bluuebunny/crossref_metadata_2025 huggingface dataset:
Observation of dynamic behavior of lithographically induced self-assembly of supramolecular periodic pillar arrays in a homopolymer film
Authors: Deshpande Paru , Sun Xiaoyun , Chou Stephen Y.
Published: July 2002

The dynamic behavior of lithographically induced self-assembly (LISA), a newly discovered pattern formation phenomenon, was recorded and analyzed with real time video. Two surprising and intriguing phenomena were observed. First, a LISA pillar array was formed in an orderly manner starting under the corners of a mask pattern, then the edges, and later propagating to the center of the mask pattern. Second, the time interval between the formation of two subsequent LISA pillars has a shell structure. The observation presents evidence critical to uncovering the physical origin of this phenomenon, which is believed to be related to the interplay of electrodynamics, fluid hydrodynamics, and polymer chemistry at the nanometer scale.
Tunnel magnetoresistance and magnetic ordering in ion-beam sputtered Co80Fe20/Al2O3 discontinuous multilayers
Authors: Kakazei G. N. , Pogorelov Yu. G. , Lopes A. M. L. , Sousa J. B. , Cardoso S. , Freitas P. P. , Pereira de Azevedo M. M. , Snoeck E.
Published: July 2002

Discontinuous multilayered Co80Fe20(t)/Al2O3(30 Å) thin films have been prepared by ion-beam sputtering. We report on structural, magnetic, and transport (for current in plane geometry) results obtained in this system. With growing nominal thickness t of the metal layers, which effectively characterizes the granular structure, a transition from tunnel to metallic conductance is observed, indicating the onset of infinite conducting paths at t>18 Å. At t<18 Å, that is within the range of tunnel regime, a different characteristic value t>13 Å was detected from the magnetization data which display here a transition from superparamagnetic to ferromagnetic behavior. The measurements of tunnel magnetoresistance (MR) show that a sharp maximum of MR sensitivity to field takes place at this thickness, reaching ∼24%/kOe at room temperature. At least, MR itself as a function of t has a break at the same value. All these features suggest that some specific kind of percolation with respect to magnetic order occurs in our system when the disordered granular structure is still well separated, as confirmed by the data of high resolution transmission electron microscopy. Hence such magnetic percolation is clearly distinct from usual electrical percolation in these discontinuous layers. At the same time, the highest MR (∼6.5% at room temperature) in this series is attained with decreasing t only at t=10 Å.
Magnetic flux trapping and frozen magnetoresistance in bismuth-containing high-temperature superconducting films
Authors: Sukhanov A. A. , Omel’chenko V. I.
Published: July 2002

Hall-probe and magnetoresistance studies of trapped magnetic fields are carried out on Bi-containing high-Tc superconducting films obtained by magnetron sputtering. It is shown that the trapped magnetic fields are alternating in sign in the plane of the film on a characteristic scale of less than 50 μm. The temperature dependence of the frozen magnetoresistance and trapped magnetic fields is investigated. It is found that heating the films causes the trapped magnetic fields and frozen magnetoresistance to decrease to zero at a temperature substantially lower than the temperature at which the magnetoresistance vanishes, and for trapping after cooling in zero field a maximum appears on the curve of the trapped magnetic fields versus the trapping temperature. The experimental data are discussed and compared with the results of calculations in the model of superconducting loops, making it possible to determine the form and parameters of the distribution function of the loops over critical fields and the temperature dependence of the critical fields.
Inter-conduction band electron relaxation dynamics in 6H–SiC
Authors: Tomita T. , Saito S. , Suemoto T. , Harima H. , Nakashima S.
Published: July 2002

The ultrafast inter-conduction band carrier dynamics in 6H–SiC was observed by using pump and probe transient absorption technique. Probe wavelength dependence of the bleaching was compared with the steady-state absorption profiles for polarizations parallel and perpendicular to the c axis, and these bleachings were ascribed to the decrease of electron populations in the lowest conduction band. The relaxation time from the higher to the lowest conduction band due to the inter-conduction band electron–phonon scattering is 1.25 ps.
Displacement of domain walls under a nanocontact current: Mechanism for magnetoresistance asymmetric switching
Authors: Osipov V. V. , Ponizovskaya E. V. , Garcı́a N.
Published: July 2002

We study the action of a magnetic field induced by nanocontact current pulses on the domain walls in thin magnetic films. We show that the pulses of a certain current direction shift the wall to the contact. Such an effect of attraction of the wall to the nanocontact does not depend on the initial position of the wall relative to the contact and results in an increase of nanocontact magnetoresistance. The opposite pulses repel this wall from the contact, i.e., the field action depends on the current direction. Our calculations explain experimental data relating to magnetoresistance devices.
Mechanisms of noise increase in direct-coupled high Tc superconducting quantum interference device magnetometers exposed to magnetic fields
Authors: Yokosawa Koichi , Kuriki Shinya , Hirano Satoru , Oyama Hiroshi , Suzuki Daisuke , Tsukada Keiji
Published: July 2002

We have investigated the behavior of high-critical-temperature (high Tc) direct-coupled superconducting quantum interference device (SQUID) magnetometers in static and fluctuating magnetic fields. The magnetometers consist of narrow-linewidth superconducting films to prevent flux trap during field cooling. Moreover, they have no superconducting films crossing the bicrystal lines of the substrates (except at the Josephson junctions); i.e., they have no flux dams. When one of these magnetometers was cooled in a static magnetic field Bcool, the low-frequency noise when Bcool<100 μT was as low as that under zero-field cooling, but above 100 μT the noise increased substantially. On the other hand, when a field Bext of less than 4 μT was applied after zero-field cooling, the low-frequency noise increased in proportion to Bext. It returned to its original value reversibly when Bext was turned off. However, when Bext was greater than or equal to 4 μT, the output of the flux-locked-loop started to drift with time and the low-frequency noise increased further. This additional noise increase remained after turning off Bext. These results suggested that there are two modes of increase for the low-frequency noise induced by flux penetration due to the shielding current: a “reversible” mode and an “irreversible” mode. We found that the low-frequency noises of the two modes were additive with respect to their power, suggesting that the two noises derived from independent sources at different sites on the magnetometer. We also found that the reversible-mode noise could be reduced by improving the profile of the film edge.
Metallic contamination in hydrogen plasma immersion ion implantation of silicon
Authors: Chu Paul K. , Fu Ricky K. Y. , Zeng Xuchu , Kwok Dixon T. K.
Published: July 2002

In plasma immersion ion implantation (PIII), ions bombard all surfaces inside the PIII vacuum chamber, especially the negatively pulsed biased sample stage and to a lesser extent the interior of the vacuum chamber. As a result, contaminants sputtered from these exposed surfaces can be reimplanted into or adsorb on the silicon wafer. Using particle-in-cell theoretical simulation, we determine the relative ion doses incident on the top, side, and bottom surfaces of three typical sample chuck configurations: (i) a bare conducting stage with the entire sample platen and high-voltage feedthrough/supporting rod exposed and under a high voltage, (ii) a stage with only the sample platen exposed to the plasma but the high-voltage feedthrough protected by an insulating quartz shroud, and (iii) a bare stage with a silicon extension or guard ring to reduce the number of ions bombarding the side and bottom of the sample platen. Our simulation results reveal that the ratio of the incident dose impacting the top of the sample platen to that impacting the side and bottom of the sample stage can be improved to 49% using a guard ring. To corroborate our theoretical results, we experimentally determine the amounts of metallic contaminants on 100 mm silicon wafers implanted using a bare chuck and with a 150 mm silicon wafer inserted between the 100 mm wafer and sample stage to imitate the guard ring. We also discuss the effectiveness of a replaceable all-silicon liner inside the vacuum chamber to address the second source of contamination, that from the interior wall of the vacuum chamber. Our results indicate a significant improvement when an all-silicon liner and silicon guard ring are used simultaneously.
Permanent and transition dipole moments in CaF and CaCl
Authors: Raouafi S. , Jeung G.-H. , Jungen Ch.
Published: July 2002

Permanent and transition dipole moments have been calculated for various states of CaF and CaCl by using an effective one-electron variational eigenchannel R-matrix approach combined with generalized quantum defect theory. The ion core dipole moment has been evaluated ab initio. The calculations reproduce the existing measurements involving the lowest states of these compounds to within about 5%. The calculations yield for the first time electronic one-photon transition moments for excitation of the higher Rydberg series members, from the A 2Π, C 2Π or D 2Σ+ intermediate states. Permanent dipole moments for the higher Rydberg states have also been evaluated. It is found that the dipolar ion core causes an “internal” Stark effect to appear, such that certain series are characterized by strong positive dipole moments and other series are characterized by strong negative dipole moments.
The potential energy surface of excited states by time-dependent density functional theory: The reaction of sulfur atom and nitrogen dioxide
Authors: Chen Wei-Chen , Yu Chin-hui
Published: July 2002

The reaction of sulfur atom and nitrogen dioxide is studied by B3LYP/cc-pVTZ method. The potential energy surfaces of the reaction on several electronic states are computed by time-dependent density functional theory (TD–DFT). The complexes of the precursor of sulfur atom, OCS and CS2, and NO2 are also studied to probe all possible reactive routes. The possible products in the ground state are SNO2, SONO, a mixture of SO and NO, as well as cis- and trans-OSNO. Calculations show that the most possible product is SNO2 because it is the product of a barrierless reaction and is trapped in a well of 34 kcal/mol. Even this most probable product, SNO2, might not be detected in matrix-isolation experiment for the precursor of sulfur atom chelates with NO2 and the formation of SNO2 is directionally prohibited. Other products are not likely to be generated in matrix-isolation experiment from the ground state of sulfur atom and NO2 since the barrier of 12 kcal/mol obstructs the reaction route. In the same experimental condition, products other than SNO2 are probably initiated with singlet sulfur. The calculation suggests that the sulfur atom in the D1 state attacks the oxygen atom of NO2 in the A12 state, then reacts without barrier, and after a crossing and quenching falls into the lowest A′2 state around the geometry of a transition structure in the ground state. Afterward, the reaction proceeds to yield the products SO, NO, cis-OSNO, and trans-OSNO. This reaction path bypasses the formation of SNO2; thus, when the sulfur atom is in the D1 excited state, the SNO2 will not be observed.
Batalin–Tyutin quantization of the spinning particle model
Authors: Ghosh Subir
Published: July 2002

The spinning particle model for anyon is analyzed in the Batalin–Tyutin scheme of quantization in extended phase space. Here additional degrees of freedom are introduced in the phase space such that all the constraints in the theory are rendered first class, that is, commuting in the sense of Poisson brackets. Thus the theory can be studied without introducing the Dirac brackets which appear in the presence of noncommuting or second class constraints. In the present case the Dirac brackets make the configuration space of the anyon noncanonical and also, being dynamical variable dependent, poses problems for the quantization program. We show that previously obtained results (e.g., gyromagnetic ratio of anyon being 2) are recovered in the Batalin–Tyutin variable independent sector in the extended space. The Batalin–Tyutin variable contributions are significant and are computable in a straightforward manner. The latter can be understood as manifestations of the noncommutative space–time in the enlarged phase space.
Millisecond electron–phonon relaxation in ultrathin disordered metal films at millikelvin temperatures
Authors: Gershenson M. E. , Gong D. , Sato T. , Karasik B. S. , Sergeev A. V.
Published: July 2002

We have measured directly the thermal conductance between electrons and phonons in ultrathin Hf and Ti films at millikelvin temperatures. The experimental data indicate that electron–phonon coupling in these films is significantly suppressed by disorder. The electron cooling time τε follows the T−4 dependence with a record-long value τε=25 ms at T=0.04 K. The hot-electron detectors of far-infrared radiation, fabricated from such films, are expected to have a very high sensitivity. The noise-equivalent power of a detector with the area 1 μm2 and the noise limited by fluctuations of the temperature are expected to be (2–3)×10−20 W/Hz, which is two orders of magnitude smaller than that of the state-of-the-art bolometers.
Low leakage current and good ferroelectric properties of SrBi2(Ta0.7Nb0.3)2O9–Bi3TiTaO9 solid solution thin film
Authors: Mitsuya Masatoshi , Nukaga Norimasa , Watanabe Takayuki , Funakubo Hiroshi , Saito Keisuke , Osada Minoru
Published: July 2002

(1−x) SrBi 2 (Ta 0.7 Nb 0.3 ) 2 O 9 +x Bi 3 TiTaO 9  (x=0–0.5) solid-solution (SBTN+BTT) films of low defect contents were directly crystallized on (111)Pt/Ti/SiO2/Si substrates at 650 °C by metalorganic chemical vapor deposition. The deposited films showed a strong (103) orientation. The remanent polarization (Pr) of the directly crystallized SBTN (x=0) was very small. However, the Pr value increased to 7.1 μC/cm2 by adding 30% of BTT (x=0.3) and was almost equal to that of Sr0.8Bi2.2(Ta0.7Nb0.3)2O9(S0.8B2.2TN), which is widely studied for nonvolatile memory applications. The leakage current density of the SBTN+BTT solid solution was on the order of 10−8 A/cm2 for fields up to 200 kV/cm due to its low defect contents character, while that of S0.8B2.2TN was above 10−6 A/cm2 due to the existence of defects in the Sr sites. The solid-solution film showed a fatigue-free character.
Spectral inhomogeneity induced by vacancies and thermal phonons and associated observables in time- and frequency-domain nonlinear spectroscopy: I2 isolated in matrix argon
Authors: Bihary Z. , Karavitis M. , Gerber R. B. , Apkarian V. A.
Published: July 2002

The vibrational self-consistent field method is used to analyze the inhomogeneous spectral distribution of transitions caused by vacancies and thermally populated phonons, specializing to molecular iodine isolated in an Ar matrix. At experimentally relevant temperatures, for a vacancy concentration of 1.4%, both defect-induced and phonon-induced spectral shifts contribute to the spectral distribution. Both contributions scale linearly with vibrational overtone number. The predicted widths are consistent with reported resonant Raman spectra. In time-resolved coherent anti-Stokes Raman scattering (TRCARS) measurements, spectral indistinguishability implies that all members of the inhomogeneous ensemble contribute coherently to the detectable homodyne signal. The connection between spectral distribution and the observable in TRCARS is derived. The predicted polarization beats and free induction decay due to the inhomogeneous ensemble are in qualitative agreement with experiments.
A classification method for vortex sheet and tube structures in turbulent flows
Authors: Horiuti Kiyosi
Published: July 2002

A new classification method for structures in turbulent flow is proposed and applied to the analysis of homogeneous isotropic turbulence. The criteria for the classification of the structures into three groups, namely, the group of structures similar to the core region of the Burgers’ vortex tube in which vorticity is predominant, that of the structures similar to the curved sheet in the circumference of the tube core in which strain is predominant, and that of the flat sheets similar to the Burgers’ vortex layer in which vorticity and strain are comparably large, were considered. This method was developed based on the eigenvalue solutions of the λ2 method [Jeong and Hussain, J. Fluid Mech. 285, 69 (1995)] on the basis of the principal strain eigenvectors, which were reordered according to the degrees of alignment with the vorticity vector. Assessment of the proposed method was carried out in fully developed homogeneous isotropic turbulence and in the process of rolling up of the vortex layer in ABC flow. It was shown that the spiral vortex sheet emanating from the tube core, which was generated during the rolling up of the layer, can be accurately educed using the proposed method, and its performance was markedly better than that of eduction obtained using the second-order invariant of the velocity gradient tensor Q. The process of formation of a vortex tube was investigated using the proposed method. In the analysis of turbulence statistics, it was shown that the characteristic differences of the three regions for contributions of strain and vorticity were correctly educed using the proposed method, and that the flat sheet region is primarily responsible for the generation of turbulence.
Electron detection by resistive charge division based position sensitive detectors
Authors: Räisänen J. , Harju O. , Tjurin G. , Riihimäki I. , Lindholm L.
Published: July 2002

The use of resistive charge division based solid-state detectors for position sensing of electrons has been studied. The operational conditions having importance for device performance are considered for one- and two-dimensional detectors manufactured by SiTek®. The parameter of main interest has been position resolution. Good linearity with a spatial resolution of 0.2 mm is obtained under optimum conditions. The advantages and limitations of the present approach and alternative techniques that could be considered for individual electron detection are discussed. A possible application of the detector is foreseen in its use for lattice location studies of impurity atoms by emission channeling.
Thermal hysteresis loop, dynamical breakdown, and emission-current spike in quantum-well photodetectors
Authors: Huang Danhong , Morath Christian , Cardimona D. A. , Singh Anjali
Published: July 2002

A nonadiabatic sequential-tunneling model is developed and applied to explore the common origin of the transient behavior of electrons in quantum-well photodetectors in the presence of different time-dependent external sources, including device temperature, electric field, and incident optical flux. For the time-dependent temperature, a counterclockwise hysteresis loop in the tunneling current as a function of the swept temperature is predicted and attributed to a blockade or an enhancement of the sequential tunneling of electrons between quantum wells by the space-charge-field effect when the device temperature is swept up and then down. When a time-dependent electric field is applied, a dynamical breakdown of the photodetectors is predicted, where the peak of total current linearly increases with the frequency of an ac electric field from its static value under a dc field. This is due to the presence of an additional dielectric current, which is proportional to the oscillation frequency of the ac electric field and whose peak value becomes larger than the value of the saturated tunneling-current peak in the high-frequency domain. Under the dynamical-breakdown condition, the quantum-well photodetectors behave just like a uniform dielectric medium. In the presence of a time-dependent optical flux, an emission-current spike is predicted as a result of the dominant enhancement of the escape probability of electrons from quantum wells over the loss of electron density when an applied dc electric field is small. The experimental observations of the transient behavior of electrons in quantum-well photodetectors are successfully reproduced by our numerical calculations.
Micromodification of silicon dioxide in a variable pressure/environmental scanning electron microscope
Authors: Stevens-Kalceff Marion A.
Published: July 2002

Electron irradiation in the ionized gaseous environment of a variable pressure/environmental scanning electron microscope induces modifications of poorly conducting specimens. In particular it is shown, using nondestructive depth-resolved cathodoluminescence microanalysis, that environmental ions can penetrate into the bulk of the irradiated specimen and modify the local microstructure of the irradiated specimen. The observed modifications are attributed to electric fields associated with trapped electrons and environmental ions. These effects can be controlled by varying the environmental gas and/or electron beam parameters.
Bit error rate test accessory makes errors audible
Authors: Green Samuel I.
Published: July 2002

A simple accessory for bit error rate test instruments provides a means to monitor error rate audibly without watching a display so that the user may observe and make adjustments to equipment under test while hearing the effects of these adjustments. Normally inaudible narrow error pulses trigger a toggle flip-flop to provide a signal rich in audible components.
Novel method for the production of finely spaced Bradbury–Nielson gates
Authors: Kimmel Joel R. , Engelke Friedrich , Zare Richard N.
Published: July 2002

Bradbury–Nielson gates for the modulation of beams of charged particles, particularly ion beams in mass spectrometry, have been produced with an adjustable wire spacing down to 0.075 mm. The gates are robust, they can be fabricated in less than 3 h, and the method of production is reproducible. In time-of-flight mass spectrometers, fine wire spacing leads to improvements in mass resolution and modulation rates. Gates that were produced using this new method have been installed in a Hadamard transform time-of-flight mass spectrometer in order to demonstrate their utility.
The linear stability of a drop of fluid during spin coating or subject to a jet of air
Authors: McKinley I. S. , Wilson S. K.
Published: July 2002

In this paper we investigate the linear stability of an initially axisymmetric thin drop of Newtonian fluid either on a uniformly rotating substrate (the simplest model for spin coating) or on a stationary substrate under the influence of an axisymmetric jet of air directed normally towards the substrate. Drops both with and without a dry patch at their center are considered. For each problem we examine both the special case of quasistatic motion (corresponding to zero capillary number) analytically and the general case of nonzero capillary number numerically. In all cases the drop is found to be unconditionally unstable, but the growth rate and wavenumber of the most unstable mode depend on the details of the specific problem considered.
Quantitative characterization of intermolecular dipolar interactions of two-component systems in solution nuclear magnetic resonance
Authors: Chen Zhong , Chen Zhiwei , Zhong Jianhui
Published: July 2002

Liquid nuclear magnetic resonance behaviors related to intermolecular dipolar interactions were investigated theoretically and experimentally in highly polarized two-component spin systems. A modified CRAZED pulse sequence was designed to investigate relative signal intensities with considerations of spin transverse relaxation, longitudinal relaxation, molecular diffusion, and optimal radio-frequency flip angles. The dissipation of the demagnetizing field due to relaxation and diffusion processes during the detection period was taken into account as well. For the first time, vigorous analytical expressions of the spin dynamics, including all the effects mentioned above, were derived from the combination of the demagnetizing field model and product operator formalism. In the regime where the linear approximation (γμ0M0t≪1) is valid, these explicit analytical expressions can quantitatively describe the signal behaviors related to intermolecular dipolar interactions. All the theoretical predictions based on the analytical expressions for the directly excited component are in excellent agreement with experimental observations reported previously. Several valuable insights for the indirectly excited component were gained from the analytical expressions and verified by experimental measurements, including optimal radio-frequency flip angles, unusual relative signal intensities for n=−2 and n=2, and unconventional diffusion and multi-exponential longitudinal relaxation processes, where n is the ratio of the coherence-selection gradient areas in the CRAZED pulse sequence. In addition, n-order diffusion coefficients of the directly and indirectly excited spins during the evolution period predicted by the demagnetizing field picture are found to be the same as those obtained with the combination of the intermolecular multiple-quantum picture and Gaussian phase distribution approximation which is valid in the case of unrestricted isotropic diffusion. These results suggest that a combination of the demagnetizing field model and product operator formalism provides excellent predictive power and computational convenience for diffusion and relaxation behaviors in two-component systems. These quantitative studies may also provide an opportunity to probe specific sources of new contrast for medical MR imaging.
Kinetic approach to fatigue phenomenon in ferroelectrics
Authors: Shur Vladimir Ya. , Rumyantsev Evgenii L. , Nikolaeva Ekaterina V. , Shishkin Eugene I. , Baturin Ivan S.
Published: July 2002

We propose an approach to the explanation of the fatigue effect as an evolution of the switching area during cyclic switching as a result of self-organized domain kinetics due to retardation of bulk screening of the depolarization field. The formation of spatially nonuniform internal bias field during cycling (kinetic imprint effect) slows the domain kinetics in some regions leading to formation of the kinetically frozen domains. Presented fatigue and rejuvenation experimental data measured in sol-gel PbZrxTi1−xO3 thin films are in accordance with the results of computer simulation.
Thermal characteristics of optical gain for GaInNAs quantum wells at 1.3 μm
Authors: Kim Chang Kyu , Lee Yong Hee
Published: July 2002

The gain characteristics of 1.3-μm-wavelength GaInNAs, InGaAlAs, and InGaAsP single-quantum-well structures are studied and compared. Among these quantum wells, GaInNAs offers the lowest carrier density over a wide range of temperature (300–400 K) for applications that require high gain because of the highest differential gain. It is due to the large electron effective mass originating from the nitrogen incorporation. The change in threshold carrier density with temperature is smallest for GaInNAs because of the large conduction band offset and the large differences in the band gap energy between the well and the barrier. The interaction with the temperature-independent nitrogen states makes the shift of gain with temperature slowest as well. For these reasons, the threshold current of GaInNAs is expected to be more temperature independent than those of other materials.
Collisional effects on the radio-frequency sheath dynamics
Authors: Qiu Hua-Tan , Wang You-Nian , Ma Teng-Cai
Published: July 2002

Using the hydrodynamic model, we have derived a self-consistent model describing the dynamics of a collisional rf sheath driven by a sinusoidal current source. Numerical results have been obtained for the dependence of ion density, ion kinetic energy, sheath thickness, and sheath electric field, on the collision parameter and the current source parameter.
A semi-infinite bubble advancing into a planar tapered channel
Authors: Halpern D. , Jensen O. E.
Published: July 2002

We consider two problems in which a semi-infinite bubble moves into a uniformly convergent two-dimensional channel under creeping-flow conditions. In the first (steady) problem, the bubble is stationary with respect to the channel walls, which move away from the channel vertex parallel to themselves, each entraining a liquid film of uniform thickness; the appropriate flux is provided by a line source at the channel vertex. In the second (unsteady) problem, the channel walls are fixed and the bubble is drawn at a constant speed towards the channel vertex by a line sink. In this case the bubble interface assumes a self-similar shape, and wedge-shaped films are deposited on the channel walls. The boundary-element method, supported by asymptotic approximations, is used to compute flows over a range of bubble speeds (measured by a capillary number Ca) and wedge angles α. In the steady problem, the deposited film thickness increases monotonically with α at low Ca, but diminishes with increasing α at sufficiently high Ca. In the unsteady problem, the film thickness was found always to increase with both α and Ca. In both cases, the dimensionless pressure drop across the bubble tip can be nonmonotonic in α. Implications of these results in modeling coating and peeling flows are discussed.
Single-mode tunable organic laser based on an electroluminescent oligothiophene
Authors: Zavelani-Rossi M. , Lanzani G. , De Silvestri S. , Anni M. , Gigli G. , Cingolani R. , Barbarella G. , Favaretto L.
Published: July 2002

Single-mode tunable laser emission is obtained from a stable cavity based on a prototype compound of a class of functionalized electroluminescent oligothiophenes. Laser emission is demonstrated in the red spectral region with a tunability wavelength range of 30 nm, centered at 607 nm. The laser exhibits a well-defined pump threshold and good emission characteristics. The results suggest the use of the family of functionalized oligothiophenes as optical amplifiers covering the whole visible spectral region.
Electrical isolation of AlxGa1−xAs by ion irradiation
Authors: v. Lippen T. , Boudinov H. , Tan H. H. , Jagadish C.
Published: July 2002

The evolution of sheet resistance Rs of n-type and p-type conductive AlxGa1−xAs layers (x=0.3, 0.6, and 1.0) during proton irradiation was investigated. The threshold dose Dth to convert a conductive layer to a highly resistive one is slightly different for n- and p-type samples with similar initial free carrier concentration and does not depend on the Al content. The thermal stability of the isolation, i.e., the temperature range for which the Rs is maintained at ≈109 Ω/sq, was found to be dependent on the ratio of the carrier trap concentration to the original carrier concentration. The thermal stability of isolated p-type samples is limited to temperatures lower than 450 °C. The temperature of ≈600 °C is the upper limit for the n-type samples thermal stability.
Characterization of spectroscopic and laser properties of Pr3+ in Sr5(PO4)3F crystal
Authors: Sardar Dhiraj K. , Castano Francisco
Published: July 2002

Spectroscopic and laser properties have been characterized for Pr3+ incorporated into Sr5(PO4)3F crystal belonging to the apatite structure family. The standard Judd–Ofelt analysis has been applied to the measured optical absorption intensities to determine the radiative decay rates, branching ratios, and emission cross sections of principal intermanifold transitions of Pr3+ from the D21 and P03 states to the lower-lying manifolds in the visible region. The measured room temperature fluorescence lifetimes of the D21→3H4 (594 nm) and P03→3F2 (625 nm) transition are 325 and 105 μs, respectively, while the Judd–Ofelt analysis predicts the radiative lifetimes for the D21 and P03 states to be 822 and 116 μs, respectively. The room temperature emission cross sections of the D21→3H4 and P03→3F2 intermanifold transitions have been also determined to be 0.54×10−20 and 4.15×10−20 cm2, respectively.
Kinetics of a two-dimensional lattice gas mixture in a color field
Authors: Woodward C. E. , Campion M. , Isbister D. J.
Published: July 2002

Using computer simulations, we have investigated the kinetics of a two-dimensional, two-component fluid mixture on a lattice in the presence of a field which drives the different species in opposite directions. A kinetic instability arises when the field strength and fluid density are sufficiently large. The nonuniform state which emerges has zero flux and consists of layers of particles, essentially close-packed and perpendicular to the field direction. The layers each contain particles of essentially a single species, but the layer composition alternates between the different species with periodicity depending on the fluid density and the field strength. A mean-field analysis suggests this is a first-order dynamic phase transition, which is consistent with the computer simulations.
Stochastic model of reaction rate oscillations in the CO oxidation on nm-sized palladium particles
Authors: Peskov N. V. , Slinko M. M. , Jaeger N. I.
Published: July 2002

A mesoscopic stochastic model of the catalytic reaction 2CO+O2→2CO2 on the surface of a metal particle is considered. The model is a Markovian chain of elementary reaction steps, which mimics the catalytic oxidation of CO on a nm-sized Pd particle. The model takes into account the effect of the particle size on the reaction rate and the role of temporal fluctuations of the concentrations of the reactants. The main goal of the paper is the comparison of the dynamics produced by the stochastic model and the deterministic model obtained via averaging of the master equation, while the catalyst particle size is reduced. Intrinsic fluctuations during the reaction are shown to change the reaction kinetics drastically for small metal particles with only several hundreds of surface atoms.
Dependence of relativistic self-guiding and Raman forward scattering on duration and chirp of an intense laser pulse propagating in a plasma
Authors: Yau T.-W. , Hsu C.-J. , Chu H.-H. , Chen Y.-H. , Lee C.-H. , Wang J. , Chen S.-Y.
Published: July 2002

Relativistic self-guiding and Raman forward scattering of an intense ultrashort laser pulse propagating in an underdense plasma were studied using a terawatt Ti:sapphire laser system. The dependence of these processes on the duration and frequency chirp of the laser pulse was investigated by detuning the pulse compressor. It was observed that the efficiency of Raman forward scattering is enhanced for a positively chirped pulse and diminished for a negatively chirped pulse. In addition, as a result of the dependence on pulse duration and peak power, an optimal duration for Raman forward scattering was found for fixed pulse energy and spectral bandwidth. On the other hand, relativistic self-guiding of the laser pulse was affected by the pulse duration while no dependence on the chirp was observed.
Pressure sensor based on freely suspended ferroelectric liquid crystal film
Authors: Yablonskii S. V. , Nakano K. , Mikhailov A. S. , Ozaki M. , Yoshino K.
Published: July 2002

We report on a pressure gauge based on a vibrating, freely suspended ferroelectric liquid crystal film, which shows some promise of being sensitive to the pressures in the range from 50 to 105 Pa. The gauge indicates changes in resonant frequency of the film caused by changes of air pressure. Pressure measurements using such sensor have been carried out.
Relaxation phenomena in pulsed discharges
Authors: Daube Th. , Meyer P. , Riemann K.-U. , Schmitz H.
Published: July 2002

The sheath and presheath relaxation in front of an electrode biased to high negative voltage pulses is investigated on the basis of ion fluid equations as well as of a particle-in-cell/Monte Carlo simulation. The electrons are assumed to be Boltzmann distributed and the ions are governed by charge exchange collisions. The electron Debye length is small compared to the ion mean free path. Switching on a high negative voltage, three phases on different time scales may be distinguished: the matrix extraction phase, the sheath expansion phase, and the presheath relaxation initiated by a rarefaction wave. Correspondingly, switching off a high negative voltage results in a fillup process followed by a sheath constriction and by a compression wave rearranging the presheath. All these phenomena are mixed if voltage pulses of finite duration are applied. We present numerical results exhibiting typical relaxation phenomena for single pulses as well as for periodic pulses with various frequencies and pulse forms. Results for the particularly important matrix extraction phase are compared with an analytic step model of the homogeneous matrix sheath [K.-U. Riemann and Th. Daube, J. Appl. Phy. 86, 1202 (1999)].
Local density augmentation in attractive supercritical solutions. III. How important is the solute–solvent interaction range?
Authors: Egorov S. A.
Published: July 2002

We study the local solute–solvent structure in dilute supercritical solutions, using as a model system a dilute Yukawa solute in a supercritical Lennard-Jones fluid. Our primary interest is in the effect of the solute–solvent interaction range on the local solvent density around the solute. We employ the integral equation theory for inhomogeneous fluids to calculate the solute–solvent structural properties. The theory is shown to be in excellent agreement with Monte Carlo simulations and to provide a substantial improvement over the integral equation theory formulated for homogeneous fluids. In particular, it is demonstrated that the homogeneous theory greatly overestimates the local density enhancement for long-ranged solute–solvent interactions in the highly compressible supercritical regime.
Raman scattering and x-ray absorption studies of Ge–Si nanocrystallization
Authors: Kolobov A. , Oyanagi H. , Usami N. , Tokumitsu S. , Hattori T. , Yamasaki S. , Tanaka K. , Ohtake S. , Shiraki Y.
Published: July 2002

We have studied the local structure of GeSi nanocrystals embedded in SiO2 prepared by co-sputtering of Ge, Si, and SiO2 targets onto a Si(100) substrate. From Raman scattering, we conclude that the formation of the isotropic crystalline Ge phase starts at about 800 °C followed by the formation of a GeSi phase at higher temperatures. The formed nanocrystals, whose size depends on the annealing temperature, are randomly oriented. The local structure of the nanocrystals has been studied by x-ray absorption fine structure spectroscopy. They are found to consist of a relaxed Ge core with a typical diameter of ∼4 nm and the Ge–Ge bond length of 2.45 Å and of a GeSi outer shell, the Ge–Si bond length being 2.39 Å. The average composition of the grown nanocrystals is estimated to be Ge0.75Si0.25.
Nonlinear response effects in continuum models of the hydration of ions
Authors: Sandberg Lars , Edholm Olle
Published: July 2002

The evaluation of the free energy of hydration depends essentially upon a calculation of the difference in polarization energy in a vacuum and in water. This is feasible by electrostatic continuum theory which is lucid and computationally effortless. It is, however, insufficient to apply the frequently used linear response approximation and nonlinear response effects must be integrated with the continuum model. We start from the time-honored Langevin–Debye theory and modify it to even describe polar liquids such as aqueous solvents. The modified Langevin–Debye model is then applied to study the hydration of cations. It is to be noted that reaction-field models are unapplicable to a nonlinear dielectric in an inhomogeneous field. The nonlinear effects are less than 10 kJ/mol or 2% for monovalent ions. However, for multiply charged ions the effect can be as large as 1000 kJ/mol or 15% and the nonlinear response model reduces the mean error of the calculated hydration free energy by at least 60%, which results in a mean error of only 4% for such ions. The mean error is of the same magnitude as the electrostatic free energy uncertainty.
Single InP/GaInP quantum dots studied by scanning tunneling microscopy and scanning tunneling microscopy induced luminescence
Authors: Håkanson U. , Johansson M. K.-J. , Persson J. , Johansson J. , Pistol M.-E. , Montelius L. , Samuelson L.
Published: July 2002

We have studied the optical and structural properties of single, self-assembled InP quantum dots (QDs) overgrown with nominally 5 nm of GaInP, using an ultrahigh-vacuum scanning tunneling microscope (STM) operating at low temperatures. The STM is combined with an optical detection system, which allows us to detect the emission from individual quantum dots with high spatial resolution. We find that the InP QDs act as nucleation points for the GaInP overgrowth, where the strain induced by the overlayer give rise to a QD emission around 1.46 eV.
H
2
O
adsorption on BaF2(111) at ambient temperatures
Authors: Sadtchenko Vlad , Conrad Peter , Ewing George E.
Published: July 2002

Adsorption of H2O on the BaF2(111) surface was investigated at temperatures from −1 to 25 °C using infrared spectroscopy. At coverages near a monolayer, adsorbed films have spectra that are strikingly similar to those theoretically predicted for the surface bilayer of Ih ice. At multilayer coverages the spectra become similar to those of bulk liquid water. The coverage dependent values for enthalpy and entropy of H2O adsorption were derived from adsorption isotherm measurements. The enthalpy of formation of the monolayer is greater than that for water condensation to either the liquid or solid phases. The absolute entropy of the monolayer reveals a high degree of order having a value near that expected for ice if it were to exist at room temperature. Both the enthalpy and entropy approach the values for bulk liquid water for increases to multilayer coverages. Our results are consistent with an icelike hexagonal H2O adlayer on BaF2(111) at surface temperatures far above the ice melting point, if the adlayer thickness does not exceed one monolayer. The structure of the H2O adlayers at higher coverages is also discussed.
Ablation of solids by femtosecond lasers: Ablation mechanism and ablation thresholds for metals and dielectrics
Authors: Gamaly E. G. , Rode A. V. , Luther-Davies B. , Tikhonchuk V. T.
Published: July 2002

The mechanism of ablation of solids by intense femtosecond laser pulses is described in an explicit analytical form. It is shown that at high intensities when the ionization of the target material is complete before the end of the pulse, the ablation mechanism is the same for both metals and dielectrics. The physics of this new ablation regime involves ion acceleration in the electrostatic field caused by charge separation created by energetic electrons escaping from the target. The formulas for ablation thresholds and ablation rates for metals and dielectrics, combining the laser and target parameters, are derived and compared to experimental data. The calculated dependence of the ablation thresholds on the pulse duration is in agreement with the experimental data in a femtosecond range, and it is linked to the dependence for nanosecond pulses.
Effect of Ru thickness on spin flop in synthetic spin valves
Authors: Lu Zhengqi , Pan Genhua , Al-Jibouri Abdul , Hoban Mike
Published: July 2002

Synthetic spin valves with structure of sub/Ta/NiFe/IrMn/AP2/Ru/AP1/Cu/CoFe/Ta were systematically studied by varying the Ru thickness in the artificial antiferromagnetic structure. Spin-valve films were deposited using a Nordiko 9606 PVD system and annealed in a magnetic field of 6 kOe parallel to the easy axis of the free layer. MR(H) and M(H) curves of these spin valves were experimentally measured and quantitatively simulated using Boltzmann transport equation and coherent magnetization rotation model. The corresponding interlayer exchange coupling and pinning angle of these spin valves were determined from simulation. Both experimental measurement and simulation results have shown that the characteristics of synthetic spin valves depend strongly on the thickness of Ru. The pinning field for spin valves with ultrathin Ru layer was canted. This was evidenced in the reduction of the measured MR ratio and shape of MR curve when Ru is thinner than 0.5 nm. It is concluded that the pinning angle is determined by the spin–flop angle of one of the pinned layers (AP2) at spin–valve annealing temperature, which is in turn determined by the final balance between the interlayer antiferromagnetic coupling field and the external applied magnetic field during annealing.
Stability of pinning centers in fatigued lead–zirconate–titanate
Authors: Nuffer Jürgen , Lupascu Doru C. , Rödel Jürgen
Published: July 2002

After bipolar electric fatigue of bulk lead–zirconate–titanate ceramics, the well known reduced remnant polarization values are observed as well as an asymmetric strain hysteresis reflecting a macroscopic offset polarization. Thermal treatment and high electric fields each liberate the domain system from the microscopic defects responsible for the macroscopic offsets. High electric fields (4 Ec) only allow one to remove the type of pinning center inducing an offset polarization, while thermal treatment also removes more stable agglomerates and rejuvenates the material to almost its original properties.
Direct observation of photoinduced magnetization in a relaxor ferromagnet
Authors: Okimoto Y. , Ogimoto Y. , Matsubara M. , Tomioka Y. , Kageyama T. , Hasegawa T. , Koinuma H. , Kawasaki M. , Tokura Y.
Published: July 2002

Persistent ferromagnetic magnetization (M) as large as 0.6 μB per Mn site can be induced by irradiating visible laser pulses on a thin film of 1% Cr-doped Pr0.5Ca0.5MnO3. The enhancement of M upon the photoexcitation was also confirmed by the scanning superconducting quantum interference device microscope as an increase of magnetic field from ferromagnetic domains. The observed photoinduced transition from a metastable charge/orbital ordered state into a ferromagnetic one is likely to be assisted by the microscopic phase separation characteristic of such a relaxor ferromagnet as the present Cr-doped manganite.
Above-room-temperature optically pumped 4.12 μm midinfrared vertical-cavity surface-emitting lasers
Authors: Zhao F. , Wu H. , Jayasinghe Lalith , Shi Z.
Published: July 2002

Optically pumped lead salt vertical-cavity surface-emitting lasers (VCSELs) with a nine period PbSe/PbSrSe quantum well active region operating above room temperature in pulsed mode are reported. The gain peak and cavity mode of the VCSEL structure are in resonance at 300 K. A power output of 40 mW is obtained at room temperature and it does not show saturation. The room-temperature threshold pump density is 200 kW/cm2. The lasing wavelength of λ=4.12 μm remains nearly constant over a temperature range of 280–310 K.
Influence of laser pulse duration on relativistic channels
Authors: Delfin C. , Lokhnygin V. , Mauritsson J. , Sjögren A. , Wahlström C.-G. , Pukhov A. , Tsakiris G. D.
Published: July 2002

A high-power (10 TW) laser is employed to generate relativistic channels in an underdense plasma. The lengths of the channels are measured by imaging the Thomson-scattered light, and the gas densities are determined through the forward Raman scattered light. The laser-pulse parameters are varied and their impact on the channel formation is studied. It is found that increasing the laser pulse duration in many cases produces longer channels, even as this implies reducing the laser peak power. A theoretical discussion is presented, proposing an explanation of the experimental results.