423 research outputs found
Detection of non-Gaussian Fluctuations in a Quantum Point Contact
An experimental study of current fluctuations through a tunable transmission
barrier, a quantum point contact, are reported. We measure the probability
distribution function of transmitted charge with precision sufficient to
extract the first three cumulants. To obtain the intrinsic quantities,
corresponding to voltage-biased barrier, we employ a procedure that accounts
for the response of the external circuit and the amplifier. The third cumulant,
obtained with a high precision, is found to agree with the prediction for the
statistics of transport in the non-Poissonian regime.Comment: 4 pages, 4 figures; published versio
Measurement of Counting Statistics of Electron Transport in a Tunnel Junction
We present measurements of the time-dependent fluctuations in electrical
current in a voltage-biased tunnel junction. We were able to simultaneously
extract the first three moments of the tunnel current counting statistics.
Detailed comparison of the second and the third moment reveals that counting
statistics is accurately described by the Poissonian distribution expected for
spontaneous current fluctuations due to electron charge discreteness, realized
in tunneling transport at negligible coupling to environment.Comment: bibliography expande
On the Cooling of Electrons in a Silicon Inversion Layer
The cooling of two-dimensional electrons in silicon-metal-oxide semiconductor
field effect transistors is studied experimentally. Cooling to the lattice is
found to be more effective than expected from the bulk electron-phonon coupling
in silicon. Unexpectedly, the extracted heat transfer rate to phonons at low
temperatures depends cubically on electron temperature, suggesting that
piezoelectric coupling (absent in bulk silicon) dominates over deformation
potential. According to our findings, at 100 mK, electrons farther than 0.1 mm
from the contacts are mostly cooled by phonons. Using long devices and low
excitation voltage we measure electron resistivity down to 100 mK and find that
some of the "metallic" curves, reported earlier, turn insulating below about
300 mK. This finding renders the definition of the claimed 2D metal-insulator
transition questionable. Previous low temperature measurements in silicon
devices are analyzed and thumb rules for evaluating their electron temperatures
are provided.Comment: 5 pages, 4 figures. Discussion corrected and a few references adde
Colored noise in the fractional Hall effect: duality relations and exact results
We study noise in the problem of tunneling between fractional quantum Hall
edge states within a four probe geometry. We explore the implications of the
strong-weak coupling duality symmetry existent in this problem for relating the
various density-density auto-correlations and cross-correlations between the
four terminals. We identify correlations that transform as either ``odd'' or
``anti-symmetric'', or ``even'' or ``symmetric'' quantities under duality. We
show that the low frequency noise is colored, and that the deviations from
white noise are exactly related to the differential conductance. We show
explicitly that the relationship between the slope of the low frequency noise
spectrum and the differential conductance follows from an identity that holds
to {\it all} orders in perturbation theory, supporting the results implied by
the duality symmetry. This generalizes the results of quantum supression of the
finite frequency noise spectrum to Luttinger liquids and fractional statistics
quasiparticles.Comment: 14 pages, 3 figure
Effect of long-range Coulomb interaction on shot-noise suppression in ballistic transport
We present a microscopic analysis of shot-noise suppression due to long-range
Coulomb interaction in semiconductor devices under ballistic transport
conditions. An ensemble Monte Carlo simulator self-consistently coupled with a
Poisson solver is used for the calculations. A wide range of injection-rate
densities leading to different degrees of suppression is investigated. A sharp
tendency of noise suppression at increasing injection densities is found to
scale with a dimensionless Debye length related to the importance of
space-charge effects in the structure.Comment: RevTex, 4 pages, 4 figures, minor correction
Transport of a Luttinger liquid in the presence of a time dependent impurity
We show that the macroscopic current and charge can be formulated as a
Quantum Mechanical zero mode problem. We find that the current is given by the
velocity operator of a particle restricted to move around a circle. As an
explicit example we investigate a Luttinger liquid of length which is
perturbed by a time dependent impurity. Using the statistical mechanics of zero
modes we computed the non-equilibrium current. In particular we show that in
the low temperature limit, , the zero mode method introduced here
becomes essential for computing the current
Regulating Access to Adult Content (with Privacy Preservation)
In the physical world we have well-established mechanisms for keeping children out of adult-only areas. In the virtual world this is generally replaced by self declaration. Some service providers resort to using heavy-weight identification mechanisms, judging adulthood as a side effect thereof. Collection of identification data arguably constitutes an unwarranted privacy invasion in this context, if carried out merely to perform adulthood estimation. This paper presents a mechanism that exploits the adult's more extensive exposure to public media, relying on the likelihood that they will be able to recall details if cued by a carefully chosen picture. We conducted an online study to gauge the viability of this scheme. With our prototype we were able to predict that the user was a child 99% of the time. Unfortunately the scheme also misclassified too many adults. We discuss our results and suggest directions for future research
Nonuniversal behavior of scattering between fractional quantum Hall edges
Among the predicted properties of fractional quantum Hall states are
fractionally charged quasiparticles and conducting edge-states described as
chiral Luttinger liquids. In a system with a narrow constriction, tunneling of
quasi-particles between states at different edges can lead to resistance and to
shot noise. The ratio of the shot noise to the backscattered current, in the
weak scattering regime, measures the fractional charge of the quasi-particle,
which has been confirmed in several experiments. However, the non-linearity of
the resistance predicted by the chiral Luttinger liquid theory was apparently
not observed in some of these cases. As a possible explanation for these
discrepancies, we consider a model where a smooth edge profile leads to
formation of additional edge states. Coupling between the current carrying edge
mode and the additional phonon like mode can lead to {\it nonuniversal}
exponents in the current-voltage characteristic, while preserving the ratio
between shot noise and the back-scattered current, for weak backscattering. For
special values of the coupling, one may obtain a linear I-V behavior.Comment: 10 pages, 3 figure
Low frequency admittance of a quantum point contact
We present a current and charge conserving theory for the low frequency
admittance of a quantum point contact. We derive expressions for the
electrochemical capacitance and the displacement current. The latter is
determined by the {\em emittance} which equals the capacitance only in the
limit of vanishing transmission. With the opening of channels the capacitance
and the emittance decrease in a step-like manner in synchronism with the
conductance steps. For vanishing reflection, the capacitance vanishes and the
emittance is negative.Comment: 11 pages, revtex file, 2 ps figure
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