bluuebunny/crossref_metadata_2025 huggingface dataset:
(Ga,Mn)As: A new diluted magnetic semiconductor based on GaAs
Authors: Ohno H. , Shen A. , Matsukura F. , Oiwa A. , Endo A. , Katsumoto S. , Iye Y.
Published: July 2002

A new GaAs-based diluted magnetic semiconductor, (Ga,Mn)As, was prepared by molecular beam epitaxy. The lattice constant of (Ga,Mn)As films was determined by x-ray diffraction and shown to increase with the increase of Mn composition, x. Well-aligned in-plane ferromagnetic order was observed by magnetization measurements. Magnetotransport measurements revealed the occurrence of anomalous Hall effect in the (Ga,Mn)As layer.
Interband cascade light emitting diodes in the 5–8 μm spectrum region
Authors: Yang Rui Q. , Lin Chih-Hsiang , Murry S. J. , Pei S. S. , Liu H. C. , Buchanan M. , Dupont Emmanuel
Published: July 2002

Interband cascade electroluminescence in the 5–8 μm spectrum region is observed from a staircase of Sb-based type-II quantum well structures. The structure was grown by molecular beam epitaxy on a GaSb substrate and comprises 15 periods of active regions separated by digitally graded multilayer injection regions. The device has been operated at 300 and 77 K with an output optical power up to 700 nW. The strong blue shift of the electroluminescent peak with the applied bias due to the Stark effect has also been observed.
Chains composed of nanosize metal particles and identifying the factors driving their formation
Authors: Zhang L. , Manthiram A.
Published: July 2002

In an attempt to identify the driving force for the formation of chains composed of nanometer size Fe alloy particles that were recently reported, experiments have been carried out with the magnetic metals Fe, Co, and Ni and a nonmagnetic metal Cu. The samples obtained by a reduction of aqueous metal chloride solutions with potassium borohydride have been characterized by x-ray diffraction, transmission electron microscopy, and superconducting quantum interference device magnetometer measurements. The results reveal that the magnetic interaction between the adjacent particles plays a determining role in the chain structure formation. Although both crystallinity and boron content do not play a direct role, they influence significantly the magnetic properties, and thereby have an indirect role in the formation of the chain structures.
Tunable magnetic regenerator alloys with a giant magnetocaloric effect for magnetic refrigeration from ∼20 to ∼290 K
Authors: Pecharsky V. K. , Gschneidner K. A.
Published: July 2002

A giant magnetocaloric effect (ΔSmag) has been discovered in the Gd5(SixGe1−x)4 pseudobinary alloys, where x⩽0.5. For the temperature range between ∼50 and ∼280 K it exceeds the reversible (with respect to alternating magnetic field) ΔSmag for any known magnetic refrigerant material at the corresponding Curie temperature by a factor of 2–10. The two most striking features of this alloy system are: (1) the first order phase transformation, which brings about the large ΔSmag in Gd5(SixGe1−x)4, is reversible with respect to alternating magnetic field, i.e., the giant magnetocaloric effect can be utilized in an active magnetic regenerator magnetic refrigerator; and (2) the ordering temperature is tunable from ∼30 to ∼276 K by adjusting the Si:Ge ratio without losing the giant magnetic entropy change.
Anisotropic short-range structure of Co0.16Pd0.84 alloy films having perpendicular magnetic anisotropy
Authors: Kim Sang-Koog , Chernov V. A. , Kortright J. B. , Koo Y. M.
Published: July 2002

Polarized Co K edge extended x-ray absorption fine structure measurements obtained with electric vector parallel and perpendicular to the film plane indicate differences in Co bonding along these two directions in room-temperature evaporated Co0.16Pd0.84 alloy films. A local modulation of the Co fraction whose amplitudes are 0.05–0.09 exists along the growth direction in the alloy films. Pd underlayer and Pd spacer layers alternated with the alloy layer induce coherency strains resulting in an anisotropic effective strain state. Both anisotropic strain and local compositional modulation are likely to contribute to the perpendicular magnetic anisotropy observed in these alloys.
p
-type arsenic doping of Hg1−xCdxTe by molecular beam epitaxy
Authors: Zandian M. , Chen A. C. , Edwall D. D. , Pasko J. G. , Arias J. M.
Published: July 2002

Growth of in situ As doped Hg1−xCdxTe by molecular beam epitaxy and activation of As at 250 °C is reported. We have used elemental arsenic, As4, as the p-type dopant source. The activation of As was observed in the 1016–1018 cm−3 range after a low temperature annealing step at 250 °C. However, for doping levels above 5×1018 cm−3, we have observed that the As activation efficiency drops. It is speculated at this time that self-compensation and formation of neutral As complexes may limit doping efficiency at very high levels. We also report our data on the structural and electrical characteristics of these As doped p-type layers using secondary ion mass spectroscopy analysis, and Hall effect measurements. An acceptor activation energy of 5.4 meV was obtained based on the dependence of the Hall coefficient on temperature. This value was attributed to singly ionized As located on a Te site (AsTe•) acting as an acceptor. A brief discussion on activation mechanism of As doped p-type HgCdTe material is also presented.
Nitrogen plasma annealing for low temperature Ta2O5 films
Authors: Alers G. B. , Fleming R. M. , Wong Y. H. , Dennis B. , Pinczuk A. , Redinbo G. , Urdahl R. , Ong E. , Hasan Z.
Published: July 2002

A low temperature oxygen/nitrogen plasma process is reported that substantially reduces leakage currents in chemical vapor deposited (CVD) and physical vapor deposited (PVD) films of tantalum oxide. We show that a combination of nitrogen and oxygen in a remote downstream microwave plasma source reduces leakage currents in CVD films of tantalum oxide and also reduces trap densities as measured by charge pumping. The as deposited CVD films show a high level of photoluminescence that is substantially lowered by the plasma anneal due to a reduction in the density of midgap states. For films deposited by PVD in the thickness range of 100 nm we find low leakage currents with a substantial improvement from the introduction of nitrogen into the plasma. However, PVD films in the thickness range of 20 nm show larger relative leakage currents and less of an improvement from the addition of nitrogen. The role of nitrogen in lowering leakage currents and charge trapping is thought to occur from a reduction in the density of bulk trap states in the oxide due to partial incorporation of nitrogen in the oxide. Both of these low temperature deposition and annealing processes are compatible with integration into the upper levels of metallization for high density circuits.
Sound velocity of AlxGa1−xN thin films obtained by surface acoustic-wave measurements
Authors: Deger C. , Born E. , Angerer H. , Ambacher O. , Stutzmann M. , Hornsteiner J. , Riha E. , Fischerauer G.
Published: July 2002

To determine the sound velocity in wurtzite AlxGa1−xN, we have used surface acoustic-wave (SAW) delay lines on AlxGa1−xN/c-Al2O3. AlxGa1−xN films with compositions from x=0 to x=1 were grown by plasma-induced molecular beam epitaxy. Starting from published data, we fine tuned the values of the elastic moduli used in numerical calculations such that the simulated and measured dispersion of the SAW were in good agreement. Based on these values, the surface and bulk acoustic-wave velocities of single-crystal AlxGa1−xN were determined as functions of the composition. The resulting SAW velocities ranged from 3700 to 5760 m/s for GaN and AlN, respectively.
Solvent-driven chemical motor
Authors: Mitsumata Tetsu , Ikeda Kazuo , Gong Jian Ping , Osada Yoshihito
Published: July 2002

A solvent-driven chemical motor using amphiphilic polymer gel has been fabricated. The driving force of the gel originates from the surface tension of spreading organic fluid which is pumped out by osmotic and hydrostatic pressures in the gel. A tetrahydrofurane-swollen gel equipped with a spouting hole made a controlled translational motion with a velocity of 77 mm/s or rotational motion with a maximum speed of 400 rpm and a torque of 10−9–10−7 Nm on the water surface. A generator to produce an electric power with a maximum electromotive force of 15 mV and electric power of 0.2 μW has also been constructed. The successful fabrication of gel motor may produce a new era of soft machine systems which work without pollution and unnecessary intermediates.
High-resolution depth profiling in ultrathin Al2O3 films on Si
Authors: Gusev E. P. , Copel M. , Cartier E. , Baumvol I. J. R. , Krug C. , Gribelyuk M. A.
Published: July 2002

A combination of two complementary depth profiling techniques with sub-nm depth resolution, nuclear resonance profiling and medium energy ion scattering, and cross-sectional high-resolution transmission electron microscopy were used to study compositional and microstructural aspects of ultrathin (sub-10 nm) Al2O3 films on silicon. All three techniques demonstrate uniform continuous films of stoichiometric Al2O3 with abrupt interfaces. These film properties lead to the ability of making metal-oxide semiconductor devices with Al2O3 gate dielectric with equivalent electrical thickness in the sub-2 nm range.
Effect of nitric oxide annealing on the interface trap densities near the band edges in the 4H polytype of silicon carbide
Authors: Chung G. Y. , Tin C. C. , Williams J. R. , McDonald K. , Di Ventra M. , Pantelides S. T. , Feldman L. C. , Weller R. A.
Published: July 2002

Results of capacitance–voltage measurements are reported for metal–oxide–semiconductor capacitors fabricated using the 4H polytype of silicon carbide doped with either nitrogen (n) or aluminum (p). Annealing in nitric oxide after a standard oxidation/reoxidation process results in a slight increase in the defect state density in the lower portion of the band gap for p-SiC and a significant decrease in the density of states in the upper half of the gap for n-SiC. Theoretical calculations provide an explanation for these results in terms of N passivating C and C clusters at the oxide–semiconductor interface.
The solvation of Na+ in water: First-principles simulations
Authors: White Jody A. , Schwegler Eric , Galli Giulia , Gygi François
Published: July 2002

First-principles molecular dynamics simulations have been performed on the solvation of Na+ in water. Consistent with the available experimental data, we find that the first solvation shell of Na+ contains on average 5.2 water molecules. A significant number of water exchanges between the first and second solvation shells are observed. However, the simulations are not long enough to reliably measure the rate of water exchange. Contrary to several previous studies, we do not find any effect of Na+ on the orientation of water molecules outside of the first solvation shell. Furthermore, the complete set of structural properties determined by first-principles molecular dynamics is not predicted by any of the known classical simulations.
Water ordering around methane during hydrate formation
Authors: Koh Carolyn A. , Wisbey Robert P. , Wu Xiaoping , Westacott Robin E. , Soper Alan K.
Published: July 2002

The structure of water around methane during hydrate crystallization from aqueous solutions of methane is studied using neutron diffraction with isotopic substitution over the temperature range 18 °C to 4 °C, and at two pressures, 14.5 and 3.4 MPa. The carbon–oxygen pair correlation functions, derived from empirical potential structure refinement of the data, indicate that the hydration sphere around methane in the liquid changes dramatically only once hydrate has formed, with the water shell around methane being about 1 Å larger in diameter in the crystal than in the liquid. The methane coordination number in the liquid is around 16±1 water molecules during hydrate formation, which is significantly smaller than the value of 21±1 water molecules found for the case when hydrate is fully formed. Once hydrate starts to form, the hydration shell around methane becomes marginally less ordered compared to that in the solution above the hydrate formation temperature. This suggests that the hydration cage around methane in the liquid may be different from that when hydrate is forming and from that found in the hydrate crystal structure. Methane–methane radial distribution functions show that methane molecules can adopt a range of separations during hydrate formation, corresponding to the more distorted nature of the methane–water correlations. There is noticeable ordering of the methane molecules with a monolayer of water molecules between them once hydrate has formed. The dipole moments of the hydrating water molecules lie mostly tangential to the methane–water axis, both before, during, and after hydrate formation.
CC2 excitation energy calculations on large molecules using the resolution of the identity approximation
Authors: Hättig Christof , Weigend Florian
Published: July 2002

A new implementation of the approximate coupled cluster singles and doubles method CC2 is reported, which is suitable for large scale integral-direct calculations. It employs the resolution of the identity (RI) approximation for two-electron integrals to reduce the CPU time needed for calculation and I/O of these integrals. We use a partitioned form of the CC2 equations which eliminates the need to store double excitation cluster amplitudes. In combination with the RI approximation this formulation of the CC2 equations leads to a reduced scaling of memory and disk space requirements with the number of correlated electrons (n) and basis functions (N) to, respectively, O(N2) and O(nN2), compared to O(n2N2) in previous implementations. The reduced CPU, memory and disk space requirements make it possible to perform CC2 calculations with accurate basis sets on large molecules, which would not be accessible with conventional implementations of the CC2 method. We present an application to vertical excitation energies of alkenes C2nH2n+2, for n=1–12, and report results for the lowest lying dipole-allowed transitions for the TZVPP basis sets, which for n=12 contain 1108 basis functions. Comparison with conventional CC2 results for the smaller alkenes show that for CC2 ground state energies and for excitation energies of valence states, the error due to the RI approximation is negligible compared to the usual basis set error, if auxiliary basis sets are used, which have been optimized for MP2 energy calculations.
Capacitance–voltage characterization of AlN/GaN metal–insulator–semiconductor structures grown on sapphire substrate by metalorganic chemical vapor deposition
Authors: Hashizume Tamotsu , Alekseev Egor , Pavlidis Dimitris , Boutros Karim S. , Redwing Joan
Published: July 2002

Electrical characterization of AlN/GaN interfaces was carried out by the capacitance–voltage (C–V) technique in materials grown by metalorganic chemical vapor deposition. The high-frequency C–V characteristics showed clear deep-depletion behavior at room temperature, and the doping density derived from the slope of 1/C2 plots under the deep depletion condition agreed well with the growth design parameters. A low value of interface state density Dit of 1×1011 cm−2 eV−1 or less around the energy position of Ec−0.8 eV was demonstrated, in agreement with an average Dit value estimated from photoassisted C–V characteristics.
Deep levels and compensation in γ-irradiated CdZnTe
Authors: Cavallini A. , Fraboni B. , Dusi W. , Zanarini M. , Siffert P.
Published: July 2002

The behavior of detector-grade Cd0.9Zn0.1Te in a radiation-hostile environment has been investigated by studying the effects on the material defective states induced by γ irradiation. The detector performance is strongly affected by the presence of charge-trapping centers which may also intervene in the material compensation properties. We have investigated by photoinduced current transient spectroscopy analyses the evolution with increasing irradiation dose of the deep levels both present in the as-grown material and induced by the ionizing radiation. A significant correlation between the material resistivity and some deep levels behavior has been observed. We have compared this trend to the results obtained from γ-irradiated CdTe:Cl to better understand the role deep traps play in the compensation process of II–VI materials.
Shape corrections to exchange-correlation potentials by gradient-regulated seamless connection of model potentials for inner and outer region
Authors: Grüning M. , Gritsenko O. V. , van Gisbergen S. J. A. , Baerends E. J.
Published: July 2002

Shape corrections to the standard approximate Kohn-Sham exchange-correlation (xc) potentials are considered with the aim to improve the excitation energies (especially for higher excitations) calculated with time-dependent density functional perturbation theory. A scheme of gradient-regulated connection (GRAC) of inner to outer parts of a model potential is developed. Asymptotic corrections based either on the potential of Fermi and Amaldi or van Leeuwen and Baerends (LB) are seamlessly connected to the (shifted) xc potential of Becke and Perdew (BP) with the GRAC procedure, and are employed to calculate the vertical excitation energies of the prototype molecules N2, CO, CH2O, C2H4, C5NH5, C6H6, Li2, Na2, K2. The results are compared with those of the alternative interpolation scheme of Tozer and Handy as well as with the results of the potential obtained with the statistical averaging of (model) orbital potentials. Various asymptotically corrected potentials produce high quality excitation energies, which in quite a few cases approach the benchmark accuracy of 0.1 eV for the electronic spectra. Based on these results, the potential BP-GRAC-LB is proposed for molecular response calculations, which is a smooth potential and a genuine “local” density functional with an analytical representation.
Time dependent self-diffusion coefficient of molecules in porous media
Authors: Valiullin Rustem , Skirda Vladimir
Published: July 2002

The time dependence of the self-diffusion coefficient D(t) in porous media is investigated by Monte Carlo simulation of autocorrelation functions 〈f(t)f(0)〉, where f(t) is the force of interaction between a molecule and the surface at time t. At short times, D(t) is governed by the surface population of the molecules and the probability of their return to the surface. At times t>t*, where t* is the characteristic time for the autocorrelation function to converge to zero, the apparent dynamics of the molecules is completely determined by the geometry of the surface on the length scale of D0t*, where D0 is the bulk self-diffusion coefficient. D(t) in this limit is the sum of a constant D∞=limt→∞ D(t) and a time-dependent term R̄p2/2d⋅t, where R̄p2 is the mean-squared size of an effective unit cell of the porous space and d is the dimensionality of the space. The meaning of tortuosity for self-diffusion is discussed.
A scaling analysis of turbulent flows driven by Rayleigh–Taylor and Richtmyer–Meshkov instabilities
Authors: Zhou Ye
Published: July 2002

We investigate inertial subrange energy spectra associated with turbulent flows developed by the Rayleigh–Taylor instability and Richtmyer–Meshkov instability (RMI). We argue that the extended Kolmogorov–Kraichnan phenomenology originally developed for turbulent flows with an external agent should also be applicable to these instability driven turbulent flows. A prediction of the mixing zone width for the RMI induced turbulent flow is presented using the RMI modified energy spectrum and a two-equation turbulence model. A possible application to subgrid modeling for large-eddy simulation is discussed briefly.
Modification of a vortex street by a polymer additive
Authors: Cressman J. R. , Bailey Q. , Goldburg W. I.
Published: July 2002

A Kármán vortex street is created in a flowing soap film by a rod penetrating the film. The velocity field generated by this rod is modified by the addition of the polymer polyethylene oxide having a molecular weight of 5×106 and a concentration of 30 wppm. The rms velocity fluctuations behind the rod are strongly suppressed by the polymer additive and the power spectrum of the velocity fluctuations is modified as well. The experiments show that the polymer additive decreases the rate at which energy is injected into the flow. The measurements further indicate that the polymer introduces an elongational viscosity term into the Navier–Stokes equation.
Polar cap study during northward interplanetary magnetic field on 19 January 1998
Authors: Slinker S. P. , Fedder J. A. , McEwen D. J. , Zhang Y. , Lyon J. G.
Published: July 2002

The interplanetary magnetic field (IMF) was northward for an extended period on 19 January 1998. This caused the open polar cap of the ionosphere to become very small and the auroral emission to move poleward. The auroral emission at 630 nm was observed by the meridian scanning photometer located at Eureka near the north magnetic pole. Effects of changes in sign of the dawn–dusk component of the IMF were also observed. A magnetohydrodynamic simulation model of the magnetosphere and ionosphere was used to study these events. The model was driven using data from the Wind and IMP-8 spacecraft. The simulation results show a very small open polar cap indicating that the magnetosphere is nearly closed. Moreover, in response to the shift from dawnward to duskward IMF, a narrow strip of closed field breaks off from the dawn boundary and convects across the polar cap and into the dusk boundary.
Fast-growth thermodynamic integration: Error and efficiency analysis
Authors: Hummer Gerhard
Published: July 2002

Equilibrium free energy differences can be calculated accurately from repeated fast-growth thermodynamic integration (TI) based on Jarzynski’s identity [Phys. Rev. Lett. 78, 2690 (1997)]. We derive expressions for the free energy differences. Error estimates allow us to quantify the relative efficiency of performing many fast-growth vs few slow-growth TIs for a given total computational cost. Fast-growth TI is illustrated through the calculation of the potential of mean force between two methane molecules and compared to umbrella sampling analyzed by using the weighted histogram analysis method. Fast-growth TI is well suited for parallel computer architectures, requiring only the simplest parallelism with repeated runs for different starting conditions.
Diameter-controlled synthesis of single-crystal silicon nanowires
Authors: Cui Yi , Lauhon Lincoln J. , Gudiksen Mark S. , Wang Jianfang , Lieber Charles M.
Published: July 2002

Monodisperse silicon nanowires were synthesized by exploiting well-defined gold nanoclusters as catalysts for one-dimensional growth via a vapor–liquid–solid mechanism. Transmission electron microscopy studies of the materials grown from 5, 10, 20, and 30 nm nanocluster catalysts showed that the nanowires had mean diameters of 6, 12, 20, and 31 nm, respectively, and were thus well defined by the nanocluster sizes. High-resolution transmission electron microscopy demonstrated that the nanowires have single-crystal silicon cores sheathed with 1–3 nm of amorphous oxide and that the cores remain highly crystalline for diameters as small as 2 nm.
Magnetovolume anomalies in Ce2Fe17−xMnx
Authors: Prokhnenko O. , Arnold Z. , Kamarad J. , Medvedeva I. , Kuchin A.
Published: July 2002

The small nonmonotonic changes of the lattice parameters upon the substitution of Mn for Fe in Ce2Fe17 lead to significant changes of the magnetic properties. In order to determine the role of the volume, we made ac susceptibility and thermal expansion measurements on the polycrystalline compounds Ce2Fe16.65Mn0.35 and Ce2Fe16Mn1 under hydrostatic pressure up to 10 kbar in the temperature range 10–300 K. The substitution of Mn for Fe leads to significant narrowing of the range of stability of the low-temperature ferromagnetic phase characterized by ΘT=48 K (x=0.35) in comparison with binary Ce2Fe17, where ΘT=94 K. Applying a relatively small pressure of 1.5 kbar leads to suppression of both the ferromagnetic order and large positive spontaneous volume magnetostriction in Ce2Fe16.5Mn0.35. The pressure effect on ΘT is significant, dΘT/dp=−30 K/kbar. The effect of pressure on the Néel temperature is moderate and increases slightly, with increasing Mn content (dTN/dp=−2.2 K/kbar for Ce2Fe16.65Mn0.35). No ferromagnetic order was observed in Ce2Fe16Mn1 at ambient pressure, but an unexpected reentrant increase of the ac susceptibility was detected at pressures higher than 1.5 kbar below 50 K. It indicates the existence of a pressure-induced ferromagnetic phase at low temperature. The results show not only a strong correlation between the magnetic properties and the lattice volume but also a significant role of the Mn content.
The effect of hydrogen on the magnetostriction of rare-earth compounds TbxDy1−xFe2
Authors: Nikitin S. A. , Tereshina I. S. , Touliakov A. P. , Tereshina E. A. , Verbetsky V. N. , Salamova A. A.
Published: July 2002

The crystal structure and the magnetic and magnetoelastic properties of TbxDy1−xFe2Hy (x=0.27; 0.41; y⩽3) are investigated. The hydrides are obtained by the interaction of TbxDy1−xFe2 samples with hydrogen gas at room temperature. It is established that the hydrides have the same cubic type of structure as the parent compounds but the lattice parameter in the hydrides is larger than in the hydrogen-free samples. Both the Curie temperature and the magnetic moment decrease drastically as the hydrogen concentration increases. Measurements of the longitudinal λ∥ and transverse magnetostriction λ⊥ are carried out in magnetic fields up to 12 kOe in the temperature range 78–300 K using strain gauges. The thermal expansion coefficients as functions of temperature are obtained for hydrides with high hydrogen concentration. The magnetostriction of hydrides is strongly different from that of hydride-free samples. The small and negative value of the magnetostriction at low temperatures proves that hydrogen atoms make a strong contribution to the crystal field acting on the rare-earth ions.
Grain growth and strain release in nanocrystalline copper
Authors: Lu L. , Tao N. R. , Wang L. B. , Ding B. Z. , Lu K.
Published: July 2002

Grain growth and strain release processes in the electrodeposited nanocrystalline (nc) Cu specimen with a high purity were investigated by means of differential scanning calorimetry, x-ray diffraction, electrical resistance measurement, and high-resolution transmission electron microscopy. It was found that for the as-deposited nc Cu, the grain growth started at about 75 °C, at which the microstrain in (111) plane (e111) began to release, while the mean microstrain and that in (100) plane (e100) began to release at a higher temperature (150 °C). With an increment in microstrain in the nc Cu introduced by cold rolling, the grain growth onset temperature increased while the strain release onset temperature dropped obviously. These results showed an evident correlation between the grain size stability and the microstrain in the nc materials. The activation energy for the grain growth was determined by using Kissinger analysis and isothermal kinetics analysis, being about 86 kJ/mol, implying that the grain growth process is dominated by grain boundary diffusion.
Micro-Raman scattering and dielectric investigations of phase transition behavior in the BaTiO3–BaZrO3 system
Authors: Dobal P. S. , Dixit A. , Katiyar R. S. , Yu Z. , Guo R. , Bhalla A. S.
Published: July 2002

In this study, the phase transition behavior of the BaTiO3–BaZrO3 system was studied using micro-Raman scattering and dielectric measurement techniques. BaZrxTi1−xO3 ceramics were prepared for x=0.00, 0.05, 0.08, 0.15, 0.20, and 1.00 compositions using a solid-state reaction technique. A single-phase perovskite structure of the ceramics was identified by the x-ray diffraction technique. The basic phase transition temperatures in these compositions were studied in the temperature range of 70–575 K. The tetragonal to cubic transition temperature was found to decrease with increasing Zr content. The orthorhombic to tetragonal transition temperature that increases with an initial increase in Zr content merges with the tetragonal–cubic transition for x⩾0.15 compositions. Raman spectra of rhombohedral and orthorhombic phases could not be distinguished. Excellent agreement between the crystallographic transition temperatures obtained by both techniques suggested that Zr substituted octahedra were uniformly distributed in the material without introducing any mechanical stress.
Effect of Ar addition to an O2 plasma in an inductively coupled, traveling wave driven, large area plasma source: O2/Ar mixture plasma modeling and photoresist etching
Authors: Takechi K. , Lieberman M. A.
Published: July 2002

We report on the effect of Ar addition to an O2 plasma on photoresist etching in an inductively coupled, traveling wave driven, large area plasma source (LAPS). We also develop a simplified spatially varying O2/Ar mixture discharge model corresponding to the LAPS in a two-dimensional geometry in order to account for the effect of Ar addition. A photoresist etch kinetics model and spatially varying O2/Ar mixture discharge model are used to explain the experimental data. We find that the addition of 50% Ar increases the plasma density and etch rate approximately by a factor of 2. From the simulation we find that argon metastables (Ar*) play an important role in the mixture plasma. The simulation predicts an enhancement in O-atom density due to Ar addition, even in the presence of dilution of the feed gas. The experimental data and predicted etch rates from the simulation are generally in good agreement, indicating that the increase in the etch rate with Ar addition is due to both the increase in the plasma density and the enhancement in O-atom density attributable to the dissociation of O2 by Ar*.
On the calculation of entropy from covariance matrices of the atomic fluctuations
Authors: Andricioaei Ioan , Karplus Martin
Published: July 2002

An ad hoc method for calculating the entropy of a biomolecular system from the covariance matrix of the atomic fluctuations is analyzed. It is shown that its essential assumption can be eliminated by a quasiharmonic analysis. The computer time required for use of the latter is of the same order as that of the former.
Selective etching of GaN polar surface in potassium hydroxide solution studied by x-ray photoelectron spectroscopy
Authors: Li Dongsheng , Sumiya M. , Fuke S. , Yang Deren , Que Duanlin , Suzuki Y. , Fukuda Y.
Published: July 2002

Etching characteristics of nondoped GaN films with the polar surface in KOH solution have been investigated. It is confirmed that the continuous etching in KOH solution takes place only for the GaN films with N-face (−c) polarity independent of the deposition method and growth condition. It is found by x-ray photoelectron spectroscopy (XPS) analysis for the Ga face (+c) and N-face (−c) GaN films that the atomic composition of the +c surface is not changed before and after dipping in KOH solution and that on the other hand, the amount of oxygen (oxide) on the −c surface is significantly decreased by the etching. It is also found that the band bending increases by −0.4±0.2 and 0.6±0.2 eV for the +c and −c surfaces after etching, respectively. This is discussed in terms of the surface chemistry. Based on the XPS result, the selective etching of the GaN polar surface is pointed out to originate from bonding configuration of nitrogen at the surface.
Influence of Si growth temperature for embedding β-FeSi2 and resultant strain in β-FeSi2 on light emission from p-Si/β-FeSi2 particles/n-Si light-emitting diodes
Authors: Suemasu T. , Negishi Y. , Takakura K. , Hasegawa F. , Chikyow T.
Published: July 2002

We have fabricated Si/β-FeSi2 particles/Si structures by reactive deposition epitaxy for β-FeSi2 and molecular-beam epitaxy (MBE) for Si. It was found that the photoluminescence (PL) intensity of β-FeSi2 strongly depended on MBE-Si growth temperature for embedding β-FeSi2 in Si. Room temperature 1.6 μm electroluminescence was realized from a Si-based light-emitting diode by embedding the β-FeSi2 active region with Si grown at 500 °C. Observation with transmission electron microscopy revealed that the a axis of β-FeSi2 from which PL was observed was about 9% longer than that of β-FeSi2 without PL or of bulk β-FeSi2.
Effect of interfaces on Gilbert damping and ferromagnetic resonance linewidth in magnetic multilayers
Authors: Berger L.
Published: July 2002

In magnetic multilayers, the presence of sharp interfaces causes a local increase of the interaction between spin waves and conduction electrons. This leads to an increase of the Gilbert spin-damping parameter near an interface. In turn, the ferromagnetic-resonance linewidth is increased over its value in single-layer films. In addition, the precession of magnetic spins during ferromagnetic resonance induces conduction-electron transitions from the spin-up to the spin-down band. As a result, the spin-up Fermi level differs from the spin-down Fermi level by an amount Δμ̄. At high precession amplitudes, the existence of Δμ̄ causes a measurable decrease of the Gilbert parameter. At precession-cone angles exceeding 4°, the Gilbert parameter returns nearly to its single-layer value. Ferromagnetic-resonance line shapes are predicted to be non-Lorentzian, narrower and sharper near the top. This line-narrowing effect increases with increasing microwave power. The effect of Δμ̄ spreads into the entire multilayer, so that precession in one magnetic layer can cause a reduction or increase of the Gilbert parameter in other layers, or even a spontaneous spin precession in these layers.
Taylor cone and jetting from liquid droplets in electrospinning of nanofibers
Authors: Yarin A. L. , Koombhongse S. , Reneker D. H.
Published: July 2002

Sessile and pendant droplets of polymer solutions acquire stable shapes when they are electrically charged by applying an electrical potential difference between the droplet and a flat plate, if the potential is not too large. These stable shapes result only from equilibrium of the electric forces and surface tension in the cases of inviscid, Newtonian, and viscoelastic liquids. In liquids with a nonrelaxing elastic force, that force also affects the shapes. It is widely assumed that when the critical potential φ0* has been reached and any further increase will destroy the equilibrium, the liquid body acquires a conical shape referred to as the Taylor cone, having a half angle of 49.3°. In the present work we show that the Taylor cone corresponds essentially to a specific self-similar solution, whereas there exist nonself-similar solutions which do not tend toward a Taylor cone. Thus, the Taylor cone does not represent a unique critical shape: there exists another shape, which is not self-similar. The experiments of the present work demonstrate that the observed half angles are much closer to the new shape. In this article a theory of stable shapes of droplets affected by an electric field is proposed and compared with data acquired in our experimental work on electrospinning of nanofibers from polymer solutions and melts.
Crystal and electronic structures of Bi4−xLaxTi3O12 ferroelectric materials
Authors: Shimakawa Y. , Kubo Y. , Tauchi Y. , Asano H. , Kamiyama T. , Izumi F. , Hiroi Z.
Published: July 2002

The crystal structures of Bi4Ti3O12 and Bi3.25La0.75Ti3O12 were refined by neutron powder diffraction. Large structural distortions were revealed, and ferroelectric polarizations along the a and c axes were calculated from the displacements of the constituent ions. In Bi3.25La0.75Ti3O12, La atoms substitute for Bi atoms in a perovskite-type unit only, and the substitution causes less distortion of the structure, resulting in smaller spontaneous polarization and lower ferroelectric Curie temperature. Electronic-structure calculations revealed that covalent interaction, which originates from the strong hybridization between Ti 3d and O 2p orbitals, plays an important role in the structural distortion and ferroelectricity of the materials. Changes in ceramic-sample density with sintering temperature give information concerning device fabrication temperature; that is, substituting La for Bi atoms appears to “increase” the synthesis temperature of the Bi4Ti3O12 and Bi3.25La0.75Ti3O12 systems.
Alignment mechanism of carbon nanofibers produced by plasma-enhanced chemical-vapor deposition
Authors: Merkulov Vladimir I. , Melechko Anatoli V. , Guillorn Michael A. , Lowndes Douglas H. , Simpson Michael L.
Published: July 2002

We report experimental evidence showing a direct correlation between the alignment of carbon nanofibers (CNFs) prepared by plasma-enhanced chemical-vapor deposition and the location of the catalyst particle during CNF growth. In particular, we find that CNFs that have a catalyst particle at the tip (i.e., growth proceeds from the tip) align along the electric-field lines, whereas CNFs with the particle at the base (i.e., growth proceeds from the base) grow in random orientations. We propose a model that explains the alignment process as a result of a feedback mechanism associated with a nonuniform stress (part tensile, part compressive) that is created across the interface of the catalyst particle with the CNF due to electrostatic forces. Furthermore, we propose that the alignment seen recently in some dense CNF films is due to a crowding effect and is not directly the result of electrostatic forces.
Cavity ringdown spectroscopy measurements of the infrared water vapor continuum
Authors: Cormier John G. , Ciurylo Roman , Drummond James R.
Published: July 2002

We report measurements of the water vapor continuum using infrared cavity ringdown spectroscopy at frequencies of 931.002, 944.195, and 969.104 cm−1. Our values of the water vapor continuum coefficients for self-broadening at T=296 K are Cs0 (931 cm−1)=2.23±0.17, Cs0(944 cm−1)=2.02±0.13, and Cs0(969 cm−1)=1.79±0.21×10−22 molecules−1 cm2 atm−1. Our measurements are found to be in good agreement with the far wing line shape theory of Ma and Tipping, but we find that empirical models of the water vapor continuum, widely used in radiative transfer calculations, significantly overestimate the observed self-broadened continuum.
A new method for significantly reducing drop radius without reducing nozzle radius in drop-on-demand drop production
Authors: Chen Alvin U. , Basaran Osman A.
Published: July 2002

The lack of a simple method for generating drops whose radii (Rd) are much smaller than those (R) of nozzles which produce them has heretofore been a major limitation of the drop-on-demand technique. Therefore, the only reliable way to reduce Rd to date has been to reduce R. A new method is reported which allows an order of magnitude reduction in drop volume while using the same nozzle. It is shown that the key to forming drops with Rd<R is to judiciously control the capillary, viscous, and inertial time scales that govern the flow within the nozzle and the forming drop. The time scales are controlled in experiments by appropriately driving a piezoelectric sleeve surrounding a microcapillary tube, and the interplay between them is elucidated through computation.
Dielectric relaxor and ferroelectric relaxor: Bi-doped paraelectric SrTiO3
Authors: Ang Chen , Yu Zhi
Published: July 2002

In this article, we report the evolution of the dielectric behavior from a dielectric relaxor to a ferroelectric relaxor with variation of Bi concentration in (Sr1−1.5xBix)TiO3 (0⩽x⩽0.2). In the doping range 0.0005⩽x⩽0.002, two dielectric modes A and B are induced. The temperature (Tm) where the permittivity maximum occurs for modes A and B is independent of Bi concentration and of dc electric fields. The complex permittivity of modes A and B follows the empirical Cole–Cole equation. The relaxation time for modes A and B follows the Arrhenius law. The dielectric possessing this type of dielectric behavior is named as a “dielectric relaxor.” At x⩾0.0033, an additional mode C appears, whose Tm increases with increasing Bi concentration. The complex permittivity for mode C does not follow the Cole–Cole equation. The relaxation time of mode C follows the Vogel–Fulcher law, indicating typical relaxor-ferroelectric behavior. In this work, we refer it to a “ferroelectric relaxor” mode. In the range of 0.0033⩽x⩽0.133, the coexistence of the dielectric-relaxor modes and the ferroelectric-relaxor mode is observed. In the samples doped with higher Bi concentration, modes A and B gradually merge into mode C, and only ferroelectric-relaxor behavior remains at x⩾0.133. This system provides a composition-controlled example of evolution from a “dielectric relaxor” to a “ferroelectric relaxor.” In addition, some controversial interpretations of the dielectric behavior of the Bi doped SrTiO3 solid solutions in the literature are discussed, and the polarization relaxation species of modes A and B are attributed to Bi ions.
Mechanism of the structural phase transformations in epitaxial YHx switchable mirrors
Authors: Kooi B. J. , Zoestbergen E. , De Hosson J. Th. M. , Kerssemakers J. W. J. , Dam B. , Ward R. C. C.
Published: July 2002

The detailed mechanisms of the structural phase transformations that occur in epitaxial Y–hydride switchable mirrors are revealed with high resolution transmission electron microscopy (both cross sectional and plan view). The triangular ridge network that develops in Y prior to the α–β transformation is a result of {101̄2} deformation twinning. The basal plane that is originally parallel to the film/substrate interface is rotated by twinning over 5.6° and transformed into a prismatic plane and similarly the prismatic plane is transformed into a basal plane giving a final crystal reorientation for the ridge of 95.6°. After transformation to β, nearly vertical Σ3{111} twin boundaries arise in the ridges. In contrast, horizontal twin boundaries develop in the β domains to prevent macroscopic shape changes. Inbetween the two twin variants within the domains, Shockley partial dislocations are persistently present, which enable efficient reversible β–γ switching of the mirror.
Dependence on cation distribution of particle size, lattice parameter, and magnetic properties in nanosize Mn–Zn ferrite
Authors: Rath Chandana , Anand S. , Das R. P. , Sahu K. K. , Kulkarni S. D. , Date S. K. , Mishra N. C.
Published: July 2002

In Mn1−xZnxFe2O4 (x=0 to 1) nanosize particles prepared through hydrothermal precipitation we observe a decrease in particle size from 13 to 4 nm with increasing Zn concentration from 0 to 1. The lattice constant, a, for all Mn/Zn concentrations is found to be less than that for the corresponding bulk values. At specific compositions within x=0.35 and 0.5, the temperature dependence of the magnetization exhibits a cusp-like behavior below the temperature at which the nanoparticles undergo a ferri- to para-magnetic transition (Tc). The Curie temperatures, Tc, of the nanoparticles are in the range of 175–500 °C, which are much higher than their corresponding bulk values. To explain these unusual features, the strong preferential occupancy of cations in chemically inequivalent A and B sites and the metastable cation distribution in nanoparticles are invoked.
High pressure study of graphitization of diamond crystals
Authors: Pantea C. , Qian J. , Voronin G. A. , Zerda T. W.
Published: July 2002

Graphitization of {100} and {111} faces of diamond crystals at pressures of 0.1 and 2 GPa and various temperatures was studied by Raman spectroscopy, x-ray single crystal diffractometry, and scanning electron microscopy. Different primary mechanisms of graphitization are discussed: (a) normal growth of graphite layer by detachment of single atoms from {100} and {111} diamond surfaces and (b) lateral growth of graphite on the {111} surface by breaking off groups of atoms followed by their rearrangement into planar graphitic structures. The growth of oriented graphite crystallites was observed only on the {111} diamond faces and at p=2 GPa. In the case of synthetic diamonds, internal graphitization was observed and explained in terms of catalytic effects on the metal inclusions.
Electrical conduction mechanism in high-dielectric-constant (Ba0.5,Sr0.5)TiO3 thin films
Authors: Chang Shao-Te , Lee Joseph Ya-min
Published: July 2002

The electrical conduction mechanism of (Ba0.5,Sr0.5)TiO3 (BST) as a function of the temperature was studied. Au/BST/Pt metal–insulator–metal capacitors were fabricated. The temperature range was from 300 to 423 K. The conduction current depended on the voltage polarity. At high electrical field (>800 kV/cm) and with the Pt electrode biased negatively, the Pt/BST interface forms a Schottky barrier with a barrier height of 0.58 eV from 300 to 373 K. The Au/BST interface forms an ohmic contact. The conduction current when the Au electrode is biased negatively shows space-charge-limited-current behavior. An energy band diagram is proposed to explain the experimental results.
Pressure and temperature dependence of viscosity and diffusion coefficients of a glassy binary mixture
Authors: Mukherjee Arnab , Bhattacharyya Sarika , Bagchi Biman
Published: July 2002

Extensive isothermal-isobaric (NPT) molecular dynamics simulations at many different temperatures and pressures have been carried out in the well-known Kob–Andersen binary mixture model to monitor the effect of pressure (P) and temperature (T) on the dynamic properties such as the viscosity (η) and the self-diffusion (Di) coefficients of the binary system. The following results have been obtained: (i) Compared to temperature, pressure is found to have a weaker effect on the dynamical properties. Viscosity and diffusion coefficients are found to vary exponentially with pressure up to a certain high pressure after which the nature of exponential dependence changes. This change is rather sharp. (ii) With temperature, on the other hand, both viscosity and diffusion show super-Arrhenius dependence. Viscosity and diffusion coefficients fit well also to the mode coupling theory (MCT) prediction of a power law dependence on the temperature. The MCT critical temperature (Tc) for both the two dynamical properties are significantly higher than the corresponding critical temperature T0η obtained by fitting to the Vogel–Fulcher–Tammann (VFT) equation. (iii) The critical temperature for viscosity (T0η) is considerably larger than that for the diffusion coefficients (T0Di) implying the decoupling between diffusion and viscosity in deeply supercooled liquid. (iv) The nature of the motion of small particles change from continuous to hopping dominated once the larger ones are frozen. (v) The potential energy of the system shows a minimum against density at a relatively high density when the latter is changed by applying pressure at a constant temperature.
Defects induced in fused silica by high fluence ultraviolet laser pulses at 355 nm
Authors: Stevens-Kalceff M. A. , Stesmans A. , Wong Joe
Published: July 2002

Point defects induced at the exit surface of optical-grade fused silica by high power (>30 J/cm2) 355 nm laser pulses have been investigated to elucidate the nature of laser damage in optics for use at high peak powers (>2 GW/cm2). Six defects have been identified. Eγ′ and E74′ defects were identified using electron spin resonance spectroscopy. The nonbridging oxygen hole center, a self-trapped exciton, an oxygen-deficient center, and interstitial O2 were identified and spatially resolved in the damage craters using cathodoluminescence microanalysis. The defects are associated with the laser generated shock waves and/or thermal explosion.