1,168 research outputs found
Dephasing in Quantum Dots: Quadratic Coupling to Acoustic Phonons
A microscopic theory of optical transitions in quantum dots with
carrier-phonon interaction is developed. Virtual transitions into higher
confined states with acoustic phonon assistance add a quadratic phonon coupling
to the standard linear one, thus extending the independent Boson model. Summing
infinitely many diagrams in the cumulant, a numerically exact solution for the
interband polarization is found. Its full time dependence and the absorption
lineshape of the quantum dot are calculated. It is the quadratic interaction
which gives rise to a temperature-dependent broadening of the zero-phonon line,
being here calculated for the first time in a consistent scheme.Comment: 4 pages, 2 figure
1/f noise of Josephson-junction-embedded microwave resonators at single photon energies and millikelvin temperatures
We present measurements of 1/f frequency noise in both linear and
Josephson-junction-embedded superconducting aluminum resonators in the low
power, low temperature regime - typical operating conditions for
superconducting qubits. The addition of the Josephson junction does not result
in additional frequency noise, thereby placing an upper limit for fractional
critical current fluctuations of (Hz) at 1 Hz for
sub-micron, shadow evaporated junctions. These values imply a minimum dephasing
time for a superconducting qubit due to critical current noise of 40 -- 1400
s depending on qubit architecture. Occasionally, at temperatures above 50
mK, we observe the activation of individual fluctuators which increase the
level of noise significantly and exhibit Lorentzian spectra
K-Band Spectroscopy of an Obscured Massive Stellar Cluster in the Antennae Galaxies (NGC 4038/4039) with NIRSPEC
We present infrared spectroscopy of the Antennae Galaxies (NGC 4038/4039)
with NIRSPEC at the W. M. Keck Observatory. We imaged the star clusters in the
vicinity of the southern nucleus (NGC 4039) in 0.39" seeing in K-band using
NIRSPEC's slit-viewing camera. The brightest star cluster revealed in the
near-IR (M_K(0) = -17.9) is insignificant optically, but coincident with the
highest surface brightness peak in the mid-IR (12-18 um) ISO image presented by
Mirabel et al (1998). We obtained high signal-to-noise 2.03-2.45 um spectra of
the nucleus and the obscured star cluster at R = 1900.
The cluster is very young (age ~ 4 Myr), massive (M ~ 16E6 M_sun), and
compact (density ~ 115 M_sun pc^(-3) within a 32 pc half-light radius),
assuming a Salpeter IMF (0.1-100 M_sun). Its hot stars have a radiation field
characterized by T_eff ~ 39,000 K, and they ionize a compact HII region with
n_e ~ 10^4 cm^(-3). The stars are deeply embedded in gas and dust (A_V = 9-10
mag), and their strong FUV field powers a clumpy photodissociation region with
densities n_H > 10^5 cm^(-3) on scales of ~ 200 pc, radiating L{H_2 1-0 S(1)}=
9600 L_sun.Comment: 4 pages, 4 embedded figures, uses emulateapj.sty. To appear in ApJL.
Also available at http://astro.berkeley.edu/~agilber
Hot Stars and Cool Clouds: The Photodissociation Region M16
We present high-resolution spectroscopy and images of a photodissociation
region (PDR) in M16 obtained during commissioning of NIRSPEC on the Keck II
telescope. PDRs play a significant role in regulating star formation, and M16
offers the opportunity to examine the physical processes of a PDR in detail. We
simultaneously observe both the molecular and ionized phases of the PDR and
resolve the spatial and kinematic differences between them. The most prominent
regions of the PDR are viewed edge-on. Fluorescent emission from nearby stars
is the primary excitation source, although collisions also preferentially
populate the lowest vibrational levels of H2. Variations in density-sensitive
emission line ratios demonstrate that the molecular cloud is clumpy, with an
average density n = 3x10^5 cm^(-3). We measure the kinetic temperature of the
molecular region directly and find T_H2 = 930 K. The observed density,
temperature, and UV flux imply a photoelectric heating efficiency of 4%. In the
ionized region, n_i=5x10^3 cm^(-3) and T_HII = 9500 K. In the brightest regions
of the PDR, the recombination line widths include a non-thermal component,
which we attribute to viewing geometry.Comment: 5 pages including 2 Postscript figures. To appear in ApJ Letters,
April 200
Deep Mid-Infrared Silicate Absorption as a Diagnostic of Obscuring Geometry Toward Galactic Nuclei
The silicate cross section peak near 10um produces emission and absorption
features in the spectra of dusty galactic nuclei observed with the Spitzer
Space Telescope. Especially in ultraluminous infrared galaxies, the observed
absorption feature can be extremely deep, as IRAS 08572+3915 illustrates. A
foreground screen of obscuration cannot reproduce this observed feature, even
at large optical depth. Instead, the deep absorption requires a nuclear source
to be deeply embedded in a smooth distribution of material that is both
geometrically and optically thick. In contrast, a clumpy medium can produce
only shallow absorption or emission, which are characteristic of
optically-identified active galactic nuclei. In general, the geometry of the
dusty region and the total optical depth, rather than the grain composition or
heating spectrum, determine the silicate feature's observable properties. The
apparent optical depth calculated from the ratio of line to continuum emission
generally fails to accurately measure the true optical depth. The obscuring
geometry, not the nature of the embedded source, also determines the far-IR
spectral shape.Comment: To appear in ApJ
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