123,174 research outputs found
Surface Acoustic Wave induced Transport in a Double Quantum Dot
We report on non-adiabatic transport through a double quantum dot under
irradiation of surface acoustic waves generated on-chip. At low excitation
powers, absorption and emission of single and multiple phonons is observed. At
higher power, sequential phonon assisted tunneling processes excite the double
dot in a highly non-equilibrium state. The present system is attractive for
studying electron-phonon interaction with piezoelectric coupling.Comment: 4 pages, 3 figure
Comparative study of Cl₂, Cl₂/O₂, and Cl₂/N₂ inductively coupled plasma processes for etching of high-aspect-ratio photonic-crystal holes in InP
An extensive investigation has been performed on inductively coupled plasmaetching of InP. An important motivation for this work is the fabrication of high-aspect-ratio holes for photonic crystals. The essential chemistry is based on Cl₂ with the addition of N₂ or O₂ for sidewall passivation. The influence of different process parameters such as gas flows, temperature,pressure, ion energy, and inductively coupled plasma power on the hole geometry is presented. It is concluded that photonic crystals can be etched with Cl₂ only; however, temperature and pressure control is critical. Adding passivation gases largely broadens the window in the parameter space for hole etching. Most importantly, etching of narrow holes can be carried out at higher temperatures where the etching is mass limited and spontaneous etching of InP by Cl₂ occurs.Part of this research is supported by NanoNed, a technology program of the Dutch
Ministry of Economic Affairs
OH-selected AGB and post-AGB stellar objects II.Blue versus red evolution off the AGB
Using objects found in a systematic survey of the galactic Plane in the
1612-MHz OH line, we discuss in detail two ``sequences'' of post-AGB evolution,
a red and a blue. We argue that the red and the blue groups separate by initial
mass at 4Msun, based on evolutionary-sequence turn-off colours, spectral energy
distributions, outflow velocities and scaleheight. The higher-mass (blue)
objects may have earlier AGB termination. The lower-mass (red) objects undergo
very sudden reddening for IRAS colour R21\sim1.2; these sources must all
undergo a very similar process at AGB termination. The transition colour
corresponds to average initial masses of 1.7Msun. A combined IRAS-MSX colour
proves a very sensitive tool to distinguish lower-mass, early post-AGB objects
from sources still on the AGB and also to distinguish more evolved post-AGB
objects from star-forming regions. The high-mass blue objects are the likely
precursors of bipolar planetary nebulae, whereas the low-mass red objects will
evolve into elliptical planetary nebulae.Comment: 12 pages, LaTex, 7 figures (1 colour), AJ (accepted
Spin and spatial dynamics in electron-impact scattering off S-wave He using R-matrix with Time-Dependence theory
R-matrix with time-dependence theory is applied to electron-impact ionisation
processes for He in the S-wave model. Cross sections for electron-impact
excitation, ionisation and ionisation with excitation for impact energies
between 25 and 225 eV are in excellent agreement with benchmark cross sections.
Ultra-fast dynamics induced by a scattering event is observed through
time-dependent signatures associated with autoionisation from doubly excited
states. Further insight into dynamics can be obtained through examination of
the spin components of the time-dependent wavefunction.Comment: 6 pages, 5 figure
Pathogenic mutations in the hydrophobic core of the human prion protein can promote structural instability and misfolding
Transmissible spongiform encephalopathies, or prion diseases, are caused by misfolding and aggregation of the prion protein PrP. These diseases can be hereditary in humans and four of the many disease-associated missense mutants of PrP are in the hydrophobic core: V180I, F198S, V203I and V210I. The T183A mutation is related to the hydrophobic core mutants as it is close to the hydrophobic core and known to cause instability. We have performed extensive molecular dynamics simulations of these five PrP mutants and compared their dynamics and conformations to wild-type PrP. The simulations highlight the changes that occur upon introduction of mutations and help to rationalize experimental findings. Changes can occur around the mutation site, but they can also be propagated over long distances. In particular, the F198S and T183A mutations lead to increased flexibility in parts of the structure that are normally stable, and the short β-sheet moves away from the rest of the protein. Mutations V180I, V210I and, to a lesser extent, V203I cause changes similar to those observed upon lowering the pH, which has been linked to misfolding. Early misfolding is observed in one V180I simulation. Overall, mutations in the hydrophobic core have a significant effect on the dynamics and stability of PrP, including the propensity to misfold, which helps to explain their role in the development of familial prion diseases
The Timing Properties of Sco X-1 along its Z track as seen with EXOSAT
We present a systematic, homogeneous analysis of all the EXOSAT ME, high time
resolution data on Sco X-1. We investigate all power spectral properties of the
100 Hz quasi-periodic oscillations (QPO) and noise of Sco X-1 as a function
of position on the Z-shaped track traced out in the X-ray colour-colour
diagram. Generally, the properties of Sco X-1 vary smoothly as a function of
position along the Z track. However, some variability parameters change
abruptly at either of the vertices of the Z track indicating that the branches
of the Z track represent distinct source states not only in spectral state but
also in rapid variability characteristics. We show that the very low frequency
noise (VLFN) is consistent with being solely due to motion along the Z track.
The QPO show a remarkably rapid change in frequency at or just before the
normal-branch/flaring-branch (NB/FB) vertex. This transition happens within
1.5% of the entire extent of the Z track. The QPO themselves are visible for
17% of the Z. We find a new sort of behaviour near the NB/FB vertex, i.e.,
rapid excursions from the NB into the FB and back again taking only a few
minutes.Comment: 30 pages, 15 figures, MNRAS in pres
The connection between noise and quantum correlations in a double quantum dot
We investigate the current and noise characteristics of a double quantum dot
system. The strong correlations induced by the Coulomb interaction create
entangled two-electron states and lead to signatures in the transport
properties. We show that the interaction parameter phi, which measures the
admixture of the double-occupancy contribution to the singlet state and thus
the degree of entanglement, can be directly accessed through the Fano factor of
super-Poissonian shot noise.Comment: 5 pages, major revision, to be published in Phys. Rev.
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