5,307 research outputs found
Nonlocal resistance and its fluctuations in microstructures of band-inverted HgTe/(Hg,Cd)Te quantum wells
We investigate experimentally transport in gated microsctructures containing
a band-inverted HgTe/Hg_{0.3}Cd_{0.7}Te quantum well. Measurements of nonlocal
resistances using many contacts prove that in the depletion regime the current
is carried by the edge channels, as expected for a two-dimensional topological
insulator. However, high and non-quantized values of channel resistances show
that the topological protection length (i.e. the distance on which the carriers
in helical edge channels propagate without backscattering) is much shorter than
the channel length, which is ~100 micrometers. The weak temperature dependence
of the resistance and the presence of temperature dependent reproducible
quasi-periodic resistance fluctuations can be qualitatively explained by the
presence of charge puddles in the well, to which the electrons from the edge
channels are tunnel-coupled.Comment: 8 pages, 4 figures, published versio
Temperature-driven single-valley Dirac fermions in HgTe quantum wells
We report on temperature-dependent magnetospectroscopy of two HgTe/CdHgTe
quantum wells below and above the critical well thickness . Our results,
obtained in magnetic fields up to 16 T and temperature range from 2 K to 150 K,
clearly indicate a change of the band-gap energy with temperature. The quantum
well wider than evidences a temperature-driven transition from
topological insulator to semiconductor phases. At the critical temperature of
90 K, the merging of inter- and intra-band transitions in weak magnetic fields
clearly specifies the formation of gapless state, revealing the appearance of
single-valley massless Dirac fermions with velocity of
ms. For both quantum wells, the energies extracted from
experimental data are in good agreement with calculations on the basis of the
8-band Kane Hamiltonian with temperature-dependent parameters.Comment: 5 pages, 3 figures and Supplemental Materials (4 pages
Excitonic effects in solids described by time-dependent density functional theory
Starting from the many-body Bethe-Salpeter equation we derive an
exchange-correlation kernel that reproduces excitonic effects in bulk
materials within time-dependent density functional theory. The resulting
accounts for both self-energy corrections and the electron-hole
interaction. It is {\em static}, {\em non-local} and has a long-range Coulomb
tail. Taking the example of bulk silicon, we show that the
divergency is crucial and can, in the case of continuum excitons, even be
sufficient for reproducing the excitonic effects and yielding excellent
agreement between the calculated and the experimental absorption spectrum.Comment: 6 pages, 1 figur
Refitting of combined inner detector and muon spectrometer tracks from Monte Carlo samples by using the Kalman fitter and the STEP algorithm in the ATLAS experiment
In this paper we refit combined muon tracks using the Kalman fitter and the simultaneous track and error propagation (STEP) algorithm of the ATLAS tracking software. The muon tracks are simulated by GEANT4 in the full detector description, reconstructed by MUID, and refitted by the Kalman fitter in the ATLAS TrackingGeometry. The relative transverse momentum resolution of the refitted tracks is compared to the resolution of the refits done by the global chi-square track fitter, along with the resolution found by the MUID and STACO muon combination algorithms. Reconstructed invariant masses are compared in a similar way
Massless Dirac fermions in III-V semiconductor quantum wells
We report on the clear evidence of massless Dirac fermions in two-dimensional
system based on III-V semiconductors. Using a gated Hall bar made on a
three-layer InAs/GaSb/InAs quantum well, we restore the Landau levels fan chart
by magnetotransport and unequivocally demonstrate a gapless state in our
sample. Measurements of cyclotron resonance at different electron
concentrations directly indicate a linear band crossing at the point
of Brillouin zone. Analysis of experimental data within analytical Dirac-like
Hamiltonian allows us not only determing velocity m/s of
massless Dirac fermions but also demonstrating significant non-linear
dispersion at high energies.Comment: Main text and Supplemental Materials, 14 pages, 9 figure
An improvement of the Berry--Esseen inequality with applications to Poisson and mixed Poisson random sums
By a modification of the method that was applied in (Korolev and Shevtsova,
2009), here the inequalities
and
are proved for the
uniform distance between the standard normal distribution
function and the distribution function of the normalized sum of an
arbitrary number of independent identically distributed random
variables with zero mean, unit variance and finite third absolute moment
. The first of these inequalities sharpens the best known version of
the classical Berry--Esseen inequality since
by virtue of
the condition , and 0.4785 is the best known upper estimate of the
absolute constant in the classical Berry--Esseen inequality. The second
inequality is applied to lowering the upper estimate of the absolute constant
in the analog of the Berry--Esseen inequality for Poisson random sums to 0.3051
which is strictly less than the least possible value of the absolute constant
in the classical Berry--Esseen inequality. As a corollary, the estimates of the
rate of convergence in limit theorems for compound mixed Poisson distributions
are refined.Comment: 33 page
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