bluuebunny/crossref_metadata_2025 huggingface dataset:
Study of magnetic properties of a cryogenic temperature sensor diode
Authors: Ota S. B. , Mishra P. K. , Sahni V. C. , Singh M. R. , Bascuñán Juan
Published: July 2002

The magnetic properties of a cryogenic silicon diode temperature sensor (Lake Shore) are studied in the temperature range 5–255 K using a superconducting quantum interference device magnetometer. The M vs H curve at 5 and 250 K showed practically negligible magnetic hysteresis. The magnetization in a field of 100 Oe is found to be temperature independent to within 10% in the temperature range 5–255 K. The observed magnetization comes mainly from the packaging materials which includes ferromagnetic Kovar leads and nickel plating. We suggest a change of the construction materials. Our measurements show that, the existing diodes, if used in close proximity to the sample, in magnetization measurements, can be conveniently used only after background calibration.
Polymeric electro-optic Mach–Zehnder switches
Authors: Thackara J. I. , Chon J. C. , Bjorklund G. C. , Volksen W. , Burland D. M.
Published: July 2002

Polymeric 2×2 electro-optic switches based on the balanced bridge, or Mach–Zehnder, design are presented. Two poled nonlinear optical side-chain polymers based on the dye Disperse Red 1, one with a glass transition temperature Tg of 131 °C and the other with a Tg of 228 °C, were used as the active layers in these devices. The switch designs incorporated traveling-wave electrodes in the phase shift sections and, for the lower Tg polymer device, electrically adjustable 3 dB couplers. At the 1.32 μm operating wavelength, extinction ratios of at least 28 dB were achieved in all four states of the lower Tg switch with a propagation loss of 1.6 dB/cm and a small signal 3 dB modulation bandwidth of over 2 GHz.
Fabrication of (001) InP-based 1.55-μm wavelength lasers on a (110) GaAs substrate by direct bonding (A prospect for free-orientation integration)
Authors: Okuno Y. , Aoki M. , Tsuchiya T. , Uomi K.
Published: July 2002

We examine the direct bonding of (001) InP and (110) GaAs and demonstrate its application to device fabrication. Cross-sectional observation shows that these wafers can be united without generating dislocation. (001) InP-based 1.55-μm wavelength lasers are fabricated on (110) GaAs. The light–current characteristics of the lasers are almost identical to those of lasers fabricated on (001) GaAs, while the turn-on voltage is higher by about 0.4 V due to the large band discontinuity. The results show that the direct bonding technique is promising for allowing new concept ‘‘free-orientation integration.’’
High power mid-infrared (λ∼5 μm) quantum cascade lasers operating above room temperature
Authors: Faist Jérôme , Capasso Federico , Sirtori Carlo , Sivco Deborah L. , Baillargeon James N. , Hutchinson Albert L. , Chu Sung-Nee G. , Cho Alfred Y.
Published: July 2002

The high power operation of mid-infrared quantum cascade lasers at temperatures up to T=320 K is reported. Gain at high temperature is optimized by a design combining low doping, a funnel injector, and a three-well vertical transition active region. A molecular beam epitaxy grown InP top cladding layer is also used to optimize heat dissipation. A peak pulsed optical power of 200 mW and an average power of 6 mW are obtained at 300 K and at a wavelength λ=5.2 μm. The devices also operate in continuous wave up to 140 K.
Effective-mass Hamiltonian for strained wurtzite GaN and analytical solutions
Authors: Chuang S. L. , Chang C. S.
Published: July 2002

We present the effective-mass Hamiltonian for wurtzite crystals including the strain effects and show analytical solutions for the energy band structures. We found that the Hamiltonian can be block-diagonalized in our chosen set of basis functions such that the band structures near the zone center can be conveniently derived with analytical solutions. The constant energy surfaces and the valence-band structures are illustrated graphically. The strain effects on the band structures of the wurtzite GaN are also discussed. This Hamiltonian provides a theoretical foundation for calculating the band structures of the bulk and quantum-well wurtzite crystals under the framework of the effective mass theory.
Localized Kerr-type nonlinearities in GaAs/AlGaAs multiple quantum well structures at 1.55 μm
Authors: Hamilton C. J. , Marsh J. H. , Hutchings D. C. , Aitchison J. S. , Kennedy G. T. , Sibbett W.
Published: July 2002

We report the use of a novel impurity free vacancy disordering technique which has been used to produce waveguides with different Kerr-type nonlinear coefficients. The technique relies on standard SiO2 dielectric caps to promote disordering and Ga2O3 caps to suppress disordering. Band-gap shifts of around 40 nm and consequent changes in n2 of more than 60% are reported.
Second-harmonic generation of blue light from [112]-oriented III-V antiresonant waveguide heterostructures
Authors: Ramos P. A. , Towe E.
Published: July 2002

Second-harmonic generation of blue light by the nonlinear interaction of two counter-propagating transverse electric-polarized fundamental fields in a [112]-oriented (Al,Ga)As/GaAs antiresonant waveguide is demonstrated. Theoretical calculations of the visible output power for given fundamental inputs are shown to agree with experiment. Our results show that GaAs-based materials may offer another alternative for the development of blue/green light-emitters.
SiC rapid thermal carbonization of the (111)Si semiconductor-on-insulator structure and subsequent metalorganic chemical vapor deposition of GaN
Authors: Steckl A. J. , Devrajan J. , Tran C. , Stall R. A.
Published: July 2002

Si (111) semiconductor-on-insulator (SOI) structures have been converted to SiC by carbonization of the thin (<100 nm) Si layer using rapid thermal chemical vapor deposition with mixtures of propane and H2 at atmospheric pressure. Carbonization temperatures around 1225–1250 °C produced SiC films with optimum structural properties. X-ray diffraction (XRD) reveals a single SiC peak at 2θ=35.7° corresponding to the (111) reflection, with an uncorrected full width at half-maximum (FWHM) of ∼0.24°. Infrared spectroscopy of SiC SOI structures obtained under optimum carbonization conditions exhibited a sharp absorbtion peak produced by the Si–C bond at 795 cm−1, with FWHM=22–25 cm−1. Metalorganic chemical vapor deposition growth of GaN on the (111) SiC SOI was carried out with trimethylgallium and NH3 precursors at 1000 °C. XRD indicates highly oriented hexagonal GaN, with FWHM of the (0002) peak of ∼0.15°. The 300 °K photoluminescence (PL) spectrum of GaN films exhibits a strong near band-edge peak (at λp∼371 nm, with FWHM=100–150 meV) and weak yellow emission. Under low power excitation, the 370 nm PL emission from the GaN/SiC SOI structure displays an emission intensity ∼10× higher than that of equivalent GaN films grown on sapphire.
Low temperature deposition of patterned TiO2 thin films using photopatterned self-assembled monolayers
Authors: Collins Rochael J. , Shin Hyunjung , DeGuire Mark R. , Heuer Arthur H. , Sukenik Chaim N.
Published: July 2002

Patterned thin films of TiO2 were deposited from aqueous solution onto photopatterned self-assembled monolayer (SAM) films on Si substrates. Regions of the SAM containing sulfonate surface functionality were created by the photo-oxidation of initially deposited thioacetate groups through a mask. The nanocrystalline TiO2-on-SAM films were deposited selectively on the photolyzed regions of the SAM. The electrical properties of such films were assessed for potential microelectronic device applications. Current–voltage and capacitance–voltage measurements made on nonpatterned TiO2 films yielded values of relative permittivity ranging from 24 to 57, film resistivities of 1.0–1.5×109 Ω cm and breakdown voltages in excess of 1 MV/cm.
Microstructural characterization of solid-phase crystallized amorphous silicon films recrystallized using an excimer laser
Authors: Giust G. K. , Sigmon T. W.
Published: July 2002

The laser recrystallization of amorphous-silicon films crystallized in a furnace is investigated for single and multipulse laser irradiation. The resulting microstructure is grouped into three regimes, based on their recrystallization mechanism. For the low energy irradiation regime, the grain size in unchanged, however many intragrain defects are removed, while the high energy regime results in homogeneous nucleation of the molten film. A third regime exists between these two, characterized by large lateral grain growth. Except for the low energy regime, we conclude that the results obtained are consistent with those found for the direct laser crystallization of amorphous silicon films.
Device length dependence of optical second-harmonic generation in AlGaAs quasiphase matched waveguides
Authors: Xu C. Q. , Takemasa K. , Nakamura K. , Shinozaki K. , Okayama H. , Kamijoh T.
Published: July 2002

Device length dependence of optical second-harmonic generation (SHG) in AlGaAs semiconductor quasiphase matched (QPM) waveguides with periodically crystal domain inverted gratings has been studied both experimentally and theoretically. It is found that the SHG power depends strongly on device length. An optimum device length for obtaining maximum SHG is shown for QPM-SHG devices with non-negligible waveguide propagation loss. It is also shown that this optimum device length can be predicted theoretically, making experimental optimization unnecessary.
Calibration of strain vs nuclear quadrupole splitting in III–V quantum wells
Authors: Guerrier D. J. , Harley R. T.
Published: July 2002

A method is presented for direct measurement of the quadrupole splitting of nuclear magnetic resonances produced by uniaxial strain in III–V semiconductors. Strain is applied to nuclei in a lattice-matched GaAs/AlGaAs quantum well by bending the structure to a measured radius and the resulting splittings of resonances of nuclei within the well are obtained using optically detected nuclear magnetic resonance (ODNMR). Such calibration of the quadrupole splitting versus applied strain will provide the basis for a measurement of built-in strain in strained-layer heterostructures by means of ODNMR.
In situ monitoring during pulsed laser deposition of complex oxides using reflection high energy electron diffraction under high oxygen pressure
Authors: Rijnders Guus J. H. M. , Koster Gertjan , Blank Dave H. A. , Rogalla Horst
Published: July 2002

A suitable in situ monitoring technique for growth of thin films is reflection high energy electron diffraction (RHEED). Deposition techniques, like pulsed laser deposition (PLD) and sputter deposition, used for fabrication of complex oxide thin films use relatively high oxygen pressures (up to 100 Pa) and are, therefore, not compatible with ultrahigh vacuum RHEED equipment. We have developed a RHEED system which can be used for growth monitoring during the deposition of complex oxides at standard PLD conditions. We are able to increase the deposition pressure up to 50 Pa using a two-stage differential pumping system. Clear RHEED patterns are observable at these high pressures. The applicability of this system is demonstrated with the study of homoepitaxial growth of SrTiO3 as well as the heteroepitaxial growth of YBa2Cu3O7−δ on SrTiO3. Intensity oscillations of the RHEED reflections, indicating two-dimensional growth, are observed up to several tens of nanometers film thickness in both cases.
Local structural order around dopant atoms in Fe- and Co-doped ternary alloys based on γ′-Ni3Al
Authors: Balasubramanian Mahalingam , Lyver Rex , Budnick Joseph I. , Pease Douglas M.
Published: July 2002

We have utilized x-ray absorption fine structure (XAFS) to study the local structural order of dopants in ternary alloys of the composition Ni75(Fe/Co)xAl25−x. In γ′-Ni75Al25, the first coordination shell around Al consists of 12 Ni atoms, whereas Ni is surrounded by 8 Ni and 4 Al atoms. Cobalt absorbers exclusively occupy the Ni sublattice, yet the Co XAFS shows only Co–Ni bond contributions in the first peak of the Fourier transform, with the Al backscattering being either completely absent or so weak that it is masked by the strong Ni backscattering background. A model based on a highly distorted Al sublattice, created by dopant atoms occupying the Ni sublattice, gives a possible explanation for the absence of Al backscattering contributions, and furnishes an explanation for our results on Fe dopants in γ′-Ni3Al, as well as results from similar studies by others on doped γ′ nickel aluminides.
Lateral epitaxy of low defect density GaN layers via organometallic vapor phase epitaxy
Authors: Nam Ok-Hyun , Bremser Michael D. , Zheleva Tsvetanka S. , Davis Robert F.
Published: July 2002

Organometallic vapor phase lateral epitaxy and coalescence of GaN layers originating from GaN stripes deposited within 3-μm-wide windows spaced 3 μm apart and contained in SiO2 masks on GaN/AlN/6H–SiC(0001) substrates are reported. The extent and microstructural characteristics of the lateral overgrowth were a strong function of stripe orientation. A high density of threading dislocations, originating from the interface of the underlying GaN with the AlN buffer layer, were contained in the GaN grown in the window regions. The overgrowth regions, by contrast, contained a very low density of dislocations. The coalesced layers had a rms surface roughness of 0.25 nm.
Nonlinearly induced escape from a defect state in waveguide arrays
Authors: Peschel U. , Morandotti R. , Aitchison J. S. , Eisenberg H. S. , Silberberg Y.
Published: July 2002

We experimentally investigate the linear and nonlinear optical properties of a nonuniform waveguide array. By reducing the width of a single waveguide, we decrease its effective index and induce waveguiding along the defect. Due to the positive nonlinearity, the index difference is reduced for increasing power levels with the result that the field escapes. Waveguiding is suppressed by the action of the nonlinearity.
Tunable superstructures in hydrothermally etched iron-passivated porous silicon
Authors: Li Xin Jian , Hu Xing , Jia Yu , Zhang Yu Heng
Published: July 2002

Tunable superstructures were generated by hydrothermally etching silicon wafers in the solution of HF containing ferric nitrate. The structural units of these superstructures arrayed perpendicular to the surface and their shapes could be formed as cone-like, pillar-like, or crater-pit-like through changing the solution concentration of Fe3+ in the etching process. The photoluminescence (PL) in as-prepared porous silicon was found to be independent of the microlocations chosen to carry out the PL measurements.
Light-emitting multifunctional rhenium (I) and ruthenium (II) 2,2′-bipyridyl complexes with bipolar character
Authors: Chan Wai Kin , Ng Po King , Gong Xiong , Hou Sijian
Published: July 2002

A series of multifunctional molecules consisting of hole transport, electron transport, and light-emitting moieties were synthesized and fabricated into single-layer organic light-emitting devices. The light-emitting units were based on 2,2′-bipyridine complexes of rhenium and ruthenium. It was found that, due to the bipolar character of the molecules, the charge carrier mobilities, charge injection barrier, and the device performance were improved. The electron carrier mobilities are on the order of 10−4 cm2 V−1 s−1 and a maximum luminescence of 730 cd/m2 was observed.
Lateral intersubband photocurrent spectroscopy on InAs/GaAs quantum dots
Authors: Chu L. , Zrenner A. , Böhm G. , Abstreiter G.
Published: July 2002

We report on lateral intersubband photocurrent spectroscopy on self-assembled InAs/GaAs quantum dots in normal incidence. The in-plane polarized intersubband transition between the first exited states and wetting layer in the conduction band is peaked at 180 meV with a spectral linewidth of 20 meV. In-plane polarization dependent measurements show that the energy separation between the (100) and (010) states caused by lateral confinement is about 3.5 meV. A comparison of photoluminescence and vertical and lateral photocurrent experiments leads to a consistent picture of the energy levels in the conduction and valence band.
Changes in luminescence emission induced by proton irradiation: InGaAs/GaAs quantum wells and quantum dots
Authors: Leon R. , Swift G. M. , Magness B. , Taylor W. A. , Tang Y. S. , Wang K. L. , Dowd P. , Zhang Y. H.
Published: July 2002

The photoluminescence (PL) emission from InGaAs/GaAs quantum-well and quantum-dot (QD) structures are compared after controlled irradiation with 1.5 MeV proton fluxes. Results presented here show a significant enhancement in radiation tolerance with three-dimensional quantum confinement. Some additional radiation-induced changes in photocarrier recombination from QDs, which include a slight increase in PL emission with low and intermediate proton doses, are also examined.
Dielectric properties of pulsed-laser-deposited calcium titanate thin films
Authors: Hao Jianhua , Si Weidong , Xi X. X. , Guo Ruyan , Bhalla A. S. , Cross L. E.
Published: July 2002

We have deposited CaTiO3 thin films on SrRuO3 electrode layers on SrTiO3 and LaAlO3 substrates by pulsed laser deposition. The low-frequency dielectric properties were measured from 4 to 300 K. We found that the dielectric constants of 1-μm-thick CaTiO3 films are very close to that observed in the bulk materials. The dielectric loss depends on the temperature and is on the order of 10−4 at low temperature. The dielectric constants of the films can be tuned by electric field by 6% at room temperature and 10% at 200 K.
Undercooled liquid-to-glass transition during continuous cooling in Pd–Cu–Ni–P alloys
Authors: Nishiyama N. , Horino M. , Haruyama O. , Inoue A.
Published: July 2002

The glass-transition behavior from the supercooled liquid of a Pd40Cu30Ni10P20 alloy was investigated by employing a power-compensated differential scanning calorimetry under continuous cooling. At cooling rates of 0.83, 1.17, and 1.67 K/s, the transition was clearly detected as an abrupt decrease in heat capacity. From the difference in heat capacity between the undercooled liquid and glass, the alloy obtained at the lower cooling rate was found to have a more relaxed structure. The thermodynamic parameters determined in the present study enable us to interpret the reason for the outstandingly high glass-forming ability of the alloy.
Enhancement of hot-electron generation rate in Schottky source metal–oxide–semiconductor field-effect transistors
Authors: Uchida Ken , Matsuzawa Kazuya , Koga Junji , Takagi Shin-ichi , Toriumi Akira
Published: July 2002

Source-side hot-electron generation is experimentally demonstrated in Schottky source metal–oxide–semiconductor field-effect transistors (MOSFETs). An asymmetric n-type MOSFET having a CoSi2 layer in place of one of the n+ source/drain regions has been fabricated and intensively investigated. When the CoSi2 layer is used as the source, large gate current and negative-differential conductance (NDC) are simultaneously observed, whereas, when the n+ region is used as the source, both gate current and NDC are not observed. By comparing the device characteristics before and after the NDC observation, it is concluded that the gate current is due to hot electrons generated at the Schottky source side and the NDC is caused by trapped electrons in the oxide. These source-side hot electrons will open up the way to the realization of deca-nanoscaled high-speed devices.
Solar-blind AlGaN-based inverted heterostructure photodiodes
Authors: Tarsa E. J. , Kozodoy P. , Ibbetson J. , Keller B. P. , Parish G. , Mishra U.
Published: July 2002

True solar-blind operation with a sharp responsivity cutoff at ∼300 nm has been demonstrated in AlGaN-based photodiodes using an “inverted heterostructure photodiode” design. This structure utilizes an AlxGa1−xN(x>0.3) intrinsic or lightly doped active layer surrounded by p- and/or n-type contact layers having a narrower band gap than the active layer. By utilizing narrow band gap (e.g., GaN) contact layers, the difficulties associated with achieving high doping efficiencies in wide band gap contact layers are circumvented. This basic structure is applicable to both front- and back-side illuminated detector geometries. Front-side illuminated solar-blind photodiodes were demonstrated with a peak responsivity of 0.08 A/W at 285 nm, while back-side illuminated detectors yielded a peak responsivity of 0.033 A/W at 275 nm (both are measured without antireflection coating). Both types of detectors offered sharp spectral responsivity cutoff of at least three orders of magnitude by 325 nm.
The nonlocal and local property of the electron energy distribution function in a low-pressure inductively coupled plasma
Authors: Chung ChinWook , Chang Hong-Young
Published: July 2002

Electron energy distribution functions (EEDFs) at different radial positions are measured by a rf compensated Langmuir probe in a low-pressure solenoidal inductively coupled plasma. It is found that the measured EEDFs for trapped electrons with total energy ε<eφw (wall potential) are a function of only total energy, while the EEDFs for free electrons that can escape plasma are functions of radial position and total energy. The depletion energy of the EEDFs in the free electron range is determined not only by the wall potential but also by the electron canonical momentum pθ. These results are consistent with the nonlocal electron kinetic theory.
Pyroelectric properties in sol–gel derived barium strontium titanate thin films using a highly diluted precursor solution
Authors: Cheng Jian-Gong , Tang Jun , Chu Jun-Hao , Zhang Ai-Jun
Published: July 2002

The pyroelectric coefficient and pyroelectric infrared response in the sol–gel derived Ba0.8Sr0.2TiO3 thin films have been studied for possible infrared detector applications. The measured pyroelectric coefficient is larger than 3.1×10−4 C/m2 K at temperatures ranging from 10 to 26 °C and reaches the maximum value of 4.1×10−4 C/m2 K at 16 °C. The infrared detectivity of 4.6×106 cm Hz1/2 W−1 has been obtained at 19 °C and 10 Hz in the Ba0.8Sr0.2TiO3 films deposited on thick (500 μm) platinum coated silicon substrates. The better infrared response can be expected by the improvement in the thermal isolation of pyroelectric element and the electrode materials.
The dependence of the Fermi level on temperature, doping concentration, and disorder in disordered semiconductors
Authors: Nguyen Thanh H. , O’Leary Stephen K.
Published: July 2002

We employ an elementary model for the distribution of electronic states to develop a quantitative theory of equilibrium occupation statistics in disordered semiconductors. In particular, assuming Fermi–Dirac statistics and charge neutrality, we determine how the Fermi level position varies with temperature for various amounts of disorder and various dopant concentration levels, disorder being represented by the breadth of the tails in the conduction band and valence band distributions of electronic states. We find that as the disorder is increased the Fermi level is pulled towards the intrinsic Fermi level. An explanation for this result is provided.
Capacitance–voltage characteristics of InAs/GaAs quantum dots embedded in a pn structure
Authors: Wetzler R. , Wacker A. , Schöll E. , Kapteyn C. M. A. , Heitz R. , Bimberg D.
Published: July 2002

We study the electronic states of self-organized InAs quantum dots embedded in a pn junction by means of capacitance–voltage (C–V) characteristics. A model based on the self-consistent solution of the Poisson equation and the drift-diffusion equations is proposed for calculating the capacitance. This model allows us to determine the energy levels of the quantum dot states and their inhomogeneous broadening from a comparison with experimental C–V data. Good quantitative agreement between predictions of the model and the low-frequency C–V characteristics is obtained.
Absolute measurement of the optical polarizability of C60
Authors: Ballard A. , Bonin K. , Louderback J.
Published: July 2002

We report on a new optical technique that uses light forces and a time-of-flight spectrometer to make absolute measurements of cluster polarizabilities. This is also the first accurate report of an ac polarizability measurement of a condensable cluster in the gas phase. We have determined the optical polarizability of C60 at the fundamental wavelength of a Nd:YAG laser (1.064 μm) to be α=79±4 Å3.
Far-ultraviolet resonance Raman spectroscopy of nitrate ion in solution
Authors: Waterland Mark R. , Myers Kelley Anne
Published: July 2002

Resonance Raman spectra are presented for the nitrate anion, NO3−, in water, ethylene glycol, methanol, and acetonitrile solution at six excitation wavelengths from 246 to 204 nm, on resonance with the lowest π→π* excitation. Absolute Raman cross sections for the CH stretches of ethylene glycol and methanol at these wavelengths are also reported. The nitrate spectra in all four solvents are dominated by fundamentals, overtones, and combination bands of the totally symmetric NO stretch (ν1) near 1043 cm−1 and the out-of-phase NO stretches (ν3) at 1340–1400 cm−1, consistent with substantial changes in NO bond length upon π-electron excitation. The intensity in ν3 and the ≈60 cm−1 splitting of this nominally degenerate vibration are indicative of pronounced breaking of the isolated molecules D3h symmetry by the local solvent environment. Intensity in the overtone of the out-of-plane mode (ν2) near 830 cm−1 suggests a change in the equilibrium geometry from planar to pyramidal upon electronic excitation. The absorption spectra and absolute Raman cross sections are simulated with a model that considers resonance with two orthogonally polarized electronic states whose degeneracy is broken by the locally asymmetric environment. Both solvent reorganization and geometry changes along the nitrate molecular vibrations make major contributions to the breadth of the absorption band. No differences between resonant and nonresonant linewidths are observed for the ν1 band.
Switched matrix accelerator
Authors: Whittum David H. , Tantawi Sami G.
Published: July 2002

We describe a new concept for a microwave circuit functioning as a charged-particle accelerator at mm wavelengths, permitting an accelerating gradient higher than conventional passive circuits can withstand consistent with cyclic fatigue. The device provides acceleration for multiple bunches in parallel channels, and permits a short exposure time for the conducting surface of the accelerating cavities. Our analysis includes scalings based on a smooth transmission line model and a complementary treatment with a coupled-cavity simulation. We also provide an electromagnetic design for the accelerating structure, arriving at rough dimensions for a seven-cell accelerator matched to standard waveguide and suitable for bench tests at low power in air at 91.392 GHz. A critical element in the concept is a fast mm-wave switch suitable for operation at high power, and we present the considerations for implementation in an H-plane tee. We discuss the use of diamond as the photoconductor switch medium.
On the use of a bias-light correction for trapping effects in photoconductance-based lifetime measurements of silicon
Authors: Macdonald Daniel , Sinton Ronald A. , Cuevas Andrés
Published: July 2002

The effectiveness of a method for analytically reducing the effect of trapping centers on photoconductance-based recombination lifetime measurements in silicon is examined. The correction method involves the use of a “bias-light” term to subtract out the underlying photoconductance due to the traps. The technique extends, by approximately an order of magnitude, the range of carrier densities over which reasonably accurate (within 30%) measurements of the recombination lifetime can be made. Guidelines for determining which bias-light intensity will produce the best correction for solar grade multicrystalline silicon wafers, and the range over which it is valid, are developed for several practical cases.
Small-angle neutron scattering studies of polybutadiene/polystyrene blends as a function of pressure and microstructure: Comparison of experiment and theory
Authors: Frielinghaus Henrich , Schwahn Dietmar , Dudowicz Jacek , Freed Karl F. , Foreman K. W.
Published: July 2002

Small-angle neutron scattering (SANS) experiments have been performed for three polybutadiene/polystyrene (dPB/PS) blends of differing dPB microstructure as a function of pressure and temperature. The experimental effective SANS interaction parameters are analyzed using the mean-field lattice cluster theory (LCT). In order to provide a meaningful comparison with the LCT, contributions from the non-mean-field long-range composition fluctuations are removed from the experimental data by use of a crossover function that describes the transition between near-critical and mean-field behaviors for the extrapolated zero-angle scattering. The theory provides a good description of the overall pressure dependence of the effective interaction parameter and its small dependence on the percentage of 1,2 addition units in the dPB chains.
Epitaxial LiNbO3 thin films grown by pulsed laser deposition for optical waveguides
Authors: Balestrino G. , Martellucci S. , Medaglia P. G. , Paoletti A. , Petrocelli G. , Tebano A. , Tucciarone A. , Gelli F. , Giorgetti E. , Sottini S. , Tapfer L.
Published: July 2002

We have grown good-quality epitaxial LiNbO3 thin films on (0001) sapphire substrates using a pulsed laser deposition technique. The growth conditions permitted us to deposit c-axis oriented films avoiding the problem of the LiNb3O8 Li-deficient phase. The chemical composition of thin films was investigated by secondary ion mass spectrometry and resulted to coincide with the target composition. Guided propagation was demonstrated at 632 and 488 nm with thin films (∼1200 Å).
Multigrid methods for classical molecular dynamics simulations of biomolecules
Authors: Sagui Celeste , Darden Thomas
Published: July 2002

We present an O(N) multigrid-based method for the efficient calculation of the long-range electrostatic forces needed for biomolecular simulations, that is suitable for implementation on massively parallel architectures. Along general lines, the method consists of: (i) a charge assignment scheme, which both interpolates and smoothly assigns the charges onto a grid; (ii) the solution of Poisson’s equation on the grid via multigrid methods; and (iii) the back interpolation of the forces and energy from the grid to the particle space. Careful approaches for the charge assignment and the force interpolation, and a Hermitian approximation of Poisson’s equation on the grid allow for the generation of the high-accuracy solutions required for high-quality molecular dynamics simulations. Parallel versions of the method scale linearly with the number of particles for a fixed number of processors, and with the number of processors, for a fixed number of particles.
V-shaped defects connected to inversion domains in AlGaN layers
Authors: Pécz B. , Makkai Zs. , di Forte-Poisson M. A. , Huet F. , Dunin-Borkowski R. E.
Published: July 2002

Thick AlGaN layers and AlGaN/GaN superlattices have been grown on GaN using metalorganic chemical vapor deposition. Cross-sectional transmission electron microscopy has been used to show that V-shaped surface pits on these samples differ from similar features observed in the InGaN system. Inversion domains and segregated Al are found in the middle of each V pit, and superlattice layers are observed to follow the pit sidewalls.
Molecular simulation of the transition from liquidlike to solidlike behavior in complex fluids confined to nanoscale gaps
Authors: Cui S. T. , Cummings P. T. , Cochran H. D.
Published: July 2002

We report molecular dynamics simulations at ambient temperature and pressure of dodecane films of thickness between three and eight molecular layers confined between mica surfaces. We use an accurate united-atom model for dodecane and an effective interaction between the dodecane and the confining mica surfaces that is consistent with the surface energy of a mica surface. At ambient normal pressure, the strong surface–fluid interaction leads to increased dodecane density as the wall spacing is narrowed, crossing into a density region corresponding to bulk solid when the confined film becomes narrower than six molecular layers. Correspondingly, we observed a dramatic transition from a liquidlike to an ordered, solidlike structure when the confined dodecane film is reduced from seven to six molecular layers, consistent with experimental observation of many orders of magnitude increase in viscosity at the same film thickness. The solidlike structure is characterized by the layering as well as the in-plane orientational order of the dodecane molecules. At an extreme confinement of three molecular layers, the solidlike confined film is able to sustain a nonzero shear stress. These results with realistic models provide an improved understanding of the solidlike behavior observed in surface force apparatus experiments.
Near-infrared electroluminescence of polymer light-emitting diodes doped with a lissamine-sensitized Nd3+ complex
Authors: Slooff L. H. , Polman A. , Cacialli F. , Friend R. H. , Hebbink G. A. , van Veggel F. C. J. M. , Reinhoudt D. N.
Published: July 2002

We report 890 nm luminescence from a neodymium-doped polymer light-emitting diode. The active layer is a blend of poly(dioctylfluorene-co-benzothiadiazole), F8BT, and a lissamine-functionalized terphenyl-based neodymium complex. We detect electroluminescence from both the lissamine (580 nm) and the Nd3+ complex (890 nm). By comparison with lissamine-free devices we show that the lissamine is crucial to infrared emission. The neodymium/lissamine luminescence intensity ratio is higher under electrical excitation than under optical excitation, showing that more triplets reach Nd3+ under electrical excitation. High turn-on voltages provide a clear indication for charge trapping onto the lissamine, and we consider direct triplet formation on the lissamine to be competing efficiently with respect to slower Dexter-type triplet transfer from the F8BT to the lissamine.
Fingerprints of the nodal structure of autoionizing vibrational wave functions in clusters: Interatomic Coulombic decay in Ne dimer
Authors: Moiseyev Nimrod , Santra Robin , Zobeley Jürgen , Cederbaum Lorenz S.
Published: July 2002

The removal of an inner-valence electron from neutral neon clusters leads to autoionization and subsequent fragmentation of the cationic clusters in accordance with the interatomic Coulombic decay mechanism discovered recently. Using non-Hermitian quantum scattering theory we investigate this process in detail for the Ne dimer. We show that a pronounced structure can be observed when measuring the autoionizing electron or the Ne+ kinetic energy distributions. This phenomenon is associated with the properties of the vibrational autoionizing resonance states of the electronically excited cationic dimer. By suppressing coherence among the different vibrational autoionizing resonances, or by selectively exciting one of them, the structures in the kinetic energy distributions become more pronounced. It is demonstrated that these structures reflect the nodal structure of the wave functions of the autoionizing vibrational states most populated by the initial ionization of the neutral neon dimer. In a coherent decay we encounter substantial interference effects, but the nodal pattern of the corresponding wave functions is still present. The kinetic energy distributions are generally very sensitive to details of the potential energy curves of clusters.
Superconducting properties of MgB2 bulk materials prepared by high-pressure sintering
Authors: Takano Y. , Takeya H. , Fujii H. , Kumakura H. , Hatano T. , Togano K. , Kito H. , Ihara H.
Published: July 2002

High-density bulk materials of a recently discovered 40 K intermetallic MgB2 superconductor were prepared by high-pressure sintering. Superconducting transition with the onset temperature of 39 K was confirmed by both magnetic and resistive measurements. Magnetization versus field (M–H) curve shows the behavior of a typical type-II superconductor and the lower critical field Hc1(0) estimated from the M–H curve is 0.032 T. The bulk sample shows good connection between grains and critical current density Jc estimated from the magnetization hysteresis using sample size was 5×104 A/cm2 at 20 K and 1 T. Upper critical field Hc2(0) determined by extrapolating the onset of resistive transition and assuming a dirty limit is 18 T.
A quadrature excitation coil for longitudinally detected electron spin resonance
Authors: Resmer Frank , Nicholson Ian , Hutchison James M. S.
Published: July 2002

Longitudinally detected electron spin resonance (LODESR) is an alternative method to conventional continuous wave (cw) ESR with transverse detection. LODESR has some practical advantages over cw-ESR for in vivo experiments carried out with excitation at radio frequencies. Sample losses, however, can be a problem and generally limit the LODESR signal to noise ratio for biological samples. We have developed a LODESR coil system with quadrature excitation circuit in order to decrease the sample power deposition. The coil system consists of a high pass birdcage excitation coil operated in quadrature (320 MHz) and a solenoidal coil (325 kHz) for signal detection. Methods of decoupling the excitation and detection circuits are described. Results reported here show the effectiveness of the LODESR coil system with quadrature excitation in reducing the sample power deposition by a factor of 2. The quadrature excitation coil assembly we have described here could be used, without further modification, for LODESR imaging. The coil assembly, including rf screen, could be placed inside the field gradient coils. LODESR imaging, in contrast to nuclear magnetic resonance imaging, uses continuous rather than pulsed gradient wave forms and the copper foil used for the rf screen would not significantly distort the gradients.
Characterization of electron beams produced by ultrashort (30 fs) laser pulses
Authors: Malka V. , Faure J. , Marquès J. R. , Amiranoff F. , Rousseau J. P. , Ranc S. , Chambaret J. P. , Najmudin Z. , Walton B. , Mora P. , Solodov A.
Published: July 2002

Detailed measurements of electron spectra and charges from the interaction of 10 Hz, 600 mJ laser pulses in the relativistic regime with a gas jet have been done over a wide range of intensities (1018–2×1019 W/cm2) and electron densities (1.5×1018–1.5×1020 cm−3), from the “classical laser wakefield regime” to the “self-modulated laser wakefield” regime. In the best case the maximum electron energy reaches 70 MeV. It increases at lower electron densities and higher laser intensities. A total charge of 8 nC was measured. The presented simulation results indicate that the electrons are accelerated mainly by relativistic plasma waves, and, to some extent, by direct laser acceleration.
Surface plasmon propagation in microscale metal stripes
Authors: Lamprecht B. , Krenn J. R. , Schider G. , Ditlbacher H. , Salerno M. , Felidj N. , Leitner A. , Aussenegg F. R. , Weeber J. C.
Published: July 2002

Addressing the fundamental question of miniaturization of light guiding and routing towards nanoscale optics, we study experimentally surface plasmon propagation in silver and gold thin films of finite widths in the micrometer range. Spatially confined excitation of surface plasmons is realized by a prism coupling arrangement involving an opaque aluminum screen for a distinct separation of excitation and propagation (measurement) region. The surface plasmon propagation length as a function of film widths is measured by detecting stray light due to surface plasmon scattering with a conventional optical microscope.
Competition of intrinsic and topographically imposed patterns in Bénard–Marangoni convection
Authors: Ismagilov Rustem F. , Rosmarin David , Gracias David H. , Stroock Abraham D. , Whitesides George M.
Published: July 2002

The structure of Bénard–Marangoni convection cells can be controlled by periodic topographic patterns on the heated surface that generates the convection. When the periodicity of the topographic pattern matches the intrinsic periodicity of the convection cells, a convective pattern is formed that is 1:1 commensurate with the topographic pattern. Arrays of hexagonal, square, and triangular convection cells were generated over the appropriately designed topographic patterns, and visualized by infrared imaging. For imposed patterns with periodicity in two dimensions, as the ratio of the intrinsic and perturbing length scales changes, the pattern of the convection cells shows sharp transitions between different patterns commensurate with the imposed pattern. For imposed patterns with periodicity in one dimension (i.e., lines) the convection cells use the unconstrained dimension to adapt continuously to the external perturbation. Topographically controlled convection cells can be used to assemble microscopic particles into externally switchable regular lattices.
Photodissociation of NeBr2(B) below and above the dissociation limit of Br2(B)
Authors: Roncero Octavio , Campos-Martı́nez José , Hernández Marta I. , Delgado-Barrio Gerardo , Villarreal Pablo , Rubayo-Soneira J.
Published: July 2002

The photodissociation dynamics of the NeBr2 complex in the B electronic state is studied, for the first time, near the Br2(B) dissociation limit, below and above, when the complex is promoted from the ground T-shaped level in the X electronic state. A time-dependent treatment is used in which the initial wave packet is divided in two portions, one describing the slow predissociation dynamics below the Br2 dissociation threshold, and the second one, the fast complete dissociation in Ne+Br+Br fragments. Below that threshold, the absorption spectrum shows an increasing congestion as the vibrational energy content of Br2 increases, but narrow peaks appear again for the highest energy region of the spectrum. These peaks correspond to long lived resonances associated with “horseshoe” type states, as demonstrated by two-dimensional calculations. These resonances have a significant probability density for the linear geometry in which the Ne atom is inserted between the two bromine atoms. At this configuration the exchange of vibrational energy is rather inefficient which explains both why the spectrum is so sparse and resonances are so narrow. Above the Br2 dissociation threshold, the recombination of Br2 is found to be very inefficient, except for very low kinetic energies. The small recombination probabilities are due to vibrational couplings and not to any collisional caging effect. Since the complex remains essentially T-shaped during dissociation, extensive two-dimensional calculations are performed for longer times to better determine final vibrational distributions at low kinetic energies.
Exact solvability of superintegrable systems
Authors: Tempesta Piergiulio , Turbiner Alexander V. , Winternitz Pavel
Published: July 2002

It is shown that all four superintegrable quantum systems on the Euclidean plane possess the same underlying hidden algebra sl(3). The gauge-rotated Hamiltonians, as well as their integrals of motion, once rewritten in appropriate coordinates, preserve a flag of polynomials. This flag corresponds to highest-weight finite-dimensional representations of the sl(3)-algebra, realized by first-order differential operators.
Polyelectrolyte adsorption on charged particles: Ionic concentration and particle size effects—A Monte Carlo approach
Authors: Chodanowski Pierre , Stoll Serge
Published: July 2002

The complexation between a charged polymer and an oppositely charged spherical particle is investigated using Monte Carlo simulations. Electrostatic interactions are described in the Debye–Hückel approximation. The influence of particle size and ionic concentration on the adsorption/desorption limit, interfacial structure of the adsorbed layer, amount of adsorbed polymer, and the overcharging issue is investigated. Attention is focused on polyelectrolyte adsorption on small spherical particles whose surface curvature effects are expected to limit the amount of adsorbed monomers, large particles that allow the polyelectrolyte to spread to the same extent as on a flat surface, and particles whose radius is close to the polyelectrolyte radius of gyration so that the chain can completely wrap around it. The formation of a polyelectrolyte/particle complex and the conformations of the adsorbed polyelectrolyte are found to result from two competing effects: the electrostatic repulsions between the chain monomers which force the polyelectrolyte to adopt extended conformations and limit the number of monomers which may be attached in particular to small particles, and the electrostatic attractive interactions between the particle and the monomers forcing the charged polymer to undergo structural transition and collapse at the particle surface. It is shown that adsorption is favored by increasing particle size and decreasing ionic concentration. Trains are favored at low ionic concentrations while loops (prior desorption) are favored more when increasing the ionic strength. Below a critical particle size, by decreasing the ionic strength, electrostatic repulsions between the adsorbed monomers force the polyelectrolyte to form protuding tails in solution, hence decreasing the amount of polyelectrolyte adsorption. By decreasing the particle size still further, the low ionic concentration regime is dominated by monomer–monomer repulsions; the polymer partially wraps around or becomes tangential to the particle and two tails extend in opposite directions. The complex may or may not exhibit charge inversion depending on the particle size and ionic concentration. We find that charge reversal increases with salt concentration and reaches a maximum when the polyelectrolyte is able to wrap around the particle completely.
Formation of d-wave superconducting order in a randomly doped lattice
Authors: Loktev V. M. , Pogorelov Yu. G.
Published: July 2002

We consider the interplay between superconducting coupling and dopant impurity scattering of charge carriers in planar square lattice systems and examine the physical conditions (doping level, temperature, local symmetry of coupling and scattering potentials) necessary in this model system to obtain a d-wave superconducting order, like that observed in real doped cuprate HTSC materials. Using the Lifshitz model for the disorder introduced into the system by dopants, we analyze also the nonuniform structure of such a d-wave parameter, including both its magnitude and phase variation. The results indicate that d-wave superconductivity becomes possible in a doped metal until it is destroyed at too high doping levels.
A statistical mechanics/density functional approach to the thermodynamics of liquids
Authors: Canepa Carlo
Published: July 2002

This study describes an accurate partition function obtained evaluating the configurational integral of molecular solutes with a computationally feasible method. The key features of this method include the partition function’s factors for translation (calculated according to the approach of the Lennard-Jones–Devonshire theory of liquids) and libration. The proposed molecular partition function has been successfully tested computing entropies of vaporization and molar heat capacities at constant volume for the four common liquids dichloromethane, acetonitrile, methanol, and water. The formulation of pressure-dependent properties is also given through a virial equation, and the evaluation of its simplest term allows the estimate of enthalpies of vaporization. Structural parameters needed by the theory, such as binding energies of particles to the bulk, have been obtained through density functional theory calculations on monomers, dimers, and clusters of the four test liquids.