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Author(s): R. P. Sharma [1]; S. B. Ogale [1]; Z. H. Zhang [2]; J. R. Liu [2]; W. K. Chu [2]; Boyed Veal [3]; A. Paulikas [3]; H. Zheng [3]; [...]

Under ultrahigh vacuum conditions, extremely small Ge nanodots embedded in SiO2, i.e., Ge–SiO2 quantum dot composites, have been formed by ion implantation of 74Ge+ isotope into (0001) Z-cut quartz at a low kinetic energy of 9 keV using varying implantation temperatures. Transmission electron microscopy (TEM) images and micro-Raman scattering show that amorphous Ge nanodots are formed at all temperatures. The formation of amorphous Ge nanodots is different from reported crystalline Ge nanodot formation by high energy ion implantation followed by a necessary high temperature annealing process. At room temperature, a confined spatial distribution of the amorphous Ge nanodots can be obtained. Ge inward diffusion was found to be significantly enhanced by a synergetic effect of high implantation temperature and preferential sputtering of surface oxygen, which induced a much wider and deeper Ge nanodot distribution at elevated implantation temperature. The bimodal size distribution that is often observed in high energy implantation was not observed in the present study. Cross-sectional TEM observation and the depth profile of Ge atoms in SiO2 obtained from x-ray photoelectron spectra revealed a critical Ge concentration for observable amorphous nanodot formation. The mechanism of formation of amorphous Ge nanodots and the change in spatial distribution with implantation temperature are discussed. [ABSTRACT FROM AUTHOR]

The defect structure of radiation damage due to irradiation of Ar[sup +] ions and the regrowth of LiTaO[sub 3] single crystal has been studied using Rutherford backscattering-channeling, cross-sectional transmission electron microscopy, optical transmittance, and surface profilometer. The irradiation were carried out at 77 K with 217 keV Ar[sup +] ions to various doses between 3×10[sup 13] and 6×10[sup 15] Ar[sup +]/cm[sup 2]. Postannealing was performed in dry O[sub 2] ambient at temperatures from 648 to 823 K, which are below Curie temperature (878 K). The optical measurement showed that the as-irradiated samples are as transparent as the unirradiated sample, indicating that the radiation-induced point defects are highly mobile even at 77 K. However, the long-range migration of oxygen interstitials were suppressed at 77 K. Volumetric expansion occurred when irradiation doses exceeded 1.25×10[sup 14] Ar[sup +]/cm[sup 2]. Collective lattice distortion was observed in the sample that had received a dose of 1.5×10[sup 14] Ar[sup +]/cm[sup 2]. This study shows that both disorder overlapping and the disorder-induced strain energy play important roles in the amorphization process. The regrowth of the amorphized layers depends on the microstructure of the damage. The activation energy for the regrowth of the amorphized layer that received more irradiation (6×10[sup 15] Ar[sup +]/cm[sup 2]) is 0.74 eV, lower than the activation energy of the amorphized layer (1.20 eV) that received less irradiation (6×10[sup 14] Ar[sup +]/cm[sup 2]). © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Investigates the mechanical stress caused by flux pinning in a cylindrical superconductor with a concentric hole. Derivation of exact expressions for the radial and hoop stress using the critical-state model; Analysis of stress profiles during a magnetization process used to activate high Tc superconductors as strong trapped-field magnets.

Focuses on a study which determined the initial rates of evaporation of gallium and arsenic from the surface of single-crystal gallium arsenide during rapid thermal processing. Experimental techniques used in the study; Measurement of evaporation rate; Discussion of proximity annealing.

Presents a study which determined the substrate temperature dependence of homoepitaxial growth of silicon using mass selected ion beam deposition. Function for the reflection of high-energy electron diffraction and Rutherford backscattering spectrometry; Transition to an amorphous phase at lower film thickness; Effects of substrate temperature, contamination and surface damage on the growth mechanism.

Reports on the transition between germanium segregation and trapping during high-pressure oxidation of Ge[subx]Si[sub1-x]/Si. Analysis of the atomic fraction x of germanium; Effect of oxidation on the germanium distribution; Significance of the oxidation of Ge[subx]Si[sub1-x] alloys.

Presents a study which examined the effect of germanium on the solubility of silicon in aluminum. Method of the study; Results and discusion; Conclusion.

The effects of thermal neutron irradiation and excess oxygen on the properties of Ti-sheathed MgB2 wires were investigated separately in this paper. No significant influence was observed when thermal neutron irradiation with fluence levels of 1.23 x 1014 and 5.75 x 1014 cm2 were applied on the sample, while fluence level of 3.16 x 1015 cm2 provided a strong suppression of in-field critical current density. Excess oxygen in sample preparation process had negative effect on the properties of the MgB2 wires. The samples prepared in air possessed lower critical temperature and in-field current carrying capability in comparison with the sample prepared in glove box. [ABSTRACT FROM AUTHOR]

Self-organized extremely small Ti nanodots have been formed in the subsurface of SiO2 by implantation of isotopic 48Ti+ at a kinetic energy of 9 keV into (0001) Z-cut quartz, followed by a vacuum annealing process. Transmission electron microscopy (TEM) images show that the Ti nanodots have a two-dimensional distribution, i.e., a uniform size as small as 3.8 nm and relatively uniform positions with nearly constant inter-particle spacing of ~ 1.5 nm. A mechanism for the two-dimensional spatial distribution and the inhibition of chemical reactions between Ti and SiO2 is discussed. [ABSTRACT FROM AUTHOR]

Extremely small Sn nanodots embedded in the subsurface of SiO2, i.e., Sn–SiO2 quantum dot composites, have been formed by ion implantation of the 120Sn+ isotope into (0001) Z-cut quartz at a low kinetic energy of 9 keV at room temperature. Transmission electron microscopy images show that the Sn nanodots have an average size of about 3 nm, and both single-crystal and amorphous nanodots have been observed at room temperature. The nanodots are randomly distributed in the lateral direction but confined in a narrow layer in the depth direction. The bimodal size distribution that is often observed in high energy implantation was not observed in the present study. The spatial morphology and crystallinity of the Sn nanodots can be modified by thermal annealing, and the extent of the modification is dependent on the annealing temperature. The distribution and crystallinity of the Sn nanodots in single-crystal bulk SiO2 are different from those of Sn nanocrystals in thin SiO2 films on silicon substrates. The mechanisms that could account for these phenomena are discussed. [ABSTRACT FROM AUTHOR]

Signaling by the mammalian target of rapamycin (mTOR) has been reported to be necessary for mechanical load-induced growth of skeletal muscle. The mechanisms involved in the mechanical activation of mTOR signaling are not known, but several studies indicate that a unique [phosphotidylinositol-3-kinase (PI3K)- and nutrient-independent] mechanism is involved. In this study, we have demonstrated that a regulatory pathway for mTOR signaling that involves phospholipase D (PLD) and the lipid second messenger phosphatidic acid (PA) plays a critical role in the mechanical activation of mTOR signaling. First, an elevation in PA concentration was sufficient for the activation of mTOR signaling. Second, the isozymes of PLD (PLD1 and PLD2) are localized to the z-band in skeletal muscle (a critical site of mechanical force transmission). Third, mechanical stimulation of skeletal muscle with intermittent passive stretch ex vivo induced PLD activation, PA accumulation, and mTOR signaling. Finally, pharmacological inhibition of PLD blocked the mechanically induced increase in PA and the activation of mTOR signaling. Combined, these results indicate that mechanical stimuli activate mTOR signaling through a PLD-dependent increase in PA. Furthermore, we showed that mTOR signaling was partially resistant to rapamycin in muscles subjected to mechanical stimulation. Because rapamycin and PA compete for binding to the FRB domain on mTOR, these results suggest that mechanical stimuli activate mTOR signaling through an enhanced binding of PA to the FRB domain on mTOR. [ABSTRACT FROM AUTHOR]

Glasshouse experiments were conducted to determine the accumulation, distribution and transformation of o,p′ -DDT, p,p′ -DDT and PCBs by common reed ( Phragmites australis) and rice ( Oryza sativa L.) under hydroponic conditions. The culture solution was spiked with the organic pollutants and samples were collected daily. Analysis of the plants at harvest showed that both species had removed DDT and PCBs from the solution. DDT appeared to have accumulated within P. australis by both passive adsorption and active absorption. Both o,p′-DDT and p,p′-DDT were transformed within P. australis. DDD was the major metabolite and the transformation was mediated by reductive dehalogenation. Plant long-distance transportation systems may be involved in the translocation of PCBs within P. australis and the affinity of the PCBs for lipids is one of the major factors affecting their uptake and translocation within the plants. Similar but less pronounced results were found in O. sativa and suggest that these wetland plants may be used for the plant-mediated remediation of persistent organic pollutants. [ABSTRACT FROM AUTHOR]

Size, shape, and density of self-assembled GaN nanorods grown on Si(111) substrates by plasma-assisted molecular beam epitaxy were successfully controlled by inserting a GaN buffer layer. The structure of the GaN buffer layer plays a vital role in the nanorod growth. Only a broken buffer layer with a suitable opening size can grow nanorods. Evolution of the nanorod is traced to the initial growth stage. Crystal seed grown at the wall of the opening in the buffer layer initiates the beginning of the nanorod, and a self-catalytic vapor-liquid-solid process, triggered by the nanocapillary condensation effect, enhances the GaN nanorod growth. Furthermore, the nanorod density can be largely controlled by using the beam equivalent pressure of the N/Ga ratio. Other GaN nanostructures grown at different growth conditions are also discussed in details. [ABSTRACT FROM AUTHOR]

Metal (M) doped PrBa2Cu3O7, i.e. PrBa2(Cu1-xMx)3O7 for M=Al, Co, Fe, Ga, Ni and Zn, and x=0.05, 0.10, 0.15, and 0.20 were fabricated. No significant second phase was found for the Al, Co, Fe and Ga doped samples up to 20% doping level. Impurity phases, however, were detected for Ni and Zn doped samples when doping concentration reached 15% and higher. At 77 K the electrical resistivity for most of these compounds is several orders in magnitude higher than that of the PrBa2Cu3O7-δ. Although the lattice parameters in the doped samples changes toward tetragonal structure, all sample remained in orthorhombic structure and the parameters remain very close to those of the undoped one and YBa2Cu3O7-δ. For this reason these compounds may serve as improved I-layer materials for making YBa2Cu3O7-δ SIS Josephson junction. [ABSTRACT FROM AUTHOR]

We report the detailed microstructural study of a CaCu 3 Ti 4 O 12 (CCTO) thin film using transmission electron microscopy (TEM). The CCTO thin film studied in this work was deposited on a (001)-oriented LaAlO 3 (LAO) substrate by pulsed-laser ablation and has a high dielectric constant of about 10 4 at 1 MHz at room temperature; however, the mechanism for such a dielectric property is not yet understood. Plan-view TEM studies show that the CCTO samples have orthogonal domain structures with the edge nearly parallel to either the [100] or the [010] direction of the CCTO. A minor anatase TiO 2 phase was found at the domain boundaries. The CCTO and the TiO 2 phases are separated by an amorphous-like layer that has a thickness of several nanometres. Cross-sectional TEM studies reveal that both CCTO and TiO 2 in the films are c axis oriented with a very sharp interface to the LAO-(001) substrate and possess a unique crystallographic orientation relationship of (001) CCTO //(001) TiO 2 //(001) LAO and [100] CCTO //[100] TiO 2 //[100] LAO . The Rutherford back-scattering ion chanelling studies suggest a composition of Ca 1.5 Cu 3 Ti 5.5 O 16 for the film, in which the extra calcium, titanium and oxygen form the anatase TiO 2 phase and amorphous calcium oxide layer that separates the CCTO and TiO 2 phases. Such nanocomposite-like structures may provide an important clue to the mechanism of the dielectric property of these films. [ABSTRACT FROM AUTHOR]

This article reviews levitation devices using superconductors and magnets. Device concepts and their applications such as noncontact bearings, flywheels, and momentum wheels are discussed, following an exposition of the principles behind these devices. The basic magneto–mechanical phenomenon responsible for levitation in these devices is a result of flux pinning inherent in the interaction between a magnet and a type II superconductor, described and explained in this article by comparison with behavior expected of a perfect conductor or a nearly perfect conductor. The perfect conductor model is used to illustrate why there is a difference between the forces observed when the superconductor is cooled after or before the magnet is brought into position. The same model also establishes the principle that a resisting force or torque arises only in response to those motions of the magnet that changes the magnet field at the superconductor. A corollary of the converse, that no drag torque appears when an axisymmetric magnet levitated above a superconductor rotates, is the guiding concept in the design of superconductor magnet levitation bearings, which is the common component in a majority of levitation devices. The perfect conductor model is extended to a nearly perfect conductor to provide a qualitative understanding of the dissipative aspects such as creep and hysteresis in the interaction between magnets and superconductors. What all these entail in terms of forces, torques, and power loss is expounded further in the context of generic cases of a cylindrical permanent magnet levitated above a superconductor and a superconductor rotating in a transverse magnetic field. Then we proceed to compare the pros and cons of levitation bearings based on the first arrangement with conventional mechanical bearings and active magnetic bearings, and discuss how the weak points of the levitation bearing may be partially overcome. In the latter half, we examine designs of devices using superconductor magnet levitation, focusing more on issues specific to the application. We note that applications of superconductor magnet levitation devices tend to be most attractive in situations where energy conservation is critical. The most advanced in development are flywheel kinetic energy storage systems incorporating superconductor magnet bearings. Variations in the designs to enhance the performance in some specific regards are examined case by case. Next we present a reaction wheel for attitude control on small satellites, similar in overall design to the flywheel kinetic energy storage systems, but with subtle differences in details of emphasis, due to the difference in purpose and environment. Finally, we take a brief look at the case of vibration isolation devices as an example of a rectilinear modification of the more familiar rotational bearing applications. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

When high energy heavy ions bombard a single crystal, such as MeV Si implantation in Si, the surface region becomes vacancy-rich, while interstitials are mostly distributed near the range of the implants. We have demonstrated that vacancy retards while interstitial enhances boron thermal diffusion in silicon. In this paper we will show experimental results on the modification of boron diffusivity by point defect engineering, and its application in ultra-shallow junction (10 nm) formation. In this paper, we will also show cluster ion, such as GeB and SiB, implantation in silicon, and two-stage annealing in forming ultra shallow junction in Si. RBS, channeling, nuclear reaction, and secondary ion mass spectrometry are used for this studies. [Copyright &y& Elsevier]

We present a study of strain relaxation mechanisms in epitaxial c -axis YBa[sub 2]Cu[sub 3]O[sub 7-δ] (YBCO) films deposited on SrTiO[sub 3] (STO) and LaAlO[sub 3] (LAO). A combination of high-resolution transmission electron microscopy and electron back-scattered diffraction in scanning electron microscopy has shown that a considerable amount of 90° tilted small domains exist in films grown on LAO while only planar and/or linear defects are observed in strain-relaxed films on STO. The 90° tilted domains, which have very limited influence on the superconducting characteristics of YBCO, account for the full relief of epitaxial strain in films on LAO. As the strain relaxation originated from different stages of the film deposition, this study may also explain the different crystalline qualities, surface morphologies, and some of the transport properties of YBCO films on STO and LAO. [ABSTRACT FROM AUTHOR]

We have carried out the measurements of the magnetoresistance (MR) of YNi[sub 2]B[sub 2]C single crystals (T[sub c] = 15.3 K and H[sub c2](5.5 K) ∼ 4.7 T) in a wide range of temperature and magnetic field, T[sub c]+l < T < T[sub c]+145 K and 0 < H < 8 T. In the normal state, the in-plane MR, Δρ(H, T) = ρ(H, T) - ρ(0, T), changes from negative to positive as the temperature decreases. A pronounced sign-reversal is observed at T=80 K with H = 4 T. A negative MR supports the presence of magnetically-related scattering and could signify considerable suppression of the spin fluctuations most likely associated with nearly magnetic metals in the presence of an external field. Such novel behavior reveals the possibility of pseudo-gap formation, which plays the role of unconventional superconducting mechanism, as indicated by the recent [sup 11]B and [sup 13]C NMR studies [1]. Our results are also in quantitative agreement with the short-range magnetic moment dependence upon temperature and applied field. [ABSTRACT FROM AUTHOR]