19,504 research outputs found
CO2 laser waveguiding in proton implanted GaAs
Surface layers capable of supporting optical modes at 10.6 microns have been produced in n-type GaAs wafers through 300 keV proton implantation. The dominant mechanism for this effect appears to be free carrier compensation. Characterization of the implanted layers by analysis of infrared reflectivity spectra and synchronous coupling at 10.6 microns produced results in good agreement with elementary models. These results of sample characterization by infrared reflectivity and by CO2 laser waveguiding as implanted are presented and evaluated
Validity of adiabaticity in Cavity QED
This paper deals with the concept of adiabaticity for fully quantum
mechanically cavity QED models. The physically interesting cases of Gaussian
and standing wave shapes of the cavity mode are considered. An analytical
approximate measure for adiabaticity is given and compared with numerical wave
packet simulations. Good agreement is obtained where the approximations are
expected to be valid. Usually for cavity QED systems, the large atom-field
detuning case is considered as the adiabatic limit. We, however, show that
adiabaticity is also valid, for the Gaussian mode shape, in the opposite limit.
Effective semiclassical time dependent models, which do not take into account
the shape of the wave packet, are derived. Corrections to such an effective
theory, which are purely quantum mechanical, are discussed. It is shown that
many of the results presented can be applied to time dependent two-level
systems.Comment: 10 pages, 9 figure
Density Power Spectrum of Compressible Hydrodynamic Turbulent Flows
Turbulent flows are ubiquitous in astrophysical environments, and
understanding density structures and their statistics in turbulent media is of
great importance in astrophysics. In this paper, we study the density power
spectra, , of transonic and supersonic turbulent flows through one
and three-dimensional simulations of driven, isothermal hydrodynamic turbulence
with root-mean-square Mach number in the range of 1 \la M_{\rm rms} \la 10.
From one-dimensional experiments we find that the slope of the density power
spectra becomes gradually shallower as the rms Mach number increases. It is
because the density distribution transforms from the profile with {\it
discontinuities} having for to
the profile with {\it peaks} having for . We also find that the same trend is carried to three-dimension; that is,
the density power spectrum flattens as the Mach number increases. But the
density power spectrum of the flow with has the Kolmogorov
slope. The flattening is the consequence of the dominant density structures of
{\it filaments} and {\it sheets}. Observations have claimed different slopes of
density power spectra for electron density and cold H I gas in the interstellar
medium. We argue that while the Kolmogorov spectrum for electron density
reflects the {\it transonic} turbulence of in the warm
ionized medium, the shallower spectrum of cold H I gas reflects the {\it
supersonic} turbulence of a few in the cold neutral medium.Comment: To appear in ApJ Lett. Pdf file with full resolution figures can be
downloaded from http://canopus.cnu.ac.kr/ryu/kimryu.pd
Thermal and Fragmentation Properties of Star-forming Clouds in Low-metallicity Environments
The thermal and chemical evolution of star-forming clouds is studied for
different gas metallicities, Z, using the model of Omukai (2000), updated to
include deuterium chemistry and the effects of cosmic microwave background
(CMB) radiation. HD-line cooling dominates the thermal balance of clouds when Z
\~ 10^{-5}-10^{-3} Z_sun and density ~10^{5} cm^{-3}. Early on, CMB radiation
prevents the gas temperature to fall below T_CMB, although this hardly alters
the cloud thermal evolution in low-metallicity gas. From the derived
temperature evolution, we assess cloud/core fragmentation as a function of
metallicity from linear perturbation theory, which requires that the core
elongation E := (b-a)/a > E_NL ~ 1, where a (b) is the short (long) core axis
length. The fragment mass is given by the thermal Jeans mass at E = E_NL. Given
these assumptions and the initial (gaussian) distribution of E we compute the
fragment mass distribution as a function of metallicity. We find that: (i) For
Z=0, all fragments are very massive, > 10^{3}M_sun, consistently with previous
studies; (ii) for Z>10^{-6} Z_sun a few clumps go through an additional high
density (> 10^{10} cm^{-3}) fragmentation phase driven by dust-cooling, leading
to low-mass fragments; (iii) The mass fraction in low-mass fragments is
initially very small, but at Z ~ 10^{-5}Z_sun it becomes dominant and continues
to grow as Z is increased; (iv) as a result of the two fragmentation modes, a
bimodal mass distribution emerges in 0.01 0.1Z_sun,
the two peaks merge into a singly-peaked mass function which might be regarded
as the precursor of the ordinary Salpeter-like IMF.Comment: 38 pages, 16 figures, ApJ in pres
High-pressure study of substrate material ScAlMgO4
We report on the structural properties of ScAlMgO4 studied under
quasi-hydrostatic pressure using synchrotron high-pressure x-ray diffraction up
to 40 GPa. We also report on single-crystal studies of ScAlMgO4 performed at
300 K and 100 K. We found that the low-pressure phase remains stable up to 24
GPa. At 28 GPa, we detected a reversible phase transformation. The
high-pressure phase is assigned to a monoclinic distortion of the low-pressure
phase. No additional phase transition is observed up to 40 GPa. In addition,
the equation of state, compressibility tensor, and thermal expansion
coefficients of ScAlMgO4 are determined. The bulk modulus of ScAlMgO4 is found
to be 143(8) GPa, with a strong compressibility anisotropy. For the trigonal
low-pressure phase, the compressibility along the c-axis is twice than
perpendicular one. A perfect lattice match with ZnO is retained under pressure
in the pressure range of stability of wurtzite ZnO.Comment: 22 pages, 5 figures, 4 tables, 24 reference
Numerical studies of a one-dimensional 3-spin spin-glass model with long-range interactions
We study a p-spin spin-glass model to understand if the finite-temperature
glass transition found in the mean-field regime of p-spin models, and used to
model the behavior of structural glasses, persists in the non-mean-field
regime. By using a 3-spin spin-glass model with long-range power-law diluted
interactions we are able to continuously tune the (effective) space dimension
via the exponent of the interactions. Monte Carlo simulations of the spin-glass
susceptibility and the two-point finite-size correlation length show that deep
in the non-mean-field regime the finite-temperature transition is lost, whereas
this is not the case in the mean-field regime, in agreement with the prediction
of Moore and Drossel [Phys. Rev. Lett. 89, 217202 (2002)] that 3-spin models
are in the same universality class as an Ising spin glass in a magnetic field.
However, slightly in the non-mean-field region, we find an apparent transition
in the 3-spin model, in contrast to results for the Ising spin glass in a
field. This may indicate that even larger sizes are needed to probe the
asymptotic behavior in this region.Comment: 8 pages, 9 figures, 1 tabl
Thermal Instability and the Formation of Clumpy Gas Clouds
The radiative cooling of optically thin gaseous regions and the formation of
a two-phase medium and of cold gas clouds with a clumpy substructure is
investigated. In optically thin clouds, the growth rate of small isobaric
density perturbations is independent of their length scale. However, the growth
of a perturbation is limited by its transition from isobaric to isochoric
cooling. The temperature at which this transition occurs decreases with the
length scale of the perturbation. Consequently small scale perturbations have
the potential to reach higher amplitudes than large scale perturbations. When
the amplitude becomes nonlinear, advection overtakes the pressure gradient in
promoting the compression resulting in an accelerated growth of the
disturbance. The critical temperature for transition depends on the initial
amplitude. The fluctuations which can first reach nonlinearity before their
isobaric to isochoric transition will determine the characteristic size and
mass of the cold dense clumps which would emerge from the cooling of an
initially nearly homogeneous region of gas. Thermal conduction is in general
very efficient in erasing isobaric, small-scale fluctuations, suppressing a
cooling instability. A weak, tangled magnetic field can however reduce the
conductive heat flux enough for low-amplitude fluctuations to grow isobarically
and become non-linear if their length scales are of order 0.01 pc. Finally, we
demonstrate how a 2-phase medium, with cold clumps being pressure confined in a
diffuse hot residual background component, would be sustained if there is
adequate heating to compensate the energy loss.Comment: 26 pages, Latex, 10 postscript figures, ApJ, in pres
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