bluuebunny/crossref_metadata_2025 huggingface dataset:
Deep level defects in proton radiated GaAs grown on metamorphic SiGe∕Si substrates
Authors: González M. , Andre C. L. , Walters R. J. , Messenger S. R. , Warner J. H. , Lorentzen J. R. , Pitera A. J. , Fitzgerald E. A. , Ringel S. A.
Published: August 2006

The effect of 2MeV proton radiation on the introduction of deep levels in GaAs grown on compositionally graded SiGe∕Si substrates was investigated using deep level transient spectroscopy (DLTS). Systematic comparisons were made with identical layers grown on both GaAs and Ge substrates to directly assess the influence of threading dislocations on radiation-related deep levels for both n-type and p-type GaAs. DLTS revealed that for p+n structures, proton irradiation generates electron traps at Ec−0.14eV, Ec−0.25eV, Ec−0.54eV, and Ec−0.72eV in the n-GaAs base, and, for n+p structures, radiation-induced hole traps appear at Ev+0.18eV, Ev+0.23eV, Ev+0.27eV, and Ev+0.77eV in the p-type GaAs base, irrespective of substrate choice for both polarities. The primary influence of substituting SiGe∕Si substrates for conventional GaAs and Ge substrates is on the introduction rates of the individual traps as a function of proton radiation fluence. Substantially reduced concentrations are found for each radiation-induced hole trap observed in p-type GaAs, as well as for the Ec−0.54eV trap in n-GaAs for samples on SiGe∕Si, as a function of proton fluence. Calculated trap introduction rates reveal reductions by as much as ∼40% for certain hole traps in p-GaAs grown on SiGe∕Si. This increased radiation tolerance for GaAs grown on SiGe∕Si is attributed to interactions between the low density (∼106cm−2) of residual dislocations within the metamorphic GaAs∕SiGe∕Si structure and the radiation-induced point defects. Nevertheless, the fact that the impact of dislocations on radiation tolerance is far more dramatic for n+p GaAs structures compared to p+n structures, may have implications on future III-V∕Si space solar cell design optimization, since end-of-life versus beginning-of-life differences are critical factors for power profiling in high radiation environments.
Dielectric relaxation in pulsed laser ablated CaCu3Ti4O12 thin film
Authors: Srivastava A. , Gupta Vinay , Sarkar C. K. , Das R. R. , Bhattacharya P. , Katiyar R. S.
Published: August 2006

The dielectric constant and ac conductivity of CaCu3Ti4O12 thin films deposited on platinized silicon substrate using pulsed laser deposition technique have been measured in the metal-insulator- metal configuration over wide temperature (80–500K) and frequency (100Hz–1MHZ) ranges. The crystallographic structure and the phase purity of the deposited films were investigated using x-ray diffraction and micro-Raman spectroscopy. The dielectric dispersion data have been fitted to Debye-type relaxation with a distribution of relaxation times and an asymmetric distribution of relaxation was observed which increases with increase in ωτ.
Exchange bias between cobalt clusters and oxide thin films
Authors: Portemont Céline , Morel Robert , Brenac Ariel , Notin Lucien
Published: August 2006

This paper presents a comparison between exchange bias measurements for Co clusters on CoO, NiO, and SiO2 thin films. The proximity effect between clusters is examined by varying the amount of deposited clusters, for both ferromagnetic clusters and core-shell clusters with ferromagnetic core and antiferromagnetic shell. It is found that clusters with an antiferromagnetic shell show a stronger exchange coupling than deposited clusters resting on antiferromagnetic thin films. It is also observed that the connectivity of the antiferromagnetic shells, whether by direct contact between clusters or by indirect coupling via the antiferromagnetic substrate, is necessary to achieve large exchange anisotropy.
Analysis of SiOxCyHz polymeric materials by x-ray absorption spectroscopy: Anomalous behavior of the resonant SiKLL Auger spectra
Authors: Barranco A. , Yanguas-Gil A. , Yubero F. , González-Elipe A. R.
Published: August 2006

Silicone-like SiOxCyHz polymeric thin films with different C∕O atomic ratios have been studied by x-ray absorption spectroscopy and resonant Auger electron spectroscopy. The energy of the onset of the SiK absorption edge varies with the film composition from the value observed in SiC (i.e.,∼1840eV) to that in SiO2 (i.e.,∼1844eV). In the samples with a low C∕O atomic ratio (i.e.,∼1),the main band of the SiK spectra depicts two well defined peaks that can be associated with the Si-C and Si-O bonds present in the samples. Their relative intensity can be used to assess the concentration of these two bond types. Resonant SiKLL Auger spectra of the polymeric thin film have been measured using photon energies across the SiK absorption edge. A first Auger peak appearing at a high kinetic energy was recorded for photon energies around the onset of the absorption edge and the first peak/shoulder of the SiK spectrum associated with the Si-C bonds. This resonant SiKLL Auger signal depicts the typical behavior of a resonant Auger process, i.e., its intensity increases when increasing the photon energy, passes through a maximum and then decreases to disappear. In parallel with the evolution of this resonant spectrum, a “normal” SiKLL Auger spectrum with a lower kinetic energy appeared for photon energies around the middle of the absorption curve. This spectrum is the only one prevailing at photon energies well above the absorption edge. An outstanding behavior of the SiKLL resonant Auger peaks in the polymers with a high C∕O atomic ratio (i.e., ⩾4) is that they present a negative dispersion with photon energy, i.e., their kinetic energy decreases as the photon energy increases. This behavior is opposite to that generally found for resonant Auger peaks in most systems and particularly for SiO2. Although the origin of such behavior is still unclear, the study of the SiK absorption edge and the corresponding SiKLL resonant Auger spectra results helpful for the structural and electronic characterization of SiOxCyHz polymers.
Relationship between the microscopic morphology and the charge transport properties in poly(3-hexylthiophene) field-effect transistors
Authors: Surin M. , Leclère Ph. , Lazzaroni R. , Yuen J. D. , Wang G. , Moses D. , Heeger A. J. , Cho S. , Lee K.
Published: August 2006

We fabricate field-effect transistors (FETs) by depositing a regioregular poly(3-hexylthiophene) (RR-P3HT) active layer via different preparation methods. The solvent used in the polymer film deposition and the deposition technique determine the film microstructure, which ranges from amorphous or granular films to a well-defined fibrillar texture. The crystalline ordering of RR-P3HT into fibrillar structures appears to lead to optimal FET performances, suggesting that fibrils act as efficient “conduits” for the charge carrier transport. Treating the silicon oxide gate insulator with hexamethyldisilazane enhanced the FET performance.
Capillarity-assisted fabrication of nanostructures using a less permeable mold for nanotribological applications
Authors: Suh Kahp Y. , Jeong Hoon Eui , Kim Deok-Ho , Singh R. Arvind , Yoon Eui-Sung
Published: August 2006

A simple kinetic model is presented to describe the capillary rise of a thin polymer film into a less permeable polyurethane acrylate mold. In this model, capillarity is explained by the competition between capillary and hydrodynamic forces in the course of pattern formation. For a less permeable mold, it was found that the capillary rise increases linearly with time. In addition, the contribution of viscosity and film thickness disappears such that the kinetics is solely governed by the permeation kinetics and capillary force. The present model would be useful to describe the evolution of molded nanostructures when a less permeable mold material other than polydimethylsiloxane is used for the patterning. Moreover, nanostructures with different tip shapes (rounded or dimpled) were observed depending on the fabrication temperature. The structures were tested for potential nanotribological applications such as reduction in adhesive and friction forces.
Conductivities of heterogeneous media with graded anisotropic constituents
Authors: Duan H. L. , Karihaloo B. L. , Yi X. , Wang J.
Published: August 2006

Two methods are presented to predict the effective conductivities of heterogeneous media containing discretely suspended particles. The particles have either graded anisotropy or a graded anisotropic interphase. A differential replacement procedure based on an energy equivalency condition is presented first to replace the graded anisotropic constituents by equivalent homogeneous isotropic particles. This allows many approximate schemes to be used to predict the effective conductivities of the heterogeneous media containing graded anisotropic constituents from the conductivity of the equivalent homogeneous particles. Next, the optimized upper and lower bounds on the effective conductivities of these heterogeneous media are presented by introducing comparison materials. It is shown that the DRP predictions are within these bounds for the considered media.
Additional fluorine passivation to pyrolytic-N2O passivated ultrathin silicon oxide/Si(100) films
Authors: Yamada Hiroshi
Published: August 2006

To enhance the reliability of ultrathin silicon oxide/Si(100) films and clarify the effect of fluorine on it, in situ pyrolytic-gas passivation (PGP) using NF3 was simultaneously performed with the previously proposed PGP using N2O. As a result, the following synergistic effects of F and N passivation for the films were confirmed: The electrical characteristics, such as the time-dependent dielectric breakdown lifetime, potential barrier height energy of the oxide, and interface state density, were significantly improved. Quantitative analyses of F and N indicated that this is probably caused by microscopic structural changes in the oxide near the oxide-Si(100) substrate interface. It is, therefore, believed that F passivation effectively contributes to compensate the inconsistent-state bonding sites near the interface that remain with N passivation.
Correlation between optical and electrical properties in In0.52Al0.48As∕InxGa1−xAs metamorphic high-electron-mobility-transistor structures on GaAs substrates
Authors: Cui L. J. , Zeng Y. P. , Wang B. Q. , Zhu Z. P. , Guo S. L. , Chu J. H.
Published: August 2006

4.2 K photoluminescence (PL) and 77K standard Hall-effect measurements were performed for In0.52Al0.48As∕InxGa1−xAs metamorphic high-electron-mobility-transistor (HEMT) structures grown on GaAs substrates with different indium contents in the InxGa1−xAs well or different Si delta-doping concentrations. It was found that electron concentrations increased with increasing PL intensity ratio of the “forbidden” transition (the second electron subband to the first heavy-hole subband) to the sum of the “allowed” transition (the first electron subband to the first heavy-hole subband) and the forbidden transition. And electron mobilities decreased with increasing product of the average full width at half maximum of allowed and forbidden transitions and the electron effective mass in the InxGa1−xAs quantum well. These results show that PL measurements are a good supplemental tool to Hall-effect measurements in optimization of the HEMT layer structure.
Shear viscosity of bulk and ribbon glassy Pd40Cu30Ni10P20 well below and near the glass transition
Authors: Bobrov O. P. , Khonik V. A. , Lyakhov S. A. , Csach K. , Kitagawa K. , Neuhäuser H.
Published: August 2006

Measurements of the shear viscosity η of bulk and ribbon glassy Pd40Cu30Ni10P20 in a wide range of temperatures and heating rates Ṫ have been performed. It has been determined that η is strongly dependent on Ṫ below the glass transition temperature Tg, in agreement with earlier viscosity data on other metallic glasses, and independent of Ṫ above Tg, in contrast to previously reported data for Zr-based bulk and ribbon glasses. The heating rate dependencies of the glass transition temperature and shear viscosity at T=Tg have been studied in detail. It has been found that the inverse glass transition temperature logarithmically decreases with the heating rate while the shear viscosity at the glass transition upon glass production is by several orders of magnitude smaller than 1012Pa×s usually assumed for this case. The viscosities of bulk and ribbon samples below Tg are nearly the same, in spite of four-orders-of-magnitude difference in the production quenching rates, whereas the density is notably smaller for the ribbons. Those latter facts are shown to be in evident disagreement with the free volume model.
Carrier generation and carrier determination in dipyridyldiketopyrrolopyrrole-based H2 gas sensors
Authors: Takahashi H. , Mizuguchi J.
Published: August 2006

Operation mechanism of dipyridyldiketopyrrolopyrrole (DPPP)-based H2 sensor has been investigated with special attention to the electric-field-assisted dissociation of H2 in the presence of Pd or Pt, as well as the carrier generation and carrier determination. H2 is found to dissociate exponentially with an increase of the electric field above 102–3V∕cm as monitored by an electric current. The dissociation is greatly enhanced by the presence of DPPP which serves as a proton acceptor due to its pyridyl-N atom. The detection of H2 starts with dissociation of H2, followed by protonation of atomic hydrogen (H) at the N atom of the pyridyl ring of DPPP, ending up with a release of an electron which contributes to the electrical conduction: H+pyridyl ring→protonated pyridyl ring (N+–H)+e′. The electron conduction is confirmed by the Seebeck effect. Judging from the above process, the atomic hydrogen is pictured as an electron donor. However, the present doping is rather characterized by an instantaneous one, since the extent of doping depends on the ambient H2 concentration.
Dielectric-spectroscopic and ac conductivity investigations on copper doped layered Na1.7K0.3Ti3O7 ceramics
Authors: Maurya Deepam , Kumar Jitendra , Shripal
Published: August 2006

The dielectric losses in copper doped derivatives of layered Na1.7K0.3Ti3O7 ceramic are the cooperative contribution of electric conduction, dipole orientation, and space charge polarization. Moreover, copper doping decreases dielectric loss due to inhibition of domain wall motion. The room temperature electron paramagnetic resonance spectra reveal that copper occupies Ti4+ site with divalent oxidation state (Cu2+). For all compositions, ferroelectric phase transition having high transition temperature has been identified. It is also observed that less doping enhances electron hopping (polaron) conduction and impedes the interlayer ionic conduction. However, electron hopping conduction is dominant over lower temperature region, but as the temperature progresses, interlayer ionic conduction begins to prevail.
Optical properties of Er3+ doped Ca3(BO3)2 crystal
Authors: Lu Xiuai , You Zhenyu , Li Jianfu , Zhu Zhaojie , Jia Guohua , Wu Baichang , Tu Chaoyang
Published: August 2006

The polarized absorption, emission spectra, and the fluorescence decay curve of Er3+ ions in the anisotropic Ca3(BO3)2 crystal were investigated at room temperature. The experimental oscillator strengths for the transitions from the ground state to excited states were compared with the theoretical electric dipole oscillator strengths calculated from the Judd-Ofelt [Phys. Rev. 127, 750 (1962); J. Chem. Phys. 37, 511 (1962)] theory. The emission cross sections and gain cross sections of the π and σ spectra were determined for the H13∕24→H15∕24 transition of special interest for laser application.
Effect of the combination of electromigration and thermomigration on phase migration and partial melting in flip chip composite SnPb solder joints
Authors: Huang Annie T. , Tu K. N. , Lai Yi-Shao
Published: August 2006

Composite SnPb flip chip solder joints are used to study the combined effect of electromigration and thermomgration. The study showed that electromigration have led to the redistribution of Sn-rich phase and Pb-rich phase and displaced the low melting Sn-rich phase to the downward electron current crowding region in the contact area on the chip side. A pancake-type void forms and propagates at the contact interface and is accompanied by the migration of the Sn-rich phase followed by the partial melting of the solder joint. The migration of the Sn-rich phase was unexpected, and the location of the melting was unpredicted. We propose here that the migration of Sn-rich phase in the contact area is caused by thermomigration in the lateral direction owing to the existence of a localized hot spot at the current crowding region. The melting behavior is time dependent because it requires an incubation time for void formation, void propagation, and phase migration. Finally, the partial melting at the Sn-rich region occurs due to a large Joule heating at the end of the void growth before an electrical opening takes place.
The CiCs(SiI) defect in silicon: An infrared spectroscopy study
Authors: Potsidi M. S. , Londos C. A.
Published: August 2006

Infrared (IR) spectroscopy was employed for a thorough study of the CiCs(SiI) defect formed in neutron-irradiated carbon-doped Czochralski silicon material. Its IR signals at 987 and 993cm−1, as well as the thermal evolution of the defect were examined and discussed. Based on a previously suggested structure model of this defect its local vibrational mode frequencies were calculated. The estimated values lie very close to the experimentally detected frequencies at 987 and 993cm−1, supporting their previous assignment to the CiCs(SiI) defect. The decay of the center in the spectra was found to be governed by a second order kinetics, with an activation energy around 1.27eV.
Structural and optical properties of Zn1−xMgxO nanocrystals obtained by low temperature method
Authors: Ghosh Manoranjan , Raychaudhuri A. K.
Published: August 2006

In this paper we report structural and optical properties of magnesium substituted zinc oxide (Zn1−xMgxO) nanocrystals (∼10–12nm) synthesized by low temperature route. In the low temperature synthesis route it was possible to reach x=0.17 without segregation of Mg rich phase. The exact chemical composition has been established by quantitative analysis. Rietveld analysis of the x-ray diffraction (XRD) data confirms the wurtzite structure and a continuous compaction of the lattice (in particular, the c-axis parameter) as x increases. There is an enhancement of the strain in the lattice as the Mg is substituted. The band gap also gets enhanced as x is increased and reaches a value of 4eV for x=0.17. From the transmission electron microscope and the XRD data it has been concluded that when there is a phase segregation for x>0.17, there is a shell of Mg(OH)2 on the ZnO. The absorption also shows persistence of the excitonic absorption on Mg substitution. The nanocrystals show near band edge photoluminescence (PL) at room temperature which shows blueshift on Mg incorporation. In addition to the near band edge emission the ZnO and Zn1−xMgxO alloy nanocrystals show considerable emission in the blue-green region at wavelength of ∼550nm. We find that the relative intensity of the green emission increases with the Mg concentration for very low x (up to x=0.05) and on further increase of the Mg concentration there is a sharp decrease of relative intensity of the green emission eventually leading to a complete quenching of green emission. It is concluded that due to phase segregation (for x⩾0.20), the formation of the shell of Mg(OH)2 on the ZnO leads to quenching of the green emission. However, this shell formation does not have much effect on the near band edge PL.
Effect of cap-layer growth rate on morphology and luminescence of InAs∕InP(001) quantum dots grown by metal-organic vapor phase epitaxy
Authors: Michon A. , Sagnes I. , Patriarche G. , Beaudoin G. , Mérat-Combes M. N. , Saint-Girons G.
Published: August 2006

This work reports on the influence of the InP cap-layer growth rate on the structural and optical properties of InAs∕InP quantum dots (QDs) grown by metal-organic vapor phase epitaxy. A careful correlation between the structural and optical properties of the QDs completed by a modeling of their interband transition energy evidences the presence of different QD families with heights varying by monolayer steps. The analysis of transmission electron microscopy images and photoluminescence spectra demonstrates a drastic decrease of the QD height during the growth of the InP cap layer, due to As∕P exchange. The efficiency of this erosion mechanism is shown to be strongly related to the QD exposure time to PH3, depending on the growth rate of the InP cap layer.
Effect of amorphization and carbon co-doping on activation and diffusion of boron in silicon
Authors: Pawlak B. J. , Janssens T. , Brijs B. , Vandervorst W. , Collart E. J. H. , Felch S. B. , Cowern N. E. B.
Published: August 2006

We investigate the impact of amorphization and C co-implantation on B diffusion and activation properties after conventional spike rapid thermal annealing (RTA). We observe that after complete recrystallization at 600°C the B tail deepens by 5nm (at 5×1018at.∕cm3) due to B diffusion in a-Si. After spike RTA it becomes 12nm deeper with respect to an as-implanted profile, which proves that both diffusion mechanisms in a-Si and c-Si are important. However, the B diffusion in c-Si is sensitive to the fraction of substitutional C incorporated into c-Si. The best junction depth is Xj=16.5nm, with abruptness of 2nm/decade and Rs=583Ω∕◻.
Rydberg states of small NaArn* clusters
Authors: El Hadj Rhouma M. Ben , Ben Lakhdar Z. , Berriche H. , Spiegelman F.
Published: August 2006

The 4s and 5s Rydberg excited states of NaArn* clusters are investigated using a pseudopotential quantum-classical method. While NaArn clusters in their ground state are known to be weakly bound van der Waals complexes with Na lying at the surface of the argon cluster, isomers in 4s or 5s electronically excited states of small NaArn* clusters (n⩽10) are found to be stable versus dissociation. The relationship between electronic excitation and cluster geometry is analyzed as a function of cluster size. For both 4s and 5s states, the stable exciplex isomers essentially appear as sodium-centered structures with similar topologies, converging towards those of the related NaArn+ positive ions when the excitation level is increased. This is consistent with a Rydberg-type picture for the electronically excited cluster, described by a central sodium ion solvated by an argon shell, and an outer diffuse electron orbiting around this NaArn+ cluster core.
Multilayer green polymer light emitting diodes with improved efficiency and lifetime
Authors: Li J. , Sano T. , Hirayama Y. , Tomita T. , Fujii H. , Wakisaka K.
Published: August 2006

We report on green polymer light emitting diodes with improved efficiency and lifetime using a cross-linked hole transporting layer in this study. The poly[(9,9-dioctylfluorene)-alt-(triphenylamine)], which was used as a hole transporting material, was cross-linked by UV irradiation. The emitting layer of a green copolymer, poly[(9,9-dioctylfluorene)-co-(benzodiathiazole)], was spin coated from its solution on the top of the cross-linked hole transporting layer to prepare a multilayer device. By introducing the cross-linked hole transporting layer, the maximum luminance, efficiency, external quantum efficiency, and, especially, the lifetime of the multilayer green device were greatly increased compared with those of a single layer device. These were supposed to be caused by an improvement of carrier injection balance in the emitting layer, a decrease in the reduction-oxidation degradation of the emitting material, and fewer impurities entering the emitting layer due to the function of the cross-linked hole transporting layer.
Electric-field-induced quenching of photoluminescence in photoconductive organic thin film structures based on Eu3+ complexes
Authors: Kalinowski J. , Stampor W. , Cocchi M. , Virgili D. , Fattori V.
Published: August 2006

A large electric field effect on photoluminescence (PL) from electroluminescent emitters sandwiched between two high-work-function electrodes is reported and a model of the effect formulated. We examine the PL behavior of Eu3+ complex-based organic thin films subjected to increasing electric field applied to either two high-work-function [indium tin oxide (ITO) and Au] or two low-work-function (Al∕Al) electrodes. The progressive drop (up to 40%) of the PL for the first structure is observed while the current increases by about two orders of magnitude under illumination within the first absorption band of diamine derivative (TPD) molecules acting as an antenna harvesting exciting photons within the common matrix bonded with polycarbonate. This behavior is interpreted in terms of a field-dependent branching between ligand occupation by excited triplets and by electrons injected from negatively biased ITO or Au electrodes by TPD excitons. In contrast, the PL quenching in the Al/organic film/Al structure is due to exciton dissociation solely. This allows to extract the quenching component for the ITO/organic film/Au structure, associated with the injected charge only. The recombination of thermally injected holes with exciton-injected electrons makes the charge-induced quenching efficiency component to nonmonotonically evolve with a decrease down to negative values (PL enhancement) at high electric fields. The quenching of the emission from host (TPD) is attributed to singlet-exciton–charge-carrier interactions.
Electrostatic potentials and energy loss due to a projectile propagating through a non-Maxwellian dusty plasma
Authors: Deeba F. , Ahmad Zahoor , Murtaza G.
Published: August 2006

The electrostatic potentials (Debye and wake) and energy loss due to a charged projectile propagating through an unmagnetized collisionless dusty plasma are derived employing kappa and generalized (r,q) velocity distributions for the dust acoustic wave. It is found that these quantities in general differ from their Maxwellian counterparts and are sensitive to the values of spectral index, κ in the case of kappa distribution and to r, q in the case of generalized (r,q) distribution. The amplitudes of these quantities are less for small values of the spectral index (κ, r=0, q) but approach the Maxwellian in the limit κ→∞ (for kappa distribution) and for r=0, q→∞ [for generalized (r,q) distribution]. For any nonzero value of r, the potential and the energy loss grow beyond the Maxwellian results. The effect of kappa and generalized (r,q) distributions on potential and energy loss is also studied numerically and the results are compared with those of the Maxwellian distribution.
Performance improvement by charge trapping of doping fluorescent dyes in organic memory devices
Authors: Chen Jiangshan , Ma Dongge
Published: August 2006

We studied the memory effect in the devices consisting of dye-doped N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine sandwiched between indium-tin oxide and Ag electrodes. It was found that the on/off current ratio was greatly improved by the doped fluorescent dyes compared with nondoping devices. A mechanism of charge trapping was demonstrated to explain the improvement of the memory effect. For the off state, the conduction process is dominated by the trapping current, which is a characteristic of the space-charge limited current, whereas the on state is dominated by the detrapping current, and interpreted by Poole-Frenkel emission.
Numerical simulation of impurity desorption induced by nanosecond and femtosecond laser pulses
Authors: Chi Yinsheng , Lin Xiaohui , Chen Minhua , Chen Yunfei
Published: August 2006

A model based on a stochastic process was developed to study the impurity molecule desorption from a substrate induced by nanosecond and femtosecond lasers. The dynamics of adsorbed molecules irradiated by the laser pulses can be considered to be a Brownian motion in the bath of excited energy carriers. A two-step model was used to describe the nonequilibrium heating process induced by the femtosecond laser pulses. The difference between the desorption processes induced by nanosecond and femtosecond lasers was discussed based on the numerical results for the desorption of CO molecules from a Ru surface. Results indicate that the femtosecond laser is a much better tool for desorption than the nanosecond laser.
Shrinkage of nanocavities in silicon during electron beam irradiation
Authors: Zhu Xianfang
Published: August 2006

An internal shrinkage of nanocavity in silicon was in situ observed under irradiation of energetic electron on electron transmission microscopy. Because there is no addition of any external materials to cavity site, a predicted nanosize effect on the shrinkage was observed. At the same time, because there is no ion cascade effect as encountered in the previous ion irradiation-induced nanocavity shrinkage experiment, the electron irradiation-induced instability of nanocavity also provides a further more convincing evidence to demonstrate the predicted irradiation-induced athermal activation effect.
Effect of gold on keV x-ray emission yield from laser produced plasma of gold-copper mix-Z targets
Authors: Arora V. , Chakera J. A. , Naik P. A. , Kumbhare S. R. , Gupta P. D. , Gupta N. K.
Published: August 2006

The effect of gold on keV x-ray emission from gold-copper (Au–Cu) mix-Z plasma has been experimentally studied. The intensity of the copper L-shell line radiation (λ∼7.8–10.9Å) as well as the integrated keV x-ray yield were observed to decrease sharply with increasing atomic fraction of gold in the mix-Z target. The decrease was observed to be by a factor of ∼2.1 for 0.12 atomic fraction of gold in the Au–Cu mix-Z target with respect to the pure copper target. The results can be explained from physical consideration of the high value of free-bound opacity of gold in the spectral region of the L-shell emission of copper ions and downconversion of the absorbed keV line radiation.
A sensitive pressure sensor for diamond anvil cell experiments up to 2GPa: FluoSpheres®
Authors: Picard Aude , Oger Phil M. , Daniel Isabelle , Cardon Hervé , Montagnac Gilles , Chervin Jean-Claude
Published: August 2006

We present an optical pressure sensor suitable for experiments in diamond anvil cell in the 0.1MPa–2GPa pressure range, for temperatures between ambient and 323K. It is based on the pressure-dependent fluorescence spectrum of FluoSpheres®, which are commercially available fluorescent microspheres commonly used to measure blood flow in experimental biology. The fluorescence of microspheres is excited by the 514.5nm line of an Ar+ laser, and the resulting spectrum displays three very intense broad bands at 534, 558, and 598nm, respectively. The reference wavelength and pressure gauge is that of the first inflection point of the spectrum, located at 525.6±0.2nm at ambient pressure. It is characterized by an instantaneous and large linear pressure shift of 9.93(±0.08)nm∕GPa. The fluorescence of the FluoSpheres® has been investigated as a function of pressure (0.1–4GPa), temperature (295–343K), pH (3–12), salinity, and pressure transmitting medium. These measurements show that, for pressures comprised between 0.1MPa and 2GPa, at temperatures not exceeding 323K, at any pH, in aqueous pressure transmitting media, pressure can be calculated from the wavelength shift of two to three beads, according to the relation P=0.100 (±0.001) Δλi(P) with Δλi(P)=λi(P)−λi(0) and λi(P) as the wavelength of the first inflection point of the spectrum at the pressure P. This pressure sensor is approximately thirty times more sensitive than the ruby scale and responds instantaneously to pressure variations.
Encryption and decryption of images with chaotic map lattices
Authors: Pisarchik A. N. , Flores-Carmona N. J. , Carpio-Valadez M.
Published: August 2006

We propose a secure algorithm for direct encryption and decryption of digital images with chaotic map lattices. The basic idea is to convert, pixel by pixel, the image color to chaotic logistic maps one-way coupled by initial conditions. After small numbers of iterations and cycles, the image becomes indistinguishable due to inherent properties of chaotic systems. Since the maps are coupled, the image can be completely recovered by the decryption algorithm if map parameters, number of iterations, number of cycles, and the image size are exactly known.
Efficient implementation of the fast multipole method
Authors: Rudberg Elias , Sałek Paweł
Published: August 2006

A number of computational techniques are described that reduce the effort related to the continuous fast multipole method, used for the evaluation of Coulomb matrix elements as needed in Hartree-Fock and density functional theories. A new extent definition for Gaussian charge distributions is proposed, as well as a new way of dividing distributions into branches. Also, a new approach for estimating the error caused by truncation of multipole expansions is presented. It is found that the use of dynamically truncated multipole expansions gives a speedup of a factor of 10 in the work required for multipole interactions, compared to the case when all interactions are computed using a fixed multipole expansion order. Results of benchmark calculations on three-dimensional systems are reported, demonstrating the usefulness of our present implementation of the fast multipole method.
Anisotropic x-ray absorption effects in the optical luminescence yield of ZnO nanostructures
Authors: Rosenberg R. A. , Shenoy G. K. , Tien L.-C. , Norton D. , Pearton S. , Sun X. H. , Sham T. K.
Published: August 2006

The authors have found that the directionality of the orbital populated following core-level x-ray absorption of a hexagonal nanostructure has a strong influence on the resulting optical luminescence yield spectra. For ZnO, there is an enhancement of the band gap exciton luminescence following O 1s to 2pz relative to 2px,y excitation. The defect luminescence O 1s excitation spectrum also shows sensitivity to the nature of the defect (surface or bulk).
Finitely many Dirac-delta interactions on Riemannian manifolds
Authors: Altunkaynak Bariş , Erman Fatih , Turgut O. Teoman
Published: August 2006

This work is intended as an attempt to study the nonperturbative renormalization of bound state problem of finitely many Dirac-delta interactions on Riemannian manifolds, S2, H2, and H3. We formulate the problem in terms of a finite dimensional matrix, called the characteristic matrix Φ. The bound state energies can be found from the characteristic equation Φ(−ν2)A=0. The characteristic matrix can be found after a regularization and renormalization by using a sharp cut-off in the eigenvalue spectrum of the Laplacian, as it is done in the flat space, or using the heat kernel method. These two approaches are equivalent in the case of compact manifolds. The heat kernel method has a general advantage to find lower bounds on the spectrum even for compact manifolds as shown in the case of S2. The heat kernels for H2 and H3 are known explicitly, thus we can calculate the characteristic matrix Φ. Using the result, we give lower bound estimates of the discrete spectrum.
Crystal field effects in CeOs4Sb12 as determined by inelastic neutron scattering
Authors: Yang C. P. , Wang H. , Iwasa K. , Kohgi M. , Sugawara H. , Sato H.
Published: August 2006

The ground state and physical properties of CeOs4Sb12 are primary determined by the competition among three types of energies which are all related to interactions of the Ce3+ 4f electron with the lattice and the spin lattice, i.e., the Kondo interaction, the Ruderman-Kittel-Kasuya-Yosida interaction, and the crystal field energy. In this work, inelastic neutron scattering experiments were performed on CeOs4Sb12 powder samples at low temperatures in order to investigate the magnetic interactions, the origin of unusual physical properties and, in particular, to elucidate the ground state of crystal field splitting. The result indicates that, within the accuracy of experimental error, no well defined crystal field effect could be detected around the expected energy of 28.2meV as computed on the basis of a localized Ce3+ state model. That means that other causes are responsible for the deviation of inverse susceptibility from the linear Curie law at low temperatures and that further studies are necessary to understand the anomalous transport properties of CeOs4Sb12.
Generation of highly collimated high-current ion beams by skin-layer laser-plasma interaction at relativistic laser intensities
Authors: Badziak J. , Jabłoński S. , Głowacz S.
Published: August 2006

Generation of fast ion beams by laser-induced skin-layer ponderomotive acceleration has been studied using a two-dimensional (2D) two-fluid relativistic computer code. It is shown that the key parameter determining the spatial structure and angular divergence of the ion beam is the ratio dL∕Ln, where dL is the laser beam diameter and Ln is the plasma density gradient scale length. When dL⪢Ln, a dense highly collimated megaampere ion (proton) beam of the ion current density approaching TA∕cm2 can be generated by skin-layer ponderomotive acceleration, even with a tabletop subpicosecond laser.
Multifunctional platinum porphyrin dendrimers as emitters in undoped phosphorescent based light emitting devices
Authors: Li Yanqin , Rizzo Aurora , Salerno Marco , Mazzeo Marco , Huo Cheng , Wang Yue , Li Kechang , Cingolani Roberto , Gigli Giuseppe
Published: August 2006

The authors report on a class of platinum porphyrin based phosphorescent multifunctional first generation dendrimers (Pt-8Cn-TPP) incorporating a platinum porphyrin core as the emissive center and carbazole side groups as the hole, as well as energy transport fragments. Furthermore, the alkyl groups prevent aggregation and allow for the formation of transparent homogeneous films directly from solution. The intra- and intermolecular energy transfers by Förster and Dexter mechanisms from the carbazole side groups to the platinum porphyrin core have been investigated. Light emitting devices were fabricated and bright red electrophosphorescence was achieved from a single undoped emitting layer of Pt-8C4-TPP. The results indicate that platinum porphyrin dendrimers are very promising to make bright, red saturated and spin-coated phosphorescent devices with a simplified undoped emissive layer.
Observation of whispering gallery modes in cathode luminescence in TiO2:Eu3+ microspheres
Authors: Tomita Makoto , Totsuka Kouki , Ikari Hiroshi , Ohara Kenji , Mimura Hidenori , Watanabe Hideyuki , Kume Hiroshi , Matsumoto Takahiro
Published: August 2006

We report cavity enhanced periodic structures in cathode luminescence spectra in TiO2:Eu3+ microspheres ranging from 6.2to12.2μm in diameter. The spectral structures observed at the accelerating voltage below 10kV are attributed to the lower order whispering galley modes, while the additional components appearing at 20kV are to the higher order modes within the microsphere. These observations illustrate that the basic properties of phosphors can be advanced from the point of view of cavity quantum electrodynamics by fabricating a microcavity structure onto a single phosphor crystal.
Domain wall assisted magnetic recording
Authors: Dobin A. Yu. , Richter H. J.
Published: August 2006

Using numerical and analytical micromagnetics the authors calculated the switching fields and energy barriers of the composite (exchange spring) magnetic recording media, which consist of layers with high and low magnetocrystalline anisotropies. The authors demonstrate that the ultimate potential of the composite media is realized if the interfacial domain wall fits inside the layers. The switching occurs via domain wall nucleation, compression in the applied field, depinning, and propagation through the hard/soft interface. The authors demonstrate that the domain wall assisted magnetic recording offers up to a threefold areal density gain over conventional single layer recording.
A toy model of polymer stretching
Authors: Guardiani Carlo , Bagnoli Franco
Published: August 2006

We present an extremely simplified model of multiple-domain polymer stretching in an atomic force microscopy experiment. We portray each module as a binary set of contacts and decompose the system energy into a harmonic term (the cantilever) and long-range interaction terms inside each domain. Exact equilibrium computations and Monte Carlo simulations qualitatively reproduce the experimental sawtooth pattern of force-extension profiles, corresponding (in our model) to first-order phase transitions. We study the influence of the coupling induced by the cantilever and the pulling speed on the relative heights of the force peaks. The results suggest that the increasing height of the critical force for subsequent unfolding events is an out-of-equilibrium effect due to a finite pulling speed. The dependence of the average unfolding force on the pulling speed is shown to reproduce the experimental logarithmic law.
Nonradiative losses in Yb:KGd(WO4)2 and Yb:Y3Al5O12
Authors: Biswal Subrat , O’Connor Shawn P. , Bowman Steven R.
Published: August 2006

Photothermal deflection spectroscopy was used to measure nonradiative losses in Yb:KGW and Yb:YAG (for both crystal and for the first time in ceramic form). Fluorescent quantum efficiencies for both materials were found to be greater than 98.9%. The reduction of nonradiative losses due to annealing of Yb:YAG is also presented.
Ultra-high-frequency chaos in a time-delay electronic device with band-limited feedback
Authors: Illing Lucas , Gauthier Daniel J.
Published: August 2006

We report an experimental study of ultra-high-frequency chaotic dynamics generated in a delay-dynamical electronic device. It consists of a transistor-based nonlinearity, commercially-available amplifiers, and a transmission-line for feedback. The feedback is band-limited, allowing tuning of the characteristic time-scales of both the periodic and high-dimensional chaotic oscillations that can be generated with the device. As an example, periodic oscillations ranging from 48 to 913 MHz are demonstrated. We develop a model and use it to compare the experimentally observed Hopf bifurcation of the steady-state to existing theory [Illing and Gauthier, Physica D 210, 180 (2005)]. We find good quantitative agreement of the predicted and the measured bifurcation threshold, bifurcation type and oscillation frequency. Numerical integration of the model yields quasiperiodic and high dimensional chaotic solutions (Lyapunov dimension ∼13), which match qualitatively the observed device dynamics.
Thermal conductivity of SrBi2Nb2O9 ferroelectric thin films
Authors: Boffoué M. O. , Jacquot A. , Duclère J.-R. , Guilloux-Viry M. , Hejtmanek J. , Dauscher A. , Lenoir B.
Published: August 2006

Ferroelectric Aurivillius SrBi2Nb2O9 films of different thicknesses were prepared on Si(100) substrates by pulsed laser deposition. The films are polycrystalline and exhibit an increased grain size with film thickness. The cross-plane thermal conductivity of such SrBi2Nb2O9 films was measured in the 100–300K temperature range by the 3ω method and it was observed that the thermal properties of this family of ferroelectric materials show a glasslike behavior. The measurements performed on a polycrystalline bulk SrBi2Nb2O9 sample, in the 10–300K temperature range, led to identical conclusions. The origin of this unexpected glasslike behavior is discussed.
Effects of two-temperature ions, magnetic field, and higher-order nonlinearity on the existence and stability of dust-acoustic solitary waves in Saturn’s F ring
Authors: El-Labany S. K. , Moslem Waleed M. , Safy F. M.
Published: August 2006

Nonlinear propagation of dust-acoustic solitary waves (DASWs) in a strong magnetized dusty plasma comprising warm adiabatic variable-charged dust particles, isothermal electrons, and two-temperature ions is investigated. Applying a reductive perturbation theory, a nonlinear Zakharov-Kuznetsov (ZK) equation for the first-order perturbed potential and a linear inhomogeneous ZK-type equation for the second-order perturbed potential are derived. However, at a certain value of high-temperature ion density, the coefficient of the nonlinear terms of both ZK and ZK-type equations vanishes. Therefore, a new set of expansion physical parameters and stretched coordinates are then used to derive a modified Zakharov-Kuznetsov (mZK) equation for the first-order perturbed potential and a mZK-type equation for the second-order perturbed potential. Stationary solutions of these equations are obtained using a renormalization method. A condition for two-temperature ions assumption is examined for various cosmic dust-laden plasma systems. It is found that this condition is satisfied for Saturn’s F ring. The effects of two-temperature ions, magnetic field, and higher-order nonlinearity on the behavior of the DASWs are discussed. To obtain the stability condition of the waves, a method based on energy consideration is used and the condition for stable solitons is derived.
Self-consistent embedding theory for locally correlated configuration interaction wave functions in condensed matter
Authors: Huang Patrick , Carter Emily A.
Published: August 2006

We present new developments on a density-based embedding strategy for the electronic structure of localized feature in periodic, metallic systems [see T. Klüner et al., J. Chem. Phys. 116, 42 (2002), and references therein]. The total system is decomposed into an embedded cluster and a background, where the background density is regarded as fixed. Its effect on the embedded cluster is modeled as a one-electron potential derived from density functional theory. We first discuss details on the evaluation of the various contributions to the embedding potential and provide a strategy to incorporate the use of ultrasoft pseudopotentials in a consistent fashion. The embedding potential is obtained self-consistently with respect to both the total and embedded cluster densities in the embedding region, within the framework of a frozen background density. A strategy for accomplishing this self-consistency in a numerically stable manner is presented. Finally, we demonstrate how dynamical correlation effects can be treated within this embedding framework via the multireference singles and doubles configuration interaction method. Two applications of the embedding theory are presented. The first example considers a Cu dimer embedded in the (111) surface of Cu, where we explore the effects of different models for the kinetic energy potential. We find that the embedded Cu density is reasonably well-described using simple models for the kinetic energy. The second, more challenging example involves the adsorption of Co on the (111) surface of Cu, which has been probed experimentally with scanning tunneling microscopy [H. C. Manoharan et al., Nature (London) 403, 512 (2000)]. In contrast to Kohn-Sham density functional theory, our embedding approach predicts the correct spin-compensated ground state.
Transient grating measurement of surface acoustic waves in thin metal films with extreme ultraviolet radiation
Authors: Tobey R. I. , Siemens M. E. , Murnane M. M. , Kapteyn H. C. , Torchinsky D. H. , Nelson K. A.
Published: August 2006

The authors demonstrate the use of coherent extreme ultraviolet (EUV) light in laser induced transient grating spectroscopy. In this work, the authors study the dispersion of pulsed laser induced surface acoustic waves in a thin uniform nickel film by monitoring the diffraction of 30nm EUV light from the surface. Transient deformation of the sample surface leads to absolute diffracted intensities approaching 10−3 for 1Å displacement, an ∼700-fold increase in sensitivity compared with optical probing. EUV probing provides a convenient geometry for observing short wavelength acoustic propagation.