244 research outputs found
High Frequency Conductivity in the Quantum Hall Regime
We have measured the complex conductivity of a two-dimensional
electron system in the quantum Hall regime up to frequencies of 6 GHz at
electron temperatures below 100 mK. Using both its imaginary and real part we
show that can be scaled to a single function for different
frequencies and for all investigated transitions between plateaus in the
quantum Hall effect. Additionally, the conductivity in the variable-range
hopping regime is used for a direct evaluation of the localization length
. Even for large filing factor distances from the critical
point we find with a scaling exponent
Hopping conductivity in the quantum Hall effect -- revival of universal scaling
We have measured the temperature dependence of the conductivity
of a two-dimensional electron system deep into the localized regime of the
quantum Hall plateau transition. Using variable-range hopping theory we are
able to extract directly the localization length from this experiment. We
use our results to study the scaling behavior of as a function of the
filling factor distance to the critical point of the transition.
We find for all samples a power-law behavior
with a universal scaling exponent as proposed theoretically
Conductance fluctuations at the quantum Hall plateau transition
We analyze the conductance fluctuations observed in the quantum Hall regime
for a bulk two-dimensional electron system in a Corbino geometry. We find that
characteristics like the power spectral density and the temperature dependence
agree well with simple expectations for universal conductance fluctuations in
metals, while the observed amplitude is reduced. In addition, the dephasing
length , which governs the temperature dependence of
the fluctuations, is surprisingly different from the scaling length
governing the width of the quantum Hall plateau
transition
Bimodal Counting Statistics in Single Electron Tunneling through a Quantum Dot
We explore the full counting statistics of single electron tunneling through
a quantum dot using a quantum point contact as non-invasive high bandwidth
charge detector. The distribution of counted tunneling events is measured as a
function of gate and source-drain-voltage for several consecutive electron
numbers on the quantum dot. For certain configurations we observe
super-Poissonian statistics for bias voltages at which excited states become
accessible. The associated counting distributions interestingly show a bimodal
characteristic. Analyzing the time dependence of the number of electron counts
we relate this to a slow switching between different electron configurations on
the quantum dot
Spin-dependent shot noise enhancement in a quantum dot
The spin-dependent dynamical blockade was investigated in a lateral quantum
dot in a magnetic field. Spin-polarized edge channels in the two-dimensional
leads and the spatial distribution of Landau orbitals in the dot modulate the
tunnel coupling of the quantum dot level spectrum. In a measurement of the
electron shot noise we observe a pattern of super-Poissonian noise which is
correlated to the spin-dependent competition between different transport
channels
High-order cumulants in the counting statistics of asymmetric quantum dots
Measurements of single electron tunneling through a quantum dot using a
quantum point contact as charge detector have been performed for very long time
traces with very large event counts. This large statistical basis is used for a
detailed examination of the counting statistics for varying symmetry of the
quantum dot system. From the measured statistics we extract high order
cumulants describing the distribution. Oscillations of the high order cumulants
are observed when varying the symmetry. We compare this behavior to the
observed oscillation in time dependence and show that the variation of both
system variables lead to the same kind of oscillating response.Comment: 3 page
Noise at a Fermi-edge singularity
We present noise measurements of self-assembled InAs quantum dots at high
magnetic fields. In comparison to I-V characteristics at zero magnetic field we
notice a strong current overshoot which is due to a Fermi-edge singularity. We
observe an enhanced suppression in the shot noise power simultaneous to the
current overshoot which is attributed to the electron-electron interaction in
the Fermi-edge singularity
Noise enhancement due to quantum coherence in coupled quantum dots
We show that the intriguing observation of noise enhancement in the charge
transport through two vertically coupled quantum dots can be explained by the
interplay of quantum coherence and strong Coulomb blockade. We demonstrate that
this novel mechanism for super-Poissonian charge transfer is very sensitive to
decoherence caused by electron-phonon scattering as inferred from the measured
temperature dependence.Comment: 4 pages, 3 figures, corrected version (Figs.2 and 3
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