bluuebunny/crossref_metadata_2025 huggingface dataset:
Multiphoton ionization and density functional studies of pyrimidine–(water)n clusters
Authors: Zhang Bailin , Cai Yong , Mu Xiaolan , Lou Nanquan , Wang Xiuyan
Published: September 2002

The multiphoton ionization of pyrimidine–(water)n clusters at conditions of supersonic expansion is studied using a time-of-flight mass spectrometer at the wavelengths of 355 and 532 nm. At both wavelengths, a series of protonated C4H4N2–(H2O)nH+ cluster ions are obtained. The production of these protonated cluster ions requires an intracluster proton transfer reaction. The protonated products are also suggested to relate to the excitation or ionization of water molecules. Ab initio calculations show that the dissociation of C4H4N2–(H2O)n+ cluster ions prefer to produce protonated ions. Two ways of producing protonated cluster ions are discussed. The reaction mechanism of intracluster proton transfer and the geometric structures of pyrimidine–(water)n clusters were depicted. The proton pulling effect and the atomic charge changes in the protonated cluster ions are also discussed.
Potential energy curves and dipole transition moments for excited electronic states of XeKr and ArNe
Authors: Petsalakis Ioannis D. , Theodorakopoulos Giannoula , Liebermann Heinz-Peter , Buenker Robert J.
Published: September 2002

Relativistic core-potential calculations have been carried out on Ω states resulting from the interaction of Xe* (5p56s, 3P, 1P) with ground-state Kr atoms as well as for the system Ar* (3p54s, 3P, 1P) with ground-state Ne, using different basis sets and configuration interaction procedures. The present calculations on ArNe, employing larger sets of Rydberg functions than those of the previous calculations, yield totally repulsive potentials for the excited states of ArNe. Similar calculations on XeKr obtain shallow minima (600–860 cm−1) in the potential energy curves of the excited states at large internuclear distances (6.9–7.8 bohr). Dipole transition moments have been calculated and strong radiative transitions are predicted from excited states to the ground state. The 1(I) state, correlating with the metastable P23 state of Xe is found to have a small but nonzero dipole transition moment at short and intermediate nuclear distances leading to a radiative lifetime for the v=0 level of this state of 21.0 μs.
Calculation of the intermolecular energy of large molecules by a fragmentation scheme: Application to the 4-n-pentyl-4′- cyanobiphenyl (5CB) dimer
Authors: Amovilli Claudio , Cacelli Ivo , Campanile Silvio , Prampolini Giacomo
Published: September 2002

We present a method for computing intermolecular energies of large molecules based on a suitable fragmentation scheme, which allows one to express the complete interaction energy as a sum of interaction energies between pairs of fragments. The main advantage consists in the possibility of using standard ab initio quantum methods to evaluate the fragment energies. For the 4-n-pentyl-4′-cyanobiphenyl (5CB) dimer, the present results indicate that the most favorite arrangement corresponds to an antiparallel side-by-side geometry with a stabilization energy of about 16 kcal/mol. It is shown that, by the present method, the interaction energy of the 5CB dimer can be evaluated for all geometrical conformations and, in principle, it can be used for bulk simulations.
Wavelet approximation of correlated wave functions. II. Hyperbolic wavelets and adaptive approximation schemes
Authors: Luo Hongjun , Kolb Dietmar , Flad Heinz-Jürgen , Hackbusch Wolfgang , Koprucki Thomas
Published: September 2002

We have studied various aspects concerning the use of hyperbolic wavelets and adaptive approximation schemes for wavelet expansions of correlated wave functions. In order to analyze the consequences of reduced regularity of the wave function at the electron–electron cusp, we first considered a realistic exactly solvable many-particle model in one dimension. Convergence rates of wavelet expansions, with respect to L2 and H1 norms and the energy, were established for this model. We compare the performance of hyperbolic wavelets and their extensions through adaptive refinement in the cusp region, to a fully adaptive treatment based on the energy contribution of individual wavelets. Although hyperbolic wavelets show an inferior convergence behavior, they can be easily refined in the cusp region yielding an optimal convergence rate for the energy. Preliminary results for the helium atom are presented, which demonstrate the transferability of our observations to more realistic systems. We propose a contraction scheme for wavelets in the cusp region, which reduces the number of degrees of freedom and yields a favorable cost to benefit ratio for the evaluation of matrix elements.
Instrumentation and start up of a new elongational rheometer with a preshearing history
Authors: Rı́os Sara , Dı́az Jorge , Galindo Abraham , Soto Enrique , Calderas Fausto , Mena Baltasar
Published: September 2002

This article describes the design, start up, and some preliminary results of a rheometer that preshears the test fluid and measures its elongational viscosities. Preshearing takes place within a concentric cylinders arrangement. Fluid goes out from the lower part of the cylinders arrangement where an elongational flow is produced, by means of a vacuum chamber, which draws out the fluid filament. The influence of the preshearing history upon the elongational properties is analyzed. Data of elongational stress, uniaxial stretch rate, elongational normal stress difference, and apparent elongational viscosity are obtained. The rheometer design is based on a previous work by Sridhar and Gupta [Rheological Act 24, 207 (1985)]. Glycerin was used as the test fluid. Stretch rates up to 1250 s−1 were found. Trouton's ratios ranging from 2.6 to 3.6 were obtained.
High-frequency probing diagnostic for Hall current plasma thrusters
Authors: Litvak A. A. , Raitses Y. , Fisch N. J.
Published: September 2002

High-frequency oscillations (1–100 MHz) in Hall thrusters have not received sufficient experimental scrutiny. A diagnostic setup, consisting of a single Langmuir probe, a special shielded probe connector-positioner, and an electronic impedance-matching circuit, was successfully built and calibrated. Through simultaneous high-frequency probing of the Hall-thruster plasma at multiple locations, high-frequency plasma waves have been successfully identified and characterized (Ref. 1).
Preparation of cantilevered W tips for atomic force microscopy and apertureless near-field scanning optical microscopy
Authors: Sun W. X. , Shen Z. X. , Cheong F. C. , Yu G. Y. , Lim K. Y. , Lin J. Y.
Published: September 2002

Tip characteristics play an important role in the resolution and sensitivity of scanning probe microscopy. Extensive efforts have been devoted to tip fabrication. Most of the research is focused on scanning tunneling microscopy applications, which require sharp and short tips. Long tips that can be bent into cantilevered tips have great potential in atomic force microscopy/apertureless near-field scanning optical microscopy applications. However, the fabrication of such tips has been rarely reported. The present work is carried out with the aim of optimizing the conditions suitable for fabricating long and sharp tungsten tips. Besides topography, optical, and spectroscopic information, electrical and magnetic measurements can also be carried out with such tips obtained with the recipe reported in this article. The long tips also make it possible to measure deep grooves/trenches.
S
1
–S
vibronic spectra of benzene clusters revisited. I. The tetramer
Authors: Iimori Toshifumi , Ohshima Yasuhiro
Published: September 2002

We report the reinvestigation of the S1–S0 electronic transitions of (benzene)n clusters by two-color mass-selective resonantly enhanced two-photon ionization (R2PI) and UV–UV (ultraviolet) hole burning spectroscopies. The present paper describes the band system that has been assigned to the trimer for two decades. Hole burning measurements by monitoring the trimer ion isotopomer channels in the expansion of a mixture of C6H6 and C6D6 have shown the contribution of six spectral carriers in the R2PI spectra, two of which are isotopically pure clusters. The other hetero isotopic species containing at least one C6H6 moiety appear in two adjacent isotopomer channels. It is argued that the band system should be reassigned to the neutral tetramer having four equivalent sites, which are detected in daughter ion mass channels due to efficient fragmentation after ionization even with two-color excitation. The experimental results are consistent with a distorted tetrahedral structure with S4 symmetry predicted as the global minimum by several model calculations. This conclusion is further supported by an analysis of exciton splittings in the C6H6-localized origin band, and this analysis provides coupling constants for the excitation exchange in the S1 state. Other experimental results reported so far pertaining to this species are reconsidered on the basis of the new assignment.
On the photoelectron spectrum of AgO−
Authors: Andrews Django H. , Gianola Adam J. , Lineberger W. Carl
Published: September 2002

The 364 nm negative ion photoelectron spectrum of AgO− is reported. Transitions to the A 2Σ+ state and the X 2Π1/2 and X 2Π3/2 spin–orbit states of AgO are observed. The electronic ground state of AgO− is found to have a vibrational frequency of 497 (20) cm−1 and an equilibrium bond length of 1.935 (15) Å. The electron affinity of AgO is found to be 1.654 (2) eV.
A two-dimensional magnetohydrodynamic stability model for helicity-injected devices with open flux
Authors: Brennan D. P. , Browning P. K. , Van der Linden R. A. M.
Published: September 2002

Models of the ideal magnetohydrodynamic (MHD) stability of spheromaks and spherical tokamaks are presented, including the effects of current on the open flux which plays a key role in helicity-injected current drive. The stability of spheromak equilibria with both open and closed flux and realistic current profiles representative of helicity-injected state is investigated, where a kink instability in the open flux is shown to dominate a tilt mode in the closed flux as the open flux current density is increased. A previous one-dimensional model is extended to more realistic two-dimensional equilibria which properly incorporate a region of closed magnetic flux as well as open flux penetrating the boundaries at electrodes. A new stability code SCOTS has been developed and benchmarked which can determine the growth rates of ideal MHD modes in this geometry. The coordinate system for this code has been developed such that it extends smoothly across the separatrix between closed and open flux, thus not imposing any unphysical discontinuities in mode structures. From the results, the operating conditions of devices can be predicted since helicity-injection current drive requires fluctuations which arise from the saturated current driven instability in the open flux, and since these systems operate near the stability boundary for this mode.
Microelectromagnets for trapping and manipulating ultracold atomic quantum gases
Authors: Fortágh J. , Ott H. , Schlotterbeck G. , Zimmermann C. , Herzog B. , Wharam D.
Published: September 2002

We describe the production and characterization of microelectromagnets made for trapping and manipulating atomic ensembles. The devices consist of seven fabricated parallel copper conductors 3 μm thick, 25 mm long, with widths ranging from 3 to 30 μm, and are produced by electroplating a sapphire substrate. Maximum current densities in the wires up to 6.5×106 A cm−2 are achieved in continuous mode operation. The device operates successfully at a base pressure of 10−11 mbar. The microstructures permit the realization of a variety of magnetic field configurations and, hence, provide enormous flexibility for controlling the motion and the shape of Bose–Einstein condensates.
The β–δ phase transition in the energetic nitramine-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine: Kinetics
Authors: Smilowitz L. , Henson B. F. , Asay B. W. , Dickson P. M.
Published: September 2002

In this paper we present second harmonic generation (SHG) experiments designed to confirm the mechanism and quantify the transformation kinetics of the β–δ solid state phase transition in the organic nitramine molecule octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). The β phase adopts a centrosymmetric crystallographic configuration (P21/c) while the δ phase adopts a noncentrosymmetric one (P61(P65)). As expected, this results in a very poor generation of SHG intensity from the β phase, while the δ phase is very efficient, rivaling KH2PO4 in absolute efficiency. SHG thus provides a very high sensitivity zero background probe of the δ phase. We discuss the use of this signal as a quantitative measure of the δ phase mole fraction in ensembles of free HMX crystals and crystals embedded in a visco–elastic polymer matrix. We report imaging experiments where the spatial characteristics of the transformation are shown to be consistent with nucleation from a low density of initial sites, followed by rapid growth. We also report experiments where the total integrated SHG is measured and used to infer the transition progress as a function of time in a series of isothermal experiments on both β–δ conversion and δ–β reversion. Additionally, reversibility experiments are reported which are used to verify both the volumetric mechanism of SHG generation in this system and the independence of these results to the internal stress state of the polycrystalline samples. We compare the measured SHG intensity as a function of time for a range of temperatures with predictions of the two state kinetic model presented in the accompanying paper [B. F. Henson et al., J. Chem. Phys. 117, 3780 (2002)]. We perform a set of parameter optimization calculations based on agreement with the predictions of the model. Optimization does not significantly change the kinetic parameters that are thermodynamically constrained by the model, but there is a distribution of parameters necessary to reproduce the nucleation kinetics observed. In particular, a striking difference in nucleation kinetics is observed between samples of free crystals and crystals embedded in a visco–elastic polymer matrix.
Effects of sampling and limited data in optical tomography
Authors: Markel Vadim A. , Schotland John C.
Published: September 2002

We consider the image reconstruction problem for optical tomography in the transmission geometry. We investigate the effects of sampling and limited data on this inverse problem and derive an explicit inversion which is computationally efficient and stable in the presence of noise.
The underdamped Brownian oscillator model with Ohmic dissipation: Applicability to low-temperature optical spectra
Authors: Toutounji Mohamad M. , Small Gerald J.
Published: September 2002

The multimode Brownian oscillator (MBO) model with Ohmic dissipation has frequently been used to interpret photon echo and spectral data on solvation dynamics of chromophores in liquids at room temperature. We report on the applicability of this model to high-resolution linear electronic absorption spectra of chromophores in solid hosts at low temperatures, where the zero-phonon line (ZPL) is resolved from phonon sideband structure. The results are also relevant to frequency and time domain nonlinear spectra. In the MBO model, active BOs (phonons) are linearly coupled to bath modes. This coupling endows the bath modes with absorption intensity which, with Ohmic dissipation (white light spectrum for the bath modes), results in the bath modes contributing to absorption in the region of the ZPL. Experimental results for a multitude of molecular systems indicate that the ZPL profile is determined by electronic dephasing, which is not accounted for in the MBO model. Thus, it is important to assess the contribution of the MBO bath modes to the ZPL profile. To this end, closed-form, finite temperature expressions for the underdamped MBO (UMBO) model are derived for the linear response function, linear absorption spectrum, and width and Franck–Condon factor of the ZPL. It is proven formally that the UMBO ZPL width is zero at T=0 K. Results of calculations for model systems whose parameter values (BO damping constant, frequency and Huang–Rhys factor) are typical of real systems are presented. It is concluded that Ohmic dissipation leads to unphysically large ZPL widths as well as asymmetric ZPL profiles that appear not to have been observed. Moreover, the ZPL width adds to those of the multi-BO (phonon) transitions. Thus, use of the UMBO model with Ohmic dissipation to interpret data on relaxation dynamics of nuclear modes may result in erroneous conclusions. It is shown that Franck–Condon factors of the ZPL obtained with the UMBO model can differ significantly from those calculated with the conventional formula.
Steady, nonradiating geophysical flow past a cylinder with circulation
Authors: McDonald N. Robb
Published: September 2002

Rotating, barotropic flow past a circular cylinder with circulation in the presence of a piecewise constant distribution of potential vorticity is considered. For a given cylinder radius, linear theory is used to show that there exists a unique value of the circulation for which the downstream wake vanishes. The cylinder experiences zero drag for such flows. A numerical method, based on contour dynamics, is then used to compute steady, nonlinear, waveless flows past the cylinder. For a given upstream velocity and different choices of the cylinder radius, waveless potential vorticity interface profiles are presented and corresponding values of the circulation calculated. Possible applications to propagating geophysical vortices are discussed, including the computation of nonlinear solutions which may be interpreted as freely propagating vortices.
HfO
2
gate dielectric with 0.5 nm equivalent oxide thickness
Authors: Harris H. , Choi K. , Mehta N. , Chandolu A. , Biswas N. , Kipshidze G. , Nikishin S. , Gangopadhyay S. , Temkin H.
Published: September 2002

Hafnium dioxide films have been deposited using reactive electron beam evaporation in oxygen on hydrogenated Si(100) surfaces. The capacitance–voltage curves of as-deposited metal(Ti)–insulator–semiconductor structures exhibited large hysteresis and frequency dispersion. With post-deposition annealing in hydrogen at 300 °C, the frequency dispersion decreased to less than 1%/decade, while the hysteresis was reduced to 20 mV at flatband. An equivalent oxide thickness of 0.5 nm was achieved for HfO2 thickness of 3.0 nm. We attribute this result to a combination of pristine hydrogen saturated silicon surfaces, room temperature dielectric deposition, and low temperature hydrogen annealing.
Evolution of Ar+-damaged graphite surface during annealing as investigated by scanning probe microscopy
Authors: An B. , Fukuyama S. , Yokogawa K. , Yoshimura M.
Published: September 2002

The surface evolution of highly oriented pyrolytic graphite irradiated with Ar+ ions of 1.0 keV at doses between 5×1011 and 1×1013 ions/cm2 during annealing was investigated by scanning tunneling microscopy (STM) and atomic force microscopy (AFM) in the tapping mode. Hillocks were observed by both STM and AFM after ion irradiation, where the height of a hillock measured by STM was larger than that measured by AFM. The ion-irradiated surface was recovered in three stages during annealing: the first stage at 473–873 K, the second stage at 873–1473 K, and the third stage at 1473–1873 K. In the first stage, many of the ion-induced hillocks recovered rapidly and irregular domelike protrusions were formed due to both the recombination of the mobile interstitial clusters with the immobile vacancies and the aggregation of interstitial clusters. In the second stage, the hillocks recovered slightly and domelike protrusions aggregated to larger domelike protrusions. In the third stage, the hillocks recovered completely and domelike protrusions changed from irregular shapes to regular circles with monatomic step height of graphite due to the change from irregular carbon interstitial clusters to complete extraplane in graphite. Hexagonal hollows were also formed and became larger circular hollows above 1623 K with monatomic step height of graphite due to the vacancy clusters formed by the migration of vacancies and the following collapse of the neighboring layers in graphite.
Optical waveguides fabricated by pulsed-laser deposition of SiO2 films with different refractive indices
Authors: Okoshi M. , Kuramatsu M. , Inoue N.
Published: September 2002

We deposited pure SiO2 films with different refractive indices by pulsed-laser deposition with silicone targets. The refractive index of the films could be controlled by the deposition rate. Lowering of the deposition rate helped to make a dense film, showing higher refractive index. A 0.4-μm-thick SiO2 cladding film deposited at 0.1 nm/pulse was first formed on the whole surface of a Si wafer, and then a 1-μm-thick SiO2 core film at 0.05 nm/pulse was fabricated in a line on the sample. The sample functioned as an optical waveguide for a 633 nm line of He–Ne laser.
The use of distributed partial wave basis for accurate atom–molecule statistical distributions
Authors: Furuhama Ayako , Ten-no Seiichiro
Published: September 2002

We apply the distributed partial wave formulation to the calculation of atom–molecule statistical distributions. Site radial correlation functions are computed accurately by coupling multicenter contributions using the spherical harmonics expansion of three-dimensional intramolecular correlation functions. The results are systematically improved as the order of expansions increases. It is shown that the technique using spherical Bessel transforms with logarithmic grids, is particularly cost effective for accurate statistical distribution functions. We compared some numerical results with exact functions obtained by numerical integrations.
Application of the local self-consistent-field method to core-ionized and core-excited molecules, polymers, and proteins: True orthogonality between ground and excited states
Authors: Ferré Nicolas , Assfeld Xavier
Published: September 2002

True orthogonality between single-determinant ground state and excited states is achieved through the use of the ab initio local self-consistent-field method. This method allows to optimize the electronic wave function under the constraint of orthogonality to one or several frozen spin–orbitals, virtuals or not. Core- and valence ionization energies of small molecules are computed and show the accuracy of this new scheme. First applications deal with core ionization of macromolecular systems, like proteins or polymers.
Pressure studies of the photodimerization of oriented anthracene pairs in a dianthracene crystal: Fast tunneling of a heavy particle
Authors: Chan I. Y. , Ismail Huzeifa , Prass B. , Stehlik D.
Published: September 2002

We report a study of the photodimerization of properly arranged anthracene pairs generated by photolysis in a dianthracene crystal at 2 K. We monitor the progress of the photodimerization reaction by measuring the fluorescence lifetime of the pair excimer state, and we use pressure as an empirical parameter. Dimerization is too fast to monitor beyond 6 kbar for the normal (protonated) anthracene pairs, and beyond 10 kbar for the pedeutero sample. The results are interpreted as dimerization through a tunneling mechanism, although evidence of a photophysical retardation was observed. Pressure enhancement of the fluorescence decay rate is exponential. The pressure coefficient for rate enhancement is 0.203 (0.010) natural log units per kbar for the normal sample, and 0.2576 (0.0065) for the perdeuterated sample, respectively (with the standard deviation of the mean given in parentheses). The reaction may be formally construed as nanosecond tunneling of a very heavy particle. The origin of the “reverse” deuterium isotope effect is discussed.
Photoelectron spectroscopy of HCCN− and HCNC− reveals the quasilinear triplet carbenes, HCCN and HCNC
Authors: Nimlos Mark R. , Davico Gustavo , Geise C. Michael , Wenthold Paul G. , Lineberger W. Carl , Blanksby Stephen J. , Hadad Christopher M. , Petersson George A. , Ellison G. Barney
Published: September 2002

Negative ion photoelectron spectroscopy has been used to study the HCCN− and HCNC− ions. The electron affinities (EA) of cyanocarbene have been measured to be EA(HCCN X̃ 3Σ−=2.003±0.014 eV and EA(DCCN X̃ 3Σ−)=2.009±0.020 eV. Photodetachment of HCCN− to HCCN X̃ 3Σ− shows a 0.4 eV long vibrational progression in ν5, the H–CCN bending mode; the HCCN− photoelectron spectra reveal excitations up to 10 quanta in ν5. The term energies for the excited singlet state are found to be T0(HCCN ã 1A′)=0.515±0.016 eV and T0(DCCN ã 1A′)=0.518±0.027 eV. For the isocyanocarbene, the two lowest states switch and HCNC has a singlet ground state and an excited triplet state. The electron affinities are EA(HCNC X̃ 1A′)=1.883±0.013 eV and EA(X̃ 1A′ DCNC)=1.877±0.010 eV. The term energy for the excited triplet state is T0(HCNC ã 3A″)=0.050±0.028 eV and T0(DCNC ã 3A″)=0.063±0.030 eV. Proton transfer kinetics in a flowing afterglow apparatus were used to re-measure the enthalpy of deprotonation of CH3NC to be ΔacidH298(CH3NC)=383.6±0.6 kcal mol−1. The acidity/EA thermodynamic cycle was used to deduce D0(H–CHCN)=104±2 kcal mol−1 [ΔfH0(HCCN)=110±4 kcal mol−1] and D0(H–CHNC)=106±4 kcal mol−1 [ΔfH0(HCNC)=133±5 kcal mol−1].
Modeling triangular titration fronts in the O2+H2 reaction on a catalytic Rh(111) surface
Authors: Monine Michael , Pismen Len , Bär Markus , Or-Guil Michal
Published: September 2002

We present a model for the titration of an oxygen saturated catalytic Rh(111) surface with hydrogen. Oxygen is removed by reaction-diffusion fronts. Experimentally, these fronts have been observed to be either isotropic or triangular depending on the conditions of preparation of the oxygen layer as well as on temperature and hydrogen pressure. If we model only the surface reaction and the diffusion of hydrogen, we obtain isotropic fronts with velocities in the range of 2–3 μm s−1. These results are in line with experimental measurements for surfaces exposed to oxygen for a short period. To correctly reproduce the possible triangular shape of the titration fronts and the smaller front velocities of 0.1–1 μm s−1 for experiments with the surface exposed to oxygen for a long time (>1 h), we have to include the formation of a subsurface oxygen-rich phase and its removal. It is assumed that the phase transition between the oxygen-rich and oxygen-free subsurface phases occurs via front propagation, and the front speed has triangular symmetry compatible with the symmetry of the crystalline bulk. By fitting parameters describing the propagation of the phase transition front, its anisotropy and its coupling to the reaction-diffusion front on the surface, we have been able to quantitatively reproduce all experimental observations presented by Schaak and Imbihl in Chem. Phys. Lett. 283, 386 (1998).
Dynamic force microscopy in superfluid helium
Authors: Callaghan F. D. , Yu X. , Mellor C. J.
Published: September 2002

Piezoelectric quartz tuning forks have been used for topographic dynamic force imaging in superfluid helium and in high magnetic fields. This has been achieved by immobilizing one tine of the tuning fork to stabilize its behavior in superfluid. Images acquired at room temperature and at 50 K are also presented. Frequency–distance curves are shown to be markedly different in superfluid than in air due to a long-range fork–sample interaction in liquid. Evidence is presented that this is due to a change in the hydrodynamic effective mass of the fork as the gap between the fork and sample is reduced. In addition, Q-control has been implemented and used to both increase and decrease the quality factors of tuning forks in both vacuum and superfluid helium.
Spin–phonon interaction and mode softening in NiF2
Authors: Lockwood D. J.
Published: September 2002

The temperature dependence of the four Raman-active phonons in NiF2 is investigated at temperatures above and below the antiferromagnetic ordering temperature of TN=73 K. All four modes exhibit distinct anomalies in their intensities and frequencies near TN due to spin–phonon coupling. The phonon linewidths also exhibit weak anomalies. From the temperature dependences of the phonon frequencies, estimates are made of the spin–phonon coupling coefficients. The B1g phonon exhibits anomalous mode softening with decreasing temperature from 300 K to TN.
Nonlinear optical spectroscopy of epitaxial magnetic garnet films
Authors: Pavlov V. V. , Pisarev R. V. , Fiebig M. , Fröhlich D.
Published: September 2002

Second and third harmonic optical spectra are studied in epitaxial magnetic thin films in the spectral ranges 1.7–3.2 eV and 2.4–4.2 eV, respectively. No significant increase of the intensity of the nonlinear spectra is found above the bandgap near 3.2 eV, where the linear absorption increases by two orders of magnitude. Large magnetic contributions to the second harmonic spectra and magnetic contrast as high as 100% are observed at selected photon energies. Contrary to that, no magnetic contribution to the third harmonic spectra is found.
Optical study of 4f excitations in rare earth cuprates
Authors: Nekvasil V.
Published: September 2002

Several recent examples are used to demonstrate that Raman and infrared spectroscopy can be successfully used as a novel experimental tool to study microscopic processes involving 4f electrons in rare earth (RE) cuprates. Raman-active crystal field (CF) excitations in Nd2CuO4 were measured under hydrostatic pressure up to ∼7 GPa. The observed pressure-induced shifts of the CF levels were interpreted using density-functional-theory-based ab initio calculations and the superposition model. An infrared transmission study of the IJ4, J=9/2, 11/2, 13/2 multiplets of Nd3+ in Nd2CuO4 reveals a splitting of the Kramers doublets of the order of a few cm−1 due to the Nd-Cu exchange interaction. This study shows that these splittings can be described by an effective anisotropic exchange Hamiltonian for the Nd3+ ion expressed in terms of spherical tensor operators up to the sixth order. The isotropic term in the exchange Hamiltonian vanishes for symmetry reasons in this case. An analysis of the infrared transmission spectra in RE1+xBa2−xCu3O6+δ (RE=Nd,Sm) up to ∼ 10 000 cm−1 indicates that, besides the regular sites, the RE ions also occupy Ba sites, even in the samples with the cation stoichiometry 1–2–3.
Specific heat study of magnetic excitations in a one-dimensional S=1 Heisenberg magnet with strong planar anisotropy
Authors: Feher A. , Orendáč M. , Orendáčová A. , Čižmár E.
Published: September 2002

The results of experimental studies of the specific heat of the magnetic chain compounds Ni(C2H8N2)2Ni(CN)4, Ni(C11H10N2O)2Ni(CN)4, and Ni(C10H8N2)2Ni(CN)4⋅H2O are reported. All compounds are identified as S=1 planar Heisenberg magnetic chains with large planar anisotropy and different values of the in-plane anisotropy constant. The low-temperature specific heat data are interpreted assuming the existence of noninteracting excitons and antiexcitons as elementary excitations from the singlet-ground state. The extended strong-coupling model is used for analysis of the data at higher temperatures. The applicability of the models used with respect to the value of the in-plane anisotropy is discussed.
Gaussian ellipsoid model for confined polymer systems
Authors: Eurich Frank , Maass Philipp , Baschnagel Jörg
Published: September 2002

Polymer systems in slab geometries are studied on the basis of the recently presented Gaussian ellipsoid model [F. Eurich and P. Maass, J. Chem. Phys. 114, 7655 (2001)]. The potential of the confining walls has an exponential shape. For homogeneous systems in thermodynamic equilibrium we discuss density, orientation, and deformation profiles of the polymers close to the walls. For strongly segregated mixtures of polymer components A and B equilibrium profiles are studied near a planar interface separating A and B rich regions. Spinodal decomposition processes of the mixtures in the presence of neutral walls show upon strong confinement an increase of the lateral size of A and B rich domains and a slowing down of the demixing kinetics. These findings are in agreement with predictions from time dependent Ginzburg–Landau theory. In the case, where one wall periodically favors one of the two mixture components over the other, different equilibrium structures emerge and lead to different kinetic pathways of spinodal decomposition processes in such systems.
Evaluation of the viscoelastic properties of a nematic dimer by cone-and-plate rheo-nuclear magnetic resonance spectroscopy and comparison with Leslie–Ericksen theory
Authors: Lukaschek Michail , Kothe Gerd , Schmidt Claudia , Gomes Alexandre E. , Polimeno Antonino
Published: September 2002

The viscoelastic behavior of a nematic dimer is studied by deuteron (2H) nuclear magnetic resonance (NMR) spectroscopy using a cone-and-plate rheometer. From the H2 NMR spectra, monitored as a function of the shear rate, the orientation of the director relative to the magnetic field is obtained. Analysis of this experiment, using an analytical and a numerical solution of the Leslie–Ericksen equations, gives detailed information on the viscoelastic properties of the dimer. In particular, the two Leslie viscosities α2 and α3 and the three-dimensional director distribution are determined. The analysis reveals that α2 and α3 are both positive, that is, the dimer is of the flow-aligning type, and the director is tilted by an angle of about 33° from the magnetic field toward the velocity axis.
Trading quantum for classical resources in quantum data compression
Authors: Hayden Patrick , Jozsa Richard , Winter Andreas
Published: September 2002

We study the visible compression of a source E={|φi〉,pi} of pure quantum signal states or, more formally, the minimal resources per signal required to represent arbitrarily long strings of signals with arbitrarily high fidelity, when the compressor is given the identity of the input state sequence as classical information. According to the quantum source coding theorem, the optimal quantum rate is the von Neumann entropy S(E) qubits per signal. We develop a refinement of this theorem in order to analyze the situation in which the states are coded into classical and quantum bits that are quantified separately. This leads to a trade-off curve Q*(R), where Q*(R) qubits per signal is the optimal quantum rate for a given classical rate of R bits per signal. Our main result is an explicit characterization of this trade-off function by a simple formula in terms of only single-signal, perfect fidelity encodings of the source. We give a thorough discussion of many further mathematical properties of our formula, including an analysis of its behavior for group covariant sources and a generalization to sources with continuously parametrized states. We also show that our result leads to a number of corollaries characterizing the trade-off between information gain and state disturbance for quantum sources. In addition, we indicate how our techniques also provide a solution to the so-called remote state preparation problem. Finally, we develop a probability-free version of our main result which may be interpreted as an answer to the question: “How many classical bits does a qubit cost?” This theorem provides a type of dual to Holevo’s theorem, insofar as the latter characterizes the cost of coding classical bits into qubits.
Surface-modified GaAs terahertz plasmon emitter
Authors: Darmo J. , Strasser G. , Müller T. , Bratschitsch R. , Unterrainer K.
Published: September 2002

We studied the THz emission from n-GaAs plasmon emitters modified by low-temperature-grown (LT) GaAs surface layers. The THz emission is increased since the LT GaAs pins the Fermi level at a midgap position, increasing the surface depletion field. For a THz emitter with a 70-nm-thick LT GaAs layer we observe without external fields a THz emission intensity of 140 nW. In addition, the long-term performance of the modified emitters is improved by the LT GaAs surface layer.
Pure-carbon ring transistor: Role of topology and structure
Authors: Cuniberti Gianaurelio , Yi Juyeon , Porto Markus
Published: September 2002

We report results on the rectification properties of a carbon nanotube (CNT) ring transistor, contacted by CNT leads, whose novel features have been recently communicated by H. Watanabe, C. Manabe, T. Shigematsu, and M. Shimizu [Appl. Phys. Lett. 78, 2928 (2001)]. This letter contains results which are validated by the experimental observations. Moreover, we report on additional features of the transmission of this ring device which are associated with the possibility of breaking the lead inversion symmetry. The linear conductance displays a chessboardlike behavior alternated with anomalous zero lines which should be directly observable in experiments. We are also able to discriminate the structural properties showing up in our results (quasione-dimensional confinement) from pure topological effects (ring configuration), thus helping to gain physical intuition on the rich ring phenomenology.
Degradation of LaMnO3−y surface layer in LaMnO3−y/metal interface
Authors: Plecenik A. , Fröhlich K. , Espinós J. P. , Holgado J. P. , Halabica A. , Pripko M. , Gilabert A.
Published: September 2002

We report electrical measurements showing the degradation processes of LaMnO3−y (LaMnO) in LaMnO/normal metal interface in both point contact and planar-type junctions. The time evolution of the junction resistance is characterized by the appearance of a second maximum in the resistance versus temperature dependence (R–T) that occurs at a temperature lower than that for the metal–insulator transition in the bulk. These effects are explained in terms of the formation of a depleted interface layer in LaMnO caused by an out-diffusion of oxygen from the manganite surface to the normal metal. This was confirmed by XPS measurements. Similar results on LaSrMnO3−y/metal interfaces were also obtained.
Reduction of the light-onset voltage of light-emitting diodes based on a soluble poly(p-phenylene vinylene) by grafting polar molecules onto indium–tin oxide
Authors: Morgado J. , Charas A. , Barbagallo N.
Published: September 2002

We report on the reduction of the light-onset voltage of light-emitting diodes based on poly[(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] with aluminum cathodes down to 2.2 V by grafting polar molecules onto indium-tin oxide. This value is close to Eg/e=2.13 V, Eg being the energy gap estimated from the absorption onset and e the electron charge, in spite of the poor electron injection ability of the aluminum cathode. In addition, there is a very efficient suppression of the commonly observed low-voltage current peaks.
Organic single-layer electroluminescent devices fabricated on CuOx-coated indium tin oxide substrate
Authors: Hu Wenping , Matsumura Michio
Published: September 2002

Organic single-layer devices were fabricated by depositing tris(8-quinolinato)aluminum (ALQ) layer on CuOx-coated indium tin oxide (ITO) substrates. These organic single-layer devices showed luminance as high as 2500 cd/m2, and the luminance-current efficiency reached 1.2 cd/A, which was higher than that of single-layer devices fabricated on conventional ITO substrates by eight times. The improvement in the electroluminescent properties of these devices was attributed to the enhanced hole injection from the CuOx-coated ITO into the ALQ layer.
Passivation of GaAs(110) with Ga2O3 thin films deposited by electron cyclotron resonance plasma reactive molecular beam epitaxy
Authors: Kim D. , Chen G. , Mei X. Y. , Ruda H. E.
Published: September 2002

Gallium oxide thin films deposited by electron cyclotron resonance plasma molecular beam epitaxy on GaAs(110) surfaces are reported. Room temperature photoluminescence spectra show an enhancement over as-is surfaces by greater than an order of magnitude for semi-insulating wafers. This enhancement is corroborated by low temperature photoluminescence spectra, showing a reduction in AsGa, OAs, and carbon-related emissions. The bonding configuration at the interface to GaAs was investigated by x-ray photoelectron spectroscopy depth profiling and secondary ion mass spectroscopy: Arsenic oxide related compounds were below the sensitivity limits of the former technique, while carbon (both in the film and in the vicinity of the interface) was below the sensitivity limit of the latter technique. Photoluminescence enhancement is also attributed to hydrogen passivation of EL2 defects, which is found to be stable following deposition at temperatures of 400 °C on semi-insulating and p-type wafers.
A revised MRCI-algorithm coupled to an effective valence-shell Hamiltonian. II. Application to the valence excitations of butadiene
Authors: Strodel Paul , Tavan Paul
Published: September 2002

In Paper I of this work we have sketched an improved MRCI algorithm and its coupling to the effective valence-shell Hamiltonian OM2. To check the quality of the resulting OM2/MRCI approach, it is applied here to the excited valence states of all-trans butadiene. As is explained by a review of previous theoretical work, proper descriptions of these states posed severe problems within correlated ab initio treatments but seemed to be trivial within simple correlated π-electron models. We now show that an extended MRCI treatment of the correlations among all valence electrons as described by OM2 closely reproduces the experimental evidence, placing the vertical 2 1Ag excitation by about 0.2 eV below the 1 1Bu excitation. By an analysis of σ–π interactions we explain the corresponding earlier success of correlated π-electron theory. Exploiting the enhanced capabilities of the new approach we investigate the potential surfaces. Here, OM2/MRCI is shown to predict that the 2 1Ag state is energetically lowered about four times more strongly than the 1 1Bu state upon geometry relaxation constrained to the C2h symmetry. We conclude that OM2/MRCI should be well-suited for the study of excited state surfaces of organic dye molecules.
From amplified spontaneous emission to microring lasing using nanocrystal quantum dot solids
Authors: Malko A. V. , Mikhailovsky A. A. , Petruska M. A. , Hollingsworth J. A. , Htoon H. , Bawendi M. G. , Klimov V. I.
Published: September 2002

We study different emission regimes in close-packed films of chemically synthesized CdSe nanoparticles [nanocrystal quantum dots (NQDs)]. We observe that the NQD photoluminescence is dominated by excitons and biexcitons, respectively, before and after the threshold for stimulated emission. Furthermore, we demonstrate the regime of microring lasing into sharp, whispering-gallery modes using NQD solids incorporated into microcapillary tubes. This result indicates a feasibility of miniature, solid-state laser devices based on chemically synthesized NQDs.
Top-emission ultraviolet light-emitting diodes with peak emission at 280 nm
Authors: Yasan A. , McClintock R. , Mayes K. , Darvish S. R. , Kung P. , Razeghi M.
Published: September 2002

We demonstrate light emission at 280 nm from UV light-emitting diodes consisting of AlInGaN/AlInGaN multiple quantum wells. Turn-on voltage of the devices is ∼5 V with a differential resistance of ∼40 Ω. The peak emission wavelength redshifts ∼1 nm at high injection currents.
Spatial dependence of ultrafast carrier recombination centers of phosphorus-implanted and annealed silicon wafers
Authors: Othonos Andreas , Christofides Constantinos
Published: September 2002

In this letter, the spatial dependence of the carrier recombination centers induced in phosphorus-implanted and annealed silicon wafers have been examined. Ultrafast time-resolved reflectivity measurements of a set of phosphorus-implanted annealed silicon wafers (1016 P+/cm2) as a function of position on the wafer have been carried out, and an x–y map of the carrier lifetime for each of the samples has been obtained. Measurements reveal distinct features of the distribution of carrier recombination centers for the nonannealed and annealed samples between 350 °C and 1100 °C in an area of 36×36 μm2 with resolution better than 3 μm. The presence of islands of clusters in ion-implanted and annealed samples is also discussed in this letter.
Low-damage etched/regrown interface of strain-compensated GaInAsP/InP quantum-wire laser fabricated by CH4/H2 dry etching and regrowth
Authors: Yagi Hideki , Muranushi Kengo , Nunoya Nobuhiro , Sano Takuya , Tamura Shigeo , Arai Shigehisa
Published: September 2002

GaInAsP/InP strain-compensated five-layered quantum-wire lasers with the wire width of 23 nm were fabricated by electron-beam lithography, CH4/H2-reactive ion etching and organometallic vapor-phase-epitaxial regrowth, and the quality of the etched/regrown interface was evaluated from the spontaneous emission efficiency dependence on the active region width in comparison with that of unetched quantum-well lasers. As a result, the product of the surface recombination velocity and the carrier lifetime at the etched/regrown interface was estimated to be less than 3 nm at room temperature. Finally, no noticeable degradation in the spontaneous emission efficiency of this quantum-wire laser was observed within measurement temperature from 25 °C to 85 °C.
Gallium arsenide photodetectors for imaging in the far ultraviolet region
Authors: Caria M. , Barberini L. , Cadeddu S. , Giannattasio A. , Rusani A. , Sesselego A. , Lai A. , D’Auria S. , Dubecky F.
Published: September 2002

The aim of the present work is to systematically investigate the response and stability of commercial GaAs devices in the 200–400 nm UV range with a view to establishing their potentiality in imaging devices. The irradiation results of GaAs detectors with various geometries are presented and discussed. The detectors were reverse biased in fully depleted condition and in partially depleted condition (5 V reverse bias) in order to investigate the possibilities of integration with the standard bias values of read-out-integrated circuits. The results show that fabrication technology for nondedicated devices is still immature.
Epitaxial PbZr.52Ti.48O3 films on SrTiO3/(001)Si substrates deposited by sol–gel method
Authors: Talin A. A. , Smith S. M. , Voight S. , Finder J. , Eisenbeiser K. , Penunuri D. , Yu Z. , Fejes P. , Eschrich T. , Curless J. , Convey D. , Hooper A.
Published: September 2002

We report on the sol–gel deposition and characterization of high-quality, epitaxial films of PbZr.52Ti.48O3 (PZT) on (001)Si substrates, with a thickness range of 400 Å to 1 μm. The epitaxial growth of PZT on (001)Si is achieved using a thin template layer of SrTiO3, grown by molecular-beam epitaxy. The sol–gel PZT films have a typical surface roughness of 5 Å and exhibit well defined reflective high-energy electron diffraction patterns characteristic of smooth, epitaxial films. Using high-resolution transmission electron microscopy and double-crystal x-ray diffraction, we find that the PZT films are oriented with the c axis normal to the (001)Si plane and with the a axis lying along 〈110〉Si direction. Finally, we measure the electromechanical coupling coefficients and the surface acoustic wave velocities for our films as a function of thickness and compare our experimental data to previously published theoretical values for this system.
Photophysical studies of mixed furan, pyrrole, and thiophene-containing oligomers with three and five rings
Authors: Seixas de Melo J. , Elisei Fausto , Becker Ralph S.
Published: September 2002

The photophysics of several oligomers containing mixed furan, pyrrole, and thiophene heterocyclic systems is reported. The mixed systems contain three rings and five rings of the heterocycles. Comprehensive spectroscopic and photophysical data were obtained and all of the rate constants kF, kIC, and kISC were evaluated. The lowest singlet excited state is of (1)B-like origin in any solvent. It is possible to have a reasonable understanding of the photophysics of the mixed ring systems compared to all the thiophene analogs if it is considered that some π–electron decoupling occurs at the site of the pyrrole or furan substitution, although this cannot be the total answer, as is discussed.