bluuebunny/crossref_metadata_2025 huggingface dataset:
Tailorable, visible light emission from silicon nanocrystals
Authors: Wilcoxon J. P. , Samara G. A.
Published: July 2002

Crystalline, size-selected Si nanocrystals in the size range 1.8–10 nm grown in inverse micellar cages exhibit highly structured optical absorption and photoluminescence (PL) across the visible range of the spectrum. The most intense PL for the smallest nanocrystals produced (∼2 nm) was in the blue (∼365 nm) with a radiative lifetime of ∼1 ns and is attributed to direct recombination at zone center.
Temperature-dependent micro-photoluminescence of individual CdSe self-assembled quantum dots
Authors: Kim J. C. , Rho H. , Smith L. M. , Jackson Howard E. , Lee S. , Dobrowolska M. , Furdyna J. K.
Published: July 2002

We use micro- and nano-photoluminescence to study the temperature-dependent excitonic emission from CdSe quantum dots embedded in a ZnSe matrix. By varying the spatial resolution from 200 nm to 1.7 μm, we are able to study the temperature dependence of the ultranarrow (∼200 μeV) emission from excitons confined to single quantum dots, as well as statistical ensembles of up to 200 dots. By measuring the quenching of the photoluminescence (PL) with temperature, we find compelling evidence that the PL emission from these samples results from two different kinds of states. Similar to previous work, we find that a broad PL line persists to 300 K with an activation energy of ∼40 meV. However, we find that the ultranarrow lines are quenched at about 60 K, indicating an effective activation energy of only 4.0 meV.
Localized vibrational modes of carbon-hydrogen complexes in GaN
Authors: Manasreh M. O. , Baranowski J. M. , Pakula K. , Jiang H. X. , Lin Jingyu
Published: July 2002

Localized vibrational modes of carbon-hydrogen complexes in metalorganic chemical vapor deposition grown GaN on sapphire were studied using a Fourier-transform infrared spectroscopy technique. Three distinctive localized vibrational modes were observed around 2850, 2922, and 2959 cm−1 for undoped, Si-, and Mg-doped samples. These peaks are related to CH, CH2, and CH3 defect complexes, respectively. However, the localized vibrational modes were not observed in some undoped samples, which is indicative of high quality grown epitaxial layers. It is also observed that the frequencies and intensities of the localized vibrational modes are sample dependent.
Multiple gated InAs dot ensembles
Authors: Austing D. G. , Tarucha S. , Main P. C. , Henini M. , Stoddart S. T. , Eaves L.
Published: July 2002

We adapt a multiple gating technology to characterize electrically an ensemble of “self-assembled” InAs dots embedded in a plane within an Al0.20Ga0.80As tunneling barrier. Although the μm-sized mesa incorporates several hundred dots, we find that only a few of them contribute to the current close to threshold. Gating allows us to probe the origin of the sharp current peaks, and we can classify these peaks into families in a simple way according to their gate voltage dependence.
Low-threshold optically pumped lasing at 444 nm at room temperature with high characteristic temperature from Be-chalcogenide-based single-quantum-well laser structures
Authors: Chang J. H. , Cho M. W. , Godo K. , Makino H. , Yao T. , Shen M. Y. , Goto T.
Published: July 2002

We have achieved low-threshold optically pumped lasing at 444 nm at room temperature with high characteristic temperature (T0) from ZnSe/ZnMgBeSe single-quantum-well structures. The threshold intensity is as low as 15 kW cm−2, while T0 value is as high as 166 K. Lasing is observed up to 473 K. Lasing wavelength of 444 nm at room temperature is the shortest wavelength ever achieved in ZnSe-based laser diode structures. The laser structure includes a single ZnMgBeSe/ZnSe/ZnMgBeSe quantum well with a ZnSe well thickness of 4 nm. The (004) x-ray diffraction rocking curve of the ZnMgBeSe quaternary cladding layers shows a sharp diffraction peak with a full width at half maximum of 21 arcsec which is in contrast to that from a ZnMgSSe cladding layer showing much broader multiple peaks. The observed lasing features are partly ascribed to high crystal quality of the ZnMgBeSe layers and type-I band alignment, as has been supported by photoluminescence in addition to x-ray diffraction measurements.
Thin crystalline functional group copolymer poly(vinylidene fluoride–trifluoroethylene) film patterning using synchrotron radiation
Authors: Choi Jaewu , Manohara H. M. , Morikawa E. , Sprunger P. T. , Dowben P. A. , Palto S. P.
Published: July 2002

The photodegradation mechanism due to synchrotron radiation exposure of crystalline poly[vinylidene fluoride–trifluoroetylene, P(VDF–TrFE)] copolymer thin films has been studied with ultraviolet photoemission spectroscopy (UPS) and mass spectroscopy. Upon increasing exposure to x-ray white light (hν⩽1000 eV), UPS measurements reveal that substantial chemical modifications occur in P(VDF–TrFE) 5 monolayer films, including the emergence of new valence band features near the Fermi level, indicating a semimetallic photodegradeted product. The photodetached fragments of the copolymer consist mainly of H2, HF, CHF, CH2. This x-ray exposure study demonstrates that P(VDF–TrFE) films, possessing unique technologically important properties, can be directly patterned by x-ray lithographic processes.
Electronic states and band alignment in GalnNAs/GaAs quantum-well structures with low nitrogen content
Authors: Hetterich M. , Dawson M. D. , Egorov A. Yu. , Bernklau D. , Riechert H.
Published: July 2002

We investigate the electronic states in strained Ga0.62In0.38N0.015As0.985/GaAs multiple- quantum-well structures using photoluminescence and (polarized) photoluminescence excitation measurements at low temperature. From a theoretical fit to the experimental data, a type-I band alignment for the heavy holes with a strained conduction-band offset ratio of about 80% is obtained, while the light holes show an approximately flat band alignment. Additionally, our results suggest an increased effective electron mass in GaInNAs, possibly due to the interaction of the conduction band with nitrogen-related resonant states, an observation prospectively of benefit for GaInNAs-based diode lasers.
Hole transport in Mg-doped GaN epilayers grown by metalorganic chemical vapor deposition
Authors: Kim K. S. , Cheong M. G. , Hong C.-H. , Yang G. M. , Lim K. Y. , Suh E.-K. , Lee H. J.
Published: July 2002

A two-band model involving the heavy- and light-hole bands was adopted to analyze the temperature-dependent Hall effect measured on Mg-doped p-type GaN epilayers. At 300 K, the hole concentration was determined to be nearly twice the Hall concentration estimated from the measured Hall coefficient, meanwhile the Hall mobility of heavy hole turned out to be only half of the measured one. It is shown that the scattering by space charge and acoustic deformation potential is anomalously enhanced in Mg-doped GaN, and that the light hole affects conspicuously the observed transport parameters.
Observation of electronic Raman scattering from Mg-doped wurtzite GaN
Authors: Tsen K. T. , Koch C. , Chen Y. , Morkoc H. , Li J. , Lin J. Y. , Jiang H. X.
Published: July 2002

Electronic Raman scattering experiments have been carried out on both molecular beam epitaxy and metal-organic chemical vapor deposition-grown Mg-doped wurtzite GaN samples. Aside from the expected Raman lines, a broad structure (full width at half maximum≅15 cm−1) observed at around 841 cm−1 is attributed to the electronic Raman scattering from neutral Mg impurities in Mg-doped GaN. Our experimental results demonstrate that the energy between the ground and first excited states of Mg impurities in wurtzite GaN is about one-half of its binding energy.
Controlling the concentration and position of nitrogen in ultrathin oxynitride films formed by using oxygen and nitrogen radicals
Authors: Watanabe K. , Tatsumi T.
Published: July 2002

The formation of oxynitride films less than 2.0 nm by using oxygen and nitrogen radicals produced by an electron cyclotron resonance plasma in an ultrahigh-vacuum system has been studied. We found that the N concentration can be controlled at values up to 15% and that, although the interface roughness tends to increase with increasing N concentration, supplying oxygen and nitrogen radicals simultaneously decreases the roughness of the film and increases its nitrogen concentration (N: 12.1%, root mean square: 0.12 nm). We also could easily control the nitrogen profile in the oxynitride less than 2.0-nm-thick by using different processing sequences.
Room-temperature operation at 333 nm of Al0.03Ga0.97N/Al0.25Ga0.75N quantum-well light-emitting diodes with Mg-doped superlattice layers
Authors: Kinoshita A. , Hirayama H. , Ainoya M. , Aoyagi Y. , Hirata A.
Published: July 2002

We demonstrate room-temperature deep ultraviolet (UV) current injection emission from Al0.03Ga0.97N/Al0.25Ga0.75N multiquantum-well (MQW) light-emitting diodes (LEDs) fabricated by metalorganic vapor phase epitaxy. The electroluminescence (EL) peaked at 333.0 nm under pulsed current injection. To our knowledge, this is the shortest wavelength ever reported for nitride QW LEDs. A Mg-doped GaN/AlGaN superlattice (SL) hole conductive layer was used as a p-type layer in order to enable current injection into such deep-level AlGaN QWs. We observed single-peak near band-edge emission from the QWs. The output intensity did not saturate up to current densities of 0.33 kA/cm2. We obtained a reasonable well width dependence on the EL peak wavelength of Al0.01Ga0.99N MQW LEDs, which confirms that the main emission peak originates from the QW regions. These results revealed that the Mg-doped SL hole conductive layers are highly suitable for application to GaN-based UV light-emitting devices.
Determination of the resistivity components ρab and ρc from multiterminal measurements in Bi2Sr2CaCu2O8+x crystals
Authors: Esposito M. , Muzzi L. , Sarti S. , Fastampa R. , Silva E.
Published: July 2002

A general method for the determination of the resistivity tensor from multiterminal measurements is developed, based on the solution of a Laplace equation for the potential inside the sample with suitable boundary conditions. We have performed the calculation in the general case of an arbitrary contact geometry, with finite width of the contacts on the sample surfaces. The precision and the accuracy of the calculation and of the hypothesis of the model have been checked by means of a self-consistent method based on the reconstruction of voltage measurements.
Covering of substrate holes through particle deposition
Authors: Lu Shih-Yuan
Published: July 2002

A methodology is proposed to determine the minimum coating thickness for covering substrate holes through particle deposition. The particle deposition process is carried out with a combined, two-dimensional, on-lattice model, in which both deterministic and nondeterministic driving forces are taken into account. The normalized covering thickness (hc/Dw) is investigated subject to variation in the Peclet number (Pe) and the size of the depositing particle, normalized hole size (Dw/L), and degree of postcontact restructuring allowed. It is found that ballistic particle movement gives a slightly better covering structure for smaller holes while diffusive particle movement much more efficiently covers larger holes. The normalized covering thickness increases with increasing normalized hole size and with decreasing normalized depositing particle size. Postcontact restructuring first improves the covering capability of the depositing particle, but then impairs it if further restructuring is allowed. It is further found that the normalized covering thickness scales with the normalized hole size as hc/Dw∼(Dw/L)E with E=0.67, 0.56, and 0.36 for Pe of infinity, 0.5, and 0.1, respectively.
Investigation on anisotropy of vertical-cavity surface-emitting lasers
Authors: Liu Shian , Lin Shiming , Cheng Peng , Zhang Guangbin , Wang Qiming , Chen Ye , Li Guohua , Han Hexiang
Published: July 2002

We have studied the spontaneous emission of polarized excitons in the GaInP/AlGaInP vertical-cavity surface-emitting lasers from 50 K to room temperature. It is observed that the spontaneous emission peak enters and leaves the resonant regime. At the resonant regime, the emission intensities of the perpendicularly and horizontally polarized excitons are enhanced and their proportions are different from that in nonresonant regime. These experimental results are explained by the dressed exciton theory of the semiconductor microcavity device. Based on this theory, the intensity enhancement and the polarization dependence are understood as cooperative emission and the microcavity anisotropy.
Electron spin resonance study of a temperature sensitive magnetic fluid
Authors: Parekh Kinnari , Upadhyay R. V. , Mehta R. V. , Srinivas D.
Published: July 2002

Electron spin resonance spectra of a temperature sensitive magnetic fluid involving polydispersed Mn0.5Zn0.5Fe2O4 ferrite particles are scanned from 200 to 400 K. For such a polydispersed system, the deconvolutions of the spectra suggest coexistence of two different resonance modes: (i) a broad line due to ferrimagnetic resonance and (ii) a sharp line at g=2 due to intrinsic superparamagnetic phase. From the intensity variation of the line at g=2, it is found that the contribution due to the superparamagnetic phase increases with temperature. A study of peak-to-peak linewidth variation with temperature indicates that the weakening of the magnetic moment is responsible for the observed reduction in the linewidth.
Characterization of thick cryogenic fuel layers using convergent-beam interferometry: A numerical investigation
Authors: McKenty P. W. , Wittman M. D. , Goncharov V. N.
Published: July 2002

The inner-surface roughness of thick cryogenic-fuel layers in inertial confinement fusion (ICF) targets plays a critical role in determining the overall success of an ICF capsule implosion. Imperfections at this surface affect the growth of Raleigh–Taylor hydrodynamic instabilities during both the acceleration and deceleration phases of the implosion. Characterization of this surface is performed using a Mach–Zehnder interferometer that illuminates the target with a wavefront that is convergent to a point near the targets’ rear focal point, thereby reducing the strong negative-lens effects of the thick cryogenic fuel layer. The construction of this interferometer is described in the text. Phase-shifting interferometry is utilized to acquire the perturbed wavefronts that have passed through the target. These wavefronts are subsequently sampled around the target perimeter and decomposed into a one-dimensional Fourier spectrum, which is Abel transformed into a two-dimensional (2D) spectrum. The validity of convergent-beam interferometry is demonstrated by analyzing numerically generated perturbed wavefronts. The wavefronts are analyzed, and the (2D) spectrum obtained is compared to the actual spectrum imposed on the interior of the ice surface of the target model. Agreement between these spectra is >80% for Legendre modes between 2 and 50.
The laminar flow tube reactor as a quantitative tool for nucleation studies: Experimental results and theoretical analysis of homogeneous nucleation of dibutylphthalate
Authors: Mikheev Vladimir B. , Laulainen Nels S. , Barlow Stephan E. , Knott Michael , Ford Ian J.
Published: July 2002

A laminar flow tube reactor was designed and constructed to provide an accurate, quantitative measurement of a nucleation rate as a function of supersaturation and temperature. Measurements of nucleation of a supersaturated vapor of dibutylphthalate have been made for the temperature range from −30.3 to +19.1 °C. A thorough analysis of the possible sources of experimental uncertainties (such as defining the correct value of the initial vapor concentration, temperature boundary conditions on the reactor walls, accuracy of the calculations of the thermodynamic parameters of the nucleation zone, and particle concentration measurement) is given. Both isothermal and the isobaric nucleation rates were measured. The experimental data obtained were compared with the measurements of other experimental groups and with theoretical predictions made on the basis of the self-consistency correction nucleation theory. Theoretical analysis, based on the first and the second nucleation theorems, is also presented. The critical cluster size and the excess of internal energy of the critical cluster are obtained.
On the representation of inhomogeneous linear force-free fields
Authors: Clegg J. R. , Browning P. K. , Laurence P. , Bromage B. J. I. , Stredulinsky E.
Published: July 2002

It is shown that there is a false assumption hidden in the description of a relaxed state with inhomogeneous boundary conditions as the vector sum of a potential field, satisfying the boundary conditions, and a sum of eigenfunctions of the associated eigenvalue problem expanded by certain coefficients. In particular, although the Jensen and Chu formula (1984) can provide the correct expansion coefficients, it contains an implicit paradox in its derivation according to a general vector theorem. The same paradox led Chu et al. (1999) to be concerned about a contradiction obtained by taking the curl of their magnetic field expansion which, if permitted, becomes inconsistent with a current normal to the surface. The assumption that the curl can be commuted across an infinite sum of terms is the mechanism leading to these, apparently paradoxical, conclusions. Two mechanisms for resolving this apparent paradox are possible, one of which will be described in some detail below and the other discussed further in a forthcoming, more theoretical paper (Laurence et al., 2000). The decomposition of the magnetic field above is valid with convergence in the mean squared sense, but a decomposition of the current needs to be reinterpreted in terms of negative Sobolev spaces. To avoid this, and remain in a more easily managable and familiar setting, we derive the expansion coefficients in a way that involves the commuting of the inverse curl (as opposed to the curl) and the series. The resulting series converges in a mean square sense. When this is done the calculation can conform to the general vector theorem and a new gauge-invariant expression for the coefficients is obtained. However the consequence of the non-commutability is nullified in the Jensen and Chu formula, in both simply and multiply connected domains, by the important extra requirement of a boundary condition on the vector potential eigenfunctions; this excludes magnetic field eigenfunctions that carry flux, but there remains a complete set for the expansion and all flux is carried by the potential field. The two formulas are then identical. On a different issue, it is shown that if the general expansion is taken over a half-space, by combining positive and negative eigenvalue terms, then the coefficients are anisotropic, that is they are tensors except when evaluated at the first eigenvalue. A specific example is presented to illustrate the situation and to validate the new method of deriving the coefficients.
Electromigration saturation in a simple interconnect tree
Authors: Hau-Riege Stefan P. , Thompson Carl V.
Published: July 2002

Electromigration-saturation experiments have been carried out on passivated “L”-shaped interconnects which had three contacts through W-filled vias at the ends and at the corners of the L’s. Currents of different magnitudes and directions were independently applied in the two limbs of the L’s. Variations in the steady-state electrical resistances were observed, and attributed to a variation in the location of the voids along the lines. Variations in the rate of approach to the steady state were also observed, and are attributed to variations in void shapes. The effects of electromigration in L-shaped interconnects were simulated using an electromigration simulator based on the one-dimensional Korhonen et al. [J. Appl. Phys. 73, 3790 (1993)] model for electromigration, modified to include the effects of junctions. It is shown that, using a single set of input parameters and information about void shapes, the simulation can be used to make accurate worst-case predictions of the evolving and the steady state values of electromigration-induced resistance changes in these simple L-shaped interconnect trees.
Efficient calculation of the current–voltage characteristic of a resistively shunted Josephson junction
Authors: Colclough M. S.
Published: July 2002

We describe a method for calculating the current–voltage characteristics given by the resistively shunted Josephson junction model. The method is based on a continued fraction, and allows rapid calculation of the characteristic for arbitrary temperatures, in the low-capacitance regime.
Photoluminescence from silicon nano-particles synthesized by laser-induced decomposition of silane
Authors: Botti S. , Coppola R. , Gourbilleau F. , Rizk R.
Published: July 2002

This work deals with photoluminescence study of silicon nanoparticles produced by CO2-laser-induced decomposition of SiH4 mixed to helium in a controlled atmosphere reactor. By adjusting the pressure of both reactor and precursor gas and its dilution rate in helium, we were able to control, to a certain extent, the silicon growth rate and hence the particle diameter. This latter was determined by both small angle neutron scattering techniques and high resolution transmission electron microscopy observations. Particles with mean diameter ranging between 3 and 10 nm were submitted to photoluminescence and infrared absorption spectroscopy measurements. The photoluminescence spectra revealed two main peaks at about 1.7 and 2.1 eV. The peak position of the former was insensitive to the change of particle size, while its intensity increased after oxidation. The latter showed, however, a slight size dependence but had undergone a drastic decrease after oxidation. These features enabled us to ascribe the red peak (1.7 eV) to some radiative surface defect, while the yellow peak (2.1 eV) appeared consistent with an emission from an oxygen-related defect such as the nonbridging oxygen hole center.
Anomalous temperature dependence of photoluminescence from InAs quantum dots
Authors: Jiang W. H. , Ye X. L. , Xu B. , Xu H. Z. , Ding D. , Liang J. B. , Wang Z. G.
Published: July 2002

Self-assembled InAs quantum dots are fabricated on a GaAs substrate by molecular beam epitaxy. The dots are covered by several monolayers of In0.2Ga0.8As before a GaAs cap layer and an in situ postgrowth annealing is performed to tune the emission to higher energy. The temperature dependence of photoluminescence from this structure demonstrates a slower redshift rate of the peak position, a gradual broadening of the linewidth and an abnormal enhancement of integrated intensity as the temperature is increased from 15 to 300 K. These phenomena are closely related to the introduction of an InGaAs layer and to the intermixing of In and Ga atoms during annealing. We propose a model to explain the unusual increase in PL intensity, which fits the experimental data well.
Alternating current losses in YBa2Cu3O7−x coated conductors on technical substrates
Authors: Ashworth S. P. , Maley M. , Suenaga M. , Foltyn S. R. , Willis J. O.
Published: July 2002

We report on measurements of the ac losses of YBa2Cu3O7−x conductors on technical substrates. The samples are taken from conductors 1 m long produced with an ion beam assisted deposition step in the manufacturing process yielding 1 cm wide conductors with critical currents approaching 100 A at 77.4 K. The losses due to combinations of ac transport currents and ac magnetic fields were measured calorimetrically and the results compared to theoretical estimates. While the results of our measurements are similar to the theoretical estimates, there are some important differences in the qualitative behavior.
Bubble dynamics and shock waves generated by laser absorption of a photoacoustic sphere
Authors: Sun J. M. , Gerstman Bernard S. , Li Bin
Published: July 2002

Theoretical analysis and extensive numerical simulations are performed on the system of a spherical absorber illuminated by a pulsed laser. Both bubble formation and shock wave generation are investigated. We find that a strong shock front forms just outside the absorber and attenuates quickly into an acoustic wave as it propagates away from the absorber. The formation and growth of the bubble is shown to have a gentle threshold dependence and to be a continuous function of the energy of the laser pulse with no explosive threshold. There is a weak dependence of the threshold fluence for bubble formation on the pulse duration of the laser. The bubble growth dynamics are also pulse-duration dependent for pulse lengths shorter than the thermal conduction time. For ultrashort pulses (shorter than the acoustic transit time across the absorber), the bubble dynamics approach a limiting growth trajectory. The process is a cooperative effect of both thermal and pressure interactions. We also find that variation of the absorber’s mechanical parameters, such as the thermal expansion coefficient, can significantly alter not only the shock waves but also the bubble growth dynamics.
Optical properties of plasma species absorbed during diamond deposition on steel
Authors: Shahedipour F. , Conner B. P. , White H. W.
Published: July 2002

Diamond thin films have been deposited directly on stainless steel substrates at low pressure and low temperature conditions over the range of methane (CH4) to hydrogen (H2) ratios of 4% to 9% using electron-cyclotron-resonance microwave plasma-assisted chemical vapor deposition (PACVD) technique. In situ Fourier transform infrared spectroscopy, employed in a reflection-absorption geometry (FTIRRAS), and optical emission spectroscopy have been used to study the plasma species adsorbed on the substrate surfaces as well as the species above the substrates surfaces both before and during the nucleation and film growth. It is demonstrated that these techniques can provide useful information on the early stages of diamond growth. When correlated with film properties measured by Raman spectroscopy and scanning electron microscopy, the results from FTIRRAS indicate that the absorption of the graphitic and diamond phases are related to the ratio of CH4 to H2 and can be identified at the early stages of film growth.
Symmetries in covariant classical mechanics
Authors: Saller Dirk , Vitolo Raffaele
Published: July 2002

In the framework of covariant classical mechanics (i.e., general relativistic classical mechanics on a space–time with absolute time), developed by Jadczyk and Modugno, we analyze systematically the relationship between symmetries of geometric objects. We show that the (holonomic) infinitesimal symmetries of the cosymplectic structure on space–time and of its horizontal potentials are also symmetries of spacelike metric, gravitational and electromagnetic fields, Euler–Lagrange morphism and Lagrangians. Then, we provide a definition for a covariant momentum map associated with a group of cosymplectic symmetries by means of a covariant lift of functions of phase space. In the case of holonomic symmetries, we see that any covariant momentum map takes values in the quantizable functions in the sense of Jadczyk and Modugno, i.e., functions quadratic in velocities with leading coefficient proportional to the spacelike metric. Finally, we illustrate the results by some examples.
Photoacoustic and photoluminescence studies of porous silicon etched by low-concentration hydrofluoric acid
Authors: Toyoda T. , Takahashi T. , Shen Q.
Published: July 2002

We applied photoacoustic (PA), photoluminescence (PL), photoluminescence excitation (PLE), and atomic force microscopy (AFM) techniques on porous silicon (PS) layers to study the influence of chemical etching by low-concentration hydrofluoric acid. The chemical etching reveals the formation of PS layers of small dimensions by AFM observations, indicating the possibility of a strong quantum confinement effect. PA spectroscopy is useful to obtain the optical absorption characteristic for strongly scattering media such as PS and it helps to confirm the above speculation by indicating the blueshift of the fundamental absorption edge for the PS layer with chemical etching. PL spectroscopy also confirms the possibility of a quantum confinement effect by revealing the strong intensity and blueshift for the PS layer with chemical etching. PLE measurements suggest that the site for the radiative processes is different from that for the recombination of carriers and the PL of PS layers were dominated only by small crystallites in various size distributions.
Double-resonance spectroscopy of the high Rydberg states of HCO. V. Rovibronic interactions and l-uncoupling in the (010) manifold
Authors: Zückerman Eric J. , Mayer Eric E. , Foltynowicz Robert J. , Robinson Jason D. , Jen Shi Hui , Konopka Michael C. , Sanford Todd , Hedderich Hartmut G. , Chen I-Chia , Grant Edward R.
Published: July 2002

We report the ionization-detected absorption spectra of autoionizing Rydberg states converging to the (010) vibrational level of HCO+. Sharp second-photon resonances appear in transitions from first-photon-prepared originating states that have total angular momentum (less-spin) from N′=0 to 5, selected from the Σ+ and Σ− components of the (010) band of the 3pπ2Π Rydberg state. We systematically compare spectra in order to characterize observed resonances in terms of the good total angular momentum quantum number, N. Rydberg analysis establishes the convergence of series to detailed cation-core rotational quantum numbers, N+. Observed series are found to fit well with simulations employing a limited set of constant quantum defects (δ=1.062, 0.794, 0.606, 0.253, 0.015, 0.002, −0.027 and −0.076). The strengths of observed transitions as a function of initial and final total angular momentum provide a purely experimental indication of the appropriate assignment of the approximately good orbital angular momentum quantum number, l, for each series. Interactions between Rydberg orbital and core rotational angular momentum are found to conform with a coupling case intermediate between Hund’s cases (b) and (d). Splitting patterns further assign certain features according to predominant case (b) composition.
Conducting ballistic magnetoresistance and tunneling magnetoresistance: Pinholes and tunnel barriers
Authors: Garcı́a N.
Published: July 2002

This letter shows that conducting ballistic and tunneling magnetoresistances have identical physical treatments and, therefore, it is not possible to distinguish whether an experiment is measuring, in oxide barriers, electron conduction through pinholes or tunneling because both types of conduction are governed by the same physics. Currents in both cases are calculated by matching wave functions at the electrodes (both are ballistic). Therefore, pinholes or small ballistic nanocontacts in the tunnel barrier allow conducting ballistic electrons that account for most of the current. We also show that, in the case of nonsuperconducting materials, e.g., magnetic materials, current–voltage characteristics are not suitable to distinguish one process from the other.
Light-cone expansion of the Dirac sea in the presence of chiral and scalar potentials
Authors: Finster Felix
Published: July 2002

We study the Dirac sea in the presence of external chiral and scalar/pseudoscalar potentials. In preparation, a method is developed for calculating the advanced and retarded Green’s functions in an expansion around the light cone. For this, we first expand all Feynman diagrams and then explicitly sum up the perturbation series. The light-cone expansion expresses the Green’s functions as an infinite sum of line integrals over the external potential and its partial derivatives. The Dirac sea is decomposed into a causal and a noncausal contribution. The causal contribution has a light-cone expansion which is closely related to the light-cone expansion of the Green’s functions; it describes the singular behavior of the Dirac sea in terms of nested line integrals along the light cone. The noncausal contribution, on the other hand, is, to every order in perturbation theory, a smooth function in position space.
A nonlinear two-fluid model for toroidal plasmas
Authors: Sugiyama L. E. , Park W.
Published: July 2002

A nonlinear numerical model for the two-fluid (electron and ion fluid) description of the evolution of a plasma in toroidal geometry, MH3D-T, is described. The model extends the “drift” ordering for small perturbations to arbitrary perturbation size. It is similar, but not identical, to the collisional Braginskii equations. The ion gyroviscous stress tensor, Hall terms, temperature diamagnetic drifts, and a separate electron pressure evolution are included. The model stresses the (fluid) parallel dynamics by solving the density evolution together with the temperature equations, including the thermal equilibration along the magnetic field. It includes the neoclassical, collisional parallel viscous forces for electrons and ions. The model has been benchmarked against the stabilizing effects of the ion diamagnetic drift ω*i on the m=1, n=1 reconnecting mode in a cylinder. The stabilization mechanism is shown to be poloidal rotation of the global kink flow of the plasma mass vi within q<1, relative to the location of the magnetic field X-point within the reconnection layer. The ion ω*i-drift is also shown to cause frequency-splitting for the toroidal Alfvén eigenmode (TAE). Basic diamagnetic and neoclassical magnetohydrodynamic (MHD) effects on magnetic island evolution and rotation are discussed. The dynamics of the plasma along the magnetic field, when compressibility, parallel thermal conductivity, plasma density evolution, and full toroidal geometry are kept, are found to have strong effects on both linear growth rates and nonlinear evolution. The nonlinear coupling of magnetic islands, driven by perturbations of different toroidal mode number, is enhanced by the density evolution in both MHD and two-fluids.
Properties of Mg activation in thermally treated GaN:Mg films
Authors: Lin C. F. , Cheng H. C. , Chang C. C. , Chi G. C.
Published: July 2002

The Mg acceptors of GaN films are activated as p-type GaN with rapid thermal annealing (RTA) and furnace treatments. The GaN:Mg films are activated successfully by using the RTA system below 1000 °C for 1 min. After the RTA treatment, we observed a sharper linewidth and stronger emission intensities from donor to acceptor peaks in the photoluminescence spectra. By comparing the electrical properties of GaN:Mg films at optimum conditions made with RTA (800 °C) and furnace (700 °C) treatments, we find similar activated hole concentration and a higher hole mobility for GaN:Mg films with RTA treatment at 800 °C. A higher bulk resistivity caused by increasing nitrogen vacancies is found at higher temperature and longer time RTA treatments. Faster treatment times and lower temperatures for the GaN:Mg films were achieved with the RTA activation process.
Three-configurational surface magneto-optical Kerr effect measurement system for an ultrahigh vacuum in situ study of ultrathin magnetic films
Authors: Lee J.-W. , Jeong J.-R. , Kim D.-H. , Ahn J. S. , Kim J. , Shin S.-C.
Published: July 2002

We have constructed a three-configurational surface magneto-optical Kerr effect system, which provides the simultaneous measurements of the “polar,” “longitudinal,” and “transverse” Kerr hysteresis loops at the position where deposition is carried out in an ultrahigh vacuum growth chamber. The present system enables in situ three-dimensional vectorial studies of ultrathin film magnetism with a submonolayer sensitivity. We present three-configurational hysteresis loops measured during the growth of Co films on Pd(111), glass, and Pd/glass substrates.
Water pair potential of near spectroscopic accuracy. II. Vibration–rotation–tunneling levels of the water dimer
Authors: Groenenboom G. C. , Wormer P. E. S. , van der Avoird A. , Mas E. M. , Bukowski R. , Szalewicz K.
Published: July 2002

Nearly exact six-dimensional quantum calculations of the vibration–rotation–tunneling (VRT) levels of the water dimer for values of the rotational quantum numbers J and K ⩽2 show that the SAPT-5s water pair potential presented in the preceding paper (paper I) gives a good representation of the experimental high-resolution far-infrared spectrum of the water dimer. After analyzing the sensitivity of the transition frequencies with respect to the linear parameters in the potential we could further improve this potential by using only one of the experimentally determined tunneling splittings of the ground state in (H2O)2. The accuracy of the resulting water pair potential, SAPT-5st, is established by comparison with the spectroscopic data of both (H2O)2 and (D2O)2: ground and excited state tunneling splittings and rotational constants, as well as the frequencies of the intermolecular vibrations.
Effect of the geometrical parameters on the electric field of pixelated two-dimensional arrays of γ-ray spectrometers
Authors: Nemirovsky Y. , Ifraimov M. , Ludwig A.
Published: July 2002

We study the effect of the geometrical parameters of pixelated two-dimensional (2D) arrays of γ-ray spectrometers upon the distribution of the electric field lines, which are induced by the applied voltage. It is shown that the size and in particular the separation between anodes strongly affect the field uniformity. The electric field is calculated by three methods including: (i) a commercial software for 3D field analysis based on finite element method; (ii) a 3D finite difference approach; and (iii) the Moments method. The required boundary conditions and the calculation efficiency of each method are compared. Since the uniformity of the electric field strongly affects the spectral performance of pixelated 2D arrays of γ-ray spectrometers, the results presented here are important for the optimal design and modeling of such arrays.
A complete active space self-consistent field multireference configuration interaction study of the low-lying excited states of BrO
Authors: Li Yumin , Francisco Joseph S. , Peterson Kirk A.
Published: July 2002

A theoretical study of the excited states of the BrO radical has been carried out for the first time using high level ab initio molecular orbital methods. The vertical excitation energies for the low-lying excited states (2 2Π, 1 2Σ+, 1 2Σ− and 1 2Δ) are calculated using the internally contracted multireference configuration interaction (MRCI) method. Based on vertical excitation energies, the ordering of the first few excited states are determined to be 2 2Π, 1 2Δ, 1 2Σ−, and 1 2Σ+. The potential energy curves for the ground state (X 2Π) and several low-lying excited states are examined at the MRCI+Q/aug-cc-pVQZ level of theory. Several excited states are calculated to intersect the 2 2Π state, resulting in predissociation into Br(2P)+O(3P). Results for the quartet states of BrO are also included, as well as calculations of the effects of spin–orbit coupling on the electronic states of BrO.
Stresses in piezoceramics undergoing polarization switchings
Authors: Fotinich Y. , Carman G. P.
Published: July 2002

Piezoelectric materials exhibit nonlinear behavior when subjected to large electrical or mechanical loads. This strong nonlinear material behavior is induced by localized polarization switching (i.e., change of the polarization direction) at the subgrain level. In this article, a nonlinear finite element code is described to model polarization switchings in piezoceramics subjected to large electromechanical loads. The local polarization switching criterion is based on electric displacement combined with stability arguments. To evaluate the model, three cases are studied in this article: a partially electroded specimen simulating 180° polarization switching, a fully electroded specimen simulating 90° switching and a plate containing a void. The analytical results are compared with experimental data obtained from Moiré interferometer with reasonable agreement. Stresses in the vicinity of the voids are computed and the results are used to explain fatigue behavior observed in piezoceramics. It has been determined that, for a material undergoing full polarization switching, compressive stress along the polarization direction reduces internal stresses and extends fatigue life. If the applied electric field value is below the coercive field limit, the compressive prestress can increase the stresses thus reducing fatigue life.
Water–wire clusters: Vibronic spectra of 7-hydroxyquinoline⋅(H2O)3
Authors: Bach Andreas , Coussan Stéphane , Müller Andreas , Leutwyler Samuel
Published: July 2002

The supersonically cooled 7-hydroxyquinoline⋅(H2O)3 cluster was investigated by mass- and isomer-selected S1←S0 resonant two-photon ionization and S1→S0 fluorescence spectroscopy. UV(ultraviolet)/UV-holeburning measurements prove that a single cluster isomer is formed, although different tautomers (7-keto- and 7-hydroxyquinoline), rotamers (cis and trans) and isomers (cyclic and water–wire) are possible. Ab initio calculations of structures and vibrations of different tautomers and isomers of this cluster predict that the cis-enol-7-hydroxyquinoline⋅(H2O)3 “water–wire” cluster is the most stable species. The experimental S0 and S1 inter- and intramolecular vibrational frequencies are in good agreement with the calculated harmonic frequencies. S1←S0 excitation leads to contraction of all four hydrogen bonds along the hydrogen bonded water wire, inducing intense intermolecular stretching vibrations of the O–H⋯O and HO–H⋯N(quinoline) hydrogen bonds which terminate the water–wire. There are no spectroscopic signs of proton transfer.
Extracting knowledge-based energy functions from protein structures by error rate minimization: Comparison of methods using lattice model
Authors: Xia Yu , Levitt Michael
Published: July 2002

We describe a general framework for extracting knowledge-based energy function from a set of native protein structures. In this scheme, the energy function is optimal when there is least chance that a random structure has a lower energy than the corresponding native structure. We first show that subject to certain approximations, most current database-derived energy functions fall within this framework, including mean-field potentials, Z-score optimization, and constraint satisfaction methods. We then propose a simple method for energy function parametrization derived from our analysis. We go on to compare our method to other methods using a simple lattice model in the context of three different energy function scenarios. We show that our method, which is based on the most stringent criteria, performs best in all cases. The power and limitations of each method for deriving knowledge-based energy function is examined.
Electron-beam irradiation-induced gate oxide degradation
Authors: Cho Byung Jin , Chong Pei Fen , Chor Eng Fong , Joo Moon Sig , Yeo In Seok
Published: July 2002

Gate oxide degradation induced by electron-beam irradiation has been studied. A large increase in the low-field excess leakage current was observed on irradiated oxides and this was very similar to electrical stress-induced leakage currents. Unlike conventional electrical stress-induced leakage currents, however, electron-beam induced leakage currents exhibit a power law relationship with fluency without any signs of saturation. It has also been found that the electron-beam neither accelerates nor initiates quasibreakdown of the ultrathin gate oxide. Therefore, the traps generated by electron-beam irradiation do not contribute to quasibreakdown, only to the leakage current.
Modulational instability of electrostatic ion-cyclotron waves by dust modes in a magnetoactive dusty plasma
Authors: Banerjee A. K. , Salahuddin M. , Islam M. K. , Cairns R. A. , Salimullah M.
Published: July 2002

A high-amplitude electrostatic ion-cyclotron wave is shown to be unstable against modulational instability by the electrostatic dust modes in a magnetized, homogeneous, and collisional dusty plasma. The presence of the dust component has strong effects on the nonlinear coupling via the dust modes. The electron–neutral and ion–neutral collisions are seen to have significant effects on the damping and consequent overall growth of the modulational instability of the ion-cyclotron wave in the dusty plasma.
Time-dependent quantum wave packet studies of the F+HCl and F+DCl reactions
Authors: Tang Bi-Yu , Yang Ben-Hui , Han Ke-Li , Zhang Rui-Qin , Zhang John Z. H.
Published: July 2002

Full three-dimensional time-dependent quantum wave-pack calculations have been carried out for the F+HCl and F+DCl reactions on a many-body expansion of the ground 2A′HClF potential energy surface. The calculated energy-dependence of reaction probability exhibits oscillating structure in the F+HCl reaction but not in the F+DCl system. The effects of initial state excitation on the total reaction probabilities as a function of collision energy are investigated for reactions from various initial vibrational and rotational states of HCl and DCl. Our results show that reagent vibrational and/or rotational excitation can generally lead to an increase in reaction probability at low collision energy and a slight decrease at relatively high collision energy. Thermal rate constants for the title reactions are calculated and they are in generally good agreement with experimental measurement. Investigation of steric effects for the reactions indicates that the H (or D) side of HCl (or DCl) molecule is only slightly favored for reactive attack and reaction proceeds from almost all attack angles. The present results indicate that the H/D kinetic isotope effect should not be totally neglected.
Photoinduced intermolecular electron transfer in complex liquids: Experiment and theory
Authors: Tavernier H. L. , Kalashnikov M. M. , Fayer M. D.
Published: July 2002

Photoinduced intermolecular electron transfer between Rhodamine 3B and N,N-dimethylaniline has been studied in a series of seven liquids: acetonitrile, ethanol, propylene glycol, and mixtures of ethanol, 2-butanol, ethylene glycol, propylene glycol, and glycerol. In each liquid, the donor and acceptors have different diffusion constants and experience distinct dielectric properties. Ps time-dependent fluorescence measurements and steady-state fluorescence yield measurements were made and analyzed using a detailed statistical mechanical theory that includes a distance-dependent Marcus rate constant, diffusion with the hydrodynamic effect, and solvent structure. All solvent-dependent parameters necessary for calculations were measured, including dielectric constants, diffusion constants, and redox potentials, leaving the electronic coupling unknown. Taking the distance-dependence of the coupling to be β=1 Å−1, data were fit to a single parameter, the coupling matrix element at contact, J0. The theory is able to reproduce both the functional form of the time-dependence and the concentration-dependence of the data in all seven liquids by fitting only J0. Despite the substantial differences in the properties of the experimental systems studied, fits to the data are very good and the values for J0 are very similar for all solvents.