bluuebunny/crossref_metadata_2025 huggingface dataset:
Observation of stimulated emission in an ultrashort-period nonsymmetric GaAs/AlAs superlattice
Authors: Litovchenko V. G. , Korbutyak D. V. , Bercha A. I. , Kryuchenko Yu. V. , Krylyuk S. G. , Grahn H. T. , Hey R. , Ploog K. H.
Published: July 2002

Nonsymmetric short-period GaAs/AlAs superlattices, for which the well thickness is at least a factor of 2 larger than the barrier thickness, have been shown to exhibit a direct band gap for any well thickness. These superlattices are characterized by an enhanced intensity of the luminescence as compared to their symmetric indirect-gap counterparts with the same well width, and, thus, may be used as light-emitting devices, in particular, as low-threshold lasers in the red visible spectrum. This conjecture is supported by the observation of stimulated emission at T=80 K for a GaAs/AlAs superlattice with six monolayers well and three monolayers barrier width.
Ultralow frequency waves and total electron content fluctuations
Authors: Hammond Richard T. , Meier Ben , Atkins Scott , Davis Jon , Bobb Lloyd
Published: July 2002

Low frequency electromagnetic waves in the ionosphere are examined, and we calculate the refraction angles and other properties as these waves propagate through the ionosphere. The correlation to fluctuations in the total electron content is derived, and the results are compared to observed results. © 2001 American Institute of Physics.
Dynamic process during axial magnetic compression of field-reversed configuration for equilibrium shape control
Authors: Kitano K. , Maeshima S. , Okada S. , Goto S.
Published: July 2002

Adiabatic magnetic compression experiments on a field-reversed configuration (FRC) plasma are reported. The compression is performed on a long FRC held in a straight mirror field. The separatrix length of the FRC is limited by the distance between the magnetic mirrors. The mirror distance is compressed in time to 35% of the original one by the compression coil installed inside the chamber, and the separatrix length is shortened to 38%. The separatrix radius of the compressed FRC increases by 56% and the aspect ratio (separatrix length/separatrix diameter) changes from 12.6 to 4.1. Magnetic probes with the compensation circuit are utilized to investigate the dynamic transition phase during the compression. The transition process is found from these measurements to be divided into three stages, where a new equilibrium state is achieved in the final stage.
The effect of substituents on the vibronic structure of the electronic spectra of α,ω-dithienylpolyenes: A computational study
Authors: Negri Fabrizia , Zgierski Marek Z.
Published: July 2002

We present a quantum chemical investigation of the vibronic structure in the electronic spectra of three α,ω-dithienylpolyenes. Equilibrium structures and vibrational force fields of ground and excited electronic states are computed at semiempirical and ab initio levels of theory. The molecular parameters are employed to calculate displacement parameters for totally symmetric modes which are then used to model the vibronic structure of emission and absorption spectra. The observed spectral features are well reproduced by the computed spectra and two characteristic frequency regions are identified for the emission spectra. The first, above 1500 cm−1, is due to the C=C stretching activity typical of trans polyenes. In addition, the strong activity in emission around 1430 cm−1 is rationalized by considering that the two etherocyclic rings add, to the central polyenic chain, a conjugated carbon skeleton with cis linkages. In analogy with the oligomers of thiophene, the cis linkages, along with vibronic coupling, are responsible for the remarkable activity observed in emission in this frequency region.
RAIL: Reaction-path and variational rate constants using the integrated molecular orbital with harmonic cap method
Authors: Espinosa-Garcı́a J. , Corchado José C.
Published: July 2002

We introduce and test a new computational approach to calculating the intrinsic reaction-path and, from there, kinetic and dynamic information for reactions involving the breaking–forming of covalent bonds in large molecules. It is an extension of the integrated molecular orbital method with harmonic cap to follow the intrinsic reaction-path. We have named the method RAte constant calculation with Integrated Levels (RAIL). We show that the new scheme reproduces the behavior of a benchmark calculation for some kinetic and dynamic properties, such as, the minimum energy path and ground-state vibrationally adiabatic curves, the localization of the bottleneck on the reaction path, and the non-Arrhenius behavior of the rate constants with tunneling effect over the temperature range 298–1500 K. Moreover, the new scheme shows little dependency on which “low-level” is used in the computation.
The phase diagram of molten randomly grafted copolymers
Authors: Qi Shuyan , Chakraborty Arup K.
Published: July 2002

We study the microphase segregation of molten randomly grafted copolymers (RGCs) using a Landau field theory. Under one wave number approximation, we find three equilibrium ordered microphases: lamellar phase (LAM), hexagonal cylinder phase (HEX), and bcc sphere phase (BCC). The stability of these phases strongly depends on the architectural parameters describing the RGC chains (e.g., the backbone length, the branch length, and the number of branches). Our calculation reveals that RGCs with high average composition of backbone monomers or with low branching density tend to form LAM microstructures. For a small average composition of backbone monomers, HEX and BCC microphases appear in turn with increasing branching density. Independent of the architectural parameters and composition, the disorder to order transition for molten RGCs is always from the disordered phase to the LAM microphase. The physical reasons underlying this behavior and experimentally testable predictions are discussed.
Deactivation kinetics of supersaturated boron:silicon alloys
Authors: Luo Weiwei , Yang Shenzhi , Clancy Paulette , Thompson Michael O.
Published: July 2002

The effect of laser annealing on the electrical activity of boron-doped silicon wafers has been investigated as a function of boron concentration, annealing time, and annealing temperature (from 600 °C to 1050 °C). Metastable supersaturated alloys were produced by the laser annealing of ion-implanted Si 〈100〉 wafers using an excimer laser with a pulse duration of 30 ns. The extent of dopant activation, deactivation, and tendency towards precipitation were subsequently studied following rapid thermal annealing in an argon ambient using a four-point probe of the sample resistance. Sheet resistances as low as 15 Ω/□ were achieved in 200 nm layers. Following laser anneals, boron atoms remained active at concentrations of 7.5×1020/cm3 up to 800 °C for 210 s. A two-mode relaxation model including defect association and precipitation was proposed to describe the annealing behavior. These results show that laser processing can produce metastable B-doping levels, stable to moderate thermal processing, at concentrations adequate for all anticipated device structures.
Structure of the exact wave function. III. Exponential ansatz
Authors: Nakatsuji Hiroshi
Published: July 2002

We continue to study exponential ansatz as a candidate of the structure of the exact wave function. We divide the Hamiltonian into ND (number of divisions) parts and extend the concept of the coupled cluster (CC) theory such that the cluster operator is made of the divided Hamiltonian. This is called extended coupled cluster (ECC) including ND variables (ECCND). It is shown that the S(simplest)ECC, including only one variable (ND=1), is exact in the sense that it gives an explicit solution of the Schrödinger equation when its single variable is optimized by the variational or H-nijou method. This fact further implies that the ECCND wave function with ND⩾2 should also have a freedom of the exact wave function. Therefore, by applying either the variational equation or the H-nijou equation, ECCND would give the exact wave function. Though these two methods give different expressions, the difference between them should vanish for the exact wave function. This fact solves the noncommuting problem raised in Paper I [H. Nakatsuji, J. Chem. Phys. 113, 2949 (2000)]. Further, ECCND may give more rapidly converging solution than SECC because of its non-linear character, ECCND may give the exact wave function at the sets of variables different from SECC. Thus, ECCND is exact not only for ND=1, but also for ND⩾2. The operator of the ECC, exp(S), is an explicit expression of the wave operator that transforms a reference function into the exact wave function. The coupled cluster including general singles and doubles (CCGSD) proposed in Paper I is an important special case of the ECCND. We have summarized the method of solution for the SECC and ECCND truncated at order n. The performance of SECC and ECC2 is examined for a simple example of harmonic oscillator and the convergence to the exact wave function is confirmed for both cases. Quite a rapid convergence of ECC2 encourages an application of the ECCND to more general realistic cases.
An ab initio and experimental study of vibrational effects in low energy O++C2H2 charge-transfer collisions
Authors: Fukuzawa Kaori , Matsushita Toshio , Morokuma Keiji , Levandier Dale J. , Chiu Yu-hui , Dressler Rainer A. , Murad Edmond , Midey Anthony , Williams Skip , Viggiano Albert A.
Published: July 2002

Theoretical and experimental studies are performed to elucidate the low energy charge-transfer dynamics of the reaction, O+(4S)+C2H2(X 1Σg+)→O+C2H2+. In particular, the role of the low-frequency acetylene bending modes (612 and 730 cm−1) in promoting charge transfer was examined. High-temperature guided-ion beam measurements are carried out over the energy range from near-thermal to 3 eV at 310 and 610 K. The charge-transfer cross sections are found to decrease up to 0.5 eV, to have a constant value at intermediate energies between 0.5 and 1.5 eV, and then to dramatically increase above a threshold of a spin-allowed process determined to be at 1.7 eV. A bending vibrational enhancement of ∼8 is observed at intermediate energies. Thermal energy rate co-efficients are measured in a variable temperature-selected ion flow drift tube apparatus from 193 to 500 K. At each temperature, a negative energy dependence is observed. In order to elucidate the reaction mechanism in detail, high level ab initio calculations using Complete Active Space Self-Consistent Field and Multi-Reference Single- and Double-excitation Configuration Interaction methods have been performed. The results indicate that the charge transfer reaction occurs at an early stage via nonadiabatic transition between quartet and doublet states. There is a weak van der Waals minimum at the entrance channel between O+(4S) and C2H2 with the relative energy of −1.51 kcal/mol. The minimum of the quartet/doublet crossing seam (Q/D MSX), where the spin-forbidden nonadiabatic transition is most likely to take place, lies very near this minimum at RCO=4.06 Å, RCC=1.20 Å, and ∠CCH=166.6° with a relative energy of −1.48 kcal/mol. After the nonadiabatic transition, the system propagates on the doublet surface to reach the exothermic O(1D)+C2H2+(X̃ 2Πu) products. No energy barrier exists on the reaction pathway, strongly suggesting that the reaction should occur at low energy with a negative energy dependence, which is consistent with the experiment. The Q/D MSX has a bent acetylene moiety, which suggests that the excitation in bending modes will enhance the reaction, in agreement with the experiment.
From classical to quantum mechanics: “How to translate physical ideas into mathematical language”
Authors: Bergeron H.
Published: July 2002

Following previous works by E. Prugovečki [Physica A 91A, 202 (1978) and Stochastic Quantum Mechanics and Quantum Space–time (Reidel, Dordrecht, 1986)] on common features of classical and quantum mechanics, we develop a unified mathematical framework for classical and quantum mechanics (based on L2-spaces over classical phase space), in order to investigate to what extent quantum mechanics can be obtained as a simple modification of classical mechanics (on both logical and analytical levels). To obtain this unified framework, we split quantum theory in two parts: (i) general quantum axiomatics (a system is described by a state in a Hilbert space, observables are self-adjoints operators, and so on) and (ii) quantum mechanics proper that specifies the Hilbert space as L2(Rn); the Heisenberg rule [pi,qj]=−iℏδij with p=−iℏ∇, the free Hamiltonian H=−ℏ2Δ/2m and so on. We show that general quantum axiomatics (up to a supplementary “axiom of classicity”) can be used as a nonstandard mathematical ground to formulate physical ideas and equations of ordinary classical statistical mechanics. So, the question of a “true quantization” with “ℏ” must be seen as an independent physical problem not directly related with quantum formalism. At this stage, we show that this nonstandard formulation of classical mechanics exhibits a new kind of operation that has no classical counterpart: this operation is related to the “quantization process,” and we show why quantization physically depends on group theory (the Galilei group). This analytical procedure of quantization replaces the “correspondence principle” (or canonical quantization) and allows us to map classical mechanics into quantum mechanics, giving all operators of quantum dynamics and the Schrödinger equation. The great advantage of this point of view is that quantization is based on concrete physical arguments and not derived from some “pure algebraic rule” (we exhibit also some limit of the correspondence principle). Moreover spins for particles are naturally generated, including an approximation of their interaction with magnetic fields. We also recover by this approach the semi-classical formalism developed by E. Prugovečki [Stochastic Quantum Mechanics and Quantum Space–time (Reidel, Dordrecht, 1986)].
Geometries and spectroscopic properties of germanium and tin hexamers (Ge6, Ge6+, Ge6−, Sn6, Sn6+, and Sn6−)
Authors: Zhao Cunyuan , Balasubramanian K.
Published: July 2002

Ground and excited electronic states of neutral, cationic and anionic germanium and tin hexamers (Ge6 and Sn6) are investigated. Different geometries such as tetragonal bipyramid (D4h,D2h) and edge-caped trigonal bipyramid (C2v) (ECTBP) were studied. We have employed a number of high level electron correlation techniques such as large scale multireference singles+doubles configuration interaction (MRSDCI) computations that included up to 16 million configurations, complete active-space multiconfiguration self-consistent filed (CAS-MCSCF), and Møller-Plesset (MP2) techniques to investigate the low-lying electronic states, their geometries and energy separations. A 1A1g tetragonal bipyramid structure (D4h symmetry) is found as the ground state for both Ge6 and Sn6, in accord with the previously suggested experimental assignments. The excitation energies, atomization energies, ionization potentials and vertical and adiabatic electron affinities are computed and compared with the experimental results. Our computations of the excited states of these species have facilitated assignment of the anion photodetachment spectra of the anions of these clusters and comparison with the photoionization spectra of the neutral species.
Formation and stability of free charged lanthanum hexaboride clusters at field evaporation
Authors: Boustani I. , Buenker R. , Shrednik V. N. , Loginov M. V. , Korsukova M. M. , Gurin V. N.
Published: July 2002

Field evaporation of LaB6 single crystals resulting in a large variety of ionized clusters of LaBnm+ type, where n=1,2,…,6 and m=1, 2, 3, 4, is investigated in a collaborative experimental and theoretical study. The aim of the present work was to compare the measured appearance probability of various clusters in a flux with the calculated energetic stability of corresponding free clusters. The experiments were carried out with a time-of-flight atom probe. The appearance probability of cluster ions in the atom probe spectra was estimated by calculating its rating in many selected spectra. The methods for determining such ratings have been developed and discussed. The theoretical calculations are based on ab initio quantum chemical Hartree–Fock self-consistent field and configuration interaction methods. A double ζ basis set was used to determine the cluster geometries, energies, and relative stability. For the inner-shell electrons of lanthanum we have used relativistic compact effective potentials. The geometry optimization was carried out using the analytical gradient method. The experimentally obtained ratings of some ionic species were compared with the energetic stability of free clusters calculated as described previously. The main conclusion of the present study is that unstable clusters, e.g., LaB3+, or almost unstable LaBnm+ clusters were preferably observed in the ion flux. The physical explanation of this phenomenon lies in the possibility of a stable cluster to redistribute its interatomic bonds and thus to remain attached to the surface. An unstable cluster does not have this characteristic and as a result this must leave the surface.
Noncontact surface tension measurement by drop rotation
Authors: Rhim Won-Kyu , Ishikawa Takehiko
Published: July 2002

Validity of the surface tension measurement technique that was proposed by Elleman et al. was experimentally verified. The technique was based on Brown and Scriven’s work on the shape evolution of rotating drops. Molten tin and aluminum drops were levitated in high vacuum by the electrostatic levitator and rotated by applying a rotating magnetic field. This technique offers an alternative technique for those liquids where the drop oscillation technique cannot be used. As a demonstration, the technique was applied to a glass-forming alloy (Zr41.2Ti13.8Cu12.5Ni10.0Be22.5) and its surface tension was measured down to the lower temperature where the drop oscillation technique could not be applied due to its high viscosity.
Conformational effects on optical charge transfer in the emeraldine base form of polyaniline from electroabsorption measurements and semiempirical calculations
Authors: Premvardhan Lavanya L. , Wachsmann-Hogiu Sebastian , Peteanu Linda A. , Yaron David J. , Wang Pen-Cheng , Wang Wei , MacDiarmid Alan G.
Published: July 2002

The electroabsorption (Stark) spectrum of the emeraldine base form of polyaniline (EB) in a polymethyl methacrylate (PMMA) matrix at room temperature is presented. For the lowest-energy (1.98 eV) band, the absolute value of the change in dipole moment on excitation (|Δμ⃗|) is 6.1 D and the trace of the change in polarizability (Tr Δα⇊) is 200 Å3. Similar values were obtained for the corresponding electronic transition of a five-ring oligomer of EB that is symmetrically terminated with phenyl rings, also in a PMMA matrix. In order to understand the origin of the substantial value of |Δμ⃗| measured for these nominally symmetric species, a series of calculations were performed in which the geometries of model oligomers were varied in a systematic fashion and resulting effects on the values of |Δμ⃗| were determined. Numerous low-energy conformers were identified by this method having substantial values of |Δμ⃗|, suggesting that the large |Δμ⃗| measured experimentally results from conformational heterogeneity. Our findings suggest that Stark spectroscopy is a useful probe of structural disorder in this technologically important polymer in the solid state.
Treatment of soft threshold in impact ionization
Authors: Tan C. H. , David J. P. R. , Rees G. J. , Tozer R. C. , Herbert D. C.
Published: July 2002

The avalanche multiplication and excess noise properties of a range of submicron Si diodes have been measured and analyzed using a model for impact ionization which includes the effect of dead space, modified to allow for a gradual onset of ionization, with realistic threshold energies. Good agreement is achieved between the predictions of this “soft dead space” model and measurements of multiplication and excess noise, both on a range of submicron diodes with uniform electric fields and also on a p+n diode with a highly nonuniform electric field.
Electromigration-induced failure of single layered NiFe Permalloy thin films for a giant magnetoresistive read head
Authors: Bae Seongtae , Judy Jack H. , Tsu I-Fei , Murdock Edward S.
Published: July 2002

Electromigration-induced failures have been investigated in single layered NiFe Permalloy thin films, which are used in giant magnetoresistance (GMR) read heads in high-density magnetic recording disk drives. The time-to-failure and median-time-to-failure (MTTF, t50) of NiFe thin films were found to be strongly dependent on the applied current density, film thickness, and ambient temperature. The activation energy (ΔQ) of NiFe films derived from the “Black equation” was found to be 0.8 eV. The MTTF and standard deviation (σ) of a NiFe thin film electromigration test stripe with 10 nm thickness, 5 μm width, and 20 μm length were 3.74 and 1.74 h, respectively, at 160 °C ambient temperature. Typical electromigration failures such as “voids and cracks” were observed at the cathode or center region of NiFe film test stripes. These results suggest that the electromigration-induced failures are one of the crucial factors in determining the reliability of GMR spin-valve read heads used in high-density magnetic recording disk drives.
Stress dependence of the electromechanical properties of 〈001〉-oriented Pb(Mg1/3Nb2/3)O3–PbTiO3 crystals: Performance advantages and limitations
Authors: Viehland D. , Ewart L. , Powers J. , Li J. F.
Published: July 2002

The electromechanical properties of 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 〈001〉-oriented crystals have been investigated under unaxial stress (σ). The results demonstrate enhanced piezoelectric coefficients and electroacoustic power densities with increasing σ. However, these increases are accompanied by significant increases in hysteretic losses. The results also demonstrate that the electromechanical coupling coefficient (∼0.92) is relatively independent of σ between 0 and 6×107 N/m2. Future transducer designs can rely on piezocrystals offering a k33 of 0.92, 10 dB more power, and low losses under operational conditions of σ<107 N/m2 and 5<T<40 °C.
Sample holder for high-temperature vibrating sample magnetometer
Authors: Mikhov M.
Published: July 2002

A sample holder for vibrating sample magnetometer operating in the temperature range between 300 and 1100 K is described. It permits bulk and powder samples to be fixed safely and replaced easily in the magnetometer. The temperature hysteresis, when magnetization is measured upon heating and cooling with a sweep of 3 K/min is less than 1 K at 625 K.
A technique to investigate inhomogeneity in materials: An arrangement of microtip and scanning electron microscope
Authors: Paul S. , Clough F. J
Published: July 2002

Amorphous and polycrystalline materials show different electrical properties when the contact area is in the submicron range. A large number of instruments (such as combinations of scanning electron microscope and scanning tunneling microscope) have been employed for such an investigation. A distinct disadvantage of these instruments is that they are rather expensive. In view of this, we have developed an instrumentation, which can be used to measure electrical behavior of any material in the submicron scale. This was achieved by installing a simple tip-holding arm in the scanning electron microscope; electrical meters (picoammeter and an LCR bridge) were used to investigate the behavior. We have studied amorphous carbon films grown in our laboratory, using this technique.
Sensitivity enhancement of Quantum Design dc superconducting quantum interference devices in two-stage configuration
Authors: Mezzena R. , Vinante A. , Falferi P. , Vitale S. , Bonaldi M. , Prodi G. A. , Cerdonio M. , Simmonds M. B.
Published: July 2002

The energy sensitivity of a direct current (dc) superconducting quantum interference device (SQUID) can be improved if it is operated in a two-stage configuration. Employing this technique, a commercial dc SQUID system was modified and made competitive with other sensors especially designed for very low noise applications. We report the noise measurements performed in the temperature range 4.2 K–25 mK. At 4.2 K, the coupled energy sensitivity obtained with the two-stage dc SQUID was approximately ten times better than with a conventional readout electronics. The noise energy decreases linearly until approximately 300 mK, in good agreement with theoretical previsions. At lower temperature the hot-electron effect produces a saturation and the best energy sensitivity measured with open input coil is 35 ℏ.
Arrays of epitaxial Co submicron particles: Critical size for single-domain formation and multidomain structures
Authors: Kazakova O. , Hanson M. , Blomquist P. , Wäppling R.
Published: July 2002

Arrays of Co particles with well-defined geometry and lateral sizes in the range 0.1–0.5 μm were prepared by electron beam lithography and studied by magnetization measurements and magnetic force microscopy. The starting materials were epitaxial Co films, with in-plane magnetic anisotropy, having thicknesses in the range 10–50 nm. The particles change from multidomain behavior to that of stable single domains in zero field, as the lateral dimension decreases within these ranges. For particles of circular shape, i.e., without in-plane shape anisotropy, the critical size for forming single domains was found to be about 200 nm in lateral extension and 20 nm thickness. Elliptical particles prepared with 20 nm thickness and lateral dimensions 150 by 450 nm were found to be stable single domains, of good uniformity, in zero field. The switching of these particles from one magnetization direction along the long axis to the other, requires an applied field BS≈(100±20)mT. A comparison with other published results leads to the conclusion that although the critical volume for single domains may be about the same, the final microscopic magnetic structure and the processes for magnetic switching for particles in the actual range of sizes, are strongly governed by the intrinsic properties of the Co films, which may vary widely.
Processing of carbon-fiber-reinforced Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 bulk metallic glass composites
Authors: Kim C. P. , Busch R. , Masuhr A. , Choi-Yim H. , Johnson W. L.
Published: July 2002

Carbon-fiber-reinforced bulk metallic glass composites are produced by infiltrating liquid Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 into carbon fiber bundles with diameter of the individual fiber of 5 μm. Reactive wetting occurs by the formation of a ZrC layer around the fibers. This results in a composite with a homogeneous fiber distribution. The volume fraction of the fibers is about 50% and the density of the composite amounts to 4.0 g/cm3.
Admittance dispersion of n-type GaAs/Ga(As, N)/GaAs heterostructures grown by molecular beam epitaxy
Authors: Krispin P. , Spruytte S. G. , Harris J. S. , Ploog K. H.
Published: July 2002

The electrical admittance of rectifying metal–semiconductor contacts on n-type GaAs/Ga(As, N)/GaAs heterostructures depends strongly on frequency and temperature. The distinct dispersion is due to the relatively high diffusion barrier around the Ga(As, N) layer. As long as the admittance is controlled by electrons inside the Ga(As, N) layer, their response to the ac electric field is dependent on frequency and temperature. Under appropriate conditions, capacitance–voltage measurements can be used to examine depth-resolved electrical characteristics of n-type GaAs/Ga(As, N)/GaAs heterostructures. The experimental depth profiles of the carrier concentration are compared with calculations based on self-consistent solutions of the Poisson equation. For 3% GaN mole fraction, the conduction band offset between GaAs and Ga(As, N) is found to be −(0.40±0.01) eV, i.e., about 95% of the total band gap difference. The heterointerfaces are of type I. At the Ga(As, N)-on-GaAs interface, negative charges of about 1.3×1011 cm−2 are observed, which are not removed by postgrowth annealing. The as-grown GaAs-on-Ga(As, N) interface is free of fixed charges. However, acceptor-like interfacial traps are generated at this interface by annealing, probably due to the conversion of those donor-like levels, which were detected at the inverted interface of as-grown p-type GaAs/Ga(As, N)/GaAs heterostructures.
Scattering matrix approach to electronic dephasing in long-range electron transfer
Authors: Li Xin-Qi , Yan YiJing
Published: July 2002

Based on the Büttiker dephasing model, we propose an analytical scattering matrix approach to the long-range electron transfer phenomena. The present efficient scheme smoothly interpolates between the superexchange and the sequential hopping mechanisms. Various properties such as the drastic dephasing-assisted enhancement and turnover behaviors are demonstrated in good agreement with those obtained via the dynamical reduced density-matrix methods. These properties are further elucidated as results of the interplay among the dephasing strength, the tunneling parameter, and the bridge length of the electron transfer system.
Laser pointing stability measured by an oblique-incidence optical transmittance difference technique
Authors: Gray J. , Thomas P. , Zhu X. D.
Published: July 2002

We describe an oblique-incidence optical transmittance technique for determining the pointing stability of a laser. In this technique, we follow the angular drift of a monochromatic laser beam by measuring the relative changes in transmittance through a parallel fused quartz window for s and p polarized components of the beam in response to the drift. This method is shown in the present experiment to have the sensitivity to detect angular changes in the range of 2 μ radians (1 μ radian=10−6 radian). To demonstrate this technique, we measured the angular drifts of two commercial intensity-stabilized He–Ne lasers.
Calculation of the free energy of polymeric systems in terms of compositional and orientational order parameters
Authors: Fukuda Jun-ichi , Yokoyama Hiroshi
Published: July 2002

We present a general framework for the calculation of the free energy functional of inhomogeneous polymeric systems in terms of the compositional scalar order parameter φ and the orientational order parameters of second-rank tensor Qij. Our formalism is a straightforward extension of the density functional theory developed by Tang and Freed [J. Chem. Phys. 94, 1572 (1991)] and the contribution of the conformational entropy is given in terms of the gradient expansion. As a demonstration, we give an exact expression for the free energy of Gaussian chains up to second order in the gradients.
Fizeau interferometer for phase shifting interferometry in ultrahigh vacuum
Authors: Kaiser S. , Maier T. , Grossmann A. , Zimmermann C.
Published: July 2002

We describe a phase shifting interferometer which is based on a frequency-tunable diode laser in a Littrow configuration and is suitable for operation in ultrahigh vacuum. A wave front distortion measurement of a thin glass plate with a repeatability of λ/1000 is presented.
Anisotropic photodissociation of CH3Cl+•
Authors: Won Dong Shin , Kim Myung Soo , Choe Joong Chul , Ha Tae-Kyu
Published: July 2002

Photodissociation of the methyl chloride ion has been investigated using mass-analyzed ion kinetic spectrometry (MIKES). The MIKE spectrum for the chlorine atom loss from the methyl chloride ion has been measured as a function of the laser polarization angle at 357, 488.0, and 514.5 nm. The anisotropy parameters and kinetic energy release distributions have been determined. At all the wavelengths used, an anisotropic dissociation (β=1.2) in the repulsive excited electronic state has been observed. Results from quantum chemical calculations carried out at the various levels suggest that the methyl chloride ion is excited to the first excited electronic state Ã, and dissociates repulsively in this state.
Applications of photoinduced electron spin polarization at room temperature to microwave technology
Authors: Blank Aharon , Levanon Haim
Published: July 2002

We present a method for controlling the bulk permeability via spin polarization generated by light excitation. This process involves the magnetic interaction of photoexcited triplets with stable radicals in liquid solution. The resulting non-Boltzmann spin population of the stable radical is reflected by a significant change in the permeability of the chemical system. We demonstrate how these light-driven changes result in conspicuous changes in the reflection coefficient (amplitude and phase) of a microwave cavity in which the active chemical system is placed. This effect can lead the way to ultralow noise microwave amplifiers and low-loss microwave phase shifters, operating at room temperature with very low spin temperature (<16 K). Moreover, the nonlinear character of the phenomenon can be utilized for devices, which protect sensitive instrumentation from a strong destructive microwave pulse.
The asymptotic decay of pair correlations in the path-integral quantum hard-sphere fluid
Authors: Bailey Lorna E. , Sesé Luis M.
Published: July 2002

A study of the asymptotic decay of the pair radial correlations that can be defined in the path-integral quantum hard-sphere fluid is presented. These distinct quantum pair correlations arise from the breaking of the classical spherical symmetry of the particles under the quantum effects. The three types of correlations analyzed are the so-called linear response, instantaneous and center-of-mass, which correspond to distinct averaging criteria over the thermal packets associated with the quantum particles. The basic methodology employed to perform this analysis, based on the fixing of the complex poles of the static structure factor, was put forward by Tago and Smith [Can. J. Phys. 55, 761 (1977)] and independently by Evans et al. [Mol. Phys. 80, 755 (1993); J. Chem. Phys. 100, 591 (1994)]. To apply this method it is required the knowledge of the direct correlation functions connected to the pair radial correlations involved, which over a wide range of conditions are available in the literature [J. Chem. Phys. 108, 9086 (1998); Mol. Phys. 99, 585 (2001)]. In the quantum hard-sphere fluid both pure imaginary and complex conjugate poles are possible, and the properties of this system depend on the density and the temperature. However, no Fisher–Widom line has been obtained. The decay of the correlations in this fluid is of the exponentially damped oscillatory type, in agreement with the purely repulsive character of the interparticle potential. The linear response and instantaneous decay properties follow the same pattern, albeit slight differences can be observed. Comparison with the Percus–Yevick classical results and with those that can be derived from Tarazona and Vicente’s model [Mol. Phys. 56, 557 (1985)] is made. Besides, it is proven that the above theory of asymptotic behavior evinces the capability for resolving fine-drawn features of quantum changes of phase in the hard-sphere system.
A fast x-ray photoelectron spectroscopy study of the adsorption and temperature-dependent decomposition of propene on Ni(100)
Authors: Whelan Caroline M. , Neubauer Ralf , Borgmann Dieter , Denecke Reinhard , Steinrück Hans-Peter
Published: July 2002

Using synchrotron radiation, the adsorption and decomposition of propene (C3H6) on the Ni(100) surface has been investigated in situ by time-resolved and temperature-programmed x-ray photoelectron spectroscopy. At 105 K, high-resolution C 1s spectra indicate precursor mediated occupation of a single adsorption state from submonolayer to monolayer coverage with evidence of adsorbate–adsorbate interactions and dispersed phase growth. High exposures lead to the formation of multilayers which desorb above 105 K leaving a chemisorbed monolayer. Between 105 and 150 K, a shift of the binding energies in the C 1s spectra is attributed to the transition from π- to di-σ-bonded propene. An abrupt change in the photoemission spectra occurs at 200 K due to the conversion of di-σ-bonded propene to a C3 intermediate containing a methyl group. Formation of this C2HxCH3 surface species is complete at 300 K and is immediately proceeded by dehydrogenation to carbidic carbon which is the final decomposition product above 370 K.
Thermal stability of antiferromagnetically coupled multilayers with Ru/Co and Cu/Co interfaces
Authors: Bal K. , van den Berg H. A. M. , Deck D. , Rasing Th.
Published: July 2002

The long-term thermal stability of the hard artificial antiferromagnetic subsystem (AAF) found in giant magnetoresistance (GMR) and tunnel magnetoresistance sensors based on Cu/Co/Ru/Co/Cu has been investigated using the complete sensor stack as well as of selected single layers. A striking asymmetry between both types of Co layers in the single Co layer samples with respect to magnetic and transport properties is observed, which show a different dynamics for each upon annealing. Directly after deposition, the Co/Ru interface is stronger mixed as compared to the Ru/Co interface, due to difference in sputter energies and order of growth. The different behavior upon annealing has been interpreted by the difference in concentration gradients and lattice mismatch at the interfaces. Despite the strong mixing found at the Co/Ru interface, the impact on the characteristics of the AAF, like the GMR signal and the rigidity is limited. Several causes, like the cohesion in the AAF induced by its Co seed layer on the Cu and the limited availability of Ru, are pointed out as likely origins of the improved stability of the AAF as compared to the single Co layers. Possibilities to even further improve the rigidity by carefully matching of anneal procedure and layer thicknesses are discussed.
Structure and properties of the YBa2Cu3O7−x/LaAlO3 superlattices
Authors: Lebedev O. I. , Hamet J.-F. , VanTendeloo G. , Beaumont V. , Raveau B.
Published: July 2002

Superconductor/insulator superlattices have been prepared using pulsed laser deposition. Two different [(YBa2Cu3O7−x)n/LaAlO3)m]×N superlattices with N=15 and N=30 have been studied by high resolution electron microscopy. Both supperlattices show a zero resistivity transition at T=82 K and T=86 K, respectively. High resolution transmission electron microscopy leads us to propose two models for the YBa2Cu3O7−x–LaAlO3 interface layers. These models describe the interfaces as a stacking of CuO–AlO layers and BaO–AlO layers. High resolution transmission electron microscopy observations also reveal the presence of antiphase boundaries in the lanthanum aluminate deposited layers. Their formation is attributed to partial stress relaxation in the superlattice.
On the direct evaluation of the equilibrium distribution of clusters by simulation. II
Authors: Kusaka Isamu , Oxtoby David W. , Wang Zhen-Gang
Published: July 2002

We clarify some of the subtle issues surrounding the observational cluster method, a simulation technique for studying nucleation. The validity of the method is reaffirmed here. The condition of the compact cluster limit is quantified and its implications are elucidated in terms of the correct enumeration of configuration space.
Random dopant model for three-dimensional drift-diffusion simulations in metal–oxide–semiconductor field-effect-transistors
Authors: Sano Nobuyuki , Tomizawa Masaaki
Published: July 2002

We investigate the dopant model employed in drift-diffusion device simulations for the study of statistical threshold voltage variations associated with discrete random dopants. It is pointed out that the conventional dopant model, when extended to the extreme “atomistic” regime, becomes physically inconsistent with the length-scale presumed in drift-diffusion simulations. Splitting the Coulomb potential of localized dopants between the long-range and short-range parts, we propose a dopant model appropriate for three-dimensional drift-diffusion simulations.
Four-wave mixing in two-level atoms with stochastically modulated transition frequencies
Authors: Green Jennifer C. , Kemple Marvin D. , Vemuri Gautam
Published: July 2002

We investigate nonlinear wave mixing in two-level atoms wherein, using a dressed state formalism, it is shown that the signal arises as a consequence of quantum interference between relevant probability amplitudes for photon emission. We then report a theoretical study of the modifications that are induced in the four-wave mixing signals when the transition frequency of the atoms is stochastically modulated. Such modulations, which arise from perturbations in the immediate environment of the atoms, are shown to cause significant alterations in the nonlinear response of the atoms. We also describe how one can, via an investigation of the four-wave mixing signal, differentiate between Markov and non-Markov fluctuations in the transition frequency of the two-level system. The calculations rely on a recently developed Monte Carlo procedure that can be used to obtain exact solutions to a wide variety of spectroscopic problems, with minimal restrictions on atom, field, and noise parameters.
Optical investigations of β-FeSi2 with and without Cr addition
Authors: Rebien M. , Henrion W. , Stauss P. , Diesner K. , Panknin D.
Published: July 2002

Thin films of semiconducting iron disilicide (β-FeSi2) with up to 8 at. % Cr addition grown on Si(001) and Si(111) substrates were studied by spectroscopic ellipsometry as well as transmission and reflection measurements at room temperature. The dielectric function was deduced in the interband spectral range. In molecular beam epitaxy (MBE) preparation part of the Fe atoms were substituted by Cr during deposition. For a low Cr amount in the doping range up to ∼0.4 at.%, Cr was found to modify epitaxial growth on Si(111) substrates with a change in dominating β-FeSi2 grain orientation. Higher amounts of Cr lead to the precipitation of CrSi2, which was detected optically and confirmed by x-ray diffraction measurements, and a deterioration of film morphology. Furthermore, β-FeSi2 thin films were implanted with Cr and subsequently annealed at various temperatures. In these samples also CrSi2 was detected. The results suggest that it is impossible to produce β-(Fe1−x,Crx)Si2 alloys (x>0.003) by MBE or ion implantation.
Sm-doped CdWO4 thin films synthesized by pulsed laser deposition
Authors: Tanaka Katsumi , Ohga Koh-ichi , Choo Cheow-Keong , Nakata Ryouhei
Published: July 2002

Samarium doped CdWO4 (Sm-CdWO4) thin films were synthesized by pulsed laser deposition (PLD) using pressed powder targets composed of Sm2O3, CdO, and WO3 mixtures. Congruent, stoichiometric, and crystalline thin films were obtained on glass substrates at 873 K using targets with 0.01 at. % Sm. The x-ray activated photoluminescence (x-ray PL) spectra were redshifted by about 40 nm at 298 K. Changes of the PL component at 2.85 eV into those at 2.20 and 2.52 eV were typical in their convolutions. These PL components showed very similar dependencies on excitation wavelengths, indicating that CdWO4 related PL emissions in the Sm-doped PLD films are relaxed from the same midband excited state. The PL peaks characteristic to Sm3+ had two sidebands shifted to both lower and higher energy sides as much as 0.03 eV. Plume emission spectra were analyzed to elucidate effects of Sm on our PLD processes. It was interpreted that energy transfer should occur between excited neutral Cd to the excited state of Sm3+ in laser plume.
Edge localized mode particle losses from the DIII-D tokamak
Authors: Porter G. D. , Casper T. A. , Moller J. M.
Published: July 2002

Particle losses associated with edge localized mode (ELM) activity on the DIII-D tokamak [J. Luxon et al., Proceedings of the 11th International Conference on Plasma Physics and Controlled Nuclear Fusion (International Atomic Energy Agency, Vienna, 1986, Vol. I, p. 159] are evaluated quantitatively using density profile data obtained from a Thomson scattering system. It is shown that up to 10% of the total core particle content is lost with each ELM. The particle loss varies inversely with ELM frequency. The temporally averaged ELM particle loss is shown to be about 25% of the total particle loss from the confined region under a wide variety of plasma conditions. Although this ELM loss is a small fraction of the total ion flux, it is large compared to the particle input from neutral beam heating. Hence ELM particle losses are sufficient to control the density rise associated with H-mode plasma operation with neutral beam heating. In addition to controlling the average density by enhancing the total ion flow, albeit only by 25%, it is posited that the ELMs play a role in determining the density profile in the H-mode pedestal region.
Infrared spectra of OCS–hydrogen complexes
Authors: Tang Jian , McKellar A. R. W.
Published: July 2002

Five distinct species of OCS–hydrogen van der Waals complexes (OCS–orthoH2,–paraH2,–paraD2,–orthoD2, and –HD) have been observed by infrared spectroscopy in the region of the OCS ν1 vibration (≈2062 cm−1) in a pulsed supersonic jet expansion using a tunable diode laser probe. The spectra are those of prolate asymmetric rotors, with observed a-type transitions (ΔK=0), but no b-type transitions (ΔK=±1). The fitted rotational parameters are consistent with structures having intermolecular separations in the range of 3.6–3.8 Å and angles of about 70° between the OCS axis and the hydrogen center of mass. The band origins are slightly red-shifted (−0.05 to −0.20 cm−1) relative to that of the free OCS molecule. The results are of interest due to recent observations of the same complexes trapped in ultracold helium droplets; there is a remarkably close correspondence of the vibrational band origins of the free and trapped species.
Atomic orbital Laplace-transformed second-order Møller–Plesset theory for periodic systems
Authors: Ayala Philippe Y. , Kudin Konstantin N. , Scuseria Gustavo E.
Published: July 2002

We present an atomic-orbital formulation of second-order Møller–Plesset (MP2) theory for periodic systems. Our formulation is shown to have several advantages over the conventional crystalline orbital formulation. Notably, the inherent spatial decay properties of the density matrix and the atomic orbital basis are exploited to reduce computational cost and scaling. The multidimensional k-space integration is replaced by independent Fourier transforms of weighted density matrices. The computational cost of the correlation correction becomes independent of the number of k-points used. Focusing on the MP2 quasiparticle energy band gap, we also show using an isolated fragment model that the long range gap contributions decay rapidly as 1/R5, proof that band gap corrections converge rapidly with respect to lattice summation. The correlated amplitudes in the atomic orbital (AO) basis are obtained in a closed-form fashion, compatible with a semidirect algorithm, thanks to the Laplace transform of the energy denominator. Like for its molecular counterpart, the Laplace quadrature can be accurately carried out by using few quadrature points, 3–7 depending on the application. In particular, MP2 quasiparticle energy band gaps are computed accurately with 3 Laplace quadrature points. All these traits indicate that robust calculations of the correlation correction to the Hartree–Fock (HF) energy and band gap of large systems can be carried out. We present benchmark periodic MP2 calculations on polyacetylene, polyphenylenevinylene, hexagonal boron nitride, and stacked polyacetylene.