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Magneto-optic characterizations of superlattices and wedged sandwiches with oscillatory interlayer magnetic coupling

Description: Three examples of magnetic coupling across metallic spacer layers are considered. Fe/Nb sputtered superlattices are observed to have as many as five antiferromagnetic oscillations, but a weak magnetoresistive anomaly. Epitaxial trilayers of Fe/Mo/Fe grown on Mo(100) and Co/Cu/Co grown on Cu(100) are observed to have short- and long-period oscillations, respectively. The trilayers are grown with wedged spacer layers and characterized in-situ by means of the magneto-optic Kerr effect.
Date: July 1, 1992
Creator: Bader, S. D.
Partner: UNT Libraries Government Documents Department

Incommensurate lattice-instability in K/sub 2/SeO/sub 4/

Description: Successive phase transformations in K/sub 2/SeO/sub 4/ at T/sub i/ = 130K and T/sub c/ = 93K have been studied by the triple-axis neutron scattering technique. Above T/sub i/ a SIGMA/sup 2/ optic phonon branch along (xi,0,0) shows striking softening and ..omega..(vector q) for vector q approximately (1/3,0,0) tends to zero at T/sub i/. This softening results from a temperature dependent decrease of the interlayer forces with range a/2 and a (a is one unit cell length along the a axis) in the presence of strong and persisting forces with a range 3a/2. The critical scattering above T/sub i/ has a maximum at an incommensurate position: vector q/sub delta/ = (1-delta) vector a*/3 with delta approximately 0.08 and peaks at E = 0 near T/sub i/. At T/sub i/ superlattice reflections appear at incommensurate positions, vector q/sub delta/. The deviation delta decreases with decreasing temperature with an apparently discontinuous jump to delta = 0 at T/sub c/. Below this temperature the crystal remains commensurate and is ferroelectric. The incommensurate transition, the simultaneous lock-in of the commensurate phase and the ferroelectricity are discussed using a Landau type expansion of the free energy.
Date: January 1, 1976
Creator: Iizumi, M; Axe, J D; Shirane, G & Shimaoka, K
Partner: UNT Libraries Government Documents Department

HREM defects in GaAs/Ga sub 1-x In sub x As strained layer superlattices

Description: GaAs/Ga{sub 1{minus}x}In{sub x}As strained layer superlattices with well-widths of 7nm, barrier widths of 14nm and periods of 10 to 30 have been examined by HREM both in plan view and cross section. Strain release occurs mostly by dislocation generation but twins are also observed, especially at the substrate interface. High resolution images of dislocations, twins and interfaces are analyzed to elucidate mechanisms of strain release. 5 refs., 7 figs.
Date: January 1, 1990
Creator: Unal, O.; Laurich, B. K. & Mitchell, T. E.
Partner: UNT Libraries Government Documents Department

Lattice parameters and structures of iron-silicon single crystals and the elastic constants of Fe/sub 3/Si

Description: The room temperature lattice parameters of thirteen slowly cooled iron-silicon single crystals containing from 3.61 to 25 atom percent silicon were measured. In addition, x-ray diffraction patterns were obtained from (100), (110), and (111) faces of each crystal. The lattice parameter versus composition curve has a newly discovered, and well defined, plateau extending from approximately 9.5 to 12 atom percent silicon. The inflection point at 9.5 atom percent silicon coincides with the first observance of superlattice reflections in these alloys. The inflection point at 12 atom percent silicon coincides with known changes in slope in the composition dependence of both the electrical resistivity and the elastic constants. In addition the elastic constants of a Fe/sub 3/Si single crystal were measured over the temperature range of 4.2 to 300 degrees Kelvin. The electrical resistivity of this same alloy was also measured over the temperature range of 4.2 to 400 degrees Kelvin.
Date: October 1, 1976
Creator: Rausch, J. B.
Partner: UNT Libraries Government Documents Department

High temperature resistive phase transition in A15 high temperature superconductors. [433/sup 0/K in Nb/sub 3/Sn; 485/sup 0/K in Nb/sub 3/Ge]

Description: Resistive measurements were made on A15 high temperature superconductors. Anomalies indicative of a phase transition were observed at 433/sup 0/K in a single crystal Nb/sub 3/Sn and at 485/sup 0/K in an unbacked Nb/sub 3/Ge sputtered thin film. Results are compared with the high temperature transmission electron diffraction studies of Nb/sub 3/Ge films by Schmidt et al. A possible instability in the electron energy spectrum is discussed. (auth)
Date: January 1, 1976
Creator: Chu, C. W.; Huang, C. Y.; Schmidt, P. H. & Sugawara, K.
Partner: UNT Libraries Government Documents Department

Oscillatory interlayer magnetic coupling of sputtered Fe/Nb superlattices

Description: The saturation field of sputtered Fe/Nb superlattices oscillates as a function of the Nb thickness with a periodicity of [approximately]9 [Angstrom]. In contrast to the case of Fe/Cr superlattices, the concurrent magnetoresistance oscillations were found to be very weak. Yet polarized neutron reflection measurements confirm that the Fe/Nb superlattices with high saturation field possess a magnetic ground state of the [plus minus][plus minus] type. Neutron and x-ray measurements indicate that, while the crystalline and antiferromagnetic order is well developed along the thickness of the film, the average lateral size of the crystallites (as well as of the magnetic domains) is quite small. This effect (thought to be related to the gross mismatch of the iron and the niobium crystal lattices) may be the cause of the high overall resistance of the material, and its weak dependence on the magnetization.
Date: September 1, 1992
Creator: Mattson, J. E.; Fullerton, E. E.; Sowers, C. H.; Huang, Y. Y.; Felcher, G. P. & Bader, S. D.
Partner: UNT Libraries Government Documents Department

Lattice instability in single-crystal La/sub 2-x/Sr/sub x/CuO/sub 4/

Description: The structural phase transition from the tetragonal to the orthorhombic phase of doped and undoped samples of La/sub 2-x/Sr/sub x/CuO/sub 4/ has been investigated by using inelastic neutron scattering techniques. The rotational nature of the soft mode leads to moderate electron-phonon coupling and the mode is unlikely to enhance significantly conventional phonon mediated superconductivity. 13 refs., 3 figs., 1 tab.
Date: January 1, 1988
Creator: Boeni, P.; Axe, J. D.; Shirane, G.; Birgeneau, R. J.; Gabbe, D. R.; Jenssen, H. P. et al.
Partner: UNT Libraries Government Documents Department

Development of Direct and Optical Polarized Nuclear Magnetic Resonance (NMR) Methods for Characterization and Engineering of Mesophased Molecular Structures

Description: The development of NMR methods for the characterization of structure and dynamics in mesophase composite systems was originally proposed in this LDRD. Mesophase systems are organic/inorganic hybrid materials whose size and motional properties span the definition of liquids and solids, such as highly viscous gels or colloidal suspensions. They are often composite, ill defined, macromolecular structures that prove difficult to characterize. Mesophase materials are of broad scientific and programmatic interest and include composite load bearing foams, aerogels, optical coatings, silicate oligomers, porous heterogeneous catalysts, and nanostructured materials such as semiconductor quantum dot superlattices. Since mesophased materials and precursors generally lack long-range order they have proven to be difficult to characterize beyond local, shortrange order. NMR methods are optimal for such a task since NMR observables are sensitive to wide ranges of length (0-30{angstrom}) and time (10{sup -9}-10{sup 0}sec) scales. We have developed a suit of NMR methods to measure local, intermediate, and long range structure in a series of mesophase systems and have constructed correlations between NMR observables and molecular size, topology, and network structure. The goal of this research was the development of a strong LLNL capability in the characterization of mesophased materials by NMR spectroscopy that will lead to a capability in rational synthesis of such materials and a fundamental understanding of their structure-property relationships. We demonstrate our progress towards attaining this goal by presenting NMR results on four mesophased model systems.
Date: January 29, 2002
Creator: Maxwell, R; Baumann, T & Taylor, B
Partner: UNT Libraries Government Documents Department

Superlattice effects on the amorphization of ni-ti multilayers

Description: A phenomenological model is proposed to correlate the onset of solid-state amorphization with the loss of interfacial stability in Ni-Ti multilayers. Additionally, a temperature dependence to the onset of amorphization is attributed to the effect of interfacial coherency that varies with the Ni-Ti layer pair spacing.
Date: February 10, 2000
Creator: Jankowski, A F
Partner: UNT Libraries Government Documents Department

Macroscopic Superlattices of CdSe Colloidal Nanocrystals: Appearance and Optical Properties

Description: Two and three dimensional assemblies of colloidal nanocrystals (NCs) have been of great interest during recent years [1-3]. While size-dependent optical and electronic properties of isolated particles are particularly important for fundamental research, studies of their ordered assemblies provide a transition path to the engineering of materials and devices for future practical applications. Assemblies of NCs of different materials, such as semiconductors, metals and metal oxides, have been reported in the literature during recent years [4-7]. However, perfect, crystallographic-ordered assemblies of colloidal NCs or colloidal superlattices (SLs) have been observed so far only using transmission (TEM) and scanning electron microscopy (SEM) in a very small scale of a few hundred nanometers, while macroscopic characterization and device application demonstrations have been performed mainly on amorphous, randomly packed powders of NCs [8, 9]. To make SLs available for traditional methods of characterization, they should be obtained in a sufficiently large size. For colloidal NCs soluble in variety of solvents, simple growth from solution seems to be an appropriate choice to produce SLs. In solution, NCs act as large molecules that, as shown previously [1, 8], can form nanoscale ordered assemblies by the classical Frank-Cabrerra mechanism [10] of crystal growth. It is, however, not clear how such ordered structures will look at larger scale. Will they grow as 3-D faceted shapes of extended sizes, or will they form poly-domain structures with local crystallographic arrangement? Taking into account the complex nature of colloidal NCs consisting of relatively big crystalline cores surrounded by large organic surfactant molecules, it is hard to imagine easy formation of large-scale faceted SLs. Spontaneous growth of perfectly faceted crystals requires precisely uniform size, shape and orientation of building units within the crystallographic lattice. This makes the distribution of sizes and shapes that always exist in NCs one more reason to ...
Date: March 25, 2004
Creator: Zaitseva, N; Manna, L; Leon, F; Gerion, D; Saw, C & Galli, G
Partner: UNT Libraries Government Documents Department

Low-Temperature Aging Mechanisms in U-6wt% Nb

Description: Phase stability and aging mechanisms in a water-quenched (WQ) U-6wt% Nb (U-14at% Nb) alloy artificially aged at 200 C and naturally aged at ambient temperature for 15 years have been investigated and studied using Vickers-hardness measurement, X-ray diffraction (XRD) analysis, and transmission electron microscopy (TEM) techniques. Age hardening/softening phenomenon is recorded from the artificially aged samples based upon the microhardness measurement. The age hardening can be readily rationalized by the occurrence of fine-scaled Nb segregation, or spinodal decomposition, within the {alpha}'' domains, which results in the formation of a modulated structure containing nano-scaled Nb-rich and Nb-lean domains. Prolonged aging leads to age softening of the alloy by coarsening of the modulated structure. Chemical ordering, or disorder-order phase transformation, is found within the naturally aged alloy according to TEM observations of antiphase domain boundaries (APBs) and superlattice diffraction patterns. A possible superlattice structure for the ordered {alpha}'' phase observed in the naturally aged sample and underlying low-temperature aging mechanisms are proposed.
Date: December 7, 2004
Creator: Hsiung, L L
Partner: UNT Libraries Government Documents Department

Low-Temperature Agining Behavior of U-6 wt% Nb

Description: Phase stability and aging mechanisms in a water-quenched (WQ) U-6wt% Nb (U-14at% Nb) alloy artificially aged at 200 C and naturally aged at ambient temperature for 15 years have been investigated and studied using Vickers-hardness measurement, X-ray diffraction (XRD) analysis, and transmission electron microscopy (TEM) techniques. Age hardening/softening phenomenon is recorded from the artificially aged samples based upon the microhardness measurement. The age hardening can be readily rationalized by the occurrence of fine-scaled Nb segregation, or spinodal decomposition, within the {alpha}'' domains, which results in the formation of a modulated structure containing nano-scaled Nb-rich and Nb-lean domains. Prolonged aging leads to age softening of the alloy by coarsening of the modulated structure. Chemical ordering, or disorder-order phase transformation, is found within the naturally aged alloy according to TEM observations of antiphase domain boundaries (APBs) and superlattice diffraction patterns. A possible superlattice structure for the ordered {alpha}'' phase observed in the naturally aged sample and underlying low-temperature aging mechanisms are proposed.
Date: January 26, 2005
Creator: Hsiung, L L
Partner: UNT Libraries Government Documents Department

InGaAs/GaAs QD Superlattices: MOVPE Growth, Structural and Optical Characterization, and Application in Intermediate-Band Solar Cells

Description: We report on the growth and characterization of InGaAs/GaAs quantum dot (QD) superlattices for application in intermediate-band solar cells (IBSCs). Good optical and structural quality QD superlattices with up to 50 periods were obtained by metal-organic vapor-phase epitaxy (MOVPE) growth on {l_brace}113{r_brace}B GaAs substrates. Solar cells containing Si {partial_derivative}-doped and undoped QD superlattice absorption regions have been fabricated and their performance compared with control cells containing undoped GaAs or undoped InGaAs/GaAs superlattice absorption regions. The QD superlattice cells exhibited photoresponses extended to longer wavelengths than the control cells. The introduction of QDs to the absorbing region of the solar cells resulted in a decrease in the open-circuit voltages and, in some cases, a decrease in the short-circuit currents of the cells.
Date: February 1, 2005
Creator: Norman, A. G.; Hanna, M. C.; Dippo, P.; Levi, D. H.; Reedy, R. C.; Ward, J. S. et al.
Partner: UNT Libraries Government Documents Department

Low-Temperature Aging and Phase Stability of U6Nb

Description: Aging behavior and phase stability of a water-quenched U-6wt%Nb (U-14at%Nb) alloy artificially aged at 200 C and naturally aged at ambient temperature for 15 years have been investigated using Vickers hardness test, X-ray diffraction analysis, and transmission electron microscopy techniques. Age hardening/softening phenomenon is observed from the artificially aged samples according to microhardness measurement. The age hardening can be rationalized by the occurrence of spinodal decomposition, or fine scale of Nb segregation, which results in the formation of a nano-scale modulated structure within the artificially aged samples. Coarsening of the modulated structure after prolonged aging leads to the age softening. The occurrence of chemical ordering (disorder-order transformation) is found in the naturally aged sample based upon the observations of antiphase domain boundaries (APB's) and superlattice diffraction patterns. A possible superlattice structure is accordingly proposed for the chemically ordered phase observed in the naturally aged alloy sample.
Date: November 21, 2003
Creator: Hsiung, L M; Briant, C L & Chasse, K R
Partner: UNT Libraries Government Documents Department

GaInNAs Structures Grown by MBE for High-Efficiency Solar Cells: Final Report; 25 June 1999--24 August 2002

Description: The focus of this work is to improve the quality of GaInNAs by advanced thin-film growth techniques, such as digital-alloy growth techniques and migration-enhanced epitaxy (MEE). The other focus is to further investigate the properties of such materials, which are potentially beneficial for high-efficiency, multijunction solar cells. 400-nm-thick strain-compensated Ga0.92In0.08As/GaN0.03As0.97 short-period superlattices (SPSLs) are grown lattice-matched to GaAs substrates. The photoluminescence (PL) intensity of digital alloys is 3 times higher than that of random alloys at room temperature, and the improvement is even greater at low temperature, by a factor of about 12. The room-temperature PL intensity of the GaInNAs quantum well grown by the strained InAs/GaN0.023As SPSL growth mode is higher by a factor 5 as compare to the continuous growth mode. The SPSL growth method allows for independent adjustment of the In-to-Ga ratio without group III competition. MEE reduces the low-energy tail of PL, and PL peaks become more intense and sharper. The twin peaks photoluminescence of GaNAs grown on GaAs was observed at room temperature. The peaks splitting increase with increase in nitrogen alloy content. The strain-induced splitting of light-hole and heavy-hole bands of tensile-strained GaNAs is proposed as an explanation of such behavior.
Date: August 1, 2003
Creator: Tu, C. W.
Partner: UNT Libraries Government Documents Department

Optical Properties of CdSe Nanoparticle Assemblies

Description: We report on three-dimensional fluorescence imaging of micron-size faceted crystals precipitated from solutions of CdSe nanocrystals. Such crystals have previously been suggested to be superlattices of CdSe quantum dots [1,2]. Possible applications for these materials include their use in optical and optoelectronic devices. The micron-size crystals were grown by slow evaporation from toluene solutions of CdSe nanocrystals in the range of 3-6 nm, produced by traditional wet-chemistry techniques. By using a confocal microscope with laser illumination, three-dimensional raster-scanning and synchronized hyper-spectral detection, we have generated spatial profiles of the fluorescence emission intensity and spectrum. The fluorescence data of the micro-crystals were compared with spectra of individual nanocrystals obtained from the same solution. The results do not support the assertion that these microcrystals consist of CdSe superlattices.
Date: November 24, 2003
Creator: Huser, T; Gerion, D; Zaitseva, N; Krol, D M & Leon, F R
Partner: UNT Libraries Government Documents Department

Non-oscillatory antiferromagnetic coupling in sputtered Fe/Si superlattices

Description: A series of sputtered Fe(30{Angstrom})/Si(x) superlattices were grown for x=10--40{Angstrom}. Magnetization and Kerr hysteresis loops, and neutron-reflectively measurements identify antiferromagnetic (AF) coupling of the Fe layers at room temperature for x=15{Angstrom} nominal thickness, with switching fields of 6kOe. X-ray structural analysis indicate that the spacer medium is crystalline for x<20{Angstrom}, while sputtered Si is amorphous (a). Failure to detect oscillations in the AF coupling for thicker Si layers is due to the formation of a-Si, as opposed to the crystalline silicide responsible for the coupling.
Date: June 1, 1992
Creator: Fullerton, E. E.; Mattson, J. E.; Lee, S. R.; Sowers, C. H.; Huang, Y. Y.; Felcher, G. et al.
Partner: UNT Libraries Government Documents Department

Semiconductor isotope engineering

Description: Isotopic control of semiconductor crystals offers a wide range of scientific and technical opportunities. We review neutron transmutation doping of natural and isotopically controlled semiconductor structures, special properties of isotope superlattices, the effect of host isotopes on local vibrational modes of low mass impurities, and intrinsic properties which depend on isotope mass and isotopic composition of single crystals.
Date: May 1, 1993
Creator: Haller, E. E.
Partner: UNT Libraries Government Documents Department

Deposition and properties of novel nitride superlattice coatings. Progress report, 28 September 1990--October 1994

Description: The major effort during this report period was to elucidate the hardening mechanisms in polycrystalline superlattices. Since the last progress report, the authors have been working on theoretical modeling of hardening mechanisms in superlattices and have carried out detailed studies of two new polycrystalline superlattice coatings, NbN/VN and CrN/TiN, which were produced using an opposed, dual-cathode, high-rate, reactive, unbalanced-magnetron sputtering system. Significant hardness enhancement the above rule-of-mixture hardness value was found for the CrN/TiN, but not for the NbN/VN. Correlating these results with prior results for TiN/NbN and TiN/VN shows that the major hardening mechanism in polycrystalline superlattices is the difference in layer elastic moduli, which is minimal in the NbN/VN case. The modulus difference provides a barrier to dislocation flow across the layers. In the absence of modulus-difference hardening in NbN/VN, it was found that other hardening mechanisms such as coherency strains and the small grain sizes observed in the superlattices play a minor role. The results achieved in this report period are described in this paper along with publications and presentations.
Date: October 1, 1994
Creator: Barnett, S. A.; Sproul, W. D. & Wong, M. S.
Partner: UNT Libraries Government Documents Department

Quantum dynamical phenomena of independent electrons in semiconductor superlattices subject to a uniform electric field

Description: This report discusses the following topics: Bloch oscillations and other dynamical phenomena of electrons in semiconductor superlattices; solvable dynamical model of an electron in a one-dimensional aperiodic lattice subject to a uniform electric field; and quantum dynamical phenomena of electrons in aperiodic semiconductor superlattices.
Date: July 27, 1994
Creator: Bouchard, A. M.
Partner: UNT Libraries Government Documents Department

Energetics, bonding mechanism and electronic structure of metal/ceramic interfaces. Annual progress report, April 1, 1992--March 31, 1993

Description: Progress are reported on: electronic structure of PdO, PtO, and AgO (band structure calculations); ab initio calculations of electronic structure of TiO{sub 2}(110) surface; and electronic structure of VO{sub 2} and TiO{sub 2} thin films and multilayers. (DLC)
Date: December 31, 1993
Creator: Freeman, A. J.
Partner: UNT Libraries Government Documents Department

Photo- and thermal-induced antiferromagnetic interlayer coupling in Fe/(Fe-Si) superlattices

Description: We have grown via sputtering, Fe - iron silicide superlattices that exhibit novel thermal and photomagnetic effects. For {approximately}15 {Angstrom} nominal thickness of the silicide layer the superlattices are antiferromagnetically (AF) coupled at room temperature. Upon cooling below 100K, they gradually revert to ferromagnetic alignment, but the AF coupling is restored at low temperature by exposure to visible light off sufficient intensity (> 10 mW/mm{sup 2}). These effects are due to charge carriers in the silicide spacer layer which, when thermally or photo-generated, are capable of communicating spin information between the Fe layers.
Date: August 1, 1993
Creator: Mattson, J. E.; Fullerton, E. E.; Kumar, S.; Lee, S. R.; Sowers, C. H.; Grimsditch, M. et al.
Partner: UNT Libraries Government Documents Department

Excitons in semiconducting superlattices, quantum wells, and ternary alloys. Performance report, September 15, 1987--January 31, 1994

Description: Semiconducting layered structures can be fabricated with precisely defined layer thicknesses down to one monolayer; an example is superlattices (SLs), in which two semiconductors with different band gaps are interleaved. Research aims to understand optically excited states in SLs, particularly Type II indirect SLs (in which the excited electron and hole are separated both in real and in momentum space); time resolved tunable laser spectroscopy is used. Optical experiments were made in SLs with only a few atomic layers per period, to probe the interface quality. From the study of the exciton spectra of the mixed Type I-Type II CdTe/CdZnTe SLs, the band offset at the CdTe/CdZnTe interface was obtained to high accuracy. Analysis of exciton-phonon coupling at isoelectronic traps, which provide a 3-D analog of the exciton traps formed by well width fluctuations in SLs, has corrected errors and given a self-consistent model of phonon-assisted transitions at these centers.
Date: February 1, 1994
Creator: Sturge, M. D.; Nahory, R. E. & Tamargo, M. C.
Partner: UNT Libraries Government Documents Department

Forward scattering of neutrons from polymeric and magnetic multilayers

Description: Grazing incidence neutrons are not only reflected and refracted from imperfect layers, but also partially scattered by lateral dishomogeneities. In general, scattering may take place both in the reflection plane ({open_quotes}forward scattering{close_quotes}) and out of it. The forward scattering from multilayers is highly structured in ridges, whose maxima can be indexed in terms of the multilayer spacings. In experiments on diverse diblock copolymers two kind of ridges were found, either at constant-k{sub z} loci or else at constant-q{sub z} loci. The relative intensity of the ridges appears to be related to the type and size of imperfections of the lamellar structure. Constant-q{sub z} streaks of magnetic nature were also found in the forward scattering of metallic superlattices (Fe/Cr, Co/Ru, Fe/Nb) in the antiferromagnetic state. Here the diffuse scattering appears around the antiferromagnetic peaks while absent from the structural peaks, as if the crystalline superlattice were to partition into antiferromagnetic domains of limited lateral extension and columnar character.
Date: August 1, 1993
Creator: Felcher, G. P.
Partner: UNT Libraries Government Documents Department