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Articles 331 - 360 of 378
Full-Text Articles in Astrophysics and Astronomy
Photoluminescence Studies In Znxcd1-Xte Single Crystals, David D. Allred, Jesus González-Hernández, Elías López-Cruz, Worth P. Allred
Photoluminescence Studies In Znxcd1-Xte Single Crystals, David D. Allred, Jesus González-Hernández, Elías López-Cruz, Worth P. Allred
Faculty Publications
The crystalline quality of ZnxCd1-xTe single crystals prepared by a modified Bridgman method with 0≤x≤0.05 has been analyzed using photoluminescence. The spectrum of a typical sample is dominated by lines originating from the recombination of free and bound excitons. Lines due to free excitons in their ground and first excited states are observed in both the pure CdTe and the mixed crystals. Excitons bound to Dc vacancies are observed in the pure CdTe crystal but not in the mixed crystal. Weaker and broader features appearing at energies below the exciton emission range were associated with transitions involving free-to-bound and bound-to-bound …
Design Of High Performance Soft X-Ray Windows, Raymond T. Perkins, David D. Allred, Larry V. Knight, James M. Thorne
Design Of High Performance Soft X-Ray Windows, Raymond T. Perkins, David D. Allred, Larry V. Knight, James M. Thorne
Faculty Publications
X-ray windows are used in sources and detectors to separate the neighborhood of the x-ray generation or detection from the use environment. While each use has its own requirements, there are some principles that should be used in designing an optimal x-ray window. Because x-rays are absorbed to some extent by all materials, minimizing absorption is one criterion in preparing windows. Also, for most uses there is a pressure difference across the window so that ensuring pinhole-free structure and sufficient mechanical strength to support the differential is another criterion for window design. Traditionally, absorption is minimized by fabricating the window …
Photoconductive Characterization Of Znxcd1-Xte (0≤X≤0.25) Single Crystal Alloys, David D. Allred, Elías López-Cruz, Jesus González-Hernández, Worth P. Allred
Photoconductive Characterization Of Znxcd1-Xte (0≤X≤0.25) Single Crystal Alloys, David D. Allred, Elías López-Cruz, Jesus González-Hernández, Worth P. Allred
Faculty Publications
Large grain polycrystalline and single crystals of ZnxCd1-xTe grown by a modified Bridgman method were studied using the photoluminescence and photoconductivity techniques. The temperature dependence of the band gap, as determined by photoluminescence, follows the Varshni equation for measuring temperature in the range of 15-290 K. One of the fitted parameters, the Debye temperature, monotonically decreases with the increase of the atomic zinc concentration. A close correlation between the photoluminescence and photoconductivity measurements is also found. Samples in which the photoluminescence spectra exhibit emission bands associated to cadmium vacancies and other structural defects, show a photoresponse curve which includes, in …
Advanced Infrared Optically Black Baffle Materials, David D. Allred, Ronald D. Seals, C. M. Egert
Advanced Infrared Optically Black Baffle Materials, David D. Allred, Ronald D. Seals, C. M. Egert
Faculty Publications
Infrared optically black baffle surfaces are an essential component of many advanced optical systems. All internal surfaces in advanced infrared optical sensors that require stray light management to achieve resolution are of primary concern in baffle design. Current industrial materials need improvements to meet advanced optical sensor systems requirements for optical, survivability, and endurability. Baffles are required to survive and operate in potentially severe environments. Robust diffuse-absorptive black surfaces, which are (1) thermally and mechanically stable to threats of x-ray, launch, and in-flight maneuver conditions, with specific densities to allow an acceptable weight load, (2) handleable during assembly, (3) cleanable, …
Annealing Behavior Of The Photoluminescence Lines In Cdte And Znx Cd1-X Te Single Crystals, David D. Allred, A. Reyes-Mena, Jesus González-Hernández, Elías López-Cruz, Worth P. Allred
Annealing Behavior Of The Photoluminescence Lines In Cdte And Znx Cd1-X Te Single Crystals, David D. Allred, A. Reyes-Mena, Jesus González-Hernández, Elías López-Cruz, Worth P. Allred
Faculty Publications
The main lines in the photoluminescence spectra of Zn1Cd1-xTe single crystals grown by a modified Bridgman method in the compositional range of 0≤X≤0.25 have been identified. All crystals show only near-band-edge emission. To assist in the identification, various samples with different compositions were annealed under a Cd atmosphere. In the pure crystals, the prominent (A°, X) bound exciton line, as well as the doublet at longer wavelengths, disappear after the annealing. In contrast, the treatments do not change significantly the PL spectra of the mixed crystals.
Characterization Of Metal/Carbon Multilayers By Raman Spectroscopy, David D. Allred, Qi Wang, Jesus González-Hernández
Characterization Of Metal/Carbon Multilayers By Raman Spectroscopy, David D. Allred, Qi Wang, Jesus González-Hernández
Faculty Publications
Laser Raman spectroscopy has been found to be useful for characterizing amorphous semiconductor multilayers, especially the interfaces of multilayers. Recently, we have extended this technique to the characterization of magnetron sputtered multilayers commonly used as reflectors in soft x-ray optics. Unlike the multilayers previously studied which contained only semiconductors and dielectrics, these are generally semiconductor/metal multilayers. We report here on the Raman characterization of the most common class of multilayers used in soft x-ray optics, those that contain a high density metal like tungsten interspersed with layers of carbon. In all of the metal/carbon multilayers the dominate feature in the …
Deposition Of Zinc Selenide By Atomic Layer Epitaxy For Multilayer X-Ray Optics, J.K. Shurtleff, David D. Allred, R. T. Perkins, J. M. Thorne
Deposition Of Zinc Selenide By Atomic Layer Epitaxy For Multilayer X-Ray Optics, J.K. Shurtleff, David D. Allred, R. T. Perkins, J. M. Thorne
Faculty Publications
Thin film deposition techniques currently being used to produce multilayer x-ray optics (MXOs) have difficulty producing smooth, uniform multilayers with d-spacings less than about twelve angstroms. We are investigating atomic layer epitaxy (ALE) as an alternative to these techniques. ALE is a chemical vapor deposition technique which deposits an atomic layer of material during each cycle of the deposition process. The thickness of a film deposited by ALE depends only on the number of cycles. Multilayers deposited by ALE should be smooth and uniform with precise d-spacings which makes ALE an excellent technique for producing multilayer x-ray optics. We have …
Diffuse Absorbing Beryllium Coatings Produced By Magnetron Sputtering, David D. Allred, C. M. Egert
Diffuse Absorbing Beryllium Coatings Produced By Magnetron Sputtering, David D. Allred, C. M. Egert
Faculty Publications
Beryllium coatings with varying thicknesses and columnar grain sizes were deposited by low temperature magnetron sputtering and wet chemically etched to enhance diffuse absorption of light. After etching these coatings exhibited a matte black surface finish and low specular reflectance (below 2%) in the IR up to a critical wavelength dependent upon the original grain size of the coating. Extremely thick coatings (350 µm) with original grain sizes of 10 to 12 µm were produced which exhibited specular reflectances below 0.5% up to 50 µm wavelength and a Lambertian BRDF at 10.6 µm averaging 4.3x10-3 ster-1. Scanning electron micrographs are …
Nonlocal Symmetries And Backlund Transformations For The Self-Dual Yang-Mills System, C. J. Papachristou, Kent B. Harrison
Nonlocal Symmetries And Backlund Transformations For The Self-Dual Yang-Mills System, C. J. Papachristou, Kent B. Harrison
Faculty Publications
The observation is made that generalized evolutionary isovectors of the self-dual Yang–Mills equation, obtained by "verticalization'' of the geometrical isovectors derived in a previous paper [J. Math. Phys. 28, 1261 (1987)], generate Bäcklund transformations for the self-dual system. In particular, new Bäcklund transformations are obtained by "verticalizing'' the generators of point transformations on the solution manifold. A geometric ansatz for the derivation of such (generally nonlocal) symmetries is proposed.
Comparison Of Effective Medium Procedures For Optical Modeling Of Laminar Structures, David D. Allred, Robert F. Edgerton
Comparison Of Effective Medium Procedures For Optical Modeling Of Laminar Structures, David D. Allred, Robert F. Edgerton
Faculty Publications
This study addresses the question, "How can the optical properties of matter in ultrathin amorphous nonmetallic films in multilayers best be determined from reflectance (R) and transmission (T) measurements?" A blue shift in the band gap of plasma CVD a-Si:H/a-SiNx:H multilayers was reported several years ago. It was suggested that the shift in the band gap, Eg, Relative to bulk a-Si:H as given by the Tauç plot was due to quantum confinement effects. The purpose of this study is to evaluate the usefulness of various effective media theories (EMT) for determining the optical constants of materials in a multilayer and …
Surface Conductance Of A Copper Wire In A Fluid At High Pressure, Daniel L. Decker
Surface Conductance Of A Copper Wire In A Fluid At High Pressure, Daniel L. Decker
Faculty Publications
The three-dimensional flow of heat in a wire carrying a current and immersed in a liquid is solved in detail. Using this exact result the surface conductance of copper in petroleum ether has been measured as a function of pressure to 40 kbars. The measured surface conductance for copper in a fluid is very small, justifying approximations which yield results that are in agreement with a simplified one-dimensional heat-flow problem. Surprisingly, even at 40 kbars pressure a very large fraction of the joule heating within a wire with a length-to-diameter ratio of ~100 is dissipated through the ends of the …
Superconductivity At 155 K, David D. Allred, S. R. Ovshinsky, R. T. Young, G. Demaggio, G. A. Van Der Leeden
Superconductivity At 155 K, David D. Allred, S. R. Ovshinsky, R. T. Young, G. Demaggio, G. A. Van Der Leeden
Faculty Publications
Transition to a superconducting zero-resistance state at 155 K is observed for the first time in bulk material. A new five-element compound has been synthesized with nominal composition Y1Ba2Cu3F2Oy. Fluorine plays a critical role in achieving this effect. X-ray diffraction and electron microprobe analysis indicate that the samples are multiphasic. Evidence is presented that the samples contain superconducting phases with onset temperatures considerably above 155 K. Magnetic measurements suggest a flux-trapping effect below 260 K, and diamagnetic deviations from Curie-Weiss behavior in the range from 250 K≤T≤100 K indicate that Meissner effect in a small superconducting volume fraction.
Some Aspects Of The Isogroup Of The Self-Dual Yang-Mills System, C. J. Papachristou, B. Kent Harrison
Some Aspects Of The Isogroup Of The Self-Dual Yang-Mills System, C. J. Papachristou, B. Kent Harrison
Faculty Publications
A generalized isovector formalism is used to derive the isovectors and isogroup of the self-dual Yang-Mills (SDYM) equation in the so-called J formulation. In particular, the infinitesimal "hidden symmetry'' transformation, a linear system, and a well-known Backlund transformation of the SDYM equation are derived in the process. Thus symmetry and integrability aspects of the SDYM system appear in natural relationship to each other within the framework of the isovector approach.
Anneal Induced Changes In Amorphous Semiconductor Multilayers, David D. Allred, Jesus González-Hernández, O. V. Nguyen
Anneal Induced Changes In Amorphous Semiconductor Multilayers, David D. Allred, Jesus González-Hernández, O. V. Nguyen
Faculty Publications
We have prepared, heat treated and characterized various amorphous semiconductor periodic multilayers and ultrathin films. These were prepared by several vapor deposition techniques at substrate temperatures ranging from 25°C to 300°C and possessed periodicities from 22 to 400Å. Films were subjected to isochronal thermal treatments at progressively higher temperatures. Two effects were observed: enhanced diffusion and retarded crystallization. Interdiffusion, at rates which are many orders of magnitude higher than those anticipated from crystalline data, was observed in a-Si/a-Ge multilayers. Crystallization of germanium, the more readily crystallized member of the couple, is retarded; the extent depends on the thickness of the …
Equilibrium Properties Of Short Field-Reversed Configurations, Ross L. Spencer, M. Tuszewski
Equilibrium Properties Of Short Field-Reversed Configurations, Ross L. Spencer, M. Tuszewski
Faculty Publications
Some features of short field-reversed configuration (FRC) equilibria relevant to transport and stability are studied numerically and analytically. It is shown that magnetic field curvature effects significantly increase the FRC magnetization for plasma elongations epsilon is less than or equal to 4.
Considerations Of Temperature Stability In A Cylindrical Plasma, S. Neil Rasband, Yao-Hui Xu
Considerations Of Temperature Stability In A Cylindrical Plasma, S. Neil Rasband, Yao-Hui Xu
Faculty Publications
A variational formulation is used to consider thermal equilibrium and stability for a cylindrical plasma modeled by dissipative magnetohydrodynamics. The parameter dependence of equilibrium solutions and their stability is followed using homotopy techniques. Solution branches and their connections are studied in relation to the applied fields.
A Generalized Molien Function For Field Theoretical Hamiltonians, Jeffrey W. Felix, Dorian M. Hatch
A Generalized Molien Function For Field Theoretical Hamiltonians, Jeffrey W. Felix, Dorian M. Hatch
Faculty Publications
A generating function, or Molien function, the coefficients of which give the number of independent polynomial invariants in G, has been useful in the Landau and renormalization group theories of phase transitions. Here a generalized Molien function for a field theoretical Hamiltonian (with short-range interactions) of the most general form invariant in a group G is derived. This form is useful for more general renormalization group calculations. Its Taylor series is calculated to low order for the FGamma-2 representation of the space group R[3 bar]c and also for the l=1 (faithful) representation of SO(3).
Example Of A Group Action Determined Phase Transition, Jeffrey W. Felix, Dorian M. Hatch
Example Of A Group Action Determined Phase Transition, Jeffrey W. Felix, Dorian M. Hatch
Faculty Publications
The principles of the group action approach to structural phase transitions are outlined. It is assumed that all properties of the transition are determined by the action of a single physically irreducible represention of the space group of the more symmetric phase. We determine the isotropy groups using the image space of the representation. The free energy minima are determined to fourth order and to all orders using the results of Gufan and then compared. This theory is applied to Calcite (Roverline3c) to determine all possible continuous commensurate phase transitions.
Isee/Ice Data Reduction And Analysis: Magnetic Fields, Edward J. Smith, Douglas E. Jones, George L. Siscoe, Bruce T. Tsurutani
Isee/Ice Data Reduction And Analysis: Magnetic Fields, Edward J. Smith, Douglas E. Jones, George L. Siscoe, Bruce T. Tsurutani
Faculty Publications
In accordance with the instructions provided by GSFC, the work has been divided into a number of individual study tasks. For each task, a brief narrative description and a budget are provided. Task I, the Core Task, is treated as a single task. Task II, concerning the comet, is a single task. Task III is subdivided into three major categories: (A) on-going interplanetary studies begun in prior years (B) on-going magnetotail studies begun in FY'84 and (C) new studies proposed for FY'85 and '86.
Isotropy Group Description Of A Phase Transition In Nano3, W. Scott Stornetta, Dorian M. Hatch
Isotropy Group Description Of A Phase Transition In Nano3, W. Scott Stornetta, Dorian M. Hatch
Faculty Publications
Predictions of the possible post-transition symmetries of an R 3c zone center continuous phase transition are made using recent extensions of the Landau theory. The Landau theory is summarized and direct group theoretical methods are applied to the R 3c phase. General techniques for solving systems of nonlinear polynomial equations are reviewed and these techniques are applied to the minimization of the free energy F for the aforementioned possible symmetries. The results of the direct methods and the minimization procedures are shown to be compatible. It is shown that the transition to R 3c in NaNO3 is consistent with the …
Unification Of Ernst-Equation Backlund Transformations Using A Modified Wahlquist-Estabrook Technique; Wahlquist-Estabrook, B. Kent Harrison
Unification Of Ernst-Equation Backlund Transformations Using A Modified Wahlquist-Estabrook Technique; Wahlquist-Estabrook, B. Kent Harrison
Faculty Publications
The three known Backlund transformations for the Ernst equation are derived using a modification of the Wahlquist-Estabrook prolongation procedure. The modification requires that the equation to be studied be cast into a set of differential forms and their exterior derivatives, such that all coefficients are constant (a "CC ideal''). Analysis of the resulting equations produces 16 solutions composed of the three basic transformations combined with identity and other essentially trivial transformations. The group structure of the transformations is discussed. A Backlund transformation (already known) for the Ernst-Maxwell equations can be found by the same method. Promising generalizations are mentioned.
West Mountain Observatory: Planning And Construction Pictorial History, Gene Libutti
West Mountain Observatory: Planning And Construction Pictorial History, Gene Libutti
BYU West Mountain Observatory Project
No abstract provided.
West Of Utah Lake: Building Nears Finish, Marc Enggas
West Of Utah Lake: Building Nears Finish, Marc Enggas
BYU West Mountain Observatory Project
No abstract provided.
Landau Free Energy Form At The F Point Of The Roverline3 Structure, Dorian M. Hatch, Daniel L. Decker
Landau Free Energy Form At The F Point Of The Roverline3 Structure, Dorian M. Hatch, Daniel L. Decker
Faculty Publications
A free energy form compatible with the F point of the trigonal D63d(Roverline3c) symmetry is constructed. It is a function of the order parameter, elastic strain, and the stress. The resulting free energy exhibits a close formalistic analogy to the stressed cubic perovskite structures and their associated multicriticality.
Program For New Observatory Facility For Brigham Young University, Provo, Utah, Byu Department Of Physics And Astronomy
Program For New Observatory Facility For Brigham Young University, Provo, Utah, Byu Department Of Physics And Astronomy
BYU West Mountain Observatory Project
No abstract provided.
Chemical Vapor Deposited Molybdenum Films For Use In Photothermal Conversion, David D. Allred, G. E. Carver, B. O. Seraphin
Chemical Vapor Deposited Molybdenum Films For Use In Photothermal Conversion, David D. Allred, G. E. Carver, B. O. Seraphin
Faculty Publications
High infrared reflectance, coupled with higher solar absorptance, is required for efficient photothermal conversion. Converters can be fabricated by depositing an absorber on a highly reflecting metal. The absorber functions in the visible, yet becomes transparent in the near infrared, allowing the metal to suppress the thermal emittance. Economic considerations demand the use of thin films, rather than bulk materials. The thin film reflector must be capable of withstanding high temperatures of operation. Compatibility of the reflector with the substrate below, and the absorber above, is required for long-time service. Highly reflective silver films suffer reflectance losses by agglomeration, and …
Chemical Vapor Deposited Amorphous Silicon For Use In Photothermal Conversion, David D. Allred, D. C. Booth, M. Janai, G. Weiser, B. O. Seraphin
Chemical Vapor Deposited Amorphous Silicon For Use In Photothermal Conversion, David D. Allred, D. C. Booth, M. Janai, G. Weiser, B. O. Seraphin
Faculty Publications
Efficient photothermal conversion requires surfaces of high solar absorptance and low thermal emittance. This can be accomplished by the tandem action of a good infrared reflector overlaid by a film of sufficient solar absorptance that is transparent in the infrared. Crystalline silicon is a suitable candidate for the absorber layer. Its indirect band gap, however, results in a shallow absorption edge that extends to far into the visible. In contrast, the absorption edge of amorphous silicon is steeper and located farther into the infrared, resulting in a larger solar absorptance. We report on the fabrication of amorphous silicon absorbers by …
The Use Of Nuclear Reactions And Sims For Quantitative Depth Profiling Of Hydrogen In Amorphous Silicon, David D. Allred, G. J. Clark, C. W. White, B. R. Appleton, C. W. Magee, D. E. Carlson
The Use Of Nuclear Reactions And Sims For Quantitative Depth Profiling Of Hydrogen In Amorphous Silicon, David D. Allred, G. J. Clark, C. W. White, B. R. Appleton, C. W. Magee, D. E. Carlson
Faculty Publications
Depth profiles for hydrogen in amorphous silicon have been determined by the use of resonant nuclear reactions [1H(15N,αγ)12C and 1H(19F,αγ)16O] and by secondary ion mass spectroscopy (SIMS). Independent calibration procedures were used for the two techniques. Measurements were made on the same amorphous silicon film to provide a direct comparison of the two hydrogen analysis techniques. The hydrogen concentration in the bulk of the film was determined to be about 9 at % H. The SIMS results agree with the resonant nuclear reaction results to within 10%, which demonstrates that quantitative hydrogen depth profiles can be obtained by SIMS analysis …
Computer Model Of A Fast Toroidal Plasma Compression, With Application To The Topolotron, H. Mark Nelson, Keith H. Brown, Charles A. Hart
Computer Model Of A Fast Toroidal Plasma Compression, With Application To The Topolotron, H. Mark Nelson, Keith H. Brown, Charles A. Hart
Faculty Publications
The method is developed for a comuputer sequence which models a fast toroidal theta or screw pinch for a highly conducting axially symmetric plasma. The computer sequence takes into account the self-inductances of the plasma and the external conductors which drive the compression as well as the mutual inductance which electromagnetically couples the two. The computer sequence is divided into three phases: a snowplow compression phase, an adiabatic compression phase, and a crowbarred circuit phase. The computer sequence is applied to a topolotron and an example is given of a magnetohydrodynamic equilibrium geometry for which the plasma surface possesses an …
Behavior Of A Manganin Coil At Elevated Temperatures And Pressures, Jonathan D. Weiss, Daniel L. Decker, H. B. Vanfleet
Behavior Of A Manganin Coil At Elevated Temperatures And Pressures, Jonathan D. Weiss, Daniel L. Decker, H. B. Vanfleet
Faculty Publications
We have determined the temperature dependence in the first-order pressure coefficient of a manganin coil to be dBetaB/dT= (4.2±0.8) ×10^-7 (kbar degrees C)^-1 in the equation R (T,P) =R (T,0)(1+BetaP+GammaP2), where R (T,P) is the coil resistance at temperature T and pressure P. The measurements were performed by simultaneously measuring the coil resistance and temperature at the transitions of mercury and bismuth, which were observed by differential thermal analysis.