bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical study of finite-temperature spectroscopy in van der Waals clusters. I. Probing phase changes in CaArn
Authors: Calvo F. , Spiegelman F. , Heitz M.-C.
Published: May 2003

The photoabsorption spectra of calcium-doped argon clusters CaArn are investigated at thermal equilibrium using a variety of theoretical and numerical tools. The influence of temperature on the absorption spectra is estimated using the quantum superposition method for a variety of cluster sizes in the range 6⩽n⩽146. At the harmonic level of approximation, the absorption intensity is calculated through an extension of the Gaussian theory by Wadi and Pollak [J. Chem. Phys. 110, 11890 (1999)]. This theory is tested on simple, few-atom systems in both the classical and quantum regimes for which highly accurate Monte Carlo data can be obtained. By incorporating quantum anharmonic corrections to the partition functions and respective weights of the isomers, we show that the superposition method can correctly describe the finite-temperature spectroscopic properties of CaArn systems. The use of the absorption spectrum as a possible probe of isomerization or phase changes in the argon cluster is discussed at the light of finite-size effects.
Theoretical study of finite-temperature spectroscopy in van der Waals clusters. II. Time-dependent absorption spectra
Authors: Calvo F. , Spiegelman F. , Wales D. J.
Published: May 2003

Using approximate partition functions and a master equation approach, we investigate the statistical relaxation toward equilibrium in selected CaArn clusters. The Gaussian theory of absorption [J. Chem. Phys., 118, 8739 (2003)] is employed to calculate the average photoabsorption intensity associated with the 4s2→4s14p1 transition of calcium as a function of time during relaxation. In CaAr6 and CaAr10 simple relaxation is observed with a single time scale. CaAr13 exhibits much slower dynamics, and the relaxation occurs over two distinct time scales. CaAr37 shows even slower relaxation with multiple transients, reminiscent of glassy behavior due to competition between different low-energy structures. We interpret these results in terms of the underlying potential energy surfaces for these clusters.
Theoretical study of the finite-temperature spectroscopy in van der Waals clusters. III. Solvated chromophore as an effective diatomics
Authors: Calvo F. , Spiegelman F. , Mestdagh J.-M.
Published: May 2003

The absorption spectroscopy of calcium-doped argon clusters is described in terms of an effective diatomic molecule Ca–(Arn), in the framework of semiclassical vertical transitions. We show how, upon choosing a suitable reaction coordinate, the effective finite-temperature equilibrium properties can be obtained for the ground and excited surfaces from the potential of mean force (PMF). An extension of the recent multiple range random-walk method is used to calculate the PMF over continuous intervals of distances. The absorption spectra calculated using this single-coordinate description are found to be in good agreement with the spectra obtained from high-statistics Monte Carlo data, in various situations. For CaAr13, we compare the performances of two different choices of the reaction coordinate. For CaAr37, the method is seen to be accurate enough to distinguish between different low-energy structures. Finally, the idea of casting the initial many-body problem into a single degree of freedom problem is tested on the spectroscopy of calcium in bulk solid argon.
Nuclear magnetic resonance chemical shifts with the statistical average of orbital-dependent model potentials in Kohn–Sham density functional theory
Authors: Poater Jordi , van Lenthe Erik , Baerends Evert Jan
Published: May 2003

In this paper, an orbital-dependent Kohn–Sham exchange-correlation potential, the so-called statistical average of (model) orbital potentials, is applied to the calculation of nuclear magnetic resonance chemical shifts of a series of simple molecules containing H, C, N, O, and F. It is shown that the use of this model potential leads to isotropic chemical shifts which are substantially improved over both local and gradient-corrected functionals, especially for nitrogen and oxygen atoms. This improvement in the chemical shift calculations can be attributed to the increase in the gap between highest occupied and lowest unoccupied orbitals, thus correcting the excessively large paramagnetic contributions, which have been identified to give deficient chemical shifts with both the local-density approximation and with gradient-corrected functionals. This is in keeping with the improvement by the statitical average of orbital model potentials for response properties in general and for excitation energies in particular. The present results are comparable in accuracy to those previously reported with self-interaction corrected functionals by Patchovskii et al., but still inferior to those obtained with accurate Kohn–Sham potentials by Wilson and Tozer. However, the present approach is computationally expedient and routinely applicable to all systems, requiring virtually the same computational effort as local-density and generalized-gradient calculations.
Theory of adiabatic hexaamminecobalt self-exchange
Authors: Endres R. G. , LaBute M. X. , Cox D. L.
Published: May 2003

We have reexamined the thermally induced Co(NH3)62+/3+ [Co(II/III)] redox reaction using the first-principles density-functional-theory method, semiclassical Marcus theory, and known charge transfer parameters. We find support for a previously suggested mechanism involving the excited state (2Eg) of Co(II) which becomes lower than the ground state (4T1g) in the transition state region. This lowers the transition-state barrier considerably by ∼6.9 kcal/mol and leads to a spin-allowed and adiabatic electron exchange process. Our calculations are consistent with previous experimental results regarding the spin-excitation energy (3T1g) of Co(III) and the fact that an optical absorption peak (2Eg) of the Co(II) species could not be found experimentally. Our rate is of order 6×10−3 (M s)−1 and hence two orders of magnitude faster than determined previously by experiments.
Phase behavior of a lattice protein model
Authors: Combe Nicolas , Frenkel Daan
Published: May 2003

We present a numerical simulation of the phase behavior of a simple model for a protein solution. We find that this system can occur in three phases, namely a dilute liquid, a dense liquid and a crystal. The transition from dilute-liquid to dense-liquid takes place in the regime where the fluid phase is metastable with respect to the crystal. We have computed the relative stabilities of different crystal morphologies. In addition, we have analyzed the “nucleation” of the native state of an isolated lattice protein. Using a “Gō” model [N. Gō, J. Stat. Phys. 30, 413 (1983)] to describe the protein, we show that a first order transition exists between the native and the coil state. We show this by analyzing the free energy barrier for the coil-to-native transition.
Field-theoretic simulations of confined polymer solutions
Authors: Alexander-Katz Alfredo , Moreira André G. , Fredrickson Glenn H.
Published: May 2003

We used field-theoretic simulations to study the equilibrium behavior of a polymer solution under good solvent conditions confined to a slit of width L. In particular, we obtained the chemical potential and the density profiles across the slit for different values of the monomer excluded volume over a wide range of concentrations C. We also obtained mean field results for the chemical potential and the density profiles. The effective correlation length ξeff was calculated from the density profiles and compared to the mean field result (valid in the limit of high concentrations). For small excluded volume parameters we found that ξeff is well described by the mean field result for all concentrations. For larger excluded volume parameters the correlation length exhibits a C−3/4 scaling behavior for intermediate concentrations, which is compatible with the behavior expected for this system in the semidilute regime.
Ab initio molecular dynamics study of aqueous solvation of ethanol and ethylene
Authors: van Erp Titus S. , Meijer Evert Jan
Published: May 2003

The structure and dynamics of aqueous solvation of ethanol and ethylene are studied by density functional theory based Car–Parrinello molecular dynamics. We did not find an enhancement of the structure of the hydrogen bonded network of hydrating water molecules. Both ethanol and ethylene can easily be accommodated in the hydrogen-bonded network of water molecules without altering its structure. This supports the conclusion from recent neutron diffraction experiments that there is no hydrophobic hydration around small hydrophobic groups. Analysis of the electronic charge distribution using Wannier functions shows that the dipole moment of ethanol increases from 1.8 D to 3.1 D upon solvation, while the apolar ethylene molecule attains an average dipole moment of 0.5 D. For ethylene, we identified configurations with π-H bonded water molecules, that have rare fourfold hydrogen-bonded water coordination, yielding instantaneous dipole moments of ethylene of up to 1 D. The results provide valuable information for the improvement of empirical force fields, and point out that for an accurate description of the aqueous solvation of ethanol, and even of the apoler ethylene, polarizable force fields are required.
Modal analysis of q-space—relaxation correlation experiments
Authors: Marinelli Luca , Hürlimann M. D. , Sen P. N.
Published: May 2003

Correlations between nuclear magnetic resonance based relaxation and diffusion-diffraction measurements in fluid-saturated isolated pores with relaxing surfaces are investigated. These correlations can be measured in two-dimensional experiments using previously reported pulse sequences that consist of an initial subsequence that probes diffusion-diffraction properties with pulsed gradient spin echo (PGSE), followed by another subsequence that measures relaxation effects. The magnetization is expressed in terms of diffusion eigenmodes of the restrictive geometry with the proper boundary condition. In general, each relaxation mode displays a different diffusion-diffraction behavior. For small values of the wave-vector q=γgδ (g is the pulsed gradient amplitude, and δ its time duration), an effective time dependent modal diffusion coefficient for each eigenmode is defined. In the fast-diffusion regime, in which surface relaxation is weak compared to diffusion across the pore, the lowest eigenmode dominates and the diffusion and relaxation dependencies of the magnetization approximately factorize. When surface relaxation is strong, higher eigenmodes contribute significantly and diffusion and relaxation effects cannot be factorized anymore. A correlation between the short-time modal diffusion coefficients and relaxation rates can be used to extract the properties of surface relaxation and the pore-size. The shift of the “diffraction” minima at q≈ integer multiples of 2π/ls (pore-size is ls) at weak relaxation values to higher wave-vectors at strong relaxation is associated with significant contributions from the higher modes to the magnetization at large wave vectors.
The vibrational spectrum of HONO: Fully coupled 6D direct dynamics
Authors: Luckhaus David
Published: May 2003

The vibrational spectrum of nitrous acid (HONO) in the electronic ground state is derived from fully coupled quantum-mechanical calculations. A global six-dimensional potential hypersurface is directly interpolated from density-functional calculations. A potential-optimized generalized coordinate discrete variable representation is combined with multidimensional successive truncation to yield the complete vibrational spectrum up to the first OH stretching overtone of both stable isomers. Higher OH stretching overtones up to 7νOH are calculated selectively applying the adiabatic contraction technique. The ab initio prediction of the vibrational spectrum is in remarkably good agreement with available experimental data. The results indicate the onset of the intramolecular (1,3)-H-transfer between the first and second OH stretching overtone of cis-HONO.
A mixed quantum-classical semirigid vibrating rotor target approach to methane dissociation on Ni surface
Authors: Xiang Yun , Zhang John Z. H.
Published: May 2003

A mixed quantum-classical semirigid vibrating rotor target (QC-SVRT) approach is presented to study dissociative chemisorption of polyatomic molecules on solid surface. In the four-dimensional flat surface study, the translational motion of the polyatom is treated classically while the rovibrational motion is treated quantum mechanically using the semirigid vibrating rotor target (SVRT) model. The QC-SVRT approach is computationally efficient and is ideally suited to treating polyatomic molecules due to their heavy translational masses. This QC-SVRT method is applied to calculating dissociation probability of methane on a Ni(111) surface which was studied previously by the full quantum-mechanical semirigid vibrating rotor target (QM-SVRT) method. Numerical results show that the current mixed method gives dissociation probabilities that are in reasonable agreement with the quantum results and the reaction threshold is correctly produced. The agreement of the QC result with the quantum one is good for the initial ground state, but less satisfactory for the excited state.
Laser-induced surface cleaning of molybdenum field emitter arrays for enhanced electron emission
Authors: Seo D. S. , Kim C. O. , Hong J. P. , Shin J. S. , Song B. K. , Cha S. N. , Chung J. W. , Kim J. M. , Baik H. S.
Published: May 2003

Visible laser irradiation on molybdenum field emission arrays (Mo–FEAs) was performed as one efficient cleaning method in order to etch off any unnecessary oxidation layers on the FEA surface. Scanning electron microscopy and high-resolution transmission electron microscopy showed clear removal of ultrathin MoO oxide layers at the emitter edge through a photoinduced thermal process. A sharp surface morphology of the emitter tips was also observed due to the crystallization or thermalmigration effect during the laser exposure. The structural enhancement of the FEA was strongly confirmed by a remarkable increase of 40% in the emission current after laser exposure.
Local “hybrid” functionals based on exact-expression approximate exchange
Authors: Karasiev Valentin V.
Published: May 2003

We propose to replace the nonlocal Hartree–Fock exchange in conventional exchange-correlation hybrid functionals by the local exact-expression approximate exchange (EEAX) terms, such as, for example, the self-consistent α (SCα), the asymptotically-adjusted self-consistent α (AASCα), the localized Hartree-Fock method introduced by Görling or the approximation to the optimized effective potential method introduced by Krieger, Li, and Iafrate. Such replacement is justified by the fact that the exchange energy expression of the EEAX functionals is equal to the Hartree–Fock one (exact exchange) while the corresponding EEAX exchange potential is a local, multiplicative operator. Applications to diatomic molecules for the case of EEAX=SCα, AASCα are presented. The performance of new totally local “hybrid” exchange-correlation functionals is quite close to that of the original nonlocal hybrid models.
Atomic structure, binding energy, and magnetic properties of iron atoms supported on a polyaromatic hydrocarbon
Authors: Senapati L. , Nayak S. K. , Rao B. K. , Jena P.
Published: May 2003

The atomic structure, energetics, and properties of gas-phase cluster complexes containing coronene (C24H12) molecule and up to two iron atoms are studied for the first time using density functional theory and generalized gradient approximation for exchange and correlation. The geometries of the neutral and cationic iron–coronene complexes are optimized without symmetry constraint and by examining the possibility that iron atoms could occupy various sites via individual π or bridging interactions. In both neutral and cationic complexes a single Fe atom is found to preferentially occupy the on-top site above the outer ring, while two Fe atoms dimerize and reside on the top of center of the outer rings. The binding energy of neutral Fe2–coronene defined with respect to dissociation into coronene and Fe2 is larger than that of Fe–coronene while reverse is true for the corresponding cations. Although the ionization potentials of these complexes are not very sensitive to the number of adsorbed Fe atoms, they are significantly reduced from those of the Fe atom or the coronene molecule. The photodecomposition of cationic (Fen–coronene)+ complexes proceeds through the ejection of either coronene+ or (Fe–coronene)+ cations while in the case of neutral Fe2–coronene, the ejection of Fe2 is energetically preferred. The coupling between the Fe atoms remains ferromagnetic although the magnetic moment/atom is reduced from the free-atom value. The results compare well with recent mass ion intensity and photofragmentation experiments.
Molecular dynamics study of the nano-rheology of n-dodecane confined between planar surfaces
Authors: Cui S. T. , McCabe C. , Cummings P. T. , Cochran H. D.
Published: May 2003

Realistic molecular simulations agree with previously published surface force experiments that n-dodecane confined between mica surfaces displays shear-thinning starting at shear rate orders of magnitude less than in the bulk fluid. We probe the origin of this behavior by studying rotational and diffusional relaxations in the simulated fluid and find a freezing-out of the rotational degrees of freedom and a power-law diffusional relaxation, resulting in over seven orders of magnitude increase in the relaxation time.
Growth and characterization of TiO2 as a barrier for spin-polarized tunneling
Authors: Bibes M. , Bowen M. , Barthélémy A. , Anane A. , Bouzehouane K. , Carrétéro C. , Jacquet E. , Contour J.-P. , Durand O.
Published: May 2003

We report on the elaboration and characterization of tunnel junctions based on La2/3Sr1/3MnO3 and TiO2. The structural analysis shows that TiO2 grows epitaxially in the anatase phase and forms flat interfaces with the adjacent layers. Resistance maps of a La2/3Sr1/3MnO3/TiO2 bilayer reveal a homogeneous resistance level. After patterning tunnel junctions, we obtain a large positive tunneling magnetoresistance (TMR) at low temperature for La2/3Sr1/3MnO3/TiO2/La2/3Sr1/3MnO3 junctions and a negative TMR in the case of La2/3Sr1/3MnO3/TiO2/Co. This negative TMR reflects a negative spin polarization of Co at the interface with TiO2, in analogy with recent experimental results for the Co/SrTiO3 interface.
Condensation limited cooling in supersonic expansions
Authors: Hillenkamp Matthias , Keinan Sharon , Even Uzi
Published: May 2003

Supersonic expansions of pure and seeded rare gases have been investigated experimentally, measuring the translational and rotational temperatures. The lowest achievable translational temperature in the jet depends on both gas properties as well as on experimental boundary conditions like nozzle shape and nozzle–skimmer distance. We show that there is a limit to the lowest temperature achieved, under practical conditions, set by condensation in the jet. A large cluster binding energy enhances the formation of clusters and they release their condensation energy into the beam. The spatial confinement of the jet extends to long distances, and is sensitive to the shape of the nozzle. The confined jet forms a narrow cone of high intensity, and results in increased collision probability and cluster formation.
Molecular dynamics calculations of melting rates with a novel order parameter: The diatomic Pa3 crystal
Authors: Tepper H. L. , Scheinhardt-Engels S. M. , Briels W. J.
Published: May 2003

A method is presented to design order parameters that can be used as discriminator in two-phase crystal-liquid molecular dynamics simulations. The proposed methodology is an extension to molecular crystal structures of a previously introduced discriminator for the atomic fcc environment [Phys. Rev. Lett. 79, 5074 (1997)] and can be readily applied to any crystal structure with both translational and orientational order. As an example, the discriminator is applied to the molecular Pa3 environment and subsequently used to study crystal melting rates with a diatomic carbon dioxide potential. The system’s melting temperature proves to be below the roughening transition which is exemplified by faceted growth. The dynamically corrected melting rates are easily fitted to a rate law for two-dimensional nucleation and growth from which the melting temperature is deduced. The feasibility of the method for the example system holds promise for more extensive microscopic investigations of molecular crystal growth and melting.
Convection-driven phase segregation of deeply quenched liquid mixtures
Authors: Mauri Roberto , Califano Filomena , Calvi Erika , Gupta Rajan , Shinnar Reuel
Published: May 2003

Observing the phase separation of deeply quenched, low viscosity liquid mixtures we inferred that the process is driven by the convection due to capillary forces, and not by molecular diffusion neither by gravity, heat or surface effects. After quenching a partially miscible, initially homogeneous, off-critical liquid mixture to a temperature T deeply below its critical point of miscibility Tc, with |T−Tc|/Tc≈0.1, we observed the formation of rapidly coalescing droplets of the minority phase, whose size grows linearly with time. Following the motion of isolated 10 μm droplets, we saw that they move in random directions at speeds exceeding 100 μm/s, showing that during most of the process the system is far from local equilibrium. Eventually, when their size reaches the capillary length, the nucleating drops start sedimenting as gravity becomes the dominant force. This behavior was observed for both density-segregated and density-matched systems, irrespectively whether they were kept in horizontal or vertical cells. The experiments were repeated using both untreated (i.e., hydrophilic) and modified (i.e., hydrophobic) cell walls, with identical results and, in addition, no bulk motion was observed when the mixture was replaced with water, showing that the observed convection is not induced by gravity, neither by surface or temperature effects. Using a simple dimensional analysis of the governing equations based on the diffuse interface model, we showed that convection is induced by the coalescence among drops which, in turn, is the result of a nonequilibrium capillary force that indeed dominates both diffusion and gravity forces.
Depletion forces near a soft surface
Authors: Bickel Thomas
Published: May 2003

We investigate excluded-volume effects in a bidisperse colloidal suspension near a flexible interface. Inspired by a recent experiment by Dinsmore et al. [Phys. Rev. Lett. 80, 409 (1998)], we study the adsorption of a mesoscopic bead on the surface and show that depletion forces could in principle lead to particle encapsulation. We then consider the effect of surface fluctuations on the depletion potential itself and construct the density profile of a polymer solution near a soft interface. Surprisingly we find that the chains accumulate at the wall, whereas the density displays a deficit of particles at distances larger than the surface roughness. This nonmonotonic behavior demonstrates that surface fluctuations can have major repercussions on the properties of a colloidal solution. On average, the additional contribution to the Gibbs adsorbance is negative. The amplitude of the depletion potential between a mesoscopic bead and the surface increases accordingly.
Synthesis of well-ordered CuO nanofibers by a self-catalytic growth mechanism
Authors: Hsieh Chien-Te , Chen Jin-Ming , Lin Hung-Hsiao , Shih Han-Chang
Published: May 2003

One-dimensional and well-ordered CuO nanofiber arrays have been synthesized by a processing of self-catalytic growth. Using polycarbonate (PC) membrane as a template, copper nuclei sites Cu(111) were uniformly deposited on copper substrate via a high voltage input (electric field: 15 V/cm) in a copper sulphate solution. According to the pore diameter of PC membranes, two different sizes of copper nuclei could be well-controlled in ranges of 50 to 60 and 100 to 150 nm. With heat treatment in oxygen atmosphere, the electrodeposited copper nuclei were transformed into CuO nanofiber arrays. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy analyses showed that the nanocrystalline CuO nanofibers with a mean length of 8 μm had an average order of 107–108/cm2 in density, and their average diameters were accorded with the size of Cu nuclei.
Proton Moiré fringes for diagnosing electromagnetic fields in opaque materials and plasmas
Authors: Mackinnon A. J. , Patel P. K. , Price D. W. , Hicks D. , Romagnani L. , Borghesi M.
Published: May 2003

High contrast proton moiré fringes have been obtained in a laser-produced proton beam. Moiré fringes with modulation of 20%–30% were observed in protons with energies in the range of 4–7 MeV. Monte Carlo simulations with simple test fields showed that shifts in the moiré fringes can be used to give quantitative information on the strength of transient electromagnetic fields inside plasmas and materials that are opaque to conventional probing methods.
Tuning the response of magnetic suspensions
Authors: Chen M. , Sun L. , Bonevich J. E. , Reich D. H. , Chien C. L. , Searson P. C.
Published: May 2003

Electrochemical template synthesis of multilayer nanowires consisting of alternating ferromagnetic and nonmagnetic layers provides an approach to control the properties of magnetic particles in suspension. Copper/nickel multilayer nanowires were fabricated by electrochemical deposition from a solution containing both nickel and copper ions. We demonstrate that the magnetic shape anisotropy and dipolar interactions between magnetic layers can be exploited to tailor the magnetic response in ferromagnetic/nonmagnetic multilayer nanowires in a suspension.
Wide photonic band gaps at the visible in metallic nanowire arrays embedded in a dielectric matrix
Authors: Garcia N. , Ponizowskaya E. V. , Zhu Hao , Xiao John Q. , Pons A.
Published: May 2003

We present experimental verification of very wide band gaps at the visible by using a composite material consisting of an array of metallic nanowires embedded in a dielectric matrix. When the wavelength of the incident electromagnetic wave is long in such that the effective permittivity of the composite is negative, the electromagnetic waves will be reflected. At short wavelengths, the electromagnetic waves will go through the dielectric matrix. Therefore, the cutoff frequency is controlled by the average separation between metallic nanowires and their radii, allowing us to tune the band gap at will. These experimental results, in very good agreement with theoretical calculations, are at variant with many results that claim that the randomness will destroy the band gaps. In addition, the insulating nature of the composite minimizes the energy loss. Finally, it is easy and inexpensive to prepare samples. This work opens ground in the development of optical devices nanotechnology.
Submicron electrical current density imaging of embedded microstructures
Authors: Schrag B. D. , Xiao Gang
Published: May 2003

We have developed a scanning magnetic microscopy technique for noninvasively imaging submicron magnetic fields from embedded microscopic electrical circuits. We are able to extract from the field data a complete profile of current densities using a mathematical algorithm. As an example, we provide current density images of micron-scale passivated conductors undergoing electromigration.
Strain-gradient-induced electric polarization in lead zirconate titanate ceramics
Authors: Ma Wenhui , Cross L. Eric
Published: May 2003

Strain-gradient-induced polarization or flexoelectricity was investigated in unpoled soft lead zirconate titanate (PZT) ceramic where the texture symmetry m∞∞ forbids macropiezoelectricity. Even under high strain gradient (1 m−1) the induced polarization is small (1.6 μC/m2) at 20 °C. Higher strain gradients induce ferroelastic poling and an additional extrinsic contribution to the flexoelectric coefficient μ12 raising the value from 0.5 to 2.0 μC/m. Cooling through the Curie point (TC) under maximum stress (80 MPa) where the peak permittivity (∼20 000) could raise μ12 to 20 μC/m, the equivalent electric field is still only ∼1 kV/m, inadequate to achieve significant ferroelectric poling. The situation may be different in thin PZT films where much larger strain gradients can occur.
Simultaneous generation of red, green, and blue quasi-continuous-wave coherent radiation based on multiple quasi-phase-matched interactions from a single, aperiodically-poled LiTaO3
Authors: Liao Jun , He Jing-Liang , Liu Hui , Wang Hui-Tian , Zhu S. N. , Zhu Y. Y. , Ming N. B.
Published: May 2003

We report the experimental result of simultaneous generation of red, green, and blue (RGB) quasi-continuous-wave coherent radiation from a single, aperiodically poled LiTaO3 (APPLT), with a diode-pumped dual-wavelength Nd:YVO4 laser oscillating at 1064 and 1342 nm simultaneously. Red at 671 nm and blue at 447 nm were achieved by frequency doubling and tripling of a 1342-nm fundamental laser, respectively, and green at 532 nm was achieved by frequency doubling of a 1064-nm fundamental laser. The aperiodic structure provided three effective reciprocals to compensate the phase mismatches of the above three nonlinear processes at the same time. The result indicates that the APPLT may be employed to construct an all-solid-state RGB laser.
Far-infrared (λ≃87 μm) bound-to-continuum quantum-cascade lasers operating up to 90 K
Authors: Scalari Giacomo , Ajili Lassaad , Faist Jérôme , Beere Harvey , Linfield Edmund , Ritchie David , Davies Giles
Published: May 2003

We report terahertz frequency (3.5 THz, λ≃87 μm) emission from quantum-cascade lasers employing a bound-to-continuum transition in the active region. The maximum operating temperature is in excess of 90 K. Peak powers of 20 mW at 20 K and 10 mW at 77 K are achieved. The same devices show continuous-wave operation up to 55 K with measured optical powers of 15 mW at 10 K.
Nondestructive determination of current–voltage characteristics of superconducting films by inductive critical current density measurements as a function of frequency
Authors: Yamasaki Hirofumi , Mawatari Yasunori , Nakagawa Yoshihiko
Published: May 2003

A nondestructive inductive method was developed to determine the current–voltage characteristics of superconducting films by measuring the critical current density Jc as a function of the frequency of ac magnetic fields. In well-known inductive methods that measure the Jc distribution of large-area films, Jc is measured at a frequency and the electric field E induced in the superconductor is proportional to the frequency; therefore, the E versus current density relation can be determined by measuring Jc at various frequencies. Current–voltage characteristics of YBa2Cu3O7 films measured by this method agree well with those determined by measuring magnetic-field-sweep rate dependence of magnetization.
Bragg-diffraction-type acousto-optic device using Pb(ZrTi)O3 film
Authors: Lee Changho , Kim Sangho , No Kwangsoo
Published: May 2003

We fabricated a hybrid-type acousto-optic (AO) device [Pb(Zr,Ti)O3(PZT)/TiOx/SiO2/SiNx/SiO2/Si] and analyzed AO Bragg diffraction efficiency (ζ) to estimate the applicability of piezoelectric PZT to a Bragg-diffraction-type AO device. A surface acoustic wave (SAW) was generated on the PZT film, and an optical beam (He–Ne laser) was guided in the SiNx film. ζ was varied to 9% in the AO devices fabricated with various AO quality factors (Q) and film thicknesses of top SiO2 and PZT film, respectively. Using the results, we qualitatively analyzed the decay of SAW power to the film depth direction and AO figure of merit (M2=0.75×10−18 s3/g) of the SiNx film.
Improvement of discharge capacity of LiCoO2 thin-film cathodes deposited in trench structure by liquid-delivery metalorganic chemical vapor deposition
Authors: Cho Sang-In , Yoon Soon-Gil
Published: May 2003

Pt collector and LiCoO2 cathode films were deposited onto trench-SiO2/Si substrates by liquid-delivery metalorganic chemical vapor deposition (LDMOCVD). The 100-nm-thick Pt thin films deposited at 350 °C showed the lowest resistivity and roughness and step-coverage of 57% in trench structure (aspect ratio-1). The LiCoO2 cathode films (step-coverage=51%) deposited onto a Pt collector showed an increase of approximately two and half times the discharge capacity compared with those of planar-cathode films. The LDMOCVD process is suitable to improve the discharge capacity of LiCoO2 cathode films using a trench structure in lithium rechargeable batteries.
Horn resonator boosts miniature free-electron laser power
Authors: Bakhtyari A. , Brownell J. H.
Published: May 2003

The Dartmouth miniature grating-based free-electron laser is a benchtop device producing coherent, tunable terahertz radiation. Two resonator designs, the planar grating horn and the grating horn are described and their performance compared with the simple planar grating case. The grating horn increases output intensity by a factor of 100 without limiting the tuning range of the device.
Enhancement of laser-ultrasound/electromagnetic-acoustic transducer signals from Rayleigh wave interaction at surface features
Authors: Boonsang S. , Dewhurst R. J.
Published: May 2003

The enhancement of signals from Rayleigh wave interaction at solid surface features is described, for the case when signals arise from laser-ultrasound/electromagnetic-acoustic transducer (EMAT) detection system. Transient surface acoustic waves were detected by an in-plane EMAT receiver. B-scan images from surface slot or surface edge interactions revealed increased signal amplitude from these features of a factor ∼1.59. An explanation is based on the interaction of the EMAT sensor with the Rayleigh wave.
Impact of substrate hot hole injection on ultrathin silicon dioxide breakdown
Authors: Heh Dawei , Vogel Eric M. , Bernstein Joseph B.
Published: May 2003

The impact of hot holes on gate oxide breakdown is studied by investigating devices under constant voltage stress with a different amount of preinjected substrate hot holes. The results show that oxide breakdown is independent of the amount of those preinjected hot holes, which suggests that defects generated by hot holes are not directly related to oxide breakdown during constant voltage stress conditions.
Intensity and spatial modulation of spontaneous emission in GaAs by field aperture selecting transport
Authors: Boone Thomas D. , Tsukamoto Hironori , Woodall Jerry M.
Published: May 2003

A potential technique for modulating the light emission resulting from excess minority carrier recombination in a semiconductor device is introduced. This process utilizes an electric field to transport a packet of minority carriers past an optical output aperture defined on the surface of the semiconductor. A short burst of light is allowed to escape through the surface of the device as the packet drifts past the opening in the aperture. To first order, the temporal length of the optical pulse will be a function of the width of the excess minority carrier packet, the width of the aperture, and the drift velocity of the excess minority carriers. In p-type gallium arsenide, geometric scales of 5 μm should make possible pulse widths near 100 ps. Initial experimental results are presented confirming the spatial displacement and the attenuation of the external luminescence intensity as a function of applied bias voltage.
High-quality InAsyP1−y step-graded buffer by molecular-beam epitaxy
Authors: Hudait M. K. , Lin Y. , Wilt D. M. , Speck J. S. , Tivarus C. A. , Heller E. R. , Pelz J. P. , Ringel S. A.
Published: May 2003

Relaxed, high-quality, compositionally step-graded InAsyP1−y layers with an As composition of y=0.4, corresponding to a lattice mismatch of ∼1.3% were grown on InP substrates using solid-source molecular-beam epitaxy. Each layer was found to be nearly fully relaxed observed by triple axis x-ray diffraction, and plan-view transmission electron microscopy revealed an average threading dislocations of 4×106 cm−2 within the InAs0.4P0.6 cap layer. Extremely ordered crosshatch morphology was observed with very low surface roughness (3.16 nm) compared to cation-based In0.7Al0.3As/InxAl1−xAs/InP graded buffers (10.53 nm) with similar mismatch and span of lattice constants on InP. The results show that InAsyP1−y graded buffers on InP are promising candidates as virtual substrates for infrared and high-speed metamorphic III–V devices.
Energy of free excitons in CuInSe2 single crystals
Authors: Yakushev M. V. , Mudryi A. V. , Tomlinson R. D.
Published: May 2003

CuInSe 2 single crystals grown by the vertical Bridgman method were studied using photoluminescence (PL) and optical reflection (OR) at temperatures 4.2 and 78 K. A and B free- exciton peaks were well resolved in both the PL and OR spectra measured at a low temperature from a number of samples. The energy, spectral separation, and the width of the excitonic lines varied considerably for samples with different elemental compositions. The closer to the ideal stoichiometry, the higher the energies and the smaller the A–B separation. Similar changes in the excitonic spectra were revealed for a temperature rise from 4.2 up to 78 K. These measurements explain the scatter in the excitonic parameters and the band-gap values measured in the present work and reported in the literature.
Electroluminescence of self-assembled Ge hut clusters
Authors: Stoffel M. , Denker U. , Schmidt O. G.
Published: May 2003

We have fabricated Si-based light-emitting diodes operating in the near infrared. The active layers of the devices consist of either one or ten layers of Ge/Si self-assembled hut clusters grown by molecular-beam epitaxy. Luminescence is observed in the spectral range between 1.4 and 1.5 μm. For the ten layer stack of Ge islands, electroluminescence is observed up to room temperature. A direct comparison with a pure Si reference p-i-n diode allows us to attribute the luminescence to radiative recombinations between holes localized in the Ge islands and electrons localized in the strained Si above and below the islands.
InAs/GaAs(001) quantum dots close to thermodynamic equilibrium
Authors: Costantini G. , Manzano C. , Songmuang R. , Schmidt O. G. , Kern K.
Published: May 2003

InAs/GaAs(001) quantum dots are grown at high temperature and extremely low flux and analyzed by in situ scanning tunneling microscopy. A bimodal distribution of dots is observed, composed of “small” and “large” islands. While the former show a broad distribution of sizes and shapes, the latter appear to be highly uniform and have a truncated pyramid shape with irregular octagonal base. (110) and (111) facets are identified and atomically resolved showing (1×1) and (2×2) surface reconstructions, respectively. The shape of the large quantum dots is in excellent agreement with recent theoretical predictions, proving that the chosen deposition conditions are close to thermodynamic equilibrium.
Fabrication of a single-electron inverter in multiwall carbon nanotubes
Authors: Ishibashi K. , Tsuya D. , Suzuki M. , Aoyagi Y.
Published: May 2003

A single-electron inverter based on the Coulomb blockade effects has been fabricated in multiwalled carbon nanotubes, by using the Ar irradiation method for the tunnel barrier formation in the quantum dots. The device characteristics were measured at 2.6 K, and an indication of inverter-like transfer characteristics has been obtained. The performance of the device was not completely satisfactory for the inverter operation in terms of the voltage gain and the voltage swing, but the present technique turns out to be useful for the fabrication of single-electron logic devices in carbon nanotubes.
Optical nanowriting on azobenzene side-chain polymethacrylate thin films by near-field scanning optical microscopy
Authors: Likodimos V. , Labardi M. , Pardi L. , Allegrini M. , Giordano M. , Arena A. , Patanè S.
Published: May 2003

Optical writing and subsequent reading of information on thin films of azobenzene side-chain polymethacrylates on the 100-nm scale are demonstrated by near-field scanning optical microscopy (NSOM) with polarization control. Polarized blue light at 488 nm coupled to the NSOM aperture probe activates trans–cis–trans isomerization cycles of the side chains, causing their alignment and thus locally inducing optical birefringence. Red light at 690 nm with modulated polarization is coupled to the same aperture and used to detect optical anisotropy on the local scale. Lines of width on the 100-nm scale were optically inscribed and detected even with no concurrent topographic modification.
A high-temperature shape-memory alloy Ni54Mn25Ga21
Authors: Xu Huibin , Ma Yunqing , Jiang Chengbao
Published: May 2003

A high-temperature shape-memory alloy, Ni54Mn25Ga21, was developed with a shape-memory effect of 6.1% and a martensitic transformation temperature higher than 250 °C for single crystals. The measured compressive strength and strain were 845 MPa and 20.5%, respectively, with a compressive axis along the growth direction of the rods at room temperature. One thousand thermal cycles were performed on the Ni54Mn25Ga21 without obvious changes of the martensitic structure, transformation behavior, and shape-memory effect, indicating an excellent thermal stability for the present alloy.
Parametric modulation mechanism of surface acoustic wave on a partially closed crack
Authors: Kim J.-Y. , Yakovlev V. A. , Rokhlin S. I.
Published: May 2003

Low-frequency parametric modulation of a pulsed surface acoustic wave by a partially closed fatigue crack is described. The crack is initiated in the plastic-yielding zone induced by a surface cavity and clamped due to the constraint of the surrounding elastic medium. Small periodic loading, superimposed on a static crack opening load, changes periodically the crack closure state and produces a nonlinear modulation of the reflected ultrasonic pulses. We describe quantitative experimental and modeling results relating the reflected modulation spectrum to crack length, closure, and modulation load. Surface acoustic wave measurements, which were performed in situ during a fatigue test, show that the modulation response varies significantly depending on the ratio of the fatigue crack length to the size of the plastic-yielding zone.
Pi-stacked pentacene thin films grown on Au(111)
Authors: Kang Joo H. , Zhu X.-Y.
Published: May 2003

Scanning tunneling microscopy with high impedance has been used to image the growth of pentacene thin films on Au(111). Instead of the herringbone structure in bulk solid, pentacene molecules in these thin films form a cofacial, π-stacked crystalline phase with their molecular planes parallel to the surface. The growth of this crystalline phase is attributed to the formation of a close-packed, crystalline monolayer which seeds the growth of the π-stacked multilayer film.
Isothermal capacitance transient spectroscopy of pseudomorphic high-electron-mobility transistors
Authors: Maruno Shigemitsu , Abe Yuji , Ozeki Tatsuo , Nakamoto Takahiro , Yoshida Naohito
Published: May 2003

The surface electronic properties of AlGaAs/InGaAs pseudomorphic high-electron-mobility transistors were investigated by isothermal capacitance transient spectroscopy (ICTS) and gate-leakage current characteristic measurements. Both hole- and electron-like trap spectra were observed by ICTS measurements on gate–source/drain capacitance. We observed enhancement of leakage current and drastic change of static and transient capacitance behavior around a pinch-off voltage. The leakage characteristics and ICTS results were explained in terms of a surface states model.
Dynamic modeling and scaling of nanostructure formation in the lithographically induced self-assembly and self-construction
Authors: Wu Lin , Chou Stephen Y.
Published: May 2003

We numerically studied the dynamical formation process and the scaling of the nanostructures in the lithographically induced self-assembly and self-construction of thin polymer films. Our studies show that the period of the self-assembled pillars depends on the ratio between the surface tension force and the electrostatic force. The viscosity of the polymer has no effect on the final pillar shape. When the feature width of the mold is comparable to or smaller than the most unstable disturbance wavelength of the system, the initially self-assembled pillars will merge to form a self-constructed mesa.
Charge carrier mobility in doped semiconducting polymers
Authors: Arkhipov V. I. , Heremans P. , Emelianova E. V. , Adriaenssens G. J. , Bässler H.
Published: May 2003

An analytic model of the equilibrium hopping conductivity in a disordered organic semiconductor at large charge carrier densities is formulated. Calculated dependences of the equilibrium hopping mobility upon the carrier density are compared with recent experimental data obtained on doped poly(3-hexylthiophene) films. Doping is shown to create additional energy disorder due to potential fluctuations caused by the Coulomb field of randomly distributed dopant ions.
Monte Carlo simulation of remote-Coulomb-scattering-limited mobility in metal–oxide–semiconductor transistors
Authors: Gámiz F. , Roldán J. B. , Carceller J. E. , Cartujo P.
Published: May 2003

An improved theory for remote-charge-scattering-limited mobility in silicon inversion layers is developed. The model takes into account the effects of image charges, screening, inversion layer quantization, the contribution of different subbands, oxide thickness, the actual distribution of charged centers inside the structure, the actual distribution of carriers in the inversion layer, the correlation of charged centers, and the charged centers sign. The model is implemented in a Monte Carlo simulator, where the effects of the ionized impurities charge, the interface trapped charge, and the contribution of other scattering mechanisms are taken into account simultaneously. Our results show that remote Coulomb scattering cannot be neglected for oxide thicknesses below 2 nm, but that its effects for tox>5 nm are negligible. Good agreement with experimental results has been obtained.
Superconducting quantum interference device amplifiers at gigahertz frequencies
Authors: Mück Michael , Welzel Christian , Clarke John
Published: May 2003

A series of five dc superconducting quantum interference devices (SQUIDs) have been operated as microstrip amplifiers at frequencies ranging from 2.2 to 7.4 GHz. In these devices, the signal is connected between the SQUID washer and coil, which acts as a microstrip resonator. The gain measured at 4.2 K ranged from 12±1 to 6±1 dB. The noise temperature of three devices at 4.2 K in the frequency range 2.2–4 GHz was between 1 and 2 K, and the saturation temperature was between 150 and 250 K. Applications of these devices include readout for axion detectors, and intermediate-frequency amplifiers for superconductor–insulator–superconductor and hot-electron bolometer mixers.
Polarizability of water clusters: An ab initio investigation
Authors: Ghanty Tapan K. , Ghosh Swapan K.
Published: May 2003

Ab initio calculations have been performed to obtain the polarizability of water clusters (H2O)n up to n=20. It is shown that a simple linear relation involving the aggregation number fits the polarizability results extremely well (with correlation coefficient >0.999) indicating a near additive nature of this quantity for weakly bonded molecular clusters. Calculated dynamic polarizabilities are also shown to follow the same trend. The effect of electron correlation on the static polarizability of water clusters has also been investigated.