bluuebunny/crossref_metadata_2025 huggingface dataset:
Evidence of repulsive interactions between NO, O, and N on Ir(110). A fast x-ray photoelectron spectroscopy study
Authors: de Wolf C. A. , Bakker J. W. , Wouda P. T. , Nieuwenhuys B. E. , Baraldi A. , Lizzit S. , Kiskinova M.
Published: July 2002

The adsorption of NO on Ir(110) was studied in the temperature range from 200 to 500 K with fast x-ray photoelectron spectroscopy (XPS) using synchrotron radiation. Only three surface species were observed: molecular NO, atomic O, and atomic N. NO dissociation starts at temperatures higher than 300 K and molecular NO can coadsorb with Oads in the whole temperature region studied. By collecting O 1s and N 1s spectra within a few seconds we obtained information on the interaction between Nads, Oads, and NOads with increasing coadsorbate coverage. The Nads–Oads interaction has been found to be strongly repulsive. As a consequence, the Nads is destabilized already at moderate coverages and temperatures above 350 K leading to the removal of Nads from the surface in the form of N2 desorption.
Thermodynamics and mechanical properties of random copolymer networks
Authors: Schulz Michael
Published: July 2002

We analyze of the thermodynamic properties of random crosslinked polymer networks made of phantom chains with compositional disorder. We start our investigations from an extended Edwards Hamiltonian and end with the discussion of the free energy and mechanical properties. Using replica field theory we determine and solve the saddle point equations. The physically relevant ground state is invariant against translations in the real space, but it depends strongly on the coupling parameters for interactions, topology and compositional disorder of the network. We predict three different thermodynamic states for random copolymer networks. For two regimes the saddle point shows a rotational symmetry after elimination of translation effects. Here the network behaves similar to a homogeneous network. The third regime corresponds to a ground state with broken replica symmetry. Here, the behavior of the random copolymer network shows similarities to the thermodynamical properties of spin glasses. The network state is comparable diluted proteins.
Energy disposal in the two-photon laser-assisted reaction in xenon and chlorine gas mixtures
Authors: Kohel J. , Keto J. W.
Published: July 2002

The two-photon laser assisted reaction (LAR) in low pressure xenon and chlorine gas mixtures has been studied over a broad range of excitation wavelengths in order to characterize the role of the entrance channel in determining the vibrational state distribution of the reaction products. We measure a high degree of vibrational excitation in the XeCl* product, confirming observations from previous studies of the LAR of Xe+Cl2 collision pairs [Ku et al., J. Phys. Chem. 87, 2989 (1983)], and in distinct contrast with the vibrationally cold excimer observed following laser excitation in molecular beam experiments [Boivineau et al., Chem. Phys. Lett. 128, 528 (1986)]. The mean vibrational energy in the XeCl excimer depends strongly on laser wavelength, increasing with decreasing wavelength. Moreover, an increasing fraction of the available energy from the reaction is observed as vibration in the XeCl* product as the laser is tuned toward shorter wavelengths. The reaction outcomes are interpreted in terms of a selectivity of initial conditions on the ionic potential surface that mediates the reactive collision.
Cluster photofragmentation dynamics: Quasiclassical trajectory studies of Arn–H2S and Arn–SH (n=1,2)
Authors: Fair Joanna R. , Nesbitt David J.
Published: July 2002

Quasiclassical trajectory calculations with model potential energy surfaces have been used to elucidate the formation dynamics of open-shell radical clusters by “gentle-recoil” photolysis of closed-shell hydride clusters. Specifically, model surfaces for Ar–H2S and Ar2–H2S have been constructed and used to explore photofragmentation dynamics at 193 and 248 nm for comparison with previous experimental results. A remarkable efficiency (as high as 25%) for forming highly excited radical Ar–SH and Ar2–SH clusters is calculated, despite photolysis recoil energies more than 100-fold in excess of the dissociation limit. This surprisingly high survival probability is traced to two dynamical sources. First, ejection of the light H atom from Arn–H2S effectively removes all but a small fraction of the excess photolysis energy from the nascent radical cluster in the center-of-mass frame. Second, although trajectory calculations indicate that nearly 50% of the surviving clusters contain energies up to two-fold higher than the dissociation limit, these clusters are classically bound due to novel angular momentum barriers predicted by Pollak [J. Chem. Phys. 86, 1645 (1987)] for a polyatomic system. Finally, an analysis is presented that indicates the “gentle-recoil” photolysis mechanism may permit efficient formation of highly internally excited, chemically reactive radical clusters of OH and SH with light species such as H2 and D2.
Multimode electrometer for atmospheric ion measurements
Authors: Harrison R. Giles , Aplin Karen L.
Published: July 2002

A computer-controlled electrometer capable of either high impedance voltage measurements or femtoampere current measurements has been developed. It is intended for use with a Gerdien ion counter in atmospheric temperatures. Temperature variations can cause significant thermal errors arising from offset voltage and bias current changes that compromise the measurements. Use of high impedance switching permits the offset voltage and input bias currents of the electrometer to be measured, and allows regular software compensations to be applied in both modes.
Molecular beam epitaxy of MnAs/ZnSe hybrid ferromagnetic/semiconductor heterostructures
Authors: Berry J. J. , Chun S. H. , Ku K. C. , Samarth N. , Malajovich I. , Awschalom D. D.
Published: July 2002

We report the use of molecular beam epitaxy to create hybrid ferromagnetic/semiconductor heterostructures composed of MnAs and ZnSe, with a Curie temperature of 325 K. The presence of a ZnSe buffer layer exclusively stabilizes the type-B orientation of MnAs, in which the (1̄101) MnAs and (001) ZnSe planes are parallel. Single-phase type-B αMnAs/ZnSe heterostructures yield magnetic properties comparable to those reported in the literature for MnAs/GaAs heterostructures. Variations in growth conditions also permit the stabilization of a strained, nonferromagnetic phase that can coexist with the ferromagnetic phase even at room temperature.
Spin-resolved off-specular neutron scattering maps from magnetic multilayers using a polarized He3 gas spin filter
Authors: Nickel B. , Rühm A. , Donner W. , Major J. , Dosch H. , Schreyer A. , Zabel H. , Humblot H.
Published: July 2002

We report on the application of the polarized He3 gas spin filter technique for the measurement of spin-polarized diffuse neutron reflectivity with spin analysis. The spin filter together with a one-dimensional position-sensitive detector enables the simultaneous investigation of sections in reciprocal space while exploiting spin sensitivity. An example of diffuse neutron scattering from a Fe/Cr superlattice demonstrates the potential of the method. In addition we present a first step towards the interpretation of diffuse scattering from magnetic multilayers by exploiting the supermatrix formalism.
Why the Ising and continuous-space models of ionic systems exhibit essentially different critical behavior
Authors: Ciach A. , Stell G.
Published: July 2002

A Landau–Ginzburg–Wilson functional of two order-parameters—the density of charge φ and the density of mass ρ—is constructed for ionic systems in which the positions of ions are either in the Euclidean space or are restricted to the lattice sites. We find two phase-transitions: (i) a line of continuous transitions to the charge-ordered phase, induced by the fluctuations φ(r)∝cos(r⋅k) with 2π/k of molecular size, which terminates at a tricritical point and (ii) a transition between two uniform, ion-poor and ion-rich phases, induced by the whole spectrum of the charge fluctuations. Due to the dominant role of the short-wavelength charge fluctuations, the positions of the transitions depend significantly on the short-distance properties of the system. In different systems (continuous or on different lattices) one or the other transition may be preempted by the occurrence of the other, by which qualitatively different phase diagrams are obtained.
Quantum-dot phonons in self-assembled InAs/GaAs quantum dots: Dependence on the coverage thickness
Authors: Artús L. , Cuscó R. , Hernández S. , Patanè A. , Polimeni A. , Henini M. , Eaves L.
Published: July 2002

We study the phonons of self-assembled InAs/GaAs quantum dots for different coverage thicknesses L. The additional Raman feature detected between the GaAs transverse optical and the InAs longitudinal optical modes, which we assign to phonons of the dots, exhibits an upward frequency shift with L. This shift is attributed to compressive strain in the dots and, on the basis of its dependence on L, we show that these phonons arise from the quantum dots and not from the wetting layer.
Proton transfer reaction of 4-methyl-2,6-diacetylphenol and an analysis with AM1 potential-energy surfaces
Authors: Mandal A. , Guha D. , Das R. , Mitra S. , Mukherjee S.
Published: July 2002

The ground and excited state proton transfer processes of 4-methyl-2,6-diacetylphenol (MAOH) have been studied by means of steady-state absorption, emission and time resolved spectroscopy in different protic and aprotic solvents at room temperature and 77 K. The relative fluorescence quantum yield measurements are made at different excitation wavelength and both in presence and absence of added base. The emission properties of MAOH at 77 K have been examined in relation to those of 4-methyl-2,6-diformylphenol (MFOH). At this temperature, unlike MFOH, MAOH show phosphorescence only in presence of base like triethylamine in all the solvents studied here. From nanosecond measurements and fluorescence quantum yield we have estimated the decay rate constants. The nonradiative decays are always dominant in the decay processes of the excited states. The energetics of the ground- and excited-state proton transfer in MAOH molecule has been studied by the configuration interaction method at AM1 level of approximation. The ground singlet and the excited triplet are predicted to have considerable barriers on the respective proton transfer paths, while the barrier height is much lower on the corresponding singlet surface. The process is predicted to be endothermic in the ground state and exothermic in the excited singlet and triplet state.
Hard magnetic properties of Fe–Co–Nd–Dy–B nanocrystalline alloys containing residual amorphous phase
Authors: Zhang Wei , Matsushita Mitsuhide , Inoue Akihisa
Published: July 2002

The glass transition and supercooled liquid region were observed in the composition range of 4 to 56 %Co, 2 to 4 %Nd, and 18 to 30 at%B in melt-spun Fe–Co–Nd–0.5 at%Dy–B amorphous alloys. The largest value of the supercooled liquid region defined by the difference between the glass transition temperature (Tg) and the crystallization temperature (Tx), ΔTx(=Tx−Tg), was 47 K for Fe67Co9.5Nd3Dy0.5B20 and the Tg and Tx are 798 and 845 K, respectively. The crystallized structure consists of Fe3B, Nd2Fe14B, α-Fe, and remaining amorphous phases in the annealing temperature up to about 903 K for 420 s and changes to Fe3B, Nd2Fe14B, and α-Fe phases in the higher temperature range. The grain sizes after annealing at 903 K are about 20 nm for Fe3B, 10 nm for Nd2Fe14B, 30 nm for α-Fe, and 5 nm for the remaining amorphous phase. The interparticle spacing of the Nd2Fe14B phase is less than 40 nm. The maximum energy product (BH)max was obtained for the alloy containing the residual amorphous phase subjected to annealing for 420 s at 903 K. The magnetization at a field of 1256 kA/m, remanence, intrinsic coercive force, and (BH)max are 1.60 T, 1.36 T, 227 kA/m, and 110 kJ/m3, respectively. The hard magnetic properties are interpreted to result from the exchange magnetic coupling among Nd2Fe14B, Fe3B, α-Fe, and remaining amorphous phases with ferromagnetism at room temperature. The formation of the finely mixed structure caused by the residual amorphous phase is concluded to result in the good hard magnetic properties with (BH)max above 100 kJ/m3 for the B-rich-type Fe–Co–Nd–Dy–B alloy.
Coherent magnetic reversal in half-metallic manganite tunnel junctions
Authors: Jo Moon-Ho , Mathur N. D. , Evetts J. E. , Blamire M. G.
Published: July 2002

We report coherent magnetization reversal in the La0.7Ca0.3MnO3/NdGaO3/La0.7Ca0.3MnO3 tunnel junctions probed by spin-polarized tunneling. The junction resistance shows an extremely sharp switching with distinct binary resistances and its two-dimensional angular dependence reveals two different magnetic anisotropies in play in the top and bottom La0.7Ca0.3MnO3 electrodes. The large bottom electrode (50×2000 μm) shows the effective twofold crystalline anisotropy and the top electrode (6×6 μm) shows an additional fourfold anisotropy. The geometrical expression of the switching fields in two-dimensional polar coordinates provides evidence that the coherent magnetization reversal of the junctions, particularly the top electrode, closely resembles the ideal coherent rotation model.
Divertor bypass in the Alcator C-Mod tokamak
Authors: Pitcher C. S. , LaBombard B. , Danforth R. , Pina W. , Silveira M. , Parkin B.
Published: July 2002

The Alcator C-Mod divertor bypass has for the first time allowed in situ variations to the mechanical baffle design in a tokamak. The design utilizes small coils which interact with the ambient magnetic field inside the vessel to provide the torque required to control small flaps of a Venetian blind geometry. Plasma physics experiments with the bypass have revealed the importance of the divertor baffling to maintain high divertor gas pressures. These experiments have also indicated that the divertor baffling has only a limited effect on the main chamber pressure in C-Mod.
Lock-in technique for concurrent measurement of adhesion and friction with the scanning force microscope
Authors: Krotil H.-U. , Stifter Th. , Marti O.
Published: July 2002

Regardless of all the great progress in new scanning probe microscopy techniques, the concurrent measurement of adhesive and frictional forces with local resolution using scanning force microscopy (SFM) has not been possible until now. In this paper, we present a novel scanning probe microscopy mode, called combined dynamic x mode or CODYMode®. In CODYMode® SFM at least two oscillations with sufficiently different frequencies and amplitudes are superimposed and interact with the sample surface. This enables the concurrent measurement of the topography, adhesive and frictional forces beside further mechanical surface properties of the sample. By means of the characterization of plasma treated biaxially oriented polypropylene foils the benefits of the new modulation technique are pointed out where common SFM techniques are not adequate. As second application high-velocity friction experiments (in the range of several centimeters per second) on silicon under controlled environmental conditions are introduced and the role of the native water film on it is discussed under friction and viscoelastic aspects.
Design and application of a zone plate monochromator for laboratory soft x-ray sources
Authors: Vogt U. , Wieland M. , Wilhein T. , Beck M. , Stiel H.
Published: July 2002

In this article we describe the test of a zone plate monochromator for a laboratory soft x-ray source which is a laser produced plasma on a liquid jet target. The monochromator consists of a zone plate and a pinhole. Due to the special zone plate used (condensor zone plate KZP7) the monochromator is particularly suitable for laboratory sources since it collects a relatively large solid angle in the present setup. Depending upon the diameter of the pinhole a monochromaticity of up to λ/Δλ=600 can be achieved. The usefulness of the linear monochromator was proven on the basis of a filter transmission measurement. The monochromator can be used for several applications. In particular it is suitable for time-resolved x-ray absorption spectroscopy and pump and probe experiments. The use for such investigations is discussed.
Ultrahigh vacuum high-pressure reaction system for 2-infrared 1-visible sum frequency generation studies
Authors: Kung K. Y. , Chen P. , Wei F. , Rupprechter G. , Shen Y. R. , Somorjai G. A.
Published: July 2002

We designed an ultrahigh vacuum high-pressure (UHVHP) reaction system for in situ sum frequency generation studies. This system allows for pressure (10−9 Torr–1 atm), and temperature (150–1100 K) dependent investigation of adsorbates on single crystals or polycrystalline foils and of catalytic reactions. By combining two optical parametric systems, we are able to simultaneously detect CO and ethylene on Pt(111) surface. A gas chromatograph is used to monitor the kinetics of ethylene hydrogenation on Pt(111).
The “Strain-Gauge Thermocouple”: A novel device for simultaneous strain and temperature measurement
Authors: Cappa Paolo , Marinozzi Franco , Sciuto Salvatore Andrea
Published: July 2002

A novel methodology for simultaneous strain and temperature measurements by means of an ac powered electrical resistance strain gauge connected to a strain conditioner using thermocouple wires is proposed and validated. To this aim a specific device has been designed and implemented; the characteristics of the electronic circuit for signal conditioning have then been tested in order to determine the overall performances in temperature and strain measurements. The field verification of the method is conducted by imposing strain fields in the range from 0 to about 700 μm/m and temperature variations in the range from −10 to 100 °C. The difference between the strain measured by the proposed device and the one evaluated by a conventional digital strain meter was always less than 4 μm/m while the mean temperature discrepancy was 0.5 °C with respect to the reference temperature measured with a K-type thermocouple. Finally, compensation of temperature effects on the actual strain value has been performed while the temperature ranges from ambient to 100 °C with a residual error value of 1.4±1 μm/m.
Electronic states of the manganese dimer ion probed by photodissociation spectroscopy
Authors: Terasaki Akira , Matsushita Akira , Tono Kensuke , Yadav Ramkuber T. , Briere Tina M. , Kondow Tamotsu
Published: July 2002

The optical spectrum of the manganese dimer ion, Mn2+, was obtained by measurement of the photodissociation action spectrum in the photon-energy range from 1.9 through 5.6 eV. The spectrum was analyzed by calculating its electronic and geometric structures using density functional theory including nonlocal corrections. The simulation was in reasonable agreement with the experimental result, allowing the assignment of the electronic states involved in the optical transitions. The ground state was shown to be a Σg+12 state. The excited electronic states corresponding to the transitions around 2.9, 4.0, and 5.3 eV were assigned to Σu+12, Σu+12 together with Πu12, and Πu12, respectively. The high-spin character indicates a ferromagnetic coupling of all the 3d electrons.
Optical properties of undoped and modulation-doped AlGaN/GaN single heterostructures grown by metalorganic chemical vapor deposition
Authors: Kwon Ho Ki , Eiting C. J. , Lambert D. J. H. , Shelton B. S. , Wong M. M. , Zhu T. G. , Dupuis R. D.
Published: July 2002

The optical properties of undoped and modulation-doped AlGaN/GaN single heterostructures (SHs) grown by metalorganic chemical vapor deposition are investigated at low temperature using photoluminescence measurements. The formation of a two-dimensional electron gas at the heterojunction is verified by temperature-dependent Hall mobility and 300 K capacitance-voltage measurements. Radiative recombination is observed between the electrons in two-dimensional quantum states at the heterointerface and the holes in the flat-band region or bound to residual acceptors both in undoped and modulation-doped AlGaN/GaN SHs. These peaks disappear when the top AlGaN layer is removed by reactive ion etching. In addition, the photoluminescence results under different laser excitation intensity and lattice temperature are also described for undoped and modulation-doped AlGaN/GaN SHs with various Al compositions and growth interrupt times.
Role of surface-roughness scattering in double gate silicon-on-insulator inversion layers
Authors: Gámiz F. , Roldán J. B. , Cartujo-Cassinello P. , López-Villanueva J. A. , Cartujo P.
Published: July 2002

The effect of surface-roughness scattering on electron transport properties in extremely thin double gate silicon-on-insulator inversion layers has been analyzed. It is shown that if the silicon layer is thin enough the presence of two Si–SiO2 interfaces plays a key role, even for a very low transverse effective field, where surface-roughness scattering is already noticeable, contrary to what happens in bulk silicon inversion layers. We have studied the electron transport properties in these devices, solving the Boltzmann transport equation by the Monte Carlo method, and analyzed the influence of the surface-roughness parameters and of the silicon layer thickness. For low transverse effective fields, μSR decreases as the silicon layer decreases. However, at higher transverse effective fields, there is a different behavior pattern of μSR with Tw since it begins to increase as Tw decreases until a maximum is reached; for lower silicon layer thicknesses, surface-roughness mobility abruptly falls. Finally we have compared the behavior of μSR versus Tw for double gate silicon-on-insulator and single gate silicon-on-insulator inversion layers.
Electrical conductivity of water compressed dynamically to pressures of 70–180 GPa (0.7–1.8 Mbar)
Authors: Chau R. , Mitchell A. C. , Minich R. W. , Nellis W. J.
Published: July 2002

The electrical conductivity of water was measured at high pressures (70 to 180 GPa) and temperatures (4000 to 11 000 K) using a reverberating shock wave technique. The measured electrical conductivity of water varies from 39 to 200 Ω−1 cm−1 between 70 and 180 GPa. The relatively weak pressure dependence of the electrical conductivity is consistent with water being fully ionized chemically and the primary conduction mechanism is highly mobile protons. The results are in contrast to hydrogen, in which electrons are the dominant charge carriers.
Secondary ion emission from Ar, Kr, Xe, N2, O2, CO, and SF6 adsorbed on Pt(111): Relation between neutralization of sputtered ions and bond nature of adsorbates
Authors: Souda Ryutaro
Published: July 2002

On the basis of sputtering experiments, it is demonstrated that the neutralization of secondary ions is closely related to the nature of bonding of parent atoms and molecules adsorbed on a Pt(111) surface. The secondary ions are emitted from all of gas-condensed multilayers examined, such as Ar, Kr, Xe, O2, N2, CO, and SF6 because the valence hole tends to be localized in the physisorbed species undergoing sputtering. Ar and Kr are physisorbed when they are in direct contact with the Pt(111) surface as evidenced by the emission of intense secondary ions, but very little secondary ions are emitted from Xe, CO, and O2 in a submonolayer coverage regime because the valence hole is delocalized due to chemisorption or the overlap of the wave functions with those of the Pt(111) substrate. The N2 molecule is chemisorbed at a very initial stage and then the physisorbed layer grows in the island form before completion of the first monolayer as determined from the evolution of the secondary N+ and N2+ ion yields. The SF6 molecule is chemisorbed in a submonolayer coverage regime as evidenced by that the SFx+ fragment ions are absent. The F+ ion arises even from chemisorbed F adatoms or dissociatively chemisorbed SFx fragments. This is because F+ is not initiated by the valence hole state but arises from the screened F 2s core–hole state having an antibonding character, which is followed by intraatomic Auger decay after breakage of the chemisorptive bond.
Langevin stabilization of molecular dynamics
Authors: Izaguirre Jesús A. , Catarello Daniel P. , Wozniak Justin M. , Skeel Robert D.
Published: July 2002

In this paper we show the possibility of using very mild stochastic damping to stabilize long time step integrators for Newtonian molecular dynamics. More specifically, stable and accurate integrations are obtained for damping coefficients that are only a few percent of the natural decay rate of processes of interest, such as the velocity autocorrelation function. Two new multiple time stepping integrators, Langevin Molly (LM) and Brünger–Brooks–Karplus–Molly (BBK–M), are introduced in this paper. Both use the mollified impulse method for the Newtonian term. LM uses a discretization of the Langevin equation that is exact for the constant force, and BBK–M uses the popular Brünger–Brooks–Karplus integrator (BBK). These integrators, along with an extrapolative method called LN, are evaluated across a wide range of damping coefficient values. When large damping coefficients are used, as one would for the implicit modeling of solvent molecules, the method LN is superior, with LM closely following. However, with mild damping of 0.2 ps−1, LM produces the best results, allowing long time steps of 14 fs in simulations containing explicitly modeled flexible water. With BBK–M and the same damping coefficient, time steps of 12 fs are possible for the same system. Similar results are obtained for a solvated protein–DNA simulation of estrogen receptor ER with estrogen response element ERE. A parallel version of BBK–M runs nearly three times faster than the Verlet-I/r-RESPA (reversible reference system propagator algorithm) when using the largest stable time step on each one, and it also parallelizes well. The computation of diffusion coefficients for flexible water and ER/ERE shows that when mild damping of up to 0.2 ps−1 is used the dynamics are not significantly distorted.
Monte Carlo test of the lattice cluster theory: Thermodynamic properties of binary polymer blends
Authors: Buta Dorel , Freed Karl F. , Szleifer Igal
Published: July 2002

The particle exchange method is used to compute the exchange chemical potential for binary mixtures of linear polymers in Monte Carlo lattice model simulations. The internal energy and the specific heat are also calculated for each thermodynamic state. Simulations are performed for nine blend compositions, with the total monomer volume fraction held fixed at φ=0.8 or φ=0.7. Both polymer species have the same polymerization indices, M=40, 50, or 100. The two species are distinguished by their van der Waals interaction strengths, ε11, ε12, and ε22. Several ratios ε22/ε11 are considered, and the simulation temperatures range from almost athermal conditions to near-critical states. The parameter ε12 is either computed as the geometrical average ε11ε22 or is assigned an independent value. Three approximate solutions to the model are compared to the simulations: Flory–Huggins theory, Guggenheim’s random mixing approximation, and the lattice cluster theory (LCT). The results confirm the higher accuracy of the LCT. However, some predictions for the specific heat are again found to be inaccurate because of the low order cutoff of the high temperature perturbative expansion.
Quantum theory of infrared detectors based on intrasubband transitions in III–V quantum wells
Authors: Hagston W. E. , Stirner T. , Rasul F.
Published: July 2002

One of the perceived drawbacks of multiquantum well (QW) infrared detectors based on intrasubband conduction band transitions in III–V materials (e.g., GaAs/AlxGa1−xAs) is that photons at normal incidence to the multiquantum well interfaces will either not be absorbed or, at best, will be absorbed very weakly. The present article reviews briefly certain key aspects of the experimental evidence and theoretical arguments that both support and challenge this view, prior to developing a completely different approach to the problem. It is shown that the questions at issue relate to basic concepts of quantum mechanics. On this basis it is argued that in QW structures of appropriate design normal incidence absorption should occur, thus enabling, in principle, two-dimensional infrared detector imaging arrays to be fabricated in a simple manner.
Acoustic emission induced by dislocation annihilation during plastic deformation of crystals
Authors: Polyzos B. , Douka E. , Trochidis A.
Published: July 2002

A description of the dislocation annihilation component of acoustic emission (AE) during plastic deformation of crystals is presented. The annihilation of dislocation kink–antikink pairs during Frank–Read source operation is investigated within the framework of a dynamic model describing the interaction between dislocations and acoustic waves. This model consists of a d’Alembert wave equation nonlinearly coupled to a sine-Gordon equation. The AE by annihilating dislocation kinks is considered and both the emitted energy and the power spectral density of the emitted waves are calculated. It is shown that the lifetime of the generating event is inversely proportional to the square of the applied stress in agreement with existing measurements. The stress corresponding to maximum emission is calculated and is found also to be in agreement with existing experimental data. The obtained results support the hypothesis that the annihilation of dislocations can also be a source of considerable AE during plastic deformation of crystals. The proposed model, furthermore, provides a better physical insight to the emission mechanisms involved.
Low-temperature scanning-tunneling microscope for luminescence measurements in high magnetic fields
Authors: Kemerink M. , Gerritsen J. W. , Hermsen J. G. H. , Koenraad P. M. , van Kempen H. , Wolter J. H.
Published: July 2002

We have designed and built a low-temperature (1.3–4.2 K) scanning-tunneling microscope which is capable of collecting light that is generated in the tunneling region. Light collection is done by means of two fibers whose cleaved front is in close proximity (≈1 mm) to the tunneling region. The whole system can be operated in high magnetic fields (11 T) without loss of optical signal strength. As a demonstration, we measured the electroluminescence spectra of an InGaAs quantum well at various temperatures. At 4.2 K, we found an electron-to-photon conversion factor that is three orders of magnitude higher than at room temperature.
Intersubband transitions in bismuth nanowires
Authors: Black M. R. , Padi M. , Cronin S. B. , Lin Y.-M. , Rabin O. , McClure T. , Dresselhaus G. , Hagelstein P. L. , Dresselhaus M. S.
Published: July 2002

Optical absorption associated with the one-dimensional joint density of states of an intersubband transition in bismuth nanowires is reported. The previously observed strong absorption in bismuth nanowires at ∼1000 cm−1 is here shown to depend on the wire diameter and on the polarization of the incident light. The absorption line shape, the decreasing frequency with increasing wire diameter, and the polarization dependence of the reflectivity, all indicate that this resonance is due to an intersubband absorption resulting from quantum-confinement effects.
Electric-field-dependent carrier capture and escape in self-assembled InAs/GaAs quantum dots
Authors: Fry P. W. , Finley J. J. , Wilson L. R. , Lemaître A. , Mowbray D. J. , Skolnick M. S. , Hopkinson M. , Hill G. , Clark J. C.
Published: July 2002

Photoluminescence and complementary photocurrent spectroscopy, both as a function of electric field, are used to probe carrier capture and escape mechanisms in InAs/GaAs quantum dots. Carrier capture from the GaAs matrix is found to be highly field sensitive, being fully quenched in fields of only 15 kV/cm. For fields less than 20 kV/cm, carriers excited in the wetting layer are shown to be captured by the dots very effectively, whereas for fields in excess of 50 kV/cm tunnel escape from the wetting layer into the GaAs continuum is dominant. For excitation directly into the dots, radiative recombination dominates up to 100 kV/cm.
Particle dynamics of debris produced during laser–plasma soft x-ray generation
Authors: Letardi T. , Lo D. , Zheng C.-E.
Published: July 2002

The dynamics of debris in a buffer gas after ejection from the solid target during laser–plasma soft x-ray generation is studied. Analytical formulas for the flight range of cylindrical and spherical debris are derived using the incompressible flow theory. The validity range of the formulas and the related error margins are discussed. Finally, the flight range formula for spherical particles is extended to two times the Mach number at the expenses of a somewhat higher error margin (up to 25%).
Double proton transfer in the complex of acetic acid with methanol: Theory versus experiment
Authors: Fernández-Ramos Antonio , Smedarchina Zorka , Rodrı́guez-Otero Jesús
Published: July 2002

To test the approximate instanton approach to intermolecular proton-transfer dynamics, we report multidimensional ab initio bimolecular rate constants of HH, HD, and DD exchange in the complex of acetic acid with methanol in tetrahydrofuran-d8, and compare them with the NMR (nuclear magnetic resonance) experiments of Gerritzen and Limbach. The bimolecular rate constants are evaluated as products of the exchange rates and the equilibrium rate constants of complex formation in solution. The two molecules form hydrogen-bond bridges and the exchange occurs via concerted transfer of two protons. The dynamics of this transfer is evaluated in the complete space of 36 vibrational degrees of freedom. The geometries of the two isolated molecules, the complex, and the transition states corresponding to double proton transfer are fully optimized at QCISD (quadratic configuration interaction including single and double substitutions) level of theory, and the normal-mode frequencies are calculated at MP2 (Møller-Plesset perturbation theory of second order) level with the 6-31G (d,p) basis set. The presence of the solvent is taken into account via single-point calculations over the gas phase geometries with the PCM (polarized continuum model). The proton exchange rate constants, calculated with the instanton method, show the effect of the structure and strength of the hydrogen bonds, reflected in the coupling between the tunneling motion and the other vibrations of the complex. Comparison with experiment, which shows substantial kinetic isotopic effects (KIE), indicates that tunneling prevails over classic exchange for the whole temperature range of observation. The unusual behavior of the experimental KIE upon single and double deuterium substitution is well reproduced and is related to the synchronicity of two-atom tunneling.
Determination of exciton transition energy and bowing parameter of AlGaN alloys in AlGaN/GaN heterostructure by means of reflectance measurement
Authors: Jiang H. , Zhao G. Y. , Ishikawa H. , Egawa T. , Jimbo T. , Umeno M.
Published: July 2002

The normal-incidence reflectance measurement was employed to obtain the free exciton transition energy (EFX) of AlGaN alloys in AlxGa1−xN/GaN/sapphire heterostructure grown by metalorganic chemical vapor deposition. It was found that the thickness variation of the AlGaN layer may cause a noticeable change in the line shape of reflectance spectrum and impede the identification of the desired excitonic position. By using a reflection model of two absorbing layers with a transparent substrate, the experimental reflectance spectra were theoretically simulated and utilized to explain the reflection mechanism in AlxGa1−xN/GaN heterostructures. On the basis of the above analysis, the feasibility of the reflectance measurement for such heterostructures is confirmed. At room temperature, the EFXs obtained from the fitting showed an excellent agreement with the corresponding peak energies in the photoluminescence spectra. Furthermore, at the optical energy position about 100 meV above the EFX, the spectral feature of exciton-LO phonon interaction was observed in the reflectance spectrum record for low Al composition (x⩽0.16). Using the Al mole fraction derived from x-ray diffraction measurement, the bowing parameter of the epitaxial AlGaN layer was determined. In the range of 0⩽x<0.3, the resulting bowing parameter shows a downward value of 0.53 eV.
Dispersion of the pinning field direction of a ferromagnet/antiferromagnet coupled system
Authors: Hou Chunhong , Chen Jian , Kief Mark T. , Gao Zheng , Mao Sining , Pokhil Taras
Published: July 2002

The exchange bias field of a ferromagnet/antiferromagnet bilayer is usually measured by the unidirectional shift of the hysteresis loop of the ferromagnetic layer. The exchange bias field results from interfacial exchange coupling between the spins in the ferromagnetic and antiferromagnetic layers. In general, the spins of the antiferromagnetic layer can vary locally in their anisotropy directions resulting in a distribution in local pinning directions. This letter reports a quantitative study on this distribution or dispersion using the anisotropic magnetoresistive (AMR) effect in a small rotating magnetic field. The AMR data as a function of the rotating angle of the field measured at a low field reveal the dispersion inside the ferromagnetic layer and at the interface of a ferromagnet/antiferromagnet system. The interaction between the domains within the ferromagnetic layer also plays a significant role in the dispersion inside the ferromagnetic layer.
Generalized solvent boundary potential for computer simulations
Authors: Im Wonpil , Bernèche Simon , Roux Benoı̂t
Published: July 2002

A general approach has been developed to allow accurate simulations of a small region part of a large macromolecular system while incorporating the influence of the remaining distant atoms with an effective boundary potential. The method is called the Generalized Solvent Boundary Potential (GSBP). By representing the surrounding solvent as a continuum dielectric, both the solvent-shielded static field from the distant atoms of the macromolecule and the reaction field from the dielectric solvent acting on the atoms in the region of interest are included. The static field is calculated once, using the finite-difference Poisson–Boltzmann (PB) equation, and the result is stored on a discrete grid for efficient simulations. The solvent reaction field is developed using a basis-set expansion whose coefficients correspond to generalized electrostatic multipoles. A matrix representing the reaction field Green’s function between those generalized multipoles is calculated only once using the PB equation and stored for efficient simulations. In the present work, the formalism is applied to both spherical and orthorhombic simulation regions for which orthonormal basis-sets exist based on spherical harmonics or cartesian Legendre polynomials. The GSBP method is also tested and illustrated with simple model systems and two detailed atomic systems: the active site region of aspartyl-tRNA synthetase (spherical region) and the interior of the KcsA potassium channel (orthorhombic region). Comparison with numerical finite-difference PB calculations shows that GSBP can accurately describe all long-range electrostatic interactions and remain computationally inexpensive.
Compliant effect of low-temperature Si buffer for SiGe growth
Authors: Luo Y. H. , Wan J. , Forrest R. L. , Liu J. L. , Jin G. , Goorsky M. S. , Wang K. L.
Published: July 2002

Relaxed SiGe attracted much interest due to the applications for strained Si/SiGe high electron mobility transistor, metal-oxide-semiconductor field-effect transistor, heterojunction bipolar transistor, and other devices. High-quality relaxed SiGe templates, especially those with a low threading dislocation density and smooth surface, are critical for device performance. In this work, SiGe films on low-temperature Si buffer layers were grown by solid-source molecular-beam epitaxy and characterized by atomic force microscope, double-axis x-ray diffraction, and photoluminescence spectroscopy. It was demonstrated that, with the proper growth temperature and Si buffer thickness, the low-temperature Si buffer became tensily strained and reduced the lattice mismatch between the SiGe and the Si buffer layer. This performance is similar to that of the compliant substrate: a thin substrate that shares the mismatch strain in heteroepitaxy. Due to the smaller mismatch, misfit dislocation and threading dislocation densities were lower.
He–Xe microdischarges: Comparison of simulation results with experimental data
Authors: Veronis G. , Inan U. S. , Pasko V. P.
Published: July 2002

A comparison of predictions of a one-dimensional simulation model with the results of a recent experimental study [Appl. Phys. Lett. 76, 544 (2000)] of a dc He–Xe microdischarge is presented. The experimental results are remarkably reproduced by the model but only when unusually high values are used for the unknown rate coefficients of formation and recombination reactions of HeXe+ heteronuclear ions.
Oxidation kinetics in SrTiO3 homoepitaxy on SrTiO3(001)
Authors: Zhu X. D. , Si Weidong , Xi X. X. , Jiang Qidu
Published: July 2002

Using an oblique-incidence optical reflectivity difference technique, we investigated kinetic processes in SrTiO3 homoepitaxy on SrTiO3(001) under pulsed-laser deposition conditions. Depending upon growth temperature and oxygen ambient pressure, we found that the oxidation of an as-grown SrTiO3 monolayer may take a much longer time to complete than the recrystallization of the monolayer. The oxidation reaction was found to be characterized by an effective activation energy barrier of 1.35 eV and a large preexponential factor.
Self-consistent integral equation theory for polyolefins: Comparison to molecular dynamics simulations and x-ray scattering
Authors: Pütz Mathias , Curro John G. , Grest Gary S.
Published: July 2002

We report on self-consistent polymer reference interaction site model (PRISM) calculations as well as molecular dynamics (MD) simulations for several types of polyolefins. For all polymer types one single set of potential parameters was used. In general we find good semi-quantitative agreement between PRISM and MD results. Further we compare both MD and PRISM results to experimental x-ray scattering data and show that the potentials used give a good to excellent description of these data. From the quality of the PRISM calculations it is clear that PRISM can be used as an efficient tool in model development.
High-density (>1023/cm3) relativistic electron plasma confined between two laser pulses in a thin foil
Authors: Shen Baifei , Meyer-ter-Vehn J.
Published: July 2002

The nonlinear interaction of two counterpropagating high-intensity laser pulses in thin foil targets is investigated theoretically and by simulation. The two laser pulses have circular polarization and illuminate the foil normally from both sides. They create a spatially confined relativistic electron plasma. An analytical stationary solution is derived for the special case in which the polarization of the two beams rotates in the same direction. The solutions depend on the relative phase of the two pulses. In general, the foil oscillates as a whole; the oscillation frequency is derived analytically. High order harmonics are found when frequencies and/or polarizations are changed. For same polarization and frequencies ωL and 2ωL of the two incident pulses, a harmonics spectrum is found in which each third harmonic (third, sixth, etc.) is absent. Under certain conditions, an instability occurs which can destroy the foil.
An experimental study of the occupied volume in polyethylene terephthalate
Authors: Consolati G. , Quasso F.
Published: July 2002

Combined use of positron annihilation lifetime spectroscopy—an experimental tool to investigate the intermolecular sub-nanometric spaces (free volume) in polymers—and of specific volume data supply unique information on the occupied volume in a semicrystalline polymer—poly(ethylene terephthalate)—, in particular, on its thermal behavior. From our data an expansion coefficient for the occupied volume αo=2.9⋅10−5 K−1 is obtained, when the dependence of ortho-positronium intensity on the temperature is taken into account. The free volume fraction at the glass transition temperature results f=0.04, in agreement with analogous estimates in other semicrystalline polymers.
Effect of implant temperature on secondary defects created by MeV Sn implantation in silicon
Authors: Wong-Leung J. , Jagadish C. , Conway M. J. , Fitz Gerald J. D.
Published: July 2002

Secondary defects induced by ion implantation in silicon after annealing have been previously shown to vary with the implantation and annealing conditions. However, in the low dose implants, well below the amorphization dose, the defects have been predominantly characterized to be interstitial in nature. In this article, we study the effect of implant temperature on secondary defects created by 1 MeV Sn implantation to a dose of 3×1013 cm−2 after subsequent annealing. We report a variation in the defect microstructure with implant temperature showing preferential formation of small interstitial loops for −191 °C and only rod-like defects for similar implants carried out at 300 °C. We conclude that these microstructures are a result of the dense cascades created by heavy Sn ions, creating local amorphous pockets in the implant damage region at the lowest implant temperatures. The variation of the microstructure with implant temperature is interpreted in terms of the effect of dynamic annealing over the defects formed in silicon.
Towards the classification of static vacuum spacetimes with negative cosmological constant
Authors: Chruściel Piotr T. , Simon Walter
Published: July 2002

We present a systematic study of static solutions of the vacuum Einstein equations with negative cosmological constant which asymptotically approach the generalized Kottler (“Schwarzschild–anti-de Sitter”) solution, within (mainly) a conformal framework. We show connectedness of conformal infinity for appropriately regular such spacetimes. We give an explicit expression for the Hamiltonian mass of the (not necessarily static) metrics within the class considered; in the static case we show that they have a finite and well-defined Hawking mass. We prove inequalities relating the mass and the horizon area of the (static) metrics considered to those of appropriate reference generalized Kottler metrics. Those inequalities yield an inequality which is opposite to the conjectured generalized Penrose inequality. They can thus be used to prove a uniqueness theorem for the generalized Kottler black holes if the generalized Penrose inequality can be established.
Conformational behavior of isolated polymers under an external field in good solvents
Authors: Wenczel Rob , Shew Chwen-Yang
Published: July 2002

We present theoretical studies for the conformational behavior of isolated polymers under an external field in good solvents. We assume the monomers of the polymer chain are subject to a three-dimensional external field, which is modeled as a spherical harmonic potential, and monomers interact via hard-core repulsions. We begin with Monte Carlo simulations to investigate the effects of the external field. The chain conformation changes insignificantly if the external field is weak. When the strength of the external field is increased, the chain contracts drastically. A first-order perturbation theory (with ideal-chain reference system) is proposed, with the hard-core repulsion modeled as a perturbation. This approach yields good agreement with the simulations, but with the discrepancy between these two approaches becoming pronounced for strong external field. The perturbation theory makes the prediction that the size of a polymer chain contracts approximately one order of magnitude before collapsing to its globular conformation. Further, to account for behavior under strong fields, a theory of mean-field type is developed, which predicts 〈R2〉∼(N/k)2/5 (where k denotes field strength), consistent with the simulations. These studies are of importance in understanding supercoiled long chain DNA in a tiny cellular nucleus, and deformation of polymer materials under external fields.