bluuebunny/crossref_metadata_2025 huggingface dataset:
Absorption spectra of nanocrystalline silicon embedded in SiO2 matrix
Authors: Ma Zhixun , Liao Xianbo , Kong Gonglin , Chu Junhao
Published: July 2002

Nanocrystalline silicon embedded SiO2 matrix is formed by annealing the SiOx films fabricated by plasma enhanced chemical vapor deposition technique. Absorption coefficient and photoluminescence of the films have been measured at room temperature. The experimental results show that there exists an exponential absorption in the spectral range of 2.0–3.0 eV. The relationship of (αhν)1/2∝(hν−Eg) demonstrates that the luminescent nanocrystalline silicon has an indirect band structure. The existence of Stokes shift between photoluminescence and absorption edge indicates that radiative combination can take place not only between shallow trap states of electrons and holes but also between quantized states of electrons and holes.
Geometrical structure and electronic properties of atomically resolved multiwall carbon nanotubes
Authors: Hassanien A. , Tokumoto M. , Ohshima S. , Kuriki Y. , Ikazaki F. , Uchida K. , Yumura M.
Published: July 2002

Multiwall carbon nanotubes (MWNTs) are predicated to exhibit various electronic properties depending on their diameters and chiralities. The existence of multishells offers the possibility of switching between high and low conducting states within the same nanotube. So far, it has been a big challenge to probe the structure and the electronic properties of different MWNT shells. Here we report the nanostructure measurements of MWNT together with their electronic properties as revealed by a scanning tunneling microscope. The images show tubes with different chiralities (0°–30°) and diameters (4–9 nm). Upon thermal oxidation in air, we have observed diameter-dependent tube opening. MWNTs with large diameters (7–9 nm) show open ends while smaller diameters have closed ends. The structure of open end shows unsaturated bonds, which offers rich chemistry to develop molecular technology based on carbon nanotubes. Finally, we present the structure of the inner shells of MWNT.
Femtosecond near-field spectroscopy of a single GaAs quantum wire
Authors: Guenther T. , Emiliani V. , Intonti F. , Lienau C. , Elsaesser T. , Nötzel R. , Ploog K. H.
Published: July 2002

Quasi-two-color femtosecond pump and probe spectroscopy and near-field scanning optical microscopy are combined to study the carrier dynamics in single semiconductor nanostructures. In temporally, spectrally, and spatially resolved measurements with a time resolution of 200 fs and a spatial resolution of 200 nm, the nonlinear change in reflectivity of a single quantum wire is mapped in real space and time. The experiments show that carrier relaxation in a single quantum wire occurs on a 100 fs time scale at room temperature.
Photoluminescence from nanocrystalline silicon in Si/SiO2 superlattices
Authors: Photopoulos P. , Nassiopoulou A. G. , Kouvatsos D. N. , Travlos A.
Published: July 2002

Si/SiO 2 superlattices on oxidized silicon wafers were fabricated by successive cycles of silicon deposition and high-temperature thermal oxidation. Silicon films were deposited by low-pressure chemical vapor deposition at 580 °C. As-deposited silicon was amorphous, and it exhibited two weak photoluminescence (PL) peaks in the visible range, a stable one at ≅650 nm and an unstable one at 530–550 nm. By oxidation at 900 °C, a drastic increase of the stable PL peak was observed with an initial redshift from 650 to 800–900 nm and a subsequent blueshift down to 680–700 nm. Its position and intensity depended on the oxidation time. For prolonged oxidation, PL disappeared. The observed PL is attributed to silicon crystallites passivated by oxygen. Localized states at the Si/SiO2 interface limit the emitted wavelength. The PL peak from superlattices was at the same position as from one bilayer, while a superlinear increase in PL intensity was observed by increasing the number of bilayers.
Infrared absorption spectra by H2–He collisional complexes: The effect of the anisotropy of the interaction potential
Authors: Gustafsson Magnus , Frommhold Lothar , Meyer Wilfried
Published: July 2002

As an extension of previous work which was based on the isotropic interaction approximation, absorption spectra in the rotational and fundamental bands of H2, induced by collisions with He, are calculated by numerical integration of the close-coupled Schrödinger equation to account for the anisotropy of the interaction potential. A refined quantum chemical dipole surface of interacting H2–He pairs is also obtained with an extended grid of molecular geometries. This dipole surface agrees generally well with previous results, but is smaller by about 5% in the isotropic overlap term which is significant only in the fundamental band. The effects of the anisotropy of the interaction are to reduce the peak intensities of the Q and S lines by roughly 10% and to increase absorption in the far wings by a similar amount. The accuracy of the dipole surface as well as that of the ab initio interaction potential that enters the calculations of the spectra are believed to permit the prediction of absolute spectral intensities with an accuracy of about ±5%. Comparisons with the available measurements show very good agreement of the shapes of the spectral profiles, but the absolute intensities differ by up to 10% in some cases. These remaining differences between theory and measurements appear to be random and are generally smaller than the differences among comparable measurements. Our results should therefore provide a reliable basis for predicting absorption by H2–He pairs for temperatures and frequencies for which no laboratory measurements exist. This fact is of a special interest, for example, for the spectroscopic analyses of the atmospheres of the outer planets.
Surface-induced dissociation of singly and multiply charged fullerene ions
Authors: Biasioli F. , Fiegele T. , Mair C. , Herman Z. , Echt O. , Aumayr F. , Winter HP , Märk T. D.
Published: July 2002

Collisions of singly and multiply charged ions C60z+ (z=1,2,3,4,5) with a hydrocarbon-covered stainless steel surface have been investigated; product ions of fragmentation and pickup reactions were determined as a function of the collision energy (100–500 eV) and the projectile charge z. All ions scattered off the surface are singly charged. The extent of fragmentation increases with the collision energy and the projectile charge. However, the increase of fragmentation with the charge of the projectile is less pronounced than expected from a full conversion of electronic energy, gained in the neutralization process, into internal energy of the ion.
Simultaneous measurement of the solubility of gases in polymers and of the associated volume change
Authors: Hilic Sacha , Pádua Agı́lio A. H. , Grolier Jean-Pierre E.
Published: July 2002

A new instrument to measure simultaneously the solubility of gases in polymers and the associated volume changes of the polymer is presented. The quantity of penetrating fluid introduced in the system is evaluated by pressure decay measurements in a calibrated volume, while a vibrating-wire force sensor is employed to weigh the polymer sample during sorption inside the high-pressure equilibrium cell. The combined use of the two techniques is required by the nature of the sorption process, since the effects of swelling and solubility processes cannot be isolated by performing one single gravimetric or pressure decay measurement. The knowledge of the equation of state of the gas is required, as well as an estimation of its viscosity. The present instrument accepts polymer samples of about 5 g, and operates at temperatures up to 473 K and pressures up to 100 MPa. The technique was validated by measuring the sorption of nitrogen in polystyrene. The instrument showed a sensitivity of the order of 1 mg of dissolved gas and is capable of detecting a volume change of 0.05% of the initial volume of the polymer. The same principles exposed in the present work can be adapted to other kinds of sorption studies.
Thermoelectric figure-of-merit barrier at minimum lattice thermal conductivity?
Authors: Min Gao , Rowe D. M.
Published: July 2002

An analysis of the transport properties data on established thermoelectric materials indicates that the ratio of the electrical power factor to optimum electrical conductivity appears constant. This result, when combined with the previously proposed concept of a minimum thermal conductivity, enables realistic maximum dimensionless figures-of-merit to be calculated. It is concluded that a barrier to increasing the dimensionless figure-of-merit appears to exist which is around 2 at room temperature and that overcoming this barrier will require the development of materials with radically different transport properties from those currently available.
Optical control of third-harmonic generation in azo-doped polymethylmethacrylate thin films
Authors: Churikov Victor M. , Hsu Chia Chen
Published: July 2002

The possibility of controlling third-harmonic generation in polymethylmethacrylate doped with disperse red 1 via exposition by frequency-doubled and fundamental radiation of a Nd:YAG laser is demonstrated. It was possible to change the third-harmonic efficiency up to four times depending on pumping intensity. The change was completely reversible after 15 min of dark relaxation in the case of pumping the sample with the second harmonic. This effect is attributed to the reversible change of third-order susceptibility due to trans-to-cis isomerization of azo-bond molecules.
In-plane polarization anisotropy of the spontaneous emission of M-plane GaN/(Al,Ga)N quantum wells
Authors: Rau B. , Waltereit P. , Brandt O. , Ramsteiner M. , Ploog K. H. , Puls J. , Henneberger F.
Published: July 2002

We study the in-plane polarization of wurtzite GaN/(Al, Ga)N multiple quantum wells. Identical M-plane (11̄00) and C-plane (0001) structures are grown by plasma-assisted molecular-beam epitaxy on γ-LiAlO2(100) and 6H–SiC(0001), respectively. While the emission from the conventional [0001] oriented wells is isotropic within the growth plane, we observe a strong polarization anisotropy of over 90% for the M-plane sample. The luminescence is polarized normal to [0001] and shows no spectral shift with polarization angle, i.e., it originates solely from A excitons (px and py valence band states). The deviation of the polarization degree from unity is attributed to the mixing with pz valence band states due to quantum confinement.
Continuous-wave operation of distributed feedback AlAs/GaAs superlattice quantum-cascade lasers
Authors: Schrenk W. , Finger N. , Gianordoli S. , Gornik E. , Strasser G.
Published: July 2002

We report on continuous-wave operation of first-order distributed feedback quantum-cascade lasers at λ=11.8 μm, based on interminiband transitions in a chirped AlAs/GaAs superlattice. Short devices operate in continuous-wave up to ∼30 K. The single-mode emission wavelength is continuously tunable with the temperature. A metallized surface-relief grating is used for feedback to achieve single-mode emission.
Grazing-incidence diffraction strain analysis of a laterally-modulated multiquantum well system produced by focused-ion-beam implantation
Authors: Grenzer J. , Darowski N. , Pietsch U. , Daniel A. , Rennon S. , Reithmaier J. P. , Forchel A.
Published: July 2002

Focused Ga+ ion beam implantation was used to define a laterally periodic modulation of the electronic band gap in a GaAs/Ga0.97In0.03As/Al0.2Ga0.8As/GaAs [001] multiquantum well structure. The samples were investigated as-implanted and after a rapid thermal annealing (60 s at 650 and 800 °C) by means of x-ray grazing-incidence diffraction. The method provides a separate inspection of the induced strain and the damage profiles as a function of depth below the sample surface. For samples with an ion dose of 5×1013 cm−2, we found a nearly uniform lateral strain amplitude of about 2×10−3 up to the maximum information depth of about 500 nm. It was accompanied by the appearance of structural defects. Rapid thermal annealing at 650 °C has reduced the strain amplitude by a factor of five as well as the density of volume defects. The maximum strain amplitude were found in a depth of about 100 nm. After rapid thermal annealing at 800 °C, the strain has disappeared.
Strain-engineered self-assembled semiconductor quantum dot lattices
Authors: Lee H. , Johnson J. A. , He M. Y. , Speck J. S. , Petroff P. M.
Published: July 2002

We demonstrate a self-assembling method for growing semiconductor quantum dots into ordered lattices. The quantum dot nucleation and positioning into lattices was achieved using a periodic subsurface stressor lattice. Three different two-dimensional (2D) square lattices are demonstrated. The unit cell dimensions, orientation, and the number of quantum dots in the basis are tunable. We find that the 2D lattice can be replicated at periodic intervals along the growth direction to form a three-dimensional (3D) lattice of quantum dots.
Population and coherence transfer in half-integer quadrupolar spin systems induced by simultaneous rapid passages of the satellite transitions: A static and spinning single crystal nuclear magnetic resonance study
Authors: Schäfer Hartmut , Iuga Dinu , Verhagen Rieko , Kentgens Arno P. M.
Published: July 2002

We have recently shown that utilizing double frequency sweeps (DFSs) instead of pulses can lead to increased efficiencies in population and coherence transfer in half-integer quadrupolar spin systems. Cosine modulation of the carrier amplitude corresponds to the simultaneous irradiation of two frequencies symmetrically around the rf-carrier frequency. Convergent or divergent DFSs can be generated by appropriate time-dependent cosine modulation of the rf field. Population and coherence transfer induced by sweeping the modulation frequency through the quadrupolar satellite transitions is investigated in detail. The time dependence of such passages determines the adiabaticity of the transfer processes. Insight into the involved spin dynamics is of utmost importance in the design and optimization of experiments based on amplitude modulation, such as DFS enhanced multiple-quantum magic angle spanning, where multiple to single-quantum conversion is performed by a DFS. Vega and co-workers have provided a theoretical basis of adiabatic coherence transfer in spin-3/2 systems induced by the combined action of simple time independent cosine amplitude modulation (CAM) of the rf field and sample spinning [Madhu et al., J. Chem. Phys. 112, 2377 (2000)]. In our report we will extend this theory to DFS induced adiabatic transfer phenomena in spin-3/2 and spin-5/2 systems. A fully analytical description will be presented covering the whole adiabaticity range resulting in an accurate description of actual experiments. In this context it will be shown that both population and coherence transfer are governed by the same principles and one unique adiabaticity parameter for each pair of spectral satellites. The transfer phenomena derived for spin-3/2 systems will be studied and quantified experimentally for 23Na in a single crystal of NaNO3. In a static and spinning sample the combination with DFS and CAM irradiation will be studied showing the equivalence of the transfer in all these situations. Further we will demonstrate the greater flexibility of a DFS compared to a CAM pulse to manipulate the adiabaticity and thus to maximize the transfer efficiency. Finally, the 27Al resonance in an α-Al2O3 single crystal will be inspected to demonstrate that the efficiency of DFS-induced population and coherence transfer in spin-5/2 systems depends on the direction of the DFS.
The dispersion of the polarizability of C60: A confirmation of recent experimental results through theoretical calculations
Authors: Ruud Kenneth , Jonsson Dan , Taylor Peter R.
Published: July 2002

We present frequency-dependent linear response calculations of the polarizability of C60 using a Hartree–Fock reference wave function. The results indicate that the apparent dispersion of the polarizability as extracted from two recent, independent experiments are slightly too large, although our results are consistent with both experiments to within experimental uncertainty.
CASSCF and MRCI studies of the electronic excited states of CH2Cl and CH2Br
Authors: Li Yumin , Francisco Joseph S.
Published: July 2002

The potential energy curves for the low-lying excited states of CH2Cl and CH2Br have been computed using the complete active space self-consistent field (CASSCF) with the cc-pVTZ basis sets. The vertical excitation energies for the excited states were calculated using the multireference internally contracted configuration interaction (MRCI) method with the cc-pVTZ basis set for CH2Cl, and cc-pVTZ and ECP basis sets for CH2Br. For CH2Cl and CH2Br, all the excited states studied are found to be repulsive along the C–Cl and C–Br coordinate.
Large-scale production of nanocrystals by laser ablation of microparticles in a flowing aerosol
Authors: Nichols W. T. , Keto J. W. , Henneke D. E. , Brock J. R. , Malyavanatham G. , Becker M. F. , Glicksman H. D.
Published: July 2002

We experimentally demonstrate the production of nanoparticles by laser ablation of microparticles entrained at high density in a flowing aerosol. The currently measured production rate of 20 grams per hour could be scaled to industrially useful rates. We have characterized the size distribution of particles and found nearly monodisperse distributions where mean sizes were smaller and varied less with laser fluence than was observed for ablation of microparticles held on flat plates. Mean size was controlled from 4–20 nm by varying the type and pressure of carrier gas. We found Ag and CdSe nanoparticles were crystalline having few dislocations. Materials tested included metals (Ag, Au, and W), semiconductors (Si, CdSe, GaN, and ZnO), ceramics (WC, SiC, and YBa2Cu3O7), and a ferroelectric. Two types of collection processes are described that preserve the nonagglomerated nature of the particles, even at high mass densities.
Influence of noncontact dissipation in the tapping mode: Attempt to extract quantitative information on the surface properties with the local force probe method
Authors: Aimé J. P. , Boisgard R. , Nony L. , Couturier G.
Published: July 2002

In the Tapping mode, a variation of the oscillation amplitude and phase as a function of the tip sample distance is the necessary measurement to access quantitatively to the properties of the surface. In the present work, we give a systematic comparison between experimental data recorded on two surfaces, phase and amplitude, and theoretical curves. With an interaction between the tip and the surface taking into account an attractive and a repulsive term, the analytical approach is unable to properly describe the relationship between the phase variation and the oscillation amplitude variation. When an additional dissipation term is involved, due to the attractive interaction between the tip and the surface, the model gives a good agreement with the recorded data. Particularly, the trends in the phase variations related to the noncontact situations have been found to be amenable to an analysis based upon a simple viscoelastic behavior of the surface.
Low resistance magnetic tunnel junctions and their interface structures
Authors: Fujikata J. , Ishi T. , Mori S. , Matsuda K. , Mori K. , Yokota H. , Hayashi K. , Nakada M. , Kamijo A. , Ohashi K.
Published: July 2002

Effects of interface structure and oxidation state were studied in stacked magnetic tunnel junction (MTJ) structures with top and bottom antiferromagnetic layers to obtain optimum resistance and high tunneling magnetoresistance (TMR) ratios for read heads. The roughness of the NiFe surface and the Al coverage were significantly improved by introduction of O2 surfactant gas on the Ta-seed-layer surface, which increased TMR ratios of the MTJ with low resistance area (RA) products of less than 10 Ω μm2. Furthermore, it was found that avoidance of Ni oxidation and Co oxidation at the tunnel barrier interface is essential to obtaining high TMR ratios, and that a good Al coverage and Fe–oxide formation may enhance TMR ratios when Fe-rich magnetic materials are used. For the top-type and bottom-type structures, a TMR ratio of 12%–17% with RA products of 6–7 Ω μm2 was obtained, which provides sufficient performance for read heads.
On completeness of random exponentials in the Bargmann–Fock space
Authors: Chistyakov G. , Lyubarskii Yu. , Pastur L.
Published: July 2002

We study the completeness/incompleteness properties of a system of exponentials EΛ={eπλz; λ∈Λ}, viewed as elements of the Bargmann–Fock space of entire functions. We assume that the index set Λ is a realization of a random point field in ℂ (the support of a random measure). We prove that the properties are determined by the density of the field, i.e., by the mean number of the field points per unit area. We also discuss certain implications and motivations of our results, in particular, the jumps of the integrated density of states of the Landau Hamiltonian with the random potential, equal to the sum of point scatters.
Terahertz radiation from magnetoresistive Pr0.7Ca0.3MnO3 thin films
Authors: Kida Noriaki , Tonouchi Masayoshi
Published: July 2002

Terahertz (THz) radiation with its spectrum extending up to 1 THz has been observed by an illumination of femtosecond optical pulses to optical switching devices fabricated on magnetoresistive manganite thin films; Pr0.7Ca0.3MnO3. The THz radiation strongly depends on temperature T and its T trend reverses sign across charge-orbital and spin-ordering T.
Application of classical models of chirality to surface second harmonic generation
Authors: Hache F. , Mesnil H. , Schanne-Klein M. C.
Published: July 2002

Two classical models (Kuhn and Kauzmann) are extended to calculate the second-order nonlinear response of an isotropic layer of chiral molecules. Calculation of the various nonlinear susceptibilities (electric dipolar, magnetic dipolar, and electric quadrupolar) is performed and applied to the derivation of the second harmonic field radiated by the molecules. It is shown that the two models give strikingly different results about the origin of the chiral response in such experiments. Previously published results are analyzed in view of this calculation which allows to understand the different interpretations proposed. This calculation emphasizes the interest of surface second harmonic generation to access information about the microscopic origin of optical activity in chiral molecules.
Low-resistance spin-dependent tunnel junctions with ZrAlOx barriers
Authors: Wang Jianguo , Freitas P. P. , Snoeck E.
Published: July 2002

Spin-dependent tunnel junctions with ZrAlOx barriers were formed by natural oxidation (5 min at 10 Torr) of 7-Å-thick Zr–Al films. Resistance×area products of 6 Ω μm2 were achieved with a 15.3% tunnel magnetoresistance (TMR) signal. Bottom-pinned (MnIr) junctions were deposited on top of 600-Å-thick, ion-beam-smoothed, low-resistance, Al electrodes. Effective average barrier height and thickness are 0.28 eV and 8.2 Å, respectively, and breakdown voltage is 0.41 V for 1 μm2 junctions. The TMR signal decreases by half at a bias voltage of 210 mV. Junction TMR decreases for anneals above 250 °C. High-resolution transmission electron microscopy indicates that ZrAlOx forms an amorphous barrier that is smoother than pure crystalline ZrOx or pure amorphous AlOx barriers. These low-resistance tunnel junctions are attractive for read head applications above 100 Gbit/in.2 where competitive signal-to-noise ratios imply resistance×area products of few Ω μm2 and TMR signals near or above 20%.
Structural and optical properties of silicon–germanium alloy nanoparticles
Authors: Lin Chung-Wei , Lin Shih-Yen , Lee Si-Chen , Chia Chih-Ta
Published: July 2002

Silicon–germanium alloy nanoparticles with spherical shape have been prepared by the thermal evaporation method. The shape and structure of these dots have been studied. Transmission electron microscopy images show that the SiGe nanoparticles grown at 100 Torr may be composed of two half-moon shaped sections. Furthermore from the transmission electron diffraction patterns and Raman spectra, a change of structure from amorphous to crystalline at the 0.4 Torr growth pressure was observed. It is also found that if the growth pressure increases, the Ge composition of SiGe dots will decrease. A model is proposed to explain this phenomenon.
The onset of thermal convection in a horizontal fluid layer heated from below with time-dependent heat flux
Authors: Yang Dae Jun , Choi Chang Kyun
Published: July 2002

The onset of convective motion in an initially quiescent, horizontal fluid layer heated from below is analyzed by using propagation theory. When the heat flux is changed suddenly from a constant to a linear ramp with time, cellular convection will set in due to buoyancy forces at a certain time. In this system predictions of the onset time of convective motion are compared with available experimental data and the effect of the aspect ratio is discussed. It is very interesting that propagation theory covers the whole domain of time and the results represent experimental data reasonably well.
Ab initio line shape cross sections: On the need of off-the-energy shell calculation
Authors: Boissoles J. , Boulet C. , Bruet X.
Published: July 2002

Most of the all-quantum-mechanical calculations of line shape relaxation cross sections that have been previously published were based on the “impact approximation” allowing us to express off-diagonal elements in terms of collision theory S-matrices. Two consequences of that approximation are pointed out. The first is that impact cross sections do not obey detailed balance except in limited cases. The second, which is shown of more severe consequence for line coupling applications, is the loss of a sum rule relating the off-diagonal elements (coupling a given line to all the others) to the diagonal one. Finally, it appears clearly that off-the-energy shell calculations with realistic potential surfaces are absolutely necessary for ab initio computing line shapes of overlapping lines.
Strain distribution control on the silicon wafer scale for advanced nanostructure fabrication
Authors: Omi H. , Bottomley D. J. , Ogino T.
Published: July 2002

Looking to the long-term future of Si semiconductor technology, we propose, fabricate, and demonstrate strain distribution control on the planar Si wafer scale for advanced nanostructure self-assembly. Oxygen ions are implanted through patterned layers on the Si wafer; the sample is then annealed at 1325 °C to produce bulk oxide inclusions which yield a strain distribution. Strained epitaxial growth of Ge on the Si(001) substrate surface at 550 °C in ultrahigh vacuum produces three-dimensional islands whose location and size distribution are well controlled. The degree of localization control is in agreement with simulations of the elastic strain distribution.
Effect of intergranular interactions on thermal energy barrier distribution in perpendicular media
Authors: Zhou Hong , Bertram H. Neal , Schabes Manfred E.
Published: July 2002

Micromagnetic simulations have been performed to simulate dynamic hysteresis loops in perpendicular media. Thermal energy barrier distributions have been calculated. For a fixed percentage of magnetization switched, a linear variation between the scaled applied field Ha(t)/HK and ln(f0t)1/2 is found. Without including intergranular magnetostatic interactions, intergranular exchange coupling reduces the thermal energy barrier distribution width, compared to the physical volume distribution width. However, the magnetostatic interactions increase the energy barrier width substantially. The increase is significantly reduced at smaller magnetostatic interactions. For the effective volume of 50% magnetization switched (V50), the results show V50=〈V2〉 almost independent of intergranular magnetostatic and exchange interactions.
Lateral stress and shear strength measurements in polycrystalline potassium chloride, above and below the B1/B2 phase transition during shock loading
Authors: Millett J. C. F. , Bourne N. K. , Rosenberg Z.
Published: July 2002

Manganin stress gauges have been placed in potassium chloride plates, in such orientation that they are sensitive to the lateral component of stress during shock loading. At ∼2 GPa, this material undergoes a dynamic phase change from the B1, NaCl structure under ambient conditions to the B2 CsCl structure. Impact conditions were chosen such that measurements were made both above and below this stress. In combination with known longitudinal stresses, the shear strengths were determined and plotted as a function of impact stress. In the low pressure phase, it would appear that shear strength is constant or only increases marginally, while at higher stresses, strengthening is clearly observed. Both observations are discussed in terms of dislocation motion, generation, and storage.
Intrinsic reduction of the depolarization loss in solid-state lasers by use of a (110)-cut Y3Al5O12 crystal
Authors: Shoji Ichiro , Taira Takunori
Published: July 2002

The depolarization caused by thermally induced birefringence in Y3Al5O12 lasers is intrinsically reduced without any compensation by use of rods cut in other directions than (111). Using a (110)-cut crystal, one can expect the depolarization to become less than 1/10 of that caused in a conventional (111)-cut crystal.
Direct generation of local orbitals for multireference treatment and subsequent uses for the calculation of the correlation energy
Authors: Maynau Daniel , Evangelisti Stefano , Guihéry Nathalie , Calzado Carmen J. , Malrieu Jean-Paul
Published: July 2002

We present a method that uses the one-particle density matrix to generate directly localized orbitals dedicated to multireference wave functions. On one hand, it is shown that the definition of local orbitals making possible physically justified truncations of the CAS (complete active space) is particularly adequate for the treatment of multireference problems. On the other hand, as it will be shown in the case of bond breaking, the control of the spatial location of the active orbitals may permit description of the desired physics with a smaller number of active orbitals than when starting from canonical molecular orbitals. The subsequent calculation of the dynamical correlation energy can be achieved with a lower computational effort either due to this reduction of the active space, or by truncation of the CAS to a shorter set of references. The ground- and excited-state energies are very close to the current complete active space self-consistent field ones and several examples of multireference singles and doubles calculations illustrate the interest of the procedure.
Microairplane propelled by laser driven exotic target
Authors: Yabe T. , Phipps C. , Yamaguchi M. , Nakagawa R. , Aoki K. , Mine H. , Ogata Y. , Baasandash C. , Nakagawa M. , Fujiwara E. , Yoshida K. , Nishiguchi A. , Kajiwara I.
Published: July 2002

We propose a propulsion concept to drive a microairplane by laser that can be used for observation of climate and volcanic eruption. Since it does not have to develop thrust for vertical takeoff, and it has no engine in the normal sense, the microairplane can be very light, with its payload consisting only of observation and communication equipment. In order to demonstrate the concept, we succeeded in flying a paper microairplane driven by a 590 mJ, 5 ns pulse yttrium–aluminum–garnet laser that impinges on a double-layer “exotic target.” The coupling efficiency agrees well with simulations and with experiments.
Thermal noise limitations to 2×20-μm2 magnetoresistive memory element thresholds
Authors: Waite R. I. , Pohm A. V. , Comstock C. S.
Published: July 2002

The effect of thermal agitation on the abruptness of the switching threshold for transverse magnetoresistive memory elements was examined. When the switching fields were within 1% or 2% of the threshold, thermal effects were observed.
The current-voltage characteristics of point-to-ring corona
Authors: Giubbilini Pierluigi
Published: July 2002

In this communication the point-to-ring corona is considered. The experimental results obtained from this new geometry have been compared particularly with the ones that refer to point-to-plane corona. This comparison has provided evidence of the similar features between point-to-ring and point-to-plane corona. In particular, the Townsend relation for the current-voltage characteristics is also valid for the point-to-ring symmetry.
Surface and volume anisotropy from dipole-dipole interactions in ultrathin ferromagnetic films
Authors: Draaisma H. J. G. , de Jonge W. J. M.
Published: July 2002

At the boundary of a ferromagnetic material, the local change in the surroundings of the atomic magnetic moments induces an additional magnetic anisotropy. The dipole-dipole interaction, responsible for the form-dependent demagnetizing field inside the ferromagnet, differs for magnetic moments at the boundary and magnetic moments inside the bulk material. By calculation it is shown that the demagnetization factor for an ultrathin ferromagnetic film is thickness dependent. However, the anisotropy resulting from the dipole-dipole interaction can be interpreted as a surface and a volume anisotropy which depend on the crystalline structure and orientation of the film, but are independent of the thickness of the film.
Annealing behavior of undoped Hg0.8Cd0.2Te epitaxial films at low temperatures
Authors: Vydyanath H. R. , Hiner C. H.
Published: July 2002

Hall-effect measurements were carried out on undoped Hg0.8Cd0.2Te films which were grown liquid-phase epitaxially from Te-rich solutions and were annealed under different partial pressures of Hg at temperatures varying from 150 to 500 °C. Within the phase boundary limits of the material, the hole concentration in the films was found to increase with decrease in the partial pressure of Hg in agreement with the previously reported behavior of undoped bulk Hg0.8Cd0.2Te crystals at temperatures in the range of 400–655 °C. The hole concentration was found to be independent of the partial pressure of Hg at extremely low Hg pressures and is interpreted to be the consequence of the crystals reaching the phase boundary limits. Limits of partial pressure of Hg under Te-saturated conditions at temperatures below 500 °C have been deduced from the electrical data. Previously published values of the equilibrium constants Ki and K″VHg for the intrinsic excitation reaction and for the incorporation of the doubly ionized native acceptor defect deduced for the bulk crystals at temperatures in excess of 400 °C, also satisfactorily explain the electrical data on the epitaxial films at temperatures below 400 °C.
The pressure effect of the Curie temperature in TbAl2
Authors: Sato K. , Nishimura K. , Isikawa Y. , Kamigaki K. , Yoshida H. , Kaneko T.
Published: July 2002

The pressure effect of the ferromagnetic Curie temperature in Tb(Al1−xCox)2, 0≤x≤0.1 was measured by the induction method under hydrostatic pressure. The Curie temperatures of these compounds increased with increasing pressure and the derivative dTc/dp as a function of cobalt concentration did not change so much compared with that of Gd(Al1−xCox)2 [Sato et al., J. Phys. (Paris) Colloq. 49, C8-453 (1988)]. These results are discussed on the basis of the RKKY model.
Mechanical method of iron loss measurement in a rotational field and analysis of iron loss in a motor
Authors: Kaido Chikara
Published: July 2002

A mechanical method of iron loss measurement in a rotational field was developed and, using this method, the mechanism of iron losses of motor cores was investigated. Iron losses in rotational fields of angular velocity ω(rad/s) induce torque T  (Nm) in specimens, and the total losses are calculated from ωT. This new method of iron loss measurement is unrestricted by the configuration of specimen and the exciting conditions, and the iron losses of cores can be measured in the condition used in the motor. Using this new method, the iron losses of 0.5-mm-thick nonoriented electrical steels with 0.3% and 3% Si were measured in rotational fields at frequencies of 50–900 Hz. In disks, the ratios of iron loss in a rotational field to that in a one-direction field decrease as induction increases. The ratio is about 2 at less than 1T and becomes about 1 or less as induction approaches saturation. The effects of teeth on iron loss were investigated. The iron losses of a core with teeth are higher than those of a disk, and increase with lower Si and higher frequency. Iron losses at the teeth are larger than at the other parts, and increase with lower Si and higher frequency. The increase in iron loss is caused by the eddy current loss due to the flux wave form distortion and the flux concentration near the teeth and by hysteresis loss due to the flux distribution which is not uniform near the teeth.
Spin-polarized band structure of magnetically coupled multilayers
Authors: Herman Frank , Sticht Juergen , Van Schilfgaarde Mark
Published: July 2002

The magnetic properties of idealized bcc Fe[001]/Cu[001] and Fe[001]/Cr[001] superlattices have been investigated by carrying out self-consistent spin-polarized linearized muffin-tin orbital/atomic sphere approximation and linearized augmented spherical wave methods/atomic sphere approximation calculations. Structures composed of 2–4 Fe, 2–11 Cu, and 2–6 Cr monolayers were considered. In Fe/Cu, the exchange coupling between successive Fe slabs does not exhibit the FM to AFM crossover observed experimentally at about 9 Cu monolayers. In Fe/Cr superlattices having atomically abrupt interfaces, the exchange coupling exhibits short-range oscillations. When interfacial roughness is taken into account, these oscillations are smoothed. However, the averaged exchange coupling does not switch from FM to AFM at about 4–5 Cr monolayers as observed. Since experimental samples differ markedly from the idealized models employed, it is likely that structural and chemical imperfections will have to be taken into account in order to reconcile first-principles theory with experiment. In this context the structural distortion from bcc to bct in Fe/Cu, and interfacial roughness and localized Fe impurities in Fe/Cr and discussed.
Etching processes of tungsten in SF6-O2 radio-frequency plasmas
Authors: Peignon M. C. , Cardinaud Ch. , Turban G.
Published: July 2002

The reactive ion etching of chemical vapor deposited tungsten in SF6/O2 radio-frequency plasma has been studied by means of optical emission spectroscopy, mass spectrometry, and in situ x-ray photoelectron spectroscopy. Two etch products are detected: WF6 and WOF4. A correlation is found between their concentration in the gas phase and the amount of atomic fluorine and oxygen, as measured by actinometry. In an atomic F-rich plasma, WF6 dominates over WOF4, the latter appearing as soon as oxygen is introduced in the plasma. After etching, the tungsten surface contains three chemical elements: sulfur, oxygen and fluorine; their concentration depends on the reactor parameters (gas mixture, cathode material). Various species have been observed on this surface: S—W (with S 2p at 162 eV), Ox—S—Fy (with S 2p at 170 eV, O 1s at 533 eV, F 1s at 686.4 eV). Two types of tungsten fluorides have been identified: chemisorbed WFn (F 1s at 684.5 eV) and physisorbed WFn species (F 1s at 687.7 eV). The latter are thought to be the precursors of WF6 and O■W—F4 etch products. The role of sulfur, oxygen and fluorine during the etching process of tungsten is discussed.
Atomic mixing in ion-bombardment-induced temperature spikes in metals
Authors: Koponen I.
Published: July 2002

The theory of temperature spikes is generalized to account for the coupling between conduction electrons and lattice atoms. This theory is used to revise the phenomenological model of atomic mixing. The revised model predicts mixing efficiencies in agreement with experimental observations and is capable of explaining the widely different values of mixing efficiencies in near-neighbor transition metals.
Interfacial dynamics and formation of porous structures
Authors: Yan Hong , Hu Xiao
Published: July 2002

A computer simulation study of a two-dimensional two-component resistor network model is presented. The model is constructed to elucidate the relationship between interfacial dynamics and formation of porous structures. The interfacial dynamics, which are stochastic in nature, have been studied and the effect of both global and regional characteristics in interfacial dynamics on the resulting morphologies has been investigated. In the simulation, the advancing interface exhibits either an unstable front as dictated by the Mullins–Sekerka instability, or a stable one that is roughened by kinetic noise. The relevance of the simulation to the experimental observations of porous silicon formation is demonstrated and the implications are discussed.
Coherent Foucault imaging: A method for imaging magnetic domain structures in thin films
Authors: Chapman J. N. , Johnston A. B. , Heyderman L. J.
Published: July 2002

A method for imaging magnetic domain structures in the transmission electron microscope is described. Coherent Foucault imaging provides a direct means of producing a magnetic interferogram which reveals the quantitative distribution of magnetic induction across the specimen. The technique requires the high coherence of a field-emission gun system and the ability to position an aperture accurately to cut part of the diffraction pattern. A simple analytical theory, together with computer simulations and experimental results, is presented to demonstrate the power of the technique.
Optically tunable fiber optic delay generator utilizing photochromic doped sol-gel gel-glass delay line
Authors: Levy D. , del Monte F. , López-Amo M. , Otón J. M. , Datta P. , Matías I.
Published: July 2002

A tunable fiber optic delay generator has been demonstrated utilizing a novel photochromic material embedded in a silica sol-gel matrix. This material has been placed between two multimode commercial fibers filling up the V-shaped groove of a removable connector. With this simple structure, the ability to control optically propagation times of optical signals is achieved. Measurements of input wavelength and optical power required for the desired delay time are presented.
Structural and dielectric properties of spin-on barium-strontium titanate thin films
Authors: Ivanov D. , Caron M. , Ouellet L. , Blain S. , Hendricks N. , Currie J.
Published: July 2002

Barium-strontium-titanate (Ba0.5Sr0.5)TiO3 thin film capacitors fabricated by a spin-on technique were studied. At room temperature the films have high dielectric constant ε, ranging from 22 to 151 at 1 MHz. Complex impedance spectroscopy was used in order to measure the frequency dependence of their dielectric properties. Cole–Cole reactance versus resistance of experimental data in the frequency range 10 Hz–10 MHz show semicircular behavior indicative of current leakage. The current leakage is proportional to the dielectric constant and in particular the lowest leakage current we measured at 1 MHz was 3×10−5 A/cm2 on samples with ε=22.
Non-Gaussian surface roughness distribution of magnetic media for minimum friction/stiction
Authors: Bhushan Bharat , Chilamakuri Sameera
Published: July 2002

Contact of two rough surfaces at an interface occurs at a small fraction of the nominal area. The real area of contact and interfacial adhesion primarily control the friction of an interface. With the presence of a thin film of liquid lubricant or adsorbed water layer at the interface, menisci form around the contacting and near-contacting asperities. The meniscus formation results in stiction problems in the head–medium interfaces. The objective of this research is to predict optimum surface roughness for minimum friction/stiction. For the first time, the effects of skewness and kurtosis of surfaces with non-Gaussian distribution on contact area and meniscus contribution have been studied. A numerical model that accounts for the effects of roughness distribution and liquid film was used for this study. Based on this study, a surface with non-Gaussian distribution with positive skewness between 0 and 0.2 and/or with high values of kurtosis (K≥5) should be used for low friction and stiction.
Thermal lensing measurements in line-focus end-pumped neodymium yttrium aluminium garnet using holographic lateral shearing interferometry
Authors: Blows Justin L. , Dawes Judith M. , Omatsu Takashige
Published: July 2002

Thermal lensing in compact neodymium yttrium aluminium garnet slabs and rods end pumped with a beam brought to a line focus has been measured using holographic lateral shearing interferometry. This powerful technique enabled us to directly measure thermal lenses with focal lengths in the range 0.02–12 m with an accuracy of 12% and good spatial resolution. It was found that the line focused pump beam resulted in severe astigmatic thermal lensing. The measured thermal lens strongly deviated from the ideal parabolic profile along both axes, introducing an important additional consideration for laser-diode end-pumped laser resonator design. The degree of longitudinal heat flow was found to strongly influence thermal lens power. It is anticipated that results obtained from the measurement technique presented here will enable optimal design of high-power end-pumped solid-state laser cavities.
Plasma transport around dust agglomerates having complex shapes
Authors: Keiter Eric R. , Kushner Mark J.
Published: July 2002

Dust particles generated in low temperature plasmas as used for microelectronics fabrication are often agglomerates of smaller monodisperse particles. The transport of these agglomerates, and the subsequent contamination of surfaces, depends on the details of ion-momentum transfer (ion drag) to, and electrostatic forces on, the agglomerate. Given that the charge distribution on the surface of the agglomerate and local electric fields in the vicinity of the agglomerate depend on its shape, the subsequent forces on the agglomerate will also be a function of shape. In this article, we describe results from a simulation in which plasma transport around, and the charging of, agglomerates are investigated. We find that the charge distribution on the agglomerates is generally nonuniform, a consequence of both shadowing and charge depletion. The ion-momentum transfer cross section, calculated using a Monte Carlo simulation, also depends on the shape of the agglomerate.
Resonant excitation of intraband absorption in InAs/GaAs self-assembled quantum dots
Authors: Sauvage S. , Boucaud P. , Gérard J.-M. , Thierry-Mieg V.
Published: July 2002

We have investigated the infrared absorption between confined levels in the conduction and valence bands of undoped InAs/GaAs self-assembled quantum dots. The intraband absorption, which is measured by photoinduced spectroscopy, is analyzed under resonant and nonresonant optical excitation of the quantum dots. The assignment of electron and hole intraband transitions is achieved on the basis of experimental results obtained with n-and p-doped quantum dots. A careful analysis of the absorption spectra shows that several hole transitions and one electron transition with a large broadening are evidenced in the mid-infrared spectral range. We show that the amplitude of the intraband absorption depends on the pump excitation wavelength and exhibits a maximum when the dots are populated via the wetting layer. The spectral shape of the hole intraband absorption is very weakly dependent on the excitation wavelength. The amplitude of the photoinduced hole intraband absorption exhibits a sublinear behavior with the pump intensity. This feature is explained by the state filling of the quantum dots.