2,990 research outputs found
Ultraviolet C II and Si III Transit Spectroscopy and Modeling of the Evaporating Atmosphere of GJ436b
Hydrogen gas evaporating from the atmosphere of the hot-Neptune GJ436b
absorbs over 50% of the stellar Ly emission during transit. Given the
planet's atmospheric composition and energy-limited escape rate, this hydrogen
outflow is expected to entrain heavier atoms such as C and O. We searched for C
and Si in the escaping atmosphere of GJ436b using far-ultraviolet HST COS G130M
observations made during the planet's extended H I transit. These observations
show no transit absorption in the C II 1334,1335 \AA\ and Si III 1206 \AA\
lines integrated over [-100, 100] km s, imposing 95% (2) upper
limits of 14% (C II) and 60% (Si III) depth on the transit of an opaque disk
and 22% (C II) and 49% (Si III) depth on an extended, highly asymmetric transit
similar to that of H I Ly. C is likely present in the outflow
according to a simulation we carried out using a spherically-symmetric,
photochemical-hydrodynamical model. This simulation predicts a 2% transit
over the integrated bandpass, consistent with the data. At line center, we
predict the C II transit depth to be as high as 19%. Our model predicts a
neutral hydrogen escape rate of g s (
g s for all species) for an upper atmosphere composed of hydrogen and
helium.Comment: 7 pages, 4 figures, 1 table; accepted to ApJ Letter
Spin Density Matrix of Spin-3/2 Hole Systems
For hole systems with an effective spin j=3/2, we present an invariant
decomposition of the spin density matrix that can be interpreted as a multipole
expansion. The charge density corresponds to the monopole moment and the spin
polarization due to a magnetic field corresponds to a dipole moment while heavy
hole-light hole splitting can be interpreted as a quadrupole moment. For quasi
two-dimensional hole systems in the presence of an in-plane magnetic field B
the spin polarization is a higher-order effect that is typically much smaller
than one even if the minority spin subband is completely depopulated. On the
other hand, the field B can induce a substantial octupole moment which is a
unique feature of j=3/2 hole systems.Comment: 8 pages, 1 figure, 3 table
Enhancement of bulk second-harmonic generation from silicon nitride films by material composition
We present a comprehensive tensorial characterization of second-harmonic
generation from silicon nitride films with varying composition. The samples
were fabricated using plasma-enhanced chemical vapor deposition, and the
material composition was varied by the reactive gas mixture in the process. We
found a six-fold enhancement between the lowest and highest second-order
susceptibility, with the highest value of approximately 5 pm/V from the most
silicon-rich sample. Moreover, the optical losses were found to be sufficiently
small (below 6 dB/cm) for applications. The tensorial results show that all
samples retain in-plane isotropy independent of silicon content, highlighting
the controllability of the fabrication process.Comment: 4 pages, 3 figures, 2 tables; Re-submitted to Optics Letter
Magnetism in one-dimensional quantum dot arrays
We employ the density functional Kohn-Sham method in the local spin-density
approximation to study the electronic structure and magnetism of quasi
one-dimensional periodic arrays of few-electron quantum dots. At small values
of the lattice constant, the single dots overlap, forming a non-magnetic
quantum wire with nearly homogenous density. As the confinement perpendicular
to the wire is increased, i.e. as the wire is squeezed to become more
one-dimensional, it undergoes a spin-Peierls transition. Magnetism sets in as
the quantum dots are placed further apart. It is determined by the electronic
shell filling of the individual quantum dots. At larger values of the lattice
constant, the band structure for odd numbers of electrons per dot indicates
that the array could support spin-polarized transport and therefore act as a
spin filter.Comment: 11 pages, 6 figure
The First Year IceCube-DeepCore Results
The IceCube Neutrino Observatory includes a tightly spaced inner array in the
deepest ice, called DeepCore, which gives access to low-energy neutrinos with a
sizable surrounding cosmic ray muon veto. Designed to be sensitive to neutrinos
at energies as low as 10 GeV, DeepCore will be used to study diverse physics
topics with neutrino signatures, such as dark matter annihilations and
atmospheric neutrino oscillations. The first year of DeepCore physics
data-taking has been completed, and the first observation of atmospheric
neutrino-induced cascades with IceCube and DeepCore are presented.Comment: 4 pages, 3 figures, TAUP 2011 (Journal of Physics: Conference Series
(JCPS)
Universal vortex formation in rotating traps with bosons and fermions
When a system consisting of many interacting particles is set rotating, it
may form vortices. This is familiar to us from every-day life: you can observe
vortices while stirring your coffee or watching a hurricane. In the world of
quantum mechanics, famous examples of vortices are superconducting films and
rotating bosonic He or fermionic He liquids. Vortices are also observed
in rotating Bose-Einstein condensates in atomic traps and are predicted to
exist for paired fermionic atoms. Here we show that the rotation of trapped
particles with a repulsive interaction leads to a similar vortex formation,
regardless of whether the particles are bosons or (unpaired) fermions. The
exact, quantum mechanical many-particle wave function provides evidence that in
fact, the mechanism of this vortex formation is the same for boson and fermion
systems.Comment: 4 pages, 4 figure
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