Advanced development of Pb-salt semiconductor lasers for the 8.0 to 15.0 micrometer spectral region

The technology was studied for producing Pb-salt diode lasers for the 8-51 micron spectral region suitable for use as local oscillators in a passive Laser Heterodyne Spectrometer (LHS). Consideration was given to long range NASA plans for the utilization of the passive LHS in a space shuttle environment. The general approach was to further develop the method of compositional interdiffusion (CID) recently reported, and used successfully at shorter wavelength. This technology was shown to provide an effective and reproducible method of producing a single-heterostructure (SH) diode of either the heterojunction or single-sided configuration. Performance specifications were exceeded in several devices, with single-ended CW power outputs as high as 0.88 milliwatts in a mode being achieved. The majority of the CID lasers fabricated had CW operating temperatures of over 60K; 30% of them operated CW above the boiling temperature of liquid nitrogen. CW operation above liquid nitrogen temperature was possible for wavelengths as long as 10.3 microns. Operation at 77K is significant with respect to space shuttle operations since its allows considerable simplification of cooling method

Reliability improvements in tunable Pb1-xSnxSe diode lasers

Recent developments in the technology of Pb-salt diode lasers which have led to significant improvements in reliability and lifetime, and to improved operation at very long wavelengths are described. A combination of packaging and contacting-metallurgy improvements has led to diode lasers that are stable both in terms of temperature cycling and shelf-storage time. Lasers cycled over 500 times between 77 K and 300 K have exhibited no measurable changes in either electrical contact resistance or threshold current. Utilizing metallurgical contacting process, both lasers and experimental n-type and p-type bulk materials are shown to have electrical contact resistance values that are stable for shelf storage periods well in excess of one year. Problems and experiments which have led to devices with improved performance stability are discussed. Stable device configurations achieved for material compositions yielding lasers which operate continuously at wavelengths as long as 30.3 micrometers are described

Development of lead salt semiconductor lasers for the 9-17 micron spectral region

Improved diode lasers of Pb sub 1-x Sn sub x Se operating in the 9-17 micrometers spectral region were developed. The performance characteristics of the best lasers exceeded the contract goals of 500 microW/mode at T 30K in the 9-12 micrometers region and 200 microW/mode at T 18K in the 16-17 micrometers region. Increased reliability and device yields resulted from processing improvements which evolved from a series of diagnostic studies. By means of Auger electron spectroscopy, laser shelf storage degradation was shown to be characterized by the presence of In metal on the semiconductor crystal surfaces. Studies of various metal barrier layers between the crystals and the In metal led to the development of an improved metallurgical contacting technology which has resulted in devices with performance stability values exceeding the contract goal of a one year shelf life. Lasers cycled over 500 times between 300K and 77K were also shown to be stable. Studies on improved methods of fabricating striped geometry lasers indicated that good spectral mode characteristics resulted from lasers which stripe widths of 12 and 25 micrometers

Comment on "Valence QCD: Connecting QCD to the Quark Model"

I criticize certain conclusions about the physics of hadrons drawn from a "valence QCD" approximation to QCD.Comment: 12 pages, 8 figures; some minor improvements made to the tex

How did the Welsh government manage to reform council tax in 2005?

Repeated calls have been made for council tax (CT) in the UK to be reformed. A ‘tyranny of the status quo’ suggests that politicians will avoid this because they fear a backlash from the losers of reform. This paper claims that the tyranny of the status quo is not a fixed law. The Welsh government revalued CT in 2005 but did not communicate the complexity of reform sufficiently. Reform requires greater efforts to communicate the complexity of winning and losing

Single-Domain Grain Size Limits for Metallic Iron

Theoretical examination of possible nonuniform spin configurations in metallic iron indicates that circular spin (CS) is the lowest-energy nonuniform arrangement. The upper grain size limit (d0) to single-domain (SD) behavior is thus defined by the SD to CS transition. Superparamagnetic (SP) behavior marks the lower grain size limit to the stable SD range, and the SP to SD threshold size (ds) can be determined by Néel\u27s relaxation theory. Calculations of d0 and ds for spherical metallic iron particles at 290°K indicate that d0 ( = 173 Å) \u3c ds ( =260 Å), and no stable SD range exists. A stable SD range does exist for prolate ellipsoids of elongation q \u3e 1.1 but remains very constricted. For a prolate ellipsoid of q = 1.67, a stable SD range occurs between the SP critical length LS = 150 Å and d0 = 360 Å. Both d0and ds increase with temperature, but the stable SD range decreases. The size and shape criteria for the stable SD behavior of metallic iron help to explain (1) the low SD content of lunar samples, (2) the widespread occurrence of SP behavior and viscous magnetization in lunar soils and low metamorphic grade breccias, (3) the changes in the magnetic properties of breccias during annealing, and (4) the increased SD content of shocked breccias. The narrow grain size limits for SD behavior also suggest that magnetostatic interaction between metal grains in the solar nebula is not a viable mechanism for iron-silicate fractionation

Theoretical Single-Domain Grain Size Range in Magnetite and Titanomagnetite

A theoretical model of single-domain (SD) grain sizes is applied to magnetite and titanomagnetite. In this model, transition to a two-domain configuration takes place at the SD threshold d0. This two-domain configuration is shown to be more applicable to fine-grained magnetites in igneous rocks than previous models involving transition to a circular spin configuration at d0. Calculations of the stable SD grain size range were accomplished by calculating the superparamagnetic threshold size ds by Néel\u27s relaxation equation and calculating the SD threshold d0 at which SD to two-domain transition occurs. For cubic magnetite particles the SD range is extremely narrow and occurs at very small grain size. At room temperature, ds ≃ 0.05 μm, and d0 ≃ 0.076 μm. For cubic magnetite particles just above d0 a two-domain configuration is predicted in which a 180° domain wall occupies ∼60% of the particle volume. No SD range exists for cubic magnetites at T \u3e 450°K. These results are in good agreement with experimental determinations of SD limits in equant magnetites and also agree with experimental observations of thermoremanent magnetization in submicron pseudo-single-domain (PSD) magnetites. The SD range increases rapidly with particle elongation. For a length : width ratio of 5 : 1, SD limits of ds ≃ 0.05 μm and d0 ≃ 1.4 μm are calculated. Both d0 and the SD range for titanomagnetites (Fe3−x Tix04) increase with Ti content. For cubic titanomagnetites of x = 0.6, ds≃ 0.08 μm, and d0 ≃ 0.3 μm. Comparison of the calculated SD range with the available high-resolution grain size distributions of opaque grains in igneous rocks suggests that elongated SD grains or submicron PSD grains are the major carriers of stable natural remanence in igneous rocks

Updated opacities from the opacity project

Using the code autostructure, extensive calculations of inner-shell atomic data have been made for the chemical elements He, C, N, O, Ne, Na, Mg, Al, Si, S, Ar, Ca, Cr, Mn, Fe and Ni. The results are used to obtain updated opacities from the Opacity Project (OP). A number of other improvements on earlier work have also been included. Rosseland-mean opacities from the OP are compared with those from OPAL. Differences of 5-10 per cent occur. The OP gives the 'Z-bump', at log(T) 5.2, to be shifted to slightly higher temperatures. The opacities from the OP, as functions of temperature and density, are smoother than those from OPAL. The accuracy of the integrations used to obtain mean opacities can depend on the frequency mesh used. Tests involving variation of the numbers of frequency points show that for typical chemical mixtures the OP integrations are numerically correct to within 0.1 per cent. The accuracy of the interpolations used to obtain mean opacities for any required values of temperature and density depends on the temperature-density meshes used. Extensive tests show that, for all cases of practical interest, the OP interpolations give results correct to better than 1 per cent. Prior to a number of recent investigations which have indicated a need for downward revisions in the solar abundances of oxygen and other elements, there was good agreement between properties of the Sun deduced from helioseismology and from stellar evolution models calculated using OPAL opacities. The revisions destroy that agreement. In a recent paper, Bahcall et al. argue that the agreement would be restored if opacities for the regions of the Sun with 2 × 106T 5 × 106 K (0.7-0.4 R) were larger than those given by OPAL by about 10 per cent. In the region concerned, the present results from the OP do not differ from those of OPAL by more than 2.5 per cent

Numerical Evidence for the Observation of a Scalar Glueball

We compute from lattice QCD in the valence (quenched) approximation the partial decay widths of the lightest scalar glueball to pairs of pseudoscalar quark-antiquark states. These predictions and values obtained earlier for the scalar glueball's mass are in good agreement with the observed properties of $f_J(1710)$ and inconsistent with all other observed meson resonances.Comment: 12 pages of Latex, 3 PostsScript figures as separate uufil