bluuebunny/crossref_metadata_2025 huggingface dataset:
Equation of state for star polymers in good solvents
Authors: Patrickios C. S. , Lue L.
Published: July 2002

We develop a free-energy model for star polymers in good solvents that accurately describes concentrated polymer solutions and displays the correct universal scaling behavior, in the limit of infinite molecular weight, for dilute and semidilute polymer concentrations. The architecture of the polymer molecules enters the model through the value of the second virial coefficient and the rescaled penetration function Ψ̄, the ratio of the penetration function Ψ(f) to its asymptotic, infinite-molecular-weight value Ψ*(f), where f is the number of arms on the star polymer. The direction of approach of the equation of state to the universal, infinite-molecular-weight scaling limit depends on the relative magnitude of Ψ̄. For Ψ̄>1, the scaling equation of state is approached from “above,” while for Ψ̄<1, the scaling equation of state is approached from “below.” We also perform new Monte Carlo simulations for the pressure and mean-square radius of gyration of star polymers composed of tangent-hard-spheres. The theory compares well with the Monte Carlo simulation data for the equation of state.
Thermodynamics of anisotropic surfactant micelles. II. A molecular interpretation of the micellar curvature free energy
Authors: Bergström Magnus
Published: July 2002

Explicit expressions have been derived for the two curvature constants k1 and k2 that mainly influence the size and shape, respectively, of generally shaped micelles (triaxial tablets with a thickness<width<length). It is found that geometrical packing constraints, together with an unfavorable hydrocarbon/water interfacial tension γhc/w, give rise to a large and positive contribution to both k1 and k2 that increases with the micellar thickness (equivalent to surfactant tail length) as well as with γhc/w. The constant k1 that mainly influence the size of the micelles may be brought down by electrostatics to values where rather small micelles are able to form. However, electrostatics also have a tendency to increase the constant k2 that mainly affects the length-to-width ratio of the micelles so that the smaller micelles are predicted to be rather disklike, i.e., with a low length-to-width ratio. In addition, residual head group effects as well as the free energy contribution due to the hydrocarbon chains may influence the micellar size and shape. In particular, it is demonstrated that mixing of two surfactants reduces k2 and promotes the formation of elongated micelles with a large length-to-width ratio whereas k1, and thus the overall size of the micelles, is only slightly influenced. The effect of mixing is predicted to increase with increasing asymmetry between the surfactants mixed in the micelles with respect to head group size and charge number as well as hydrophobic tail volume.
Vibrational dephasing dynamics at hydrogenated and deuterated semiconductor surfaces: Symmetry analysis
Authors: Wang J.-K. , Tsai C.-S. , Lin C.-E. , Lin J.-C.
Published: July 2002

Raman scattering has been performed on single crystal hydrogenated silicon and germanium surfaces to investigate the temperature dependence of their vibrational spectral profiles. Based on a single-mode dephasing model for pure vibrational dephasing, the frequency shift and line broadening were analyzed to extract the following dephasing parameters: exchange mode frequency, coupling strength, and friction parameter. The exchange modes for the XH stretches on hydrogenated X(100) surfaces (X=Ge, Si, and C) are found to match their respective bending frequencies. The corresponding ones for hydrogenated X(111) surfaces, on the other hand, are located within the bulk phonon. This surface dependence of the exchange mode in surface vibrational dephasing dynamics is correlated with the structural relaxation and its associated symmetry variation at surfaces. It is further confirmed by the experiments performed on deuterated semiconductor surfaces. A site-symmetry induced representation method is exploited to analyze the symmetry properties of the surface vibrations on these two surfaces. This group-theoretical analysis has revealed the selection rules of choosing the dominant exchange modes on hydrogenated semiconductor surfaces. We have identified the exchange modes in the surface phonon spectra of these surfaces according to their symmetry properties.
Experimental study of the bifurcation nature of the electrostatic potential of a toroidal helical plasma
Authors: Fujisawa A. , Iguchi H. , Minami T. , Yoshimura Y. , Tanaka K. , Itoh K. , Sanuki H. , Lee S. , Kojima M. , Itoh S.-I. , Yokoyama M. , Kado S. , Okamura S. , Akiyama R. , Ida K. , Isobe M. , Nishimura S. , Osakabe M. , Nomura I. , Shimizu A. , Takahashi C. , Toi K. , Matsuoka K. , Hamada Y. , Fujiwara M.
Published: July 2002

The bifurcation nature of the electrostatic structure is studied in the toroidal helical plasma of the Compact Helical System (CHS) [K. Matsuoka et al., Proceedings of the 12th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Nice, 1988 (International Atomic Energy Agency, Vienna, 1989), Vol. 2, p. 411]. Observation of bifurcation-related phenomena is introduced, such as characteristic patterns of discrete potential profiles, and various patterns of self-sustained oscillations termed electric pulsation. Some patterns of the electrostatic structure are found to be quite important for fusion application owing to their association with transport barrier formation. It is confirmed, as is shown in several tokamak experiments, that the thermal transport barrier is linked with electrostatic structure through the radial electric field shear that can reduce the fluctuation resulting in anomalous transport. This article describes in detail spatio-temporal evolution during self-sustained oscillation, together with correlation between the radial electric field and other plasma parameters. An experimental survey to find dependence of the temporal and spatial patterns on plasma parameters is performed in order to understand systematically the bifurcation property of the toroidal helical plasma. The experimental results are compared with the neoclassical bifurcation property that is believed to explain the observed bifurcation property of the CHS plasmas. The present results show that the electrostatic property plays an essential role in the structural formation of toroidal helical plasmas, and demonstrate that toroidal plasma is an open system with a strong nonlinearity to provide a new attractive problem to be studied.
Collision-induced bound state motion in I2. A classical molecular dynamics study
Authors: Ermoshin V. A. , Engel V. , Meier C.
Published: July 2002

We use three-dimensional classical molecular dynamics to simulate femtosecond time-resolved experiments on the caging dynamics of I2 in rare gas environments [Wan et al., J. Chem. Phys. 106, 4353 (1997)]. The calculated pump–probe signals are in excellent agreement with experiment. Prominent oscillatory structures as observed in the pump–probe signals are interpreted in terms of caustics which appear in the classical bound state dynamics of the caged molecules. The results confirm conclusions based on a simple statistical model which treats the collisions as random perturbations involving hard sphere scattering.
Lattice sums for gratings and arrays
Authors: McPhedran R. C. , Nicorovici N. A. , Botten L. C. , Grubits K. A.
Published: July 2002

Lattice sums arising in quasiperiodic Green’s functions for the Helmholtz equation, over general two-dimensional arrays are investigated. The array sums are related to those over a single quasiperiodic line of sources, and their difference is be expressed in terms of exponentially convergent series. It is shown that our expressions can be used to generate the sums pertaining to the case of photonic gap states, associated with complex quasiperiodicity (Bloch) vectors. The accuracy and computational speed of our expressions are illustrated.
Selective preparation of enantiomers by laser pulses: From optimal control to specific pump and dump transitions
Authors: González L. , Hoki K. , Kröner D. , Leal A. S. , Manz J. , Ohtsuki Y.
Published: July 2002

Starting from optimal control, various series of infrared, ultrashort laser pulses with analytical shapes are designed in order to drive a preoriented molecule from its ground torsional state, which represents the coherent superposition of left and right atropisomers, towards a single enantiomer. Close analysis of the population dynamics, together with the underlying symmetry selection rules for the laser induced transitions, yields the mechanism. Namely, the molecule is driven from its ground vibrational state towards the coherent superposition of the lowest doublet of states via a doublet of excited torsional states with opposite symmetries. This pump-and-dump mechanism can be achieved by simpler series of analytical laser pulses. This decomposition of the optimal pulse into analytical subpulses allows us to design different scenarios for the selective preparation of left or right enantiomers. Exemplary this is demonstrated by quantum simulations of representative wave packets for the torsional motions of the model system, H2POSH, in the electronic ground state, based on the ab initio potential energy surface, and with ab initio dipole couplings.
Athermal annealing of phosphorus-ion-implanted silicon
Authors: Grun J. , Fischer R. P. , Peckerar M. , Felix C. L. , Covington B. C. , DeSisto W. J. , Donnelly D. W. , Ting A. , Manka C. K.
Published: July 2002

A 1 cm2 area in phosphorus-implanted silicon samples is annealed by irradiation of a much smaller 0.002 cm2 area with a single laser pulse. Resistivity of the annealed region is uniform and similar to that measured after thermal annealing. Electrically activated donors did not diffuse into the sample and only slightly towards the sample surface. The process is 100% reproducible. We present evidence that the annealing is not caused by heat.
Contrast enhancement in the detection of defects in transparent layered structures: The use of optothermal interference technique in solar cell investigation
Authors: Batista J. A. , Mansanares A. M. , da Silva E. C. , Vaz C. C. , Miranda L. C. M.
Published: July 2002

This article shows the enhanced sensitivity of the optothermal interference technique in the detection of local differences (nonhomogeneity in thickness and optothermal parameters), compared to the conventional optical interference, when investigating layered transparent structures. The measured signal is sensitive to the reflectance variation at the distinct interfaces, function of temperature, as well as to the optical phase lag between the reflected beams. Measurements made on solar cells show contrast of the order of 100% in the optothermal interference, while the conventional optical interference presents a contrast of only 15%. A model based on the reflectance variation at each interface describes the signal behavior as a function of modulation frequency. Theoretical calculation based on this model evidences the influence of the optothermal parameters in the signal contrast.
Magnetic observation of deformation substructure in cyclically deformed Ni3Fe single crystals
Authors: Yasuda Hiroyuki Y. , Furuta Daizoh , Sasaki Akiko , Umakoshi Yukichi
Published: July 2002

The magnetic anisotropy of permeability and high-field susceptibility in Ni3Fe single crystals due to dislocations and antiphase boundary (APB) was theoretically calculated and measured by a vibrating sample magnetometer to determine the cyclic hardening and deformation substructure during cyclic deformation. Two types of anisotropy in magnetic properties were observed due to the atomic rearrangement near APB and the internal stress field around dislocations. In an early stage of fatigue where strong cyclic hardening occurred, the magnetic anisotropy was mainly due to the APB, while in a further stage of fatigue where cyclic softening occurred, the anisotropy was caused by dislocations. The cyclic softening was induced by the disordering in the localized region of coarse slip bands.
On a small structure in velocity field within a contact region
Authors: Onishi Yoshimoto , Takeshi Ooshida , Umemura Noriko
Published: July 2002

In a contact region (contact surface in Euler terms) connecting the two uniform regions in a flow field such as caused by the sudden break of a membrane in a shock-tube, there exists a small structure in velocity field, which has been reported earlier by the calculation based on the Boltzmann equation of the BGK type. Probably, the accepted view would be that the velocity within this region is uniform. Here this velocity structure is investigated thoroughly based on not only the Boltzmann equation but also the Navier–Stokes equations. Actually the velocity field has a hump or a pit, the width of which is found to be the same as the thickness of the region. The magnitude of the hump or the pit decreases with time, inversely proportional to the square root of time. At the so-called tailoring condition at which the contact region has so far been thought not to manifest itself, actually it does exist; the velocity structure does not appear of course but the temperature and, hence, the density still have structures, although they are small in magnitude.
Nonmonotonic depletion of phase separation under strong supercooling
Authors: Hecht I. , Gitterman M.
Published: July 2002

We analyze the growth of a droplet of a stable phase (a plane wave front in one-dimensional case) under strong supercooling in the framework of the Landau–Ginzburg potential for both a conserved and a nonconserved order parameter coupled to thermal diffusion. When the coupling between changes in temperature and the order parameter is strong enough, and the heat conductivity is sufficiently small, the following nontrivial phenomenon may occur: the latent heat released with the growth of a droplet is taken away very slowly, and the local overheating transforms the initially overcooled state into an overheated one. As a result, the rate of droplet growth decreases, and even may change its sign, so that droplets of radius larger than the critical one, nevertheless, may shrink. For the steady-state growth in the one-dimensional case this phenomenon was analyzed analytically, while beyond the steady regime and in the two-dimensional case, the equations were solved numerically. The presence of a thermal bath with heat loss through thermal dissipation narrows the areas of occurence of this phenomenon.
Effect of optical saturation on pulsed photothermal deflection signals in flowing media
Authors: Li B. C. , Gupta R.
Published: July 2002

A two-level transient-state model has been developed which describes the effect of optical saturation on the photothermal signals, valid for conditions where a steady state may not exist. The theory is quite general, as it considers arbitrary pulse shapes, homogeneously and inhomogeneously broadened lines, and arbitrary spectral profile of the excitation. Equations for the temporally and spatially dependent absorption coefficient are derived, and, for generality, time-dependent photothermal deflection signals in a flowing medium are calculated (stationary medium being just a special case). The saturation behaviors of the photothermal signals obtained by monochromatic rectangular and Q-switched pulse excitations for homogeneously and inhomogeneously broadened lines are evaluated and compared. Differences between the predictions of the steady-state and transient-state models are discussed. Experiments have been performed using NO2 as a sample gas in a flowing medium. The experimental results are in agreement with the predictions of the transient state model.
Methanol–pyridine complexes trapped in argon and nitrogen matrices: Infrared induced isomerization and theoretical calculations
Authors: Coussan S. , Brenner V. , Perchard J. P. , Zheng W. Q.
Published: July 2002

Selective vibrational excitations of OH stretching modes of methanol–pyridine hydrogen-bonded complexes trapped in solid argon or nitrogen at 7 K were carried out in the range 3400–3280 cm−1. This proved an efficient way for inducing isomerization processes within heterodimers and larger aggregates. The main photoproducts are non-H-bonded species, with OH and CO frequencies, respectively, close to 3665 and 1020 cm−1. These unbonded species are unstable, with lifetimes strongly temperature dependent in the range 7–20 K. The possible structures of the heterodimer have been calculated theoretically according to a two-step method. The potential energy surface is firstly explored using a semiempirical method, then the properties of its minima are calculated in the framework of the density functional method. The deepest minimum corresponds to the quasilinear N⋯HO hydrogen-bonded structure, with vibrational properties in good agreement with the matrix data obtained after deposition at low concentration in both dopants. Several other minima were examined in which H bonding is either weak (OH… π electrons) or nonexistent. The vibrational perturbations are weak in all cases, but the identification of the photoproduct of the stable dimer to the form involving the OH… π interaction can be discarded. The stable forms of the mixed trimers were also calculated. (Methanol)2–pyridine has a cyclic structure, the methanol dimer being tied to pyridine through a strong OH⋯N hydrogen bond and a weak CH⋯O interaction. Methanol–(pyridine)2 is also cyclic, with a OH⋯N bond slightly stronger than in the heterodimer. Comparison with experimental data allows identification of this heterotrimer in nitrogen matrices, in the presence of an excess of pyridine with respect to methanol. Among its photolysis products, one class in which the OH group is perturbed on the oxygen atom has been identified.
Low energy ion assisted control of interfacial structure: Ion fluence effects
Authors: Zhou X. W. , Wadley H. N. G.
Published: July 2002

Multilayered thin films consisting of high electrical conductivity copper layers sandwiched between pairs of low coercivity ferromagnetic alloys can exhibit giant magnetoresistance. The magnitude of the magnetoresistance increases with the structural and chemical perfection of the interfaces. Recent atomistic modeling and experimental observations have shown that nickel and cobalt atoms in the ferromagnetic layer readily exchange with underlying copper atoms during the deposition of the ferromagnetic layer upon the copper spacer. This results in mixing at the ferromagnetic metal on copper interface. Low energy (1–20 eV) inert gas ions can be used during deposition to flatten the surface of layers, in some cases without causing interlayer mixing. Here we use the molecular dynamics simulation method to investigate the effects of the assisting ion fluence upon the surface roughness and interlayer mixing of a model Ni/Cu/Ni multilayer system. The results reveal that the surface roughness initially drops rapidly with ion fluence and then approaches a limiting roughness that is dependent upon the surface type, the ion energy, and the ion mass. For a Cu on Ni surface irradiated by 2.0 eV Xe+ ions, the flattening transition occurs at a fluence of about 0.2 ions/Å2 (corresponding to an ion to metal deposition flux ratio of about 5). The same transition was seen at a similar fluence for a Ni on Cu surface, but at a higher Xe+ ion energy of 14.0 eV. Threshold energies for flattening and mixing were identified for various surfaces. The probabilities of both flattening and mixing were found to increase with ion fluence and ion energy. Because the threshold energy for mixing was lower than that for smoothing, significant interfacial mixing was only seen during ion assisted flattening of the Ni on Cu interface. Simple models have been developed to establish the functional dependence of interfacial structural parameters upon the assisting ion fluence.
Structural transition of Ge dots induced by submonolayer carbon on Ge wetting layer
Authors: Wakayama Yutaka , Gerth Gerhard , Werner Peter , Gösele Ulrich , Sokolov Leonid V.
Published: July 2002

We have investigated the influence of carbon on Ge dot growth on Si(100) substrates. To modify the Ge dot structure, submonolayers of carbon were deposited on Ge wetting layers. The Ge deposited on the carbon-covered wetting layer tends to form dome structures instead of hut structures even at a substrate temperature of 500 °C. The main effect of C is to enhance a structural transition from huts to domes by influencing the configurational energy of the Ge dots. The dominant factor to determine the dot size is the substrate temperature. Accordingly, small domes with 10–20 nm in diameter were formed by combining techniques of the submonolayer C on the Ge wetting layer and low-temperature deposition.
Rate and efficiency of spontaneous emission in metal-clad microcavities
Authors: Worthing P. T. , Wasey J. A. E. , Barnes W. L.
Published: July 2002

In this article, we report experimental results on the control of spontaneous emission from sources embedded within metal-clad microcavities. The microcavity samples consist of an Eu3+ doped emissive monolayer embedded with a planar dielectric slab, bounded by a silver film on each side. We investigate the effect of both the cavity thickness and the location of the emissive monolayer within the cavity on the spontaneous emission rate of the emitters, observing both enhancement and inhibition. By varying the thickness of one of the silver films we also investigate the role of the reflectivity of the mirrors of the cavity on the control of spontaneous emission. In addition to the emission rate, results are presented on the spatial distribution of the emitted radiation. We find good agreement between our experimental results and those predicted by a classical theory over a large range microcavity structures. This agreement allows us to assess the contribution of the decay mediated by the electromagnetic modes of the cavity and the intrinsic nonradiative decay to the spontaneous emission rate. We then compare the ability of metal-clad and hybrid (one metal mirror and one distributed Bragg reflector) microcavity designs to optimize the efficiency of radiative emission. We show that enhancements, due to the increased confinement of the electromagnetic modes supported by the metal-clad microcavities, can be sufficient to overcome the loss associated with the absorption in the metal layers. We discuss the implication these results may have for device design.
Double-periodic arrays of vortices
Authors: Kuvshinov Boris N. , Schep Theo J.
Published: July 2002

Analytical solutions to the sinh-Poisson equation are discussed. This equation plays a role in the theory of vortex dynamics [Mallier and Maslowe, Phys. Fluids A 5, 1074 (1993)] and in the discussion of the most probable states of inviscid two-dimensional flows in fluids and plasmas [Montgomery and Joyce, Phys. Fluids 17, 1139 (1974)]. We present a family of double-periodic solutions on a rectangular grid. In limiting cases these solutions reproduce Mallier–Maslowe vortex streets and arrays of Greenhill’s point vortices.
Suppressed shot noise in trap-assisted tunneling of metal–oxide–semiconductor capacitors
Authors: Iannaccone G. , Crupi F. , Neri B. , Lombardo S.
Published: July 2002

We show that the tunneling current of a metal–oxide–semiconductor capacitor subjected to voltage stress exhibits suppressed shot noise with respect to the “full” shot noise level associated with the same current before stress. We provide experimental results exhibiting a suppression down to about 70% and a theoretical model for transport and noise in the stress induced leakage current regime based on trap assisted tunneling, which is able to reproduce such reduction. Numerical results from the model are compared with measurements.
III-N semiconductor growth with activated nitrogen: State-specific study of A3Σu+ metastable N2 molecules
Authors: Jordan D. C. , Tsong I. S. T. , Smith David J. , Wilkens B. J. , Doak R. B.
Published: July 2002

High quality epitaxial III-N semiconductor films, ranging in thickness from 300 to 900 Å, have been grown using A3Σu+ metastable nitrogen molecules. The work employed a corona discharge supersonic free-jet to generate a molecular beam containing exclusively the A3Σu+ activation state in an otherwise ground state N2 beam. AlN films were grown on 6H–SiC(0001) and Si(001) substrates. GaN films were grown on the same substrates and on buffer layers of AlN deposited in situ on 6H–SiC(0001). The N-atom incorporation efficiency (the number of N atoms attaching to a III–N surface per incident A3Σu+ molecule) approached 100% and was independent of substrate temperature from 600 to 900 °C, implying direct molecular chemisorption of the A3Σu+. These measurements support theoretical predictions that A3Σu+ is an ideal precursor for III–N growth.
Calculation of the ground state of shallow donors in GaAs1−xNx
Authors: Wu J. , Walukiewicz W. , Haller E. E.
Published: July 2002

The ground-state binding energy and the Bohr radius of shallow donors in GaAs1−xNx alloys are determined as a function of the nitrogen atomic fraction using variational principle calculations. The calculations show that the ground state is strongly affected by the N-induced changes of the conduction band structure and the possible increase of the dielectric constant. Most importantly, large effects are predicted for the case of a Coulomb potential screened by free charge carriers.
Modular theory and the reconstruction of four-dimensional quantum field theories
Authors: Kähler Ralf , Wiesbrock Hans-Werner
Published: July 2002

In this letter we construct a representation of the 3+1-dimensional Poincaré group by modular groups of von Neumann algebras lying in a specified modular position with respect to each other. Combining this new result with an old one of Bisognano–Wichmann [J. Math. Phys. 16, 985 (1975)] we obtain a net of local observables of a 3+1-dimensional quantum field theory out of a finite set of algebras.
Ion transport phenomena in oxide layer on the silicon surface and electron–ion exchange effects at the SiO2/Si interface
Authors: Goldman E. I. , Zhdan A. G. , Chucheva G. V.
Published: July 2002

The results of ion depolarization experiments for SiO2 insulating layers on Si under isothermal and thermostimulated conditions have been analyzed. Additionally, the ion currents under the continuous change of gate voltage (dynamic current–voltage characteristics) have been investigated. It is shown that, depending on the type of experiment, an insulating layer shows itself either as an ion trap system with the wide distribution of lifetimes (energies of trap activation are distributed in an interval of 0.75–1.5 eV), or as a medium with free ions, which have a mobility μ (423 K)=2.7×10−8 cm2/V s and an activation energy of mobility Eμ=0.80±0.05 eV. These values correspond to those found for Na+ ions. The model resolving this apparent contradiction has been offered. Each ion and isolator matrix surrounding it are expected to generate a localized electronic state, but ion+electron neutral associates (NAs) are formed by filling these states owing to electron tunnel transitions from the semiconductor. NAs play the role of ion traps. The wide distribution of ion lifetimes on such traps is connected with spread of an electron tunnel length at the associate decay. The increase of NA ionization degree at the quasistationary electrical field change provides a smooth transition from a set of a few mobile NAs to an ensemble of free ions. The theory developed on the basis of model given and taking into account both NAs diffusion and their formation and decay processes allows the experimental data to be described qualitatively and quantitatively. Undamped circulation of particles in an oxide layer of a metal–oxide–semiconductor system under a stationary polarization electrical field is predicted and found: NAs formed as a result of ion neutralization at the semiconductor surface diffuse into the insulator volume, where they decay thermally to ions and electrons; the latter leave for a gate almost instantly compared to ion transport times; the ions formed again come back to the insulator–semiconductor interface under the electrical field action. The NA diffusion coefficient D(423 K)≈8.6×10−18 cm2 s−1 is a minimum on 8 orders of magnitude less than the free ion diffusion coefficient at the temperature 423 K.
Angular distribution of ejected atoms from Nd:YAG laser irradiating metals
Authors: Torrisi L. , Andò L. , Ciavola G. , Gammino S. , Barnà A.
Published: July 2002

A Nd:YAG pulsed laser is employed to irradiate different metals in vacuum at the ECLISSE facility of the Laboratorio Nazionale del Sud, Catania, INFN. Laser pulse energy, 9 ns in width, ranges between 100 and 900 mJ. The ejection of atoms by means of laser irradiation is studied in terms of angular distribution, laser etching yield and film thickness deposited on a substrate. Light elements (Ni, Cu) show an angular distribution that is larger than heavy ones (W, Pb). A theoretical approach, applied to fit experimental data, indicates that the distribution depends on the high power of cos θ and that the flow velocity of ejected atom ranges between 27 000 and 88 000 m/s and the kinetic energy of ejected species ranges between 0.7 and 4.4 keV.
In situ measurement of the optical absorption coefficient of porous silicon
Authors: Diesinger Heinrich , Bsiesy Ahmad , Hérino Roland
Published: July 2002

Detailed experimental procedure of an in situ photoelectrical technique applied to porous silicon absorption coefficient measurement is described. In this technique, the underlying silicon substrate is used as a photodetector of the monochromatic light intensity transmitted by the porous layer. In particular, the investigation of spectral range validity of this technique has evidenced that at high photon energy, the porous silicon photoluminescence (PL) is excited and contributes to the photocurrent. It is shown that this PL contribution can be readily separated from that of the directly transmitted light, providing two benefits: first, the high accuracy of the measurement remains unaffected by the PL at high excitation energy, which justifies the application to photon energy as high as 3 eV, and second, the PL component can be further exploited to measure the porous silicon PL quantum efficiency. Typical absorption coefficient spectra obtained by this technique are then recalled. They are discussed in the frame of a model based on the analysis of the absorption coefficient of an ensemble of different quantum-size silicon crystallites.
Fluorescence extended x-ray absorption fine structure study on local structures around Mn atoms in thin (In, Mn)As layer and (In, Mn)As quantum dots
Authors: Ofuchi H. , Kubo T. , Tabuchi M. , Takeda Y. , Matsukura F. , Guo S. P. , Shen A. , Ohno H.
Published: July 2002

We have investigated thin (In, Mn)As layer and (In, Mn)As quantum dots (with Mn mole fraction lower than 0.02) on GaAs(001) by fluorescence extended x-ray absorption fine structure (EXAFS) in order to study the local structures formed around the Mn atoms. The EXAFS analysis revealed that in a 10 nm thick (In, Mn)As layer, the In-site substitutional Mn and the NiAs-type MnAs coexisted, while the majority of the Mn atoms were substituted in the In-sites of InAs in (In, Mn)As quantum dots. It is considered that different growth modes for the thin layer and the quantum dots affect the local structures.
Fredholm indices and the phase diagram of quantum Hall systems
Authors: Avron J. E. , Sadun L.
Published: July 2002

The quantized Hall conductance in a plateau is related to the index of a Fredholm operator. In this paper we describe the generic “phase diagram” of Fredholm indices associated with bounded and Toeplitz operators. We discuss the possible relevance of our results to the phase diagram of disordered integer quantum Hall systems.
Electrical and optical properties of the B+ and P+ implanted C60 thin films
Authors: Dharmarasu N. , Sakai T. , Kojima N. , Yamaguchi M. , Ohshita Y. , Narayanan K. L.
Published: July 2002

Conductivity control of the boron and phosphorus ion implanted C60 films subjected to various doses with multiple energy has been investigated. Temperature dependent conductivity shows the semiconducting nature of the B+ and P+ implanted films. Fourier transform infrared spectra of the B+ and P+ implanted C60 films show the structural change of the C60 into amorphous carbon. Optical gaps of the implanted films have been observed as a function of implantation dose. The dramatic increase in the conductivity and the reduction in the optical gap on ion implantation are thought to be attributed to an increase in sp2 bounded states, lattice damage, and substitutional impurity doping with ion implantation. The B+ and P+ implantation into C60 and amorphous carbon suggests the possibility of fabricating carbon based homojunction solar cells.
Screening and strain effects on the ground-state energy of a quasi-two-dimensional quantum well system
Authors: Yang Tzuen-Rong , Kim Mi-Ra
Published: July 2002

We discuss the screening and strain effects on the ground-state energy of the strained-layer semiconductor quantum wells, which forms a quasi-two-dimensional electron-hole plasma within the screened Hartree-Fock approximation. The exchange self-energy is evaluated within the Thomas-Fermi screening scheme for variation of the sheet density and the biaxial compressive strain. Such evaluation includes the solving of the Luttinger-Kohn Hamiltonian, which leads to obtaining the valence band structure [Phys. Rev. 97, 869 (1955)]. The dependence of the exchange self-energy on the sheet density and the strain is shown to be significantly reduced as the screening effects are taken into account. Also, the screening plays an important role in determining the ground-state energy and the renormalization of the single-particle energies. In addition, we observe that the ground-state energy increases with the biaxial compressive strain.
Systematic study of the normal and pumped state of high efficiency diamond particle detectors grown by chemical vapor deposition
Authors: Marinelli Marco , Milani E. , Paoletti A. , Tucciarone A. , Verona Rinati G. , Angelone M. , Pillon M.
Published: July 2002

The efficiency and charge collection distance (CCD) of nuclear particle detectors based on high quality diamond films grown by chemical vapor deposition (CVD) have been systematically studied as a function of the methane content in the growth gas mixture and for varying film thickness. The effects of preirradiation with β particles (pumping) have been thoroughly studied. The results fully support a recently proposed model [Marinelli et al., Appl. Phys. Lett. 75, 3216 (1999)] discussing the role of in-grain defects and grain boundaries in determining the charge collection spectra of CVD diamond films both in the normal and in the pumped state. The model allows us to quantitatively explain the dependence of CCD and efficiency on film thickness, giving a microscopic picture of the effects of preirradiation with ionizing radiation in CVD diamond films. The highest average CCD obtained is 145 μm in a 160 μm thick detector (corresponding to about 50% average efficiency), while the maximum value (about 70% efficiency) is close to 370 μm. In addition, CCD is shown to be higher than film thickness and to monotonically increase with thickness, indicating margins for further improvements.
Detection of field-dependent antiferromagnetic domains in exchange-biased Fe3O4/NiO superlattices
Authors: Borchers J. A. , Ijiri Y. , Lind D. M. , Ivanov P. G. , Erwin R. W. , Qasba Aron , Lee S. H. , O’Donovan K. V. , Dender D. C.
Published: July 2002

Neutron diffraction studies of Fe3O4/NiO superlattices reveal that the field dependence of domains in the antiferromagnetic NiO is correlated with the presence or absence of exchange biasing. Measurements of the full width at half maximum of the (111) NiO reflection show that after cooling in zero field, the antiferromagnetic domain size both parallel and perpendicular to the growth axis is sensitive to the strength of the applied field. In contrast, after cooling from room temperature in a field of 5 T, the domain size is generally smaller than the zero-field-cooled value and does not vary with field. These data suggest that exchange biasing originates from domain walls frozen into the antiferromagnet upon field cooling.
High-power electroabsorption modulator using intra-step-barrier quantum wells
Authors: Shin Dong-Soo , Yu Paul K. L. , Pappert S. A.
Published: July 2002

We propose an efficient high-optical-saturation-power electroabsorption modulator (EAM) using intra-step-barrier quantum wells (IQWs) incorporating a step-barrier inside the well for both the conduction and the valence bands. From the simulation, we show that in the IQW (a) the onset of the redshift of the quantum-confined Stark effect is delayed to a higher electric field; and after the onset, (b) the transition energy and the oscillator strength respond sharply to the electric field. As the screening effect by the photogenerated carriers is reduced at the higher electric field, the optical saturation effect in the IQWs can be significantly reduced. Due to the higher optical power handling capability and better modulation efficiency, the EAM with IQWs is expected to substantially improve the fiber link performance.
Origins of both insulator–metal transition and colossal magnetoresistance in doped manganese perovskites
Authors: Yuan S. L. , Zhao W. Y. , Zhang G. Q. , Tu F. , Peng G. , Liu J. , Yang Y. P. , Li G. , Jiang Y. , Zeng X. Y. , Tang C. Q. , Jin S. Z.
Published: July 2002

Based on experiments of both transport and paramagnetic resonance for (La1−xYx)2/3Ca1/3MnO3, the ground state above Tc and possible origins of both insulator–metal transition and colossal magnetoresistance near Tc are discussed. Modeling the system as a network of junctions, each consisting of a paramagnetic region sandwiched between two ferromagnetic domains, a phenomenological expression is proposed for resistance as a function of temperature and magnetic field. We show that the observed transport and magnetotransport phenomena can be quantitatively explained by the present model for the whole temperature range studied.
Electric bidirectional stress effects on metal–oxide–silicon capacitors
Authors: El-Hdiy A. , Ziane Dj.
Published: July 2002

Bidirectional electron injections were performed in thick and thin oxides of metal–oxide–silicon capacitors under a constant oxide electric field. Results show that dissymmetry, due to electron trapping near both oxide interfaces, is accentuated when the oxide is thin. However electron trapping is more marked when oxide is thick. Two kinds of thick oxide were used: wet and dry technologies. Wet oxide contains relatively more defects than dry. This is due to hydrogen incorporation in wet oxide during the oxidation process. But this difference is not well marked, because boron atoms injected after the oxidation process would deactivate hydrogen atoms. We describe the electron trapping by a well-known power law. The exponent of this law depends on oxide field polarity and also on oxide thickness but is technology independent. Results also show a nonexpecting behavior of tunneling current during bidirectional stress at a constant voltage. Here, we describe its causes and effects.
Monte Carlo simulation of the generation of terahertz radiation in GaN
Authors: Starikov E. , Shiktorov P. , Gružinskis V. , Reggiani L. , Varani L. , Vaissière J. C. , Zhao Jian H.
Published: July 2002

The conditions for microwave power generation at low temperatures under optical phonon emission are analyzed by Monte Carlo simulations of both small- and large-signal responses in bulk zinc blende and wurtzite GaN. As a result of the high optical phonon energy and the strong interaction of electrons with optical phonons in GaN a general improvement on the transit-time resonance and a considerable increase in the maximum generation frequency and power can be achieved in comparison to the widely studied III–V materials such as GaAs and InP. A dynamic negative differential mobility caused by transit-time resonance occurs in a wide frequency range of about 0.05–3 THz and persists in the THz frequency range up to the liquid nitrogen temperature with doping levels up to about 5×1016 cm−3. The efficiency of the amplification and generation is found to depend nonmonotonously on static and microwave electric field amplitudes, generation frequency, and doping level so that for each generation frequency there exists an optimal range of parameter values. Under optimal conditions a generation efficiency of about 1% to 2% can be achieved in the 0.5–1.5 THz frequency range.
Mechanics of laser-assisted debonding of films
Authors: Tavernier P. R. , Clarke D. R.
Published: July 2002

Films of GaN and ZnO can be separated from the substrates on which they are grown by the use of a laser-assisted debonding process in which a pulsed laser is shone through the substrate and absorbed in the film. Experience shows that the success in separating the films intact and damage free depends sensitively on the laser parameters used and the physical and geometric properties of the films. In this contribution, the mechanics of the laser-assisted debonding of GaN films are presented and used to construct process maps that delineate the conditions for damage-free film separation. The key variable is the nondimensional group ΩEp/(dp2τ), where Ω is a lumped material constant, Ep is the laser pulse energy, dp is the diameter of the illuminated area and τ is the laser pulse length. Experimental observations of UV/excimer laser assisted debonding of GaN films from sapphire substrates are used to illustrate the types of deformations and cracking modes on which the process maps are based.
Geometrical constraints on finite-time Lyapunov exponents in two and three dimensions
Authors: Thiffeault Jean-Luc , Boozer Allen H.
Published: July 2002

Constraints are found on the spatial variation of finite-time Lyapunov exponents of two- and three-dimensional systems of ordinary differential equations. In a chaotic system, finite-time Lyapunov exponents describe the average rate of separation, along characteristic directions, of neighboring trajectories. The solution of the equations is a coordinate transformation that takes initial conditions (the Lagrangian coordinates) to the state of the system at a later time (the Eulerian coordinates). This coordinate transformation naturally defines a metric tensor, from which the Lyapunov exponents and characteristic directions are obtained. By requiring that the Riemann curvature tensor vanish for the metric tensor (a basic result of differential geometry in a flat space), differential constraints relating the finite-time Lyapunov exponents to the characteristic directions are derived. These constraints are realized with exponential accuracy in time. A consequence of the relations is that the finite-time Lyapunov exponents are locally small in regions where the curvature of the stable manifold is large, which has implications for the efficiency of chaotic mixing in the advection–diffusion equation. The constraints also modify previous estimates of the asymptotic growth rates of quantities in the dynamo problem, such as the magnitude of the induced current.
Thermal deformation measurement of electronic packages using the atomic force microscope scanning moiré technique
Authors: Lu Y. G. , Zhong Z. W. , Yu J. , Xie H. M. , Ngoi B. K. A. , Chai G. B. , Asundi A.
Published: July 2002

In this article, the feasibility of atomic force microscope (AFM) scanning moiré on a cross-line diffraction grating has been studied. The AFM scanning moiré technique has been applied to measure the thermal deformation of electronic packages successfully. This technique is convenient to perform the mismatch method, also it could obtain a higher resolution than any other moiré method.
The stretching vibrational overtone spectra of PH3: Local mode vibrational analysis, dipole moment surfaces from density functional theory and band intensities
Authors: He Sheng-Gui , Zheng Jing-Jing , Hu Shui-Ming , Lin Hai , Ding Yun , Wang Xiang-Huai , Zhu Qing-Shi
Published: July 2002

The infrared spectra of PH3 molecule were recorded on a Bruker IFS 120HR Fourier transform spectrometer from 4000 to 9500 cm−1. The P–H stretching vibrational frequencies and intensities were derived from the experimental data. The Morse oscillator parameters De and α in the anharmonically coupled anharmonic oscillator local mode model were determined by the least-squares fitting with the observed vibrational band centers. The ab initio three-dimensional P–H stretching dipole moment surfaces were calculated by the density functional theory method. The dipole moment vectors were projected to three kinds of molecule-fixed reference systems. The corresponding dipole moment components were fitted to polynomial functions in terms of the P–H bond length displacements with the molecular symmetry taken into account. The absolute band intensities were obtained and then compared with the experimental data. The results showed that a proposed improved bond dipole model can predict the absolute band intensities within a factor of 2 for most of the observed transitions, indicating a reasonably good agreement.
The spectral linewidth of tunable semiconductor InAsSb/InAsSbP lasers emitting at 3.2–3.6 μm (2800–3100 cm−1)
Authors: Imenkov A. N. , Kolchanova N. M. , Yakovlev Yu. P. , Kubát P. , Civiš S.
Published: July 2002

A method was used to measure the width of emission lines of a type of semiconductor laser with composition InAsSb/InAsSbP. This type of laser was manufactured specially for absorption high-resolution spectroscopy of gases absorbing in the 2800–3100 cm−1 region. Parameters used for calculation of spectral emission linewidths were obtained using selected rotation-vibration lines of the gaseous molecules N2O, CH3Cl, and OCS. The estimated spectral emission linewidths varied in the range 10–30 MHz in dependence of the current passing and the type of laser.
Electronic spectroscopy of the alkaline-earth halide cluster Ca2Cl3
Authors: Lobo Julio D. , Deev Andrei , Wong Chi-Kin , Spotts James M. , Okumura Mitchio
Published: July 2002

A visible spectrum of the cluster Ca2Cl3 was observed from 651 to 630 nm by 1+1′ resonant multiphoton ionization spectroscopy. Spectra were obtained for each of the four isotopomers: Ca2 35Cl3, Ca2 35Cl2 37Cl, Ca2 35Cl 37Cl2, and Ca2 37Cl3. The spectra were composed of a strong origin band at 15 350.8 cm−1 and several very weak vibronic bands. All of the bands were sharp with partially resolved rotational band contours. Density functional calculations predicted three minimum energy isomers. The spectrum was assigned to the B22←X̃ 2A1 transition of the lowest energy isomer, a planar C2v structure having a ring of two Cl and two Ca atoms and a terminal Cl atom. The ring isomer of Ca2Cl3 has the unpaired electron localized on one Ca2+ ion to form a Ca+ chromophore. The two other predicted isomers, a D3h trigonal bipyramid and a C2v planar V-shaped structure, were not consistent with the observations.
X-ray reflectivity and nanotribological study of deposition-energy-dependent thin CNx overcoats on CoCr magnetic films
Authors: Liu W. J. , Zhou J. N. , Rar A. , Barnard J. A.
Published: July 2002

Using a combination of high-resolution x-ray reflectivity (XRR), nanotribological, and x-ray photoelectron spectroscopy (XPS) techniques, a thorough description of the structural parameters of ultrathin CNx overcoats on CoCr magnetic films is obtained. In addition, the tribological performance as a function of deposition energy is shown to correlate well with trends found by XRR and XPS.