10,374 research outputs found
Variations on Kaluza-Klein Cosmology
We investigate the cosmological consequences of having quantum fields living
in a space with compactified dimensions. We will show that the equation of
state is not modified by topological effects and so the dynamics of the
universe remains as it is in the infinite volume limit. On the contrary the
thermal history of the universe depends on terms that are associated with
having non-trivial topology. In the conclusions we discuss some issues about
the relationship between the non-critical string-inspired cosmology and
the result obtained with matter given by a hot massless field in S^{1}\times
\mbox{\bf
R}.Comment: 22 pages, 7 figures in a uuencoded file (using uufiles), LaTeX,
FTUAM-93/13 (LaTeX errors corrected
Dynamical Properties of Multi-Armed Global Spirals in Rayleigh-Benard Convection
Explicit formulas for the rotation frequency and the long-wavenumber
diffusion coefficients of global spirals with arms in Rayleigh-Benard
convection are obtained. Global spirals and parallel rolls share exactly the
same Eckhaus, zigzag and skewed-varicose instability boundaries. Global spirals
seem not to have a characteristic frequency or a typical size ,
but their product is a constant under given experimental
conditions. The ratio of the radii of any two dislocations (,
) inside a multi-armed spiral is also predicted to be constant. Some of
these results have been tested by our numerical work.Comment: To appear in Phys. Rev. E as Rapid Communication
Continuous-wave and Transient Characteristics of Phosphorene Microwave Transistors
Few-layer phosphorene MOSFETs with 0.3-um-long gate and 15-nm-thick Al2O3
gate insulator was found to exhibit a forward-current cutoff frequency of 2 GHz
and a maximum oscillation frequency of 8 GHz after de-embedding for the
parasitic capacitance associated mainly with the relatively large probe pads.
The gate lag and drain lag of the transistor was found to be on the order of 1
us or less, which is consistent with the lack of hysteresis, carrier freeze-out
or persistent photoconductivity in DC characteristics. These results confirm
that the phosphorene MOSFET can be a viable microwave transistor for both
small-signal and large-signal applications.Comment: Accepted for oral presentation at IMS 201
StarGO: A New Method to Identify the Galactic Origins of Halo Stars
We develop a new method StarGO (Stars' Galactic Origin) to identify the
galactic origins of halo stars using their kinematics. Our method is based on
self-organizing map (SOM), which is one of the most popular unsupervised
learning algorithms. StarGO combines SOM with a novel adaptive group
identification algorithm with essentially no free parameters. In order to
evaluate our model, we build a synthetic stellar halo from mergers of nine
satellites in the Milky Way. We construct the mock catalogue by extracting a
heliocentric volume of 10 kpc from our simulations and assigning expected
observational uncertainties corresponding to bright stars from Gaia DR2 and
LAMOST DR5. We compare the results from StarGO against that from a
Friends-of-Friends (FoF) based method in the space of orbital energy and
angular momentum. We show that StarGO is able to systematically identify more
satellites and achieve higher number fraction of identified stars for most of
the satellites within the extracted volume. When applied to data from Gaia DR2,
StarGO will enable us to reveal the origins of the inner stellar halo in
unprecedented detail.Comment: 11 pages, 7 figures, Accepted for publication in Ap
Simultaneous cooling of an artificial atom and its neighboring quantum system
We propose an approach for cooling both an artificial atom (e.g., a flux
qubit) and its neighboring quantum system, the latter modeled by either a
quantum two-level system or a quantum resonator. The flux qubit is cooled by
manipulating its states, following an inverse process of state population
inversion, and then the qubit is switched on to resonantly interact with the
neighboring quantum system. By repeating these steps, the two subsystems can be
simultaneously cooled. Our results show that this cooling is robust and
effective, irrespective of the chosen quantum systems connected to the qubit.Comment: 5 pages, 3 figure
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