16,451 research outputs found
WFPC2 Observations of NGC 454: an Interacting Pair of Galaxies
We present WFPC2 images in the F450W, F606W and F814W filters of the
interacting pair of galaxies NGC 454. Our data indicate that the system is in
the early stages of interaction. A population of young star-clusters has formed
around the late component, and substantial amounts of gas have sunk into the
center of the earlier component, where it has not yet produced significant
visible star formation or nuclear activity. We have photometric evidence that
the star-clusters have strong line emission, which indicate the presence of a
substantial component of hot, massive stars which formed less than 5-10 Myrs
ago.Comment: 14 pages, 4 figures, Latex (AAS macros), ApJL in pres
Structural and electronic properties of Li intercalated graphene on SiC(0001)
We investigate the structural and electronic properties of Li-intercalated
monolayer graphene on SiC(0001) using combined angle-resolved photoemission
spectroscopy and first-principles density functional theory. Li intercalates at
room temperature both at the interface between the buffer layer and SiC and
between the two carbon layers. The graphene is strongly -doped due to charge
transfer from the Li atoms and two -bands are visible at the
-point. After heating the sample to 300C, these -bands
become sharp and have a distinctly different dispersion to that of
Bernal-stacked bilayer graphene. We suggest that the Li atoms intercalate
between the two carbon layers with an ordered structure, similar to that of
bulk LiC. An AA-stacking of these two layers becomes energetically
favourable. The -bands around the -point closely resemble the
calculated band structure of a CLiC system, where the intercalated Li
atoms impose a super-potential on the graphene electronic structure that opens
pseudo-gaps at the Dirac points of the two -cones.Comment: 9 pages, 7 figure
Lower limit on the achievable temperature in resonator-based sideband cooling
A resonator can be effectively used as a cooler for another linear oscillator
with a much smaller frequency. A huge cooling effect, which could be used to
cool a mechanical oscillator below the energy of quantum fluctuations, has been
predicted by several authors. However, here we show that there is a lower limit
T* on the achievable temperature that was not considered in previous works and
can be higher than the quantum limit in realistic experimental realizations. We
also point out that the decay rate of the resonator, which previous studies
stress should be small, must be larger than the decay rate of the cooled
oscillator for effective cooling.Comment: 6 pages, 4 figures, uses psfra
Equilibrium states and invariant measures for random dynamical systems
Random dynamical systems with countably many maps which admit countable
Markov partitions on complete metric spaces such that the resulting Markov
systems are uniformly continuous and contractive are considered. A
non-degeneracy and a consistency conditions for such systems, which admit some
proper Markov partitions of connected spaces, are introduced, and further
sufficient conditions for them are provided. It is shown that every uniformly
continuous Markov system associated with a continuous random dynamical system
is consistent if it has a dominating Markov chain. A necessary and sufficient
condition for the existence of an invariant Borel probability measure for such
a non-degenerate system with a dominating Markov chain and a finite (16) is
given. The condition is also sufficient if the non-degeneracy is weakened with
the consistency condition. A further sufficient condition for the existence of
an invariant measure for such a consistent system which involves only the
properties of the dominating Markov chain is provided. In particular, it
implies that every such a consistent system with a finite Markov partition and
a finite (16) has an invariant Borel probability measure. A bijective map
between these measures and equilibrium states associated with such a system is
established in the non-degenerate case. Some properties of the map and the
measures are given.Comment: The article is published in DCDS-A, but without the 3rd paragraph on
page 4 (the complete removal of the paragraph became the condition for the
publication in the DCDS-A after the reviewer ran out of the citation
suggestions collected in the paragraph
Searching for degeneracies of real Hamiltonians using homotopy classification of loops in SO()
Topological tests to detect degeneracies of Hamiltonians have been put
forward in the past. Here, we address the applicability of a recently proposed
test [Phys. Rev. Lett. {\bf 92}, 060406 (2004)] for degeneracies of real
Hamiltonian matrices. This test relies on the existence of nontrivial loops in
the space of eigenbases SO. We develop necessary means to determine the
homotopy class of a given loop in this space. Furthermore, in cases where the
dimension of the relevant Hilbert space is large the application of the
original test may not be immediate. To remedy this deficiency, we put forward a
condition for when the test is applicable to a subspace of Hilbert space.
Finally, we demonstrate that applying the methodology of [Phys. Rev. Lett. {\bf
92}, 060406 (2004)] to the complex Hamiltonian case does not provide any new
information.Comment: Minor changes, journal reference adde
Scattering of coherent states on a single artificial atom
In this work we theoretically analyze a circuit QED design where propagating
quantum microwaves interact with a single artificial atom, a single Cooper pair
box. In particular, we derive a master equation in the so-called transmon
regime, including coherent drives. Inspired by recent experiments, we then
apply the master equation to describe the dynamics in both a two-level and a
three-level approximation of the atom. In the two-level case, we also discuss
how to measure photon antibunching in the reflected field and how it is
affected by finite temperature and finite detection bandwidth.Comment: 18 pages, 7 figure
Atomic Processes in Planetary Nebulae and H II Regions
Spectroscopic studies of Planetary Nebulae (PNe) and H {\sc ii} regions have
driven much development in atomic physics. In the last few years the
combination of a generation of powerful observatories, the development of ever
more sophisticated spectral modeling codes, and large efforts on mass
production of high quality atomic data have led to important progress in our
understanding of the atomic spectra of such astronomical objects. In this paper
I review such progress, including evaluations of atomic data by comparisons
with nebular spectra, detection of spectral lines from most iron-peak elements
and n-capture elements, observations of hyperfine emission lines and analysis
of isotopic abundances, fluorescent processes, and new techniques for
diagnosing physical conditions based on recombination spectra. The review is
directed toward atomic physicists and spectroscopists trying to establish the
current status of the atomic data and models and to know the main standing
issues.Comment: 9 pages, 1 figur
Quantum chaos and critical behavior on a chip
The Dicke model describes N qubits (or two-level atoms) homogenously coupled
to a bosonic mode. Here we examine an open-system realization of the Dicke
model, which contains critical and chaotic behaviour. In particular, we extend
this model to include an additional open transport qubit (TQ) (coupled to the
bosonic mode) for passive and active measurements. We illustrate how the
scaling (in the number of qubits N) of the superradiant phase transition can be
observed in both current and current-noise measurements through the transport
qubit. Using a master equation, we also investigate how the phase transition is
affected by the back-action from the transport qubit and losses in the cavity.
In addition, we show that the non-integrable quantum chaotic character of the
Dicke model is retained in an open-system environment. We propose how all of
these effects could been seen in a circuit QED system formed from an array of
superconducting qubits, or an atom chip, coupled to a quantized resonant cavity
(e.g., a microwave transmission line).Comment: 7 page
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