10,504 research outputs found
Tracing the development of dust around evolved stars: The case of 47 Tuc
We observed mid-infrared (7.5-22 mum) spectra of AGB stars in the globular
cluster 47 Tuc with the Spitzer telescope and find significant dust features of
various types. Comparison of the characteristics of the dust spectra with the
location of the stars in a logP-K-diagram shows that dust mineralogy and
position on the AGB are related. A 13 mum feature is seen in spectra of low
luminosity AGB stars. More luminous AGB stars show a broad feature at 11.5 mum.
The spectra of the most luminous stars are dominated by the amorphous silicate
bending vibration centered at 9.7 mum. For 47 Tuc AGB stars, we conclude that
early on the AGB dust consisting primarily of Mg-, Al- and Fe oxides is formed.
With further AGB evolution amorphous silicates become the dominant species.Comment: 2 figures, accepted for publication in ApJ Letter
Ultrasound sensing using the acousto-optic effect in polymer dispersed liquid crystals
Acousto-optic effects are demonstrated in polymer dispersed liquid crystal (PDLC) films, showing promise for applications in ultrasound sensing. The PDLC films are used to image two displacement profiles of an air-coupled flexural transducers resonant modes at 295 kHz and 730 kHz. Results are confirmed using laser vibrometry. The regions on the transducers with the largest displacements are clearly imaged by the PDLC films, with the resolution agreeing well with laser vibrometry scanning. Imaging takes significantly less time than a scanning system (switching time of a few seconds, as compared to 8 hours for laser vibrometry). Heating effects are carefully monitored using thermal imaging, and are found not to be the main cause of PDLC clearing
Ultrasound sensing using the acousto-optic effect in polymer dispersed liquid crystals
Electronic structure and chemical bonding in Ti2AlC investigated by soft x-ray emission spectroscopy
The electronic structure of the nanolaminated transition metal carbide Ti2AlC
has been investigated by bulk-sensitive soft x-ray emission spectroscopy. The
measured Ti L, C K and Al L emission spectra are compared with calculated
spectra using ab initio density-functional theory including dipole matrix
elements. The detailed investigation of the electronic structure and chemical
bonding provides increased understanding of the physical properties of this
type of nanolaminates. Three different types of bond regions are identified;
the relatively weak Ti 3d - Al 3p hybridization 1 eV below the Fermi level, and
the Ti 3d - C 2p and Ti 3d - C 2s hybridizations which are stronger and deeper
in energy are observed around 2.5 eV and 10 eV below the Fermi level,
respectively. A strongly modified spectral shape of the 3s final states in
comparison to pure Al is detected for the buried Al monolayers indirectly
reflecting the Ti 3d - Al 3p hybridization. The differences between the
electronic and crystal structures of Ti2AlC, Ti3AlC2 and TiC are discussed in
relation to the number of Al layers per Ti layer in the two former systems and
the corresponding change of the unusual materials properties.Comment: 14 pages, 7 figures; PACS:78.70.En, 71.15.Mb, 71.20.-
Dust in the Photospheric Environment: Unified Cloudy Models of M, L, and T Dwarfs
We address the problem of how dust forms and how it could be sustained in the
static photospheres of cool dwarfs for a long time. In the cool and dense gas,
dust forms easily at the condensation temperature, T_cond, and the dust can be
in detailed balance with the ambient gas so long as it remains smaller than the
critical radius, r_cr. However, dust will grow larger and segregate from the
gas when it will be larger than r_cr somewhere at the lower temperature, which
we refer to as the critical temperature, T_cr. Then, the large dust grains will
precipitate below the photosphere and only the small dust grains in the region
of T_cr < T < T_cond can be sustained in the photosphere. Thus a dust cloud is
formed. Incorporating the dust cloud, non-grey model photo- spheres in
radiative-convective equilibrium are extended to T_eff as low as 800K. Observed
colors and spectra of cool dwarfs can consistently be accounted for by a single
grid of our cloudy models. This fact in turn can be regarded as supporting
evidence for our basic assumption on the cloud formation.Comment: 50 pages with 14 postscript figures, to be published in Astrophys.
Experimental electronic heat capacities of and Plutonium; heavy-fermion physics in an element
We have measured the heat capacities of PuAl and
Pu over the temperature range 2-303 K. The availability of data below
10 K plus an estimate of the phonon contribution to the heat capacity based on
recent neutron-scattering experiments on the same sample enable us to make a
reliable deduction of the electronic contribution to the heat capacity of
PuAl; we find
mJKmol as . This is a factor larger than that
of any element, and large enough for PuAl to be
classed as a heavy-fermion system. By contrast,
mJKmol in Pu. Two distinct anomalies are seen in the
electronic contribution to the heat capacity of
PuAl, one or both of which may be associated with
the formation of the martensitic phase. We suggest that the large
-value of PuAl may be caused by proximity to
a quantum-critical point.Comment: 4 pages, 4 figure
Two-axis control of a singlet-triplet qubit with an integrated micromagnet
The qubit is the fundamental building block of a quantum computer. We
fabricate a qubit in a silicon double quantum dot with an integrated
micromagnet in which the qubit basis states are the singlet state and the
spin-zero triplet state of two electrons. Because of the micro magnet, the
magnetic field difference between the two sides of the double dot is
large enough to enable the achievement of coherent rotation of the qubit's
Bloch vector about two different axes of the Bloch sphere. By measuring the
decay of the quantum oscillations, the inhomogeneous spin coherence time
is determined. By measuring at many different values of
the exchange coupling and at two different values of , we provide
evidence that the micromagnet does not limit decoherence, with the dominant
limits on arising from charge noise and from coupling to nuclear
spins.Comment: 10 pages, 9 figure
Enhancement mode double top gated MOS nanostructures with tunable lateral geometry
We present measurements of silicon (Si) metal-oxide-semiconductor (MOS)
nanostructures that are fabricated using a process that facilitates essentially
arbitrary gate geometries. Stable Coulomb blockade behavior free from the
effects of parasitic dot formation is exhibited in several MOS quantum dots
with an open lateral quantum dot geometry. Decreases in mobility and increases
in charge defect densities (i.e. interface traps and fixed oxide charge) are
measured for critical process steps, and we correlate low disorder behavior
with a quantitative defect density. This work provides quantitative guidance
that has not been previously established about defect densities for which Si
quantum dots do not exhibit parasitic dot formation. These devices make use of
a double-layer gate stack in which many regions, including the critical gate
oxide, were fabricated in a fully-qualified CMOS facility.Comment: 11 pages, 6 figures, 3 tables, accepted for publication in Phys. Rev.
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