bluuebunny/crossref_metadata_2025 huggingface dataset:
A growing string method for determining transition states: Comparison to the nudged elastic band and string methods
Authors: Peters Baron , Heyden Andreas , Bell Alexis T. , Chakraborty Arup
Published: April 2004

Interpolation methods such as the nudged elastic band and string methods are widely used for calculating minimum energy pathways and transition states for chemical reactions. Both methods require an initial guess for the reaction pathway. A poorly chosen initial guess can cause slow convergence, convergence to an incorrect pathway, or even failed electronic structure force calculations along the guessed pathway. This paper presents a growing string method that can find minimum energy pathways and transition states without the requirement of an initial guess for the pathway. The growing string begins as two string fragments, one associated with the reactants and the other with the products. Each string fragment is grown separately until the fragments converge. Once the two fragments join, the full string moves toward the minimum energy pathway according to the algorithm for the string method. This paper compares the growing string method to the string method and to the nudged elastic band method using the alanine dipeptide rearrangement as an example. In this example, for which the linearly interpolated guess is far from the minimum energy pathway, the growing string method finds the saddle point with significantly fewer electronic structure force calculations than the string method or the nudged elastic band method.
Measurement of orientation and alignment moment relaxation by polarization spectroscopy: Theory and experiment
Authors: Costen Matthew L. , Crichton Hilary J. , McKendrick Kenneth G.
Published: April 2004

A diagrammatic perturbation theory description of one-color polarization spectroscopy (PS) is developed which emphasizes the significance of orientation and alignment tensor moments of the rotational angular momentum, and their collisional evolution. The influences of Doppler motion, velocity-changing collisions, decay of population, orientation and alignment, and nuclear hyperfine depolarization on the calculated PS signal are discussed. Illustrative simulations are presented of the evolution of the PS signal as a function of pump–probe laser delay. These are generated by a Monte Carlo integration of the derived equations for the signal electric field over typical experimental pump and probe laser temporal profiles and velocity distributions for a commonly studied system, the OH A 2Σ+–X 2Π (0,0) band. These predictions are compared with a preliminary set of results obtained in an experimental apparatus designed for one-color polarization spectroscopy using independent pump and probe lasers. Measurements are presented using linearly polarized pump light on the Q1(2.5) transition of the OH A 2Σ+–X 2Π (0,0) band with He as the collision partner. The decay of the experimental PS pump–probe signal is discussed with reference to inelastic collisional population transfer rates in the literature. It is concluded that the collisional depolarization of rotational alignment is rapid, with a rate approximately twice that of population transfer. This is consistent with previous measurements in atmospheric pressure flames. PS is shown to be a viable novel spectroscopic method for determining rotational angular momentum orientation and alignment relaxation rates, which are valuable quantities because they are sensitive probes of the forces involved in inelastic collisions.
Electronic properties of CrF and CrCl in the X 6Σ+ state: Observation of the halogen hyperfine structure by Fourier transform microwave spectroscopy
Authors: Katoh Kaoru , Okabayashi Toshiaki , Tanimoto Mitsutoshi , Sumiyoshi Yoshihiro , Endo Yasuki
Published: April 2004

The rotational spectra of the CrF and CrCl radicals in the X 6Σ+ state were observed by employing a Fourier transform microwave spectrometer. The CrF and CrCl radicals were generated by the reaction of laser-ablated Cr with F2 and Cl2, respectively, diluted in Ar. A chromium rod made of chromium powder pasted with epoxy resin was ablated by a Nd:YAG laser. Rotational transitions were measured in the region between 8 and 26 GHz. Several hyperfine constants due to the halogen nuclei were determined by a least-squares analysis. The electronic properties of CrF and CrCl were derived from their hyperfine constants and were compared with those of other 3d transition metal monohalides: TiF, MnF, FeF, CoF, NiF, and FeCl.
Quantum model simulations of symmetry breaking and control of bond selective dissociation of FHF− using IR+UV laser pulses
Authors: Elghobashi Nadia , González Leticia , Manz Jörn
Published: April 2004

Symmetry breaking and control of bond selective dissociation can be achieved by means of ultrashort few-cycle-infrared (IR) and ultraviolet (UV) laser pulses. The mechanism is demonstrated for the oriented model system, FHF−, by nuclear wave packets which are propagated on two-dimensional potential energy surfaces calculated at the QCISD/d-aug-cc-pVTZ level of theory. The IR laser pulse is optimized to drive the wave packet coherently along alternate bonds. Next, a well-timed ultrashort UV laser pulse excites the wave packet, via photodetachment of the negative bihalide anion, to the bond selective domain of the neutral surface close to the transition state. The excited wave packet is then biased to evolve along the pre-excited bond toward the target product channel, rather than bifurcating in equal amounts. Comparison of the vibrational frequencies obtained within our model with harmonic and experimental frequencies indicates substantial anharmonicities and mode couplings which impose restrictions on the mechanism in the domain of ultrashort laser fields. Extended applications of the method to randomly oriented or to asymmetric systems XHY− are also discussed, implying the control of product directionality and competing bond-breaking.
Relaxation processes in mixtures of liquid crystals and polymers near phase boundaries and during phase separation
Authors: Freyssingeas E. , Graca M. , Wieczorek S. A. , Hołyst R.
Published: April 2004

We present experimental studies of the relaxation of concentration fluctuations in a semidilute solution of polystyrene (PS) (30% by weight) in 4-cyano-4′-n-octyl-biphenyl (8CB) (70% by weight) using the photon correlation spectroscopy (PCS). In the homogeneous phase there are two modes of relaxation. The slow one (typical time scale is τs=0.001 s) is due to the diffusion of polymer chains (of molecular mass 65 000) in the LC matrix (of molecular mass 290), while the fast one has the time scale of the order of τf≈0.00001 s. The amplitude of the fast mode is much weaker than the one for the slow mode. Moreover it does not depend on the scattering wave vector, q. The value of the diffusion coefficient, Dc=1/(τsq2) for the slow mode decreases with temperature according to the Arhenius law until we reach the coexistence curve. Its value close to the coexistence is Dc=4×105 nm2/s and the activation energy in the homogeneous mixture is Ec=127 kJ/mol. If we gradually undercool the mixture below the coexistence into the metastable two-phase region without inducing the phase separation we find unexpectedly that Dc does not change with temperature even 4° below the coexistence curve. The characteristic time of the fast mode does not depend on the scattering wave vector indicating that it is related to the transient gel structure. We have shown that it is possible to measure the short time relaxation of concentration fluctuations during the phase separation in the mixture. At low temperature close to the isotropic–nematic phase transition we have observed that the relaxation is well separated in time from the typical time of the domain growth. This relaxation mode is characterized by the large diffusion coefficient D=2×108 nm2/s. The mode probably comes from the coupling between the orientational dynamics of liquid crystals and the transient gel structure of polymers.
Simulations of time-dependent fluorescence in nano-confined solvents
Authors: Thompson Ward H.
Published: April 2004

The time-dependent fluorescence of a model diatomic molecule with a charge-transfer electronic transition in confined solvents has been simulated. The effect of confining the solvent is examined by comparing results for solutions contained within hydrophobic spherical cavities of varying size (radii of 10–20 Å). In previous work [J. Chem. Phys. 118, 6618 (2002)] it was found that the solute position in the cavity critically affects the absorption and fluorescence spectra and their dependence on cavity size. Here we examine the effect of cavity size on the time-dependent fluorescence, a common experimental probe of solvent dynamics. The present results confirm a prediction that motion of the solute in the cavity after excitation can be important in the time-dependent fluorescence. The effects of solvent density are also considered. The results are discussed in the context of interpreting time-dependent fluorescence measurements of confined solvent systems.
Positive electron affinity of fullerenes: Its effect and origin
Authors: Luo Ji , Peng L.-M. , Xue Z. Q. , Wu J. L.
Published: April 2004

The universal variation pattern of the total energy of various fullerenes including single-walled carbon nanotubes with respect to their extra charge is revealed by the density-functional-theory calculations. The parabolic energy-charge curve with its lowest energy value corresponding to a negatively charged fullerene indicates that these carbon materials have positive electron affinity and are not in the most stable state. The positive electron affinity seems to originate from the π-electrons and is found to be related to the aggregation property of fullerenes.
Extracting atoms from molecular electron densities via integral equations
Authors: Gilbert Andrew T. B. , Gill Peter M. W. , Taylor Stephen W.
Published: April 2004

The observation that a molecular electron density is close to the superposition of its constituent atoms leads naturally to the idea of modeling a density by a sum of nuclear-centered, spherically symmetric functions. The functions that are optimal in a least-squares sense are known as Stewart atoms. Previous attempts to construct Stewart atoms by expanding them in an auxiliary basis have been thwarted by slow convergence with respect to the size of the auxiliary basis used. We present a method for constructing Stewart atoms via convolution integrals which bypasses the need for an auxiliary basis, and is able to produce highly accurate approximations to Stewart atoms.
Adapting the nudged elastic band method for determining minimum-energy paths of chemical reactions in enzymes
Authors: Xie Li , Liu Haiyan , Yang Weitao
Published: April 2004

Optimization of reaction paths for enzymatic systems is a challenging problem because such systems have a very large number of degrees of freedom and many of these degrees are flexible. To meet this challenge, an efficient, robust and general approach is presented based on the well-known nudged elastic band reaction path optimization method with the following extensions: (1) soft spectator degrees of freedom are excluded from path definitions by using only inter-atomic distances corresponding to forming/breaking bonds in a reaction; (2) a general transformation of the distances is defined to treat multistep reactions without knowing the partitioning of steps in advance; (3) a multistage strategy, in which path optimizations are carried out for reference systems with gradually decreasing rigidity, is developed to maximize the opportunity of obtaining continuously changing environments along the path. We demonstrate the applicability of the approach using the acylation reaction of type A β-lactamase as an example. The reaction mechanism investigated involves four elementary reaction steps, eight forming/breaking bonds. We obtained a continuous minimum energy path without any assumption on reaction coordinates, or on the possible sequence or the concertedness of chemical events. We expect our approach to have general applicability in the modeling of enzymatic reactions with quantum mechanical/molecular mechanical models.
Onset of decoherence: Six-wave mixing measurements of vibrational decoherence on the excited electronic state of I2 in solid argon
Authors: Bihary Z. , Karavitis M. , Apkarian V. Ara
Published: April 2004

Pump–probe, four-wave, and six-wave mixing measurements of I2 isolated in solid argon are used to provide a clear experimental measure for the onset of vibrational quantum decoherence on the excited electronic state. The electronically resonant, six-wave mixing measurements bypass the rapid electronic dephasing, and measure the quantum cross-correlation between two packets launched on the B-state. The vibrational quantum coherence survives one period of motion, 400 fs, during which ∼2000 cm−1 of energy is transferred to the lattice. The decoherence occurs during the second cycle of motion, while classically coherent motion measured via pump–probe spectroscopy using the same electronic resonances continues for ∼15 periods. This is contrasted with vibrational dephasing on the ground electronic surface, which lasts for 102 periods, as measured through time-resolved coherent anti-Stokes Raman scattering. The measurements and observables are discussed through time-circuit diagrams, and a mechanistic description of decoherence is derived through semiclassical analysis and simulations that reproduce the experiments.
Density and energy relaxation in an open one-dimensional system
Authors: Jose Prasanth P. , Bagchi Biman
Published: April 2004

A new master equation to mimic the dynamics of a collection of interacting random walkers in an open system is proposed and solved numerically. In this model, the random walkers interact through excluded volume interaction (single-file system); and the total number of walkers in the lattice can fluctuate because of exchange with a bath. In addition, the movement of the random walkers is biased by an external perturbation. Two models for the latter are considered: (1) an inverse potential (V∝1/r), where r is the distance between the center of the perturbation and the random walker and (2) an inverse of sixth power potential (V∝1/r6). The calculated density of the walkers and the total energy show interesting dynamics. When the size of the system is comparable to the range of the perturbing field, the energy relaxation is found to be highly nonexponential. In this range, the system can show stretched exponential (e−(t/τs)β) and even logarithmic time dependence of energy relaxation over a limited range of time. Introduction of density exchange in the lattice markedly weakens this nonexponentiality of the relaxation function, irrespective of the nature of perturbation.
The role of symmetry and optical selection rules in revealing the molecular structure of the lowest Rydberg and ionic states of the 1,4-diazabicyclo[2.2.2]octane–Arn (n=1,2,3) van der Waals complexes
Authors: Belcher David E. , Watkins Mark J. , Tonge Nicola , Cockett Martin C. R.
Published: April 2004

The 1,4-diazabicyclo[2.2.2]octane–Arn (n=1,2,3) van der Waals complexes (DABCO–Arn) have been investigated using a combination of (1+1′) resonance enhanced multiphoton ionization (REMPI) and zero electron kinetic energy (ZEKE) spectroscopy. The additivity of the spectral shifts observed in both REMPI and ZEKE spectra, taken together with analysis of vibrational structure, suggest that in both DABCO–Ar and DABCO–Ar2 the argon atoms bind in equivalent equatorial (face) locations between two adjacent (CH2)2 bridges. However, the cumulative evidence from both REMPI and ZEKE spectra, together with ab initio results, suggests that the DABCO–Ar3 complex does not revert to D3h symmetry, but rather adopts a C2v structure in which all three argon atoms bind to one side of the DABCO framework. The exceptionally low wave-number vibrational structure observed in the REMPI spectra suggest that the van der Waals interaction in the excited state is extremely weak. However, ionization necessarily increases the strength of the interaction by virtue of the introduction of charge-induced dipole forces, as revealed by a consistent increase in vibrational wave numbers of the modes observed in the resultant ZEKE spectra.
Ab initio study on structural and electronic properties of BanOm clusters
Authors: Chen G. , Liu Z. F. , Gong X. G.
Published: April 2004

Density-functional calculation within local density approximation, shows that the electronic property of a barium oxide cluster is strongly correlated with its equilibrium structure. The ground-state structures of BanOm (4⩽n⩽9,m⩽n) clusters can be classified into four categories: (a) compact, (b) dangling state, (c) F—center, and (d) stoichiometric. The compact cluster is metallic, almost no energy gap exists between the highest occupied and the lowest unoccupied molecular orbitals. The energy gap for the dangling state cluster is larger than that for the F—center cluster, while the stoichiometric cluster has the largest energy gap.
Mass analyzed threshold ionization spectroscopy of p-fluorostyrene
Authors: Georgiev S. , Neusser H. J. , Chakraborty Tapas
Published: April 2004

Adiabatic ionization energy (AIE) and two-color threshold ion vibrational spectra of p-fluorostyrene have been measured by mass analyzed threshold ionization (MATI) method via three different intermediate levels in the first excited state, vibrationless S1 origin, 421411, and 231 vibronic levels. Features of the ion vibrational spectra indicates that the geometry of the molecular ion including the conformation of the vinyl chain in the ionic ground state (D0) is almost identical to that of its neutral ground state (S0), and ionization has very little effect on the vibrational potentials of the aromatic ring modes. Comparison of the AIE with the reported value of styrene shows that fluorination at the para position of the aromatic ring has little effect on energy of the electron ejected in ionization process from the styrene chromophore.
Metal-nanoparticle single-electron transistors fabricated using electromigration
Authors: Bolotin K. I. , Kuemmeth F. , Pasupathy A. N. , Ralph D. C.
Published: April 2004

We have fabricated single-electron transistors from individual metal nanoparticles using a geometry that provides improved coupling between the particle and the gate electrode. This is accomplished by incorporating a nanoparticle into a gap created between two electrodes using electromigration, all on top of an oxidized aluminum gate. We achieve sufficient gate coupling to access more than ten charge states of individual gold nanoparticles (5–15 nm in diameter). The devices are sufficiently stable to permit spectroscopic studies of the electron-in-a-box level spectra within the nanoparticle as its charge state is varied.
Yield of electronically excited N2 molecules from the dissociative recombination of N2H+ with e−
Authors: Rosati Richard E. , Johnsen Rainer , Golde Michael F.
Published: April 2004

Quantitative spectroscopic observations of the N2 first positive band system (N2(B 3Πg−A 3Σu+))/electron in a recombining N2H+ flowing-afterglow plasma indicate that a substantial fraction of the product N2 molecules are formed in one or more of the low-lying triplet states, B 3Πg, A 3Σu+, and W 3Δu. The total measured N2(B−A) emission intensity from N2(B,v′⩾1) is equivalent to a yield of (19±8)%. The effect of rapid collision-induced transitions between states of the triplet manifold is discussed.
Crystal nucleation for a model of globular proteins
Authors: Shiryayev Andrey , Gunton James D.
Published: April 2004

A continuum model of globular proteins proposed by Talanquer and Oxtoby [J. Chem. Phys. 109, 223 (1998)] is investigated numerically, with particular emphasis on the region near the metastable fluid–fluid coexistence curve. Classical nucleation theory is shown to be invalid not only in the vicinity of the metastable critical point but also close to the liquidus line. An approximate analytic solution is also presented for the shape and properties of the nucleating crystal droplet.
A path integral influence functional for excess electron in fluids: Density-functional formulation
Authors: Sumi Tomonari , Sekino Hideo
Published: April 2004

In this paper, we propose a path integral influence functional from a solvent to determine a self-correlation function of a quantum particle in classical simple fluid. It is shown that the influence functional is related to a grand potential functional of the pure solvent under a three-dimensional external field arising from a classical isomorphic polymer, on which the quantum particle is mapped. The influence functional can be calculated from the self-correlation function, the solute–solvent and the solvent–solvent pair correlation function. The obtained equation of the self-correlation function is applied to an excess electron problem in fluid helium. The Fourier path-integral Monte Carlo method is employed to perform the path integral of the electron. The solute–solvent pair correlation function is estimated from a reference interaction site model integral equation. These results obtained form our proposed influence functional and from that proposed by Chandler, Singh, and Richardson are compared with those provided by a path integral Monte Carlo simulation with the explicit helium solvent.
Rapid fluorescence quenching of S2-xanthione by 3,3-diethylpentane in perfluorohydrocarbons
Authors: Krystkowiak Ewa
Published: April 2004

Rapid fluorescence quenching of S2-xanthione by 3,3-diethylpentane has been studied in three perfluorohydrocarbons of different viscosities. The donor fluorescence decay in the presence of a quencher was fitted using the Smoluchowski–Collins–Kimball (SCK) model. The molecular parameters, R (the sum of the molecular radii), D (the sum of diffusion coefficients), and the specific rate constant of the process, κ, were determined. The values of parameter D for all systems studied differed from the sum of the macroscopic diffusion coefficients Dx measured independently. This behavior is explained by the dependence of the molecular diffusion coefficient (as determined from the SCK model) on the distance traveled by the donor and quencher molecules during the excited donor lifetime.
Electrical characterization of zinc oxide thin films by electrochemical capacitance–voltage profiling
Authors: Tang X. , Clauzonnier A. , Campbell H. I. , Prior K. A. , Cavenett B. C.
Published: April 2004

The growth of ZnO by epitaxial techniques is important for the development of new optoelectronic devices. In order to establish control over the growth, it is important to be able to measure dopant profiles through the layers to supplement Hall effect data. A standard method for many semiconductors is by electrochemical capacitance–voltage (C–V) profiling. In this letter, C–V profiles from metal organic chemical vapor deposition grown ZnO thin films are presented. It is shown that carrier density profiles can be obtained routinely and reproducibly by using 0.1 M ZnCl2.
Bulk and interfacial properties of binary polymer mixtures
Authors: Bryk P. , Sokołowski S.
Published: April 2004

A microscopic density functional theory is used to investigate a binary mixture of polymers, built of freely jointed tangent hard spheres. The difference in the chain length and in the segment diameter of polymers gives rise to a demixing transition. We evaluate the bulk fluid phase equilibria (binodal) and the limit of stability of a mixed state (spinodal) for selected systems, and analyze the decay of the critical packing fraction, critical mole fraction, and critical pressure with an increase of the chain length. The bulk results are subsequently used in the calculations of the density profiles across the fluid–fluid interface. The obtained profiles are smooth and do not exhibit any oscillations on the length scale of the segment diameter. Upon approaching the critical point the interfacial tension vanishes as (Δρ)3, where Δρ is the difference between bulk densities of one component in bulk phases rich and poor in that species. This indicates that the microscopic density functional theory applied here is of a mean-field type.
Tunable distributed feedback color center laser using stabilized F2+** color centers in LiF crystal
Authors: Martyshkin D. V. , Parker J. G. , Fedorov V. V. , Mirov S. B.
Published: April 2004

We report a room temperature tunable color center distributed feedback (DFB) laser using stabilized F2+** centers in LiF (LiF:F2+**) as a gain medium. Tunable oscillation of LiF:F2+** DFB lasing was achieved in the near infrared region (882–962 nm) with a lasing linewidth of less than 0.2 cm−1. Also the lasing threshold and slope efficiency with respect to pump energy were found to be 1.2 mJ and up to 3%, respectively.
Phase locking of a two-dimensional laser array by controlling the far-field pattern
Authors: Zhou Yi , Liu Liping , Etson Candice , Abranyos Yonatan , Padilla Angela , Chen Y. C.
Published: April 2004

We report phase locking of a two-dimensional laser array through beam control in the far field of the emitters. Compared to the near-field techniques, the far-field approach involves simpler beam profiles and does not rely on having a large number of emitters in a perfectly periodic structure. Our experimental results and computer simulation also point to much higher feedback efficiency, less sensitivity to power variations among the emitters, and the ability to repair a missing element.
Exploration of artificial multiferroic thin-film heterostructures using composition spreads
Authors: Chang K.-S. , Aronova M. A. , Lin C.-L. , Murakami M. , Yu M.-H. , Hattrick-Simpers J. , Famodu O. O. , Lee S. Y. , Ramesh R. , Wuttig M. , Takeuchi I. , Gao C. , Bendersky L. A.
Published: April 2004

We have fabricated a series of composition spreads consisting of ferroelectric BaTiO3 and piezomagnetic CoFe2O4 layers of varying thicknesses modulated at nanometer level in order to explore artificial magnetoelectric thin-film heterostructures. Scanning microwave microscopy and scanning superconducting quantum interference device microscopy were used to map the dielectric and magnetic properties as a function of continuously changing average composition across the spreads, respectively. Compositions in the middle of the spreads were found to exhibit ferromagnetism while displaying a dielectric constant as high as ≈120.
Enhanced electromechanical properties in all-polymer percolative composites
Authors: Li Jiang Yu , Huang Cheng , Zhang Qiming
Published: April 2004

In this letter, we discuss the enhanced electrostriction and dielectric constant in an all-polymer percolative composite consisting of poly(vinylidene fluoride-trifluoroethylenechlorotrifluoroethylene) [P(VDF–TrFE–CTFE)] terpolymer matrix and polyaniline (PANI) conductive particles. Using a self-consistent approach, we calculate the dielectric constant and dielectric loss tangent of the composite in excellent agreement with experiments, and demonstrate that the electrostriction enhancement is due to the electric field fluctuation in P(VDF–TrFE–CTFE) matrix, which becomes dominant when the dielectric constant of the second phase is much larger than that of the matrix. The inhomogeneous field distribution in the matrix has also been used to estimate the breakdown field of the composite, which agrees well with experimental measurement. The study could be used to design and optimize electrostrictive composites with optimal electromechanical properties.
Interfacial reactions of Mn/GaAs thin films
Authors: Hilton J. L. , Schultz B. D. , McKernan S. , Palmstrøm C. J.
Published: April 2004

Mn thin films were deposited in situ on molecular beam epitaxy grown GaAs(001) surfaces at 0 °C. Postgrowth anneals of the Mn/GaAs samples were done at 200, 300, and 400 °C for times ranging from 0.5 to 30 h. Rutherford backscattering spectrometry, x-ray diffraction, and transmission electron microscopy show that for samples annealed at 300 °C the interfacial reactions initially result in the formation of an epitaxial two phase region (Mn2As and MnGa) with an average composition of Mn0.6Ga0.2As0.2. The rate of reaction between the Mn and GaAs shows a square root of time dependence, indicating that the reactions are diffusion limited.
Transport mechanisms in atomic-layer-deposited Al2O3 dielectrics
Authors: Specht M. , Städele M. , Jakschik S. , Schröder U.
Published: April 2004

We analyze the field and temperature dependence of electron currents through atomic-layer-deposited thin (3.6–6 nm) sheets of Al2O3 which were annealed above the crystallization temperature. On the basis of electrical characterization and numerical simulation that includes trap-assisted transport as well as the band bending in the contact regions, we have identified three characteristic field regions in which the currents are dominated by elastic trap-assisted tunneling, Frenkel–Poole hopping, or Fowler–Nordheim tunneling. We find that the Frenkel–Poole traps lie in a narrow band about 1.2 eV below the conduction band minimum of Al2O3, whereas the energetic distribution of the elastic traps is broad and has a tail that reaches far into the band gap. The numerical results are compatible with a Si/Al2O3 conduction band offset of 2.7 eV.
Surface band bending in as-grown and plasma-treated n-type GaN films using surface potential electric force microscopy
Authors: Cho Sang-Jun , Doğan Seydi , Sabuktagin Shahriar , Reshchikov Michael A. , Johnstone Daniel K. , Morkoç Hadis
Published: April 2004

The surface band bending, as well as the effect of plasma-induced damage on band bending, on GaN surfaces, was investigated. The upward band bending, measured by surface potential electric force microscopy (a variant of atomic force microscopy), for the as-grown n-type GaN was about 1.0 eV which increased to ∼1.4 eV after reactive ion etching (RIE). UV illumination decreased the band bending by 0.3 eV with time constants on the order of seconds and hundreds of seconds for the as-grown and RIE treated GaN, respectively. This implies that there is a higher density of the surface states in the samples subjected to the RIE process. After the RIE treatment, the shape of the photoluminescence spectrum remained unchanged, but the intensity dropped by a factor of 3. This effect can be attributed to nonradiative defects created near the surface by the RIE treatment.
Continuous-wave second-harmonic generation in modal phase matched semiconductor waveguides
Authors: Ducci S. , Lanco L. , Berger V. , De Rossi A. , Ortiz V. , Calligaro M.
Published: April 2004

We report the observation of continuous-wave second-harmonic generation in a modalphase-matched GaAs/AlGaAs waveguide. The heterostructure has been designed to generate a second-harmonic signal via a type-II process with a fundamental signal around 1.55 μm. Continuous wave conversion enables a quantitative estimation of the conversion efficiency. In our case, η=30%±5% W−1 cm−2 is found experimentally.
Gradual facet degradation of (Al,In)GaN quantum well lasers
Authors: Kümmler V. , Lell A. , Härle V. , Schwarz U. T. , Schoedl T. , Wegscheider W.
Published: April 2004

In our study, III-nitride laser diodes with uncoated facets obtained by cleavage show a much faster degradation than coated ones. An increase in threshold current and drop of slope efficiency suggest increased absorption losses. Degradation experiments in different atmospheres prove the influence of the respective atmosphere and indicate the growth of an oxide film leading to increased absorption. Because the observed degradation is insensitive to the photon density we suggest nonradiative centers, which are saturated at low photon densities, to be at the origin of degradation. No evidence for photon enhanced degradation of coated laser diodes was found. A dielectric coating efficiently protects the facets.
High-resolution imaging of cardiac biomagnetic fields using a low-transition-temperature superconducting quantum interference device microscope
Authors: Fong L. E. , Holzer J. R. , McBride K. , Lima E. A. , Baudenbacher F. , Radparvar M.
Published: April 2004

We have developed a multiloop low-temperature superconducting quantum interference device sensor with a field sensitivity of 450 fT/Hz−1/2 for imaging biomagnetic fields generated by action currents in cardiac tissue. The sensor has a diameter of 250 μm and can be brought to within 100 μm of a room-temperature sample. Magnetic fields generated by planar excitation waves are associated with a current component parallel to the wave front, in agreement with predictions of the bidomain model. Our findings provide a new basis for interpreting the magnetocardiogram.
Effect of impurities on the mobility of single crystal pentacene
Authors: Jurchescu Oana D. , Baas Jacob , Palstra Thomas T. M.
Published: April 2004

We have obtained a hole mobility for the organic conductor pentacene of μ=35 cm2/V s at room temperature increasing to μ=58 cm2/V s at 225 K. These high mobilities result from a purification process in which 6,13-pentacenequinone was removed by vacuum sublimation. The number of traps is reduced by two orders of magnitude compared with conventional methods. The temperature dependence of the mobility is consistent with the band model for electronic transport.
Band alignment at the i-ZnO/CdS interface in Cu(In,Ga)(S,Se)2 thin-film solar cells
Authors: Weinhardt L. , Heske C. , Umbach E. , Niesen T. P. , Visbeck S. , Karg F.
Published: April 2004

The interface between the i-ZnO layer and the CdS buffer in Cu(In,Ga)(S,Se)2 thin-film solar cells from the Shell Solar baseline process has been investigated using ultraviolet- and x-ray photoelectron spectroscopy and inverse photoemission. Combining both techniques, a direct determination of the conduction and valence band offsets at the interface is possible. Different from existing models, we find a flat conduction band alignment (i.e., a conduction band offset of 0.10±0.15 eV), ∼0.5 eV above the Fermi level, and a valence band offset of 0.96±0.15 eV.
Strong elasticoluminescence from monoclinic-structure SrAl2O4
Authors: Xu Chao-Nan , Yamada Hiroshi , Wang Xusheng , Zheng Xu-Guang
Published: April 2004

Elastico-deformation luminescence in strontium aluminates was investigated systematically using precisely controlled pure-phase Eu-doped strontium aluminates of SrAl12O19, Sr4Al14O25, SrAl4O7, α-SrAl2O4, β-SrAl2O4, Sr3Al2O6 and their mixed phases. This study revealed that only the α-SrAl2O4 phase produces strong elastico-deformation luminescence; other strontium aluminates show no deformation luminescence. Correlation of deformation luminescence and crystal structure was found. The α-SrAl2O4 has the lowest symmetry, crystallizing in a monoclinic structure. This finding can be applied in designing strong elastico-deformation-luminescent materials.
Effect of tungsten implantation on electron field emission properties of ion-beam-synthesized SiC layers
Authors: Tsang W. M. , Wong S. P. , Lindner J. K. N.
Published: April 2004

The effect of tungsten implantation on the electron field emission (FE) properties of ion-beam-synthesized (IBS) SiC/Si heterostructures was studied and correlated to their microstructures and surface morphology. The turn-on field of the IBS SiC layers is improved from 25 to 14 V/μm after W implantation and is further decreased to 10 V/μm after thermal annealing at 1200 °C for 1 h. The improvements in FE properties after W implantation can be attributed to the formation of WC as well as two types of local field enhancement mechanisms. In the as-implanted sample, the local field enhancement effect is attributed to electrical inhomogeneity due to the formation of conducting WC nanoclusters embedded in the SiC layers. In the annealed samples, the dominated field enhancement is attributed to a surface morphology effect due to the formation of small protrusion structures at the surface.
The role of the InGaAs surface in selective area epitaxy of quantum dots by indium segregation
Authors: Yeoh T. S. , Swint R. B. , Elarde V. C. , Coleman J. J.
Published: April 2004

The surface of strained InGaAs films for selective regrowth of InAs nanostructures is investigated by atomic force microscopy and Rutherford backscattering. 3.3-nm-thick In0.33Ga0.67As films were annealed at temperatures between 400 and 800 °C. Significant indium desorption was found to occur at temperatures above 550 °C. The optimum parameters are presented for selective growth of InAs quantum dots having densities of 6.6×1010 cm−2 on In0.33Ga0.67As films.
Coulomb oscillations based on band-to-band tunneling in a degenerately doped silicon metal-oxide-semiconductor field-effect transistor
Authors: Kim Kyung Rok , Kim Dae Hwan , Lee Jong Duk , Park Byung-Gook
Published: April 2004

We report Coulomb oscillations based on band-to-band tunneling through a valence band in silicon metal-oxide-semiconductor field-effect transistors. Degenerately p+-doped channel and n+-doped source/drain enables band-to-band tunneling, which can play a major role in the transport between the channel and source/drain. The formation of tunnel barriers and a quantum dot in a single-electron transistor structure originates from two p+–n+ tunnel junctions and a p+-doped channel with mesoscopic dimension, respectively. Coulomb-blockade oscillations with multiple peaks were clearly observed at liquid nitrogen temperature. Using the electrical and thermal characterization of the quantum dot, single-electron charging effect based on band-to-band tunneling is confirmed.
Deep-ultraviolet emission from an InGaAs semiconductor laser
Authors: Wang S. M. , Shen Y. H. , Xu J. X. , Hu L. G. , Zhu J. , Yang D. R. , Zhang H. , Zeng Y. W. , Yao J. Q.
Published: April 2004

It is reported that a normal InGaAs laser diode (LD) operating at 980 nm possesses a second harmonic at 490 nm and a strong deep-UV emission at room temperature. By comparing with the radiation from cadmium sulphide (CdS) nanoparticles and nanowires, it is found that the UV emission from the LD can be attributed to a characterized radiation of nanoscaled semiconductors. By doping different semiconductor nanomaterials into the active layers of the LD and taking some effective techniques, such as cavity optimization, the wavelength, and the power output of the UV emission could be adjusted.
Simulation and design of GaN/AlGaN far-infrared (λ∼34 μm) quantum-cascade laser
Authors: Jovanović V. D. , Indjin D. , Ikonić Z. , Harrison P.
Published: April 2004

Designs of GaN/AlGaN quantum-cascade lasers emitting at 34 and 38 μm (ΔE∼36 and 34 meV) are presented, assuming either a- or c-plane crystal growth orientation. In the calculation of the quasibound state energies and wave functions, we account for the intrinsic electric field induced by piezoelectric and (in case of c-plane growth) the spontaneous polarization. The quantum-cascade structures were simulated, and their output characteristics extracted, using a fully self-consistent rate equation model with all relevant intra- and interperiod scatterings included. Both electron–LO-phonon and electron–electron scattering mechanisms are taken into account. Maximal population inversions between active laser states of up to 19% for the a-plane, and up to 40% for the c-plane design, are predicted and, based on estimated modal gain and waveguide/mirror losses in suitably designed structures, these indicate the feasibility of laser action in GaN/AlGaN cascades.
Comparison of the electrical activation of P+ and N+ ions co-implanted along with Si+ or C+ ions into 4H-SiC
Authors: Schmid F. , Pensl G.
Published: April 2004

The annealing behavior of P+ and N+ ions implanted into p-type 4H-SiC epilayers is studied by a temperature-dependent Hall-effect. Detailed investigations reveal that the electrical activation of implanted P+ ions is governed by the site competition effect, while implanted N+ ions react with intrinsic defects during the annealing step and can form thermally stable and electrically inactive complexes. In this way, the electrical activation of implanted N+ ions is strongly reduced.
Polarization switching of submicron ferroelectric capacitors using an atomic force microscope
Authors: Prasertchoung S. , Nagarajan V. , Ma Z. , Ramesh R. , Cross J. S. , Tsukada M.
Published: April 2004

We report on the measurement of switchable pulse polarization of micron and submicron ferroelectric capacitors contacted using an atomic force microscope. Fast square pulses with rise time on the order of tens of nanoseconds are used to obtain the switchable polarization (ΔP) of discrete polycrystalline Pb(ZrTi)O3 capacitors of 21.5, 0.69, and 0.19 μm2 prepared by sputtering and reactive ion etching. Our studies show that the switching characteristics of these capacitors are well behaved, indicating that high-speed and high-density ferroelectric memory capacitors are not limited by scaling down the capacitor area.
Anomalous hot-carrier-induced degradation of offset gated polycrystalline silicon thin-film transistors
Authors: Hatzopoulos A. , Dimitriadis C. A. , Pananakakis G. , Ghibaudo G. , Kamarinos G.
Published: April 2004

Hot-carrier effects in offset gated n-channel polycrystalline silicon thin-film transistors of channel length L=10 μm and intrinsic offset lengths ΔL=0.5 and 1 μm are investigated. The gate- and drain-bias conditions for maximum device degradation were determined from substrate current measurements. The experimental data show that hot-carrier stress provokes an anomalous threshold voltage and on-state current degradation, exhibiting a “staircase-like” degradation with stress time. These results lead to the conclusion that, at the initial stages of stress, a small offset region from the drain end is damaged due to charging of the grain boundaries. As the stress proceeds further and the grain boundary traps are filled with electrons generated by impact ionization, the damage is transferred to the neighboring offset region, resulting in a staircase-like degradation of the device parameters with stress time.
Scattering mechanisms in high-mobility strained Ge channels
Authors: Rössner B. , Chrastina D. , Isella G. , von Känel H.
Published: April 2004

We report on the low-temperature mobility in remotely doped p-type strained Ge layers on relaxed Si0.3Ge0.7 virtual substrates, grown by low-energy plasma-enhanced chemical vapor deposition. A maximum mobility of 120 000 cm2 V−1 s−1 has been reached at 2 K, at a carrier sheet density of 8.5×1011 cm−2. Analysis of the mobility and Dingle ratio τ/τq as a function of sheet density suggests that remote impurity scattering is the limiting factor at low sheet densities, but that interface impurities become more important as the sheet density increases.
Diagnostics of doping integrity in n+/p/n+ transistor-channel structure by scanning nonlinear dielectric microscopy
Authors: Matsukawa Takashi , Yasumuro Chiaki , Masahara Meishoku , Tanoue Hisao , Kanemaru Seigo
Published: April 2004

Scanning nonlinear dielectric microscopy was used to diagnose doping integrity in a transistor channel. The carrier state at various points in a pn junction was defined as n-type, p-type, or depletion through nonlinear capacitance (dC/dV) profiling and pinpoint capacitance–voltage analysis. Carrier state analysis was applied to the n+/p/n+ transistor channel of structures with different process parameters. An increase in the n+ activation temperature from 800 to 950 °C caused shrinkage in channel length of the p-type region. Decreasing the substrate acceptor concentration from 1018 to 1017 cm−3 caused depletion of the entire channel when the gate length was less than 200 nm.
Positive temperature coefficient resistance and humidity sensing properties of Cd-doped ZnO nanowires
Authors: Wan Q. , Li Q. H. , Chen Y. J. , Wang T. H. , He X. L. , Gao X. G. , Li J. P.
Published: April 2004

Cd-doped ZnO nanowires in mass production were synthesized by evaporating metal zinc (Zn) and cadmium (Cd) at 900 °C. Devices using the synthesized nanowires were fabricated on microstructured substrates. Cd-doped ZnO nanowires show a clear positive temperature coefficient of resistance effect, which is quite abnormal as compared to pure ZnO nanowires. At room temperature, resistance change of more than three orders of magnitude was measured when Cd-doped ZnO nanowire device was exposed to a moisture pulse of 95% relative humidity.
Femtosecond studies of electron capture times in InGaN/GaN multiple quantum wells
Authors: Fan W. H. , Olaizola S. M. , Wells J.-P. R. , Fox A. M. , Wang T. , Parbrook P. J. , Mowbray D. J. , Skolnick M. S.
Published: April 2004

Subpicosecond time-resolved differential transmission spectroscopy has been used to investigate the carrier density and temperature dependence of the quantum well electron capture time of blue-emitting InGaN/GaN multiple quantum well structures. It is found that the capture time varies significantly with both temperature and carrier density, the latter effect being consistent with carrier-induced band bending or increased carrier–carrier scattering. At room temperature, the electron capture time is in the range 0.4–0.8 ps for carrier densities ⩽5×1018 cm−3.
Spin momentum transfer in current perpendicular to the plane spin valves
Authors: Covington M. , Rebei A. , Parker G. J. , Seigler M. A.
Published: April 2004

We present experimental and numerical micromagnetic data on the effect of spin momentum transfer in current perpendicular to the plane spin valves. Starting from a configuration with orthogonal free- and pinned-layer magnetizations, the free-layer magnetization exhibits abrupt current-induced switching that is qualitatively consistent with the spin torque model. When operating the spin valve as a field sensor, spin transfer can produce a change in resistance that mimics an effective magnetic field and induce magnetic instability that requires a larger bias field in order to stabilize the device.
Observation of spin-transfer switching in deep submicron-sized and low-resistance magnetic tunnel junctions
Authors: Huai Yiming , Albert Frank , Nguyen Paul , Pakala Mahendra , Valet Thierry
Published: April 2004

The spin-transfer effect has been studied in magnetic tunnel junctions (PtMn/CoFe/Ru/CoFe/Al2O3/CoFe/NiFe) with dimensions down to 0.1×0.2 μm2 and resistance–area product RA in the range of 0.5–10 Ω μm2 (ΔR/R=1%–20%). Current-induced magnetization switching is observed with a critical current density of about 8×106 A/cm2. The attribution of the switching to the spin-transfer effect is supported by a current-induced ΔR/R value identical to the one obtained from the R versus H measurements. Furthermore, the critical switching current density has clear dependence on the applied magnetic field, consistent with what has been observed previously in the case of spin-transfer-induced switching in metallic multilayer systems.
Low-temperature fabrication of carbon nanofibers by self-assembling of polycyclic aromatic hydrocarbon molecules
Authors: Suen Shich-Chang , Whang Wha-Tzong , Wu Bo-Wei , Lai Yi-Fan
Published: April 2004

Carbon nanofibers (CNFs) with diameter ∼66 nm have been synthesized from coronene by low-temperature (∼45 °C) vacuum sublimation without the aid of a catalyst. Our method makes use of the property of polycyclic aromatic hydrocarbon (PAH) molecules, which can self-assemble into columnar aggregates. This polycyclic aromatic hydrocarbon-carbon nanofibers (PAH-CNFs) reveal much better thermal stability than commercial carbon nanotubes and exhibit field-emission characteristics with the onset of an electric field of 5.4 V/μm and field enhancement factor of 1326 cm−1 for 100 μm interelectrode distance. The relative high thermal stability and easier process open up a possibility to fabricate large-scale field-emission devices in an efficient way and to provide a broad range of applications in nanoscience and technology.