bluuebunny/crossref_metadata_2025 huggingface dataset:
Spin polarization of the conduction bands and secondary electrons of Gd(0001)
Authors: Li Dongqi , Pearson J. , Bader S. D. , McIlroy D. N. , Waldfried C. , Dowben P. A.
Published: July 2002

Angle- and spin-resolved photoemission was utilized to investigate the 5d bulk bands and the surface state of Gd(0001) in the temperature range of 130–350 K. The bulk bands at 1–2 eV below the Fermi energy EF show Stoner-like behavior, while the temperature dependence of the surface state near EF indicates spin-mixing behavior due to fluctuating local 5d moments. The secondary electron spectra of the Gd surfaces both before and after initial oxygen adsorption show a polarization dip at low kinetic energies due to the extra scattering channel for minority electrons via the unoccupied 4f level. The temperature dependencies of the surface and bulk magnetization are separated using the spin polarization of the surface state and the bulk exchange splitting.
Emission enhancement in single-layer conjugated polymer microcavities
Authors: Grüner J. , Cacialli F. , Friend R. H.
Published: July 2002

We have studied the luminescence properties of microcavities which are formed by a single layer of poly(para-phenylenevinylene) sandwiched between a dielectric mirror coated with a conducting indium tin oxide layer and a semitransparent metal electrode. Compared with a device without cavity structure, the spectral and spatial emission are significantly narrowed, and the forward emission intensity is enhanced. We measure a spectral linewidth (full width at half maximum) of the cavity modes of about 4 nm in photoluminescence and 20 nm in electroluminescence and an enhancement of luminescence intensity in the forward direction of more than an order of magnitude. The implications of the narrowing of the emission and possible transfer mechanisms for excitation energy are discussed.
Chemically functionalized surfaces from ultrathin block-copolymer films
Authors: Meiners J. C. , Elbs H. , Ritzi A. , Mlynek J. , Krausch G.
Published: July 2002

Supramolecular self-assembly of polystyrene–poly(vinylpyridine) diblock copolymers adsorbed from a selective solvent onto smooth substrates was used to create surfaces with regularly varying chemical composition. Well-ordered arrays of localized chemical reaction sites are formed on such surfaces with sizes and intersite distances of molecular length scales. We demonstrate site-specific complex formation with transition metal salts resulting in a nanoscopic ordered array of metal salt–polymer complexes. Selective etching of the metal-loaded microdomains is shown. The effect of block copolymer molecular architecture on the supramolecular structure of these ‘‘chemically functionalized’’ surfaces was studied.
Microstructure and optical absorption properties of Si nanocrystals fabricated with low-pressure chemical-vapor deposition
Authors: Nakajima A. , Sugita Y. , Kawamura K. , Tomita H. , Yokoyama N.
Published: July 2002

We report a simple technique for fabricating a layer of isolated Si quantum dots on SiO2 glass substrates. This technique uses conventional low-pressure chemical-vapor deposition for an extremely short deposition time in the early stage of poly-Si film growth. The layer after a deposition time of 60 s has isolated Si nanocrystals 5–20 nm in diameter and 2–10 nm in height. The measurements of optical absorption coefficient α show that the absorption edge for Si nanocrystals shifts to higher energies compared to that of bulk Si, indicating a widening of the energy gap caused by quantum size effects. The linear relationship (αhν)1/2 against hν suggests that the Si nanocrystal, whose diameter is as small as 10 nm, basically maintains the properties of an indirect band-gap semiconductor. Special attention must be paid to the Brownian migration of Si nanocrystals for fabricating Si quantum dots.
Parametric tip model and force–distance relation for Hamaker constant determination from atomic force microscopy
Authors: Argento C. , French R. H.
Published: July 2002

Hamaker constants and dispersion forces interactions of materials are of increasing interest and the advent of atomic force microscopy (AFM) force measurements represents a new opportunity for quantitative studies of these interactions. A critical problem is the determination of a force–distance relation for realistic AFM probes. Due to the inadequacies of existing power-law sphere–plane models to describe the probe–sample system, we present a new parametric tip force–distance relation (PT/FDR). A surface integration method is developed to compute the interactions between arbitrarily shaped bodies. The method is based on the Hamaker pairwise integration in a continuous fashion, reducing the six-dimensional integration to a four-dimensional scheme. With this method, the PT/FDR is obtained and a nonlinear fitting routine is used to extract the model parameters and the Hamaker constant from AFM force–distance data. From the sensitivity analysis of the fitting of synthesized AFM force–distance data, one finds that, for large tip radius (compared to separation), the force is proportional to the product of the Hamaker constant and tip radius. Unique determination of the Hamaker constant can be achieved if a small radius tip is used in the AFM scan. By fitting to literature data, the effectiveness of the PT/FDR is shown.
Reconstruction of the SiO2 structure damaged by low-energy Ar-implanted ions
Authors: Garrido B. , Samitier J. , Bota S. , Moreno J. A. , Montserrat J. , Morante J. R.
Published: July 2002

The damage created in SiO2 layers by low-energy Ar ions (130 keV) and the reconstruction of the structure after various annealing steps have been characterized as a function of the implantation dose. Quantitative determinations of the damage produced have been performed from infrared spectroscopy. We show that two dose thresholds for damage are encountered: At 1014 cm−2 damage saturates and for doses above 1017 cm−2 sputtering effects dominate. Annealing at high temperatures (1100 °C) restores the structure of the initial nonimplanted oxide only for doses below the second threshold, although some disorder remains. Electroluminescence measurements show that annealing is able to eliminate electrically active defects. For implantation doses greater than 1017 cm−2, annealing is unable to restore the structure completely as sputtering effects create a depleted oxygen layer at the surface and substoichiometric defects appear. The presence of microcavities created by the Ar atoms at such high doses may affect the annealing behavior.
Alternating and direct current field effects on the dielectric response of La-modified lead zirconate titanate
Authors: Tan Qi , Viehland Dwight
Published: July 2002

The complex dielectric responses of various La-modified lead zirconate titanate ceramic specimens have been measured under various ac drive and dc bias conditions. Investigations were performed for different La concentrations on specimens with Zr/Ti ratios of 40/60 and 65/35. These investigations demonstrated that the influence of increasing ac drive and dc bias are in opposition: increasing drive results in an increase in the dielectric constant, whereas increasing bias results in a decrease. Investigations as a function of La content revealed a strong decrease in the ac-induced nonlinearities with increasing La content in the compositional range near a crossover between micron-sized domains and tweedlike structures. The ac drive dependent dielectric data were then empirically fitted to a power series expansion in the ac field. An anomalous change in sign of the second-order nonlinearity was found for lower La-content compositions near the temperature of the dielectric maximum. It is believed that this change in sign results from a suppression of ferroelectric order and the stabilization of a paraelectric state at temperatures close to the ferroelectric phase transformation.
Exciton linewidth due to scattering by free carriers in semiconducting quantum well structures: Finite confining potential model
Authors: Koh Tong San , Feng Yuan Ping , Spector Harold N.
Published: July 2002

The dependence of the exciton linewidth broadening due to scattering by free carriers on carrier concentration, temperature, and well width, in semiconducting quantum well structures is theoretically studied using a finite confining potential model. The contribution to the linewidth due to ionization scattering of the exciton to the continuum electron-hole states, is included in the present calculations, which is shown to further enhance the importance of free carrier scattering in the broadening of the exciton linewidth. Quasi 3 dimensional features in the very narrow wells due to the finite confinement are also presented and discussed.
Hydrogen elimination and phase transitions in pulsed-gas plasma deposition of amorphous and microcrystalline silicon
Authors: Srinivasan Easwar , Parsons Gregory N.
Published: July 2002

Removal of hydrogen from the growth surface during silane plasma deposition of silicon is correlated with the transition from amorphous to microcrystalline film structure. Plasma deposition experiments were performed using a pulsed gas technique, where repeated steps of thin amorphous silicon film deposition, and atomic hydrogen (or deuterium) exposure are used to form microcrystalline and polycrystalline thin films at substrate temperatures below 250 °C. Infrared absorption and Raman spectroscopy are used to estimate the silicon-hydrogen bonding concentrations, and characterize crystal structure, respectively. Hydrogen elimination probed using real-time differentially pumped mass spectroscopy demonstrates that during atomic deuterium exposure, hydrogen abstraction by deuterium, rather than silicon etching, is the primary mechanism for hydrogen removal from the depositing surface. Polycrystalline material, with no shoulder at 480 cm−1 in the Raman spectrum, and grain sizes greater than 1000 Å, as determined by transmission electron microscopy, have been formed at temperatures below 250 °C. The amorphous to crystal transition is observed at substrate temperatures as low as 25 °C, with longer hydrogen exposure required at lower temperatures. Hydrogen is shown to be preferentially abstracted from monohydride (Si–H) units as compared to dihydride (SiH2) units at or near the depositing growth surface, consistent with ab initio energy calculations of hydrogen interactions with silicon hydrides. A transition in hydrogen removal kinetics is observed upon film crystallization, where the rate of hydrogen removal is reduced for more crystalline materials. These results are valuable for understanding surface reactions in low temperature crystalline silicon deposition, for example, for fabrication of high mobility thin film transistor structures on glass.
Ultraviolet-induced densification in fused silica
Authors: Schenker Richard E. , Oldham William G.
Published: July 2002

Ultraviolet-induced densification in fused silica is investigated using birefringence measurements to detect compaction-induced strain. This technique is capable of measuring compaction in the 10 parts per billion range in cm-sized specimens. A universal relation describing ultraviolet-induced compaction is discovered in which, using the total energy absorbed from two-photon absorption as the dose parameter, density changes equal a material dependent constant times the dose parameter to a power of about 0.65. This dose dependence is consistent with past compaction studies using electron beam and gamma radiation. Moreover, the density change per ionization “event” is found to be consistent for all radiation types. By comparing density measurements to refractive index change measurements found with interferometry, polarizability changes from ultraviolet-induced densification are extracted and compared with previous results using other radiation sources and inelastic densification methods.
Measurement of thermal diffusivities of thin metallic films using the ac calorimetric method
Authors: Yamane Tsuneyuki , Mori Yuji , Katayama Shin-ichiro , Todoki Minoru
Published: July 2002

The thermal diffusivities of thin films of evaporated metal less than 1×10−6 m thick have been measured using the light-irradiation ac calorimetric method. The thermal diffusivities of metallic thin films become smaller than those of the bulk as the thickness of the metallic thin films decreases. It is determined, using x-ray diffraction measurement, that the decrease of thermal diffusivity of thin films of evaporated metal is caused by a decrease in crystalline size.
Leakage currents in Ba0.7Sr0.3TiO3 thin films for ultrahigh-density dynamic random access memories
Authors: Dietz G. W. , Schumacher M. , Waser R. , Streiffer S. K. , Basceri C. , Kingon A. I.
Published: July 2002

(Ba,Sr)TiO3 (BST) thin films grown by chemical vapor deposition and with platinum (Pt) top and bottom electrodes have been characterized with respect to the leakage current as a function of temperature and applied voltage. The data can be interpreted via a thermionic emission model. The Schottky approximation accounts for superohmic behavior at higher fields, but the barrier lowering is stronger than expected from this theory. While the leakage mechanism is comparable to SrTiO3 thin films prepared by chemical solution deposition, the absolute values of the leakage current are significantly lower for the metalorganic chemical vapor deposition (MOCVD) prepared BST film. This is presumably due to a more homogeneous microstructure of the latter and may also be due to different electrode processing. The influence of the film thickness on the leakage in combination with additional findings is used to discuss the field distribution in the films under a dc voltage stress.
Absorption and luminescence of the surface states in ZnS nanoparticles
Authors: Chen Wei , Wang Zhanguo , Lin Zhaojun , Lin Lanying
Published: July 2002

A broad absorption band around 500 nm is observed in ZnS nanoparticles. The absorption becomes more intensive and shifts to the blue as the particle size is decreased. The absorption energy is lower than the band gap of the particles and is considered to be caused by the surface states. This assignment is supported by the results of the fluorescence and of the thermoluminescence of the surface states. Both the absorption and the fluorescence reveal that the surface states are size dependent. The glow peak of the semiconductor particles is not varied as much upon decreasing size, indicating the trap depth of the surface states is not sensitive to the particle size. Considering these results, a new model on the size dependence of the surface states is proposed, which may explain our observations reasonably.
Piezoresistive cantilever designed for torque magnetometry
Authors: Willemin M. , Rossel C. , Brugger J. , Despont M. H. , Rothuizen H. , Vettiger P. , Hofer J. , Keller H.
Published: July 2002

New piezoresistive silicon cantilevers designed specifically for torque magnetometry on microscopic samples have been microfabricated and tested. These levers have been optimized to detect the torque in two directions corresponding to flexion and torsion. Torque resolution of ∼10−14 N m can be achieved depending on the operating mode. In one version an integrated loop allows an absolute calibration of the device with an accuracy of ∼1%. This loop can also be used to excite the lever mechanically. One application is the determination of the mass of nanogram samples by measuring the resonance frequency shift (nanobalance).
Optical properties of Si clusters and Si nanocrystallites in high-temperature annealed SiOx films
Authors: Inokuma T. , Wakayama Y. , Muramoto T. , Aoki R. , Kurata Y. , Hasegawa S.
Published: July 2002

Structure, optical absorption and photoluminescence (PL) properties of SiOx films subjected to thermal annealing at 750–1100 °C are investigated. Si crystallites with a few nanometers in size are observed in the SiO1.3 and SiO1.65 films annealed at 1100 °C. Threshold energies in optical absorption of the Si nanocrystallites are higher than that for bulk Si, suggesting a contribution from quantum confinement effects. The PL spectrum shows a remarkable increase in intensity after annealing at temperatures above 1000 °C. This PL behavior is closely related to the formation of Si nanocrystallites by the annealing. The PL peak energy of the annealed films shifts to higher energy with decreasing crystallite size but does not follow the blueshift for the absorption threshold energy. These results suggest that a localized state contributes to the PL mechanism. The SiO1.8 film annealed at 1100 °C, which contains no Si crystallites, exhibits an intense PL similar to the annealed SiO1.3 and SiO1.65 films. It is implied that noncrystalline Si nanoparticles are formed in the SiO1.8 film under high-temperature annealing.
Orientation dependence of shock-induced heating in anharmonic molecular crystals
Authors: Jindal V. K. , Dlott Dana D.
Published: July 2002

A significant development in the field of detonics is the observation of direction dependence of shock wave initiation to detonation of single crystals of energetic materials. Various mechanisms have been proposed to account for this direction dependence, but the seemingly simplest possibility not previously considered, that the direction dependence is due mainly to the anisotropy in the temperature increase generated by the shock, is investigated in this work. The temperature increase depends on the anharmonic potential. In the model system studied here, naphthalene, the anharmonic intermolecular potential is available from the literature. Using this potential, we calculate the directional compressibility, the mode Grüneisen parameters, and the bulk Grüneisen parameters along different crystallographic directions. These results show the temperature increase is quite a bit greater for a shock of given pressure along the a or b axes than along the c axis, mainly because the compressibility along the c axis is smaller. Using thermochemical data for energetic materials, it is shown the temperature increase anisotropy is large enough to lead to reaction rates which differ by several orders of magnitude, when typical initiating shocks are incident along different crystallographic directions.
Stress relaxation in Si-doped GaN studied by Raman spectroscopy
Authors: Lee In-Hwan , Choi In-Hoon , Lee Cheul-Ro , Shin Eun-joo , Kim Dongho , Noh Sam Kyu , Son Sung-Jin , Lim Ki Yong , Jae Lee Hyung
Published: July 2002

We report the Si-doping-induced relaxation of residual stress in GaN epitaxial layers grown on (0001) sapphire substrate by the metalorganic vapor phase epitaxy technique. Micro-Raman spectroscopy is used to assess stress situation in the films with systematically modulated doping concentration from 4.0×1017 up to 1.6×1019 cm−3. As the Si-doping concentration increases, a monotonic decrease of the E2 phonon frequency is observed, which signifies gradual relaxation of the stress in the film. The layers are fully relaxed when electron concentration exceeds 1.6×1019 cm−3. The linear coefficient of shift in Raman frequency (ω) induced by the in-plane biaxial compressive stress (σ∥) is estimated to be Δω/Δσ∥=7.7 cm−1/GPa. We suggest that Si doping increases density of misfit dislocation, judging from linewidth of x-ray rocking curve.
Photoluminescence from B-doped Si nanocrystals
Authors: Fujii Minoru , Hayashi Shinji , Yamamoto Keiichi
Published: July 2002

Boron-doped Si nanocrystals as small as 3.5 nm were prepared and their photoluminescence (PL) properties were studied. The PL properties were found to be very sensitive to the B concentration. For the sample without B doping the temperature-dependent shift of the PL peak was almost the same as that of the bulk band gap. As the B concentration increased, the temperature dependence deviated from that of the bulk band gap, and the peak exhibited a low-energy shift as the temperature decreased. The anomalous temperature dependence is considered to be due to the contribution of the PL from excitons bound to the neutral B states.
Gallium nitride/conjugated polymer hybrid light emitting diodes: Performance and lifetime
Authors: Zhang Chi , Heeger Alan J.
Published: July 2002

We report on pure white light emission from GaN/conjugated polymer hybrid light emitting diodes (LEDs) using a single layer of conjugated polymer. When the conjugated polymer is properly encapsulated, the hybrid LEDs can operate at least 5000 h, with decay in output luminosity comparable to that of commercial blue GaN lamps. By using different conjugated polymers, emission with a full range of colors is demonstrated with the hybrid LED.
Neutron diffraction analysis on FINEMET alloys
Authors: Zhu J. , Clavaguera N. , Clavaguera-Mora M. T. , Howells W. S.
Published: July 2002

Neutron diffraction has been used to study in situ the nanocrystallization process of Fe73.5Cu1Nb3Si22.5−xBx (x=5, 9, and 12) amorphous alloys. Nanocrystallization results in a decrease of both the silicon content and the grain size of the Fe(Si) phase with increasing value of x. By comparing the radial distribution function peak areas with those predicted for ideal bcc and DO3 structure, it can be concluded that the ordering in DO3 Fe(Si) crystals increases with the silicon content.
Half-leaky guided wave determination of azimuthal anchoring energy and twist elastic constant of a homogeneously aligned nematic liquid crystal
Authors: Yang F. , Sambles J. R. , Bradberry G. W.
Published: July 2002

Optical excitation of half-leaky guided modes has been used to explore the director distortion of a homogeneously aligned nematic liquid crystal (BDH E7) under application of an in-plane electric field. Homogeneous alignment is realized by obliquely evaporated SiOx layers with an in-plane field being applied across a 3 mm gap between gold electrodes. The director is initially aligned approximately parallel to the edges of the electrode gap. By modeling the director profile in the cell to fit the half-leaky guided mode reflectivity data, both with and without applied fields, and then comparing the field induced distortions to continuum theory, we obtain both the twist elastic constant k22 and the surface azimuthal anchoring coefficient, Wa. At 23.7 °C we find that k22=6.5×10−12 N and Wa=2.3×10−6 Jm−2.
Multibarrier heterostructure GaAs/AlAs switch
Authors: Reklaitis A.
Published: July 2002

A multibarrier heterostructure GaAs/AlAs current switching diode has been proposed and investigated by Monte Carlo particle simulations. The switching phenomenon is based on electron tunneling and thermoemission from the GaAs wells to the AlAs barriers, electron drift across the thin AlAs barriers followed by a subsequent impact ionization in the undoped GaAs layers. The calculated switching voltage is close to 100 V for the diode involving four AlAs barriers at 300 K lattice temperature. The estimated switching time is in the order of 10 ps.
Mössbauer investigation of Fe–Ni fine particles
Authors: Liu Bin , Huang Runsheng , Wang Jinhui , Widatallah H. M. , Lu Huaixian , Zhang Jinrong , Liu Jinxiang
Published: July 2002

Fine particles about 20 nm in average diameter of Fe1−xNix (x=0.10–0.85) alloys prepared by evaporation in nitrogen atmosphere were studied using Mössbauer technique, x-ray diffraction, and magnetic measurement. Their different phases were identified, and their hyperfine field distribution, specific saturation magnetization, remnant magnetization, and coercivity were determined. The fine particles show some properties, which are sensitive to the Ni content x and different from those of corresponding bulk alloys. The results suggest that the fine particles may be useful from an applied viewpoint.
Low temperature crystallization of lead zirconate titanate thin films by a sol-gel method
Authors: Huang Z. , Zhang Q. , Whatmore R. W.
Published: July 2002

Pyroelectric lead zirconate titanate (PZT) thin films have been prepared by a sol-gel method and characterized by x-ray diffraction and transmission electron microscopy (TEM). A metastable Pt3Pb intermetallic phase has been identified. The formation of this metastable phase was found to depend on the drying temperature, the thickness of the as-deposited film, annealing temperature, and annealing time. Perovskite PZT was found to nucleate on top of the intermetallic phase, rather than directly on Pt. The improved lattice match between the intermetallic (a0=4.05 Å) and perovskite PZT(a0=4.035 Å) as compared to between Pt(a0=3.9231 Å) and the perovskite is believed to substantially reduce the activation energy for the nucleation of perovskite on Pt. Using this effect, (111) perovskite PZT has been grown at a temperature as low as 440 °C. The formation of the intermetallic phase is believed to facilitate the (111) film orientation. The growth kinetics of the PZT were analyzed using the Avrami model, and from this, the crystallization activation energy was determined as 179 kJ/mol for the phase transformation from pyrochlore to perovskite for this materials system. TEM examination and measurement of electrical property indicated that the films crystallized at 480 °C were good quality, with a pyroelectric coefficient of 1.8×10−4 C m−2 K−1 and a remnant polarization of 24 μC m−2.
Low-field microwave magnetoimpedance in amorphous microwires
Authors: Lofland S. E. , Bhagat S. M. , Domı́nguez M. , Garcı́a-Beneytez J. M. , Guerrero F. , Vázquez M.
Published: July 2002

We have measured the low-field (<40 Oe) microwave losses in 5 μm diam glass-covered amorphous wires fabricated by the Taylor–Ulitovsky method at 9.8 and 32.5 GHz and their hysteresis loops at very low frequency (0.1 Hz). They exhibit well-differentiated hysteresis loops depending on the magnetostrictive character of the alloy compositions [positive (FeSiBC), vanishing (CoMnSiB), and negative (CoSiB) magnetostriction] as a consequence of the particular magnetoelastic anisotropy distribution and hence the spontaneous domain structure. In the low-field region for which the microwires are not magnetically saturated, as is typical in ferromagnetic resonance experiments, the microwave losses evolve with applied field in a similar manner to that of the magnetization process, and denote a correlation between microwave losses and the magnetization process of magnetically nonsaturated samples.
Energy and charge transfer in organic light-emitting diodes: A soluble quinacridone study
Authors: Shaheen S. E. , Kippelen B. , Peyghambarian N. , Wang J.-F. , Anderson J. D. , Mash E. A. , Lee P. A. , Armstrong N. R. , Kawabe Y.
Published: July 2002

A soluble derivative of quinacridone, N,N′-di-isoamyl quinacridone (DIQA), has been synthesized and used to study the mechanisms of Förster energy transfer and charge transfer in organic light-emitting diodes (OLEDs) based on 8-hydroxyquinoline (Alq3). Quantum efficiencies and spectra were measured for both photoluminescence (PL) and electroluminescence (EL) for films of poly(9-vinylcarbazole) (PVK) doped with Alq3 and DIQA. Both PL and EL showed an efficiency enhancement in films of PVK:Alq3:DIQA compared to films of PVK:Alq3. However, the optimal DIQA doping concentration was found to be lower for EL than for PL. Examination of the spectra revealed that more emission originated from DIQA for EL than for PL at a given doping level. We conclude that Förster energy transfer from Alq3 to DIQA occurs in both cases of PL and EL, but that charge transfer to DIQA occurs in the operation of the OLED resulting in additional pathways to DIQA emission. Ultraviolet photoelectron spectroscopy measurements showed that electron transfer from Alq3 to DIQA, hole transfer from PVK to DIQA, and hole transfer from Alq3 to DIQA are all energetically favorable processes. These results suggest that charge transfer is an important mechanism in the efficiency enhancement seen in OLEDs based on a host–dopant scheme, and that both the electronic properties and the optical properties of the dopant material are important parameters for device optimization.
ZnMgSe/ZnCdSe and ZnMgSe/ZnSeTe distributed Bragg reflectors grown by molecular beam epitaxy
Authors: Peiris F. C. , Lee S. , Bindley U. , Furdyna J. K.
Published: July 2002

This article investigates distributed Bragg reflectors (DBRs) based on two wide-gap II–VI semiconductor alloy combinations: ZnMgSe/ZnCdSe and ZnMgSe/ZnSeTe. Prior to fabrication of the DBRs, a prism coupler technique was used to determine the indices of refraction n of the above ternary alloys of various compositions prepared by molecular beam epitaxy (MBE). Using these values of n, two DBR systems, Zn0.66Mg0.34Se/Zn0.74Cd0.20Se and Zn0.62Mg0.38Se/ZnSe0.56Te0.44, were fabricated, each with a relatively large difference in the indices of refraction between its layer materials. It was found that although a higher reflectivity could be achieved using the ZnMgSe/ZnSeTe combination (since it manifests a larger difference in their indices of refraction Δn=0.35), the number of periods which can be deposited in this DBR system is limited due to growth difficulties that arise when combining ZnMgSe and ZnSeTe. Therefore the ZnMgSe/ZnSeTe DBR system, which was restricted to just 10 periods, yielded a modest reflectivity of 85%. On the other hand, although Δn in the ZnMgSe/ZnCdSe DBR system is smaller (Δn=0.20), it poses fewer growth difficulties, making it possible to grow DBR stacks consisting of a large number of layers without compromising the crystal quality of the structure. By growing 20 periods of the ZnMgSe/ZnCdSe DBR system, we obtained a DBR with a reflectivity as high as 98%.
Characterization of treated indium–tin–oxide surfaces used in electroluminescent devices
Authors: Mason M. G. , Hung L. S. , Tang C. W. , Lee S. T. , Wong K. W. , Wang M.
Published: July 2002

The influence of oxidative and reductive treatments of indium–tin–oxide (ITO) on the performance of electroluminescent devices is presented. The improvement in device performance is correlated with the surface chemical composition and work function. The work function is shown to be largely determined by the surface oxygen concentration. Oxygen-glow discharge or ultraviolet–ozone treatments increase the surface oxygen concentration and work function in a strongly correlated manner. High temperature, vacuum annealing reduces both the surface oxygen and work function. With oxidation the occupied, density of states (DOS) at the Fermi level is also greatly reduced. This process is reversible by vacuum annealing and it appears that the oxygen concentration, work function, and DOS can be cycled by repeated oxygen treatments and annealing. These observations are interpreted in terms of the well-known, bulk properties of ITO.
Composition modulation of optical absorption in AgxAu1−x alloy nanocrystals in situ formed within pores of mesoporous silica
Authors: Shi Huazhong , Zhang Lide , Cai Weiping
Published: July 2002

Nanoscaled AgxAu1−x (0.2<x<0.8) alloy particles were synthesized by putting gold and silver particles into pores of monolithic mesoporous silica by soaking and annealing method. It has been found that in optical absorption spectrum, the surface plasmon resonance (SPR) maximum of AgxAu1−x alloy particles can shift from about 524 nm (SPR maximum of Au nanoparticles) to about 400 nm (SPR maximum of Ag nanoparticles) with increasing concentration of Ag in AgxAu1−x alloy particle. The simulation of optical absorption spectrum of AgxAu1−x alloy particles was conducted based on Mie theory. The calculated values are found to be consistent with the experimental results.
A large low field colossal magnetoresistance in the La0.7Sr0.3MnO3 and CoFe2O4 combined system
Authors: Yan Chun-Hua , Xu Zhi-Gang , Zhu Tao , Wang Zhe-Ming , Cheng Fu-Xiang , Huang Yun-Hui , Liao Chun-Sheng
Published: July 2002

La 0.7 Sr 0.3 MnO 3 and CoFe2O4 samples have been prepared by a sol-gel process. A mixture of the two samples was annealed at 800 °C and the scanning electron micrograph shows large CoFe2O4 grains well surrounded by La0.7Sr0.3MnO3 grains. The powder x-ray diffraction patterns indicate no evidence of reaction between the La0.7Sr0.3MnO3 and CoFe2O4. The resistivity of the combined sample (with 20 wt% CoFe2O4) is about an order of magnitude larger than that of the same-grain-sized La0.7Sr0.3MnO3 sample, while a large low field magnetoresistance (MR) has been obtained in the combined samples. Under an applied field of 5 kOe, the MR of the combined sample is 10% even at 280 K and 5% at 290 K, whereas the same nanosized La0.7Sr0.3MnO3 sample is only 2% and 1%, respectively.
High-speed dynamics, damping, and relaxation times in submicrometer spin-valve devices
Authors: Russek Stephen E. , Kaka Shehzaad , Donahue Michael J.
Published: July 2002

The dynamical response of spin-valve devices with linewidths of 0.8 μm has been measured after excitation with 160 ps magnetic impulses. The devices show resonant frequencies of 2–4 GHz which determine the upper limit of their operation frequency. The dynamical response can be fit with Landau–Lifshitz models to extract an effective uniform-mode damping constant, αum. The measured values of αum were between 0.04 and 0.01 depending on the magnitude of the longitudinal bias field. The appropriate damping coefficient for use in micromagnetic modeling, αmm, was extracted from the dynamical response with large longitudinal bias field. This value was used to model the switching of a 0.1 μm×1.0 μm magnetoresistive random access memory cell. The micromagnetic model included shape disorder that is expected to be found in real devices. The simulations showed that, while the magnetization reverses rapidly (<0.5 ns), it took several nanoseconds for the energy to be removed from the magnetic system. The switching energy was stored in short wavelength magnetic fluctuations that could dramatically affect the re-reversal process 1–2 ns after the first reversal.
Stress-induced enhancement of second-order nonlinear optical susceptibilities of barium titanate films
Authors: Zhao Tong , Lu Huibin , Chen Fan , Yang Guozhen , Chen Zhenghao
Published: July 2002

A method introducing stress is suggested to enhance optical nonlinearity. Three stress induced samples of BaTiO3 thin film, BaTiO3/SrTiO3 superlattice, and cerium doped BaTiO3/SrTiO3 superlattice were epitaxially grown on SrTiO3 (100) substrates by laser molecular beam epitaxy. The measurement of the dependence of the effective second-harmonic generation (SHG) coefficients on the fundamental polarization angle and incident angle indicates that SHG coefficients d33 of the three samples are greatly enhanced, with the maximum value being one order of magnitude larger than that of the bulk BaTiO3 crystal. Our results clearly show that the introduction of stress is a very powerful means to enhance nonlinear optical susceptibility.
Analysis of nuclear quadrupole interaction effects in electron spin-echo modulation spectra by second-order perturbation methods
Authors: Heming M. , Narayana M. , Kevan Larry
Published: July 2002

The incorporation of nuclear quadrupole interaction into electron spin-echo modulation spectra by second-order perturbation methods has been developed for both two-pulse and three-pulse echo sequences. The results have been compared with exact numerical diagonalization of the Hamiltonian and with a first-order perturbation treatment. Model calculations have been carried out for deuterium (I=1) and aluminum (I=5/2) nuclei. It is shown that the second-order expressions can be used to obtain relatively accurate values for the number and distance of interacting nuclei at electron–nuclear distances greater than 0.26 nm. The procedure is more limited when the quadrupolar interaction exceeds the dipolar interaction when neither can be neglected.
Temperature dependence of the rate constants for deexcitation of He(2 1P) by SF6 and Ar
Authors: Ukai Masatoshi , Tanaka Yasushi , Koizumi Hitoshi , Shinsaka Kyoji , Hatano Yoshihiko
Published: July 2002

The temperature dependence of the rate constants for deexcitation of He(2 1P) by SF6 and Ar has been measured in the temperature range from 133 to 300 K using a pulse radiolysis method, thus obtaining the collisional energy dependence of the deexcitation cross sections. The results agree well with the calculations of the Penning ionization cross sections based on a long-range dipole–dipole interaction. It is also suggested that bent trajectory due to an attractive potential is important, which influences greatly the absolute value and the collisional energy dependence of the cross sections.
Raman study of low frequency lattice modes in dipalmitoylphosphatidylcholine (DPPC) multilayers
Authors: Jürgens E. , Geurts J. , Richter W. , Sackmann E.
Published: July 2002

Low frequency Raman spectra (6–30 cm−1) of macroscopically ordered multilayers of dipalmitoylphosphatidylcholine (DPPC) were taken as a function of temperature (80–270 K) and for different water contents. A weak band is observed which is shifted from 18.5 to 14 cm−1 between 80 and 270 K. This band is shifted to lower frequencies with increasing water content. In addition, a strong background appears at the Rayleigh line position which grows strongly with increasing temperature. The low frequency band can be explained (1) in terms of an optical mode associated with an antiphase in-plane vibration of two opposing monolayers or it can (2) be attributed to rotational vibrations of the hydrocarbon chains. The softening of this band is attributed to a decrease in the chain orientational order with increasing temperature. The increasing rotational freedom is expected to cause a transition from a monoclinic or orthorhombic to a hexagonal lattice and eventually triggers the Lβ′ → Lα chain melting transition (at T=340 K for 5.3 wt. % of water). The appearance of the background can also be attributed to an increasing anharmonicity of the lattice potential upon approaching the phase transition, which leads to mode coupling. The possibility of ferroelectric order due to a collective orientation of the dipole moments of the C–O groups of the glycerol backbone is discussed.
Molecular dynamics simulations of a sodium octanoate micelle in aqueous solution
Authors: Jönsson Bo , Edholm Olle , Teleman Olle
Published: July 2002

Molecular dynamics simulations of a sodium octanoate micelle in aqueous solution are reported. Simple analytical expressions have been used for the interatomic interactions, including a Lennard-Jones term and a Coulombic interaction betwen partial charges on each site. Two different interaction potentials have been investigated, and amphiphilic aggregation is shown to depend strongly on the electrostatic properties of the water model. For both potentials the micelle shows a broad transition region between the aqueous and hydrocarbon regions. The hydration of different carbon atoms in the chain is largest for the carboxylic atom, then decreases along the chain, reaching a minimum and then increases again at the end of the chain. The micellar translational and rotational diffusion are too fast, probably due to deficiencies in the water model. The rotational diffusion of the entire micelle is found to be an order of magnitude slower than that of the monomers. Reorientational time correlation functions and order parameters for C–H vectors in the chains are reported and agree qualitatively with a recently proposed two-step model for NMR relaxation in micelles. The dynamics of water molecules close to the micellar surface is found to be similar to bulk water.
Phase equilibria and solvation forces for fluids confined between parallel walls
Authors: Evans R. , Marini Bettolo Marconi U.
Published: July 2002

The thermodynamics of fluids confined between two adsorbing solid substrates (walls) is revisited. Attention is focused on the phase equilibria of an open system characterized by the variables μ (chemical potential), T (temperature), and H (wall separation). Clapeyron equations for the shape of lines of coexistence are derived and used to interpret the results of earlier calculations of two first-order transitions, namely capillary condensation of an undersaturated ‘‘gas’’ to ‘‘liquid’’ and prewetting (thick–thin film transition) at finite H. At such transitions the adsorption Γ and the solvation force f jump discontinuously. Criticality of a confined fluid is associated with the divergence of the derivatives (∂Γ/∂μ)T,H and (∂2Γ/∂μ2)T,H or equivalently, with the divergence of (∂f/∂H)T,μ and (∂2f/∂H2)T,μ. The presence of the additional field variable H, and its conjugate density f, implies that the phase equilibria of a confined fluid can be much richer than those of a bulk fluid or of a single interface (H=∞). By extending the formalism to multicomponent systems Clapeyron equations are derived for the coexistence of phases in confined fluid mixtures. An equation for the shift in chemical potential (or concentration) of the phase separation curve of a binary liquid mixture resulting from confinement at constant pressure and temperature is presented. This equation, which becomes exact for large separations H, is the appropriate analog for mixtures of the Kelvin equation used to describe capillary condensation in pure fluids; it can also be regarded as a generalization to nonzero concentrations of the Ostwald–Freundlich formula for the dependence of solubility on particle size. Our analysis provides a framework for interpreting recent solvation force measurements on phase-separating liquid mixtures.
Magnetic coupling and dynamics in solid α and β-O2. II. Prediction of magnetic field effects
Authors: Jansen A. P. J. , van der Avoird A.
Published: July 2002

Via simple thermodynamic arguments and via quantitative lattice dynamics and spin-wave calculations, we predict that the phase transition temperature Tαβ in solid oxygen and the optical libron frequencies in the α and β phases will both be lowered by an external magnetic field. The lowering of Tαβ varies from about 0.1 K at 7.5 T to about 1.2 K at 30 T. The lowering of the Bg and Ag libron peaks in the Raman spectrum of α-O2 and of the Eg peak in the β-O2 spectrum varies between 1.5 and 2.9 cm−1 at 30 T. These shifts can be explained by the magnetic field induced changes in the sublattice magnetizations, which affect the Heisenberg exchange contribution to the intermolecular potential. From ab initio calculations it is known that the Heisenberg coupling parameter J is extremely anisotropic.
The open-shell coupled-cluster method: Excitation energies and ionization potentials of H2O
Authors: Kaldor Uzi
Published: July 2002

The open-shell coupled cluster method is used to calculate directly several electronic excitation energies and ionization potentials of the water molecule. Correlation effects are included by summing single and double virtual excitations to infinite order. Triple excitations are treated approximately, to the lowest order they appear. Their contribution is significant, 0.2–0.4 eV for excitation energies and 0.5–0.7 eV for ionization potentials. The calculated energies are in good agreement (∼0.15 eV) with experiment.
Nonlinear chemical kinetic schemes derived from mechanical and electrical dynamical systems
Authors: Samardzija Nikola , Greller Larry D. , Wasserman E.
Published: July 2002

The nonlinear transformation presented converts a mechanical or electrical system with positive and negative variables into a dynamically related chemical scheme that allows only nonnegative state variables. This transformation preserves the phase space qualitative features of the original system. The harmonic oscillator, Van der Pol, Lorenz, Rössler spiral chaos, forced negative stiffness Duffing, and a series RLC-circuit are transformed into mass action chemical schemes. For example, the harmonic oscillator is converted into the Lotka–Volterra model. Hence, the information from a variety of physical studies is applicable to chemical systems.
A quantum mechanical study of predissociation dynamics of NaI excited by a femtosecond laser pulse
Authors: Engel Volker , Metiu Horia
Published: July 2002

This work was stimulated by the experiments of Rosker, Rose, and Zewail (RRZ) who used a pump–probe sequence of femtosecond pulses to study the predissociation of NaI. We calculate quantum mechanically the nuclear wave function created by the pump pulse and its subsequent evolution. To make contact with the experiments we assume that, depending on its wavelength, the probe pulse is absorbed by either the free sodium atoms or by the bound molecule in the neutral state, and that the measured fluorescence induced by the probe pulse is proportional to the populations of these two ‘‘species.’’ The similarity between computed populations and the observed signal confirms this conjecture. We study how various factors, such as the pulse length and shape, the initial vibrational state, the temperature, and the diabatic coupling strength affect the populations (and thus the LIF signal). We also show that the RRZ analysis based on classical trajectories and the Landau–Zener formula agrees semiquantitatively with the exact quantum results.
Rotational spectrum and structure of the HCN–(CO2)3 tetramer
Authors: Gutowsky H. S. , Hajduk P. J. , Chuang Carl , Ruoff R. S.
Published: July 2002

Microwave rotational transitions have been observed for HCN–(CO2)3, DCN–(CO2)3, H13CN–(CO2)3, HC15N–(CO2)3, HCN–(13CO2)3, HCN–(18OCO)(CO2)2, and HCN–(CO2)(C18O2)2 with the pulsed Fourier transform, Flygare/Balle Mark II spectrometer. A symmetric top spectrum was observed for the parent isotopic species with rotational constants of B0=861.6392(1) MHz, DJ =0.681(5) kHz, and DJK =0.821(12) kHz. The results for isotopic substitution indicate a zero-point, vibrationally averaged geometry having the C3 symmetry of a cyclic (CO2)3 structure with the HCN along the symmetry axis and the N end closest to the (CO2)3. The C3 symmetry is confirmed by the observation of states limited to K=±3n, with n=0,1,2,..., as predicted for threefold symmetry generated by bosons only. The (CO2)3 has a pinwheel configuration, as in the free trimer, and the three carbons lie in a plane R=2.758 Å below the center of mass (c.m.) of the HCN. The C-C distance in this subunit is 3.797 Å which is 0.241 Å shorter than that found in the free (CO2)3 trimer. The individual CO2’s in the (CO2)3 pinwheel are rotated out of the C–C–C plane by γ=−6.9°, as determined from an inertial analysis, with the inner oxygens rotated away from the HCN. The HCN has an average torsional angle of 10.3°, as determined by isotopic substitution, and an observed χcc value of −3.891 MHz for the 14 N. The c.m.(HCN) to C distance is 3.525 Å, compared to 3.592 Å in the HCN-CO2 T-shaped dimer. The isotopic substitution by 18O perturbs the structure of the symmetric top clusters by a remarkable amount, decreasing γ to −28.9° and increasing R and RCC to 2.797 and 3.814 Å, respectively. In the 18O substituted species, the CO2’s are rotated in the C–C–C plane from C3v symmetry by the pinwheel angle β=∼32.5°.
Isotopic fractionation in the formation of ozone and of carbon dioxide
Authors: Bates D. R.
Published: July 2002

An explanation is proposed for the observed enrichment of the symmetric isomer OXO of heavy ozone (X being used to denote a heavy O atom). This is that the energized complex may undergo a flip in which one of the end atoms ceases to be effectively bound to the central atom and becomes effectively bound to the other end atom. The measured [OXO]:[OOX] ratio is reproduced if the flip frequency is the same as the dissociation frequency. Calculations are carried out on the variation of the enhancement with the ambient gas density. It is reasoned that the enhancement observed in the stratosphere is that arising in termolecular association and that the lower values obtained in laboratory experiments are artifacts. The measured isotopic enrichment of carbon dioxide may arise from termolecular association O+CO+CO→CO2+CO, from enriched OXO* that is generated in another association process or from both. The apparent scatter in the results is much greater than would be expected from the precision of the measurements. It is suggested that this may be an artifact (similar in origin to those for ozone).