bluuebunny/crossref_metadata_2025 huggingface dataset:
Depth-dependent magnetism of layered superconductors: Nb/Si
Authors: Yusuf S. M. , Fullerton E. E. , Osgood R. M. , Felcher G. P.
Published: July 2002

We have studied magnetic field penetration and vortex line formation in a [Nb(100 Å)/Si(15 Å)]×20 multilayer by magnetization and polarized neutron reflection. With the magnetic field applied parallel to the surface, the magnetization revealed the presence of a kink above Hcl indicative of transitions between one row of fluxoids and two rows of fluxoids parallel to the surface. The spin dependence of neutron reflectivity below Hcl was consistent with a penetration depth of 1200 Å, substantially larger than that of bulk Nb. In the mixed phase (Hext>Hcl) the field was found to penetrate the surface, with a slope as found in the case of HextHcl vortices form inside the multilayer, in addition to there being surface penetration. A modulation of the vortex fields was found with the periodicity of the Nb/Si bilayers as evidenced by the spin dependence of the reflectivity at the first Bragg peak of the multilayer.
Generation of nematic liquid crystal alignment with polyimides exposed to linearly polarized light of long wavelength
Authors: Wang Yinghan , Xu Chunying , Kanazawa Akihiko , Shiono Takeshi , Ikeda Tomiki , Matsuki Yasuo , Takeuchi Yasumasa
Published: July 2002

The alignment behavior of nematic liquid crystal (LC) on the surface of four types of polyimide (PI) films, three aromatic PIs and a PI with an alicyclic dianhydride, exposed to linearly polarized light of long wavelength (366 nm) was investigated by polarizing microscopy. It was found that the alignment of the LC was induced by all of the exposed PIs and the alignment direction of the LC was perpendicular to the polarization direction of the linearly polarized light. The degree of uniformity of LC alignment increased with increasing exposure energy incident on PI films. To produce the uniform alignment state, the PI with an alicyclic dianhydride had to be exposed to higher energy than aromatic PIs. Ultraviolet (UV) -visible spectroscopy indicated that the absorbance changes of the aromatic PIs were much smaller than that of the PI with an alicyclic dianhydride when uniform alignment of the LC was caused. Furthermore, both UV-visible and infrared spectroscopy showed the change in the absorbance of the aromatic PI with a diphenyl ether diamine was still very small even though it was exposed to linearly polarized light with a large dose. These results demonstrate that the aromatic PIs are more favorable as a photosensitive alignment layer and the uniform alignment of the LC can be induced without significant destruction of the chemical structure of PI with a diphenyl ether diamine.
New approximation for the effective energy of nonlinear conducting composites
Authors: Gibiansky Leonid , Torquato Salvatore
Published: July 2002

Approximations for the effective energy and, thus, effective conductivity of nonlinear, isotropic conducting dispersions are developed. This is accomplished by using the Ponte Castaneda variational principles [Philos. Trans. R. Soc. London Ser. A 340, 1321 (1992)] and the Torquato approximation [J. Appl. Phys. 58, 3790 (1985)] of the effective conductivity of corresponding linear composites. The results are obtained for dispersions with superconducting or insulating inclusions, and, more generally, for phases with a power-law energy. It is shown that the new approximations lie within the best available rigorous upper and lower bounds on the effective energy.
Fermi surfaces of InGaAs/InAlAs superlattices with thin delta-doped barriers
Authors: Nachtwei G. , Weber A. , Künzel H. , Böttcher J. , Jaschinski O.
Published: July 2002

We have investigated the Fermi surfaces of In0.53Ga0.47As/In0.52Al0.48As superlattices with delta-doped barriers of different thicknesses, grown by molecular beam epitaxy. The experimental data were obtained by magnetotransport measurements in tilted magnetic fields and compared with calculations using the Kronig-Penney model [Proc. R. Soc. London, Ser. A 130, 499 (1931)]. For barrier widths above 4 nm, the coupling between the wells is small, and a predominantly two-dimensional behavior of the independent electron systems occurs. For barrier widths of 4 nm and below, an anisotropic three-dimensional behavior of the electrons was found, which is attributed to strong coupling between the wells. Due to the small barrier width for the onset of coupling, the in-plane components of the magnetic field up to 12 T are not sufficient to influence the shape of the Fermi surfaces. This is a consequence of the rather high conduction band offset of about 0.35 eV, which was estimated from our experimental data.
Time resolved photocurrents in Langmuir–Blodgett films of substituted hexaalkoxytriphenylene
Authors: Vaes A. , Catry C. , Van der Auweraer M. , De Schryver F. C. , Sudiwala R. V. , Wilson E. G. , Karthaus O. , Ringsdorf H.
Published: July 2002

By successive deposition of monolayers of the hexaalkoxytriphenylene derivative Te(OH)2, organized multilayers with a thickness between 18 and 360 Å can be obtained. Photoexcitation of the Langmuir-Blodgett films, deposited on interdigitating electrodes with a spacing of 100 μm, by a laser pulse of 4 ns full width at half maximum at a wavelength of 320 nm and with an energy between 5 and 200 μJ, induces transient photocurrents. At large applied fields or upon increasing the energy of the laser pulse and at reduced pressure, besides a transient current pulse of which the features are determined by the bandwidth of the experimental setup, a second current pulse grows in and shows a maximum at 1.5 μs after excitation. At low fields, charge generation occurs mainly in the bulk of the sample. In this field range, most charge carriers disappear by trapping or recombination before they can reach the opposite electrode, making it difficult to get information about the transit time. In the high field range, the charge carriers are still due to light absorbed by the Te(OH)2 layers. In this field range, the pressure of air or nitrogen strongly influences the features of the second maximum. The observation of the second maximum above a critical voltage and under optimal pressure conditions, suggests that corona discharging is also an important process. According to this model, the second maximum is related with the transit time of the charge carriers which migrate perpendicular to the substrate.
Electron spin resonance of rhodium-vacancy complexes in solution-grown NaCl crystals
Authors: Vercammen H. , Schoemaker D. , Käss H. , Goovaerts E. , Bouwen A. , Vrielinck H. , Callens F.
Published: July 2002

Three different paramagnetic [RhCl6]4− complexes were detected in x-ray irradiated solution-grown NaCl single crystals: RhCl64−⋅nVac, n=2, 1, 0. These complexes all have a 4d7 ground state, with the unpaired electron spin mainly in a 4dz2 orbital and differ only by the presence of two, one, or none next-nearest cation vacancy (Vac). The RhCl64−⋅2Vac is formed at 77 K and partially converts into RhCl64−⋅1Vac at about 190 K. At room temperature the RhCl64−⋅0Vac is dominant, but traces of the RhCl64−⋅1Vac and RhCl64−⋅2Vac centers remain. A thermally induced reorientation motion of the 4dz2 molecular orbital is used to explain the temperature dependence of the RhCl64−⋅1Vac electron spin resonance spectrum.
Role of the plasma in the growth of amorphous carbon films by pulsed laser deposition
Authors: Dı́az Javier , Ferrer Salvador , Comin Fabio
Published: July 2002

The plasma in the plume produced by the pulsed laser evaporation with a YAG laser (532 nm) of a graphite target, and its role in the growth of amorphous carbon films have been investigated. For a laser power of 1010 W/cm2, the mean velocity of particles in the plasma was about 8 cm/μs. The plasma component of the plume was fully deflected using a static magnetic field, allowing to deposit amorphous carbon films with and without plasma. Micro-Raman spectroscopy analysis showed that the amount of sp3 hybridized carbon atoms of the nonplasma amorphous carbon decreased to about half the amount found in the full plume deposited films. Particles in the plasma constituted 23% of the total deposited mass, as estimated from the difference in thickness and density of both kinds of grown films. Langmuir probe measurements gave a similar value for the plasma proportion of 25% if particles in the plasma were singly ionized monatomic carbon. This result was consistent with the experimental ion velocity distribution, which was fitted to five Maxwell–Boltzmann distributions in which one of them carried most of the charge in the plasma.
Metalliclike mictomagnetic state in La0.55R0.15Ca0.3MnO3 (R=Y and Ho)
Authors: Xu Jun , Liang J. K. , Tang W. H. , Chen X. L. , Liu Q. L.
Published: July 2002

The magnetic and electronic transport properties of the title compounds are systematically studied. Both compounds are found to have similar properties. The magnetic measurements are consistent with the mictomagnetic behavior. An insulator-to-metal transition is observed with decreasing temperature and the low temperature resistivity shows the metalliclike feature. It is proposed that apart from the already identified ferromagnetic metal and spin glass insulator phases, the metalliclike mictomagnetic phase should exist at the boundary area between the two known phases in the universal phase diagram for the manganates.
Measurement of band offsets and interface charges by the C–V matching method
Authors: Nemirovsky Y. , Gordon G. , Goren D.
Published: July 2002

The present article describes a novel application of capacitance–voltage measurements to determine simultaneously the band discontinuities (ΔEV, ΔEC) and interface charge density (σ) of heterojunctions. The method, which we refer to as C–V matching, complements the most versatile C–V profiling technique proposed by Kroemer and successfully applied by others. In contrast to the C–V profiling which is limited to isotype heterojunctions, the new method is applicable to p-n heterojunctions as well. The methodology is based on three cardinal equations which are not controversial—the lineup of the bands relative to the common Fermi level (at equilibrium) or the quasi-Fermi levels (when voltage is applied), the charge neutrality and the expression for the total capacitance of the heterostructure. The three equations are formulated for equilibrium as well as nonequilibrium conditions, using quasi-Fermi levels and the quasi-equilibrium approximation. The three cardinal equations are defined by the two constant (albeit unknown) interface parameters (ΔEV,σ) which are assumed to be independent of the voltage and two variables (φs1, φs2), which describe the total band bending on each side of the heterointerface and vary with the applied voltage. The actual interface parameters ΔEV, σ are determined by C–V matching between the calculated and the measured curve. The metric for the optimal match between calculated and measured capacitance vectors is discussed. The methodology presented in this study is general and can be applied to semiconductor-semiconductor and semimetal-semiconductor heterojunctions. It is illustrated here for the HgTe-CdTe semimetal-semiconductor heterojunction, which cannot be evaluated by the C–V profiling. The significance of the simultaneous determination of the band discontinuities and interface charges of heterojunctions is also discussed. In addition, the methodology presented in this article models the behavior of biased heterojunctions under nonequilibrium conditions, taking into consideration the values of band offset and interface charge density of an actual heterointerface.
Optically active Er–Yb doped glass films prepared by pulsed laser deposition
Authors: Serna R. , Ballesteros J. M. , Jiménez de Castro M. , Solis J. , Afonso C. N.
Published: July 2002

Active rare-earth Er3+–Yb3+ co-doped phosphate glass films are produced in a single step by pulsed laser deposition. The films are multimode waveguides and exhibit the highest refractive index, optical density and 1.54 μm photoluminescence intensity and lifetime when deposited at low oxygen pressure (Pox⩽4×10−5 Torr). The density of the films obtained under these conditions is higher than that of the target material as a consequence of the high kinetic energy of the species generated during ablation. Luminescent emission can be excited by optical pumping the Er3+ ions either directly or through cross-relaxation of the Yb3+. Post-deposition annealing allows us to improve the luminescence performance.
Impurity gettering to secondary defects created by MeV ion implantation in silicon
Authors: Brown R. A. , Kononchuk O. , Rozgonyi G. A. , Koveshnikov S. , Knights A. P. , Simpson P. J. , González F.
Published: July 2002

Impurities in MeV-implanted and annealed silicon may be trapped at interstitial defects near the projected ion range, Rp, and also at vacancy-related defects at approximately Rp/2. We have investigated the temperature dependence of impurity trapping at these secondary defects, which were preformed by annealing at 900 °C. The binding energies of Fe, Ni, and Cu are greater at the vacancy-related defects than at extrinsic dislocation loops. During subsequent processing at temperatures up to 900 °C, the amount of these impurities trapped at Rp/2 increases with decreasing temperature while the amount trapped at Rp decreases, with most of the trapped metals located at Rp/2 in samples processed at temperatures ≲ 700 °C. However, intrinsic oxygen is trapped at both types of defects; this appears to have little effect on the trapping of metallic impurities at extrinsic dislocations, but may inhibit or completely suppress the trapping at vacancy-related defects.
Hall effect and resistivity of β-FeSi2 thin films and single crystals
Authors: Brehme S. , Lengsfeld P. , Stauss P. , Lange H. , Fuhs W.
Published: July 2002

Cr-doped and undoped β-FeSi2 thin films were deposited on (001) and (111) Si substrates by molecular beam epitaxy. Single crystals were grown by the chemical vapor transport technique. In thin films we found a significant substrate influence on the Hall voltage at room temperature which was strongly reduced at lower temperatures. In Cr-doped samples a transition between defect-band conduction and valence-band conduction was observed in the temperature range 50–100 K. The behavior of the mobility curves μ(T) suggests that defect-band conduction dominates up to near room temperature in nominally undoped thin films. Below a sample-dependent temperature, being not higher than 150 K, often a nonlinear dependence of the Hall effect on the magnetic field was found, sometimes accompanied by pronounced hysteresis behavior. It is shown that this behavior is rather related to microinhomogeneities than to a magnetic phase transition.
Temperature dependence of conventional and effective pyroelectric coefficients for compositionally graded BaxSr1−xTiO3 films
Authors: Mohammed Majed S. , Auner Gregory W. , Naik Ratna , Mantese Joseph V. , Schubring Norman W. , Micheli Adolph L. , Catalan Antonio B.
Published: July 2002

Ferroelectric thin films (∼1.2 μm) of BaxSr1−xTiO3 with gradients in composition normal to the growth surface were formed on platinum substrates by metalorganic decomposition. Effective (pseudo) pyroelectric coefficients as large as 0.06 μC/cm2 K have been obtained from these active ferroelectric devices under the application of an ac field (charge pumping). In contrast, a value of only −0.003 μC/cm2 K has been measured for the conventional pyroelectric coefficient.
Environment of Er in epitaxial Ca1−xErxF2+x thin films using local techniques
Authors: Barrière A. S. , Césaire T. , Hirsch L. , Porté B. , Villenueve G. , Lezama Luis , Rojo T. , Barberis G. E.
Published: July 2002

We present a study of Er3+ in Ca1−xErxF2+x thin films, epitaxially grown by thermal sublimation of the solid solution on silicon and CaF2 substrates. Several techniques were used in the characterization of the films, with the aim to interpret their physical properties. Extended x-rays absorption fine structure as well as the electron spin resonance show that the films present fewer clusters than Er3+ diluted in the bulk CaF2, confirming previous optical studies. These results show that the films are more convenient than the bulk materials to use in optical devices.
A comparative analysis of a-C:H by infrared spectroscopy and mass selected thermal effusion
Authors: Ristein J. , Stief R. T. , Ley L. , Beyer W.
Published: July 2002

A comparative analysis of quantitative infrared absorption spectra and mass selected thermal effusion transients measured for a large number of hydrogenated amorphous carbon (a-C:H) films is presented in order to establish reliable IR absorption cross sections for the different hydrogen bonding configurations in the material. The structural character of the material and the hydrogen concentration were varied over a wide range from soft and wide band gap polymerlike a-C:H, via so-called diamondlike carbon, to the tetrahedral form of a-C:H deposited from a plasma beam source. The results show that the IR absorption cross sections from molecular hydrocarbons can well be transferred to the solid state as long as the material is of polymeric character, but that this procedure fails as soon as the material converts into a three-dimensional cross-linked and mechanically hard structure. For the latter material an empirical average IR absorption cross section for the C–H stretch band can nevertheless be determined from a comparison between hydrogen effusion and IR spectra.
Photomodulated electron-spin resonance in amorphous silicon
Authors: Hattori K. , Ota Y. , Sato K. , Okamoto H.
Published: July 2002

Electron-spin resonance (ESR) under a modulated photoexcitation has been measured for investigating the nonequilibrium carriers trapped at band-tail states in hydrogenated amorphous silicon. The photomodulation technique is successfully applied for detecting the weak Si29 hyperfine structures involved in the light-induced ESR spectrum. Access to the recombination kinetics for band-tail carriers is obtained by the frequency-domain study of modulated ESR signal. Physical insights which led from these measurements are discussed quantitatively.
Formation and size control of a Ni cluster by plasma gas condensation
Authors: Hihara Takehiko , Sumiyama Kenji
Published: July 2002

We have constructed a plasma-gas-condensation type cluster deposition apparatus and tried to find the optimum operation conditions for controlling the cluster size. Transmission electron microscope (TEM) observation has been done to evaluate sizes of Ni clusters produced when varying the volume of a cluster growth region, sputtering power, and inert gas pressure. The mean cluster size decreases by decreasing the volume of growth region and the sputtering power. The smallest cluster obtained in this work is about 2.3 nm in diameter. We have considered the following two models for the cluster growth: (1) a cluster–cluster collision growth and (2) an atomic vapor condensation growth. The cluster growth speed estimated from the former is too slow, while that from the latter is reasonable in comparison with the present experiments. When stable embryos are made from atom collisions, they grow up faster and the final cluster sizes estimated from the latter model are consistent with those observed by TEM.
Electron beam induced changes in the refractive index and film thickness of amorphous AsxS100−x and AsxSe100−x films
Authors: Nordman Olli , Nordman Nina , Peyghambarian Nasser
Published: July 2002

In this article, electron beam induced changes in the refractive index and film thickness of time relaxed amorphous AsxS100−x (with x=30–45) and AsxSe100−x (with x=40–70) are studied. The largest index change (∼0.08) in AsxS100−x films is found when x=40. The corresponding value (∼0.06) for AsxSe100−x films is met when x=55. The difference in the best compositions is attributed to the different relaxation processes of As–S and As–Se films. Electron beam irradiation causes surface shrinkage of the films. In AsxSe100−x films contractions are deepest (∼75 nm) when x=55.
Nonlinear diffusion of electromagnetic fields and excess eddy current losses
Authors: Mayergoyz I. , Serpico C.
Published: July 2002

The purpose of the paper is to demonstrate through analytical calculations that excess eddy current losses can be (at least in part) explained by the peculiar nature of nonlinear diffusion of electromagnetic fields in magnetically nonlinear conducting laminations. This peculiar nature of nonlinear diffusion leads to the breakdown of the classical “uniformity assumption” for distribution of magnetic flux density even for low frequencies, and this results in the appearance of excess eddy current losses.
Mechanism of grain size refinement of Fe3B/Nd2Fe14B nanocomposite permanent magnet by Cu addition
Authors: Ping D. H. , Hono K. , Kanekiyo H. , Hirosawa S.
Published: July 2002

Minor addition of Cu to Nd4.5Fe77B18.5 melt-spun alloy is effective on reducing the grain size of the Fe3B/Nd2Fe14B nanocomposite permanent magnet produced via the crystallization route from the amorphous phase, thereby improving hard magnetic properties. Three-dimensional atom probe and transmission electron microscopy observations have shown that Cu clusters with a number density of ∼1024 m−3 is formed prior to the nucleation event of the Fe3B primary crystal. In the nucleation and growth stage of the Fe3B primary crystals, Cu clusters are in direct contact with the primary particles, suggesting that Cu clusters serve as heterogeneous nucleation sites for the Fe3B primary particles, thereby increasing the number density of the particles.
Ultrapressure equation of state of cerium metal to 208 GPa
Authors: Vohra Yogesh K. , Beaver Steven L. , Akella Jagannadham , Ruddle Chantel A. , Weir Samuel T.
Published: July 2002

We report static pressure compression of cerium metal to 208 GPa (volume compression V/V0=0.37) in a diamond anvil cell at room temperature. Cerium is unique in the 4f elements because of proximity of the f shell to the Fermi energy and related phase transformations induced by pressure. The energy-dispersive x-ray diffraction studies were carried out on cerium metal to 208 GPa using a synchrotron x-ray source and an internal copper pressure standard. A collapsed body centered tetragonal phase is found to be stable to the highest pressure with axial ratio remarkably constant at 1.680±0.006 in the 90–208 GPa pressure range in excellent agreement with theory. Cerium is thus isostructural and isoelectronic with 5f-band metal thorium at ultrapressures. We present equation of state parameters, which describe the compression of cerium to ultrapressures.
Radio-frequency plasma potential variations originating from capacitive coupling from the coil antenna in inductively coupled plasmas
Authors: Watanabe M. , Shaw D. M. , Collins G. J. , Sugai H.
Published: July 2002

The radio-frequency plasma potential in a stove top inductively coupled plasma source is measured by a capacitive probe. The experimental results are compared to a crude circuit model which accounts for capacitive coupling between the rf coil and the bulk plasma. The capacitive coupling model has three terms: the dielectric window capacitance, the sheath capacitance between the dielectric window and the bulk plasma, and the bulk plasma to ground sheath capacitance. The crude circuit model predictions are verified by quantitative comparison with the measured rf plasma potential in the bulk argon plasma at pressures from 1 to 20 mTorr and radio-frequency (13.56 MHz) plasma power levels from 60 to 1000 W. Finally, the measured ion energy spectrum, as determined by a retarding potential analyzer, agrees with rf plasma potential measurements over the entire range of experimental conditions.
Texture evolution in CoCrPtTa/Cr/NiAl magnetic recording media
Authors: Lu B. , Laughlin D. E. , Lambeth D. N. , Wu S. Z. , Ranjan R. , Rauch G. C.
Published: July 2002

Crystallographic textures in a CoCrPtTa/Cr/NiAl multilayer magnetic recording media were studied by electron diffraction. It was found that texture quality evolves through the thickness of the films. A large improvement of texture quality was discovered at the interface of the Cr and NiAl layers. Surprisingly, there remains a portion of randomly oriented grains in the CoCrPtTa layer, in addition to the anticipated grains of [101̄0] texture. The texture quality of the CoCrPtTa layer remains similar to that of the underlying Cr layer.
The dependence of recording performance on substrate temperatures for different crystallographically oriented thin film media
Authors: Zhang Lan , Lal Brij B. , Russak Michael A.
Published: July 2002

Thin film media with and without a NiAl seedlayer were sputter deposited over a range of substrate temperatures. Two distinctly different crystallographic and recording performance behaviors were observed. For the samples deposited with the NiAl seedlayer, which promotes the Co(10.0)/Cr(112) crystallographic orientation, the media noise was relatively low and constant throughout the temperature range investigated, whereas there was a significant reduction in noise with increasing temperature of deposition for the conventional media which have the Co(11.0)/Cr(200) structure. As a result an increase of more than 7 dB in signal-to-noise ratio was observed over the temperature range of 120–260 °C. The results suggest that the degree of Cr segregation with temperatures strongly depends on grain structures, such as grain size, grain size distribution, and grain crystallographic orientation. Magnetic characterization of both media is correlated to their recording performance as a function of substrate temperatures.
Twin structures in tetragonal SrTiO3: The ferroelastic phase transition and the formation of needle domains
Authors: Buckley A. , Rivera J. P. , Salje E. K. H.
Published: July 2002

Direct optical observation of the twin domains in ferroelastic strontium titanite at 8 K<T<105 K show that twin boundaries in the bulk do not change at temperatures below 103.4 K. At temperatures between 103.4 K and Tc=105.65 K the contrast between the birefringences of two adjacent domains is too weak to allow the visual observation of the domain wall. The temperature independent, coarse domain structure in the bulk of the crystal is in strong contrast with a fine domain structure and strain relaxations near the crystal surface. No indication for wall flip motion was found for repeated, slow cooling, and heating through the transition point.
Open volume defects (measured by positron annihilation spectroscopy) in thin film hydrogen-silsesquioxane spin-on-glass; correlation with dielectric constant
Authors: Petkov Mihail P. , Weber Marc H. , Lynn Kelvin G. , Rodbell Kenneth P. , Cohen Stephan A.
Published: July 2002

We used Doppler broadening positron annihilation spectroscopy as a nanovoid characterization tool in the study of low dielectric constant (low-k) hydrogen-silsesquioxane (HSSQ) thin films. The high void sensitivity of this method, combined with depth-resolving capability, enables one to observe changes in the local electronic environment in a thin film. We established a correlation between the annihilation parameters and the dielectric properties for a series of samples subjected to various annealing conditions in nitrogen. Qualitative differences are found between the pore structures of isochronally and isothermally annealed films, suggesting a complex relationship between the film dielectric constant and pore size and pore density. Results showing changes in the chemical environment caused by exposure to various processing environments are also presented. First, the changes in the HSSQ films were determined after a year of exposure to ambient air in which the film properties changed with time due to water absorption. Second, the intentional oxidation of the HSSQ films by exposure to an oxygen plasma were found to propagate from the film surface towards the substrate. A 10 min oxygen-plasma exposure resulted in a 130-nm-thick layer of oxidized HSSQ, with a corresponding high k (=3.6). A decrease in k (to 3.3), with a subsequent 400 °C 30 min forming gas anneal was attributed to the void formation at the HSSQ/Si interface.
Theoretical modeling of multiple quantum well lasers with tunneling injection and tunneling transport between quantum wells
Authors: Kucharczyk M. , Wartak M. S. , Weetman P. , Lau P.-K.
Published: July 2002

Multiple quantum well lasers with tunneling transport of carriers represent a new class of semiconductor lasers. Tunneling can be utilized twofold: as an injection mechanism which drives electrons from a separate confinement heterostructure into active well and also as a mechanism facilitating transport between quantum wells. Since tunneling is normally a very fast process, one can expect that employing the tunneling mechanism for transport of electrons can result in an improvement of modulation bandwidth of multiple quantum well semiconductor lasers. This assertion is justified by an analysis based on the rate equation model (analysis of the tunneling injection) and by determining differential gain (to analyze transport between wells). The analysis, done for 0.98 and 1.55 μm semiconductor lasers, suggests that in tunneling injection lasers it is possible to obtain a substantial increase of intrinsic modulation bandwidth. For the tunneling transport between wells it is shown here within a realistic model including band mixing, that an optimum range of barrier thickness exists for which the differential gain is enhanced and, consequently, the modulation bandwidth improved. A rate equation model, the choice of parameters for the model, as well as the effect of band mixing and well coupling on the optical and differential gain are described. A new formalism for the calculation of optical gain, based on work of Aversa and Iizuka [Aversa and Iizuka, IEEE J. Quantum Electron. 28, 1864 (1992)] is developed. It employs the subband energies and envelope functions determined from the Luttinger–Kohn effective mass equation including band mixing. The study performed for a system of two wells shows that the well coupling substantially shifts the spectral gain peak. The band mixing, in turn, reduces the gain peak as compared to that obtained in the parabolic model. The well coupling enhancement of the differential gain calculated with band mixing is more noticeable than that obtained in the parabolic model. It is true especially for large carrier concentrations and for barrier widths between 20 and 40 Å. Since the relaxation-oscillation frequency depends on the differential gain as fr∝G′, the predicted enhancement of G′ indicates an increase of fr, as well.
Microstructural evolution and Poisson ratio of epitaxial ScN grown on TiN(001)/MgO(001) by ultrahigh vacuum reactive magnetron sputter deposition
Authors: Gall D. , Petrov I. , Desjardins P. , Greene J. E.
Published: July 2002

ScN layers, 60–80 nm thick, were grown at 800 °C on 220-nm-thick epitaxial TiN(001) buffer layers on MgO(001) by ultrahigh vacuum reactive magnetron sputter deposition in pure N2 discharges. The films are stoichiometric with N/Sc ratios, determined by Rutherford backscattering spectroscopy and x-ray photoelectron spectroscopy, of 1.00±0.02. Plan-view and cross-sectional transmission electron microscopy analyses showed that the films are single crystals which appear defect free up to a critical thickness of ≃15 nm, above which an array of nanopipes form with their tubular axis along the film growth direction and extending to the free surface. The nanopipes are rectangular in cross section with areas of ≃1.5×5 nm2 and are self-organized along 〈100〉, directions with an average separation of ≃40 nm. Their formation is the result of periodic kinetic surface roughening which leads to atomic self-shadowing and, under limited adatom mobility conditions, to deep cusps which are the origin of the nanopipes. The ScN layers are nearly relaxed, as determined from x-ray diffraction θ-2θ scans in both reflection and transmission, with only a small residual compressive strain due to differential thermal contraction. The Poisson ratio of ScN was found to be 0.20±0.04, in good agreement with ab initio calculations.
Characteristics of electron inelastic interactions in organic compounds and water over the energy range 20–10 000 eV
Authors: Akkerman A. , Akkerman E.
Published: July 2002

Calculations of inelastic mean free paths, stopping powers, and continuous slowing down ranges for ten solid organic materials: polyethylene, guanine, poly(2-vinylpyridine), diphenyl-hexatriene, carotene, polystyrene, polymethyl(methacrylate), paraffin, polybudene sulfone, polyacetylene and water have been performed for electrons in the 20 eV–10 keV energy range. The complex dielectric formalism was used for estimating the valence part of the transport characteristics, whereas part of the electron–core interactions was evaluated using the binary encounter approximation. The calculations have been extended to account the exchange effect. Detailed comparison of the calculated data with available experimental and theoretical results is presented. The calculated mean ionization potentials for all considered materials were found in good agreement with the ICRU-37 data. Trends of the energy dependence of the inelastic mean free paths, stopping powers, and ranges are discussed. It was shown that Bethe’s nonrelativistic stopping power theory within an accuracy of 10% can be applied to these materials far below 10 keV. The presented data constitute a data base for Monte Carlo simulation of electron transport in organic materials, having a wide field of applications in microdosimetry, electron lithography, and others.
Experimental verification of prediction method for electromigration failure of polycrystalline lines
Authors: Sasagawa Kazuhiko , Naito Kazushi , Kimura Hiroki , Saka Masumi , Abé Hiroyuki
Published: July 2002

A prediction method for electromigration failure in polycrystalline lines has been proposed using the governing parameter of electromigration damage, the atomic flux divergence (AFDgen), and the usefulness has been verified by experiment where various line shapes were treated under various operating conditions. In the prediction method, lifetime and failure site in a metal line have been predicted by numerical simulation of the processes of void initiation, its growth, to line failure. The simulation has predicted accurately the lifetime as well as the failure site of the metal line. In the verification, however, the metal lines treated had the same grain size, that is, the same microstructure. In this article, the prediction method for the electromigration failure of polycrystalline lines was verified in more detail, by comparing the prediction results of lifetime and failure site with the results of experiments using not only various shaped lines but also lines whose microstructures were different.
Theoretical studies of nonlinear optical properties of compounds K4Ln2(CO3)3F4 (Ln=Pr, Nd, Sm, Eu, Gd)
Authors: Xue D. , Betzler K. , Hesse H. , Lammers D. , Zhang S.
Published: July 2002

K 4 Ln 2 ( CO 3 ) 3 F 4 (Ln=Pr, Nd, Sm, Eu, Gd) is a special type of frequency doubling compound, whose crystal structure exhibits a scarcity of fluorine ions. This leads to two different coordination polyhedrons in the general position of K(2) atoms: [K(2)O6F(1)2F(2)] and [K(21)O6F(1)2] in a 2/1 ratio. The chemical bonding structures of all constituent atoms of the compound K4Gd2(CO3)3F4 (KGCOF) are comprehensively studied; moreover, the relationship between the chemical bonding structure and the nonlinear optical (NLO) properties is investigated from the chemical bond viewpoint. The theoretical prediction of the NLO tensor coefficient d11 of KGCOF is in agreement with experimental observation. Theoretical analyses show that the nonlinearity of this crystal type mainly originates from K–O bonds. In addition, the correlation between the NLO tensor d11 and the refractive index n0 of KGCOF is discussed.
Intersubband electroabsorption spectra of semiconductor quantum wells
Authors: Kuo David M.-T. , Chang Yia-Chung
Published: July 2002

The electroabsorption spectra of semiconductor quantum wells due to intersubband (including bound to continuum) transitions are studied theoretically within the effective-mass model. Interesting oscillatory behavior in the absorption spectra due to the Franz–Keldysh effect is explored. The calculated absorption spectra due to the electric field modulation are found in good agreement with available experimental data.
Magnetic properties of mesoscopic ultrathin magnetic films with uniaxial anisotropy
Authors: Sasaki J. , Matsubara F.
Published: July 2002

We study magnetic properties of micron- and submicron-scale ultrathin ferromagnetic films with uniaxial anisotropy by means of a two-dimensional Ising model with both exchange and dipolar interactions using both a mean field approximation and a Monte Carlo method. We find that, in those films, the size of the surface area is an important parameter to determine their magnetic properties. In particular, we find three lattice size ranges A, B, and C. In the range A (small lattices), the model exhibits properties similar to those of a usual Ising model. In the range B (moderate lattices), the specific heat exhibits a double peak around T1 and T2 (<T1) and the magnetization increases abruptly around T2 like that in the first order phase transition. In the range C (large lattices), the magnetization does not increase but the specific heat still exhibits a double peak. In usual materials, the crossover length between the ranges A and B will be of order of submicrons. Then a special attention should be paid to their sizes for analyzing their magnetic properties.
Effect of ion induced damage on carrier lifetimes in strained CdZnSe/ZnSe quantum wells
Authors: Sparing L. M. , Mintairov A. M. , Hodak J. H. , Martini I. B. , Hartland G. V. , Bindley U. , Lee S. , Furdyna J. K. , Merz J. L. , Snider G. L.
Published: July 2002

Studies of the effects of reactive ion etching on molecular beam epitaxy grown CdxZn1−xSe/ZnSe strained quantum wells (QWs) using photoluminescence (PL) and time-resolved reflectivity measurements are reported. The shallow (50 nm cap layer) QW samples exhibit a blueshift in their PL peak position as a function of etch voltage up to a certain point, after which the blueshift is reduced. The reduction in the blueshift of the PL spectrum is strongly correlated with a reduction in the carrier lifetimes measured by transient reflectivity. From these experiments we suggest that the initial blueshift is a result of ion damage at the surface interacting with strain in the QW. On the other hand, the reduced carrier lifetime at higher voltages is a result of more severe structural damage in the QW.
Photon emission from metals under fast nondestructive loading
Authors: Abramova Klara B. , Rusakov Alexandr I. , Semenov Alexandr A. , Shcherbakov Igor P.
Published: July 2002

A study is reported of the mechanoluminescence excited on the backside of metal samples whose front side is irradiated by single laser pulses with an energy below the onset of spallation. The temporal and spatial distributions of temperature, thermal stresses, and thermal-stress variation rates are calculated. The temporal dependence of mechanoluminescence from metal targets is compared with calculations of the mechanical stresses initiated in the targets under pulsed loading. The dynamics of the emission process are shown to be governed by those of loading.
Interferometric measurement of gas densities behind the shock front of a laser-triggered air spark in the nanosecond regime
Authors: Najaf-Zadeh Reza A.
Published: July 2002

The growth of the shock front in an atmospheric air spark is studied using interferometry. A pair of 1 ns nitrogen laser is used for triggering the spark and probing it. An adjustable light path length controls the relative timing. It was found that the shocked shell region of the plasma column is visible about 14 ns after spark initiation. The unionized gas density inside the shocked shell grew with time and at 33 ns reached a value of 6×1022 cm−3.
Magnetic measurements on the layered III–VI diluted magnetic semiconductor Ga1−xMnxS
Authors: Pekarek T. M. , Duffy M. , Garner J. , Crooker B. C. , Miotkowski I. , Ramdas A. K.
Published: July 2002

Magnetic properties of single crystalline Ga1−xMnxS have been measured. This material is in the new class of diluted magnetic semiconductors (DMS) based on layered III–VI semiconductors, yet its magnetic behavior is remarkably different from that observed in Ga1−xMnxSe. At 10 K in a 6 T field, the magnetization for an x=0.066 sample has reached ∼10% of the expected saturation magnetization for S=5/2 and remains roughly linear with field where M/H=2×10−5 emu/g G. The prominent broad peak from 119 to 195 K in the magnetization of Ga1−xMnxSe, ascribed to direct Mn–Mn pairs, is absent in our Ga1−xMnxS data. In this temperature range, the magnetization of Ga1−xMnxS is Curie–Weiss like with Jeff/kB≈−50 K. This suggests there are no direct Mn–Mn pairs in the GaS system. However, the magnetization of Ga1−xMnxS does show a sharp cusp at 10.9±0.1 K in fields between 0.0050 and 2 T similar to the spin–glass transition in the II–VI DMS. The Curie–Weiss behavior and cusp at 10.9 K suggest the presence of Mn–S–Mn pairs in the layered III–VI DMS Ga1−xMnxS.
Grain size reduction by utilizing a very thin CrW seedlayer and dry-etching process in CoCrTaNiPt longitudinal media
Authors: Yoshimura Satoru , Djayaprawira D. D. , Kong Tham Kim , Masuda Yusuke , Shoji Hiroki , Takahashi Migaku
Published: July 2002

To decrease grain size and grain size distribution in CoCr15Ta3.5Ni10Pt5/Cr longitudinal thin film media, a very thin CrW54 seedlayer combined with a dry-etching process were utilized. In the media dry etched after CrW54 deposition, utilization of 1.5 nm CrW54 reduces the grain diameter and σ to 9.4 and 2.2 nm, respectively. By optimizing CrW54 seedlayer thickness and dry-etching process, grain diameter and σ of the magnetic layer can be reduced while maintaining coercivity, longitudinal orientation of magnetic grains, and magnetic interactions.
X-band phased array antennas using crystal yttrium–iron–garnet phase shifters
Authors: How Hoton , Shi Pin , Vittoria Carmine , Champion M. H. , Kempel Leo C. , Trott Keith D.
Published: July 2002

X-band phased array antenna containing four linear microstrip patch elements has been fabricated and tested. The elements were fed through single-crystal yttrium–iron–garnet phase shifters. By varying the bias magnetic field the input phases to the antenna elements can thus be tuned, resulting in steering of the radiation beam in one dimension.
On the estimate of eddy current losses in current carrying conductors
Authors: Friedman G. , Mayergoyz I. D.
Published: July 2002

It is shown that ohmic losses in current carrying conductors at nonzero frequency can be decomposed into two parts. One part is due to a current distribution that would have existed within the conductor at zero frequency (dc current distribution). The other is due to excess current distribution defined to be the difference of the actual current distribution at nonzero frequency and the dc current distribution. An integral equation for the computation of excess current distribution is derived and an easily computable upper bound on the excess losses is found. This upper bound is used to estimate the total eddy current losses and the accuracy of this estimate is tested for long wires of circular cross section. This estimate is shown to be accurate if the wire radius is about the same or smaller than the skin depth.
Proton-nuclear magnetic resonance study of water solvent magnetic fluid’s phase separation
Authors: Taketomi Susamu , Saito Shin-hachiro
Published: July 2002

We report proton-nuclear magnetic resonance experiments on a diluted water solvent magnetic fluid of colloidal volume fraction φ=0.30%. By sweeping the external magnetic field strength, H0, applied to the magnetic fluid around 4000 Oe, we found one major resonant field, HM, and two satellite resonant fields, HS1 and HS2, which correspond to resonant protons in three different coexisting phases. HS1 corresponds to needle-like macroclusters, or a dense phase, in which φ is evaluated to be 0.66% from HS1, while HS2 corresponds to needle-like stripes, or a diluted phase, in which φ is evaluated to be 0.24%. HM corresponds to the rest of the area, the major region. The local field which the proton felt consisted of H0, the Lorentz field and the demagnetizing field. Accordingly, in the major region, the extra field of about 2 Oe in the inverse direction was applied to the protons in addition to H0. The discovery of the three coexisting phases is not explained by the conventional two phase separation model.
Phase transitions in the quantum easy-plane antiferromagnet on the triangular lattice
Authors: Capriotti Luca , Cuccoli Alessandro , Tognetti Valerio , Verrucchi Paola , Vaia Ruggero
Published: July 2002

The quantum effects on the thermodynamics of the easy-plane triangular antiferromagnet, displaying classically both an Ising and a Berezinskii–Kosterlitz–Thouless phase transition, are evaluated by means of the pure-quantum self-consistent harmonic approximation, which allows one to deal with any spin value through classical Monte Carlo simulations. Data for the specific heat, the static spin correlation functions, and the in-plane correlation length are presented in a broad range of temperatures, for different values of the spin and of the exchange anisotropy. Our results for the quantum transition temperatures are in good agreement with the few available theoretical and numerical estimates.
From Galam–Mauger law to a powerful mean field scheme
Authors: Galam Serge
Published: July 2002

A new and powerful mean field scheme is presented. It maps to a one-dimensional finite closed chain in an external field. The chain size accounts for lattice topologies. Moreover, lattice connectivity is rescaled according to the Galam–Mauger law recently obtained in percolation theory. The associated self-consistent mean-field equation of state yields critical temperatures which are within a few percent of exact estimates. Results are obtained for a large variety of lattices and dimensions. The Ising lower critical dimension for the onset of phase transitions is dl=1+2/q. For the Ising hypercube it becomes the Golden number dl=1+5/2. The scheme recovers the exact result of no long range order for nonzero temperature Ising triangular antiferromagnets.
Effect of magnetoelastic interactions on the phase transition of two-dimensional Ising spin system
Authors: Massimino P. , Diep H. T.
Published: July 2002

Using Monte Carlo simulations, we study a two-dimensional (2D) system of particles interacting with each other through short-range cohesive and magnetic interactions, using the Lennard-Jones potential. The spin on each particle is of Ising type. Results obtained depend on the ratio Q between the cohesive and magnetic interaction strengths. For strong cohesive interaction, with Q>1.55, the magnetic phase transition remains that of the 2D Ising universality class. For 1<Q<1.55, the phase transition becomes first order.