bluuebunny/crossref_metadata_2025 huggingface dataset:
Laser-induced thermal expansion of a scanning tunneling microscope tip measured with an atomic force microscope cantilever
Authors: Huber R. , Koch M. , Feldmann J.
Published: July 2002

We investigate the transient thermal expansion of a scanning tunneling microscope tip after excitation with intense femtosecond laser pulses. The expansion dynamics are measured electrically by monitoring the time-resolved tunneling current and mechanically by use of an atomic force microscope. The tip expansion reaches values as high as 15 nm and exceeds the typical working distance of a scanning tunneling microscope by far. This results in a mechanical contact between tunneling tip and surface leading to surface modifications on a nanometer scale. Our findings clarify the mechanism of the recently proposed focusing of laser radiation in the near field of a tip technique [J. Jersch and K. Dickmann, Appl. Phys. Lett. 68, 868 (1996)] for nanostructuring.
High-temperature properties of GaInAs/AlGaAs lasers with improved carrier confinement by short-period superlattice quantum well barriers
Authors: Schäfer F. , Mayer B. , Reithmaier J. P. , Forchel A.
Published: July 2002

The influence of the carrier confinement on the output characteristics of GaInAs/AlGaAs lasers was investigated. To improve the carrier confinement, AlGaAs/GaAs short-period superlattices were used as quantum well barriers. In comparison to lasers with GaAs barriers the structures with the modified barriers show improved temperature properties at low threshold current densities without deterioration of the internal quantum efficiency (>95%). High characteristic temperatures (T0) well above 300 K were measured between 20 and 75 °C and laser operation up to 238 °C could be achieved. The large improvement in T0 is mainly attributed to the reduced thermionic emission of carriers out of the quantum well due to the increased barrier height and the carrier reflection above the barrier by the short-period superlattice.
Study of Mg diffusion during metalorganic chemical vapor deposition of GaN and AlGaN
Authors: Chang Ying-Lan , Ludowise Mike , Lefforge Dale , Perez Bill
Published: July 2002

The diffusion behaviors of Mg in GaN and AlGaN layers are investigated using otherwise undoped GaN test structures containing three Mg-dopant spikes. These simplified structures enable accurate dopant profiling by avoiding the formation of nonplanar V-shaped defects, as confirmed by atomic force microscopy measurements. We also study the Mg distribution in GaN:Mg/AlGaN heterostructures. In all cases, no significant diffusion/segregation behaviors were observed for Mg in nitride material systems at 1060 °C over a 1.25 h time.
Intersubband absorption in boron-doped multiple Ge quantum dots
Authors: Liu J. L. , Wu W. G. , Balandin A. , Jin G. L. , Wang K. L.
Published: July 2002

The intersubband absorption in self-assembled boron-doped multiple Ge quantum dots is observed. The structures used consist of 20 periods of boron-doped Ge dot layers and undoped Si barriers. The infrared absorption as a function of wavelength is measured by Fourier transform infrared spectroscopy using a waveguide geometry. Absorption peaks in the mid-infrared range have been observed, which are attributed to the transitions between the first two heavy hole states of the Ge quantum dots. The polarization dependence measurement is used to study the nature of the transitions. This observation suggests the possible use of multiple Ge quantum dots for infrared detector application.
Polymer gel as thermally responsive attenuator for ultrasonic waves
Authors: Yuan Kaihua , Hu Zhibing , Li Yong
Published: July 2002

The ultrasonic attenuation in N-isopropylacrylamide (NIPA) gel has been measured as a function of temperature at various frequencies. It is found that at room temperature, the attenuation of a longitudinal ultrasonic wave in the gel is small and close to that of pure water. However, as the temperature increases above the spinodal phase transition point of the gel, the attenuation increases drastically. This change of the attenuation is completely reversible and due to the microdomains formed in the NIPA gel above its phase transition temperature. The increase of attenuation at 15 MHz from 26 to 45 °C is about 26 dB/cm. The attenuation varies within 5 min after the temperature change is made for a sample with diameter 1 cm and height 0.8 cm. The results reported here may have potential applications from ultrasonic assisted drug release to a switch for ultrasonic signals.
Organic light-emitting diode with 20 lm/W efficiency using a triphenyldiamine side-group polymer as the hole transport layer
Authors: Shaheen S. E. , Jabbour G. E. , Kippelen B. , Peyghambarian N. , Anderson J. D. , Marder S. R. , Armstrong N. R. , Bellmann E. , Grubbs R. H.
Published: July 2002

We have used triphenyldiamine side-group polymers as hole transport layers in multilayer organic light-emitting diodes using 8-hydroxyquinoline aluminum (Alq3) as an emission layer. The device efficiency systematically increases as the ionization potential of the hole transport layer is shifted further from the work function of the indium–tin–oxide anode. We attribute this trend to better balance of hole and electron charges in the device. An optimized device consisting of a fluorinated version of the polymer as the hole transport layer, quinacridone doped Al as the emission layer, and a LiF/Al cathode results in a peak external luminous efficiency of 20 lm/W.
The spin state of the neutral silicon vacancy in 3C–SiC
Authors: Deák P. , Miró J. , Gali A. , Udvardi L. , Overhof H.
Published: July 2002

Recent theoretical studies show that the neutral silicon vacancy (VSi) in cubic silicon carbide (3C–SiC) exhibits negligible Jahn–Teller distortion. This provides an opportunity to study the energy sequence of different multiplets in a vacancy with genuine Td symmetry. Calculations using the local spin density approximation give a spin triplet as ground state. The determination of the true ground state requires, however, the incorporation of configuration interactions. Using multiconfigurational self-consistent field calculations we show that the ground state of the neutral VSi0 in 3C–SiC is a spin singlet. The calculated energy difference, ∼0.1 eV, in favor of the singlet spin state would still allow the experimental observation of the triplet state at high temperature.
Second-harmonic generation from realistic film–substrate interfaces: The effects of strain
Authors: Lyubchanskii I. L. , Dadoenkova N. N. , Lyubchanskii M. I. , Rasing Th. , Jeong Jae-Woo , Shin Sung-Chul
Published: July 2002

The optical second-harmonic generation from a thin crystalline film on a substrate is theoretically investigated for both s and p polarized incident light. The contributions of lattice misfit strain as well as of misfit dislocation strain to the second-order nonlinear optical susceptibility are described using a nonlinear photoelastic tensor and can be separated by a polarization analysis of the scattered light at the second harmonic frequency. For the s(ω)→s(2ω) and p(ω)→s(2ω) scattering geometries, the nonlinear optical signal will be determined by dislocation strain only, whereas for the s(ω)→p(2ω) and p(ω)→p(2ω) geometries both lattice misfit strain and misfit dislocation strain will contribute.
Photofabrication of three-dimensional photonic crystals by multibeam laser interference into a photopolymerizable resin
Authors: Shoji Satoru , Kawata Satoshi
Published: July 2002

Three-dimensional (3D) photonic crystal structures can be fabricated into photopolymerizable resins by using laser beam interference with high precision. Three laser beams interfere into a glass cell filled with a liquid photopolymerizable resin to form a hexagonal periodic structure. Rods are formed in a hexagonal arrangement after being photopolymerzed according to the 3D periodic light distribution which results from the laser’s interference. Two beams of another laser also interfere to form layers which cross perpendicular to the rod array. After photofabrication, the nonsolidified resin is removed by ethanol. The lattice constant can be selected by tuning the angles of the incident beams and the laser wavelength. We have fabricated a 500 μm×500 μm×150 μm photonic crystal structure, the lattice constant of which is 1 μm and contains 150 lateral layers.
Magnetic moment of Mn in the ferromagnetic semiconductor (Ga0.98Mn0.02)As
Authors: Ohldag H. , Solinus V. , Hillebrecht F. U. , Goedkoop J. B. , Finazzi M. , Matsukura F. , Ohno H.
Published: July 2002

We have studied the quasibinary ferromagnetic semiconductor (Ga0.98Mn0.02)As by magnetic circular dichroism in x-ray absorption. We find a richly structured Mn absorption spectrum typical for localized 3d electrons. An analysis of the magnetization-averaged and dichroism line shapes shows a local Mn moment of 4.6μB, which is close to the Hund’s rule moment for the half-filled 3d shell. The magnitude of the dichroism reveals that only about 1/7 of the Mn atoms participate in the ferromagnetic order. Our experiment does not show a distinction between the ferro- and paramagnetic Mn atoms.
Nano-icosahedral quasicrystalline phase formation from a supercooled liquid state in Zr–Fe–Ni ternary metallic glass
Authors: Saida Junji , Li Chunfei , Matsushita Mitsuhide , Inoue Akihisa
Published: July 2002

It was found that an icosahedral quasicrystalline phase was formed as a primary precipitation phase from the supercooled liquid region of the melt-spun Zr70Fe20Ni10 ternary metallic glass. The precipitation of an icosahedral phase takes place at 673 K at the heating rate of 0.67 K s−1. The precipitated icosahedral particle has a nearly spherical morphology and a fine grain size in the diameter range of 5–10 nm. The second crystallization reaction proceeds through a broad exothermic peak and results in the formation of Zr2Ni and Zr2Fe phases. The formation of nanoscale icosahedral phase by the addition of Fe as well as noble metals such as Pd, Au, and Pt indicates the possibility of existence of an icosahedral short-range order in the various Zr-based metallic glasses.
Nuclear shielding constants by density functional theory with gauge including atomic orbitals
Authors: Helgaker Trygve , Wilson Philip J. , Amos Roger D. , Handy Nicholas C.
Published: July 2002

Recently, we introduced a new density-functional theory (DFT) approach for the calculation of NMR shielding constants. First, a hybrid DFT calculation (using 5% exact exchange) is performed on the molecule to determine Kohn–Sham orbitals and their energies; second, the constants are determined as in nonhybrid DFT theory, that is, the paramagnetic contribution to the constants is calculated from a noniterative, uncoupled sum-over-states expression. The initial results suggested that this semiempirical DFT approach gives shielding constants in good agreement with the best ab initio and experimental data; in this paper, we further validate this procedure, using London orbitals in the theory, having implemented DFT into the ab initio code DALTON. Calculations on a number of small and medium-sized molecules confirm that our approach produces shieldings in excellent agreement with experiment and the best ab initio results available, demonstrating its potential for the study of shielding constants of large systems.
Photoionization spectroscopy of traps in GaN metal-semiconductor field-effect transistors
Authors: Klein P. B. , Binari S. C. , Freitas J. A. , Wickenden A. E.
Published: July 2002

Measurements of the spectral and intensity dependences of the optically-induced reversal of current collapse in a GaN metal-semiconductor field-effect transistor (MESFET) have been compared to calculated results. The model assumes a net transfer of charge from the conducting channel to trapping states in the high-resistivity region of the device. The reversal, a light-induced increase in the trap-limited drain current, results from the photoionization of trapped carriers and their return to the channel under the influence of the built-in electric field associated with the trapped charge distribution. For a MESFET in which two distinct trapping centers have been spectrally resolved, the experimentally measured dependence upon light intensity was fitted using this model. The two traps were found to have very different photoionization cross-sections but comparable concentrations (4×1011 cm−2 and 6×1011 cm−2), suggesting that both traps contribute comparably to the observed current collapse.
Magnetoresistance enhancement in specular, bottom-pinned, Mn83Ir17 spin valves with nano-oxide layers
Authors: Veloso A. , Freitas P. P. , Wei P. , Barradas N. P. , Soares J. C. , Almeida B. , Sousa J. B.
Published: July 2002

Bottom-pinned Mn83Ir17 spin valves with enhanced specular scattering were fabricated, showing magnetoresistance (MR) values up to 13.6%, lower sheet resistance R□ and higher ΔR□. Two nano-oxide layers (NOL) are grown on both sides of the CoFe/Cu/CoFe spin valve structure by natural oxidation or remote plasma oxidation of the starting CoFe layer. Maximum MR enhancement is obtained after just 1 min plasma oxidation. Rutherford backscattering analysis shows that a 15±2 Å oxide layer grows at the expense of the initial (prior to oxidation) CoFe layer, with ∼12% reduction of the initial 40 Å CoFe thickness. X-ray reflectometry indicates that Kiessig fringes become better defined after NOL growth, indicating smoother inner interfaces, in agreement with the observed decrease of the spin valve ferromagnetic Néel coupling.
Crossed beam reaction of cyano radicals with hydrocarbon molecules. IV. Chemical dynamics of cyanoacetylene (HCCCN; X 1Σ+) formation from reaction of CN(X 2Σ+) with acetylene, C2H2(X 1Σg+)
Authors: Huang L. C. L. , Asvany O. , Chang A. H. H. , Balucani N. , Lin S. H. , Lee Y. T. , Kaiser R. I. , Osamura Y.
Published: July 2002

The chemical reaction dynamics to form cyanoacetylene, HCCCN (X 1Σ+), via the radical–neutral reaction of cyano radicals, CN(X 2Σ+;ν=0), with acetylene, C2H2(X 1Σg+), are unraveled in crossed molecular beam experiments at two collision energies of 21.1 and 27.0 kJ mol−1. Laboratory angular distributions and time-of-flight spectra of the HCCCN product are recorded at m/e=51 and 50. Experiments were supplemented by electronic structure calculations on the doublet C3H2N potential energy surface and RRKM investigations. Forward-convolution fitting of the crossed beam data combined with our theoretical investigations shows that the reaction has no entrance barrier and is initiated by an attack of the CN radical to the π electron density of the acetylene molecule to form a doublet cis/trans HCCHCN collision complex on the A′2 surface via indirect reactive scattering dynamics. Here 85% of the collision complexes undergo C–H bond rupture through a tight transition state located 22 kJ mol−1 above the cyanoacetylene, HCCCN (X 1Σ+) and H(2S1/2) products (microchannel 1). To a minor amount (15%) trans HCCHCN shows a 1,2-H shift via a 177 kJ mol−1 barrier to form a doublet H2CCCN radical, which is 46 kJ mol−1 more stable than the initial reaction intermediate (microchannel 2). The H2CCCN complex decomposes via a rather loose exit transition state situated only 7 kJ mol−1 above the reaction products HCCCN (X 1Σ+) and H(2S1/2). In both cases the geometry of the exit transition states is reflected in the observed center-of-mass angular distributions showing a mild forward/sideways peaking. The explicit identification of the cyanoacetylene as the only reaction product represents a solid background for the title reaction to be included in reaction networks modeling the chemistry in dark, molecular clouds, outflow of dying carbon stars, hot molecular cores, as well as the atmosphere of hydrocarbon rich planets and satellites such as the Saturnian moon Titan.
Alternating current electrical properties of antiferroelectric lead zirconate thin films by pulsed excimer laser ablation
Authors: Bharadwaja S. S. N. , Krupanidhi S. B.
Published: July 2002

In situ crystalline lead zirconate (PZ) thin films were deposited on platinum metallized silicon substrates, using a 248 nm KrF pulsed excimer laser ablation technique. The antiferroelectricity in PZ thin films was confirmed by means of P vs E and C vs V measurements. The maximum observed saturated polarization (Ps) was 44 μC/cm2 at an applied field of 200 kV/cm. The calculated forward and backward switching fields were 71 and 154 kV/cm, respectively. The dielectric phase transition temperature was at ∼219 °C with zero dc bias and was increased to 290 °C in the presence of 4 V dc bias. Detailed comprehensive study was done on PZ thin films to understand the charge carrier transport with respect to frequency domain of dielectric and ac conductivity measurements. Effect of simultaneous dc bias on the frequency response of the dielectric dispersion and ac conductivity studies were done to understand the nature of charge carrier transport and their activation energies. The calculated activation energies, from ac conductivity and conductance measurements, in the absence of external dc bias were 0.42 and 0.37 eV, respectively. Measurements with the simultaneous application of 4 V dc bias gave two activation energy values 0.38 and 1.76 eV, respectively. The lower and higher activation energies were attributed to the shallow and deep trap energies respectively.
Two-dimensional x-ray focusing from compound lenses made of plastic
Authors: Piestrup M. A. , Cremer J. T. , Beguiristain H. R. , Gary C. K. , Pantell R. H.
Published: July 2002

We have measured the intensity profile and transmission of x rays focused by a series of either spherical or parabolic lenses fabricated using Mylar® (C5H4O2) or Kapton® (polyimide). The use of plastics can extend the range of operation of compound refractive lenses, improving transmission and aperture size and reducing focal length. The number of unit lenses range from 193 to 600 for each compound refractive lens. Two-dimensional focusing was obtained for photon energies 8–14 keV with imaging distances of less than 1 m. For example, full-width-half-maximum linewidths down to 16 μm at a distance of only 47 cm from the lens were achieved at 9 keV. The effective apertures of the refractive lenses were measured between 250 and 364 μm with peak transmissions between 10% and 33%.
Raman investigation of ion beam synthesized β-FeSi2
Authors: Birdwell A. G. , Glosser R. , Leong D. N. , Homewood K. P.
Published: July 2002

The Raman spectra of ion beam synthesized (IBS) β-FeSi2 are investigated and evidence for the presence of a net tensile stress is presented. Possible origins of the observed stress are suggested and a simple model is proposed in order to calculate a value of the observed stress. A correlation between the tensile stress, the nature of the band gap, and the resulting light emitting properties of IBS β-FeSi2 is suggested.
Integrated optical measurement system for fluorescence spectroscopy in microfluidic channels
Authors: Hübner Jörg , Mogensen Klaus B. , Jorgensen Anders M. , Friis Peter , Telleman Pieter , Kutter Jörg P.
Published: July 2002

A transportable miniaturized fiber-pigtailed measurement system is presented which allows quantitative fluorescence detection in microliquid handling systems. The microliquid handling chips are made in silica on silicon technology and the optical functionality is monolithically integrated with the microfluidic channel system. This results in inherent stability and photolithographic alignment precision. Permanently attached optical fibers provide a rugged connection to the light source, detection, and data processing unit, which potentially allows field use of such systems. Fluorescence measurements with two dyes, fluorescein, and Bodipy 650/665 X, showed good linear behavior over a wide range of concentrations. Minimally detected concentrations were 250 pM for fluorescein and 100 nM for Bodipy.
Quantum dot infrared photodetectors
Authors: Liu H. C. , Gao M. , McCaffrey J. , Wasilewski Z. R. , Fafard S.
Published: July 2002

Self-assembled strained semiconductor nanostructures have been grown on GaAs substrates to fabricate quantum dot infrared photodetectors. State-filling photoluminescence experiments have been used to probe the zero-dimensional states and revealed four atomic-like shells (s,p,d,f) with an excitonic intersublevel energy spacing which was adjusted to ∼60 meV. The lower electronic shells were populated with carriers by n doping the heterostructure, and transitions from the occupied quantum dot states to the wetting layer or to the continuum states resulted in infrared photodetection. We demonstrate broadband normal-incidence detection with a responsivity of a few hundred mA/W at a detection wavelength of ∼5 μm.
Impact ionization probabilities as functions of two-dimensional space and time
Authors: Plimmer S. A. , David J. P. R. , Jacob B. , Rees G. J.
Published: July 2002

The impact ionization probability functions, P(x), are effectively histograms of the positions carriers ionize along the field direction, x, which determine the avalanche multiplication properties of a semiconductor device. In this work, we use a Monte Carlo model to investigate the form of these as multidimensional functions; in not just x, but also of the elapsed time, t, and the distance traveled perpendicular to the electric field direction, y. Despite most previous temporal calculations of the avalanche process assuming that all carriers travel at the drift velocity, vd, it is shown that electrons which ionize at the shortest distances travel several times faster than vd. There is also a significant spread in possible velocities with which ionizing carriers travel along the x direction due to diffusion. Diffusive spreading of the ionization probability in both x and y is also described.
Time-resolved atomic force microscopy imaging studies of asymmetric PS-b-PMMA ultrathin films: Dislocation and disclination transformations, defect mobility, and evolution of nanoscale morphology
Authors: Hahm J. , Sibener S. J.
Published: July 2002

Time-sequenced atomic force microscopy (AFM) studies of ultrathin films of cylinder-forming polystyrene-block-polymethylmethacrylate (PS-b-PMMA) copolymer are presented which delineate thin film mobility kinetics and the morphological changes which occur in microphase-separated films as a function of annealing temperature. Of particular interest are defect mobilities in the single layer (L thick) region, as well as the interfacial morphological changes which occur between L thick and adjacent 3L/2 thick layers, i.e., structural changes which occur during multilayer evolution. These measurements have revealed the dominant pathways by which disclinations and dislocations transform, annihilate, and topologically evolve during thermal annealing of such films. Mathematical combining equations are given to better explain such defect transformations and show the topological outcomes which result from defect–defect encounters. We also report a collective, Arrhenius-type flow of defects in localized L thick regions of the film; these are characterized by an activation energy of 377 kJ/mol. These measurements represent the first direct investigation of time-lapse interfacial morphological changes including associated defect evolution pathways for polymeric ultrathin films. Such observations will facilitate a more thorough and predictive understanding of diblock copolymer thin film dynamics, which in turn will further enable the utilization of these nanoscale phase-separated materials in a range of physical and chemical applications.
Electronic structure of eka-lead (element 114) compared with lead
Authors: Landau Arie , Eliav Ephraim , Ishikawa Yasuyuki , Kaldor Uzi
Published: July 2002

The electronic level structure of eka-lead (element 114), the synthesis of which was reported last year, is studied by the recently developed intermediate Hamiltonian Fock-space coupled-cluster method. Very large basis sets are used, with l up to 8, and 36 electron are correlated. The accuracy of the resulting transition energies is tested by applying the same method to Pb; calculated ionization potentials and excitation energies agree with experiment within a few hundredths of an eV, and similar accuracy is expected for the heavier element. Ionization potentials and excitation energies of E114 are considerably higher than for Pb, due to the relativistic stabilization of the 7s and 7p1/2 orbitals. This indicates that eka-lead will probably be more inert and less metallic than lead.
Observation of crossing pores in anodically etched n-GaAs
Authors: Langa S. , Carstensen J. , Christophersen M. , Föll H. , Tiginyanu I. M.
Published: July 2002

Pores in GaAs in the micrometer range and oriented in 〈111〉 directions have been observed during the anodization of GaAs in aqueous HCl electrolytes. A direct evidence of pores intersection is presented which is a very promising feature for three-dimensional micro- and nanostructuring of III–V compounds for the production of photonic materials.
Growth of the Cr oxides via activated oxygen reactive molecular beam epitaxy: Comparison of the Mo and W oxides
Authors: Ingle N. J. C. , Hammond R. H. , Beasley M. R.
Published: July 2002

The realization of spin polarized tunnel devices made with CrO2, a theorized half-metallic ferromagnet, requires stringent control of surface and interface quality ideally obtainable via molecular beam epitaxy (MBE) growth. We have studied the MBE growth of all the di- and tri-oxides of the group VIB transition metals (Cr, Mo, and W), with the aid of a high flux atomic oxygen source and detection scheme, to help understand which oxidation states are reachable. We find that even though we can reach the +6 oxidation state of Cr (CrO3) we are unable to obtain single phase CrO2, the +4 oxidation state. One interpretation of our results is that the physical effect of pressure, not solely the oxidation potential, is important to the growth of single phase CrO2.
Laser-based processing of polymer nanocomposites for chemical sensing applications
Authors: Bubb D. M. , McGill R. A. , Horwitz J. S. , Fitz-Gerald J. M. , Houser E. J. , Stroud R. M. , Wu P. W. , Ringeisen B. R. , Piqué A. , Chrisey D. B.
Published: July 2002

Pulsed laser deposition (PLD) has been used to fabricate polymer/carbon nanocomposite thin films for use in chemical sensors (chemiresistors). Ethylene vinyl acetate copolymer (EVA) films (undoped and 20% carbon by weight) were deposited using an ArF excimer laser (193 nm) at fluences between 150 and 300 mJ/cm2. The structure and morphology of the deposited films were characterized using Fourier transform infrared spectroscopy (FTIR), Raman scattering, and transmission and scanning electron microscopy (TEM). An analysis of the FTIR spectra indicates that a film deposited using an undoped EVA target is primarily polyethylene, suggesting that the acetate group is photochemically or photothermally removed from the starting material. Gas phase measurements of the laser-evaporated material using a quadrupole time of flight mass spectrometer confirm the production of the acetyl radical on the target surface. Analysis of TEM of films deposited using C-doped targets shows that the carbon black particles (initially 50 nm particles in 1 μm agglomerates) are broken down into particles that are ⩽50 nm in the deposited film. Incorporation of carbon into the target reduces the degree of photochemical damage of the starting material, as shown in the FTIR spectra of the deposited film. The sensitivity and response time of chemiresistors fabricated from 6 μm thick composite films on top of gold electrodes were measured using toluene vapor (548 ppm). The chemiresistors exhibited a reversible and fast (<1.3 s) response to the vapor. In comparison to data reported in the literature, chemiresistors fabricated from PLD films are significantly better than devices fabricated using a more conventional polymer film growth technique.
Fronts in passive scalar turbulence
Authors: Celani A. , Lanotte A. , Mazzino A. , Vergassola M.
Published: July 2002

The evolution of scalar fields transported by turbulent flow is characterized by the presence of fronts, which rule the small-scale statistics of scalar fluctuations. With the aid of numerical simulations, it is shown that: Isotropy is not recovered, in the classical sense, at small scales; scaling exponents are universal with respect to the scalar injection mechanisms; high-order exponents saturate to a constant value; nonmature fronts dominate the statistics of intense fluctuations. Results on the statistics inside the “plateaux,” where fluctuations are weak, are also presented. Finally, we analyze the statistics of scalar dissipation and scalar fluxes.
Applications of perforated diamond anvils for very high-pressure research
Authors: Dadashev A. , Pasternak M. P. , Rozenberg G. Kh. , Taylor R. D.
Published: July 2002

To reduce the large absorption effect in diamond anvil pressure cells for soft x rays, perforated anvils employed as diamond-backing plates (DBPs) used in conjunction with miniature anvils (MAs) made of 1/100 carat diamonds were tested for high-pressure efficacy. Static pressures beyond 100 GPa were generated using a piston/cylinder cell having 0.2 mm culets. Tests were carried out in 0.1 mm cavities drilled in a Re gasket, using Ar samples and ruby chips for manometry. Except for a single failure of a 0.3 mm culet MA, no damage was detected in the DBPs drilled with truncated conical holes tapering from 0.3 mm to 1, 1.5, and 2 mm diameter. Another arrangement in which one anvil was partially drilled leaving a 0.5 mm thick wall behind the culet achieved 100 GPa. Detailed discussions are given concerning the benefits of the DBP/MA cells for high-pressure studies with soft x rays and for background reduction in Raman, IR, UV, and 57Fe Mössbauer spectroscopies.
Dielectric, hypersonic, and domain anomalies of (PbMg1/3Nb2/3O3)1−x(PbTiO3)x single crystals
Authors: Tu Chi-Shun , Tsai C.-L. , Schmidt V. Hugo , Luo Haosu , Yin Zhiwen
Published: July 2002

Dielectric permittivities, Brillouin backscattering spectra, polarization-electric field hysteresis loops, and domain structures have been measured as a function of temperature in relaxor-based ferroelectric single crystals (PbMg1/3Nb2/3O3)1−x(PbTiO3)x(PMN-xPT) for x=0.24 and 0.34. For PMN-24%PT, a diffuse phase transition which is associated with a broad frequency-dependent dielectric maximum was observed near 380 K. As the temperature increases, the PMN-24%PT crystal gradually develops cubic regions and is fully converted into the cubic state near 375 K. An extra dielectric anomaly appears at 370 K, possibly due to the percolating polar cluster induced by an external electric field. PMN-34%PT exhibits a nearly normal ferroelectric phase transition near 445 K from the tetragonal to the cubic phase. In addition, a weak diffuse phase transition observed near 280 K may result from partial conversion of rhombohedral phase to tetragonal phase. The dielectric thermal hysteresis confirms that the transitions near 280 and 445 K are diffuse first order and first order, respectively. The dielectric permittivities of PMN-24%PT and PMN-34%PT obey the relation, εm′/ε′(f,T)=1+[T−Tm(f )]γ/2δγ2, above the temperature of permittivity maximum Tm.
Correlation between cathode properties, burning voltage, and plasma parameters of vacuum arcs
Authors: Anders André , Yotsombat Banchob , Binder Robert
Published: July 2002

Burning voltages of vacuum arcs were measured for 54 cathode materials and compared with literature data. As anticipated, a correlation between the arc burning voltage and the plasma temperature was found. However, more importantly, a correlation between the cohesive energy of the cathode material and the arc burning voltage could be demonstrated. This link between a cathode material property, the cohesive energy, and a discharge property, the arc burning voltage, is essential for the operation of the vacuum arc discharge because is determines the plasma temperature. Energy balance considerations show that this “cohesive energy rule” is responsible for several other secondary relationships, such as the correlation between the mean ion charge state and the boiling temperature of the cathode.
A novel parallel-rotation algorithm for atomistic Monte Carlo simulation of dense polymer systems
Authors: Santos S. , Suter U. W. , Müller M. , Nievergelt J.
Published: July 2002

We develop and test a new elementary Monte Carlo move for use in the off-lattice simulation of polymer systems. This novel Parallel-Rotation algorithm (ParRot) permits moving very efficiently torsion angles that are deeply inside long chains in melts. The parallel-rotation move is extremely simple and is also demonstrated to be computationally efficient and appropriate for Monte Carlo simulation. The ParRot move does not affect the orientation of those parts of the chain outside the moving unit. The move consists of a concerted rotation around four adjacent skeletal bonds. No assumption is made concerning the backbone geometry other than that bond lengths and bond angles are held constant during the elementary move. Properly weighted sampling techniques are needed for ensuring detailed balance because the new move involves a correlated change in four degrees of freedom along the chain backbone. The ParRot move is supplemented with the classical Metropolis Monte Carlo, the Continuum-Configurational-Bias, and Reptation techniques in an isothermal–isobaric Monte Carlo simulation of melts of short and long chains. Comparisons are made with the capabilities of other Monte Carlo techniques to move the torsion angles in the middle of the chains. We demonstrate that ParRot constitutes a highly promising Monte Carlo move for the treatment of long polymer chains in the off-lattice simulation of realistic models of dense polymer systems.
Quantum vacuum noise in physics and cosmology
Authors: Davies P. C. W.
Published: July 2002

The concept of the vacuum in quantum field theory is a subtle one. Vacuum states have a rich and complex set of properties that produce distinctive, though usually exceedingly small, physical effects. Quantum vacuum noise is familiar in optical and electronic devices, but in this paper I wish to consider extending the discussion to systems in which gravitation, or large accelerations, are important. This leads to the prediction of vacuum friction: The quantum vacuum can act in a manner reminiscent of a viscous fluid. One result is that rapidly changing gravitational fields can create particles from the vacuum, and in turn the backreaction on the gravitational dynamics operates like a damping force. I consider such effects in early universe cosmology and the theory of quantum black holes, including the possibility that the large-scale structure of the universe might be produced by quantum vacuum noise in an early inflationary phase. I also discuss the curious phenomenon that an observer who accelerates through a quantum vacuum perceives a bath of thermal radiation closely analogous to Hawking radiation from black holes, even though an inertial observer registers no particles. The effects predicted raise very deep and unresolved issues about the nature of quantum particles, the role of the observer, and the relationship between the quantum vacuum and the concepts of information and entropy.
Carrier dynamics in low-temperature grown GaAs studied by terahertz emission spectroscopy
Authors: Němec H. , Pashkin A. , Kužel P. , Khazan M. , Schnüll S. , Wilke I.
Published: July 2002

Ultrafast dynamics of free carriers in low-temperature grown GaAs was studied using time-domain terahertz emission spectroscopy. The subpicosecond free-carrier lifetime was determined for a set of annealed samples with different growth temperatures (175–250 °C), the carrier mobility was also estimated. The influence of the growth temperature on the ultrafast carrier trapping is discussed.
Quantum dots in glass spherical microcavity
Authors: Jia Rui , Jiang De-Sheng , Tan Ping-Heng , Sun Bao-Quan
Published: July 2002

Glass spherical microcavities containing CdSexS1−x semiconductor quantum dots (QDs) are fabricated. The coupling between the optical emission of embedded CdSexS1−x QDs and spherical cavity modes is realized. When the luminescence of QDs is excited by a laser beam, the strong whispering gallery mode resonance with high Q factors is achieved in the photoluminescence spectra.
Relation between the optical properties of composite Si3N4 thin films with embedded Cu clusters and the clusters morphology: Irradiation effects
Authors: Girardeau T. , Camelio S. , Traverse A. , Lignou F. , Allain J. , Naudon A. , Guérin Ph.
Published: July 2002

Ion beam sputtering codeposition has been used to elaborate ceramic–metal (cermet) composite thin films consisting of copper nanoclusters embedded in an amorphous Si3N4 matrix. As prepared, the clusters have a size smaller than 3 nm and postirradiation by high energetic Ar+ ions leads to an homogenization of the clusters’ morphology and an increase of the clusters’ size to an average diameter of 4.5 nm. This work deals with the relation between the morphology of the clusters, characterized by extended x-ray absorption fine spectroscopy and grazing incidence small angle x-ray scattering, and the optical properties (obtained by spectroscopic ellipsometry) of the cermets, which are classically modeled with the help of the effective medium theory. In the case of the as-prepared sample, the Bruggeman effective medium theory has been successfully used. This comes from the fact that the clusters are sufficiently close to each other to create mutual interactions. On the other hand, the morphology of the postirradiated film is in agreement with the hypothesis made by the Maxwell–Garnett effective medium theory, and a resonance peak appears due to the surface plasmon excitation whose position depends on the cluster size.
Metal-insulator-vacuum type electron emission from N-containing chemical vapor deposited diamond
Authors: Okano Ken , Yamada Takatoshi , Sawabe Atsuhito , Koizumi Satoshi , Itoh Junji , Amaratunga Gehan A. J.
Published: July 2002

This letter presents a clear explanation of the electron emission mechanism of the high-resistivity N-doped diamond cathode. Due to the very low barrier to emission of electrons from the N-doped diamond conduction band into vacuum, electrons in the conduction band of diamond can establish an appreciable leakage current at very low anode voltage. When such a current starts to flow, there is a field which is developed across the diamond bulk. This field is observed as an increase in the electric field at the back contact, causing the injected tunneling current increases exponentially. This process leads to the low threshold emission from the high resistivity N-doped diamond cathode.
Thermodynamic and structural properties of liquid water around the temperature of maximum density in a wide range of pressures: A computer simulation study with a polarizable potential model
Authors: Jedlovszky Pál , Vallauri Renzo
Published: July 2002

Computer simulations of liquid water have been performed with the polarizable Brodholt–Sampoli–Vallauri (BSV) potential model at several temperatures around the temperature of maximum density (TMD) in the entire pressure range in which such a density maximum exists in thermodynamically stable liquid water. The temperature and pressure dependence of the thermodynamic and structural properties has been analyzed on the basis of these simulations. We find that the BSV model reproduces most of the important thermodynamic features of water in this temperature and pressure range. The BSV model is also found to reproduce another of the anomalous properties of liquid water, i.e., the isothermal compressibility goes through a minimum when the temperature is increased. On the other hand, it is found that above the TMD the density of the model decreases much faster with increasing temperature than in real water. However, this failure, which is a common feature of the polarizable water models, is rather unimportant in the narrow temperature range studied here. In analyzing the molecular level structure of water as a function of the thermodynamic conditions we find that the increase of the temperature as well as of the pressure has a distorting effect on the tetrahedral hydrogen bonded network, and it causes an increase of the fraction of the interstitial neighbors of the molecules. These changes result in a more compact structure and hence in an increase of the density of the system. When these changes are induced by the temperature, the increasing thermal motion of the molecules can compensate the increase of the density, and the two opposite effects result in the appearance of the density maximum.
Femtosecond laser interference technique with diffractive beam splitter for fabrication of three-dimensional photonic crystals
Authors: Kondo Toshiaki , Matsuo Shigeki , Juodkazis Saulius , Misawa Hiroaki
Published: July 2002

A simple optical interference method to fabricate microperiodic structures was demonstrated. Femtosecond laser pulse was split by a diffractive beam splitter and overlapped with two lenses. Temporal overlap of the split femtosecond pulses, which requires 10 μm order accuracy in optical path lengths, was automatically achieved by this optical setup. One-, two-, and three-dimensional periodic microstructures with micrometer-order periods were fabricated using this method.
On the dynamics of rotationally broad, spatially aligned wave packets
Authors: Seideman Tamar
Published: July 2002

The problems of rotational excitation and molecular alignment in moderately intense laser fields are addressed theoretically and numerically. Several scaling propensities are derived, which connect the achievable alignment with the underlying field and molecular parameters. An analytical model is developed, which exposes the origin of alignment in the short-pulse limit. A numerical study is presented, which quantifies the notions of adiabatic and sudden alignment and illustrates the behavior of the post-pulse alignment subsequent to slow turn-on and rapid turn-off of the laser pulse. Several new potential applications of laser alignment are proposed.
Surface photovoltage spectroscopy of semi-insulating GaAs in the 800–1100 nm range
Authors: Sharma T. K. , Kumar Shailendra
Published: July 2002

Surface photovoltage spectroscopy (SPS) studies on thick semi-insulating (SI) GaAs wafers have been done in the range 800–1100 nm using chopped light geometry. SPS peaks at 880 nm, 900 nm, and a broadband in the range 930–1080 nm have been observed. These observations are important as SPS is routinely used to study absorption-related features in InAs self-organized quantum dots grown on SI GaAs. The effect of the ambient and chopping frequencies on SPS spectra is also presented.
Room-temperature continuous photopumped laser operation of coupled InP quantum dot and InGaP quantum well InP–InGaP–In(AlGa)P–InAlP heterostructures
Authors: Walter G. , Holonyak N. , Ryou J. H. , Dupuis R. D.
Published: July 2002

Data are presented demonstrating continuous 300 K photopumped InP quantum dot (QD) laser operation (656–679 nm) realized by modifying and coupling, via tunneling, an auxiliary InGaP quantum well (QW) to the QDs of an InP–In(AlGa)P–InAlP heterostructure grown by metalorganic chemical vapor deposition. The In0.49Ga0.51P QW coupled to the InP QDs by a thin (≲20 Å) In0.5Al0.3Ga0.2P barrier overcomes the limitations of carrier collection, lateral transport, and thermalization in the QDs, thus yielding a different form of QD laser.
Direct fabrication of two-dimensional titania arrays using interference photolithography
Authors: Shishido Atsushi , Diviliansky Ivan B. , Khoo I. C. , Mayer Theresa S. , Nishimura Suzushi , Egan Gina L. , Mallouk Thomas E.
Published: July 2002

Two-dimensional (2D) titania arrays with periods of 0.8–2.0 μm were fabricated by polymerization of a photosensitive titanium-containing monomer film using interference photolithography. The 2D precursor arrays were prepared by exposing a mixture of methacrylic acid, ethyleneglycol dimethacrylate, and titanium ethoxide doped with photoinitiator to 355 nm, 15 ns pulses from a Nd-Yttrium–aluminum–garnet laser and then rinsing with methanol. Pure titania arrays were obtained from the precursor arrays by subsequent calcination at 575 °C. The structure of the arrays fabricated by this method was confirmed with optical microscopy and scanning electron microscopy.
Surface photovoltage spectroscopy characterization of a GaAs/GaAlAs vertical-cavity-surface-emitting-laser structure: Angle dependence
Authors: Liang J. S. , Huang Y. S. , Tien C. W. , Chang Y. M. , Chen C. W. , Li N. Y. , Li P. W. , Pollak Fred H.
Published: July 2002

An angle-dependent surface photovoltage spectroscopy (SPS) study has been performed at room temperature on a GaAs/GaAlAs-based vertical-cavity-surface-emitting-laser (VCSEL) structure emitting at a wavelength near 850 nm. For comparison purposes, we have also measured the angle-dependent reflectance (R). The surface photovoltage spectra exhibit both the fundamental conduction to heavy-hole (1C–1H) excitonic transition and cavity mode plus additional interference features related to the properties of the mirror stacks, whereas in the R spectra only the cavity mode and interference features are clearly visible. The energy position of the excitonic feature is not dependent on the angle of incidence, in contrast to that of the cavity mode, whose angular dependence can be fitted with a simple model. This study demonstrates the considerable potential of angle-dependent SPS for the contactless and nondestructive characterization of VCSEL structures at room temperature.
Performance of a double-multilayer monochromator at Beamline 2-BM at the Advanced Photon Source
Authors: Chu Y. S. , Liu C. , Mancini D. C. , De Carlo F. , Macrander A. T. , Lai B. , Shu D.
Published: July 2002

We describe the performance of the double-multilayer monochromator that is installed at bending magnet beamline 2-BM at the Advanced Photon Source. In order to achieve continuous operation over energies from 3.2 to 10.9 keV, four different multilayer stripes were deposited onto Si substrates using the in-house sputtering deposition facility at the APS. The optical performance of the four stripes depends on their operating energy ranges, and produces 45%–75% peak reflectivity at first-order reflection with a 1%–6% bandwidth and a total flux increase of 20–40 compared to the Si(111) double-crystal monochromator.
The electronic states of Fe2S2−/0/+/2+
Authors: Hübner Olaf , Sauer Joachim
Published: July 2002

The relative energies of a multitude of low-lying electronic states of Fe2S2−/0/+/2+ have been determined by complete active space self-consistent-field (CASSCF) calculations. For selected states dynamical correlation has been included by the multireference configuration interaction method (MRCI) and the structures of some high-spin states have been optimized by CASSCF/MRCI. Comparison is made with structures obtained by density functional calculations. In all oxidation states of Fe2S2 the numerous states are assigned to spin ladders. The ground states of Fe2S2−/0/+/2+ are Ag10, Ag1, B1u2 and Ag1, respectively. The total splittings of the lowest-energy spin ladders of Fe2S2, Fe2S2−, and Fe2S2+ are about 0.17, 0.18, and 0.35 eV, respectively. The spin ladders of Fe2S2 qualitatively reflect the picture of Heisenberg spin coupling. Some of the spin ladders of Fe2S2+ show the picture of combined Heisenberg coupling and double exchange. The calculated adiabatic electron affinity of Fe2S2 is 1.1 eV (observed 2.1 eV). Our estimate of the ionization energy of Fe2S2 is 7.9±0.5 eV. An interpretation of the observed photoelectron spectrum of Fe2S2− is given.