bluuebunny/crossref_metadata_2025 huggingface dataset:
Premelting structure: Vibrational dynamics of liquid, undercooled liquid, glassy, and crystal states in methylcyclohexane and deuterated methylcyclohexane
Authors: Abramczyk H. , Paradowska-Moszkowska K. , Wiosna G.
Published: February 2003

We present the results of the Raman studies on vibrational dynamics and static properties of methylcyclohexane (MCH) and deuterated methylcyclohexane (MCH-d14) in distinct thermodynamic states ranging from normal liquid, undercooled liquid to crystal and glassy states. We have studied the low frequency Raman spectra in the range of 15–200 cm−1 and the CH2 rocking mode of MCH and MCH-d14 as a function of cooling and warming rates and temperature in the range of 10–293 K. We have identified the premelting crystal structure (crystal β) of MCH in the temperature range of 140–147 K that proceed the melting both for the glassy state and the equilibrium crystal state (crystal α) of MCH. Deuterated MCH exists only in one crystal form (crystal β) within the studied temperature range.
On the correlation between electronic and nuclear motions in atoms
Authors: Koga Toshikatsu , Matsuyama Hisashi
Published: February 2003

Mutual correlation of the electronic and nuclear motions in an atom is discussed through the average momenta squared of electrons 〈|∑ipi|2〉 and nucleus 〈q02〉 and through the correlation coefficient Cen between electron pi and nucleus q0 momenta. The coefficient Cen is shown to be completely determined by the corresponding coefficient Cee between electron momenta pi and pj. Relations of the correlation coefficients Cen and Cee with the nuclear mass correction energies are also mentioned. Numerical results of Cen are reported for the 103 atoms from hydrogen to lawrencium.
Photomodulated thermoreflectance investigation at elevated temperatures: plasma versus thermal effect
Authors: Christofides Constantinos , Othonos Andreas , Loizidou Efi
Published: February 2003

Photomodulated thermoreflectance measurements were performed at elevated temperatures (294 to 623 K), on crystalline silicon lightly doped with boron. The temperature dependence is qualitatively and quantitatively discussed. The “competition” between thermal and plasma contribution, as a function of temperature, is one of the main subjects of this letter.
Tumbling instability in a shearing nematic liquid crystal: Analysis of broadband dielectric results and theoretical treatment
Authors: Zakharov A. V. , Vakulenko A. A. , Thoen J.
Published: February 2003

The rotational diffusion coefficient D⊥, the rotational viscosity coefficients γi (i=1,2), and the orientational relaxation time τ001 are investigated. γi are calculated by a combination of an existing statistical–mechanical approach and of broadband experimental results of the complex permittivity for the polar liquid crystal 4-n-octyl-4′-cyanobiphenyl (8CB). The flow dynamic behavior of the nematic liquid crystal in a shear flow, as well as the transition from a flow alignment regime to a tumbling instability are also investigated in 8CB near and far away of a charged surface. The relaxation of the alignment angle θ(τ) to its equilibrium θeq in a layer up to ∼1.5 μm is calculated using the Ericksen–Leslie theory for the case of planar alignment of 8CB at a charged indium tin oxide-coated glass plate and for the temperature range in which 8CB exhibits a nematic phase. In accordance with earlier Couette flow experiments, our calculations show a tumbling flow in the bulk nematic phase. The nature of the hydrodynamic instability under shear flow, in the vicinity, e.g., at a few tens of mK from a nematic–smectic A phase transition (TNA) in the nematic phase, and far from TNA is also discussed for 8CB.
Density of states simulations of proteins
Authors: Rathore Nitin , Knotts Thomas A. , de Pablo Juan J.
Published: February 2003

A modified version of a recently introduced algorithm that calculates density of states by performing a random walk in energy space has been proposed and implemented to study protein folding in a continuum. A united atom representation and the CHARMM19 [B. R. Brooks, R. E. Bruccoleri, B. D. Olafson, D. J. States, S. Swaminathan, and M. Karplus, J. Comput. Chem. 4, 187 (1983)] force field are employed for these simulations. This method permits estimation of the density of states of a protein via a random walk in the energy space, thereby allowing the system to escape from local free-energy minima with relative ease. Unlike the earlier formulation that showed slow convergence for continuum simulations, this methodology is designed to achieve better sampling and faster convergence. The modified method is used to examine folding transitions of two peptides: deca-alanine and Met-enkephalin. Protein folding both with and without an implicit solvent (solvent accessible surface area model) has been studied to validate the usefulness of the proposed algorithm.
Structural, mechanical, and vibrational properties of Ga1−xInxAs alloys: A molecular dynamics study
Authors: Branicio Paulo S. , Kalia Rajiv K. , Nakano Aiichiro , Rino Jose P. , Shimojo Fuyuki , Vashishta Priya
Published: February 2003

Structural, mechanical, and vibrational properties of Ga1−xInxAs (0⩽x⩽1) random solid solutions are investigated with classical and ab initio molecular-dynamics simulations. We find that the Ga–As and In–As bond lengths change only slightly as a function of x, despite the large lattice mismatch (∼7%) between GaAs and InAs crystals. The nearest cation–cation distance has a broad distribution, whereas the nearest neighbor anion–anion distance distribution has two distinct peaks. The elastic constants exhibit a significant nonlinear dependence on x. The phonon density-of-states exhibits two high-frequency optical modes. These results are in excellent agreement with experiments.
Lattice inversion for interionic pair potentials
Authors: Zhang Shuo , Chen Nan-xian
Published: February 2003

The ab initio total energies of alkali chloride crystals in B1 (rock salt), B2 (CsCl-type), B3 (zinc blende) and P4/mmm structures are calculated by ultrasoft pseudopotential method in this paper. Based on the different combinations of these total energies, the effective charges on ions are determined for long-range Coulomb interaction, and the short-range potential curves are obtained by the lattice inversion method. As the test for the quality of potentials, the static properties of ACl in the rock salt phase are calculated. The results are in good agreement with the experimental data. And the calculated lattice energies for ACl in wurtzite, CuAu, NiAs, and litharge structures are consistent with the predictions of pseudopotential calculations. In terms of the molecular dynamics simulations, the temperature dependences of volume, bulk modulus, and elastic constants are also investigated from room temperature to 800 K, the results are in good agreement with the experimental values. This indicates that the new interionic potentials are valid over a wide range of interionic separations and coordination numbers.
An all-optical picosecond switch in polydiacetylene
Authors: Abdeldayem Hossin , Frazier Donald O. , Paley Mark S.
Published: February 2003

A polydiacetylene derivative of 2-methyl-4-nitroaniline (PDAMNA) showed a picosecond switching property. This phenomenon was demonstrated by waveguiding a cw He–Ne laser collinearly with a mode-locked picosecond Nd:YAG laser at 532 nm through a hollow fiber coated on the inside with a thin film of PDAMNA. The Z-scan investigations of PDAMNA thin films revealed that the PDAMNA system is a three-level system and the switching is caused by excited state absorption of the He–Ne beam.
Molecular modification of an ionic semiconductor–metal interface: ZnO/molecule/Au diodes
Authors: Salomon Adi , Berkovich Dvora , Cahen David
Published: February 2003

Differences between junctions of metals on ionic or covalent semiconductors persist for junctions, prepared by wet solution methods with a molecular layer at the junctions’ interface. A series of molecules that controls the junction of Au with n-GaAs, does so even stronger with ZnO (300 instead of ∼100 mV barrier height change). With ZnO the interface behavior index is found to be 0.55, five times that with GaAs. This agrees remarkably well with results for junctions of these materials with different metals, prepared in ultrahigh vacuum. Thus, the free semiconductor surface, e.g., surface state density, rather than direct metal–semiconductor interactions, appears to dominate junction behavior.
Electronic properties of the metastable defect in boron-doped Czochralski silicon: Unambiguous determination by advanced lifetime spectroscopy
Authors: Rein S. , Glunz S. W.
Published: February 2003

By combining data from temperature- and injection-dependent lifetime spectroscopy (TDLS and IDLS) measured by means of the microwave-detected photoconductance decay technique and the quasi-steady state photoconductance technique, respectively, the exact electronic structure of the metastable defect in standard boron-doped Czochralski (Cz) silicon has been determined. A detailed Shockley–Read–Hall analysis of the entire TDLS curve reveals that the Cz-specific defect acts as an attractive Coulomb center [σn(T)=σn0T−2] which is localized in the upper band-gap half at EC−Et=0.41 eV and has an electron/hole capture cross section ratio k=σn/σp=9.3. The accuracy of this determination manifests itself by the fact that the corresponding IDLS curve can be simulated with the same parameter set.
Ni-based bulk metallic glass formation in the Ni–Nb–Sn and Ni–Nb–Sn–X (X=B,Fe,Cu) alloy systems
Authors: Choi-Yim Haein , Xu Donghua , Johnson William L.
Published: February 2003

Refractory Ni-based bulk metallic glasses are formed in the three-component Ni–Nb–Sn system near a ternary eutectic composition located within the three-phase field bounded by the three intermetallics Ni3Nb, Ni6Nb7 (μ-phase), and Ni2NbSn (BiF3-type). Bulk amorphous alloys of composition Ni60Nb40−xSnx with 3<x<9 were prepared by injection-casting the molten alloys into copper models. X-ray diffraction and differential scanning calorimetry studies show the cast strips to be fully amorphous up to thicknesses from 0.5 to 3 mm as x is varied. Maximum glass-forming ability (GFA) observed when x is between 6 and 7. These refractory bulk amorphous alloys exhibit high glass transition temperatures 881<Tg<895 K, a large, stable, undercooled liquid region with ΔT=Tx−Tg, at ∼40–60 K, very high Vickers hardness (VH∼1000–1280 Kg/mm2), and estimated yield strengths in the range of 3 to 3.8 GPa. The effects of small quaternary additions of B and Fe on the GFA of the ternary alloys are also reported.
Triplet exciton confinement in phosphorescent polymer light-emitting diodes
Authors: Chen Fang-Chung , He Gufeng , Yang Yang
Published: February 2003

A series of iridium complexes, with triplet energy levels above or below the triplet level of host polymer, were used to study the flow of excitons between the host and the dopants. The performance of phosphorescent polymer light-emitting diodes has been shown to be sensitive to the triplet energy of the dopant. When the dopant exciton level was higher than that of the host polymer, a “backward excitation energy transfer” occurred; hence, the photoluminescence is quenched and the device performance is poor. When the triplet energy level of the dopant was lower than that of the host polymer, the exciton is confined to the dopant site, and the device shows better performance due to this confinement.
InAs/AlSb quantum cascade lasers operating at 10 μm
Authors: Ohtani K. , Ohno H.
Published: February 2003

InAs/AlSb intersubband quantum cascade lasers based on bound-to-continuum transitions are fabricated and operation at 10 μm is demonstrated. A spatially indirect intersubband transition together with a double plasmon waveguide structure is employed. Threshold current density is 4.9 kA/cm2 at 4 K. Temperature dependence of the threshold current density is also presented.
PtRu–WO 3 nanostructured alloy electrode for use in thin-film fuel cells
Authors: Park Kyung-Won , Ahn Kwang-Soon , Choi Jong-Ho , Nah Yoon-Chae , Sung Yung-Eun
Published: February 2003

A PtRu–WO3 nanostructured alloy electrode consisting of alloy nanophases and amorphous tungsten oxide for use in thin-film fuel cells was designed and fabricated using a multigun sputtering system with Pt and Ru metal and a tungsten oxide target. Alloy formation and the presence of nanophases in an amorphous tungsten oxide phase was confirmed by x-ray diffraction and transmission electron microscopy, respectively. The nanostructured alloy electrode, PtRu–WO3 showed the best performance in methanol electro-oxidation because of the presence of the alloy nanophases when compared to a Pt and PtRu thin-film electrode.
Hole depth- and shape-induced radiation losses in two-dimensional photonic crystals
Authors: Ferrini R. , Lombardet B. , Wild B. , Houdré R. , Duan G.-H.
Published: February 2003

A phenomenological model is adopted to study three-dimensional radiation losses in two-dimensional (2D) photonic crystals (PhC) deeply etched in a step-index waveguide. In the case of low vertical refractive index contrast, losses are modeled in a 2D approach and out-of-plane scattering is translated into an effective imaginary index in the air holes. The case of truncated-cone holes is analytically solved. Depth and shape contributions to losses are extracted showing that the verticality of the hole sidewalls is a fundamental parameter. In order to validate the model, the case of a 2D triangular lattice etched through an InP/GaInAsP slab waveguide is studied both theoretically and experimentally.
Reflectivity of planar metallic fractal patterns
Authors: Zhou Lei , Wen Weijia , Chan C. T. , Sheng Ping
Published: February 2003

We studied the reflective properties of a small dielectric plate covered with a fractal-like metallic pattern generated by a particular type of space-filling curves. We found, both experimentally and theoretically, that the plate can reflect electromagnetic waves in a multitude of frequencies, generated from a near-field monopole antenna. Some of the reflected waves have wavelengths much larger than the lateral dimension of the plate. In comparison, a metal plate of the same size failed to reflect when its lateral size was smaller than half of the corresponding wavelength.
High-temperature elastic properties of in situ-reinforced Si3N4
Authors: Swift Geoffrey A. , Üstündag Ersan , Clausen Bjørn , Bourke Mark A. M. , Lin Hua-Tay
Published: February 2003

A high-temperature tensile stress study of a monolithic silicon nitride (Si3N4) was performed with time-of-flight neutron diffraction. A dedicated engineering diffractometer was employed at temperatures reaching 1375 °C. Rietveld refinements of diffraction spectra allowed the determination of (1) the coefficient of thermal expansion tensor during heating and (2) lattice strains during loading. The stress–strain response of individual lattice reflections was used to calculate the single-crystal elastic stiffness tensor of Si3N4 at 1375 °C via a self-consistent model.
A way to obtain visible blue light emission in porous silicon
Authors: Chen Q. W. , Zhu D. L. , Zhu C. , Wang J. , Zhang Y. G.
Published: February 2003

Strong blue emission around 435 nm from porous silicon (PS) embedded in Pb(Zrx,Ti1−x)O3 (PZT) is realized, which is stable and clearly visible to the naked eye even after exposure in an ambient atmosphere for a year. The sample is obtained by hydrothermal etching of (100)-oriented single crystal silicon wafers in an HF aqueous solution, followed by pulsed laser ablation deposition of a PZT thin layer on the surface of the PS. The electrons and holes excited from silicon nanocrystallites could be separated by the PZT-induced electrostatic field and migrate to the boundaries of PS/PZT along opposite directions, respectively. Therefore, the blue emission is interpreted in terms of the recombination of excited carriers from the silicon nanocrystallites in the interfacial region of PS/PZT.
Effect of the dot size distribution on quantum dot infrared photoresponse and temperature-dependent dark current
Authors: Kang Yong Hoon , Park Jinsung , Lee Uk Hyun , Hong Songcheol
Published: February 2003

The fit allows us to find the standard deviation and the average activation energy for electrons in the dot distribution, which is consistent with the peak energy of the photocurrent spectrum measured in the middle infrared. On the contrary, the activation energy found from a conventional Arrhenius fit is well below the photocurrent peak energy.
Solvent intermolecular polarizability response in solvation
Authors: Park Sungnam , Flanders Bret N. , Shang Xiaoming , Westervelt Robert A. , Kim Jeongho , Scherer Norbert F.
Published: February 2003

Polarizability response spectroscopy, a two-color optical Kerr effect method, has been developed and employed to study solvent intermolecular polarizability responses to photoexcited solutes. Here, we report solvent intermolecular polarizability responses in (dipolar) solvation. The time-resolved nonresonant polarizability signals are analyzed in the frequency domain where they are fit to a functional form representing diffusive reorientational, interaction-induced, and librational motions. Diffusive reorientational motion of CHCl3 was preferentially driven following photoexcitation of Coumarin 153 while interaction-induced motion was mainly driven in CH3CN solutions. The mechanism for selective solvent responses involves the relative orientation of the solvent dipole and most polarizable molecular axes and their interaction strength to the solute dipole.
Structure of surface layers of silicon heavily implanted with Ar
Authors: Ohdomari Iwao , Ikeda Masahiro , Ohno Kohsuke , Itoh Tadatsugu
Published: February 2003

A new ESR center at g=2.0029 which was first observed by Chu et al. in silicon implanted with 150-keV argon ions to doses ≳1017 ions/cm2 has been observed, in addition to an amorphous center, in the course of our investigations of amorphous Si layers damaged by ion implantation, when Ar+ implantation dose is more than about 10 times the critical dose for amorphization. These heavily argon-implanted layers of silicon were characterized using ESR and ESCA. The structure of these layers was identified using transmission electron microscopy. It is found that the new ESR center in heavily implanted layers is due to amorphous SiC.
Electrical conductance and radiation of argon plasma containing condensing natural coal ash
Authors: Sinaisky N. A. , Yavorsky I. A. , Volkov Yu. M.
Published: February 2003

A study has been made of the electrical conductance and radiation of a colloidal plasma consisting of a mixture of argon and aluminium particles condensing from the gas phase or argon with the mineral part of natural coals also condensing from the gas phase. This plasma has revealed an almost tenfold conductivity increase as compared to a colloidal plasma from prefused particles of the same minerals. It has also been found that there is a correlation between the growth in plasma conductivity and the condensation process, its radiation extremum and crystallization process. The nonequilibrium conductivity value of such a plasma (1−2 mho/m) at 2700−2500 °K, with an ash concentration up to 0.1%, makes it suitable for use in MHD generators without readily ionizable alkaline additives.
Advances in the efficient generation of intense pulsed proton beams
Authors: Humphries S. , Lee J. J. , Sudan R. N.
Published: February 2003

Recent studies on the space−charge−limited reflex vacuum triode pulsed ion accelerator show a marked improvement in the output parameters and efficiency of the device. In the current machine, a 50−nsec pulse of 130−keV protons of 6000 A is produced at a current density of 20 A/cm2. Energies up to 300 keV have been investigated. The troide efficiency, defined as proton energy output versus total energy input, has been raised from 10 to 42% by the addition of magnetic fields that prevent anomalous electron loss in the fringing fields at the edges of the electrodes. Time−of−flight measurements confirmed the peak energies. The beam was measured to have about a 10 ° half−angle of spread. The results obtained indicate that the accelerator may be scaled to the ranges of parameters currently achieved by pulsed electron accelerators.
Thermal expansion of mixed−alkali germanate glasses
Authors: Shelby J. E.
Published: February 2003

Thermal expansion measurements were made for several series of Na2O⋅K2O⋅GeO2 glasses, for one series of Na2O⋅Rb2O⋅GeO2 glasses, and for one series of Na2O⋅Cs2O⋅GeO2 glasses. Negative deviations from additivity were observed in the critical and deformation temperatures in every case, with the deviations increasing with total alkali concentration. Positive deviations from additivity were noted in the thermal expansion coefficients of the Na2O+K2O and Na2O+Rb2O glasses, whereas the Na2O+Cs2O glasses exhibited negative deviations from additivity. Changes in the coordination of the network forming elements had no effect on the nature of the deviations from additivity. Finally, since these deviations seem so similar to those observed in other alkali−free glass systems, the common usage of the term ’’mixed−alkali effect’’ to describe their occurrence is questioned.
External field effects on diffusion and solidification derived from the free−volume model
Authors: Miller Ronald I. , Ruff Rudolph C.
Published: February 2003

Expressions for diffusion coefficient D and solidification rate Uc from the free−volume model of liquids developed by Turnbull and Cohen have been used to estimate the effects which microgravity and magnetic fields will have on these quantities. The mathematical formalism decribing changes of D and Uc is the same for both the microgravity and magnetic field cases, but the difference between the magnitudes of the two effects is quite large. The change in D and Uc is found to be less than 10−4% for the microgravity case and on the order of 0.1−1.1% for the magnetic field case for four representative materials. D and Uc are found to increase under the influence of an applied magnetic field, and this is in agreement with experimental observations.
Theory of unsymmetric laminated plates
Authors: Chow T. S.
Published: February 2003

Timoshenko’s shear correction factor for an unsymmetric laminated plate is derived rigorously from a formulation involving the theory of elasticity. This approach includes the effect of nonlinear in−plane displacements and emphasizes the effect of shear deformation. As a consequence of the derivation, physically meaningful quantities including the neutral axis, mean elastic constant, and bending rigidity of a composite plate are obtained. After substituting these quantities into the homogeneous plate equation, which includes the transverse shear developed by Reissner and Mindlin, it is applied directly to unsymmetric laminates.
Prediction of dielectric breakdown in shock−loaded ferroelectric ceramics
Authors: Lysne P. C.
Published: February 2003

Recent investigations of dielectric breakdown of a poled ferroelectric ceramic suggest that the same mechanisms are acitve in shock−loading and ambient−condition experiments. Further work described here supported this view for PZT 65/35. Breakdown in shock−loaded specimens can be predicted from ambient and quasistatic−loading data using the concept of a critical particle energy. The effects of the ferroelectric−to−paraelectric transformation and other factors are discussed for shock−loaded PZT materials.
Concentration dependence of the absorption coefficient for n− and p−type GaAs between 1.3 and 1.6 eV
Authors: Casey H. C. , Sell D. D. , Wecht K. W.
Published: February 2003

The absorption coefficient α for GaAs at room temperature was determined in the spectral range from 1.3 to 1.6 eV by transmission measurements for 10⩽α⩽103 cm−1 and by a Kramers−Kronig analysis of the reflectance for α≳103 cm−1. Measurements were made on high−purity n−type samples, n−type samples with free−electron concentrations from 5×1016 to 6.7×1018 cm−3, p−type samples with free−hole concentrations from 1.5×1016 to 1.6×1019 cm−3, and p−type samples heavily doped with the amphoteric impurity Si. These data show that near the direct energy gap Eg the shape of the α−vs−photon−energy curve is strongly dependent on the impurity concentration.
Electrical conduction and dielectric breakdown in crystalline NiO and NiO(Li) films
Authors: Fuschillo N. , Lalevic B. , Leung B.
Published: February 2003

Charge transport, including dielectic breakdown in NiO films (2−10 μ thick), is studied as a function of doping (0−0.4 at.% Li), temperature (−150−+150 °C), frequency (0−108 Hz), and voltage using sputtered nickel contact electrodes. The current I is found to vary as V1 at low and as V3 at higher fields until a breakdown field Es is reached which varies with Li concentration N as Es∝T exp(−N). The field E1 at which I−V nonlinearity begins is given by E1 ?0.2(2kT/eaε′) and E1∝T exp(1/T). The ac conductivity σ (ω) ∝ω2τ2/(1+ω2τ2) and the dc conductivity σ=σ0 exp(−Eth/kT). These transport properties are shown to be consistent with the hopping−carrier model. Under current−controlled conditions, reversible dielectric breakdown (i.e., repetitive switching) was associated with the field−induced delocalization of charge carriers. The results are consistent with the view that the charge carriers prior to reversible dielectric breakdown are small polarons.
Behavior of carrier lifetime spectra (77 °K) in GaAs1−xPx
Authors: Lee M. H. , Holonyak N. , Nelson R. J. , Keune D. L. , Groves W. O.
Published: February 2003

Optical phase−shift lifetime meausrements (77 °K) on direct GaAs1−xPx doped with Te are studied to determine the dependence of the lifetime spectrum τ (λ) on doping concentration and crystal composition. The lifetime τ (λ) of lightly doped crystals (nd<1017/cm3) varies significantly with wavelength. This behavior is attributed principally to electron scattering and to the effects of electron−hole−lattice interactions, the latter occurring at the peak of the pulse excitation. The variations in τ (λ) diminish with increasing donor concentration. At doping densities nd≳1018/cm3, the lifetime spectrum is nearly flat. In all of the crystals at high−excitation levels, the carrier lifetime in the neighborhood of laser modes is shortened by stimulated emission. At crystal composition x<0.38, i.e., well below the direct−indirect transition (x≡xc?0.46, 77 °K), the behavior of the lifetime spectrum is essentially independent of crystal composition. As x approaches the direct−indirect transition xc the lifetime spectrum shows effects attributable to the influence of the X indirect conduction−band minima and the donor states associated with these minima.
Nitrogen−implanted silicon. I. Damage annealing and lattice location
Authors: Mitchell J. B. , Pronko P. P. , Shewchun J. , Thompson D. A. , Davies J. A.
Published: February 2003

The radiation damage and implanted atom location properties of nitrogen−implanted silicon have been studied. Helium−ion backscattering has been used to measure the damage for samples implanted at various doses and annealed to temperatures as high as 900 °C. The location of the implanted nitrogen in the silicon lattice has been investigated by using the 15N(p,α)12C nuclear reaction together with channeling techniques. The results indicate that ≳90% of the implanted atoms are located in nonsubstitutional positions in the silicon lattice, and that the implanted nitrogen has not outdiffused for anneals to 1185 °C. The results presented here will be used in Paper II to help explain many of the observed electrical porperties of nitrogen−implanted silicon.
On bulk waves excited at a groove by Rayleigh waves
Authors: Tuan Hang−Sheng
Published: February 2003

Characteristics of bulk waves generated by an incident Rayleigh wave at a groove discontinuity are studied in this paper. The groove, etched on top of a semi−infinite elastic substrate, is an elementary building block of a reflector−array type of signal processing devices. The mode−conversion problem is treated by a boundary perturbation technique. Two types of bulk wave are excited, i.e., p and sv waves. Their propagation and power characteristics are investigated.
Variation of impurity−to−band activation energies with impurity density
Authors: Lee T. F. , McGill T. C.
Published: February 2003

A theory of the variation of conduction electron density with the temperature for various impurity concentrations is presented. In addition to previously noted effects of condcution band edge lowering and screening of the impurity potential by the conduction electrons, the influence of a finite energy transfer integral and spatial fluctuation in the potential are included. The results show that for ND≳1017 cm−3 in silicon one must not view the activation as occurring between a single impurity level and a well−defined conduction band edge, but must include the broadening of the impurity level and tailing of the conduction−band density of states. Calculations for the shallow donors P, Sb, and As in Si are found to be in satisfactory agreement with experiment.
Cathodoluminescence of compositionally graded layers of GaAs1−xPx
Authors: Kasano Hiroyuki , Hosoki Sigeyuki
Published: February 2003

Compositionally graded layers of GaAs1−xPx grown on (100) planes of GaAs substrates were investigated as a function of the distance t from the substrate, using a 77 °K cathodoluminescence technique. It was found that luminescence intensity L of these layers was strongly dependent on the profile of the compositional gradient, dx/dt. A number of the nonradiative recombination centers are three−dimensionally distributed in the shape of a cross−grid. This is related to misfit dislocations associated with impurities in the graded layers. The gradient dependence of L demonstrates that the density of these nonradiative recombination centers, N, is proportional to (dx/dt)2. This quadratic relation shows that only grading profiles which satisfy the condition d2x/dt2<0 can successfully decrease N continuously with increasing t. Discontinuous variations of x and/or dx/dt cause a significant degradation in the subsequent layers. From the spatial variation in the ratio of the intensity of the 1.35−eV band to the near−gap band, it was found that an anomalous accumulation of Cu contaminant occurred at the substrate−epitaxial−layer interface region, where a relatively high density (∼4×108 cm−2) of these nonradiative recombination centers was present. This is because of a very steep compositional gradient (∼20% P/μ). Here, the concentration of Cu was optically estimated through cathodoluminescence data from Cu−diffused GaAs samples.
A dynamic model for flux jumps in type−II superconductors
Authors: Bussière J. F. , LeBlanc M. A. R.
Published: February 2003

The behavior of flux jumps in hard type−II superconductors can in principle be described by solutions of coupled thermal and magnetic diffusion equations. A simple magnetic diffusion equation for hard type−II superconductors based on the flux−flow concept is proposed and numerical solutions are presented for physical parameters corresponding to a sample of NbTi. Although simplifying assumptions, such as adiabatic conditions and the Jc independence of magnetic field, are made, both the threshold field of stability and the rate of flux collapse are found to agree at least qualitatively with epxerimental values obtained in a NbTi rod. Calculated sequences of magnetic field, temperature, and power density profiles occurring during flux jumps are presented.
Temperature dependence of ac loss in type−II superconductors
Authors: McConnell R. D. , Critchlow P. R.
Published: February 2003

Superconducting ac losses have been measured between 4 and 8.5 K for two single−filament NbTi wires and one multifilamentary Nb3Sn wire carrying ac current. The results demonstrate that the temperature dependence of the critical current density is the predominant factor for the NbTi samples. A temperature−dependent screening current and critical current are needed to explain the Nb3Sn behavior.
Ion diagnostics with rf probes in a warm magnetoplasma
Authors: Grimson W. T. C. , Balmain K. G.
Published: February 2003

The admittance of a parallel−plate capacitor immersed in a warm collisional electron−ion magnetoplasma is studied both experimentally and theoretically. Particular attention is given to the admittance behavior near the lower−hybrid resonance. It is shown that this resonance can be identified experimentally, and therefore that admittance measurements can be used to measure the ion plasma frequency directly, in magnetoplasmas containing one predominant ion species.
Schottky emission in thin−film sandwich structures
Authors: Soukup R. J.
Published: February 2003

Thin metal−insulator−metal structures have been studied for a number of years. The interfacial barrier−controlled conduction mechanisms are tunneling and Schottky emission. A simple method of analyzing the current−voltage characteristics for such a structure when Schottky emission is the dominant conduction mechanism is given utilizing the entire infinite series resulting from the image charge modification to the barrier.
Ultrasonic attenuation in GaAs and InSb
Authors: Kor S. K. , Tripathi N. D.
Published: February 2003

Second−and third−order elastic moduli have been used to evaluate the attenuation suffered by compressional and shear acoustic waves propagating along 〈100〉 and 〈110〉 directions and shear waves polarized along 〈100〉, 〈11̄0〉, and 〈001〉 directions for GaAs and InSb, due to phonon viscosity and thermoelastic phenomena at 298 °K. The Grüneisen mode 〈γ〉, the average square Grüneisen 〈γ2〉, nonlinearity constant D, phonon viscosity, and the dislocation drag along 〈100〉 and 〈110〉 directions are also discussed.
Addendum: Cross−sectional specimens for transmission electron microscopy
Authors: Abrahams M. S. , Buiocchi C. J.
Published: February 2003

References are provided for earlier independent uses of the TEM cross−section technique developed by us.
Interfacial edge dislocations and dislocation walls in anisotropic two−phase media
Authors: Chou Y. T. , Pande C. S. , Yang H. C.
Published: February 2003

Explicit expressions are obtained for the displacement and stress fields of an edge dislocation in the plane interface in a two−phase anisotropic medium. These expressions are used to calculate the stress field of an infinite vertical array of equally spaced interfacial edge dislocations. It is shown that in a two−phase medium, either anisotropic or isotropic, an edge−dislocation wall at or near the interface has a long−range stress field, in contrast to the result for a single−phase medium.
Acousto−optic properties of dense flint glasses
Authors: Eschler H. , Weidinger F.
Published: February 2003

The properties of dense flint glasses relevant to acousto−optic applications have been measured for longitudinal sound waves and He−Ne laser light. Some Schott glasses have proved highly suitable for acousto−optic modulators, others for wide−band optical deflectors. Besides typical acousto−optic properties such as sound velocity, figure of merit, and ultrasonic attenuation, thermal parameters such as the thermal coefficients of sound velocity and refractive index were also measured.
Stark spectroscopy by 10−μ lasers using a multipath cell
Authors: Shimizu Fujio , Shwe Htun , Tamayama T.
Published: February 2003

Extremely high−sensitivity and high−resolution Stark spectroscopy by 10−μ CO2 and N2O lasers is demonstrated using a multipath cell, which serves simltaneously as a Doppler−limited linear absorption spectrometer and as a Lamb−dip spectrometer.
Electron avalanche breakdown by laser radiation in insulating crystals
Authors: Holway Lowell H. , Fradin D. W.
Published: February 2003

Recent experimental investigations into laser−induced damage in the alkali halides have indicated that the damage is due to avalanche breakdown in its dc limit. Numerical computations for NaCl have been carried out that show that the ionization rate can be accurately computed by a classical diffusion or Fokker−Planck approximation to the Boltzmann equation for ionization rates on the order of 1012 sec−1. For hot electrons it is necessary to add a contribution to the collision frequency that is due to the deformation potential in order to explain the dc character of breakdown by 1.06−μm radiation.
Noncoaxial passing dislocation loops in ordered and disordered alloys
Authors: Sadananda K. , Marcinkowski M. J.
Published: February 2003

A detailed analysis of single dislocation loops as well as of noncoaxial passing dislocation loops in ordered and disordered alloys has been made using the piecewise approximation. The analysis shows that the passing behavior of the loops depends on y, z, and γ, where y is related to the radius of the passing loops, z is the interplanar separation, while γ is the antiphase boundary energy. When y is less than some critical value, the results show that the superlattice dislocations in ordered alloys become unstable and disassociate into partial dislocations, the motion of which generates disorder in the crystal. With a decrease of γ, superlattice dislocation instability occurs at larger y. The significance of the above results in relation to the work hardening of ordered and disordered alloys is discussed in detail.
Gas nucleation in the vicinity of solid hydrophobic spheres
Authors: Yount David E. , Kunkle Thomas D.
Published: February 2003

The theory of homogeneous nucleation predicts much larger tensile strengths for water than are actually observed. Plesset has suggested that this may result from the presence of solid impurities, and he has carried out specific calculations for hydrophobic spheres. We show that his result implies a low probability for cavitation if (i) the spheres have radii larger than 10 Å and (ii) the thicknesses of the vapor shells surrounding them are required to be at least one average intermolecular distance, about 3 Å in water. Since homogeneous nucleation is expected to be effective in producing vapor phases with radii up to about 10 Å, the presence of smooth spheres of any size, whether hydrophobic or not, can reduce the predicted tensile strength by at most 30%. This is far below the factor of 5 needed to reconcile the theory with experiments.
Skin effect in exploding-wire physics
Authors: Phung P. V. , Miles D. O.
Published: February 2003

This paper describes a method for calculating the current density in a cylindrical conductor under fast-pulse high-current conditions commonly encountered in exploding-wire studies. Due to extensive Joule heating, the transport property (resistivity-to-permeability ratio) of the conductor must be considered as a varying parameter. Basic mathematical representation of the electrodynamical process is recast into a standard nonlinear diffusion problem. An approximate analytical expression is derived for the penetration depth as a function of time. A numerical solution for the early stages of explosion is also presented for the case of a gold wire in which the permeability is temperature independent and the transport parameter varies only with the resistivity. The effect of magnetic pinch on the solid-liquid transition is not included in this illustrative computation.
Helium migration in borogermanate glasses
Authors: Shelby J. E.
Published: February 2003

Helium permeation, diffusion, and solubility have been measured in a series of B2O3-GeO2 glasses. All three parameters exhibit negative deviations from additivity at all compositions. Since intermediate glasses in this series also always have a lower free volume than would be predicted from additivity, it is postulated that the deviations observed in the helium mobility and solubility are primarily due to an increase in the compactness of the glass network. This conclusion is consistent with the results of several other recent studies of gas mobility in glass.
Thermal modelling of laser damage in 8–14-μm HgCdTe photoconductive and PbSnTe photovoltaic detectors
Authors: Bartoli F. , Esterowitz L. , Kruer M. , Allen R.
Published: February 2003

Crystalline Hg0.8Cd0.2Te and Pb0.8Sn0.2Te materials were exposed to intense 10.6-μm laser radiation for irradiation times varying by more than six orders of magnitude. Laser damage thresholds were measured and found to vary by approximately four orders of magnitude over the range of irradiation times studied. Three thermal models describing thermally induced damage in irradiated crystals are presented and discussed. Damage thresholds were calculated using a simple one-dimensional model for a uniformly irradiated semi-infinite solid. Threshold values calculated using this model are in good agreement with experimental values for irradiation times between 10−6 and 10−2 s. For long times (greater than 10−2 s), specific details of detector construction and irradiation conditions, ignored by the one-dimensional model, have a significant effect on damage thresholds. Therefore, two thermal models are presented which take into account the finite detector thickness and radial heat conduction. It is found that predictions of permanent-damage thresholds using these models are in good agreement with experimental data for all irradiation times involved. The advantages and limitations of these models are discussed. As a function of irradiation time the damage thresholds for photoconductive and photovoltaic detectors can exhibit three distinct regions of behavior. For short times (τ<10−6 s) E0 is constant and P0 is inversely proportional to τ, for intermediate times E0∝τ1/2 and P0∝τ−1/2, and for long times P0 asymptotically approaches a constant. The long-time behavior in which P0 is constant is due to the thermal profile having reached a steady-state distribution. This is brought about either because of radial heat conduction or finite detector thickness.
Hole centers in GaSe
Authors: Manfredotti C. , Rizzo A. , De Blasi C. , Galassini S. , Ruggiero L.
Published: February 2003

A systematic investigation of hole centers acting in p-GaSe single crystals has been carried out by means of the following methods: (i) dark conductivity measurements as a function of temperature; (ii) space-charge-limited-current measurements extended from 77 to 300 °K; (iii) thermally stimulated-current measurements as a function of temperature. A series of hole centers has been found with energy depths ranging from 26 to 400 meV from the valence band and densities of about 1013 cm−3. A complete summary of the energy levels present in the forbidden gap of GaSe up to 600 meV from the valence band is given. A comparison is made between the results of this work, previously reported data, results obtained by Hall-effect versus temperature measurements, and data quoted by other authors. Finally, the origin and nature of all the hole centers are discussed, taking into account their possible connections with the defective structure of GaSe.
Intensity-modulated photoconductivity. The general case
Authors: Tokarsky R. W.
Published: February 2003

The response of a photoconductor excited by sinusoidal intensity-modulated light is extended to cover the case of a continuous distribution of trapping and recombination centers. It is shown for low frequencies that a meaningful analysis is possible by dealing separately with the in phase and the quadrature components of the photoimpedance. Absolute measurements of trap densities at the quasi-Fermi level, as well as recombination frequencies, can be obtained. The components are explicitly derived for low frequencies and are either linear or independent of the modulating frequency. The approximations used in the calculations are illustrated by numerical examples.
Electron-beam effects in depth profiling measurements with Auger electron spectroscopy
Authors: Ahn J. , Perleberg C. R. , Wilcox D. L. , Coburn J. W. , Winters H. F.
Published: February 2003

Ion-beam sputter etching and simultaneous Auger electron spectroscopy have been used to obtain elemental composition profiles on some electrically insulating samples. It has been noted that the effective sputter-etch rate is substantially larger on that part of the sample area which is irradiated by the incident electron beam than on the area which is subjected to ion bombardment only. This effect, which adversely influences the depth resolution of the method, is attributed to electron-stimulated desorption of the surface oxygen which in turn causes enhanced sputtering in the electron irradiated area.
Thermal diffusivity measurement by a radial heat flow method
Authors: Donaldson A. B. , Taylor R. E.
Published: February 2003

A method is presented whereby the thermal diffusivity of a solid is measured by observing the temperature excursion which results from a radial flow of heat. The radial heat flow is produced by the instantaneous deposition of energy on a disk region of one surface of a planar specimen. Experimental results which utilized this technique are presented for Armco iron and they are compared to results based on an axial heat flow method with a radial heat flow correction.
Preferential concentration of certain elements in smaller aerosols emitted from aircraft engines
Authors: Jolly R. K. , Gupta S. K. , Randers-Pehrson G. , Buckle D. C. , Thornton W. B. , Jr. H. Aceto , Singh Jag J. , Woods David C.
Published: February 2003

Aerosols from aircraft engines were collected with an Andersen eight-stage cascade sampler for a period of 24 h. The aerosol samples from each stage were analyzed for their elemental composition using the proton-induced x-ray emission (PIXE) technique. Seventeen elements (Si, P, S, Cl, K, Ca, Ti, V, Fe, Ni, Cu, Zn, Br, Sr, Nb, Sn, and Pb) were positively identified and quantitated at each stage. Six elements (S, Ca, Fe, Zn, Sn, and Pb) showed fractional concentration increase with the decreasing aerosol size. Similar, but less well-defined, trends were also observed for V and Ni. Silicon and chlorine, on the other hand, showed an opposite trend. Neutron activation analysis of bulk aerosol samples collected every 2 h over the same period showed correlation of concentration of Si, Ca, V, Ti, Zn, Br, and Sn with the density of air traffic at the airport. Analysis of the aviation fuel samples by PIXE indicates that major fractions of Pb, Sn, Br, Zn, Ni, Fe, V, Ca, and S observed in these aerosol studies come from the aircraft engine exhaust.
Magnetomechanically excited echoes in ferrites
Authors: Kupca S. , Searle C. W.
Published: February 2003

Echoes following a two-rf-pulse excitation were observed from bulk and powder samples of polycrystalline ferrites. The echoes originate in refocusing of the elastic modes of the sample, excited by the magnetoelastic interaction.
Monte Carlo treatment of multiple Coulomb scattering in pion-beam dose calculations
Authors: Hamm R. N. , Wright H. A. , Turner J. E.
Published: February 2003

Multiple scattering of negative pions in tissue is investigated by Monte Carlo methods for an 82-MeV pencil beam by using (i) single deflections of each pion at the target surface with straight trajectories thereafter, (ii) successive angular deflections after specified distances of travel in the target, and (iii) successive angular deflections and lateral displacements after specified distances of travel. Results of the computations are compared with results predicted by empirical formulas of Fowler and Perkins for depths of 14 and 18 cm. Method (ii), with a distance of 1 cm between sucessive deflections, appears to give a satisfactory representation of multiple scattering. A calculation of isodose contours for a sample pion beam is made both with and without multiple scattering. As this example shows, multiple Coulomb scattering severely limits the degree to which a concentrated therapeutic dose can be delivered to a localized tumor region.