whole system shows anomalies due to broken time-reversal
symmetry of the surface quantum Hall state, which can be used for its
detection. In particular, we find linear bulk dc magnetoresistivity and a
quadratic field dependence of the Hall angle, shifted rf cyclotron resonance,
nonanalytic microwave transmission coefficient and saturation of the Faraday
rotation angle with increasing magnetic field or wave frequency.Comment: 5 pages, 3 figures, version as publishe
"Phase Diagram" of the Spin Hall Effect
We obtain analytic formulas for the frequency-dependent spin-Hall
conductivity of a two-dimensional electron gas (2DEG) in the presence of
impurities, linear spin-orbit Rashba interaction, and external magnetic field
perpendicular to the 2DEG. We show how different mechanisms (skew-scattering,
side-jump, and spin precession) can be brought in or out of focus by changing
controllable parameters such as frequency, magnetic field, and temperature. We
find, in particular, that the d.c. spin Hall conductivity vanishes in the
absence of a magnetic field, while a magnetic field restores the
skew-scattering and side-jump contributions proportionally to the ratio of
magnetic and Rashba fields.Comment: Some typos correcte
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