bluuebunny/crossref_metadata_2025 huggingface dataset:
Hot-electron degradation in hydrogenated amorphous-silicon-nitride thin-film diodes
Authors: Oversluizen G. , Zieren V. , Johnson M. T. , van der Put A. A. , Lodders W. H. M.
Published: July 2002

Two series of thin-film diodes (TFDs), used as switching elements in active-matrix liquid-crystal displays, have been prepared with various amorphous-silicon-nitride (a-SiNxHy) thicknesses. In a first series, with thin top metal contacts, it was observed by photon-emission spectroscopic analysis that both the effective electron temperature and the number of hot electrons increased as the nitride thickness (at constant field) or the electric field across the TFD (at constant nitride thickness) increased. A further analysis revealed that for the thicker samples, the electrons became progressively hotter on moving from the cathode to the anode. In a second series, the drift of the TFD current–voltage characteristic under dc-driving conditions has been monitored as a function of nitride thickness at various fields. The anodic drift, resulting from defect-state creation in the anodic sample region, and the field and thickness dependence of the hot-electron intensity, show very similar trends. Therefore, it is concluded that the anodic drift is driven by hot-electron-induced defect-state creation. Also, it was found that optical phonon scattering effectively limits the electron temperature up to a field of around 1.5 MV/cm, while for larger field strengths impact ionization appears a probable additional energy-loss mechanism. For the anodic drift in a TFD with an a-SiNxHy layer thickness of 104 nm, a comparable critical-field strength of around 1.5 MV/cm was determined, below which an efficient electron–phonon scattering mechanism strongly limits the production of hot electrons. For a decreasing amorphous-layer thickness, the critical-field strength increases. As a result, an improved lifetime of a-SiNxHy TFDs in their application has been obtained using a small amorphous-layer thickness.
Energy distributions of electrons emitted from GaAs(Cs, O)
Authors: Orlov D. A. , Hoppe M. , Weigel U. , Schwalm D. , Terekhov A. S. , Wolf A.
Published: July 2002

A method to map out the energy distribution N(E∥,E⊥) of an electron beam as a function of the longitudinal (E∥) and transverse (E⊥) energy has been developed and applied to study the photoemission process from GaAs(Cs, O) at 90 K. The method proceeds by “marking” electrons with fixed longitudinal energy E∥b and a subsequent measurement of the associated differential transverse energy distribution N⊥(E∥b,E⊥), applying an adiabatic magnetic compression technique. The complete energy distribution N(E∥,E⊥) of electrons from a GaAs(Cs, O) photocathode obtained by a stepwise variation of E∥b provides details about the transfer of electrons through the GaAs(Cs, O)–vacuum interface and demonstrates that not only electron energy loss, but also elastic electron scattering is of crucial importance in the escape process.
Magnetization reversal and coercivity of magnetic-force microscopy tips
Authors: Carl A. , Lohau J. , Kirsch S. , Wassermann E. F.
Published: July 2002

An experimental technique is presented that allows determining the magnetization reversal and coercivity of magnetic-force microscopy (MFM) tips. An Ω-shaped current carrying gold ring with a radius of 2400 nm fabricated by electron-beam lithography and a lift-off technique is charged with a constant current in order to produce a magnetic stray field in the z direction that is detected by MFM. While an oscillating MFM tip is continuously raster scanned across the center of the current ring, an external magnetic field is applied in the z direction and increased in magnitude in order to reverse the tip magnetization during imaging. Thus, the corresponding changes in the measured image contrast exclusively describe the magnetization reversal and coercivity of the particular part of the tip that is used for imaging. We have investigated commercially available thin-film tips and we find that the hysteresis loops measured with MFM may be significantly different as compared to hysteresis loops measured by means of superconducting quantum interference magnetometry on the respective magnetic tip coatings of the same tip.
Recrystallization in polyvinylidene fluoride upon low fluence swift heavy ion impact
Authors: Biswas A. , Gupta R. , Kumar N. , Avasthi D. K. , Singh J. P. , Lotha S. , Fink D. , Paul S. N. , Bose S. K.
Published: July 2002

Thin films (9 μm) of polyvinylidene fluoride (PVDF) are irradiated by swift heavy ions (180 MeV Ag14+) in the fluence range 1×1010–1×1012ions/cm2 with an electronic linear energy transfer LET∼11 keV/nm. In sharp contrast to the previous results, the most characteristic crystalline asymmetric and symmetric “CH2” doublets (located at 3025 and 2985 cm−1), have shown remarkable increase in their respective Fourier transform infrared (FTIR) absorbance intensities upon low fluence ion impact (1010 ions/cm2). This increase in absorbance is in consonance with the simultaneous decrease of the transmission intensities of other crystalline bending vibration bands located at 532 (CF2 bending), 614, 796, and 975 cm−1 (all due to CH2 bending) at the similar ion fluence. It appears most probable from the results that, being a polar polymer, the molecular dipoles in PVDF forming a hydrogen bond network get realigned upon irradiation into a highly ordered state of chain molecules in the crystalline regions and create volume elements as crystallites.
Strings in AdS3 and the SL(2,R) WZW model. I: The spectrum
Authors: Maldacena Juan , Ooguri Hirosi
Published: July 2002

In this paper we study the spectrum of bosonic string theory on AdS3. We study classical solutions of the SL(2,R) WZW model, including solutions for long strings with nonzero winding number. We show that the model has a symmetry relating string configurations with different winding numbers. We then study the Hilbert space of the WZW model, including all states related by the above symmetry. This leads to a precise description of long strings. We prove a no-ghost theorem for all the representations that are involved and discuss the scattering of the long string.
On the photoelectron spectrum of p-benzoquinone
Authors: Stanton John F. , Sattelmeyer Kurt W. , Gauss Jürgen , Allan Michael , Skalicky Tomas , Bally Thomas
Published: July 2002

A high-resolution photoelectron spectrum of p-benzoquinone in the low energy (9.5–11.5 eV) region is reported and analyzed with the aid of simulations based on high-level ab initio calculations. The results generally support the notion that the two prominent spectral features in this region are each due to a pair of final ion states. The lower energy feature beginning near 10 eV is due to oxygen lone-pair ionizations, while that beginning near 11 eV comes from π electron removal. Contrary to previous interpretations of the spectrum, however, the results of this study indicate that the two π states are nearly degenerate, with the strongest peak in the photoelectron spectrum representing a convolution of the corresponding pair of 0–0 ionizations.
All mode dynamics at the conical intersection of an octa-atomic molecule: Multi-configuration time-dependent Hartree (MCTDH) investigation on the butatriene cation
Authors: Cattarius Chr. , Worth G. A. , Meyer H.-D. , Cederbaum L. S.
Published: July 2002

The ground state and the first excited electronic states of the octa-atomic butatriene cation (C4H4+) exhibit a multidimensional conical intersection. This intersection is energetically low lying and is located in the vicinity of the Franck–Condon region of the neutral molecule’s ground state. The conical intersection thus dominates the nuclear dynamics in ionization processes of this molecule. This is a particularly interesting example of vibronic coupling, introducing what appears to be a new, structured band into the energy spectrum. In this work, the potential energy surfaces and their intersection are investigated by ab initio methods. A diabatic model Hamiltonian including all possible linear, quadratic and bilinear vibronic coupling terms is introduced, with coupling constants determined by a fit to the ab initio data. The nuclear dynamics of all 18 vibrational modes is then evaluated by propagating the wave packet using the multi-configuration time-dependent Hartree method. Finally, the photoelectron spectrum of butatriene is computed and compared with the experimental one.
The generic van der Waals equation of state and self-diffusion coefficients of liquids
Authors: Rah Kyunil , Eu Byung Chan
Published: July 2002

In this paper we use the generic van der Waals equation of state to define the free volume of liquids along the liquid–vapor coexistence line (liquids curve) in the case of liquid argon and along three isotherms in the high-pressure regime in the case of liquid methane. With the free volume computed from the cavity function obtained by means of a Monte Carlo simulation method, we have calculated the self-diffusion coefficients of liquid argon and liquid methane. The Cohen–Turnbull free volume theory is used to calculate them. With the empirical parameter appearing in the Cohen–Turnbull theory suitably adjusted, the theoretical and experimental values of the self-diffusion coefficients agree very well with regard to the density and temperature dependence for the cases of available data compared. A pair of analytic formulas for density dependence of the self-diffusion coefficient is obtained by using the approximate cavity functions for hard spheres and tested against the experimental data on methane. A comparison of the analytic formulas with experiment is also very good.
Dielectric relaxation of hydrogen bonded liquids: Mixtures of monohydric alcohols with n-alkanes
Authors: Schwerdtfeger Stefan , Köhler Frank , Pottel Reinhard , Kaatze Udo
Published: July 2002

The complex dielectric spectra for mixtures of unbranched monohydric alcohols with n-alkanes have been measured between 1 MHz and 18 GHz at various mole fractions xCnOH of the alcohols (0.2⩽xCnOH⩽1). Within the frequency range of measurements the spectra exhibit two dispersion/loss regions which can be analytically well represented by a Davidson–Cole-type and a Debye-type relaxation term, respectively. The relaxation parameters are discussed in terms of a wait-and-switch model of relaxation, considering also the effect of the nonpolar hydrocarbon groups in the enthalpy of activation of the dielectric relaxation process. This model allows for a consistent description of the dielectric properties of the alcohol/n-alkane mixtures which, depending on the relative length of the hydrocarbon chains, divide into two groups. Structural features of the alcohol/n-alkane mixtures are proposed to account for the characteristic behavior of both groups of liquids as a function of alcohol content.
A role for electron viscosity in plasma shock heating
Authors: Velikovich A. L. , Whitney K. G. , Thornhill J. W.
Published: July 2002

The contribution of electron viscosity to shock heating of high-Z plasmas is investigated. A classical shock profile calculation is employed to show that the electron viscosity substantially affects the structure of strong shock waves in a fully ionized plasma by reducing the overheating of the ions in the viscous subshock. This effect is present for an average ion charge Z above 10. Computer simulations of Z-pinch implosions are then employed to demonstrate that this effect of electron viscosity can also be accounted for in Lagrangian hydrodynamic simulations that use artificial viscosity. This is accomplished by dividing the corresponding viscous heat release between the ion and electron components of the plasma according to the ratio of their physical viscosities. When this is done in the time-varying, ionizing plasma environment of Z-pinch implosions that are theoretically designed to generate K-shell emission in titanium and krypton, it is found that electron viscous heating can be substantial, leading to sizable reductions in ion temperatures and 10%–20% increases in K-shell emission under certain implosion conditions.
Origin of field-dependent optical transmission of magnetic fluid films
Authors: Yang S. Y. , Chiu Y. P. , Jeang B. Y. , Horng H. E. , Hong Chin-Yih , Yang H. C.
Published: July 2002

The physical mechanism of the optical transmission of magnetic fluid films under perpendicular magnetic fields is investigated in this work. Under perpendicular magnetic fields, originally dispersed magnetic particles agglomerate to form magnetic columns. The liquid phase is transparent, whereas the columns are opaque. Hence, the liquid phase dominates the optical transmission of the magnetic fluid film. When the field strength is raised, more columns are formed, and the area of the liquid phase is reduced. This leads to the decrease in the optical transmission of the film under higher field strength. The variation in the concentration of the liquid phase under various field strengths also contributes to the transmission of the film. By taking account of the variations of the effective concentration and the area ratio of the liquid phase in the magnetic fluid film under magnetic fields, the resultant magnetic field dependence of the optical transmission was calculated and found to be consistent with the experimental results. This provides evidence for the origin of the field-dependent optical transmission of the magnetic fluid film under external fields.
Postpulse alignment of molecules robust to thermal averaging
Authors: Machholm Mette
Published: July 2002

Alignment of molecules under field free conditions with negligible vibrational or electronic excitation is created by a short off-resonant low frequency laser pulse. Typically the global maximum in postpulse alignment occurs at a rotational wave packet revival close to half a rotational period after the short pulse. The alignment effect is robust to thermal averaging at the revivals, but averaging cancels the alignment in between. The permanent dipole–field interaction can be efficient for alignment with off-resonant frequencies between the rotational and the vibrational frequencies of the molecule.
Optical spectroscopy of GaN microcavities with thicknesses controlled using a plasma etchback
Authors: Martin R. W. , Edwards P. R. , Kim H.-S. , Kim K.-S. , Kim T. , Watson I. M. , Dawson M. D. , Cho Y. , Sands T. , Cheung N. W.
Published: July 2002

The effect of an etch-back step to control the cavity length within GaN-based microcavities formed between two dielectric Bragg mirrors was investigated using photoluminescence and reflectivity. The structures are fabricated using a combination of a laser lift-off technique to separate epitaxial III-N layers from their sapphire substrates and electron-beam evaporation to deposit silica/zirconia multilayer mirrors. The photoluminescence measurements reveal cavity modes from both etched and nonetched microcavities. Similar cavity finesses are measured for 2.0 and 0.8 μm GaN cavities fabricated from the same wafer, indicating that the etchback has had little effect on the microcavity quality. For InGaN quantum well samples the etchback is shown to allow controllable reduction of the cavity length. Two etch steps of 100 nm are demonstrated with an accuracy of approximately 5%. The etchback, achieved using inductively coupled plasma and wet chemical etching, allows removal of the low-quality GaN nucleation layer, control of the cavity length, and modification of the surface resulting from lift-off.
Epitaxy and magnetotransport properties of the diluted magnetic semiconductor p-Be(1−x)MnxTe
Authors: Hansen L. , Ferrand D. , Richter G. , Thierley M. , Hock V. , Schwarz N. , Reuscher G. , Schmidt G. , Molenkamp L. W. , Waag A.
Published: July 2002

We report on the molecular-beam epitaxial growth and magnetotransport properties of p-type BeMnTe, a ferromagnetic diluted magnetic semiconductor. BeMnTe thin-film structures can be grown almost lattice matched to GaAs for Mn concentrations up to 10%. A high p-type doping with nitrogen can be achieved by using a rf plasma source. BeMnTe and BeTe layers have been characterized by magnetotransport measurements. At low temperatures, the BeMnTe samples exhibit a large anomalous Hall effect. A hysteresis in the anomalous Hall effect appears below 2.5 K in the most heavily doped sample, which indicates the occurrence of a ferromagnetic phase.
Control of a global motion on field-reversed configuration
Authors: Fujimoto K. , Hoshikawa A. , Ohmura S. , Takahashi T. , Nogi Y. , Ohkuma Y.
Published: July 2002

An n=1 mode global motion on a field-reversed configuration (FRC) plasma is observed by means of a newly developed optical system. The deviation of the FRC from the coil axis reaches 20%–40% of the plasma radius. In order to push back the FRC to the equilibrium position, a multipole field (quadrupole or hexapole field) is applied. The n=1 motion can be easily controlled by the quadrupole field, the critical field strength of which is required to be about 15% of the confinement field. It is found that the n=2 rotational instability can also be stabilized by strength of the same order. The critical strength for the n=1 motion is theoretically obtained from a model such that the driving energy of the motion given at the formation phase balances with the work done by the multipole field. The theoretical estimation agrees within a factor of 2 with the experimental results.
Optical properties of gamma irradiated visible strained quantum well lasers
Authors: Hava S.
Published: July 2002

We have investigated the effects of gamma-ray irradiation on the optical properties of visible strained multi-quantum-well lasers. It was observed that the power emission from the laser fell progressively and the wavelength linewidth increased with increase of irradiation. The laser emission properties recovered as the duration of the subsequent dc current increased. The far-field beam emission divergence was reduced after irradiation. We propose that these irradiation effects may be caused by structural changes in the strained thin layers, due to an atomic displacement and migration between layers and due to changes in the degree of strain. This affects the layer’s atomic composition, which changes the band-gaps’ energy levels and the indexes of refraction, and as a result, the carrier and optical confinements, respectively.
Characterization of the electronic structure of crystalline compounds through their localized Wannier functions
Authors: Zicovich-Wilson Claudio M. , Bert Alberto , Roetti Carla , Dovesi Roberto , Saunders Victor R.
Published: July 2002

The crystalline orbitals of seven oxygen containing compounds with increasing degree of covalent character (MgO, MnO, ZnO, Al2O3, SiO2, AlPO4, and CaSO4) are localized according to a Wannier–Boys mixed scheme recently implemented. The resulting Wannier functions are analyzed in terms of various indices (centroids positions, second-order central moment tensor, its eigenvalues and principal axes, Mulliken population analysis, and atomic localization indices). Systematic trends are observed along the series.
Direct measurement of piezoelectric field in In0.23Ga0.77N/GaN multiple quantum wells by electrotransmission spectroscopy
Authors: Lai C. Y. , Hsu T. M. , Chang W.-H. , Tseng K.-U. , Lee C.-M. , Chuo C.-C. , Chyi J.-I.
Published: July 2002

In this communication, we present experimental evidence of the piezoelectric-field-induced quantum-confined Stark effect on In0.23Ga0.77N/GaN multiple quantum wells. The optical transitions in In0.23Ga0.77N/GaN p-i-n multiple quantum wells were studied by using electrotransmission (ET) at room temperature. Quantum-well-related signals are well resolved in our ET spectra. Since the strong internal electric field breaks the symmetry of the quantum wells, both the allowed and the forbidden transitions are observed. Clear energy blueshifts in accordance with increasing reversed bias are observed in ET spectra. The strength of piezoelectric field is found to be 1.7–1.9 MV/cm in the In0.23Ga0.77N strain quantum well layer, which is comparable with the measurement reported in the literature. We have shown experimentally how the piezoelectric field affects the energy shift for the strained multiple quantum wells.
Transient and steady state photorefractive responses in dye-doped nematic liquid crystal cells
Authors: Klysubun Prapong , Indebetouw Guy
Published: July 2002

The photorefractive response of several dye-doped nematic liquid crystal samples is studied experimentally. Particular attention is given to the observation and modeling of the multiexponential dynamics of the photorefractive grating, and of the photocurrent. Both involve at least two different mechanisms with largely different time constants. These phenomena occur on time scales on the order of seconds, for applied voltages about 1 V, and light intensities as low as milliwatts/cm2.
Electronic transitions with quantum trajectories. II
Authors: Lopreore Courtney L. , Wyatt Robert E.
Published: July 2002

The quantum trajectory method (QTM) is applied to nonadiabatic electronic transitions. Equations of motion in a Lagrangian framework are derived for the probability density, velocity, position, and action functions for a discretized wave packet moving along coupled potential energy surfaces. In solving these equations of motion, we obtain agreement with exact quantum results computed by solving the time-dependent Schrödinger equation on a space-fixed grid. On each of the coupled potential energy surfaces, the dynamics of the trajectories is fully quantum mechanical, i.e., there are no “surface–hopping transitions.” We present a detailed analysis of the QTM results including density changes, complex oscillations of the wave functions, phase space analysis, and a detailed discussion of the forces that contribute to the evolution the trajectories.
Stripping dynamics in the reactions of electronically excited carbon atoms, C(1D), with ethylene and propylene—production of propargyl and methylpropargyl radicals
Authors: Kaiser R. I. , Nguyen T. L. , Mebel A. M. , Lee Y. T.
Published: July 2002

The reactions of electronically excited carbon atoms, C(1D), with ethylene and propylene were studied at three collision energies between 48 and 104 kJmol−1 employing the crossed molecular beam technique. Forward-convolution fitting of our data combined with electronic structure calculations suggests that the reactions proceed via stripping dynamics. Extremely short-lived allene and 1,2-butadiene intermediates decompose via atomic hydrogen emission to yield propargyl and methylpropargyl radicals, respectively. These production routes are of potential importance to form benzene, toluene, and o-/p-xylenes in circumstellar envelopes of carbon stars and combustion flames.
Terahertz imaging of silicon wafers
Authors: Herrmann Michael , Tani Masahiko , Sakai Kiyomi , Fukasawa Ryoichi
Published: July 2002

Silicon samples with and without implanted layers have been imaged with a standard time-domain terahertz (THz) imaging system. The carrier concentration and mobility of the substrate have been extracted from the frequency dependence of the THz transmittance using a simple model based on the Drude approximation. The carrier concentration of implanted layers could be determined simply from the relative amplitude of the main THz pluse with a spatial resolution of ≈1 mm. Both substrates and thin layers of a semiconductor were characterized with the same THz system.
Blue light-emitting devices from new conjugated poly(N-substituted-2,7-carbazole) derivatives
Authors: Morin Jean-François , Beaupré Serge , Leclerc Mario , Lévesque Isabelle , D’Iorio Marie
Published: July 2002

Light-emitting diodes derived from a new class of conjugated polymers, well-defined poly(N-substituted-2,7-carbazole) derivatives, are reported. Excimer-free electroluminescence in the blue range (424–432 nm) was observed. Good luminance (372 cd/m2 at 10 V) was reached in a device containing poly[N-(2-ethylhexyl)-2,7-carbazole] as the emitting material with indium tin oxide and Al as the electrodes. This high luminance value was achieved by adding ultrathin LiF layers next to the electrodes, and by using hole and electron transport molecules such as N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine and 2-(4-biphenylyl)-5(4-tert-butylphenyl)-1,3,4-oxadiazole.
Beam stability at light sources (invited)
Authors: Hettel R. O.
Published: July 2002

The level of photon beam stability required in an experiment to meet measurement resolution goals depends on the experiment-specific photon phase space acceptance and the sensitivity of measurement to electron beam motion within its six-dimensional phase space. Stability time scales range from hours to microseconds, depending on experiment sampling rate, data integration period, and scan duration. Meeting these requirements at third-generation storage ring light sources can imply micron and submicroradian electron orbit stability and energy (ΔE/E) oscillation amplitudes in the part-per-million range, presenting a formidable challenge for accelerator designers. Beam stability criteria, measures taken to meet them, and achievable levels of stability in present and future light sources, are reviewed.
Photonic-crystal ultrashort bends with improved transmission and low reflection at 1.55 μm
Authors: Talneau A. , Le Gouezigou L. , Bouadma N. , Kafesaki M. , Soukoulis C. M. , Agio M.
Published: July 2002

We propose and demonstrate improved designs in order to increase the transmission level through double 60°-bend photonic-crystal-based waveguides. These bends are defined in a two-dimensional photonic crystal (PC) patterned into a GaInAsP slab on InP substrate. Transmission spectra calculated using a two-dimensional finite difference time domain method that accounts for radiation losses, as well as measurements, demonstrate enhancement of the transmission when moving holes in the corner. As expected a drop in reflection is obtained. The sensitivity to the PC guide width is also evidenced.
Ideal magnetohydrodynamic ballooning stability boundaries in three-dimensional equilibria
Authors: Hegna C. C. , Hudson S. R.
Published: July 2002

The impact of three-dimensional geometry on ideal magnetohydrodynamic ballooning mode stability is studied. By using a class of “local 3D equilibria” [C. C. Hegna, Phys. Plasmas 7, 3921 (2000)], the effects of plasma shaping, profile variations and symmetry on local plasma physics properties can be addressed. As an example, a local helical axis equilibrium case is constructed that models the magnetic field spectrum of a quasihelically symmetric stellarator. In this case, the magnetic harmonic structure of the local shear (which can be manipulated via changes in the magnetic geometry) has an important impact on the stability boundaries and eigenvalue properties of three-dimensional equilibria. The presence of symmetry breaking components in the local shear produces localized field-line-dependent ballooning instabilities in regions of small average shear. These effects lower first ballooning stability thresholds and can eliminate the second stability regime. A geometric interpretation of these results is given.
One-phonon Raman scattering studies of chains of crystalline-Si nanospheres
Authors: Kohno H. , Iwasaki T. , Mita Y. , Takeda S.
Published: July 2002

Chains of crystalline-Si nanospheres were studied by means of Raman scattering spectroscopy. We found that the one-phonon Raman scattering peak from the chains was asymmetric and broader than that from bulk Si. This phenomenon can be attributed to a phonon confinement in the silicon nanospheres. The phonon confinement became more obvious by decreasing the size of the silicon nanospheres in the chains. We also found that the Si nanospheres in the chains were under compressive stress by the covering oxide layers through the analysis of the Raman shift.
Control of the tilted orientation of CoCrPt/Ti thin film media by collimated sputtering
Authors: Zheng Y. F. , Wang J. P. , Ng V.
Published: July 2002

In this work, we propose a new method to fabricate tilted magnetic thin film media which may fully use the writing head field for magnetic recording. We deposit a Ti underlayer on glass substrate using normal dc magnetron sputtering. dc magnetron sputtering with a tilted collimator is proposed to deposit the CoCrPt magnetic layer on the Ti underlayer. The tilted collimator confined the target materials approaching the substrate at oblique angles. The tilted columnar structure of the CoCrPt layer was induced by this kind of collimated sputtering. The maximum value of coercivity tilts from the normal plane when the thickness of the magnetic layer increases from 20 to 40 nm. The rocking curve results show the tilted crystallographic orientation of the CoCrPt film. With the combined effect of the tilted columnar structure and tilted easy axis, a tilted magnetic thin film was fabricated which is a potential candidate for ultrahigh density magnetic recording. A model is proposed to explain the relationship between the orientation and the film growth via the collimator.
Comparison of arsenic and phosphorus diffusion behavior in silicon–germanium alloys
Authors: Eguchi S. , Hoyt J. L. , Leitz C. W. , Fitzgerald E. A.
Published: July 2002

The diffusion behavior of ion-implanted arsenic and phosphorus in relaxed Si0.8Ge0.2 has been investigated. Under equilibrium, extrinsic conditions, both dopants are observed to diffuse faster in SiGe than in Si. Simulations of the measured profiles suggest that the ratio of the effective diffusivity in Si0.8Ge0.2 compared to that in Si is roughly seven for arsenic, and roughly two for phosphorus. Under transient diffusion conditions, the arsenic diffusivity in SiGe is retarded, and the magnitude of the diffusion is roughly the same as that in Si. This result suggests that it is possible to optimize the diffusion conditions to achieve n+ source/drain junctions that are as shallow in SiGe as in Si.
Revival structures in picosecond laser-induced alignment of I2 molecules. II. Numerical modeling
Authors: Rosca-Pruna F. , Vrakking M. J. J.
Published: July 2002

We report quantum-mechanical calculations on the formation of aligned rotational wave packets in the ground state of I2, following the excitation of a rotationally cold sample of molecules with an intense picosecond laser pulse. Using these calculations, we have studied (1) the dynamic alignment during or shortly after the intense pump pulse, (2) the alignment at characteristic revival times following the laser excitation, and (3) the alignment between the revivals, as a function of the laser-pulse duration and peak intensity and the initial rotational temperature of the molecular beam. We conclude that the alignment at a revival is maximum for intermediate pulse durations (∼3 ps for I2) that are long enough to get efficient population transfer out of the initially populated state, yet short enough for the interaction to remain partially diabatic and partially adiabatic. The alignment at the revivals increases with laser intensity, although the time-dependent structure of the revivals becomes increasingly complicated as the laser intensity is increased, and the maximum degree of alignment that is obtained is eventually limited by the initial rotational temperature of the molecular beam. The findings in this paper qualitatively agree with experimental results on short-pulse laser-induced alignment that were presented in our experimental article (preceding paper).
A study of the semiclassical initial value representation at short times
Authors: Ankerhold Joachim , Saltzer Markus , Pollak Eli
Published: July 2002

The short time dynamics of the semiclassical initial value separation are studied analytically for a one dimensional system. We find that at short times the approximation introduces spurious errors that depend on ℏ and result from the anharmonic part of the potential. This is in contrast to classical mechanics which gives the first three initial time derivatives of a coordinate dependent operator exactly. Consideration of a model system shows, though, that the error introduced is not very large and that for times which are longer than a typical period of classical motion, semiclassical initial value representation propagation is superior to classical time propagation.
Indirect-drive noncryogenic double-shell ignition targets for the National Ignition Facility: Design and analysis
Authors: Amendt Peter , Colvin J. D. , Tipton R. E. , Hinkel D. E. , Edwards M. J. , Landen O. L. , Ramshaw J. D. , Suter L. J. , Varnum W. S. , Watt R. G.
Published: July 2002

Analysis and design of indirect-drive National Ignition Facility double-shell targets with hohlraum temperatures of 200 eV and 250 eV are presented. The analysis of these targets includes the assessment of two-dimensional radiation asymmetry and nonlinear mix. Two-dimensional integrated hohlraum simulations indicate that the x-ray illumination can be adjusted to provide adequate symmetry control in hohlraums specially designed to have high laser-coupling efficiency [Suter et al., Phys. Plasmas 7, 2092 (2000)]. These simulations also reveal the need to diagnose and control localized 10–15 keV x-ray emission from the high-Z hohlraum wall because of strong absorption by the high-Z inner shell. Preliminary estimates of the degree of laser backscatter from an assortment of laser–plasma interactions suggest comparatively benign hohlraum conditions. The application of a variety of nonlinear mix models and phenomenological tools, including buoyancy-drag models, multimode simulations and fall-line optimization, indicates a possibility of achieving ignition, i.e., fusion yields greater than 1 MJ. Planned experiments on the Omega laser will test current understanding of high-energy radiation flux asymmetry and mix-induced yield degradation in double-shell targets.
Monte Carlo simulation of proteins through a random walk in energy space
Authors: Rathore Nitin , de Pablo Juan J.
Published: July 2002

A Monte Carlo algorithm that performs a random walk in energy space has been used to study random coil–helix and random coil–beta sheet transitions in model proteins. This method permits estimation of the density of states of a protein via a random walk on the energy surface, thereby allowing the system to escape from local free-energy minima with relative ease. A cubic lattice model and a knowledge based force field are employed for these simulations. It is shown that, for a given amino acid sequence, the method is able to fold long polypeptides reproducibly. Its results compare favorably with those of annealing and parallel tempering simulations, which have been used before in the same context. This method is used to examine the effect of amino acid sequence and chain length on the folding of several designer polypeptides.
Using two-photon standing waves and patterned photobleaching to measure diffusion from nanometers to microns in biological systems
Authors: Davis Sara K. , Bardeen Christopher J.
Published: July 2002

A method of measuring molecular diffusion rates in microscopic sample volumes is described. This method utilizes the standing wave interference created by colliding two counterpropagating laser beams at the focus of two opposing microscope objectives, creating a periodic light distribution in a volume on the order of 1 fl. By using a Pockels cell to vary the laser intensity with a time resolution of milliseconds, we show how this experimental geometry can be used to perform ultrahigh resolution fluorescence recovery after patterned photobleaching (FRAPP) experiments. A mathematical treatment of the experiment shows that the laser excitation profile has two characteristic length scales, the width of the focal spot and the period of the standing wave, which permits the simultaneous measurement of dynamics on two separate length scales. This feature may be used to determine whether the measured diffusion is anomalous. We present experimental results using a femtosecond Ti:sapphire laser to create a two-photon excitation profile with a fringe visibility on the order of 100. This standing wave is used to demonstrate FRAPP in both model dye/polymer systems and in more complex systems like living cells stained with a fluorescent dye. By combining the advantages of standing wave microscopy and two-photon fluorescence recovery after photobleaching, this technique permits the measurement of very short length motions in localized sample volumes, which should be useful in both biology and the study of diffusion in microscopically heterogeneous systems.
Lévy flights: Exact results and asymptotics beyond all orders
Authors: Garoni T. M. , Frankel N. E.
Published: July 2002

A comprehensive study of the symmetric Lévy stable probability density function is presented. This is performed for orders both less than 2, and greater than 2. The latter class of functions are traditionally neglected because of a failure to satisfy non-negativity. The complete asymptotic expansions of the symmetric Lévy stable densities of order greater than 2 are constructed, and shown to exhibit intricate series of transcendentally small terms—asymptotics beyond all orders. It is demonstrated that the symmetric Lévy stable densities of any arbitrary rational order can be written in terms of generalized hypergeometric functions, and a number of new special cases are given representations in terms of special functions. A link is shown between the symmetric Lévy stable density of order 4, and Pearcey’s integral, which is used widely in problems of optical diffraction and wave propagation. This suggests the existence of applications for the symmetric Lévy stable densities of order greater than 2, despite their failure to define a probability density function.
Resonances in the O(3P)+HCl reaction due to van der Waals minima
Authors: Xie Tiao , Wang Dunyou , Bowman Joel M. , Manolopoulos David E.
Published: July 2002

We present extensive exact quantum calculations of the cumulative reaction probability (CRP) for the O(3P)+HCl→OH+Cl reaction for a large range of total angular momentum, using the most recent ab initio potential energy surface of Ramachandran et al. This surface contains van der Waals minima in both the entrance and exit channels that are shown to be responsible for a number of prominent resonances in the CRP in the tunneling region. The evidence for this claim is based on an analysis of the quasibound states of the van der Waals minima and a simple overlap of these states with the region of the saddle point of the reaction. The shift of the CRP with total angular momentum is analyzed in detail, with a focus on the resonances. A rigorous test of the simple J–K shifting approximation is also made both for the total CRP and the thermal rate constant.
Chemical composition dependences of the acoustical physical constants of LiNbO3 and LiTaO3 single crystals
Authors: Kushibiki J. , Takanaga I. , Komatsuzaki S. , Ujiie T.
Published: July 2002

We determined all the independent components of the acoustical physical constants (elastic constant, piezoelectric constant, dielectric constant, and density) of LiNbO3 and LiTaO3 crystals grown from the melts of three different starting materials with the Li2O contents set to 48.0, 48.5, and 49.0 mol %, and obtained the chemical composition dependences of the constants of each single crystal around the congruent composition. All the constants as well as the measured longitudinal, shear, and leaky surface acoustic wave (LSAW) velocities varied linearly with the composition ratios in the experimental range. The composition dependences of the LSAW velocities for the 128°YX-LiNbO3, X-112°Y-LiTaO3, and 36°YX-LiTaO3 substrates, previously obtained by line-focus-beam acoustic microscopy, were well matched with the calculated ones using the constants determined. Therefore the data of the composition dependences of the determined constants enable us to easily prepare the calibration lines for evaluating the crystals for any arbitrarily cut specimen surfaces, wave propagation directions, and modes by numerical calculations.
Mode identification in spherical microcavities doped with quantum dots
Authors: Möller B. , Artemyev M. V. , Woggon U. , Wannemacher R.
Published: July 2002

Using imaging spectroscopy at the diffraction limit, a polarization-sensitive mode mapping allows the experimental identification of transverse electric and transverse magnetic modes for spherical microcavities. The method is applied to microspheres surface covered by CdSe quantum dots. A theoretical estimate of the minimum mode volume excited by a single, anisotropic quantum dot is given with Q/V larger than 1000 μm−3 for the parameters of the experimentally studied microcavities.
Field emission properties of carbon nanohorn films
Authors: Bonard J.-M. , Gaál R. , Garaj S. , Thien-Nga L. , Forró L. , Takahashi K. , Kokai F. , Yudasaka M. , Iijima S.
Published: July 2002

Self-supporting carbon films were prepared from a carbonaceous material, nanohorns. Nanohorns are spherical particles built of sharp cones of a single graphene sheet. The films show good field emission characteristics due to the sharp horn-like structures, in particular a low turn-on field and good long-term stability.
Origins of micropipes in SiC crystals
Authors: Mahajan S.
Published: July 2002

It is argued that micropipes in SiC crystals grown by the physical vapor transport process form independently of screw dislocations. A mechanism involving liquid droplets of silicon containing some carbon or graphite particles is proposed to explain the formation of micropipes. The observed strain fields associated with micropipes are attributed to the presence of screw dislocations in their vicinity. An expression is developed that relates the change in radius of a micropipe to the number of screw dislocations that are eliminated during its expansion.
Predictability of normal heart rhythms and deterministic chaos
Authors: Lefebvre J. H. , Goodings D. A. , Kamath M. V. , Fallen E. L.
Published: July 2002

The evidence for deterministic chaos in normal heart rhythms is examined. Electrocardiograms were recorded of 29 subjects falling into four groups—a young healthy group, an older healthy group, and two groups of patients who had recently suffered an acute myocardial infarction. From the measured R–R intervals, a time series of 1000 first differences was constructed for each subject. The correlation integral of Grassberger and Procaccia was calculated for several subjects using these relatively short time series. No evidence was found for the existence of an attractor having a dimension less than about 4. However, a prediction method recently proposed by Sugihara and May and an autoregressive linear predictor both show that there is a measure of short-term predictability in the differenced R–R intervals. Further analysis revealed that the short-term predictability calculated by the Sugihara–May method is not consistent with the null hypothesis of a Gaussian random process. The evidence for a small amount of nonlinear dynamical behavior together with the short-term predictability suggest that there is an element of deterministic chaos in normal heart rhythms, although it is not strong or persistent. Finally, two useful parameters of the predictability curves are identified, namely, the ‘first step predictability’ and the ‘predictability decay rate,’ neither of which appears to be significantly correlated with the standard deviation of the R–R intervals.
Hardware and software for a pulse height analyzer linked to a personal computer
Authors: Spencer C. D. , Paul S. R.
Published: July 2002

Hardware and software for a pulse height analyzer interfaced through “simple” parallel ports are described. Voltages may be analyzed up to 400 K times per second. Resolution is 1000 or 4000 channels. Each channel may store up to 224 counts. Software zeroes the memory, initiates and times data acquisition, and uploads data. © 1997 American Institute of Physics.
Two-dimensional, transient model for mass transport in laser surface alloying
Authors: Chande T. , Mazumder J.
Published: July 2002

The process of alloy generation during laser surface alloying was examined. A numerical model was developed that solved the two-dimensional, transient equation for convection diffusion of matter in the melt pool using the alternate-diagonal implicit method. Velocity distributions used were for steels for a laser power of 1.5 kW, a 0.001-m beam diameter and a traverse speed of 0.01 m/s. Calculations were made for a uniform surface mass flux of 5 and 10 kg m2 s−1 and values of 100 and 1000 for the Peclet number for diffusion. The results showed that powder particles injected into the melt pool melted practically instantaneously, that good mixing was determined by the pattern of fluid flow, that average solute contents increased linearly with increasing interaction time, and that a change in the solute diffusivity by a factor of 10 did not greatly alter the predicted solute distribution. Good mixing was also found with surface flux over only a part of the melt pool. The conclusions were that fluid flow dominates the process of mass transport and determines the nature of the solute distribution, that powder feed is an effective method for surface alloying, and that results of the model could be used to better understand the process of laser surface alloying.
Resolution of the Thomson spectrometer
Authors: Schneider R. F. , Luo C. M. , Rhee M. J.
Published: July 2002

The resolution of Thomson spectrometers is examined. Charge, mass energy, and momentum resolutions are found as functions of collimation parameters and field strengths. The results are generally applicable to all Thomson spectrometer systems. In conjunction with this analysis, a compact Thomson spectrometer with high resolving power is described.
Two-dimensional balance equations in nonlinear electronic transport and application to GaAs-GaAlAs heterojunctions
Authors: Lei X. L. , Birman Joseph L. , Ting C. S.
Published: July 2002

A non-Boltzmann theory of steady-state transport for two-dimensional systems in a strong electric field is developed, which includes a force- and an energy-balance equation. The electron temperature, impurity-, and phonon-limited mobilities are determined solely from these balance equations. The theory is applied to the calculation of ohmic and nonlinear transport in GaAs-GaAlAs heterojunctions at low temperatures. Temperature-dependent ohmic mobilities are calculated and compared with experiments. Nonlinear effects in electronic transport at low temperatures are discussed and some numerical results are presented. We also compare the present balance equations with those in the carrier temperature model.
Effect of growth temperature on the electronic energy band and crystal structure of ZnS thin films grown using atomic layer epitaxy
Authors: Lahtinen J. A. , Lu A. , Tuomi T. , Tammenmaa M.
Published: July 2002

Electroreflectance (ER) technique has been used to study the effect of growth temperature and source materials on the electronic energy band and crystal structure of zinc sulfide thin films grown using atomic layer epitaxy (ALE). The samples studied were grown by ALE at 300–500 °C from zinc acetate or zinc chloride and hydrogen sulfide. The ER spectra were measured at room temperature and at 77 K. The crystal structure of polycrystalline ZnS thin films grown from zinc acetate at 300–375 °C is mainly cubic, whereas the ZnS layers grown from zinc chloride at 425–500 °C are predominantly hexagonal. The ER spectra of the latter samples show peaks belonging to the cubic structure, too. The crystal structure of zinc sulfide thin films is found to depend both on growth temperature and on source materials.