bluuebunny/crossref_metadata_2025_split huggingface dataset:
II-VI Compounds with Fe - New Family of Semimagnetic Semiconductors
Authors: Mycielski Andrzej 
Published: March 2011

AbstractSeveral experimental methods: absorption, photoemission and transport measurements were used to determine the energy position of substitutional Fe2+ (3d6) donor state in the band structure of the semimagnetic semiconductor Hg1-v-xCdvFexSe for 0≤v≤0.7 and v+x=l, and x≤0.15. For v≤0.40, Fe2+(3d6) level is a resonant donor located in the conduction band. For v=O (HgSe) we obtain 230 meV for the position of Fe2+(3d6) level with respect to the bottom of the conduction band which coincides with the position of the Fermi level for electron concentration N ≅5x1018 cm-3. Surprisingly, the mobility of free electrons (T∼4.2K) is abnormally high and the Dingle temperature measured in quantum magnetoresistivity oscillations (SdH effect) and magnetooptical measurements is abnormally low. Because of the Coulomb interaction between the ionized donors, at low T, there will appear some correlation of their positions. This may lead to a kind of “liquefying” of the system of ions and to its “crystallisation” (i.e. formation of a superlattice or hyperlattice of ionized donors) at even lower T. The space-ordering of ionized donors influences dramatically the free-carrier scattering and correspondingly explains the high mobility and low Dingle temperature. Finally, we shall also present some magnetic properties of these new semimagnetic materials.
Electronic Transport Properties of Hg1-xFexSe
Authors: Pool F. , Kossut J. , Debska U. , Reifenberger R. , Furdyna J. K. 
Published: March 2011

AbstractThe electrical resistivity, electron concentration, and mobility of Hg1-xFexSe are reported for 4.2K < T < 300K and for 0.0001 < x < 0.12. The data are interpreted within an electronic band structure model that assumes the existence of resonant donors (due to the presence of Fe ions) whose ground state energy coincides with the conduction band continuum. The electron concentration data enable determination of the value of the donor energy as a function of the temperature and the crystal composition. The low temperature electron mobility for ∼ 0.0003 ≤ x ≤ 0.01 is considerably higher than expected and indicates a reduction of the charged impurity scattering effects at low temperatures.
Synchrotron Radiation Studies of Ternary Semimagnetic Semiconductors
Authors: Franciosi A. 
Published: March 2011

AbstractA number of spectroscopic techniques that exploit synchrotron radiation are providing new insight into the properties of ternary semimagnetic semiconducting alloys. Photon energy-dependent photoemission methods are being used to characterize the electronic structure and the elemental and orbital contributions to the valence states. Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS) is providing a detailed understanding of the atomic structure and of the local phenomena that make the virtual crystal approximation inadequate. The new experimental information is stimulating novel theoretical work on the electronic structure, lattice stability and thermodynamics of these materials. We will review the results of a number of recent experimental studies and emphasize systematic trends and promising new areas of research.
Magnetoresistance and Hall Effect Near the Metal-Insulator Transition of Cd1-xMnxSe
Authors: Shapira Y. 
Published: March 2011

AbstractA large magnetoresistance was observed in n-type Cdl-xMnxSe (x ≤ 0.10) at low temperatures. The concomitant changes in the Half coefficient as a function of H were also studied. These magnetotransport effects are caused by the s-d interaction. Some specific magnetoresistance mechanisms are discussed. The most likely dominant mechanisms are related to the giant spin splitting of the conduction band.
Strain Modification of Alloy Fluctuations in CdTe/(Cd,MnTe) Superlattice Systems
Authors: Jackson S. A. , McIntyre C. R. 
Published: March 2011

AbstractWe have calculated the effect of strains on the valence band offset and bandgap in the CdTe/(Cd,Mn)Te superlattice system for various growth directions and alloy compositions. We find that strain can modify the effect of alloy fluctuations on band offset and bandgap. The consequences of this for the formation of interface states will be discussed.
EXAFS Determination of Bond Lengths in Zn1−x MnxSe
Authors: Bunker B. A. , Pong W.-F. , Debska U. , Yoder-Short D. R. , Furdyna J. K. 
Published: March 2011

AbstractEXAFS has been used to determine bond lengths in the diluted magnetic semiconductor Zn1−xMnxSe. These alloys change from a cubic (zincblende) crystal structure to a hexagonal (wurtzite) structure as a function of composition, with the transition at x ≈ 0.3. Although x-ray diffraction measurements show the lattice parameter to change by approximately 0.2Å through the series and the crystal structure changes from cubic to hexagonal, the nearestneighbor bond distances show no change within the experimental uncertainty of 0.0Å.
Universality of the Spin-Glass Transition in the Cd1−xMnXTe System
Authors: Datta T. , Amirzadeh J. , Barrientos A. , Jones E. R. , Schetzina J. F. 
Published: March 2011

AbstractUniversality of the spin-glass (SG) transition in bulk Cd1−xMnxTe was investigated by determining the scaling behavior of the spin-glass order parameter q as a function of the reduced temperature t = (Tg-T)/Tg, where Tg is the transition temperature. q(T) was determined from the SQUID magnetometric data both above and below the transition. It can be shown that, q(T) = 1 + T[│θ│ − C/χ(T)]−1. C and θ, the Curie and Curie-Weiss parameters, were obtained from a non-linear regression of the dc low field susceptibility χ(T) above Tg. q(T) thus obtained exhibits the cannonical order parameter criteria, viz. q(T>Tg) = 0, q(T→Tg)= 0, and q(T→Tg)→0, and q(T→0)→1. In range of Mn concentration studied (0.3 ≦ x ≦ 0.55), Tg ranged between 13 K and 23.5 K. Thus q as a function of absolute temperature behaves differently for different x. However, universality is clearly evidenced when the dependence on t is determined. q(t) exhibits a universal scaling law. We observe q ˜ tβ with β ≅ 0.95. Overall good agreement was noted with the Sherrington-Kirkpatrick (SK) infinite range SG model. Observed value of β is in excellent agreement with the model prediction β −1. But in the Cd1−xMnxTe system we find θ<0, indicating a net antiferromagnetic interaction and │θ│/Tg > 1 as opposed to the SK model. We believe this to be additional evidence that the spin interactions are not distributed as a Gaussian function.
Antiferromagnetic Exchange Constants Between Mn2+ Ions in II-VI Semimagnetic Semiconductors
Authors: Aggarwal R. L. 
Published: March 2011

AbstractMn2+ ion spins in II–VI compound semimagnetic semiconductors (SMS) such as Cd1−xMn Te interact with one another through an antiferromagnetic exchange which is responsible for many interesting properties of Mn-alloyed II–VI SMS. In particular, this interaction leads to unique magnetization (M) behavior of SMS as a function of applied magnetic field (B). The nearest-neighbor (NN) Mn2+ −MN2+ interaction has been shown to yield a series of five steps in M vs. B curves above about 10 T, providing a direct determination of the NN exchange constant JNN. A detailed analysis of the magnetization steps has yielded values for the next NN exchange constant JNNN. The magnetization steps also yield information on the distribution of Mn2+ ions. Inelastic neutron scattering and Raman scattering provide alternative methods for the direct determination of the exchange constants.
Transient Spectroscopy and Related Optical Studies in Diluted Magnetic Semiconductor Superlattices
Authors: Nurmikko A. V. , Kolodziejski L. A. , Gunshor R. L. 
Published: March 2011

AbstractRecent successes in molecular beam (MBE) epitaxial growth of diluted magnetic semiconductor (DMS) artificial microstructures are now generating structures in which optically excited lower dimensional electronic and magnetic phenomena can be investigated. We consider an example which illustrates the present early state of affairs: transient magnetic polarons in connection with localized quasi 2-dimensional (2D) electronic excitations. We also comment on recent measurements and antiferromagnetic ordering in ultrathin layers in magnetic semiconductors.
Liquid Materials and Flow Processes in Reduced Gravity
Authors: Scriven L. E. 
Published: March 2011

ABSTRACTFluid states are central to melt and vapor processing. Fluids flow. As they do they convect heat, solutes, and inclusions. Liquid/gas interfaces have tension. Their importance rises when scale is small or effective gravity low.The forces that move fluids and mold fluid interfaces are recounted here. Fundamentals of viscous flow, convective transport, and capillary phenomena are summarized with special attention to the roles of gravity in liquid processing. Research needs are highlighted.
An Experimental Study of Heat Induced Surface-Tension Driven Flow
Authors: Kamotani Y. , Ostrach S. , Lowry S. 
Published: March 2011

ABSTRACTVelocity and temperature measurements are taken of heat induced surface-tension driven flows in both silicone oil and Fluorinert FC-43. Each fluid is contained in an open rectangular box and heated from above by a single strand of electrically heated nichrome wire suspended slightly above the fluid's surface. Velocity measurements are obtained for both liquids with a laser anemometer system. For silicone oil the general flow pattern is recorded using time-lapse photography. The surface temperature measurements are taken by a radiation thermometer and the bulk temperature distributions are measured by thermocouples. The velocity and temperature measurements are compared with numerical solutions obtained for the present configuration.
Overview of Containerless Processing Technologies
Authors: Barmatz M. 
Published: March 2011

ABSTRACTIn the near future, a large segment of the scientific community will have an opportunity to perform materials processing experiments on the Space Shuttle in the reduced gravity environment of space. Many of these experiments will require containerless processing techniques that provide manipulation and control of weightless (molten) materials without physical contact with container walls or other holding devices. A variety of containerless processing technologies are now being developed for space and ground-based materials processing facilities. The utilization of air jets or high intensity acoustic, electromagnetic or electrostatic fields can produce forces that support and manipulate materials. Most of the present containerless research is directed toward the development of high temperature systems capable of melting and resolidifying materials. This paper will review the materials processing capabilities and level of developmental progress of each technique. An introduction to available NASA test facilities will also be given.
Glass Processing in a Microgravity Environment
Authors: Uhlmann Donald R. 
Published: March 2011

ABSTRACTThe scientific and technological aspects of processing glasses in a microgravity environment offer considerable challenge and potential. Activities directed to obtaining information relevant to processing materials on earth and eventually processing materials in space for use in space will be considered. The features on the space environment—reduced gravitational forces, vacuum of large capacity, unique radiation conditions, and unlimited dimensions of space—will be discussed, with emphasis directed to the first. The implications of reduced gravitational forces for containerless processing, freedom from deformation, the study and control of weak forces, and the absence of convection and density segregation will be explored. On-going programs concerned with glass processing in space will be reviewed, and additional areas which merit investigation will be considered.
Bubble Behavior in Molten Glass in a Temperature Gradient
Authors: Meyyappan M. , Subramanian R. S. , Wilcox W. R. , Smith H. 
Published: March 2011

ABSTRACTGas bubble motion in a temperature gradient was observed in a sodium borate melt in a reduced gravity rocket experiment under the NASA SPAR program. Large bubbles tended to move faster than smaller ones, as predicted by theory. When the bubbles contacted a heated platinum strip, motion virtually ceased because the melt only imperfectly wets platinum. In some cases bubble diameter increased noticeably with time.
Gels and Gel-Derived Glasses in the Na2o-B2o3-Sio2 System
Authors: Mukherjee Shyama P. 
Published: March 2011

ABSTRACTThe containerless melting of high-purity multicomponent homogeneous gels and gel-monoliths offers a unique approach to making ultrapure multicomponent optical glasses in the reduced gravity environment of space. Procedures for preparing and characterizing gels and gel-derived glasses in the Na 2o-B2o3-SiO2 system are described. Preparation is based on tLe polymerization reactions of alkoxysilane with trimethyl borate or boric acid and a suitable sodium compound. The chemistry of the gelling process is discussed in terms of process parameters and the gel compositions. The physicochemical nature of gels prepared by three different procedures were found to be significantly different. Infrared absorption spectra indicate finite differences in the molecular structures of the different gels. The melting of the gel powders and the transformation of porous gel-monoliths to transparent “glass” without melting are described.
Glass Shell Manufacturing in Space
Authors: Nolen Robert L. , Downs Raymond L. , Ebner Matthias A. 
Published: March 2011

ABSTRACTHighly-uniform, hollow glass spheres, which are used for inertial-confinement fusion targets, are formed from metalorganic gel powder feedstock in a drop-tower furnace. The modelling of this gel-to-sphere transformation has consisted of three phases: gel thermochemistry, furnace-to-gel heat transfer, and gravity-driven degradation of the concentricity of the molten shell.The heat transfer from the furnace to the free-falling gel particle was modelled with forced convection. The gel mass, dimensions, and specific heat as well as furnace temperature profile and furnace gas conductivity, were controlled variables. This model has been experimentally verified. In the third phase, a mathematical model was developed to describe the gravity-driven degradation of concentricity in molten glass shells.
The Role of Fluid Flow Phenomena in the Czochralski Growth of Oxides.
Authors: Miller D.C. 
Published: March 2011

ABSTRACTIn the Czochralski growth of single crystals from large melts, fluid flow phenomena have a major effect on interface shape, growth striations, defect density and the length of crystals which can be grown from a melt of given volume and thermal geometry. Because of the technical difficulties encountered in making direct measurements in molten oxides, simulation experiments have been extensively utilized to gain insight into melt behavior.Both temperature profile and flow geometry results from simulation experiments are discussed. This data is supported by direct melt observations and results from the characterization of grown crystals. When reviewed together, this information offers new insights into the complex behavior of Czochralski growth processes, including the role of thermal gradients, crystal rotation, and surface tension driven (Marangoni) convection.
Modelling of Tgs Growth in Space
Authors: Liu L. C. , Wilcox W. R. , Kroes R. , Lal R. 
Published: March 2011

ABSTRACTTo maintain a constant growth rate in the absence of convection it is necessary to program down the temperature. Trial and error numerical computations were performed to find the first approximation for the required isothermal dissolution period, linear ramp; and polynomial temperature variation period. The linear ramp rate was limited by the specified maximum temperature gradient. The isothermal dissolution turned out to be unnecessary. The linear ramp had to be stopped before the polynomial period was begun to avoid overshooting the specified maximum growth rate. After a few hours the temperature profile approached steady state behavior.
Growth of Triglycine Sulfate (Tgs) Crystals by Solution Technique
Authors: Lal R. B. , Kroes R. L. , Wilcox W. R. 
Published: March 2011

ABSTRACTThe growth of crystals from solution is greatly influenced by buoyancy driven convection. In a low-g environment, convection is greatly suppressed and diffusion becomes the predominant mechanism for thermal and mass transport. An experiment to grow TGS crystals by solution technique during the orbital Spacelab III mission has been designed. Crystals are grown by a new and unique technique of extracting heat from the crystal through a sting. The cooling at the sting tip is responsible for the desired supersaturation near the growing crystal. Calculations indicate that the cooled sting technique for solution crystal growth is necessary in low-g to maintain a maximum growth rate of 1 mm/day. Results of groundbased work in support of the flight experiment are discussed.
Point Defects in Doped and Undoped Hgl−xCdxTe Alloys
Authors: Vydyanath H. R. 
Published: March 2011

EXTENDED ABSTRACTUsing a law of mass action approach in which lattice defects are treated as chemical species, the variation of the defect concentrations as a function of .the physicochemical conditions of preparation has been established via measurements of electrical characteristics which are directly related to the defect concentrations.
Convective and Interfacial Instabilities During Solidification of Succinonitrile Containing Ethanol
Authors: Schaefer R. J. , Coriell S. R. 
Published: March 2011

ABSTRACTAlthough slow convective flow is difficult to detect in solidifying metals, it can readily be observed in transparent materials by observing the motion of small neutrally buoyant particles. An excellent material for such studies is succinonitrile, which solidifies with an unfaceted solid/liquid interface and which has well characterized physical properties. For studies of solute-induced convection, ethanol is a useful addition to succinonitrile because it has a lower density and a somewhat similar molecular structure.Samples of high purity and ethanol-doped succinonitrile are solidified unidirectionally in a vertical temperature gradient. Latex microspheres, 2μm in diameter, are suspended in the liquid to delineate convective flow. Convective and morphological stability are observed as a function of solute concentration and growth velocity. These measurements are compared to theoretical calculations which predict the transition from stability to instability as a function of solidification conditions. The predicted transitions occur at low concentrations and solidification velocities, so that extreme care is required to eliminate the effects of impurities or thermally-induced convection.
A Dynamic Technique for Measurements of Thermophysical Properties at High Temperatures
Authors: Cezairliyan Ared 
Published: March 2011

ABSTRACTA technique is described for the dynamic measurements of selected thermophysical properties of electrically conducting substances in the range 1500 K to the melting temperature of the specimen. The technique is based on rapid resistive self-heating of the specimen from room temperature to any desired high temperature in less than one second by the passage of an electrical current pulse through it and on measuring the pertinent quantities, such as current, voltage and temperature, with millisecond resolution. The technique was applied to the measurements of heat capacity, electrical resistivity, hemispherical total emittance, normal spectral emittance, thermal expansion, melting temperature, heat of fusion, and temperature and energy of solid-solid phase transformations. Upper temperature of the measurements has been limited by the melting temperature of the specimen, at which point, the specimen collapses due to the gravitational force. In order to be able to extend the measurements to the liquid phase, performance of the dynamic experiments in a near-zero gravity environment is suggested.
Gravitationally Induced Convection During Directional Solidification of off-Eutectic Mn-Bi Alloys
Authors: Pirich Ron G. 
Published: March 2011

ABSTRACTThe effects of thermal and solute gradient, gravity induced convection during vertical directional solidification, on longitudinal macrosegregation of Bi and Mn rich off-eutectic starting compositions, has been studied as a function of composition, growth velocity and gravity vector orientation. Since the morphology of these alloys is characterized by an aligned, rodlike permanent magnet composite when grown cooperatively, the magnetic properties were used to measure composition segregation and the transition from dendritic to composite growth. Severe macrosegregation was observed in all cases studied and the degree of convection inferred by modeling the observed composition segregation using a stagnant film approach. Morphological stability was found to follow a constitutional supercooling-type law for both Bi and Mn rich compositions.
The Effect of The Natural Convection on The Transition from Columnar to Equiaxed Crystals:
Authors: Fredriksson Hasse 
Published: March 2011

ABSTRACTDuring dendritic solidification of metal alloys there normally occur temperature gradients in the liquid ahead of the solidification front. The gradient causes a natural convection ahead of the front. The natural convection causes a heat transfer from the interior of the liquid, which gives the possibility for crystals to grow ahead of the solidification front. The growth of these crystals is determined by the cooling rate and the kinetics of the solidification process. The paper will deal with the theory behind the growth of free crystals ahead of a solidification front. Some experimental results will be given. The effect of gravity on the growth process will. be discussed.
Theory and Applications of Electromagnetic Levitation
Authors: Frost R. T. , Chang C. W. 
Published: March 2011

ABSTRACTThe basic theory of electromagnetic levitation of conducting specimens by alternating fields is reviewed. Emphasis is placed upon simple approximation formulae useful in planning and interpreting laboratory measurements. Recent work involving computer solutions for forces and heating by complex field coil configurations, including those with polyphase excitation, is described. Applications and limitations of the technique both terrestrially and in a microgravity environment are discussed.
Growth and Characterization of High Quality, Low Defect, Subgrain Free Cadmium Telluride by a Modified Horizontal Bridgman Technique
Authors: Allred W. P. , Khan A. A. , Johnson C. J. , Giles N. C. , Schetzina J. F. 
Published: March 2011

ABSTRACTA low stress modified horizontal Bridgman technique has been developed and used to grow low defect, large area, subgrain free CdTe crystals for use as substrates in the epitaxial growth of HgCdTe and related IR detector materials. CdTe wafers cut from horizontal Bridgman grown boules exhibit, resistivities in the 107ohm-cm range. Etch pit counts are in the 104cm−2 range. Etch pit patterns as well as x-ray topographs indicate the absence of low-angle grain boundaries. Double crystal x-ray rocking curves are single peaked and very narrow with FWHM(333) as low as 9 arc-sec. Rocking curves of FWHM(333) = 9 to 15 arc-sec, measured at several different laboratories, have been obtained for CdTe wafers cut from several boules. This is in contrast to standard vertical Bridgman grown CdTe samples, which generally show broader x-ray rocking curves sometimes with multiple peaks as a result of subrgrain structure. Low temperature (1.6–4.5 K) photoluminescence (PL) measurements on these low defect samples reveal bright edge emission lines which are the main feature of the spectrum. Additional bound exciton lines and other sharp features associated with donor and acceptor impurities are also present. The very weak defect band luminescence (1.40–1.46 eV) provides additional evidence of sample quality.