bluuebunny/crossref_metadata_2025 huggingface dataset:
Iron–acceptor pairs in silicon: Structure and formation processes
Authors: Zhao Song , Assali L. V. C. , Justo J. F. , Gilmer G. H. , Kimerling L. C.
Published: July 2002

We performed a theoretical investigation on the properties of iron–acceptor impurity pairs (Fe–A, with A=B, Al, Ga, and In) in silicon. The calculations were performed within the framework of an ionic model, including elastic and electrostatic interactions. In contrast to the conventional point charge ionic model, our model includes a correction to the electrostatic interaction that takes into account the valence electronic cloud polarization, which adds a short range, attractive interaction to Fe–A pair bonding, and includes the silicon lattice relaxation due to the atomic size difference between the acceptor and the lattice atoms. Our results are in good agreement with the experimental trends among the Fe–A pairs, describing the increase in the pair donor energy level with increasing A principal quantum number and decreasing pair separation distance, and the pair configurational symmetries.
In
x
Ga
1−x
As/Al
y
Ga
1−y
As/Al
z
Ga
1−z
As
asymmetric step quantum-well middle wavelength infrared detectors
Authors: Wu W. G. , Chang K. , Jiang D. S. , Li Y. X. , Zheng H. Z. , Liu H. C.
Published: July 2002

In x Ga 1−x As/Al y Ga 1−y As/Al z Ga 1−z As asymmetric step quantum-well middle wavelength (3–5 μm) infrared detectors are fabricated. The components display photovoltaic-type photocurrent response as well as the bias-controlled modulation of the peak wavelength of the main response, which is ascribed to the Stark shifts of the intersubband transitions from the local ground states to the extended first excited states in the quantum wells, at the 3–5.3 μm infrared atmospheric transmission window. The blackbody detectivity (Dbb*) of the detectors reaches to about 1.0×1010 cm Hz1/2/W at 77 K under bias of ±7 V. By expanding the electron wave function in terms of normalized plane wave basis within the framework of the effective-mass envelope-function theory, the linear Stark effects of the intersubband transitions between the ground and first excited states in the asymmetric step well are calculated. The obtained results agree well with the corresponding experimental measurements.
Sheared flow amplification by vacuum magnetic islands in stellarator plasmas
Authors: Garcia L. , Carreras B. A. , Lynch V. E. , Pedrosa M. A. , Hidalgo C.
Published: July 2002

There is some experimental evidence that the E×B flows have radial structure that may be linked to rational surfaces. This flow structure may result from a self-organization process involving nonlinear flow amplification through Reynolds stress and fluctuation reduction by sheared flows. In stellarators, a large contribution to the Reynolds stress comes from the coupling of the magnetic field component of a vacuum field island with a plasma instability. In this process, the self-organization principle seems to be marginal stability for the fluctuations driving the flow.
Scattering mode displays with in-plane electrode structure
Authors: Ohura Hisanori , Okada Hiroyuki , Onnagawa Hiroyoshi
Published: July 2002

The electro-optical properties of a scattering mode display with an in-plane electrode structure are investigated. The in-plane electrodes are patterned on one side of a glass substrate, and the other side is roughened. Neither side of the substrate has an alignment layer. In the off state, this roughened surface disrupts the uniform molecular orientation and induces light scattering. In the on state, the liquid crystal molecules are uniformly aligned due to the applied electric field. This display mode does not require a polarizer. Under He–Ne laser irradiation, the contrast ratio was 17:1. The response times of transmittance rise and fall were 6.3 and 46.5 ms, respectively, which are somewhat shorter than that of conventional in-plane switching mode displays. It is considered that the disordered alignment is responsible for this improvement in response time, realized as a function of the roughened surface and/or weak surface anchoring.
Whittaker function approach to determine the impurity energy levels of coated quantum dots
Authors: Lin Ming-Chieh , Chuu Der-San
Published: July 2002

The electronic structure of a hydrogenic impurity atom located at the center of a multilayer coated quantum dot (CQD) is investigated. The electronic eigenstates of the CQD system are expressed in terms of the geometry and the material parameters by solving the Schrödinger equations analytically. Image potential effects are ignored and the effective mass approximation is employed. The ground state energy is found to be strongly influenced by the shell thickness and confined potential. In contrast to previous work, our eigenfunctions are expressed in terms of the Whittaker functions in any region, no matter where the energy levels are, i.e., whether they are higher or lower than the potential barriers. Our approach is simpler and has general significance, e.g., the bound state eigenenergies of any n-layered quantum dot can be easily determined by directly solving just “one”(2n−2)-rank secular determinant equation instead of solving 2n equations. One can also easily and quickly determine whether a system has bound states by using the Whittaker function approach.
Functionalization of silicon step arrays I: Au passivation of stepped Si(111) templates
Authors: Kirakosian A. , Lin J.-L. , Petrovykh D. Y. , Crain J. N. , Himpsel F. J.
Published: July 2002

The growth mode of Au on stepped Si(111)7×7 surfaces is determined by scanning tunneling microscopy, with the goal of providing a continuous gold layer that replicates the step morphology. Functionalization with gold allows attaching organic and biomolecules via thiol groups (e.g., alkanes and DNA). On clean Si(111), gold grows in the Stranski–Krastanov mode and produces islands with a size comparable to the step spacing. A Ti wetting layer produces smooth Au films that preserve the step topography down to a scale of a few nanometers.
Mechanisms of inert gas impact induced interlayer mixing in metal multilayers grown by sputter deposition
Authors: Zhou X. W. , Wadley H. N. G.
Published: July 2002

Control of interfacial roughness and chemical mixing is critical in nanomaterials. For example, multilayers composed of ∼20 Å conductive layer sandwiched between two ∼50 Å ferromagnetic layers can exhibit giant magnetoresistance (GMR). This property has caused a tremendous recent increase in hard disk storage capacity, and can potentially result in a new generation of nonvolatile magnetic random access memories. It has been established that good GMR properties can be obtained when the interfacial roughness and interlayer mixing of these multilayers are low. However, flat interfaces in nanoscale multilayers are not thermodynamically stable, and cannot be obtained using thermal energy deposition processes such as molecular-beam epitaxy. Hyperthermal energy sputter deposition techniques using either plasma or ion-beam gun are able to create nonequilibrium flat interfaces, and have been shown to produce better GMR multilayers. In these processes, however, inert gas ions or neutrals with energies between 50 and 200 eV can impact the growth surface. This may be a major source for interlayer mixing. By using a molecular dynamics technique and a reduced order model, the composition profile across the thickness of multiply repeated Ni/Cu/Ni multilayers has been calculated as a function of the energy and the relative flux of the inert gas ions or neutrals as well as the layer thickness. The results indicate that the 50–200 eV inert gas impact caused atomic exchange between adjacent atomic layers near the surface. The probability of exchange increased with impact energy, but decreased with the number of overlayers. The exchange between Ni overlayer and Cu underlayer atoms was much more significant than that between Cu overlayer and Ni underlayer atoms. As a result, the Ni on Cu interfaces were much more diffuse than the Cu on Ni interfaces, in good agreement with experiments. At very high inert gas flux and impact energy, an increased probability for the underlying Cu atoms to be exchanged to the surface resulted in significant Cu surface segregation.
Microscopic observation of Cu damascene interconnect grains using x-ray microbeam
Authors: Hasegawa Masaki , Hirai Yasuharu
Published: July 2002

Microscopic observation and imaging of Cu damascene interconnect grains were carried out using the x-ray microbeam of the BL-16XU at SPring-8. The x-ray focusing system of the beamline consists of a bent-cylindrical prefocusing mirror and elliptical mirrors with a Kirkpatric–Baez configuration. Diffraction spots from the microscopic area irradiated by the intense x-ray microbeam (7×6 μm full width at half maximum) at the boundary between the metal line and the dielectric were observed. The diffraction spots observed show that Cu(111) grains grow normally on the sidewall and Cu(200) grain orientation distribution is quite broad, suggesting that crystallographic texture near the trench is quite disordered. Two-dimensional images of grains with a specific orientation angle were also obtained. Grains as large as about 10 μm caused by self-annealing were observed.
Microfabrication and characteristics of double-rectangular spiral type thin-film inductors with an upper NiFe magnetic core
Authors: Kim Sang-Gi , Yun Eui-Jung , Kim Jwa-Yeon , Kim Jongdae , Cho Kyoung-Ik
Published: July 2002

Simple, double-rectangular spiral-type thin-film inductors with an upper Ni81Fe19 core for 5 MHz drive dc–dc converter application were investigated. Copper films 10-μm-thick covered by a 3-μm-thick NiFe film were used as the coils. The inductors fabricated in this study are 2.5×3.4–3.5×5.5 mm2. The inductors developed, with a small size of 2.97×3.1 mm2, exhibit L of 1.43 μH, quality factor of 15.8, and resistance of 2.88 Ω at 5 MHz, and show results comparable to those reported in recent literature for similar inductors sandwiched by more complex magnetic core materials. An analysis of the experimental data suggests an equivalent circuit of the fabricated inductors. From the equivalent circuit analysis, C was shown to have a negligibly small effect over the frequency ranges used. The calculated data obtained from the equivalent circuit and the well-known equation of Q described the high-frequency data of the L, Q, and impedance of the inductors developed well.
Role of thermal energy on the magnetic properties of laminated antiferromagnetically coupled recording media
Authors: Piramanayagam S. N. , Hee C. H. , Wang J. P.
Published: July 2002

The effect of thermal energy (kBT), which has been found to play some important roles in the magnetic properties of recently developed antiferromagnetically coupled media, is described. It was observed that the thermal energy helps to obtain an antiparallel configuration of moments at remanence. Therefore, a reduction in the remnant moment–thickness product (Mrδ) is observed, even for smaller values of J (interface coupling constant) than those used in simulations that do not consider thermal energy. The magnetic viscosity measurement helps to distinguish the magnetization decay behavior of the top and bottom layers. The magnetic moments of top and bottom layers show maximum decay at different fields and the decay rates approximately scale with their thickness. Viscosity results also point out that the magnetization reversal of the bottom layer should occur in the first quadrant, in order to obtain a low noise and thermally stable media. Micromagnetic simulation was performed by including thermal effects. In that case, Mrδ reduction could be obtained for smaller values of J than in the case where thermal energy is not included in the simulation.
28 ps recovery time in an InGaAsP/InGaAsP multiple-quantum-well saturable absorber employing carrier sweepout
Authors: Burr E. P. , Song J. B. , Seeds A. J. , Button C. C.
Published: July 2002

We describe fast recovery at a wavelength of 1550 nm in a multiple-quantum-well (MQW) saturable absorber with InGaAsP quaternary wells and barriers using electric-field-induced carrier sweepout. The MQW SA is integrated with a distributed Bragg reflector in a p-type–intrinsic–n-type structure. Pump–probe measurements show that the recovery time can be reduced from >900 to 28 ps by application of a 151 kV/cm sweepout field. We measure the dependence of the recovery time on the energy of the saturating pulse and explain our results in terms of carrier dynamics in the sweepout field.
Transport properties of submicron YBa2Cu3O7−δ step-edge Josephson junctions
Authors: Larsson Peter , Tzalenchuk Alexander Ya. , Ivanov Zdravko G.
Published: July 2002

Submicron step-edge Josephson junctions in YBa2Cu3O7−δ (YBCO) thin films were fabricated and studied. All measured junctions fall into three categories: low critical current resistively shunted Josephson junctions, intrinsic Josephson junctions, and quasiparticle tunnel junctions. The transport in the junctions is correlated with the microstructure of YBCO films grown on the step edge. We argue that the properties of conventional step-edge junctions can be understood as a superposition of those types.
Role of crystallographic structure on magnetization reversal of (101̄0) and (112̄0) textured thin film media
Authors: Holloway L. , Laidler H. , Telling N. D. , Wu S. Z.
Published: July 2002

In this study we examine the role of crystallographic structure on the magnetization reversal of two CoCrPtTa thin films grown on different underlayers which provide either a [112̄0] or a [101̄0] preferred orientation. We report synchrotron x-ray measurements and find that the stacking fault density in a CoCrPtTa thin film with a (101̄0) texture is greater than that in a sample having the (112̄0) bicrystal texture (13% compared to 8%). However, we also find that the degree of in-plane texture is poorer in the (101̄0) film. We report extensive magnetic measurements on the two films and find that increased switching at low fields correlates well with the observed level of fcc-like regions. In particular we show that a significant increase in magnetic viscosity at low fields in the (101̄0) film is most probably due to the increased level of low anisotropy fcc-like regions of Co formed by the stacking faults rather than small grains or poor texture. This is supported to some extent by the bimodal behavior observed in the activation volumes for the [101̄0] film, one peak being observed as expected around the coercivity (∼3 kOe) and another at a much lower field (∼1.2 kOe).
Direct effect of illumination on ferroelectric properties of lead zirconate titanate thin films
Authors: Kholkin A. L. , Iakovlev S. O. , Baptista J. L.
Published: July 2002

Photosensitive ferroelectric materials exhibit various photoferroelectric phenomena due to the strong influence of nonequilibrium charge carriers on polarization and phase transition. These phenomena are essential for a number of applications including photodriven actuators and sensitive photodetectors. In this work, the effect of UV illumination on dielectric and piezoelectric properties is investigated in lead zirconate titanate (PZT) thin films, which are currently the most promising material for microactuator applications. The effective piezoelectric coefficient (d33) and dielectric permittivity (ε) of PZT films are simultaneously measured under a weak ac electric field during illumination with a band gap light (λ=280–400 nm). It is shown that both d33 and ε decrease under UV illumination. The reduction of permittivity, however, is much smaller than that of d33 and demonstrates a much slower time relaxation. The d33 decrease is attributed to the reduction of average remanent polarization under the UV light. Thus a direct effect of the photoactive light on ferroelectric polarization is observed under essentially nondestructive (weak-field) conditions. The origin of the observed effect is discussed along with the possible implications for thin-film devices.
Enhancement of electrical conductivity in α-Al2O3 crystals doped with magnesium
Authors: Tardı́o M. M. , Ramı́rez R. , González R. , Chen Y. , Kokta M. R.
Published: July 2002

Direct current and alternating current electrical measurements were performed to investigate the electrical conductivity of α-Al2O3:Mg samples with different concentrations of [Mg]0 centers (Mg ions each with a trapped hole) in the temperature interval 250–800 K. The concentration of [Mg]0 centers was monitored by the optical absorption peak at 2.56 eV. These centers were produced by oxidation at temperatures above 1050 K. The formation rate of [Mg]0 centers depends on the previous thermal history of the sample in either reducing or oxidizing atmosphere. At low electrical fields, dc measurements reveal blocking contacts. At high fields, the I–V characteristic is similar to that of a diode (corresponding to a blocking contact at one side of the sample and an ohmic contact at the other side) connected in series with the bulk resistance of the sample. Steady electroluminescence is emitted at the negative electrode when a current in excess of ≈10 μA passes through the sample, indicating that the majority of carriers are holes. Low voltage ac measurements show that the equivalent circuit for the sample is the bulk resistance in series with the junction capacitance (representing the blocking contacts) connected in parallel with a capacitance, which represents the dielectric constant of the sample. The values determined for the bulk resistance in both dc and ac experiments are in good agreement. The electrical conductivity of Al2O3:Mg crystals increases linearly with the concentration of [Mg]0 centers, regardless of the amount of other impurities also present in the crystals, and is four times higher in the c∥ than in the c⊥ direction. The conductivity is thermally activated with an activation energy of 0.68 eV, which is independent of: (1) the [Mg]0 content, (2) the crystallographic orientation, and (3) the concentration of other impurities. These results favor the small-polaron-motion mechanism.
Tip shape dependence of the light emission efficiency for the scanning tunneling microscope
Authors: Uehara Y. , Suda Y. , Ushioda S. , Takeuchi K.
Published: July 2002

We have investigated the relation between the scanning-tunneling-microscope (STM) tip shape and the efficiency of STM light emission. The light intensity radiated by a current source placed in the tip-sample gap of the STM was calculated for various shapes of the tip using the finite differential time domain method. We found that the highest emission efficiency is obtained for the pyramidal tip with the flat top of 4 nm width and the apex angle of 90 °C.
Photoemission study of the effect of thermal annealing on Cs2O/Si(111) surfaces
Authors: Wu J. X. , Li F. Q. , Zhu J. S. , Ji M. R. , Ma M. S.
Published: July 2002

The Cs2O overlayers with different thickness were prepared by simultaneous oxygen–cesium adsorption on Si(111) at room temperature. Photoelectron spectroscopy and work-function measurements have been used to study the Cs2O/Si surfaces as a function of annealing temperature. The results show that the interaction of the Cs2O overlayer with the substrate is weak. The Cs2O species is sensitive to x-ray radiation, forming a new species on the top surface. The Cs2O/Si surfaces exhibit a negative electron affinity with a work-function value of 0.85±0.1 eV until the Cs2O species decomposes completely. After Cs2O disappears, both Cs–O and Si–O bonds are dominant on the surfaces with a work function of about 1.2 eV. For further annealing, the oxygen bonded to cesium gradually transfers to Si due to Cs desorption. The thickness of the SiO2 overlayer formed after Cs desorption is dependent on the mount of oxygen in the Cs2O overlayer.
Modulating fluorescence of 8-quinolinolato compounds by functional groups: A theoretical study
Authors: Sugimoto Manabu , Anzai Masaharu , Sakanoue Kei , Sakaki Shigeyoshi
Published: July 2002

Substituent dependence of a fluorescence energy of the 8-quinolinolato anion (Q−) was systematically investigated with electronic structure calculations. Large red- and blueshifts of the fluorescence were predicted for Q− with the NO2 and CN groups, respectively. For bis(8-quinolinolato)berylium with the NO2 and CN groups, the emission energies were calculated to be 1.56 eV (795 nm) and 2.66 eV (466 nm), respectively, indicating that they are potential candidates as red- and blue-light-emitting compounds.
Electron resonant tunneling with a high peak-to-valley ratio at room temperature in Si1−xGex/Si triple barrier diodes
Authors: Suda Yoshiyuki , Koyama Hajime
Published: July 2002

We have applied the triple-barrier (TB) structure to a Si1−xGex/Si electron-resonant-tunneling diode (RTD) and have demonstrated that a Si0.7Ge0.3/Si electron tunneling TB RTD exhibits a high peak-to-valley current ratio of more than 7.6 even at room temperature (RT). The results indicate that for a SiGe RTD, a combination of electron tunneling using a tensile-strained Si well structure and well doubling is very effective in enhancing the negative-differential-resistance (NDR) characteristics. Additionally, the large NDR effect at RT is suggestively explained by the simple conduction band configuration and the necessity of the double-well coresonance condition.
Two-phase radiation-affected characteristics of acoustic wave propagation in a hot gas–particle two-phase medium
Authors: Park Jae Hyun , Baek Seung Wook
Published: July 2002

A theoretical analysis has been carried out to study the acoustic wave propagation through a hot planar gas–particle two-phase medium with emphasis on two-phase radiation effect. Extended from the earlier work, radiative absorption and scattering by particles as well as gas are considered and the relaxation model was used only to describe the temporal momentum interaction between gas and particles. The differential approximation for two-phase radiation was newly formulated. The present study showed that particle radiation alone without gas radiation was able to incur the radiation-induced wave attenuation. Moreover, once the gas radiation was involved, the wave attenuation due to radiation was apparently influenced by the particle as well as gas radiation. As the gas absorption coefficient increased, the additional wave attenuation appeared only when the gas absorption coefficient became larger than the given particle absorption coefficient. The radiative absorption by particles also affected the dispersion of wave. In addition, the wave attenuation in the low-frequency region disappeared as the scattering coefficient increased. It was also observed that the forward scattering augmented the wave attenuation in the low-frequency region for a fixed scattering coefficient while the backward one reduced it. Finally, in a higher medium temperature the wave attenuation by radiation was seen to be enhanced.
Effect of a finite thickness transition layer between media with different permittivities on charged particle localization in an external electric field
Authors: Dolinsky Yu. , Elperin T.
Published: July 2002

In this study we investigate the effect of a finite thickness transition layer on the character of charged particle localization at the boundary separating domains with different electric permittivities. It is shown that the effective potential at the location of the particle can be determined using perturbation theory, and the conditions for the validity of the theory are discussed. We discuss various effects that can be observed experimentally in the behavior of charged particles located near the boundary.
On the sink/source effects in two-dimensional plasma turbulence
Authors: Galkin S. A. , Krasheninnikov S. I.
Published: July 2002

Ad hoc sinks and sources of turbulence are added in the two-dimensional (2D) magnetized plasma model to mimic excitation and damping of the modes. Depending on linear growth and damping rates and sink and source localizations, two kind of the turbulence and large scale flows evolution regimes were found: quasisteady state and dynamic. The quasisteady states with saturated averaged energy and enstrophy are characterized by strong large scale flows generated due to the inverse cascade in the nonlinear mode coupling. It is shown that for the adopted model there is not complete stabilization of high-k harmonics (microturbulence) by the generated zonal flows and convective cells. The dynamic regimes are characterized by nonvanishing energy and enstrophy fluctuations around averaged values and the energy spectrum has maximum, which corresponds to the turbulence source. The inverse cascades in these regimes are suppressed by the mode damping in the k-space sink region.
Chemical investigations of GaAs wafer bonded interfaces
Authors: Hansen D. M. , Albaugh C. E. , Moran P. D. , Kuech T. F.
Published: July 2002

The bonding chemistry of various GaAs-to-oxide/GaAs bonded samples was investigated using multiple internal transmission Fourier transform infrared spectroscopy for thermally annealed and thermocompression annealed samples. The oxides used in these investigations included a native GaAs oxide as well as two compositions of borosilicate glass (BSG) deposited by low-pressure chemical vapor deposition (LPCVD). For the thermally annealed samples, the hydrogen-bonded H2O/OH groups on the hydrophilic surface form a room temperature bond without the application of pressure. Chemical changes at the wafer-bonded interface occur in two temperature regions. For anneals between 200 and 400 °C for 1 h in N2, the H2O/OH groups react and evolve H that becomes absorbed within the oxide. The LPCVD BSG oxide was chemically unaltered during anneals in this temperature range, however, the GaAs native oxide underwent chemical modification. Initially, the GaAs oxide consisted of As(III)–O and Ga–O related oxides. The As(III)–O oxides react to form free As and Ga–O during annealing between 200 and 400 °C. For anneals between 500 and 600 °C, the reaction of H2O/OH groups continue and the H becomes infrared inactive, most likely forming H2 voids at the bonded interface. In addition, As(V)–O related oxides were observed during thermal annealing in this temperature range. No detectable chemical changes in the BSG were observed over the temperature range investigated. Samples that were annealed under an estimated 1–10 MPa of pressure had similar chemical changes to thermally annealed samples.
Self-consistent treatment of stabilization of resistive wall instabilities in reversed field pinches by radio-frequency waves
Authors: Svidzinski V. A. , Prager S. C.
Published: July 2002

Stabilization of resistive wall magnetohydrodynamic (MHD) instabilities by the force applied by injected radio-frequency (rf) waves is investigated, including the self-consistent effect of the MHD perturbation upon the rf waves in the plasma. This effect leads to the generation of Alfvénic disturbances at the frequency of the rf waves and at the wavelength (in the magnetic surface) of the MHD instability. Stabilization of the ideal external kink instability in the reversed field pinch is considered. If the self-consistent response is neglected, rf waves are confined to the thin vacuum and edge regions, and provide a restoring force which stabilizes the resistive wall instability at moderate rf wave amplitude. However, the generation of Alfvén disturbances causes the rf waves to penetrate deeply into the plasma, eliminating the stabilization.
High thermal stability of photoluminescence in a disordered quantum wire superlattice
Authors: Liu X. Q. , Sasaki A. , Ohno N. , Wang Xue-Lun , Ogura M.
Published: July 2002

A randomly disordered quantum wire superlattice (d-QWR-SL) structure was fabricated by means of metalorganic vapor phase epitaxy. Photoluminescence (PL) and the carrier lifetime properties were investigated systematically. The PL intensity was found to be independent of temperatures below 170 K. This thermally stable feature was due to dramatic suppression of the nonradiative recombination in the wire region. The carrier lifetime measurement confirmed the high thermal PL stability in the d-QWR-SL structure. This high thermal PL stability may be due to the state localization in the d-QWR-SL structures.
Temperature dependence of second-order nonlinear relaxation of a poled chromophore-doped sol–gel material
Authors: Goudket Hélène , Canva Michael , Lévy Yves , Chaput Frédéric , Boilot Jean-Pierre
Published: July 2002

Second-order optical nonlinearity relaxations of hybrid inorganic/organic sol–gel films doped with poled Disperse Red 1 chromophores were measured by second harmonic generation at several temperatures. The decay curves were fitted to biexponentials and Köhlrausch, Williams, and Watts (KWW) stretched exponentials. It can be concluded that the KWW model well describes chromophore thermal reorientation in hybrid sol–gel materials, as is known for all-organic polymers. Using the Arrhenius law, the activation energy of the reorientation process has been determined to be about 50 or 70 kcal mol−1. A stability of roughly a decade has been extrapolated at room temperature. Another experiment at room temperature gave an estimation of the stability that was found to be in qualitative agreement. It is shown that poled chromophore-doped sol–gel materials have the potential to be stable enough for applications in telecommunication devices.
Asymptotic speed ratio in a free helium jet
Authors: Bruch L. W. , Abanov Ar.
Published: July 2002

The spatial dependence of the speed ratio in an atomic helium free jet is approximated for low internal temperatures where effective-range theory becomes accurate for the atom–atom scattering.
Formation of metastable phases during heat treatment of multilayers in the Al–Pt system
Authors: Lábár János L. , Kovács András , Barna Péter B. , Gas Patrick
Published: July 2002

This communication reports that several metastable phases form subsequently during heat treatment (up to 500 °C) of Al-rich Al–Pt multilayers. Besides the known a(amorphous)-Al2Pt, formation of two metastable phases with a composition close to Al5Pt was also observed in a transmission electron microscope. One of them corresponds to a phase given by space group P4 in Pearson’s collection of intermetallic compounds. The other, a hexagonal phase (a=12.4 Å and c=26.2 Å) is the one that was observed in rapidly solidified Al–Pt alloys [L. Ma, R. Wang, and K. H. Kuo, J. Less-Common Met. 163, 37 (1990)]. Formation of these phases under different conditions is reported here.
Cr
3+
(I)
and Cr3+(II) centers in alexandrite single crystal
Authors: Yeom T. H. , Choh S. H.
Published: July 2002

Nuclear magnetic resonance spectra of the Be9 and Al27 nuclei as well as electron paramagnetic resonance spectra of Cr3+ ion in an alexandrite single crystal (BeAl2O4:Cr) have been investigated by employing a pulse nuclear magnetic resonance spectrometer and an X-band electron paramagnetic resonance spectrometer, respectively. Be has two magnetically inequivalent sites. While Al(I) at inversion symmetry has four magnetically inequivalent Al sites, Al(II) at mirror symmetry has two. The Cr3+(I) center has four magnetically inequivalent sites, whereas the Cr3+(II) center has two. All resonance sets of nuclear magnetic resonance for Be9 and Al27 nuclei and electron paramagnetic resonance for Cr3+ ion are analyzed in terms of crystal symmetry, ionic size, and charge state. It is concluded that the Cr3+(I) and Cr3+(II) ions substitute for Al3+(I) at inversion symmetry and Al3+(II) at mirror symmetry, respectively, intact of Be2+ sites.
Magneto-optic Kerr effect in a slab waveguide
Authors: Sassaroli E. , DiMarzio C. A. , He Y. , Oliver S. A.
Published: July 2002

A theoretical and numerical investigation of the magneto-optic Kerr effect in a slab optical waveguide bounded on one side by a thin magnetic film is considered. The output power has been determined as a function of the input polarization angle, output analyzer angle, thickness of the intermediate cladding layer, and the length of the magnetic film. It is found that an optimal choice of these parameters exists that produces an enhancement in polarization rotation. Moreover the Kerr effect in a confined geometry shows a richness of properties that can be experimentally checked and opens up new possibilities for the development of magnetic field sensors.
Optical properties of as-grown and annealed InAs(N)/InGaAsP strained multiple quantum wells
Authors: Chen Guan-Ru , Lin Hao-Hsiung , Wang Jyh-Shyang , Shih Ding-Kang
Published: July 2002

Optical and structural properties of as-grown and annealed InAs(N)/InGaAsP strained multiple quantum-well (MQW) structures grown by gas source molecular-beam epitaxy are investigated by photoluminescence (PL), double crystal x-ray diffraction, and photoconductivity spectroscopies. Properties of the as-grown and annealed MQW’s are studied and those of the InAs/InGaAsP MQW (C821) and the InAsN/InGaAsP MQW with the lowest nitrogen contents N=1.1% in the well (C822) are compared. For the C821 InAs/InGaAsP MQW with a very large total strain, a low energy shoulder, possibly induced by defects or impurities, can be seen in the low temperature PL spectrum, and a large density of nonradiative recombination centers is found. For nitrogen-containing MQW’s, the PL full width at half maximum and PL peak evolutions with increasing annealing temperature are influenced by the alloy inhomogeneities. The initial redshift of the PL peak after rapid thermal annealing means that the luminescence is dominated by As-rich regions in these as-grown samples. Exciton localization induced by alloy disorders is also found in high-nitrogen-content samples. By comparing the experimental results of C821 and C822, adding a little nitrogen to reduce the rather large total strain in the structure is beneficial to structural and optical quality improvement.
Characterization of the hole capacitance of hydrogenated amorphous silicon metal–insulator–semiconductor structures
Authors: Park Hyuk-Ryeol , Lee Suk-Ho , Lee Byung-Taek
Published: July 2002

The capatiance–voltage characteristics associated with the hole accumulation in hydrogenated amorphous silicon metal–insulator–semiconductor structures were investigated. The capacitance was measured by using an ac voltage and a quasistatic method. In the ac measurements, we observed the partial response of the hole capacitance, while the full response of the hole capacitance was confirmed by quasi-static measurements. The effect of illumination on the hole capacitance was also investigated. One possible mechanism accounting for the frequency and temperature dependencies of the hole capacitance is proposed.
Transient charging effects on insulating surfaces exposed to a plasma during pulse biased dc magnetron sputtering
Authors: Barnat Edward V. , Lu T.-M.
Published: July 2002

Calculations of the transient charging currents to an insulating surface exposed to a dc discharge after the application of a pulse bias to the insulating surface are made based on a quasistatic treatment of the sheath’s impedance as a nonlinear dc resistance. Measurements of these transient charging currents are shown to correlate with the calculated transient currents for the experimental conditions used in this study. Discussion is then provided to indicate the limits on the pulse, plasma, and coupling conditions in which this quasistatic, dc treatment of the sheath’s response to the applied pulse remains valid.
Low-temperature scanning near-field optical microscope for rapid measurement of thousands of absorption spectra
Authors: Fukutake Naoki , Takasaka Shigehiro , Kobayashi Takayosi
Published: July 2002

The design and performance of a developed scanning near-field optical microscope suited to the rapid measurement of absorption spectra at many pixels to obtain absorption images at low temperatures are described. An optical system composed of a tungsten–halogen lamp and a charge-coupled device camera as a broadband white light source and a multichannel detector, respectively, is employed to reduce acquisition time for a single-pixel spectrum down to 100 ms. Moreover, this microscope can be operated in both illumination and collection modes. The latter mode can provide polarization dependence of absorption spectra. The experiments with molecular J aggregates dispersed in a thin polymer film demonstrate the capability of this instrument to measure near-field absorption spectra at 50×50 scanned positions within 5 min and to obtain absorption images at different wavelengths. They offer useful information about the spatial distributions of the absorbing materials on the sample surface.
Role of interface optical phonons in magnetotunneling in asymmetric double-barrier structures
Authors: Yan Zu Wei , Liang X. X.
Published: July 2002

The role of interface-optical (IO) phonons in tunneling through an asymmetric double barrier structure in a magnetic field perpendicular to the barriers is studied. The phonon-assisted tunneling current densities are calculated and the numerical results for typical AlxGa1−xAs/GaAs/AlyGa1−yAs structures are given. The theoretical results show that the applied magnetic field sharpens and heightens the phonon-assisted tunneling peaks in agreement with experimental observations. In contrast to previous work, the calculated IO phonon-assisted tunneling current peak occurs at the energy the same as that of the bulk GaAs longitudinal-optical (LO) phonon energy. The IO phonon-assisted tunneling is much more important than the confined LO phonon-assisted tunneling. The phonon-assisted tunneling currents are sensitive to the thickness of the emitter barrier. Only one phonon-assisted tunneling peak can be easily observed for the wider well case.
Optical and xerographic properties of phthalocyanine codeposited composite film and ultrathin multilayered structure
Authors: Xu M. S. , Xu J. B. , Wang M. , Que D. L.
Published: July 2002

The optical and xerographic properties of the phthalocyanine codeposited composite (cDC) films and ultrathin multilayered (UTML) structures have been studied. Observed UV-visible absorption spectra indicate that the Q-band absorption of the cDc films is different from that of the single component films, and from that of the UTML structures. The absorption peaks are shifted with the number and thickness of the layers. This spectral variation is most likely to be due to the dipole–dipole and charge transfer interactions among different molecular assemblies. The photosensitivity of the cDC films is superior to that of the single component films. This improvement is attributed to the efficient photoinduced charge transfer and charge separation among the composites. Moreover, the photosensitivity of the UTML structures is over tenfold better than that of the cDC films, probably due to the ultrafast photoinduced charge transfer and separation and the unipolar transport of the photogenerated holes.
Structure function exponents and probability density function of the velocity difference in turbulence
Authors: Verzicco R. , Camussi R.
Published: July 2002

In this work a statistical model for the prediction of the intermittency exponents in fully developed isotropic turbulence, recently proposed by Camussi and Verzicco [Phys. Fluids 12, 676 (2000)] is clarified and improved. It was found, in fact, that the same results as those found by Camussi and Verzicco can be obtained by a neater and more general formulation. This is achieved starting from a reliable statistical description of the velocity difference probability density function (pdf). A new and more general predictive formula is obtained and the dependence on the free parameters describing the pdf shape is discussed. It is shown that for the technically measurable exponents, the results are not very sensitive to the shape of the pdf. This indicates that, within the present limitations of experimental measurements and numerical simulations, the analysis of the velocity difference statistics is not a sufficiently accurate tool for studying in detail the intermittency in turbulence.
Kinetic theory of surface waves in plasma jets
Authors: Shokri B.
Published: July 2002

The kinetic theory analysis of surface waves propagating along a semi-bounded plasma jet is presented. The frequency spectra and their damping rate are obtained in both the high and low frequency regions. Finally, the penetration of the static field in the plasma jet under the condition that the plasma jet velocity is smaller than the sound velocity is studied.
A Raman-atomic force microscope for apertureless-near-field spectroscopy and optical trapping
Authors: Anderson Mark S. , Pike William T.
Published: July 2002

An instrument that combines the analytical power of Raman spectroscopy with the spatial resolution of the Atomic Force Microscope (AFM) is presented. This instrument is capable of resolving 50 nm scale spectral features or better by using surface enhanced Raman scattering at the AFM tip. The localized spectrochemical information allows the interpretation of the concurrently acquired friction or phase contrast AFM images. This instrument has a unique combination of features including side illumination of the tip–sample interface that permits opaque samples. As a result of precise focusing of a laser at the AFM tip–sample interface this instrument is also capable of laser beam profiling and studying optical trapping at the probe tip. Applications of this versatile instrument include chemical analysis of nanometer scale phenomena, chemical separation, and the potential for targeted single molecule spectroscopy.
Compatibility between high energy particle confinement and magnetohydrodynamic stability in the inward-shifted plasmas of the Large Helical Device
Authors: Kaneko O. , Komori A. , Yamada H. , Ohyabu N. , Kawahata K. , Nakamura Y. , Ida K. , Murakami S. , Mutoh T. , Sakakibara S. , Masuzaki S. , Ashikawa N. , Emoto M. , Funaba H. , Goto M. , Idei H. , Ikeda K. , Inagaki S. , Inoue N. , Isobe M. , Khlopenkov K. , Kubo S. , Kumazawa R. , Minami T. , Miyazawa J. , Morisaki T. , Morita S. , Muto S. , Nagayama Y. , Nakajima N. , Nakanishi H. , Narihara K. , Nishimura K. , Noda N. , Notake T. , Kobuchi T. , Liang Y. , Ohdachi S. , Oka Y. , Osakabe M. , Ozaki T. , Peterson B. J. , Sagara A. , Saito K. , Sakamoto R. , Sasao M. , Sato K. , Sato M. , Seki T. , Shimozuma T. , Shoji M. , Suzuki H. , Takechi M. , Takeiri Y. , Tamura N. , Tanaka K. , Toi K. , Tokuzawa T. , Torii Y. , Tsumori K. , Yamada I. , Yamamoto S. , Yokoyama M. , Yoshimura Y. , Yoshinuma M. , Watanabe K. Y. , Watari T. , Xu Y. , Itoh K. , Matsuoka K. , Ohkubo K. , Ohtake I. , Satow T. , Sudo S. , Yamazaki K. , Hamada Y. , Motojima O. , Fujiwara M.
Published: July 2002

The experimentally optimized magnetic field configuration of the Large Helical Device [A. Iiyoshi et al., Nucl. Fusion 39, 1245 (1999)], where the magnetic axis is shifted inward by 15 cm from the early theoretical prediction, reveals 50% better global energy confinement than the prediction of the scaling law. This configuration has been investigated further from the viewpoints of high energy particle confinement and magnetohydrodynamic (MHD) stability. The confinement of high energy ions is improved as expected. The minority heating of ion cyclotron range of frequency was successful and the heating efficiency was improved by the inward shift. The confinement of passing particles by neutral beam injection was also improved under low magnetic field strength, and there could be obtained an almost steady high beta discharge up to 3% in volume average. This was a surprising result because the observed pressure gradient exceeded the Mercier unstable limit. The observed MHD activities became as high as beta but they did not grow enough to deteriorate the confinement of high energy ions or the performance of the bulk plasma, which was still 50% better than the scaling. According to these favorable results, better performance would be expected by increasing the heating power because the neoclassical transport can also be improved there.
Different behavior of photoluminescence anisotropy in porous silicon layers made by polarized-light-assisted electrochemical etching
Authors: Koyama Hideki
Published: July 2002

Luminescent porous silicon (PSi) layers have been made on n-type Si wafers by anodization in a HF solution under illumination with linearly polarized light. It is observed that the photoluminescence from samples anodized at a current density of 30 mA/cm2 and slightly oxidized electrochemically for passivation exhibits a significant anisotropy in polarization memory (PM): the degree of PM becomes maximum when the vector electric field (Eexc) of excitation laser light is parallel to that (Eetch) of the light used during anodization, and minimum when Eexc is perpendicular to Eetch. These maxima and minima are in totally different directions from those reported previously. In addition, the anisotropy is found to be weak at low anodization current density of 2.3 mA/cm2. These experimental results are discussed based on the electrochemical, rather than photochemical, thinning of Si-nanocrystal assemblies in PSi layers.
Structural dynamics in quantum solids. I. Steady-state spectroscopy of the electronic bubble in solid hydrogens
Authors: Vigliotti F. , Cavina A. , Bressler Ch. , Lang B. , Chergui M.
Published: July 2002

The structural changes due to formation of electronic bubbles in solid D2 are investigated by means of fluorescence, fluorescence-excitation, and fluorescence-depletion spectroscopy of the lowest Rydberg state, A Σ+2(3sσ), of the NO impurity. The A←X band is strongly blueshifted (∼0.7 eV) with respect to the gas phase and shows a very broad (full width at half maximum ∼2000 cm−1) and asymmetric profile. The shift results from the strong repulsion due to the overlap of the extended Rydberg orbital with the matrix species, while the width and asymmetry are governed by quantum effects on the ground-state intermolecular wave function. Fluorescence occurs with large absorption-emission Stokes shifts, bringing the A-state emission energy to its gas-phase value, which indicates a very loose cavity around the excited molecule. A line-shape analysis of the A-X absorption and emission bands allows us to extract one-dimensional intermolecular NO-matrix potentials of both involved states. We estimate the bubble radius to ∼5 Å, in good agreement with values from the literature for the bubble radius of the solvated electron. Fluorescence-depletion spectra of the A state are also presented along with the ground-state transitions to the higher C Π2Π(3pπ) and D Σ+2(3pσ) states. They are used to generate intermolecular potentials for the C and D states, which are essential ingredients for ultrafast pump–probe experiments of the bubble dynamics. The results obtained for D2 matrices are compared with those previously published for H2 matrices.
Numerical solution of the Fokker–Planck equation of fully ionized magnetized plasmas and classical transport coefficients
Authors: Bendib A. , Bennaceur-Doumaz D. , El Lemdani F.
Published: July 2002

A new numerical approach to solve the linear integrodifferential Fokker–Planck equation (FPE), which describes a collisional and magnetized plasma, is presented. For this purpose, the FPE is reduced to a simple set of ordinary differential equations, which can be easily solved, with the use of standard numerical methods. The transport coefficients induced by the first anisotropic distribution function computed by Braginskii [Reviews of Plasma Physics (Consultants Bureau, New York, 1965), Vol. 1] and improved by Epperlein and Haines [Phys. Fluids 29, 1029 (1986)], have been recovered. The viscosity coefficients are computed for arbitrary atomic numbers and arbitrary magnetic field strength and are compared to the results reported in the literature.