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Fluid flow and heat transfer in a hydrothermal crystal growth system have been modeled using fluid-superposed porous layer theory. Focus on developing a basic understanding of the transport mechanisms led to focus on the flow and temperature fields in an autoclave with no baffle. Hydrothermal synthesis using aqueous solvents with high pressure and low temperature is important for difficult-to-grow crystalline materials and a replica of the way in which crystals grow in geological circumstances. The system can be modeling as a composite fluid and porous layer.

Research was conducted to examine the low-temperature dynamic characteristics of photorefractive gratings in cobalt-doped potassium-strontium-sodium-barium-niobate crystals (Co:KNSBN). The preillumination of the sample led to a shorter write-erase cycle of the grating. Measurement of the temperature dependence of the dark decay of the photorefractive grating was performed to complete the study and to illustrate the multiple shallow levels that exist in the Co:KNSBN crystal.

Research was conducted to examine the behavior of As precipitation in low-temperature grown III-V arsenides and to correlate it with the doping level, dislocation density and crystal bond strength. Results indicate that the doping level affects the concentration of charged defects such as vacancy and antisite point defects and leads to the selective precipitation of excess As in homojunctions. Findings suggest that column III vacancies have an important role in As precipitation of low-temperature grown III-V arsenides.

Research was conducted to examine the characteristics of SiO2 film and the Si/SiO2 interface formed by remote plasma enhanced chemical vapor deposition with the addition of chlorine into SiH4-N2O. Fourier transform infrared spectroscopy, x-ray photoelectron spectroscopy, ellipsometry, secondary ion mass spectroscopy, electron spin resonance, transmission electron microscopy and atomic force microscopy were used to characterize the SiO2 films deposited with the chlorine addition.

Research was conducted to examine the electrical and optical characteristics of Be-doped InP epilayers grown at low temperature by solid source atomic layer molecular beam epitaxy (ALMBE). By controlling both the amplitude and the length of the phosphorus pulse, the residual n-type doping was reduced. Results demonstrate a well controlled p-type doping indicating that ALMBE is a useful method for the growth of p-type InP at low substrate temperature.

Research was conducted to examine the synthesization of two novel, isostructural, one-dimensional tellurites of molybdenum(VI), (NH4)4Mo6TeO22*2H2O and Rb4Mo6TeO22*2H2O using the hydrothermal approach. Characterization was also examined by means of single-crystal X-ray diffraction methods. The structure, hydrothermal syntheses, spectroscopy, powder X-ray diffraction and thermal studies of the compounds were also carried out.

Research was conducted to examine the properties of the diffuse phase transition in Mn doped Pb(Fe2/3W1/3)O3 relaxor ceramics. The freezing temperature (T(sub f)) was observed to change with the Mn content. T(sub f) increased with an increase in Mn content and an increase in 1:1 order of Fe3+ and W6+ on B sites by air annealing but decreased by oxygen annealing. Results indicate that the defect pairs will decrease and cause the opposite effect if oxygen vacancies are filled up by oxygen annealing.

Research was conducted to examine Gd(DOTA)- and Gd(DTPA)2- complexes in the crystalline state and aqueous solutions for various pH values at room temperature and other temperatures using X-ray absorption fine structure analysis. Results demonstrate that the complexes are very rigid. The experimental Fourier transforms of the complexes studied as crystals and in aqueous solutions can be easily explained by mass spectrometric calculations.

An automated particle analysis routine is implemented on an electron microprobe for analyzing the chemical composition and projective area of populations of individual silver halide microcrystals. An LN2 cryostage is used to prevent material degradation due to reaction with the impinging electron beam. The background in the EDX spectra is lowered by depositing the microcrystals on a carbon-coated copper grid, mounted in a transmission holder. The IL[Alpha]/AgL[Alpha] net X-ray intensity ratio, obtained from a spectrum-fitting algorithm, is used to determine the crystal composition by means of a standard-based calibration curve. The uncertainty on the concentration measurement of individual microcrystals is calculated using the uncertainties on the net X-ray counts and the uncertainties on the calibration curve. The area measurement is optimized by introducing a gray value histogram correction on each individual measurement. Overlapping microcrystals are scrapped from the analysis by defining a maximum shape factor, against which the shape factor of the microcrystal is tested. To minimize problems with drift of the cryostage, spectrum acquisition is carried out immediately after a single microcrystal has been located, based on the backscattered electron image. Several applications are discussed.

Research was conducted to investigate the colossal magnetoresistance in Gd1/2Sr1/2MnO3 perovskite. Irreversibility and sharp anomalies in the magnetostriction, magnetization and magnetoresistance isotherms occur at low temperatures which is attributed to the establishment of a charge-ordered state. The appearance of the anomaly is attributed to the partial delocalization of the carriers by the field.

Research was conducted to investigate the Er-related luminescence in GaP:Er, grown by organometallic vapor phase epitaxy, at low temperature under above-band gap and below-band gap excitation. Photoluminescence spectra obtained under all experimental conditions were dominated by several sharp emission lines. Results suggest that some Er-related luminescence centers are activated by direct excitation of electrons to some traps related to the luminescence centers.

Research was conducted to examine the low temperature negative magnetoresistance and magnetization of molecular beam epitaxy grown (InSb)(sub 1-x)Y(sub x). Analytical tools used include x-ray diffraction, magnetization and Hall effect measurements. Results indicate that the yttrium atoms are located as interstitials in the InSb lattice and do not hybridize their outer electrons. A

Research was conducted to examine the low temperature photoluminescence characteristics of Zn-doped InP deposited by metalorganic chemical vapor deposition. A comparison of their low temperature photoluminescence features was performed and analyzed. Two new bands emerged after a distant deep transition peak disappeared for in situ doped samples at the hole saturation level which showed that the two bands were related with the saturation of the hole concentration.

Research was conducted to examine photoemission under high photoexcitation in thick ZnSe epilayer grown by molecular beam epitaxy (MBE) using time-resolved photoluminescence spectroscopy. The existence of biexcitons was revealed under picosecond pulsed excitation as biexciton emission was observed 4 meV below the free exciton emission. Results indicate that biexciton recombination is responsible for the spontaneous emission commonly seen at lasing threshold under optical pumping at low temperatures.

Research was conducted to examine the low-temperature chlorination of GaAs(100). Soft X-ray photoelectron spectroscopy, which employs syhnchrotron radiation, was used to investigate the chemisorption and the reaction of chlorine and hydrogen chloride on the GaAs surface at 110 K. Results provide an insight into the reaction mechanism of the synchrotron radiation-induced etching of the GaAs surface by Cl2 at low temperatures.

Research was conducted to study the effects of electron impact on the oxidation of Si(100) by O2 at high and low electron energies using X-ray photoelectron and Auger electron spectroscopies. Results indicate that electron beam impact was capable of increasing the rate of silicon oxidation and, thus, led to silicon dioxide formation. Findings also suggest the involvement of dissociative electron attachment in the oxidation mechanism and the formationof O and O- species from O-(sub 2).

Terahertz radiation was generated with several designs of photoconductive antennas (three dipoles, a bow tie, and a coplanar strip line) fabricated on low-temperature-grown (LT) GaAs and semi-insulating (SI) GaAs, and the emission properties of the photoconductive antennas were compared with each other. The radiation spectrum of each antenna was characterized with the photoconductive sampling technique. The total radiation power was also measured by a bolometer for comparison of the relative radiation power. The radiation spectra of the LT-GaAs-based and SI-GaAs-based photoconductive antennas of the same design showed no significant difference. The pump-power dependencies of the radiation power showed saturation for higher pump intensities, which was more serious in SI-GaAs-based antennas than in LT-GaAs-based antennas. We attributed the origin of the saturation to the field screening of the photocarriers. Key words: Photoconductive antenna, terahertz radiation, low-temperature-grown GaAs, semi-insulating GaAs, carrier-screening effect.

Experiment was conducted to study low temperature grown GaAs by elecromodulation reflectance spectroscopy for growth temperatures between 200 and 320 degrees C. The reflectance spectra can be observed only when the concentration of the arsenic antisite defect is lower than 3.4X10 (super 19) cubic cm. Results show that the transition energies of E0 and E1 increase with the decrease of the growth temperatures while the increase of the transition energies is attributed to lattice mismatch and deviation of the stoichiometry.

Research was conducted on nuclear spin-lattice relaxations of 9Be and 27Al in an alexandrite (BeAl2-xCrxO4, x=0.02) single crystal in the temperature range 140 K-400 K. Results show a paramagnetic impurity effect on the relaxation which is negligible for both nuclei. The relaxation of the 9Be nuclear spins is found single exponential in time when the whole resonance lines are uniformly excited. The relaxation is well fitted with a linear combination of three exponential functions when only the central resonance line is excited for 27Al.

We report a structure refinement ofa [P2.sub.1]/m ferromagnesian cummingtonite crystal with Fe/(Mg + Fe) = 0.37 at 140 K; this crystal was inverted from the C2/m structure, which is stable at room temperature. The phase-transition temperature is [approximately]240 K. The most significant structural changes associated with the C2/m-to-[P2.sub.1]/m phase transition are as follows: (1) one crystallographically independent O-rotated silicate chain becomes two, with the A chain S rotated and the B chain O rotated; (2) the M4 site collapses, with a marked decrease (0.020 [Angstrom]) in the average M4-O6 bond distance; and (3) the largest decrease in the isotropic displacement factor is for the M4 cation. From 295 to 140 K, total changes of 14 and 11 [degrees] occur in the O5-O6-O5 kinking angles of the A and B chains, respectively. The volume of the M1 octahedron decreases more than that of the other three M[O.sub.6] polyhedra, whereas the volume of the M3 octahedron is essentially unchanged. All four M[O.sub.6] polyhedra become more regular as a result of the C2/m-to-[P2.sub.1]/m transformation. These results provide new evidence that the effective size of the M4 cation controls the relative stability of the C2/m and [P2.sub.1]/m structures. The results also provide a new explanation for the unresolved broadening of the cummingtonite Raman spectra measured at low temperatures by Wang et al. (1988).

Acoustic quality factor of pure niobium and vanadium crystal samples are measured as functions of heat and temperature ranging from 50 mK to 300 Kelvin. Low thermal annealing temperatures yield the highest value for the acoustic quality factor. Superconductivity component of the shear modulus is computed using high-resolution frequency measurements.

An analysis of ultrafast carrier dynamics in As-based 3-5 semiconductors shows that molecular-beam-expitaxial growth of these devices at very low temperatures introduces excess As in the layers. Since this results in large density defects in the deep layers, the trapping and recombination of carriers at these levels cause a significant decrease in carrier lifetime. GaAs and In-based layers exhibit subpicosecond carrier lifetimes, very high resistivity behavior and good carrier mobilities.

Metal-induced crystallization using a Ni standard absorption solution was performed to crystallize amorphous silicon (a-Si). Ni solution was used instead of Ni deposition on a-Si to reduce metal contamination in poly-Si. Crystallization of a-Si films spin-coated with a 5,000 ppm Ni solution was observed at as low as 500 degrees C. Moreover, needlelike morphology was detected in the MIC poly-Si. Full crystallization of a-Si can take place at 500 degrees C for 20 hours.

Piezoelectric materials have many applications at cryogenic temperatures. However, the piezoelectric response below 10 kelvin is diminished, making the use of these materials somewhat marginal. Results are presented on strontium titanate (SrTi[O.sub.3]), which exhibits a rapidly increasing piezoelectric response with decreasing temperature below 50 kelvin; the magnitude of its response around 1 kelvin is comparable to that of the best materials at room temperature. This 'giant' piezoelectric response may open the way for a broad class of applications including use in ultralow-temperature scanning microscopies and in a magnetic field-insensitive thermometer. These observations, and the possible divergence of the mechanical response to electric fields at even lower temperatures, may arise from an apparent quantum critical, point at absolute zero., The phenomenon of piezoelectricity, as a solid-state method for converting electrical signals into mechanical motion, has become ubiquitous in its applications in the laboratory as well as in everyday life. [...]

A crystal composed of super-paramagnetic manganese clusters (Mn(sub 12)-ac), at low temperature, demonstrates tunnelling between quantum mechanical behavior at the microscopic level and classical behavior at the macroscopic level. The magnetization exhibits hysteresis loops with staircase structure indicating quantization of the magnetization. The magnetic relaxation of the Mn(sub 12)-ac crystal is enhanced when the spin-up and spin-down level scheme coincide indicating the existence of resonant macroscopic quantum tunnelling in the magnetic system.

The depth dependence of bubble concentration at pressures above the transition to the air hydrate phase and the optical scattering length due to bubbles in deep ice at the South Pole are modeled with diffusion-growth data from the laboratory, taking into account the dependence of age and temperature on depth in the ice. The model fits the available data on bubbles in cores from Vostok and Byrd and on scattering length in deep ice at the South Pole. It explains why bubbles and air hydrate crystals coexist in deep ice over a range of depths as great as 800 meters and predicts that at depths below ~1400 meters the AMANDA neutrino observatory at the South Pole will operate unimpaired by light scattering from bubbles., Ancient air is known to be trapped in polar ice at depths below the layer of firn (that is, porous) ice. Early investigations showed that the air was trapped in [...]

A low-temperature solid-state synthetic method for the preparation of the tetragonal infinite layer phase of SrCuO2 based on pulsed laser deposition (PLD) is presented. The synthesis is performed in a stainless steel vacuum chamber evacuated with a turbomolecular pump. The desired SrCuO2 is synthesized from a 1:1 mixture of SrCO3 and CuO, giving the product in the orthorhombic phase. Ablation and deposition of the orthorhombic SrCuO2 gives the tetragonal layered phase.

Crystalline oxide films are important components in a wide array of electronic and optical devices, and their study and manufacture involve major aspects of current science and technology. A plethora [...]

New measurements of oxygen out-diffusion in Czochralski silicon, which verifies the previously reported time dependence of D(sub eff). It was shown that the profiles evolve by a temporal variation of the surface concentration and that the rate at which it decreases is correlated with that of the loss of isolated interstitial sites in the bulk. Findings are in agreement with an alternative model for the diffusion mechanism.

Research was conducted to examine the effect of doping and stoichiometry on the native defects in GaAs grown by molecular-beam epitaxy at 250 degrees C using a low-energy positron beam. By calculating the momentum distribution of annihilating core electrons, Ga vacancies are identified in all samples. It is proposed that the Ga vacancies are complexed with As antisites in undoped and Be-doped samples and with Si impurities in n-type material.

Research was conducted to examine the crystal structure of the xylanase from Penicillium simplicissimum. The binding of the natural, unmodified substrates to the native enzyme and the mapping of the xylose subunit binding sites using the substrate as cryoprotectant with a series of xylooligosaccharides were performed. Results demonstrate that the two catalytic residues divide the binding cleft into a substrate recognition area with strong and specific xylan binding and a product release area with considerably weaker and less specific binding.

Research was conducted to examine the effect of cooling of crystals of beta-barium metaborate (BBO) on second harmonic generation (SHG). For the calculations, a single BBO crystal, INRAD, was used and cut at 86 degrees to the c axis and was 10x8x8 cu mm. Results reveal that the 203.1-nm wavelength is the shortest ever generated in a crystal by SHG. The maximum output was at 205.5 nm which is near the 205.2-nm output convenient for probing hydrogen atoms.