374 research outputs found
Dense Gas, Massive Stars, and Ionising Radiation: Simulating Stellar Feedback in Spiral-Arm Molecular Clouds
Star formation (SF) has been continuous since the Universe was 200 million years old. It occurs in the interstellar medium (ISM) – the gas and dust between stars within galaxies. The majority of SF occurs inside giant molecular clouds (GMCs) – the most massive agglomerations of dense gas within the ISM – typically the stars form in clusters. Initially the SF is governed solely by a GMC’s morphology, but, as stars form, the energy and momentum they inject into their surroundings – stellar feedback – affects ongoing star formation within the GMC. The effects of this feedback not only help to break up the cloud, but affect the wider ISM, and hence influence both neighbouring GMC evolution and future GMC formation. This thesis explores how two forms of stellar feedback – photoionisation and supernova (SN) – affect Milky Way-like spiral arm regions through the use of nu- merical hydrodynamic simulations. The numerical initial conditions are created by extracting a 500 pc2 region from simulations of whole galaxies. This means the simulations begin with a ‘realistic’ arrangement of neighbouring GMCs. The ISM is affected by the warm (104 K) HII regions that form and expand around massive photoionising stars and the hot (106 K) SNe ejecta that are emitted from the same stars at the end of their lifetimes. In these simulations photoionisation breaks GMCs and the denser clumps in their substructure up into a larger number of objects while, at the same time, increasing the total mass of dense ISM. This results in more rapid, and partially displaced, SF when compared with simulations without stellar feedback. The main cause of these effects is the compression of dense, but non-star forming, gas from multiple sides by HII regions. SNe have little effect on SF on spiral arm scales. However, SNe are able to heat large regions of the ISM to high temperatures, but only if the gas has already been exposed to photoionising feedback
Mapping the dynamic interactions between vortex species in highly anisotropic superconductors
Here we use highly sensitive magnetisation measurements performed using a
Hall probe sensor on single crystals of highly anisotropic high temperature
superconductors to study the dynamic interactions
between the two species of vortices that exist in such superconductors. We
observe a remarkable and clearly delineated high temperature regime that
mirrors the underlying vortex phase diagram. Our results map out the parameter
space over which these dynamic interaction processes can be used to create
vortex ratchets, pumps and other fluxonic devices.Comment: 7 pages, 3 figures, to be published in Supercond. Sci. Techno
Manipulation of magnetic-flux landscapes in superconducting Bi2Sr2CaCu2O8 + δ crystals
We demonstrate experimentally that the micromagnetic profile of the out-of-plane component of magnetic induction of layered superconductors, Bz, can be manipulated by varying the in-plane magnetic field, H∥. Moving Josephson vortices, confined between layers, drag pancake vortex stacks carrying out-of-plane flux, and the magnetic profile, Bz(x), can be controllably shaped across the entire sample. Depending on the magnetic history and temperature we can increase or decrease the out-of-plane flux density at the center and near the edges of the crystal by as much as 40%, realising both “convex and concave magnetic flux lenses”. Our experimental results are well described by molecular dynamics simulations.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/58115/2/epl_76_6_1151.pd
Continuum versus discrete flux behaviour in large mesoscopic Bi(2)Sr(2)CaCu(2)O(8+delta) disks
Scanning Hall probe and local Hall magnetometry measurements have been used
to investigate flux distributions in large mesoscopic superconducting disks
with sizes that lie near the crossover between the bulk and mesoscopic vortex
regimes. Results obtained by directly mapping the magnetic induction profiles
of the disks at different applied fields can be quite successfully fitted to
analytic models which assume a continuous distribution of flux in the sample.
At low fields, however, we do observe clear signatures of the underlying
discrete vortex structure and can resolve the characteristic mesoscopic
compression of vortex clusters in increasing magnetic fields. Even at higher
fields, where single vortex resolution is lost, we are still able to track
configurational changes in the vortex patterns, since competing vortex orders
impose unmistakable signatures on "local" magnetisation curves as a function of
the applied field. Our observations are in excellent agreement with molecular
dynamics numerical simulations which lead us to a natural definition of the
lengthscale for the crossover between discrete and continuum behaviours in our
system.Comment: Submitted to Europhysics Letter
Single donor ionization energies in a nanoscale CMOS channel
One consequence of the continued downwards scaling of transistors is the
reliance on only a few discrete atoms to dope the channel, and random
fluctuations of the number of these dopants is already a major issue in the
microelectonics industry. While single-dopant signatures have been observed at
low temperature, studying the impact of only one dopant up to room temperature
requires extremely small lengths. Here, we show that a single arsenic dopant
dramatically affects the off-state behavior of an advanced microelectronics
field effect transistor (FET) at room temperature. Furthermore, the ionization
energy of this dopant should be profoundly modified by the close proximity of
materials with a different dielectric constant than the host semiconductor. We
measure a strong enhancement, from 54meV to 108meV, of the ionization energy of
an arsenic atom located near the buried oxide. This enhancement is responsible
for the large current below threshold at room temperature and therefore
explains the large variability in these ultra-scaled transistors. The results
also suggest a path to incorporating quantum functionalities into silicon CMOS
devices through manipulation of single donor orbitals
Magnetoresistance of a 2-dimensional electron gas in a random magnetic field
We report magnetoresistance measurements on a two-dimensional electron gas
(2DEG) made from a high mobility GaAs/AlGaAs heterostructure, where the
externally applied magnetic field was expelled from regions of the
semiconductor by means of superconducting lead grains randomly distributed on
the surface of the sample. A theoretical explanation in excellent agreement
with the experiment is given within the framework of the semiclassical
Boltzmann equation.Comment: REVTEX 3.0, 11 pages, 3 Postscript figures appended. The manuscript
can also be obtained from our World Wide Web server:
http://roemer.fys.ku.dk/randmag.ht
Re-entrant resonant tunneling
We study the effect of electron-electron interactions on the
resonant-tunneling spectroscopy of the localized states in a barrier. Using a
simple model of three localized states, we show that, due to the Coulomb
interactions, a single state can give rise to two resonant peaks in the
conductance as a function of gate voltage, G(Vg). We also demonstrate that an
additional higher-order resonance with Vg-position in between these two peaks
becomes possibile when interactions are taken into account. The corresponding
resonant-tunneling process involves two-electron transitions. We have observed
both these effects in GaAs transistor microstructures by studying the time
evolution of three adjacent G(Vg) peaks caused by fluctuating occupation of an
isolated impurity (modulator). The heights of the two stronger peaks exibit
in-phase fluctuations. The phase of fluctuations of the smaller middle peak is
opposite. The two stronger peaks have their origin in the same localized state,
and the third one corresponds to a co-tunneling process.Comment: 9 pages, REVTeX, 4 figure
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