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, $P_{\rho}$, 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 $P_{\rho} \propto k^{-2}$ for $M_{\rm rms} \sim 1$ to the profile with {\it peaks} having $P_{\rho} \propto k^0$ for $M_{\rm rms} \gg 1$. 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 $M_{\rm rms} \sim 1$ 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 $M_{\rm rms} \sim 1$ in the warm ionized medium, the shallower spectrum of cold H I gas reflects the {\it supersonic} turbulence of $M_{\rm rms} \sim$ 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

The Financial Status of North Dakota Farmers and Ranchers: January 1, 1985, Survey Results

Agricultural Finance,