bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermal expansion measurements of glass-forming alloys in the melt and the undercooled state under microgravity conditions
Authors: Damaschke B. , Samwer K.
Published: July 2002

We present measurements of the thermal expansion of the glass forming alloys Zr57Cu15.4Ni12.6Nb5Al10, Zr65Cu17.5Al7.5Ni10, Zr11Cu47Ti34Ni8, Zr60Cu18Al10Ni9Co3, Pd78Cu6Si16, and Pd82Si18 in the melt and the undercooled state in the TEMPUS facility under microgravity conditions. The samples were imaged with a high-resolution charge-coupled device-camera with on-board recording. After the mission, the video tapes were analyzed by digital image processing. The volume was found to be a linear function of temperature in the temperature range investigated. The thermal expansion coefficients in the melt and the undercooled state are between 5.5×10−5 and 7.9×10−5 K−1. The volume increment at the melting point was between 1% and 2.1%. Phase separations could be observed by a change of the slope of the volume versus temperature curve. The thermal expansion data can be interpreted qualitatively in terms of the free volume model.
Direct imaging of the spectral emission characteristic of an InGaN/GaN-ultraviolet light-emitting diode by highly spectrally and spatially resolved electroluminescence and photoluminescence microscopy
Authors: Fischer Peter , Christen Jürgen , Zacharias Margit , Schwegler Veit , Kirchner Christoph , Kamp Markus
Published: July 2002

The microscopic spectral emission characteristic of an InGaN/GaN double-heterostructure light-emitting diode is directly imaged by highly spectrally and spatially resolved scanning electroluminescence microscopy under operation as a function of injection current density. The luminescence intensity maps and especially the peak-wavelength scanning images provide access to the optical quality of the final device and yield direct images of the In fluctuations with 1 μm spatial resolution. Indium concentrations varying from 6% to 9% are found in the active InGaN region of the ultraviolet diode emitting at 400 nm. While for low injection current densities the electroluminescence is dominated by emission from the p GaN originating from the whole accessible area, the emission from the InGaN active layer increases and takes over for higher injection conditions showing a strongly localized spatial emission characteristic. Correlation of the results with low-temperature scanning photoluminescence microscopy enables the identification of the underlying recombination processes.
Quasi-intrinsic semiconducting state of titanyl-phthalocyanine films obtained under ultrahigh vacuum conditions
Authors: Tada Hirokazu , Touda Hiroshi , Takada Masaki , Matsushige Kazumi
Published: July 2002

We investigated electrical properties of titanyl-phthalocyanine (TiOPc) films under ultrahigh vacuum (UHV) conditions to avoid the influence of gas adsorption. The field-effect measurement revealed that TiOPc films exhibited an n-type semiconducting behavior in UHV. The electron mobility at room temperature was 9×10−6 cm2 V−1 s−1 with activation energy of 0.20 eV. The conductivity and carrier density were 9×10−8 S cm−1 and 6×1016 cm−3, respectively. A clear conversion from n-type to p-type behavior was observed when the film was exposed to oxygen. Strict control of atmosphere made it possible to obtain a quasi-intrinsic state where both p- and n-type conductions appeared simultaneously.
Micro-Raman and cathodoluminescence studies of epitaxial laterally overgrown GaN with tungsten masks: A method to map the free-carrier concentration of thick GaN samples
Authors: Kaschner A. , Hoffmann A. , Thomsen C. , Bertram F. , Riemann T. , Christen J. , Hiramatsu K. , Sone H. , Sawaki N.
Published: July 2002

Cathodoluminescence (CL) and micro-Raman spectroscopy were applied to study microscopically the optical and structural properties of two epitaxial-laterally overgrown GaN structures with tungsten masks in 〈11_00〉 and 〈112_0〉 direction, respectively. A free-carrier concentration higher than 1019 cm−3 was observed right above the masks, leading to a gradient in free-carrier concentration over the whole layer thickness. We used the normalized longitudinal-optical-phonon intensity and the broad band around 650 cm−1 as a measure for the free-electron concentration, which is a promising method to determine the free-carrier concentration justified by the good correlation with CL results.
Enhanced degradation resistance of quantum dot lasers to radiation damage
Authors: Piva P. G. , Goldberg R. D. , Mitchell I. V. , Labrie D. , Leon R. , Charbonneau S. , Wasilewski Z. R. , Fafard S.
Published: July 2002

We compare the degradation of InAs/GaAs quantum well (QW) and quantum dot (QD) laser diodes following irradiation by high energy (8.56 MeV) phosphorous ions. Over a fluence range of 108–1011 ions/cm2, the degradation of the low temperature QD photoluminescence and electroluminescence emission is greatly suppressed relative to that of QW based devices (×100 and ×1000, respectively at the highest dose studied). Irradiated QD laser diodes demonstrated lasing action over the entire range of fluences, and 2 orders of magnitude beyond the maximum dose sustainable by QW devices. The improved damage response of QD based structures results from efficient collection and localization of electrons and holes by QDs in the active region, which limit carrier transfer to nonradiative centers. This work suggests the suitability of QD device architectures for use in radiation environments, and in high power applications, wherever nonradiative processes promote the degradation or failure of traditional QW devices.
Surface enhanced sum frequency generation of carbon monoxide adsorbed on platinum nanoparticle arrays
Authors: Baldelli Steve , Eppler Aaron S. , Anderson Erik , Shen Yuen-Ron , Somorjai Gabor A.
Published: July 2002

Sum frequency generation (SFG) vibrational spectroscopy is used to study the adsorption of CO at ∼1 atm pressure on Pt nanoparticle arrays and Pt thin films. The SFG signal of CO adsorbed on platinum particles of 45 nm diameter is ∼10 000 times larger than from CO on smooth Pt films. The large enhancement is explained by plasmon resonance and Maxwell–Garnett theory. The Pt arrays are prepared using electron beam lithography to produce particles with uniform spacing and sizes on an oxidized Si(100) wafer. Further, as the Pt coverage increases the SFG signal shows a polarization dependence that is explained considering the dielectric properties of a metal film on a dielectric surface. In addition, SFG permits investigation of the CO adsorbed on the particles at ∼1 atm, which is not possible with most surface analytical techniques, that will allow for the study of the reaction of small molecules on surfaces relevant in heterogeneous catalysis.
Effective initial sorting of undulator magnets
Authors: Tanaka Takashi , Kitamura Hideo
Published: July 2002

A new method to optimize undulator magnet arrangement is considered. Field-integral values of magnet blocks instead of magnetic flux distributions are used to determine the magnet arrangement for the purpose of not only reducing integrated multipole but also improving spectral intensity. A system composed of a fast-rotating coil and a lock-in amplifier has been constructed for fast and accurate measurement of field integrals. A ten-period test undulator has been assembled with the best and the worst arrangements determined by the optimization method. Undulator performances such as integrated multipoles, orbit deviations, and phase errors have been measured at various gap values, which showed a good agreement with expectations in both cases.
Rovibrational structures of the pseudorotating lithium trimer Li321: Rotationally resolved spectroscopy and ab initio calculations of the A 2E″←X 2E′ system
Authors: Keil M. , Krämer H.-G. , Kudell A. , Baig M.A. , Zhu J. , Demtröder W. , Meyer W.
Published: July 2002

Based on resonant two-photon ionization of Li321 with cw lasers and mass-selective detection of Li3+21 ions by a quadrupole mass spectrometer, isotope-selective rotationally resolved spectra of vibronic bands in the electronic A 2E″←X 2E′ system have been recorded. The complex but completely resolved spectra could be analyzed using optical–optical double resonance techniques in combination with accurate ab initio calculations which provide potential energy surfaces and rovibronic term values for both electronic states. A detailed comparison between experimental and theoretical results for the (vs′=0, vb′=0, va′=0)←(vs″=0, vb″=0, va″=0) band demonstrates excellent agreement. The coupling between pseudorotation and rotational motion is shown to produce complex level patterns which, however, can be accurately represented by an effective pseudo/rotation Hamiltonian in terms of rotational and coupling constants that directly provide structural information. For both electronic states the nuclear dynamics is adequately described as a motion on a single adiabatic potential surface with a geometric phase π for closed loops around the conical intersection at D3h geometries.
Structures, vibrational frequencies, and infrared spectra of the hexa-hydrated benzene clusters
Authors: Lee Jin Yong , Kim Jongseob , Lee Han Myoung , Tarakeshwar P. , Kim Kwang S.
Published: July 2002

The water hexamer is known to have a number of isoenergetic structures. The first experimental identification of the O–H stretching vibrational spectra of the water hexamer was done in the presence of benzene. It was followed by the identification of the pure water hexamer structure by vibration-rotational tunneling (VRT) spectroscopy. Although both experiments seem to have located only the Cage structure, the structure of the benzene–water hexamer complex is not clearly known, and the effect of benzene in the water hexamer is unclear. In particular, it is not obvious how the energy difference between nearly isoenergetic water hexamer conformers changes in the presence of benzene. Thus, we have compared the benzene complexes with four low-lying isoenergetic water hexamers, Ring, Book, Cage, and Prism structures, using ab initio calculations. We also investigated the effects of the presence of benzene on the structures, harmonic vibrational frequencies, and infrared (IR) intensities for the four low-lying energy conformers. There is little change in the structure of the water hexamer upon its interaction with the benzene molecule. Hence the deformation energies are very small. The dominant contribution to the benzene–water cluster interaction mainly comes from the π–H interactions between benzene and a single water molecule. As a result of this π–H interaction, O–Hπ bond length increases and the corresponding stretching vibrational frequencies are redshifted. The IR spectral features of both (H2O)6 and benzene–(H2O)6 are quite similar. From both the energetics and the comparison of calculated and experimental spectra of the benzene–(H2O)6, the water structure in these complexes is found to have the Cage form. In particular, among the four different Cage structures, only one conformer matches the experimental O–H vibrational frequencies.
Chiral discrimination in hydrogen-bonded complexes
Authors: Portmann Stefan , Inauen Andreas , Lüthi Hans P. , Leutwyler Samuel
Published: July 2002

We report an accurate ab initio study of the effects of chirality on the intermolecular interactions between two small chiral molecules bound by a single hydrogen bond. The methods used are second-order Møller–Plesset theory (MP2), as well as density functional theory with the B3LYP functional. The differential interaction energy between two homochiral molecules, e.g., R⋅⋅⋅R′ and the analogous heterochiral molecules R⋅⋅⋅S′ measures the degree of chiral discrimination, termed the chirodiastaltic energy, ΔEchir. Formation of the O–H⋅⋅⋅O hydrogen bond between the chiral H-bond donor HOOH and the chiral H acceptor 2-methyl oxirane leads to four diastereomeric complexes. There are two distinct contributions to the chirodiastaltic energies, the diastereofacial contribution which controls the face or side of the acceptor to which the H bond is formed, and the diastereomeric contribution, which is the energy difference between two complexes formed by (M)- and (P)-HOOH to the same face. The largest chirodiastaltic energy is ΔEchir=0.46 kcal/mol (6% of the binding energy) between the syn-(M)- and syn-(P)-HOOH⋅2-methyl oxirane complexes. The chiral 2,3-dimethyloxirane acceptor is C2 symmetric and hence offers two identical faces. Here the chirodiastaltic energy is identical to the diastereomeric energy, and is calculated to be ΔEchir=0.36 kcal/mol or 4.5% of the binding energy.
Theoretical and experimental study of the A2Πu–X2Πg band system of C7−
Authors: Lakin Nicholas M. , Pachkov Mikhail , Tulej Marek , Maier John P. , Chambaud Gilberte , Rosmus Pavel
Published: July 2002

The A2Πu–X2Πg electronic system of gaseous C7− is examined experimentally in the light of theoretical predictions. Ab initio calculations at the RHF, RCCSD(T) and MRCI levels using the aug-cc-pVQZ basis set indicate that the transition is accompanied by a small elongation in the molecule and a significant reduction in the spin-orbit coupling constant. On the basis of these predictions the band profiles of the 000, 101, 201 and 301 transitions were recorded using photodetachment spectroscopy. These spectra revealed the spin-orbit component bands for each transition as well as providing band contours which show partially resolved rotational structure. The experimental spectra are compared to simulations based upon the calculated spectroscopic constants and the possible causes of the main features in the band contours are accessed by least-squares fitting of the profiles for the 000 and 101 transitions. The implications for the recent observation of coincidences between the A2Πu–X2Πg vibronic bands of C7− and the diffuse interstellar bands are discussed.
Effects of misfit dislocations and AlN buffer layer on the GaInN/GaN phase diagram of the growth mode
Authors: Nakajima Kazuo , Ujihara Toru , Miyashita Satoru , Sazaki Gen
Published: July 2002

The thickness-composition phase diagrams of the growth modes were determined for the GaInN-on-GaN (GaInN/GaN) and the GaInN-on-AlN-on-GaN (GaInN/AlN/GaN) structures. For this determination, the strain energy was calculated by considering the stress relaxation due to introduction of misfit dislocations, the surface energy was estimated from bonding enthalpy of the nearest-neighbor bonds on the surface, and the interface energy was estimated by considering both effects of the dangling bonds due to lattice misfit and the abrupt transition of bonding species at the heterointerface. From these phase diagrams, it was found that the layer-by-layer growth such as the Frank–van der Merwe mode was very difficult to obtain for the epitaxial growth of GaInN on GaN when the InN fraction is large. The Volmer–Weber mode is dominant in the phase diagram of the GaInN/GaN structures. The influence of an AlN buffer layer with a larger surface energy was studied by introducing an AlN layer between the GaInN layer and the GaN substrate. It was known that the layer-by-layer growth could be more easily obtained if misfit dislocations were introduced and an AlN layer was used as a buffer.
Infrared spectroscopy study of electrochromic nanocrystalline tungsten oxide films made by reactive advanced gas deposition
Authors: Solis J. L. , Hoel A. , Lantto V. , Granqvist C. G.
Published: July 2002

Nanocrystalline W oxide films were produced by advanced reactive gas deposition. The material consisted of ∼6 nm diameter tetragonal crystallites, as found from x-ray diffraction and electron microscopy. Optoelectrochemical measurements demonstrated electrochromism upon Li+ intercalation/deintercalation, and infrared absorption spectroscopy gave clear evidence for longitudinal and transversal optical modes being modified following the lithiation. Our data were consistent with ionic transport predominantly in disordered grain boundaries and intercrystalline regions and with electrochromism being associated with small polaron formation.
Cooperative energy transfer in Yb3+–Tb3+ codoped silica sol-gel glasses
Authors: Martı́n I. R. , Yanes A. C. , Méndez-Ramos J. , Torres M. E. , Rodrı́guez V. D.
Published: July 2002

Optical properties of Yb3+–Tb3+ codoped silica sol-gel samples have been studied after the gel to glass transition. Different upconversion emissions have been observed under near infrared excitation at about 1 μm. The Tb3+ ions are excited by means of energy transfer processes from Yb3+ ions. The temporal evolution of the blue-green upconversion emissions coming from Tb3+ ions and their dependence on the excitation intensity at about 1 μm has been studied. The experimental results are in good agreement with a cooperative resonant energy transfer mechanism from Yb3+ ions. An efficient backtransfer process is observed from Tb3+ ions towards Yb3+ ions. The upconversion efficiency, which is limited by this backtransfer process, has been obtained and compared with other upconversion results in similar matrix.
High internal electric field in a graded-width InGaN/GaN quantum well: Accurate determination by time-resolved photoluminescence spectroscopy
Authors: Lefebvre P. , Morel A. , Gallart M. , Taliercio T. , Allègre J. , Gil B. , Mathieu H. , Damilano B. , Grandjean N. , Massies J.
Published: July 2002

Time-resolved photoluminescence (PL), at T=8 K, is used to study a graded-width InGaN/GaN quantum well. Across the sample, the well width continuously varies from ∼5.5 to 2.0 nm corresponding to PL peak energies varying between 2.0 and 2.9 eV and to PL decay rates covering four orders of magnitude. The plot of decay times versus PL energies is very well fitted by a calculation of the electron–hole recombination probability versus well width. The only fitting parameter is the electric field in the well, which we find equal to 2.45±0.25 MV/cm, in excellent agreement with experimental Stokes shifts for this type of samples.
Quantum efficiency of exciton-to-charge generation in organic photovoltaic devices
Authors: Pettersson Leif A. A. , Roman Lucimara S. , Inganäs Olle
Published: July 2002

We present an analysis of the internal monochromatic quantum efficiency of photovoltaic devices based on polymer and polymer/fullerene thin films. A quantum efficiency of exciton-to-charge generation is defined as the external monochromatic quantum efficiency normalized to the absorption in the active materials of the device. An upper limit of the efficiency can be determined, and results show that much of the light is absorbed in photoactive layers of the device, whereas only a fraction of the generated excitons is converted to charge carriers and can be collected as photocurrent.
Magnetism in rare earth Co2 compounds under high pressures
Authors: Syshchenko O. , Sechovský V. , Diviš M. , Fujita T. , Hauser R. , Fujii H.
Published: July 2002

We have studied electrical resistivity anomalies connected with magnetism in RECo2 (RE=Nd, Tb, Er, Ho) compounds in pressures up to 8 GPa. At ambient pressure the former two compounds exhibit a second order magnetic phase transition (SOMPT) at TC, whereas a first order magnetic phase transition (FOMPT) is observed in the latter two. Although TC decreases with pressure in all four compounds, the TC(P) dependence for NdCo2 and TbCo2 differs considerably from this for HoCo2 and ErCo2. For the latter two, Tc vs P data deviate dramatically from the initial linear dependence above a critical pressure Pc to become almost pressure independent at higher pressures. We propose that this is reflecting the loss of Co metamagnetism that is also indicated by the vanishing resistivity drop at Tc for P>4 GPa and by the change from a FOMPT to a SOMPT. A scenario is discussed assuming that for P>Pc the projected Co 3d density of states at EF decreases. Hence the Co moment collapses because the RE-Co-RE exchange channel becomes ineffective to induce the itinerant 3d electron metamagnetism. The localized RE moments, however, order at a “residual” TC due to the persisting Ruderman–Kittel–Kasuya–Yosida-type exchange interaction. For NdCo2 and TbCo2 the Tc values decrease with pressure by an exponential law but remain rather high in the highest applied pressures. Results of first-principles electronic structure calculations using the full-potential linearized augmented plane wave method are presented for HoCo2 and NdCo2 compounds, as well.
Dynamics of DNA in entangled polymer solutions: An anisotropic friction model
Authors: Noguchi Hiroshi , Takasu Masako
Published: July 2002

We studied the electrophoretic behavior of DNA chains in linear-polymer solutions using Brownian dynamics with an anisotropic friction model in a three-dimensional space and projected on the x axis. For the three-dimensional model with a chain segment equal to 1/8 of the Kuhn length, a chain migrates with U-shaped conformation with low anisotropy of friction. With high anisotropy of friction, a chain always migrates with linear-shaped conformation with high segment-density regions, which remain at the same positions in space. This migration mode agrees with the observation of DNA in highly entangled solutions [Ueda et al., Biophys. Chem. 71, 113 (1998)]. The projection model also reproduces the linear-shaped motion. We clarified that the essential conditions for linear shaped motion are the sufficient chain length of DNA, the small mesh size, and strong confinement by entanglement with solvent polymers.
Mechanism of radio-frequency current collapse in GaN–AlGaN field-effect transistors
Authors: Tarakji A. , Simin G. , Ilinskaya N. , Hu X. , Kumar A. , Koudymov A. , Yang J. , Khan M. Asif , Shur M. S. , Gaska R.
Published: July 2002

The mechanism of radio-frequency current collapse in GaN–AlGaN heterojunction field-effect transistors (HFETs) was investigated using a comparative study of HFET and metal–oxide–semiconductor HFET current–voltage (I–V) and transfer characteristics under dc and short-pulsed voltage biasing. Significant current collapse occurs when the gate voltage is pulsed, whereas under drain pulsing the I–V curves are close to those in steady-state conditions. Contrary to previous reports, we conclude that the transverse electric field across the wide-band-gap barrier layer separating the gate and the channel rather than the gate or surface leakage currents or high-field effects in the gate–drain spacing is responsible for the current collapse. We find that the microwave power degradation in GaN–AlGaN HFETs can be explained by the difference between dc and pulsed I–V characteristics.
Stress engineering during metalorganic chemical vapor deposition of AlGaN/GaN distributed Bragg reflectors
Authors: Waldrip K. E. , Han J. , Figiel J. J. , Zhou H. , Makarona E. , Nurmikko A. V.
Published: July 2002

In situ stress monitoring has been employed during metalorganic chemical vapor deposition of AlGaN/GaN distributed Bragg reflectors (DBRs). It was found that the insertion of multiple AlN interlayers is effective in converting the tensile growth stress typically observed in this system into compression, thus alleviating the problem of crack generation. Crack-free growth of a 60 pair Al0.20Ga0.80N/GaN quarter-wavelength DBR was obtained over the entire 2 in. wafer; an accompanying reflectivity of at least 99% was observed near the peak wavelength around 380 nm.
Concentration dependence of the dielectric constant in mixed oxides MxOyMp′Oq
Authors: Devine R. A. B. , Revesz A. G.
Published: July 2002

The dielectric constants in mixed oxides ZrO2+Y2O3, Ta2O5+TiO2, Ta2O5+Y2O3, and ZrO2+SiO2 are examined in the context of the oxide additivity rule for molecular polarizability. The experimentally observed concentration dependence of the dielectric constant can be satisfactorily explained by taking account of effective molecular polarizability and molecular volume changes. The simple rule thus enables predictive study of the dielectric constant of oxide alloys.
Measurement of impulse response of force transducers
Authors: Fujii Yusaku
Published: July 2002

Impulse responses of force transducers are measured. An object levitated with sufficiently small friction using a pneumatic linear bearing is made to collide with a force transducer under test. The inertial force acting on the object is measured as the product of the mass and the acceleration. The acceleration of the object is measured using an optical interferometer. The output signal of the force transducer and the inertial force acting on the levitated object are then compared. Impulses with a maximum value of approximately 80–90 N with a half value width of approximately 5–19 ms have been applied to two different types of force transducers. The inertial force is measured with standard uncertainty of approximately 0.9 N, which corresponds to approximately 1% of the maximum applied impulse.
Lagrangian approaches for particle collisions: The colliding particle velocity correlation in the multiple particles tracking method and in the stochastic approach
Authors: Berlemont A. , Achim P. , Chang Z.
Published: July 2002

Two different Lagrangian approaches for particle/particle collisions are described. The first model is based on the simultaneous tracking of several particles and suitable treatment is developed on particle pairs to detect collisions on each time step of the particle trajectory realization. The second method is based on a stochastic approach where one single particle is tracked, and successive random processes are applied to generate a fictitious partner of collision. In order to validate both approaches, simulations have been carried out in homogeneous isotropic turbulence and they have been compared with LES data. The particle/particle correlated motion through the surrounding fluid is proved to be a key parameter in a particle/particle collision process.
Modified atomic force microscope applied to the measurement of elastic modulus for a single peptide molecule
Authors: Ptak Arkadiusz , Takeda Seiji , Nakamura Chikashi , Miyake Jun , Kageshima Masami , Jarvis Suzanne P. , Tokumoto Hiroshi
Published: July 2002

A modified atomic force microscopy (AFM) system, based on a force modulation technique, has been used to find an approximate value for the elastic modulus of a single peptide molecule directly from a mechanical test. For this purpose a self-assembled monolayer built from two kinds of peptides, reactive (able to anchor to the AFM tip) and nonreactive, was synthesized. In a typical experiment a single C3K30C (C=cysteine, K=lysine) peptide molecule was stretched between a Au(111) substrate and the gold-coated tip of an AFM cantilever to which it was attached via gold–sulfur bonds. The amplitude of the cantilever oscillations, due to an external force applied via a magnetic particle to the cantilever, was recorded by a lock-in amplifier and recalculated into stiffness of the stretched molecule. A longitudinal Young’s modulus for the α-helix of a single peptide molecule and for the elongated state of this molecule has been estimated. The obtained values; 1.2±0.3 and 50±15 GPa, for the peptide α-helix and elongated peptide backbone, respectively, seem to be reasonable comparing them to the Young’s modulus of protein crystals and linear organic polymers. We believe this research opens up a means by which scientists can perform quantitative studies of the elastic properties of single molecule, especially of biologically important polymers like peptides or DNA.
Influence of solute–solvent interactions on the local solvent density augmentation in supercritical fluids: An integral equation study
Authors: Perera Aurélien
Published: July 2002

The influence of small changes in solute–solvent interactions on the solvent density augmentation under supercritical conditions is examined by integral equation calculations. It is shown here, through the use of a Yukawa model for the solute–solvent interaction in a Lennard–Jones solvent, that variations in the solute size or interaction strength are not so relevant. Rather, small differences in the range of the solute–solvent interaction can lead to dramatical changes in the increase of solvation that occurs in a supercritical solvent around the critical density. It is speculated that such features may serve as an explanation to large supercritical solubility differences between structurally similar molecules such as xanthines in supercritical carbon dioxide and methanol mixtures, for example. Two temperatures emerge from the present analysis that can be used to generally characterize supercritical fluids. One is the Boyle temperature TB that is shown here to be a rigorous upper bound to the supercritical temperatures. The second, Tℵ, that is, in fact equivalent to the Boyle temperature for the solute–solvent interaction, is shown to characterize the attractiveness or repulsiveness of the infinitely dilute solute for the near-critical solvent. The magnitude of the attractiveness (repulsiveness) is defined by a parameter ℵ introduced herein, and its pertinence is equally analyzed by comparison with infinite-dilution partial molar volume calculations. The ability of various integral equations to describe the supercritical fluid state is critically examined by exploring both the subcritical and the supercritical region, particularly for the case of a Lennard–Jones solvent. It appears that, bearing few differences that have their rationale in the subcritical region, all these approximate methods are essentially qualitatively equivalent in the supercritical region. This is, however, no longer the case when a solute is inserted into the supercritical solvent, and important differences can be found between different integral equations.
Fabrication and characterization of a Pb5Ge3O11 one-transistor-memory device
Authors: Li Tingkai , Hsu Sheng Teng , Ulrich Bruce , Ying Hong , Stecker Lisa , Evans Dave , Ono Yoshi , Maa Jer-shen , Lee J. J.
Published: July 2002

A Pb5Ge3O11 metal–ferroelectric–metal–oxide–silicon memory transistor has been fabricated. The device showed a memory window of about 2 V. The memory window was almost saturated at the operation voltage of 2 V. The “off” state drain current (ID) at VD of 0.1 V and VG of 0.5 V is about 1×10−8 A. The “on” state drain current (ID) at VD of 0.1 V and VG of 0.5 V is about 1×10−6 A, which is 100 times high than that of off state.
Accuracy of free-energy perturbation calculations in molecular simulation. II. Heuristics
Authors: Lu Nandou , Kofke David A.
Published: July 2002

We examine issues involved in applying and interpreting free-energy perturbation (FEP) calculations in molecular simulation, with the aim to develop simple heuristics that can guide their use and warn when a result is likely to be inaccurate. We build on the accuracy model developed in the first paper of this series [N. Lu and D. A. Kofke, J. Chem. Phys. 114, 7303 (2001)], which emphasized the sign of the entropy difference (ΔS) between the target and reference systems as an essential indicator for the correct implementation of FEP calculations: such calculations must be performed in the “insertion” direction, for which ΔS<0, or else they are very likely to be systematically incorrect (i.e., inaccurate). We describe here an extended analysis for insertion FEP calculations, and identify the group M exp(ΔS/k), where M is the number of independent FEP samples taken and k is Boltzmann’s constant, as a relevant quantity for characterizing the accuracy of FEP result. We find that if M exp(ΔS/k) is of order 100 or larger, then one can expect the FEP calculation to yield a result of minimally acceptable accuracy; for a margin of safety a value of 1000 or greater is preferable for this group. Although the FEP-measured ΔS is required to apply this heuristic, it is “safe” in that any inaccuracy in this ΔS will be such that the group M exp(ΔS/k) is even smaller than it is for the true ΔS, and will therefore still warn of an inaccurate result. The analysis is demonstrated for a very wide range of ΔS values, considering a model FEP calculation, a hard-sphere insertion calculation, and a diameter-change FEP in the Lennard-Jones model. We apply the results of this analysis, and earlier work, to consider the question of the optimal number of intermediate stages to use in a staged FEP calculation. The analysis shows that, for optimal accuracy, stages should be selected such that the entropy difference per stage satisfies ΔS/k=−1; however, consideration of the precision instead prescribes that ΔS/k=−2. Inasmuch as the precision is the larger concern once accuracy reaches an acceptable level, the latter criterion forms our recommendation for selecting the number of intermediate stages.
Molecular dynamics simulations of the liquid–vapor interface of a molten salt. II. Finite size effects and comparison to experiment
Authors: Aguado Andrés , Scott W. , Madden Paul A.
Published: July 2002

An examination of the finite system size effects on the calculated interfacial properties of the molten salt KI is described, as an extension of the work described in the preceding paper [J. Chem. Phys. 115, 8603 (2001)]. It is shown that, for system sizes beyond ∼1000 ions, the finite size effects are accounted for by the predictions of capillary wave (CW) theory, as in van der Waals liquids. Hence, there do not appear to be additional long-range effects arising from the Coulombic interactions. Techniques used to avoid truncation effects in the Coulombic and dispersion interactions appear to have eliminated system size dependent artefacts from these sources. The scaling behavior predicted from CW theory is used to predict the bulk surface tension for the chosen interaction potential, which is then compared with experimental values, and very good agreement found. Examination of the interfacial layers show that the local structure remains strongly charge ordered, but the surface is highly roughened by clustering and, on longer length-scales, by the effect of capillary waves.
Spectroscopic properties of mixed gallium arsenide tetramers: GaAs3±, GaAs3, Ga3As±, and Ga3As
Authors: Balasubramanian K. , Zhu XiaoLei
Published: July 2002

Spectroscopic properties of the low-lying electronic states of Ga3As, As3Ga, and their anions and cations are computed by the complete active-space self-consistent field (CASSCF) followed by multireference singles+doubles configuration interaction (MRSDCI) calculations that included up to 4.4 million configurations. Whereas the ground state of Ga3As is found to be a symmetric triangular pyramidal (C3v) A11 structure, the ground state of As3Ga is predicted to undergo Jahn-Teller distortion to a Cs A′1 state with a folded geometry. The ground state of the Ga3As− ion is found to be surprisingly a planar (C2v) structure but the ground state of the GaAs3− ion exhibits a Cs nonplanar Jahn-Teller distorted geometry. The energy separations of a number of excited electronic states have been computed to predict the spectra of these species. The equilibrium geometries, vibrational frequencies, atomization energies, adiabatic ionization potentials, electron affinities, and other properties for the electronic states of Ga3As and As3Ga are computed and discussed. Large differences in the equilibrium geometries of the Ga3As− anion and the neutral Ga3As are predicted to result in considerable vibrational progression the anion photoelectron spectra.
Double-quantum-filtered nuclear magnetic resonance spectroscopy applied to quadrupolar nuclei in solids
Authors: Painter A. James , Duer Melinda J.
Published: July 2002

A double-quantum filtered nuclear magnetic resonance experiment is applied to half-integer quadrupolar nuclei in solids. Filtering through a double-quantum coherence excited in a dipolar-coupled two-spin system allows spatial correlations between spins to be determined. Theoretical and experimental investigations have been carried out to optimize the excitation of double-quantum coherence between the central transitions of two neighboring nuclei while minimizing double-quantum excitation on each individual spin. The presence of the large quadrupolar coupling in the systems under study allows multiple-quantum excitation via the dipolar coupling between two spins even under conditions of magic-angle spinning, without the need for a recoupling sequence. Two-dimensional double-quantum-filtered correlation spectra have been recorded on Na23 (I=32) for Na2ZrO3 which contains three distinct Na23 sites.
High quality two-dimensional photonic crystal slab cavities
Authors: Yoshie Tomoyuki , Vučković Jelena , Scherer Axel , Chen Hao , Deppe Dennis
Published: July 2002

We have fabricated and characterized donor-mode nanocavities formed by a single defect cavity defined within a two-dimensional photonic crystal slab. Quantum dots emitting in the 1.1–1.3 micron range were used as luminescence sources, and a design using fractional edge dislocations was used to demonstrate well-confined dipole modes with high quality factors. By applying the fractional dislocation geometry, the measured quality factor could be increased to values as high as 2800. This compares with typical quality factors of around 1500 measured from more conventional shallow donor mode cavities with larger mode volumes.
Electric field gradients of water: A systematic investigation of basis set, electron correlation, and rovibrational effects
Authors: Olsen Lars , Christiansen Ove , Hemmingsen Lars , Sauer Stephan P. A. , Mikkelsen Kurt V.
Published: July 2002

Electric field gradients at the oxygen and hydrogen nuclei of water have been calculated using high level ab initio methods. Systematic studies of basis set truncation errors have been carried out at the Hartree–Fock and coupled cluster singles and doubles (CCSD) levels using extended correlation consistent basis sets with up to 398 basis functions. Correlation effects are investigated using a hierarchy of correlation methods extending up to the approximate inclusion of triples excitations by means of the CCSD(T) method. Rovibrational effects have been calculated combining accurate ab initio electric field gradient data and accurate experimental force fields. On the basis of the most accurate results for the electric field gradients, the nuclear quadrupole coupling constants for deuterium and oxygen-17 have been discussed including the temperature dependence. The final results are discussed in view of existing experimental data. Our best values for the nuclear quadrupole coupling constants are in excellent agreement (within 1%) of recent experimental results, while some earlier experimental values are shown to be less reliable.
Gas-pressure dependence of terahertz-pulse generation in a laser-generated nitrogen plasma
Authors: Löffler T. , Roskos H. G.
Published: July 2002

Far-infrared (terahertz) pulses can be generated by photoionization of electrically biased gases with amplified laser pulses [T. Löffler, F. Jacob, and H. G. Roskos, Appl. Phys. Lett. 77, 453 (2000)]. The efficiency of the generation process can be significantly increased when the absolute gas pressure is raised because it is then possible to apply higher bias fields close to the dielectric breakdown field of the gas which increases with the pressure. The dependence of the THz output on the optical pump power does not show any indication of saturation, making the plasma emitter an interesting source for THz pulses especially in conjunction with terawatt laser systems.
Experimental results on indoor electromagnetic wave absorber using magnetic wood
Authors: Oka Hideo , Narita Koichi , Osada Hiroshi , Seki Kyoushirou
Published: July 2002

The purpose of this paper is to propose a new type of indoor electromagnetic wave absorber using magnetic wood. This magnetic wood has good electromagnetic wave absorbing characteristics, a low specific gravity, a wood texture and other wood characteristics and can be easily processed. Electromagnetic wave absorbing characteristics were measured for four types of magnetic wood. The sandwich-type magnetic wood demonstrated the best wave absorbing characteristics among the four types of magnetic wood that were studied. The experimental results showed that the proposed indoor electromagnetic wave absorber can be used to suppress the transmission and reception of cellular phone and Personal Handy Phone System (PHS) signals and can be used as a cross protection for indoor wireless Local Area Networks (LAN). This wood can be processed for use in furniture, building materials, and other applications.
Gap bowing and Stokes shift in InxGa1−xN alloys: First-principles studies
Authors: Ferhat M. , Furthmüller J. , Bechstedt F.
Published: July 2002

First-principles pseudopotential plane-wave calculations are used to investigate the electronic properties of InxGa1−xN alloys. The alloys are described within a cluster-expansion method considering configurations in large 64-atom supercells. The unusual behavior of the gap bowing is discussed as a function of composition. We show that the strongest contribution to the gap bowing is due to a structural effect, i.e., the composition-induced disorder in the bond lengths. We explain the observed huge Stokes shifts by band gap variations due to composition fluctuations and phase separation.
Three-dimensional imaging technique for direct observation of the complete velocity distribution of state-selected photodissociation products
Authors: Chichinin A. I. , Einfeld T. , Maul C. , Gericke K.-H.
Published: July 2002

We report an experimental technique provided to study the full three-dimensional velocity distribution of state-selected products of a chemical process. Time-of-flight mass spectroscopy and resonance enhanced multiphoton ionization combined with a position sensitive detector (delay-line anode) are employed. The technique has a space resolution of 0.4 mm, a time resolution better than 1 ns, and it provides the possibility to detect several products with a minimal difference between arrival times of 17 ns. One major achievement of the new technique is the possibility to determine the full three-dimensional momentum vectors of a chemical reaction product. This is especially valuable for cases where no symmetry is considered in the process. Second, the high sensitivity of the method allowing to observe single ions enables us to study physical and chemical processes at extremely low densities. Three methods for measuring the temperature of a molecular beam with the technique are demonstrated. A novel result of the present work is the study of angular distribution of NO ions due to electron recoil in the ionization of NO(A 2Σ+). Finally the advantages of the method are examined by studying the speed distributions of Cl atoms in the photolysis of Cl2 at 355 nm.
Tuning-fork-based fast highly sensitive surface-contact sensor for atomic force microscopy/near-field scanning optical microscopy
Authors: Serebryakov D. V. , Cherkun A. P. , Loginov B. A. , Letokhov V. S.
Published: July 2002

We have developed a surface-contact sensor on the basis of a tuning fork which differs from the previously described ones in that it has a high operating speed (up to 100 times as fast as the so-called Q limit), requires no external piezoelectric drive, has a sufficiently high sensitivity, and features a “soft” probe attachment which makes the lifetime of the probe equal to that of the standard atomic force microscopy. When using a “soft” probe with a rigidity of 0.5 N/m, one can reliably detect probe tip-to-sample distance variations as small as 0.1 nm. The resonance frequency resolution attained amounted to 2×10−3 Hz. The rate of transient rise is τ=1.5 ms (this refers to the response time of the sensor proper with the Z-coordinate feedback loop open and not to the response time of the microscope as a whole). We have theoretically substantiated the fact that the Q limit, where Q∼10 000 is the Q factor of the tuning fork proper, is not a fundamental restriction on the operating speed of the sensor. This sensor characteristic is governed by another independent quantity, namely, Q1∼100: the quality factor of the tuning fork preamplifier system that can be varied to suit the experimenter. In that case, the fundamental force limitation on the sensitivity of the sensor, associated with its operating speed and the Q factor of the tuning fork, is Fnoise≈10.4 nN/(√Q√Q1).
Synchronization of chaotic systems: Transverse stability of trajectories in invariant manifolds
Authors: Brown Reggie , Rulkov Nikolai F.
Published: July 2002

We examine synchronization of identical chaotic systems coupled in a drive/response manner. A rigorous criterion is presented which, if satisfied, guarantees that synchronization to the driving trajectory is linearly stable to perturbations. An easy to use approximate criterion for estimating linear stability is also presented. One major advantage of these criteria is that, for simple systems, many of the calculations needed to implement them can be performed analytically. Geometrical interpretations of the criterion are discussed, as well as how they may be used to investigate synchronization between mutual coupled systems and the stability of invariant manifolds within a dynamical system. Finally, the relationship between our criterion and results from control theory are discussed. Analytical and numerical results from tests of these criteria on four different dynamical systems are presented.
Nonlinear instability of elementary stratified flows at large Richardson number
Authors: Majda Andrew J. , Shefter Michael G.
Published: July 2002

Elementary stably stratified flows with linear instability at all large Richardson numbers have been introduced recently by the authors [J. Fluid Mech. 376, 319–350 (1998)]. These elementary stratified flows have spatially constant but time varying gradients for velocity and density. Here the nonlinear stability of such flows in two space dimensions is studied through a combination of numerical simulations and theory. The elementary flows that are linearly unstable at large Richardson numbers are purely vortical flows; here it is established that from random initial data, linearized instability spontaneously generates local shears on buoyancy time scales near a specific angle of inclination that nonlinearly saturates into localized regions of strong mixing with density overturning resembling Kelvin–Helmholtz instability. It is also established here that the phase of these unstable waves does not satisfy the dispersion relation of linear gravity waves. The vortical flows are one family of stably stratified flows with uniform shear layers at the other extreme and elementary stably stratified flows with a mixture of vorticity and strain exhibiting behavior between these two extremes. The concept of effective shear is introduced for these general elementary flows; for each large Richardson number there is a critical effective shear with strong nonlinear instability, density overturning, and mixing for elementary flows with effective shear below this critical value. The analysis is facilitated by rewriting the equations for nonlinear perturbations in vorticity-stream form in a mean Lagrangian reference frame.
Object-oriented programming applied to lattice gauge theory
Authors: Moriarty K. J. M. , Sanielevici S. , Sun K. , Trappenberg T.
Published: July 2002

A C++ version of the code written by Ardill, Moriarty, and Creutz [Comput. Phys. Commun. 29, 97 (1983)] for the Monte-Carlo simulation of SU(N) lattice gauge theories is presented. The idea is to provide a concrete example of the use of the object-oriented programming paradigm in one of the principal areas of computational physics.