1,014 research outputs found
Optical control of internal electric fields in band-gap graded InGaN nanowires
InGaN nanowires are suitable building blocks for many future optoelectronic
devices. We show that a linear grading of the indium content along the nanowire
axis from GaN to InN introduces an internal electric field evoking a
photocurrent. Consistent with quantitative band structure simulations we
observe a sign change in the measured photocurrent as a function of photon
flux. This negative differential photocurrent opens the path to a new type of
nanowire-based photodetector. We demonstrate that the photocurrent response of
the nanowires is as fast as 1.5 ps
The parabolic Anderson model and long-range percolation
This thesis has two parts. The first part deals with the parabolic Anderson model, which is a stochastic differential equation. It models the evolution of a field of particles performing independent simple random walks with binary branching. The focus of this work is on the exponential growth rate of the solution, where several basic properties are derived. The second part deals with two long-range percolation models. The occupied set of the first model is obtained by taking the union of a collection of independent Brownian motion running up to time t whose initial positions are distributed according to a Poisson Point process. Basic properties such as existence of a percolation phase transition and the uniqueness of the unbounded cluster are proven. The second model is the model of random interlacements. It is shown that the vacant set of random interlacement is transient for almost all values of the supercritical phase as the dimension tends to infinity.UBL - phd migration 201
The Complex Langevin method: When can it be trusted?
We analyze to what extent the complex Langevin method, which is in principle
capable of solving the so-called sign problems, can be considered as reliable.
We give a formal derivation of the correctness and then point out various
mathematical loopholes. The detailed study of some simple examples leads to
practical suggestions about the application of the method.Comment: 14 pages, including several eps figures and tables; clarification and
minor corrections added, to appear in PR
Chemical Chaperones Improve Protein Secretion and Rescue Mutant Factor VIII in Mice with Hemophilia A.
nefficient intracellular protein trafficking is a critical issue in the pathogenesis of a variety of diseases and in recombinant protein production. Here we investigated the trafficking of factor VIII (FVIII), which is affected in the coagulation disorder hemophilia A. We hypothesized that chemical chaperones may be useful to enhance folding and processing of FVIII in recombinant protein production, and as a therapeutic approach in patients with impaired FVIII secretion. A tagged B-domain-deleted version of human FVIII was expressed in cultured Chinese Hamster Ovary cells to mimic the industrial production of this important protein. Of several chemical chaperones tested, the addition of betaine resulted in increased secretion of FVIII, by increasing solubility of intracellular FVIII aggregates and improving transport from endoplasmic reticulum to Golgi. Similar results were obtained in experiments monitoring recombinant full-length FVIII. Oral betaine administration also increased FVIII and factor IX (FIX) plasma levels in FVIII or FIX knockout mice following gene transfer. Moreover, in vitro and in vivo applications of betaine were also able to rescue a trafficking-defective FVIII mutant (FVIIIQ305P). We conclude that chemical chaperones such as betaine might represent a useful treatment concept for hemophilia and other diseases caused by deficient intracellular protein trafficking
Real-time dynamics of lattice gauge theories with a few-qubit quantum computer
Gauge theories are fundamental to our understanding of interactions between
the elementary constituents of matter as mediated by gauge bosons. However,
computing the real-time dynamics in gauge theories is a notorious challenge for
classical computational methods. In the spirit of Feynman's vision of a quantum
simulator, this has recently stimulated theoretical effort to devise schemes
for simulating such theories on engineered quantum-mechanical devices, with the
difficulty that gauge invariance and the associated local conservation laws
(Gauss laws) need to be implemented. Here we report the first experimental
demonstration of a digital quantum simulation of a lattice gauge theory, by
realising 1+1-dimensional quantum electrodynamics (Schwinger model) on a
few-qubit trapped-ion quantum computer. We are interested in the real-time
evolution of the Schwinger mechanism, describing the instability of the bare
vacuum due to quantum fluctuations, which manifests itself in the spontaneous
creation of electron-positron pairs. To make efficient use of our quantum
resources, we map the original problem to a spin model by eliminating the gauge
fields in favour of exotic long-range interactions, which have a direct and
efficient implementation on an ion trap architecture. We explore the Schwinger
mechanism of particle-antiparticle generation by monitoring the mass production
and the vacuum persistence amplitude. Moreover, we track the real-time
evolution of entanglement in the system, which illustrates how particle
creation and entanglement generation are directly related. Our work represents
a first step towards quantum simulating high-energy theories with atomic
physics experiments, the long-term vision being the extension to real-time
quantum simulations of non-Abelian lattice gauge theories
Magnetic field imaging with atomic Rb vapor
We demonstrate the possibility of dynamic imaging of magnetic fields using
electromagnetically induced transparency in an atomic gas. As an experimental
demonstration we employ an atomic Rb gas confined in a glass cell to image the
transverse magnetic field created by a long straight wire. In this arrangement,
which clearly reveals the essential effect, the field of view is about 2 x 2
mm^2 and the field detection uncertainty is 0.14 mG per 10 um x 10 um image
pixel.Comment: 4 pages, 3 figure
Photoactivation experiment on 197Au and its implications for the dipole strength in heavy nuclei
The 197Au(gamma,n) reaction is used as an activation standard for
photodisintegration studies on astrophysically relevant nuclei. At the
bremsstrahlung facility of the superconducting electron accelerator ELBE
(Electron Linear accelerator of high Brilliance and low Emittance) of
Forschungszentrum Dresden-Rossendorf, photoactivation measurements on 197Au
have been performed with bremsstrahlung endpoint energies from 8.0 to 15.5 MeV.
The measured activation yield is compared with previous experiments as well as
with calculations using Hauser-Feshbach statistical models. It is shown that
the experimental data are best described by a two-Lorentzian parametrization
with taking the axial deformation of 197Au into account. The experimental
197Au(gamma,n) reaction yield measured at ELBE via the photoactivation method
is found to be consistent with previous experimental data using photon
scattering or neutron detection methods.Comment: 9 page
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