bluuebunny/crossref_metadata_2025 huggingface dataset:
Size-induced stability and structural transition in monodispersed indium nanoparticles
Authors: Balamurugan B. , Kruis F. E. , Shivaprasad S. M. , Dmitrieva O. , Zähres H.
Published: May 2005

The present study reports the stability and the physical significance of the size-induced crystallographic structural transition in the gas-phase synthesized monodispersed indium nanoparticles. Transmission electron microscopy and x-ray photoelectron spectroscopy studies reveal that the formation of a thin oxide shell results in enhanced stability of indium nanoparticles. These results also show a size-induced structural transition from the bulk tetragonal to face-centered-cubic structure, which is attributed to an increase in the binding energy of core electrons of indium nanoparticles due to quantum confinement effects and the presence of a thin oxide shell.
Type I-type II transition in InGaAs–GaNAs heterostructures
Authors: Schlichenmaier C. , Grüning H. , Thränhardt A. , Klar P. J. , Kunert B. , Volz K. , Stolz W. , Heimbrodt W. , Meier T. , Koch S. W. , Hader J. , Moloney J. V.
Published: May 2005

Optical interband transitions in a series of In0.23Ga0.77As–GaNxAs1−x quantum well samples are investigated. For changing nitrogen content, a type I-type II transition is identified by a detailed analysis of photoluminescence and photoreflectance spectra. Experimental results are compared systematically with spectra calculated by a microscopic theory. A valence band offset parameter of (1.5±0.5)eV is extracted for this heterostructure system.
Tunneling and nonlinear transport in a vertically coupled GaAs∕AlGaAs double quantum wire system
Authors: Bielejec E. , Seamons J. A. , Reno J. L. , Lilly M. P.
Published: May 2005

We report low-dimensional tunneling in an independently contacted vertically coupled quantum wire system. This nanostructure is fabricated in a high quality GaAs∕AlGaAs parallel double quantum well heterostructure. Using a unique flip chip technique to align top and bottom split gates to form low-dimensional constrictions in each of the independently contacted quantum wells we explicitly control the subband occupation of the individual wires. In addition to the expected two-dimensional (2D)-2D tunneling results, we have found additional tunneling features that are related to the one-dimensional quantum wires.
Adaptive illumination in computer screen assisted spectral fingerprinting
Authors: Filippini Daniel , Lundström Ingemar
Published: May 2005

The computer screen photoassisted technique (CSPT) uses displays as controlled light sources for measuring spectral fingerprints of color substances, such as those present in bioassays. Here, the possibility of tuning the illuminating sequences for matching CSPT fingerprints with target spectral transmittances is demonstrated. This approach enables robust classification; orders of magnitude form mismatches, and establish a systematic method for adapting optimum illuminating sequences to any unknown measuring platform.
Photoelectron spectroscopy of phosphorus hydride anions
Authors: Ervin Kent M. , Lineberger W. Carl
Published: May 2005

Negative-ion photoelectron spectroscopy is applied to the PH−, PH2−, P2H−, P2H2−, and P2H3− molecular anions. Franck–Condon simulations of the photoelectron spectra are used to analyze the spectra and to identify various P2Hn− species. The simulations employ density-functional theory calculations of molecular geometries and vibrational frequencies and normal modes, and coupled-cluster theory calculations of electron affinities. The following electron affinities are obtained: EA0(PH)=1.027±0.006eV, EA0(PH2)=1.263±0.006eV, and EA0(P2H)=1.514±0.010eV. A band is identified as a mixture of trans-HPPH− and cis-HPPH−. Although the trans and cis bands cannot be definitively assigned from experimental information, using theory as a guide we obtain EA0(trans-HPPH)=1.00±0.01eV and EA0(cis-HPPH)=1.03±0.01eV. A weak feature tentatively assigned to P2H3− has a vertical detachment energy of 1.74eV. The derived gas-phase acidity of phosphine is ΔacidG298(PH3)⩽1509.7±2.1kJmol−1.
Phase stability tuning in the NbxZr1−xN thin-film system for large stacking fault density and enhanced mechanical strength
Authors: Joelsson T. , Hultman L. , Hugosson H. W. , Molina-Aldareguia J. M.
Published: May 2005

The phase stability of hexagonal WC-structure and cubic NaCl-structure 4d transition metal nitrides was calculated using first-principles density functional theory. It is predicted that there is a multiphase or polytypic region for the 4d transition metal nitrides with a valence electron concentration around 9.5 to 9.7 per formula unit. For verification, epitaxial NbxZr1−xN (0⩽x⩽1) was grown by reactive magnetron sputter deposition on MgO(001) substrates and analyzed with transmission electron microscopy (TEM) and x-ray diffraction. The defects observed in the films were threading dislocations due to nucleation and growth on the lattice-mismatched substrate and planar defects (stacking faults) parallel to the substrate surface. The highest defect density was found at the x=0.5 composition. The nanoindentation hardness of the films varied between 21GPa for the binary nitrides, and 26GPa for Nb0.5Zr0.5N. Unlike the cubic binary nitrides, no slip on the preferred ⟨11¯0⟩{110} slip system was observed. The increase in hardness is attributed to the increase in defect density at x=0.5, as the defects act as obstacles for dislocation glide during deformation. The findings present routes for the design of wear-resistant nitride coatings by phase stability tuning.
Mn-rich clusters in GaN: Hexagonal or cubic symmetry?
Authors: Martínez-Criado G. , Somogyi A. , Ramos S. , Campo J. , Tucoulou R. , Salome M. , Susini J. , Hermann M. , Eickhoff M. , Stutzmann M.
Published: May 2005

In this study, we report the application of synchrotron radiation microprobe to the analysis of Mn-rich clusters in GaN. From the Mn and Ga fluorescence line intensity ratio, an average Mn concentration of 11% was deduced. A combination of fluorescence mapping with spectroscopic techniques enabled us to examine not only the cluster elemental nature but also their crystallographic orientation on the submicron scale. The strong polarization-dependent x-ray absorption near-edge structure features showed the preservation of the hexagonal symmetry in both cluster-free and Mn-rich regions. However, from the x-ray absorption data taken inside the clusters, a preferential disorder was observed in the direction parallel to the crystal growth rather than perpendicular to it.
Determination by x-ray reflectivity and small angle x-ray scattering of the porous properties of mesoporous silica thin films
Authors: Dourdain Sandrine , Bardeau Jean-François , Colas Maggy , Smarsly Bernd , Mehdi Ahmad , Ocko Benjamin M. , Gibaud Alain
Published: May 2005

Two-dimensional hexagonal silica thin films templated by a triblock copolymer were investigated by grazing incident small angle x-ray scattering (GISAXS) and x-ray reflectivity (XR) before and after removing the surfactant from the silica matrix. XR curves—analyzed above and below the critical angle of the substrate—are evaluated by the matrix technique to obtain the average electron density of the films, the wall thickness, the electron density of the walls, the radius of the pores, and subsequently the porosity of such mesoporous films. In combination with GISAXS, the surface area of the mesopores is ascertained, thereby providing a complete analysis of the porosity in thin films by x-ray scattering methods.
Performance and reproducibility enhancement of HgCdTe molecular beam epitaxy growth on CdZnTe substrates using interfacial HgTe∕CdTe superlattice layers
Authors: Chang Yong , Zhao Jun , Abad Hisham , Grein Christoph H. , Sivananthan Sivalingam , Aoki Toshihiro , Smith David J.
Published: May 2005

Interfacial layers including HgTe∕CdTe superlattices (SLs) were introduced during the molecular-beam epitaxy growth of HgCdTe on CdZnTe (211)B substrates. Transmission-electron-microscopic observations show that the SLs smooth out the substrates’ surface roughness during growth, and can also bend or block threading dislocations in a way that prevents their propagation from the substrate into the functional HgCdTe epilayers. An average etch pit density value in the low-105cm−2 range was reproducibly achieved in long wavelength HgCdTe samples, with the best value being 4×104cm−2. Photoconductive decay lifetime measurements give values approaching theoretical limits, as determined by the intrinsic radiative and Auger recombination mechanisms. The use of such interfacial layers thus leads to enhanced growth yields and material properties.
Hexagonal close-packed Ni nanostructures grown on the (001) surface of MgO
Authors: Tian W. , Sun H. P. , Pan X. Q. , Yu J. H. , Yeadon M. , Boothroyd C. B. , Feng Y. P. , Lukaszew R. A. , Clarke R.
Published: May 2005

We report the in situ microscopy observation of an unnatural phase of Ni, a highly strained hexagonal close-packed (hcp) form which we believe is stabilized by heteroepitaxial growth on the (001) face of MgO. We find that the nanosized hcp nickel islands transform into the normal face-centered cubic structure when the size of the islands exceeds a critical value (about 2.5 nm thick with a lateral size of ∼5nm). The structural transition proceeds via a martensitic change in the stacking sequence of the close-packed planes. The formation of hcp Ni nanostructures with an unusually large crystallographic c∕a ratio (∼6% larger than ideal hcp) is very interesting for spintronic and recording applications where large uniaxial anisotropies are desirable.
The vibration-rotation emission spectra of gaseous CdH2 and CdD2
Authors: Yu Shanshan , Shayesteh Alireza , Bernath Peter F.
Published: May 2005

The vibration-rotation emission spectra of CdH2 and CdD2 molecules have been recorded at high resolution using a Fourier-transform spectrometer. The molecules were generated in a furnace-discharge emission source by reaction of cadmium vapor with molecular hydrogen or deuterium. The fundamental bands for the antisymmetric stretching mode (υ3) of CdH2 and CdD2 were detected at about 1771.5 and 1278.3cm−1, respectively. In addition, the 002(Σg+)–001(Σu+) and 011(Πg)–010(Πu) hot bands were observed for CdH2. Spectroscopic constants were determined for each of the 12 observed isotopologs: Cd110H2, Cd111H2, Cd112H2, Cd113H2, Cd114H2, Cd116H2, Cd110D2, Cd111D2, Cd112D2, Cd113D2, Cd114D2, and Cd116D2. The average Cd-H and Cd-D bond distances (r0) were determined to be 1.683028(10) and 1.679161(16)Å, respectively.
Fabrication of single nickel-nitrogen defects in diamond by chemical vapor deposition
Authors: Rabeau J. R. , Chin Y. L. , Prawer S. , Jelezko F. , Gaebel T. , Wrachtrup J.
Published: May 2005

Fabrication of single nickel-nitrogen (NE8) defect centers in diamond by chemical vapor deposition is demonstrated. Under continuous-wave 745nm laser excitation single defects were induced to emit single photon pulses at 797nm with a linewidth of 1.5nm at room temperature. Photon antibunching of single centers was demonstrated using a Hanbury–Brown and Twiss interferometer. Confocal images revealed approximately 106 optically active sites∕cm2 in the synthesized films. The controlled fabrication of an NE8 based single photon source in synthetic diamond is important for fiber based quantum cryptography, and potentially linear optics quantum computing.
Stable and erasable patterning of vanadium pentoxide thin films by atomic force microscope nanolithography
Authors: Iwanaga Shiho , Darling R. B. , Cobden D. H.
Published: May 2005

Long lasting colored pattern formation on sol–gel deposited vanadium pentoxide (V2O5) thin films is demonstrated by atomic force microscope (AFM) nanolithography. The films can be locally colorized by application of a positive dc bias voltage on the conductive AFM tip. The patterns can be erased, or “bleached,” by application of a negative bias voltage. The changes in optical reflectivity, topography, and conductivity measurements made on as-deposited, colored, and bleached areas were consistent with a model in which the incorporation of hydrogen ions into V2O5 thin films is responsible for the coloration process.
Wave-packet study of H2 formation on a graphite surface through the Langmuir–Hinshelwood mechanism
Authors: Morisset S. , Aguillon F. , Sizun M. , Sidis V.
Published: May 2005

We have studied the formation of the H2 molecule on a graphite surface, when both H atoms are initially physisorbed. The graphite surface is assumed to be planar. The interaction potential is modeled to reproduce the experimental properties of H physisorption on graphite. Extending our previous work [S. Morisset, F. Aguillon, M. Sizun, and V. Sidis, J. Chem. Phys. 121, 6493 (2004)], full-dimensionality quantum calculations are presented for collision energies ranging from 4to50meV. It is shown that the reaction occurs with a large cross section and produces the H2 molecule with a considerable amount of vibrational energy. The mechanism is either direct or involves the formation of an intermediate complex.
Glassy dynamics and domains: Explicit results for the East model
Authors: van Zon Ramses , Schofield Jeremy
Published: May 2005

A general matrix-based scheme for analyzing the long-time dynamics in kinetically constrained models such as the East model is presented. The treatment developed here is motivated by the expectation that slowly relaxing spin domains of arbitrary size govern the highly cooperative events that lead to spin relaxation at long times. To account for the role of large spin domains in the dynamics, a complete basis expressed in terms of domains of all sizes is introduced. It is first demonstrated that accounting for single domains of all possible sizes leads to a simple analytical result for the two-time single-spin correlation function in the East model that is in excellent quantitative agreement with simulation data for equilibrium spin-up density values c⩾0.6. It is then shown that including also two neighboring domains leads to a closed expression that describes the slow relaxation of the system down to c≈0.3. Ingredients of generalizing the method to lower values of c are also provided, as well as to other models. The main advantage of this approach is that it gives explicit analytical results and that it requires neither an arbitrary closure for the memory kernel nor the construction of an irreducible memory kernel. It also allows one to calculate quantities that measure heterogeneity in the same framework, as is illustrated on the neighbor-pair correlation function, the average relaxation time, and the width of the distribution of relaxation times.
Surface-induced phase transitions in ultrathin films of block copolymers
Authors: Cao Dapeng , Wu Jianzhong
Published: May 2005

We study theoretically the lamellar-disorder-lamellar phase transitions of AB diblock and tetrablock copolymers confined in symmetric slitlike pores where the planar surface discriminatingly adsorbs A segments but repels B segments, mimicking the hydrophobic/hydrophilic effects that have been recently utilized for the fabrication of environmentally responsive “smart” materials. The effects of film thickness, polymer volume fraction, and backbone structure on the surface morphology have been investigated using a polymer density-functional theory. The surface-induced phase transition is manifested itself in a discontinuous switch of microdomains or a jump in the surface density dictated by the competition of surface adsorption and self-aggregation of the block copolymers. The surface-induced first-order phase transition is starkly different from the thickness-induced symmetric-asymmetric or horizontal-vertical transitions in thin films of copolymer melts reported earlier.
Observation of enhanced thermal noise due to multiple Andreev reflection in ballistic InGaAs-based superconducting weak links
Authors: Akazaki Tatsushi , Nakano Hayato , Nitta Junsaku , Takayanagi Hideaki
Published: May 2005

We have experimentally obtained clear evidence of enhanced thermal noise in a ballistic superconductor-normal metal-superconductor junction with an InGaAs-based two-dimensional electron gas (2DEG). The thermal noise was estimated from a comparison of measured current–voltage characteristics with those obtained with an extended Ambegaokar and Halperin theory. As a consequence, we have observed enhanced thermal noise that is much larger than that expected with normal reservoirs. This enhanced thermal noise can be explained by a theory that considers both the ballistic transport of the 2DEG and the thermal fluctuation in the coherent multiple Andreev reflection regime.
Stretching globular polymers. II. Macroscopic cross-linked networks
Authors: Craig A. , Terentjev E. M.
Published: May 2005

We expand upon the results for the force-extension behavior of single-collapsed polymer chains to consider the mechanical response of networks of cross-linked globular polymers in poor solvent. Force-strain curves are obtained under the affine deformation approximation for networked globules with both disordered and ordered globule conformations. Due to their large stored lengths, these networks would be capable of reaching extremely large strains. They also show anomalous nonmonotonic force-strain response, as a consequence of the nonmonotonic force-extension curves of their constituent globules. Finally, we consider the stability of ordered and disordered globules in these networks and propose means taken from biological and colloid science to stabilize networked globules.
Effect of through-space electron transfer on infrared spectrum of amorphous selenium
Authors: Nakamura Kazuma , Tsuneyuki Shinji
Published: May 2005

In this paper we present theoretical analyses on an infrared (IR) spectrum of amorphous selenium. The system is described by a 216-atom-chain model, and a set of molecular-dynamics simulations is performed to generate vitreous structures and vibrational modes. To describe an electronic structure of the system we employ a complete neglect of differential overlap model parametrized by ab initio cluster calculations. An IR intensity is evaluated with the Berry-phase formula for an electronic polarization. The effect of the through-space electron transfer on the IR spectrum is studied by artificially changing the magnitude of matrix elements associated with the electron transfer between nonbonded atoms in the chain. We find that the through-space electron transfer leads to (i) the enhancement of the bending IR peak at 135cm−1 and (ii) the appearance of a new low-frequency peak around 50cm−1, thus resulting in a good agreement with the experiment. The mechanism is discussed by a simple dipole model.
Nitrogen and indium dependence of the band offsets in InGaAsN quantum wells
Authors: Galluppi Massimo , Geelhaar Lutz , Riechert Henning
Published: May 2005

The band offsets of InGaAsN single quantum wells with varying nitrogen and indium content were quantitatively determined by surface photovoltage measurements. The experimental data directly show the different effect of nitrogen on the valence and on the conduction band states. While the conduction band offset strongly increases with increasing nitrogen concentration, the valence band offset is only weakly affected. In contrast, indium influences the valence and the conduction band states in the same way: both the valence and conduction band offsets increase with increasing indium content. In particular, the conduction band offset varies with In content as in N-free InGaAs quantum wells.
Capillary condensation of short-chain molecules
Authors: Bryk Paweł , Pizio Orest , Sokolowski Stefan
Published: May 2005

A density-functional study of capillary condensation of fluids of short-chain molecules confined to slitlike pores is presented. The molecules are modeled as freely jointed tangent spherical segments with a hard core and with short-range attractive interaction between all the segments. We investigate how the critical parameters of capillary condensation of the fluid change when the pore width decreases and eventually becomes smaller than the nominal linear dimension of the single-chain molecule. We find that the dependence of critical parameters for a fluid of dimers and of tetramers on pore width is similar to that of the monomer fluid. On the other hand, for a fluid of chains consisting of a larger number of segments we observe an inversion effect. Namely, the critical temperature of capillary condensation decreases with increasing pore width for a certain interval of values of the pore width. This anomalous behavior is also influenced by the interaction between molecules and pore walls. We attribute this behavior to the effect of conformational changes of molecules upon confinement.
Multiguide directional coupler using switchable liquid-crystalline optical channels
Authors: Kuo Chie-Tong , Huang Shuan-Yu , Jiang I-Min , Tsai Ming-Shan
Published: May 2005

A multiguide directional coupler with a gratinglike indium-tin-oxide electrode on a planar nematic liquid-crystal film is studied. The linearly polarized beam can be coupled into adjacent channels with the periodic modulation of the refractive index by applying a periodic electric field. The intensity distribution of coupling in the transverse direction varies with the distance of beam propagation. The coupling effects, which depend on the polarization of the incident beam and the temperature of the liquid crystals, are also discussed.
In situ photoemission electron spectroscopy of plasma-nitrided metal alloys
Authors: Figueroa C. A. , Alvarez F.
Published: May 2005

In this paper, we report the influence of oxygen on the structure and chemical compositions of the surface of low-energy (∼50eV) implanted stainless steel studied by in situ photoemission electron spectroscopy. The presence of oxygen at the surface forms thermodynamically stable oxides and hydroxides, degrading metallic nitrides, and preventing efficient nitrogen diffusion into the bulk material. Among these metallic nitrides, γN and FeNx are more susceptible to oxidize. Lower oxygen partial pressures augment nitrogen content at the surface determining material bulk properties.
Analytical study on exciton-longitudinal-optical-phonon coupling and comparison with experiment for ZnO quantum wells
Authors: Makino T. , Segawa Y. , Kawasaki M.
Published: May 2005

The temperature-dependent linewidths of excitons in ZnO quantum wells were studied by measuring the absorption spectra from 5 K to room temperature. We deduced experimentally the exciton-longitudinal-optical-phonon coupling strength, which showed reduction of coupling with decrease in well width. This reduction was explained in terms of the confinement-induced enhancement of the excitonic binding energy by comparing the binding energy dependence of the calculated coupling strength.
The near field of coaxial jets: A numerical study
Authors: Balarac Guillaume , Métais Olivier
Published: May 2005

The near-field behavior of coaxial jets is studied through direct numerical simulation (DNS) with a particular focus on the influence of the inner shear layer steepness characterized by its momentum thickness θ01 thus mimicking the variation in the lip thickness of a real jet nozzle. We investigate the two distinct jet regimes ru>ruc for which a recirculation bubble is present near the jet inlet and ru<ruc without any recirculation bubble, ru being the velocity ratio between the outer jet and inner jet velocities. It is shown that small values of θ01 lead to a fast transition to turbulence. The various mechanisms leading to this transition are investigated in detail: the three-dimensionality growth, the appearance of secondary vortices superimposed on the main ring vortices, and the subsequent longitudinal stretching of streamwise vortices. This stretching mechanism is shown to play a dominant role in the transition processes towards a fully developed turbulent state. For high enough values of ru, a pinching of the jet is observed near the inlet and it is shown that this pinching phenomenon lasts on a shorter downstream distance for small values of θ01 due to a more efficient turbulent mixing. In the ru>ruc case, variations of θ01 strongly affect the shape and the downstream extent of the recirculation bubble. The DNS allow to show the strong dependency of the inner and outer potential core lengths and of the critical value ruc on the jet inlet velocity profile. We finally revisit the theoretical model originally proposed by Rehab, Villermaux, and Hopfinger [“Flow regimes of large-velocity-ratio coaxial jets,” J. Fluid Mech. 345, 357 (1997)] first aimed at the prediction of the variations of various jet characteristics as a function of ru. The model is extended to determine the dependency of the jet characteristics with θ01. A very good correspondence between the theoretical predictions and the numerical results is obtained.
Enhanced nonradiative relaxation and photoluminescence quenching in random, doped nanocrystalline powders
Authors: Ruan X. L. , Kaviany M.
Published: May 2005

Nonradiative relaxation and photoluminescence quenching in nanocrystalline powders doped with rare-earth elements are of interest in optical bistability, random laser, and other optoelectronic applications. Here, the luminescence quenching of a one-dimensional random medium made of multilayer nanoparticles (Y2O3) doped with rare-earth elements (Yb3+) is analyzed by considering the transport, transition, and interaction of the fundamental energy carriers. The nonradiative decay and luminescence quenching in random media are enhanced compared to single crystals, due to multiple scattering, enhanced absorption, and low thermal conductivity. The coherent wave treatment is used to calculate the photon absorption, allowing for field enhancement and photon localization. The luminescent and thermal emission is considered as incoherent. The size-dependent absorption coefficient and penetration depth are observed. The nonradiative decay is identified as a multiphonon relaxation process, and is found to be enhanced compared to bulk materials. The luminescence quenching and nonlinear thermal emission, occurring with increasing irradiation intensity, are predicted.
Anisotropic strain relaxation in (Ba0.6Sr0.4)TiO3 epitaxial thin films
Authors: Simon W. K. , Akdogan E. K. , Safari A.
Published: May 2005

We have studied the evolution of anisotropic epitaxial strains in ⟨110⟩-oriented (Ba0.60Sr0.40)TiO3 paraelectric (m3m) thin films grown on orthorhombic (mm2) ⟨100⟩-oriented NdGaO3 by high-resolution x-ray diffractometry. All the six independent components of the three-dimensional strain tensor were measured in films with 25–1200-nm thickness, from which the principal stresses and strains were obtained. Pole figure analysis indicated that the epitaxial relations are [001]m3m‖[001]mm2 and [1¯10]m3m‖[010]mm2 in the plane of the film, and [110]m3m‖[100]mm2 along the growth direction. The dislocation system responsible for strain relief along [001] has been determined to be ∣b∣(001)=3∕4∣b∣. Strain relief along the [1¯10] direction, on the other hand, has been determined to be due to a coupled mechanism given by ∣b∣(1¯10)=∣b∣ and ∣b∣(1¯10)=3∕4∣b∣. Critical thicknesses, as determined from nonlinear regression using the Matthews–Blakeslee equation, for misfit dislocation formation along [001] and [1¯10] direction were found to be 5 and 7 nm, respectively. The residual strain energy density was calculated as ∼2.9×106J∕m3 at 25 nm, which was found to relax an order of magnitude by 200 nm. At 200 nm, the linear dislocation density along [001] and [1¯10] are ∼6.5×105 and ∼6×105cm−1, respectively. For films thicker than 600 nm, additional strain relief occurred through surface undulations, indicating that this secondary strain-relief mechanism is a volume effect that sets in upon cooling from the growth temperature.
Complete analysis of phase transitions and ensemble equivalence for the Curie–Weiss–Potts model
Authors: Costeniuc Marius , Ellis Richard S. , Touchette Hugo
Published: May 2005

Using the theory of large deviations, we analyze the phase transition structure of the Curie–Weiss–Potts spin model, which is a mean-field approximation to the nearest-neighbor Potts model. It is equivalent to the Potts model on the complete graph on n vertices. The analysis is carried out both for the canonical ensemble and the microcanonical ensemble. Besides giving explicit formulas for the microcanonical entropy and for the equilibrium macrostates with respect to the two ensembles, we analyze ensemble equivalence and nonequivalence at the level of equilibrium macrostates, relating these to concavity and support properties of the microcanonical entropy. The Curie–Weiss–Potts model is the first statistical mechanical model for which such a detailed and rigorous analysis has been carried out.
Isothermal stress relaxation in electroplated Cu films. I. Mass transport measurements
Authors: Gan Dongwen , Ho Paul S. , Huang Rui , Leu Jihperng , Maiz Jose , Scherban Tracey
Published: May 2005

Recent studies on Cu interconnects have shown that interface diffusion between Cu and the cap layer dominates mass transport for electromigration. The kinetics of mass transport by interface diffusion strongly depends on the material and processing of the cap layer. In this series of two papers, we report in Part I the interface and grain-boundary mass transport measured from isothermal stress relaxation in electroplated Cu thin films with and without a passivation layer and in Part II a kinetic model developed to analyze the stress relaxation based on the coupling of grain boundary and interface diffusion. We show that a set of isothermal stress relaxation experiments together with appropriate modeling analysis can be used to evaluate the kinetics of interface and grain-boundary diffusion that correlate to electromigration reliability of Cu interconnects. Thermal stresses in electroplated Cu films with and without passivation, subjected to thermal cycling and isothermal annealing at selected temperatures, were measured using a bending-beam technique. Thermal cycling experiments showed the effect of passivation and provided information to select the initial stresses and temperatures for isothermal stress measurements. Isothermal experiments at moderate temperatures showed a significant transient behavior of stress relaxation. Based on the kinetic model developed in Part II, grain boundary and interface diffusivities were deduced. While the deduced grain boundary diffusivity reasonably agrees with other studies, the diffusivity at the Cu∕SiN cap layer interface was found to be generally lower than the grain-boundary diffusivity at the temperature range of the present study.
Isothermal stress relaxation in electroplated Cu films. II. Kinetic modeling
Authors: Huang Rui , Gan Dongwen , Ho Paul S.
Published: May 2005

In Part I we reported experimental results obtained from isothermal stress relaxation tests of electroplated Cu thin films with and without a passivation layer and deduced grain-boundary and interface diffusivities based on a kinetic model. Here in Part II we describe the detail of the model, which is based on coupling of grain-boundary diffusion with surface diffusion for unpassivated films and with interface diffusion for passivated films. Numerical solutions are obtained for the coupled diffusion problems and analytical solutions are obtained for several limiting cases. The effects of surface diffusivity and interface diffusivity on stress relaxation of polycrystalline thin films are analyzed and compared with experiments. The model predicts a transient behavior of stress relaxation and provides a quantitative correlation between stress relaxation and the kinetics of mass transport. In particular, the models can be used together with isothermal stress relaxation tests to characterize interface diffusion and to evaluate selected cap layers for improving electromigration reliability of Cu interconnects.
Effect of surface treatments on self-trapped exciton luminescence in single-crystal CaF2
Authors: Cramer L. P. , Cumby T. D. , Leraas J. A. , Langford S. C. , Dickinson J. T.
Published: May 2005

We show that near-surface defects produced by mechanical treatments and electron irradiation can significantly enhance the intensity of luminescence due to the decay of self-trapped excitons (STEs) in single-crystal calcium fluoride during 157- and 193-nm irradiation. For example, polishing can double the intensity of the STE luminescence. Defects produced by mechanical indentation can either increase or decrease the luminescence intensity, depending on the indentation force. Electron irradiation also enhances subsequent STE luminescence. When electron-irradiated samples are annealed, additional increases in luminescence intensity are observed. Plausible mechanisms for the observed effects on STE luminescence intensity are discussed.
Grid computing and collaboration technology in support of fusion energy sciences
Authors: Schissel D. P.
Published: May 2005

Science research in general and magnetic fusion research in particular continue to grow in size and complexity resulting in a concurrent growth in collaborations between experimental sites and laboratories worldwide. The simultaneous increase in wide area network speeds has made it practical to envision distributed working environments that are as productive as traditionally collocated work. In computing power, it has become reasonable to decouple production and consumption resulting in the ability to construct computing grids in a similar manner as the electrical power grid. Grid computing, the secure integration of computer systems over high speed networks to provide on-demand access to data analysis capabilities and related functions, is being deployed as an alternative to traditional resource sharing among institutions. For human interaction, advanced collaborative environments are being researched and deployed to have distributed group work that is as productive as traditional meetings. The DOE Scientific Discovery through Advanced Computing Program initiative has sponsored several collaboratory projects, including the National Fusion Collaboratory Project, to utilize recent advances in grid computing and advanced collaborative environments to further research in several specific scientific domains. For fusion, the collaborative technology being deployed is being used in present day research and is also scalable to future research, in particular, to the International Thermonuclear Experimental Reactor experiment that will require extensive collaboration capability worldwide. This paper briefly reviews the concepts of grid computing and advanced collaborative environments and gives specific examples of how these technologies are being used in fusion research today.
A diagnostic method for real-time measurements of the density of nitrogen atoms in the postglow of an Ar–N2 discharge using a catalytic probe
Authors: Mozetic M. , Cvelbar U. , Vesel A. , Ricard A. , Babic D. , Poberaj I.
Published: May 2005

We determined the density of neutral nitrogen atoms in an Ar–N2 postglow using a fiber-optics catalytic probe. The probe, which had a catalyst made of nitrided iron, was calibrated with a NO titration. The recombination coefficient for the heterogeneous recombination of N atoms on the nitrided iron surface was determined by comparing the probe signal with the NO titration. Within the limits of experimental error the coefficient was found to be independent of the catalyst temperature between 400 and 650K and had a value of 0.21±0.04. Real-time measurements of the N-atom density were performed at a nitrogen flow of 600SCCM (standard cubic centimeter per minute) for several discharge powers between 80 and 300W, and for argon flow rates between 200 and 3000SCCM. With increasing discharge power the N-atom density increased monotonously; with increasing Ar flow the N-atom density at first increased, reaching a broad maximum at about 1.8×1021m−3 for an Ar flow of 2000SCCM, after which the N-atom density decreased with any further increase of Ar flow.
Investigation of dye-doped red emitting organic electroluminescent devices with metal-mirror microcavity structure
Authors: Sun X. Y. , Li W. L. , Hong Z. R. , Wei H. Z. , Zang F. X. , Chen L. L. , Shi Z. , Bi D. F. , Li B. , Zhang Z. Q. , Hu Z. Z.
Published: May 2005

Organic electroluminescent (EL) devices with planar microcavity structure, indium-tin-oxide/Ag∕N,N′-diphenyl-N , N ′-bis(3-methylphenyl)-1,1′-biphenyl-4 , 4′-diamine/tris(8-hydroxyquinoline)-aluminum (AlQ):4-(dicyanomethylene)-2-methyl-6-(p-dimethyl aminostyryl)-4H-pyran/AlQ∕LiF∕Al, were fabricated. The Ag and Al layers acted as not only hole-injection layer and cathode, respectively, but reflective mirrors, resulting in strong microcavity effects, such as spectral narrowing and directional emission. The effects of device parameters on the EL performance were studied in detail and were discussed in terms of conventional microcavity theory. On-axis light magnification with a coefficient (EL enhancement ratio between cavity and noncavity devices) of ∼5 was observed, which was consistent with the theoretical calculation. At the same time, optimized microcavity device with bright pure red emission showed maximum luminance of 5140cd∕m2, peak at 624nm, Commission International de I’Eclairage coordinates of x=0.663 and y=0.336, and high EL efficiency of 1.71cd∕A were obtained.
Full-spectrum nonlinear response of a sinusoidally modulated rotating disk electrode
Authors: Wilson Jamie R. , Adler Stuart B. , Schwartz Daniel T.
Published: May 2005

The nonlinear behavior of a sinusoidally modulated rotating disk electrode (RDE) is analyzed to identify the flow and concentration field interactions that lead to resonance and nullification conditions in the electrochemical response of the system. Steady periodic solutions to the von Kármán self-similar form of the Navier–Stokes equations and convective diffusion equation are examined using a combined Fourier transform/perturbation approach that yields a set of frequency dispersion equations solved using the finite element method. Experimental measurements using a RDE with sinusoidal angular velocity waveform Ω=Ω0+ΔΩcos(ω0t) are compared to theory for modulation amplitudes ΔΩ∕Ω0<1 and electrolyte Schmidt number of 1680. The modulation frequency ω0 is varied from the quasisteady to high-frequency limits to obtain the phase and amplitude of the mean, fundamental, and higher-harmonic limiting current. We show that distinct features (resonances and nullifications) in the nonlinear mean and higher harmonic components of limiting current result from constructive and destructive (in phase and 180° out of phase, respectively) interactions between cross terms of the oscillating components in the axial flow and concentration fields near the disk surface.
Polar nanodomains and local ferroelectric phenomena in relaxor lead lanthanum zirconate titanate ceramics
Authors: Shvartsman V. V. , Kholkin A. L. , Orlova A. , Kiselev D. , Bogomolov A. A. , Sternberg A.
Published: May 2005

Transparent Pb0.9125La0.0975(Zr0.65Ti0.35)0.976O3 ceramics (conventionally abbreviated as PLZT 9.75/65/35) is a typical relaxor characterized by the absence of the ferroelectric order at the macroscopic scale. In this letter, we report on the observation of complex polar structures on the surface of this material via piezoresponse force microscopy (PFM). The irregular polarization patterns are associated with the formation of a glassy state, where random electric fields destroy the long-range ferroelectric order. The measure of the disorder, the correlation length of ∼50nm, was directly deduced from the PFM images. Local poling of relaxor ceramics resulted in the formation of a stable micron-size domain that could be continuously switched under varying dc bias (local relaxor-ferroelectric phase transition). Fractal analysis was applied to analyze the origin of local order in PLZT.
Near-infrared electroluminescent devices based on colloidal HgTe quantum dot arrays
Authors: O’Connor É. , O’Riordan A. , Doyle H. , Moynihan S. , Cuddihy A. , Redmond G.
Published: May 2005

Crystalline 4.6 nm HgTe quantum dots, stabilized by 1-thioglycerol ligands, were synthesized by wet chemical methods. Room-temperature photoluminescence spectra of the dots, both in solution and as solid arrays, exhibited near-infrared emission. Light-emitting devices were fabricated by deposition of quantum dot layers onto glass∕indium tin oxide (ITO)∕3,4-polyethylene-dioxythiophene-polystyrene sulfonate (PEDOT) substrates followed by top contacting with evaporated aluminum. Room-temperature near-infrared electroluminescence from 1mm2 ITO∕PEDOT∕HgTe∕Al devices, centered at ∼1600nm, with an external quantum efficiency of 0.02% and brightness of 150nW∕mm2 at 50 mA and 2.5 V was achieved.
Miniaturizable Si-based electro-optical modulator working at 1.5 μm
Authors: Sciuto Antonella , Libertino Sebania , Coffa Salvo , Coppola Giuseppe
Published: May 2005

Optoelectronic devices are considered the innovative element for the next generation of microelectronic integrated circuits. For this purpose, both active and passive devices—extremely miniaturized—must be implemented. We fabricated and electro-optical Si-based light intensity modulator working at 1.5 μm using a bipolar mode field-effect transistor integrated within a Si rib waveguide. The principle of operation is the light absorption by a plasma of free carriers that can be opportunely moved inside or outside of the device optical channel by properly changing the control bias. The devices, only 100 μm long, were fabricated using epitaxial Si wafers and standard clean room processing. The optical characterization at 1.48 μm in static conditions shows a modulation of ∼90% while the dynamic electrical characterization provides a switching time of ≈10ns (foreseen modulation frequency of hundreds of MHz). A modulation depth above 25% is observed for modulation frequency up to 300 kHz.
Surface polarities of sputtered epitaxial CuInSe2 and Cu1In3Se5 thin films grown on GaAs (001) substrates
Authors: Yang L.-C. , Chen G. S. , Rockett A.
Published: May 2005

Epitaxial CuInSe2 and Cu1In3Se5 films have been synthesized on GaAs(001) by cosputtering Cu∕In and evaporating Se method. Scanning electron micrograph results show that surface morphologies of CuInSe2 and Cu1In3Se5 epitaxial films are substantially different. The rectangular pits of CuInSe2 films imply that the surface energy of (112)B[Se-terminated] is lower than (112)A[metal-terminated] in chalcopyrite CuInSe2 crystals. Nevertheless, the square pits of the Cu1In3Se5 films lead to the conclusion that (112)A and (112)B have almost the same surface energies in the defect-ordered chalcopyrite Cu1In3Se5 crystals. It implies that charge compensation between (112)A and (112)B facets is the driving force of the square pits formation in Cu1In3Se5 crystals.
Liquefaction of H2 molecules upon exterior surfaces of carbon nanotube bundles
Authors: Han Sang Soo , Kang Jeung Ku , Lee Hyuck Mo , van Duin Adri C. T. , Goddard William A.
Published: May 2005

We have used molecular dynamics simulations to investigate interaction of H2 molecules on the exterior surfaces of carbon nanotubes (CNTs): single and bundle types. At 80 K and 10 MPa, it is found that charge transfer occurs from a low curvature region to a high curvature region of the deformed CNT bundle, which develops charge polarization only on the deformed structure. The long-range electrostatic interactions of polarized charges on the deformed CNT bundle with hydrogen molecules are observed to induce a high local-ordering of H2 gas that results in hydrogen liquefaction. Our predicted heat of hydrogen liquefaction on the CNT bundle is 97.6kcalkg−1. On the other hand, hydrogen liquefaction is not observed in the CNT of a single type. This is because charge polarization is not developed on the single CNT as it is symmetrically deformed under the same pressure. Consequently, the hydrogen storage capacity on the CNT bundle is much higher due to liquefaction than that on the single CNT. Additionally, our results indicate that it would also be possible to liquefy H2 gas on a more strongly polarized CNT bundle at temperatures higher than 80 K.
Free energy molecular dynamics simulations of pulsed-laser-irradiated SiO2: Si–Si bond formation in a matrix of SiO2
Authors: Boero Mauro , Oshiyama Atsushi , Silvestrelli Pier Luigi , Murakami Kouichi
Published: May 2005

Recent experiments have shown that pure Si structures in a matrix of SiO2 can be formed by electron excitation techniques, with appealing applications in nanotechnology. Our ab initio simulations provide an insight into the underlying mechanism, showing that electron excitations weaken Si–O bonds in SiO2, dislodge O atoms and allow Si dangling bonds to reconstruct in stable Si–Si structures below the melting temperature. Differences in diffusivity of O (fast) and Si (slow) are shown to play a decisive role in the process.
C
O
2
detection using carbon nanotube networks and micromachined resonant transducers
Authors: Zribi Anis , Knobloch Aaron , Rao Rashmi
Published: May 2005

A CO2 sensor using single-walled carbon nanotubes (CNT) and a microelectromechanical system (MEMS) resonator is demonstrated. The MEMS transducer comprises a membrane driven into transverse vibrations by means of Lorentz forces. A downshift in the resonant frequency of the device is measured by a laser vibrometer when changes in the stress state of the CNT film∕membrane structure are caused by adsorption of CO2 onto the nanotubes. The sensor has shown excellent sensitivity, linearity, and recovery over a broad range of concentrations (0–15vol%). In comparison to resistive, dielectric, and gravimetric CNT transducers, this sensor displayed remarkable intrinsic selectivity in the presence of interferences.
Nanoscale heat conduction across tunnel junctions
Authors: Ju Y. Sungtaek , Hung M.-T. , Carey M. J. , Cyrille M.-C. , Childress J. R.
Published: May 2005

Nanoscale heat conduction across tunnel junctions prepared through natural oxidation of metal electrodes is experimentally studied. The thermal resistance of AlOx tunnel barriers increases linearly with barrier thickness, which is consistent with the prevailing theory of heat conduction in highly disordered materials. Heat conduction across tunnel junctions is strongly impeded by finite thermal resistance at interfaces between barrier and electrode layers, which can be exploited to create superior thermal barrier coatings. The thermal conductivity of nanolaminates consisting of a series of Ta∕TaOx tunnel junctions is determined to be well below the minimum thermal conductivity limit.
Morphological effects on the field emission of ZnO nanorod arrays
Authors: Zhao Q. , Zhang H. Z. , Zhu Y. W. , Feng S. Q. , Sun X. C. , Xu J. , Yu D. P.
Published: May 2005

The field-emission properties of ordered ZnO nanorod arrays with different morphologies were investigated in detail. After comparison of three different morphologies, it was found that the morphology of the ZnO nanorods has considerable effect on their field emission properties, especially the turn-on field and the emission current density. Among them, the ZnO nanoneedle arrays have the lowest turn-on field, highest current density, and the largest emission efficiency, which is ascribed to the small emitter radius on the nanoscale. On the other hand, high nanorod density remarkably reduces the local field at the emitters owing to the screening effect, which is related to the density of the emitters. The analysis results could be valuable for the application of field-emission-based devices using ZnO nanorod arrays as cathode materials.