3,987 research outputs found
Evidence for localization and 0.7 anomaly in hole quantum point contacts
Quantum point contacts implemented in p-type GaAs/AlGaAs heterostructures are
investigated by low-temperature electrical conductance spectroscopy
measurements. Besides one-dimensional conductance quantization in units of
a pronounced extra plateau is found at about which
possesses the characteristic properties of the so-called "0.7 anomaly" known
from experiments with n-type samples. The evolution of the 0.7 plateau in high
perpendicular magnetic field reveals the existence of a quasi-localized state
and supports the explanation of the 0.7 anomaly based on self-consistent charge
localization. These observations are robust when lateral electrical fields are
applied which shift the relative position of the electron wavefunction in the
quantum point contact, testifying to the intrinsic nature of the underlying
physics.Comment: 4.2 pages, 3 figure
Local Current Distribution and "Hot Spots" in the Integer Quantum Hall Regime
In a recent experiment, the local current distribution of a two-dimensional
electron gas in the quantum Hall regime was probed by measuring the variation
of the conductance due to local gating. The main experimental finding was the
existence of "hot spots", i.e. regions with high degree of sensitivity to local
gating, whose density increases as one approaches the quantum Hall transition.
However, the direct connection between these "hot spots" and regions of high
current flow is not clear. Here, based on a recent model for the quantum Hall
transition consisting of a mixture of perfect and quantum links, the relation
between the "hot spots" and the current distribution in the sample has been
investigated. The model reproduces the observed dependence of the number and
sizes of "hot spots" on the filling factor. It is further demonstrated that
these "hot spots" are not located in regions where most of the current flows,
but rather, in places where the currents flow both when injected from the left
or from the right. A quantitative measure, the harmonic mean of these currents
is introduced and correlates very well with the "hot spots" positions
Origins of conductance anomalies in a p-type GaAs quantum point contact
Low temperature transport measurements on a p-GaAs quantum point contact are
presented which reveal the presence of a conductance anomaly that is markedly
different from the conventional `0.7 anomaly'. A lateral shift by asymmetric
gating of the conducting channel is utilized to identify and separate different
conductance anomalies of local and generic origins experimentally. While the
more generic 0.7 anomaly is not directly affected by changing the gate
configuration, a model is proposed which attributes the additional conductance
features to a gate-dependent coupling of the propagating states to localized
states emerging due to a nearby potential imperfection. Finite bias
conductivity measurements reveal the interplay between the two anomalies
consistently with a two-impurity Kondo model
Why Modern Open Source Projects Fail
Open source is experiencing a renaissance period, due to the appearance of
modern platforms and workflows for developing and maintaining public code. As a
result, developers are creating open source software at speeds never seen
before. Consequently, these projects are also facing unprecedented mortality
rates. To better understand the reasons for the failure of modern open source
projects, this paper describes the results of a survey with the maintainers of
104 popular GitHub systems that have been deprecated. We provide a set of nine
reasons for the failure of these open source projects. We also show that some
maintenance practices -- specifically the adoption of contributing guidelines
and continuous integration -- have an important association with a project
failure or success. Finally, we discuss and reveal the principal strategies
developers have tried to overcome the failure of the studied projects.Comment: Paper accepted at 25th International Symposium on the Foundations of
Software Engineering (FSE), pages 1-11, 201
A chain rule for the expected suprema of Gaussian processes
The expected supremum of a Gaussian process indexed by the image of an index
set under a function class is bounded in terms of separate properties of the
index set and the function class. The bound is relevant to the estimation of
nonlinear transformations or the analysis of learning algorithms whenever
hypotheses are chosen from composite classes, as is the case for multi-layer
models
The Effects of Resonant Tunneling on Magnetoresistance through a Q uantum Dot
The effect of resonant tunneling on magnetoresistance (MR) is studied
theoretically in a double junction system. We have found that the ratio of the
MR of the resonant peak current is reduced more than that of the single
junction, whereas that of the valley current is enhanced depending on the
change of the discrete energy-level under the change of magnetic field. We also
found that the peak current-valley current (PV) ratio decreases when the
junction conductance increases.Comment: 11 pages, 3 figures(mail if you need), use revtex.st
Effect of Quantum Confinement on Electron Tunneling through a Quantum Dot
Employing the Anderson impurity model, we study tunneling properties through
an ideal quantum dot near the conductance minima. Considering the Coulomb
blockade and the quantum confinement on an equal footing, we have obtained
current contributions from various types of tunneling processes; inelastic
cotunneling, elastic cotunneling, and resonant tunneling of thermally activated
electrons. We have found that the inelastic cotunneling is suppressed in the
quantum confinement limit, and thus the conductance near its minima is
determined by the elastic cotunneling at low temperature (,
: dot-reservoir coupling constant), or by the resonant tunneling of
single electrons at high temperature ().Comment: 11 pages Revtex, 2 Postscript figures, To appear in Phys.Rev.
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