bluuebunny/crossref_metadata_2025 huggingface dataset:
Properties of the exchange hole under an appropriate coordinate transformation
Authors: Tao Jianmin , Springborg Michael , Perdew John P.
Published: September 2003

Under a general coordinate transformation, the exchange hole can be made localized, as shown by Springborg [Chem. Phys. Lett. 308, 83 (1999)]. While the conventional or untransformed hole is referenced to the position of an electron, the maximally localized one is referenced to the center of mass of an electron pair. The benefit of a more localized hole is that semilocal density functionals model it and the associated energy density more easily than they model the conventional ones. We show that, out of the class of general coordinate transformations, one can identify a subset (including the maximally localized case) which we call appropriate. Under an appropriate coordinate transformation, while the exchange hole is no longer always normalized, it retains other familiar and useful features such as the conventional on-top value and uniform-density limit. In particular, its system average remains invariant, retaining the normalization sum rule and the negativity property. Therefore, unlike the exchange energy density ex(r), the real-space analysis ẽx(u) of the exchange energy [into contributions from different electron–electron separations (u)] is uniquely defined. Thus the real-space analysis provides an alternative way to make simple and fair but detailed comparisons of approximate and exact exchange. As a byproduct, we show how to improve the accuracy of the Negele–Vautherin model for the density matrix expansion of the exchange energy by imposing negativity and sum rule constraints on the system average of its maximally localized hole.
Geminal wave functions with Jastrow correlation: A first application to atoms
Authors: Casula Michele , Sorella Sandro
Published: September 2003

We introduce a simple generalization of the well-known geminal wave function already applied in quantum chemistry to atoms and small molecules. The main feature of the proposed wave function is the presence of the antisymmetric geminal part together with a Jastrow factor. Both the geminal and the Jastrow play a crucial role in determining the remarkable accuracy of the many-body state: the former permits the correct treatment of the nondynamic correlation effects; the latter allows the wave function to fulfill the cusp conditions and makes the geminal expansion rapidly converge to the lowest possible variational energies. This ansatz is expected to provide a substantial part of the correlation energy for general complex atomic and molecular systems. The antisymmetric geminal term can be written as a single determinant even in the polarized cases. In general, therefore, the computational effort to sample this correlated wave function is not very demanding, the scaling of the algorithm with the number of atoms being comparable with the simplest Hartree–Fock calculation. We applied this Jastrow-geminal approach to atoms up to Z=14, always getting good variational energies, by particularly improving those with a strong multiconfigurational nature. Our wave function is very useful for Monte Carlo techniques, such as fixed node. Indeed, the nodal surface obtained within this approach can be substantially improved through the geminal expansion.
A quantum solute–solvent interaction using spectral representation technique applied to the electronic structure theory in solution
Authors: Yamazaki Takeshi , Sato Hirofumi , Hirata Fumio
Published: September 2003

In this paper, we present a new approach to treat the electronic structure of a molecule in solution. Unlike the hybrid-type method, such as the reference interaction site model self-consistent-field theory, the new approach describes not only the electronic structure of solute but also solute–solvent interactions in terms of the quantum chemistry based on the Hartree–Fock frozen density formulation. In the treatment, the quantum effect due to solvent, including exchange repulsion, is projected on to the solute Hamiltonian using the spectral representation method. The solvent distribution around the solute is handled by the integral equation theory of liquids. As illustrative applications of the approach, the electronic and solvation structure of noble atoms, neon and argon, in liquid neon are studied. We also investigate the electronic structure of an excess electron in liquid helium. The preliminary results demonstrate that the quantum-mechanical effect on the electronic and solvation structure of the solute due to solvent molecules is successfully represented by the new method.
On the structure of boron trifluoride in liquid and supercritical phase investigated with neutron diffraction
Authors: McLain S. E. , Siewenie J. E. , Benmore C. J. , Turner J. F. C.
Published: September 2003

The results and reverse Monte Carlo analysis of diffraction measurements of BF3 in the liquid and supercritical state are presented. Liquid BF3 displays order only in the first coordination sphere around the average molecule and there is no intermediate range order. Liquid BF3 is thus not a layered liquid. Angular analysis using reverse Monte Carlo modeling shows that the dominant structural motif in the solid state, that of pseudo trigonal bipyramidal [3+2] coordination at B is partially retained in both the liquid and supercritical phase, though in the latter, this structural preference is greatly reduced. The structure of the supercritical phase retains the same intermolecular contact as the liquid phase, though the structural sites and population of these sites is much more widely distributed.
Solution properties of charged quasi-random copolymers: Integral equation theory
Authors: Zherenkova Lubov V. , Khalatur Pavel G. , Khokhlov Alexei R.
Published: September 2003

Using the polymer integral equation method, we study the solution properties of charged quasi-random two-letter (HP) copolymers with two different types of distribution of monomer units along a copolymer chain: proteinlike copolymers and random-block copolymers. The copolymers consist of monomer units of two types: associating electroneutral hydrophobic (H) units and charged (P) units. Small mobile counterions are treated explicitly. We explore the influence of the primary structure of HP-polyelectrolyte chains on their structural behavior and aggregation in a solution, which is poor for H units and good for P units. Analysis of the static structure factors shows that there is an evident tendency to the aggregation of the hydrophobic groups belonging to different macromolecules into spatially correlated clusters. The spinodal lines and various structure diagrams are calculated for both copolymers. The characteristic temperature of counterion condensation is also estimated. The main finding is that charged proteinlike copolymers are more prone to self-organization in a poor solvent than their random-block counterparts. In particular, the apparent spinodal temperatures for proteinlike copolymers are several fold larger than for random-block copolymers with the same average block length and HP composition. The influence of the primary structure is more pronounced in the processes dominated by short-range hydrophobic interaction than in the processes mostly governed by long-range electrostatic interactions.
Electron valence-band tunneling-induced Lorentzian noise in deep submicron silicon-on-insulator metal–oxide–semiconductor field-effect transistors
Authors: Lukyanchikova N. B. , Petrichuk M. V. , Garbar N. , Mercha A. , Simoen E. , Claeys C.
Published: September 2003

In this article, the impact of several electrical and technological parameters on a particular type of Lorentzian noise, occurring in deep submicron silicon-on-insulator (SOI) metal–oxide–semiconductor field-effect transistors with an ultrathin gate dielectric is described and a semi-empirical model is proposed that captures the main features of the experimental behavior. It is shown that the noise takes place in both n- and p-channel partially depleted SOI transistors. The excess Lorentzians are also found in the n-channel fully depleted devices studied, whereby the noise plateau amplitude [SI(0)] increases for a more negative back-gate bias, putting the back interface into stronger accumulation. The dependence of the characteristic time constant τ and SI(0) on transistor length, drain, front- and back-gate bias is reported, where from a first-order model is derived. The latter is based on the idea that the excess Lorentzian noise originates from filtered shot noise induced by majority carriers, that are injected in the floating body of the transistors by electron valence-band tunneling across the ultrathin (2.5 nm) gate oxide.
Microstructural evolution and macroscopic property relationship in antiferroelectric lead lanthanum stannate zirconate titanate ceramics
Authors: Chan Wai-Hung , Xu Z. , Zhang Yong , Hung T. F. , Chen Haydn
Published: September 2003

The antiferroelectric (AFE) Pb0.97La0.02(Zr0.60Sn0.30Ti0.10)O3 ceramic has a composition near the morphotropic phase boundary that separates the AFE from the ferroelectric (FE) phase. Its structural changes as well as macroscopic properties were investigated over a temperature range of −180–300 °C using transmission electron microscopy, dielectric spectroscopy, and Sawyer–Tower polarization measurements. The previously reported tetragonal AFE phase is shown to be an incommensurate orthorhombic phase and it exhibits a sequence of phase transformations on heating from the incommensurate AFE to a multicell cubic, then to a simple cubic phase. This microstructural evolution with temperature is consistent with the corresponding macroscopic dielectric and AFE behaviors. The temperature dependence of the AFE-FE switching field is closely associated with the corresponding temperature dependence of the incommensurate modulation wavelength.
Crystallization and phase separation in Ge2+xSb2Te5 thin films
Authors: Privitera S. , Rimini E. , Bongiorno C. , Zonca R. , Pirovano A. , Bez R.
Published: September 2003

The electrical properties and the structure of isothermally annealed thin films of Ge2+xSb2Te5 (x=0 and 0.5) have been studied by in situ electrical measurements, x-ray diffraction, and transmission electron microscopy analyses. Phase separation has been observed in samples with an excess of Ge; by annealing amorphous Ge2.5Sb2Te5 films at temperatures in the range 130–160 °C, the material cannot be completely converted into the metastable face-centered-cubic (fcc) structure. At temperatures higher than 160 °C, the residual amorphous material may be converted into a fcc structure with a lower lattice parameter.
Effect of chemical substitution on the electronic properties of highly aligned thin films of Sr2−xAxFeMoO6 (A=Ca, Ba, La; x=0, 0.1)
Authors: Branford W. R. , Clowes S. K. , Bugoslavsky Y. V. , Miyoshi Y. , Cohen L. F. , Berenov A. V. , MacManus-Driscoll J. L. , Rager J. , Roy S. B.
Published: September 2003

Highly aligned films of composition Sr1.9A0.1FeMoO6 (A=Ca,Ba,La) and undoped control samples, were fabricated on (100) single-crystal LaAlO3 by ultrasonic spray pyrolysis. Compared to undoped films, the doped films showed reduced saturation magnetization and magnetoresistance and the lanthanum-doped film showed a significant increase (∼40 K) in the Curie temperature. The electrical resistivity was well described by a spin-dependent transport model for all the films. The transport spin polarization has been determined with this model, and independently by point contact Andreev reflection spectroscopy. The doped films do not have improved polarization over the undoped films and the maximum P obtained is of the order of 63%.
Spontaneous emission, light–current characteristics, and polarization bistability range in vertical-cavity surface-emitting lasers
Authors: Ryvkin B. S. , Avrutin E. A. , Pessa M.
Published: September 2003

A simple but generic phenomenological analytical theory for steady-state polarization properties of a vertical-cavity surface-emitting laser is developed. It is shown that a realistic amount of spontaneous emission in the laser mode and the saturation of the light–current curve can both significantly decrease the polarization bistability range.
Entrance channel localized states in ozone: Possible application to helium nanodroplet isolation spectroscopy
Authors: Babikov Dmitri
Published: September 2003

Accurate calculations of the bound ro-vibrational states of ozone are performed in the region of high vibrational excitation. Two unusual vibrational states localized in the far van der Waals region are discovered. They can be considered as the ground vibrational states of even and odd symmetries bound in the shallow van der Waals well. Properties of these states are presented and discussed, which opens possibilities for experimental realization of the helium nanodroplet isolation ro-vibrational spectroscopy of ozone.
Estimating Bohm’s quantum force using Bayesian statistics
Authors: Maddox Jeremy B. , Bittner Eric R.
Published: September 2003

In this paper we develop an approximate methodológy for estimating the multidimensional quantum density associated with a statistical bundle of de Broglie–Bohm trajectories. The quantum density is constructed as a discrete sum of nonequivalent Gaussian components. We incorporate the ideas of Bayesian statistical analysis and an expectation-maximization procedure to compute an approximate quantum force that drives the statistical ensemble quantum trajectories.
Soft/hard exchange-coupled layered structures with modulated exchange coupling
Authors: Yan Shi-Shen , Elkawni M. , Li D. S. , Garmestani H. , Liu J. P. , Weston J. L. , Zangari G.
Published: September 2003

Magnetically soft/hard exchange-coupled Ni80Fe20/Si3N4/Sm40Fe60 and Ni80Fe20/mixture/Sm40Fe60 layered structures with induced in-plane uniaxial anisotropy were deposited by sputtering on Si (100) substrates. The interfacial exchange coupling strength between the soft and hard layers was tailored by inserting a thin nonmagnetic insulating Si3N4 layer or by varying interfacial mixture of NiFe and SmFe. It was found that the reduction in the exchange coupling greatly reduces the nucleation field HN of the soft layer and increases the irreversible switching field Hirr of the hard layer. The simple formula used before to describe the nucleation field of the soft layer does not work in the case of the reduced interfacial exchange coupling.
Multiple photonic band gaps in the structures composed of core-shell particles
Authors: Babin V. , Garstecki P. , Hołyst R.
Published: September 2003

We present a detailed study of the full photonic band gaps for the spherical multilayered core-shell particles arranged in bcc, fcc, and diamond fcc lattices. We find that layered structure of the particles does not lead to an increase of the gap width in case of the fcc arrangement. On the other hand full photonic band gap opens up for the bcc lattice composed of such particles. The most promising is the diamond fcc arrangement. For such ordering of core particles covered by a thin shell we find three reasonably wide gaps.
Ion beam smoothening of metal surfaces
Authors: Zhong Y. , Ashkenazy Y. , Albe K. , Averback R. S.
Published: September 2003

Ion beam induced smoothening of crystalline and amorphous substrates were investigated using molecular dynamics simulations. Rough surfaces created by depositing small nanoparticles, 2–3 nm in diameter, onto flat substrates were subjected to repeated impacts with 40 keV Xe atoms. Two smoothening processes are identified: The nanoparticle either burrows into the substrate, with the underlying substrate atoms flowing around it to the surface, or it flows over the substrate surface, wetting it. Generally, these two mechanisms operate simultaneously in both amorphous and crystalline substrates. The burrowing mechanism in amorphous substrates was additionally investigated by creating low energy recoils, 20 or 200 eV in the substrate beneath the nanoparticle. Roughening of initially smooth amorphous substrates during ion impact was also studied for comparison with the smoothening process.
Trapping of negative and positive charges in Ge+ ion implanted silicon dioxide layers subjected to high-field electron injection
Authors: Nazarov A. N. , Gebel T. , Rebohle L. , Skorupa W. , Osiyuk I. N. , Lysenko V. S.
Published: September 2003

Negative and positive charge trapping in a constant current regime under high-field electron injection both from Al electrode and Si substrate in high-dose Ge+ ion implanted and then rapid thermal annealed thin-film dioxide has been studied. Negatively charged traps as well as generated positive charges with effective capture cross sections of σ1(−)>10−14 cm2, σ2(−)≈1.8×10−15, σ3(−)≈2×10−16, and σ4(−)≈3×10−18 cm2, as well as σ1(+)≈(5–7)×10−15 and σ2(+)≈3.3×10−16 cm2, respectively, are shown to be introduced into the oxide layer. A good correlation of the electron trap concentration with a cross section of σ1(−)>10−14 cm2 and the concentration of the implanted Ge atoms, determined by Rutherford backscattering spectrometry inside the oxide, is observed. The decrease of Ge concentration within the oxide layer with increasing duration of rapid thermal annealing is associated with Ge atom outdiffusion from the oxide at high-temperature annealing. The generated positive charge is shown to be collected near the SiO2/Si interface during the high field electron injection, both from the Al and Si side. A correlation of the generated positive charge with the Ge atoms embedded in the SiO2/Si interface is observed. The anode hole injection mechanism is suggested to be responsible for the observed generation of the positive charge.
Phase conjugation and perfect lensing
Authors: Maslovski Stanislav , Tretyakov Sergei
Published: September 2003

The concept of the perfect lens introduced by Pendry [Phys. Rev. Lett. 85, 3966 (2000)], i.e., a possibility to achieve optical resolution well below the wavelength limit using a backward wave (also called Veselago or double-negative) material slab lens, is reinvestigated. It is shown that there is a direct analogy between the phenomena taking place at the slab interfaces and at a couple of phase-conjugating planes placed in free space. Thus, this work introduces an alternative to Pendry’s device. The physics behind the idea is discussed and certain directions of possible realizations are outlined.
Low-intensity two-dimensional imaging of fluorescence lifetimes in living cells
Authors: Emiliani V. , Sanvitto D. , Tramier M. , Piolot T. , Petrasek Z. , Kemnitz K. , Durieux C. , Coppey-Moisan M.
Published: September 2003

The use of a time- and space-correlated single-photon counting detector enables us to perform fluorescence lifetime imaging microscopy in living cells with a temporal resolution of less than 100 ps and a spatial resolution of 500 nm. Two-dimensional (2D) maps of the fluorescence lifetimes and the corresponding prefactors are extracted by the use of a fitting program based on the Levenberg–Marquardt algorithm (Globals Unlimited). We applied this technique to extract 2D maps of protein localization in multilabeled living cells and to study protein–protein interaction by fluorescence resonance energy transfer.
Structural and optical characterization of porous anodic aluminum oxide
Authors: Gâlcă Aurelian C. , Kooij E. Stefan , Wormeester Herbert , Salm Cora , Leca Victor , Rector Jan H. , Poelsema Bene
Published: September 2003

Spectroscopic ellipsometry and scanning electron microscopy (SEM) experiments are employed to characterize porous aluminum oxide obtained by anodization of thin aluminum films. Rutherford backscattering spectra and x-ray diffraction experiments provide information on the composition and the structure of the samples. Results on our thin film samples with a well-defined geometry show that anodization of aluminum is reproducible and results in a porous aluminum oxide network with randomly distributed, but perfectly aligned cylindrical pores perpendicular to the substrate. The ellipsometry spectra are analyzed using an anisotropic optical model, partly based on the original work by Bruggeman. The model adequately describes the optical response of the anodized film in terms of three physically relevant parameters: the film thickness, the cylinder fraction, and the nanoporosity of the aluminum oxide matrix. Values of the first two quantities, obtained from fitting the spectra, are in perfect agreement with SEM results, when the nanoporosity of the aluminum oxide matrix is taken into account. The validity of our optical model was verified over a large range of cylinder fractions, by widening of the pores through chemical etching in phosphoric acid. While the cylinder fraction increases significantly with etch time and etchant concentration, the nanoporosity remains almost unchanged. Additionally, based on a simple model considering a linear etch rate, the concentration dependence of the etch rate was determined.
Improved detection of thermally induced higher resonance modes and harmonics of a microcantilever
Authors: Kar A. K. , George M. A.
Published: September 2003

Driving a microcantilever externally may not be desirable in many sensor applications. Alternatively, it is possible to extract full spectral characteristics of the anharmonic thermal motion of a microcantilever naturally vibrating at ambient temperature. Present work aims at the detection of comparatively noise free higher resonance modes and harmonics of thermal vibration for sensor applications without externally induced vibration. In microcantilever sensor based experiments with optical detection of cantilever deflection, we demonstrate the problems associated with the conventional procedure of processing photodetector signal for resonating microcantilevers and describe improvements. It has been experimentally demonstrated that isolation of the dynamic component of a position sensitive photodetector signal from its static counterpart significantly improves the resolution and limit of detection of an instrument. Outputs from conventional and proposed methods have been compared with experiments performed in both ambient air and liquid environments. A very simple and cost-effective circuit design is presented.
Grain size analysis of C54–TiSi2 under different processing conditions for deep-submicron polycrystalline silicon gate length
Authors: Pang C. H. , Hing P. , See A. , Chong Y. F.
Published: September 2003

The effect of preamorphization using silicon (Si) ion, different annealing temperatures and heating rates on nucleation density and grain size of C49 and C54–TiSi2 have been studied. The average grain size of C54–TiSi2 is found to decrease when the peak annealing temperature is changed from 700 to 850 °C. We found that the nucleation threshold temperature is about 900 °C at which the C54–TiSi2 grain size levels off at about 23 nm. We also found that the grain size of C54–TiSi2 is independent of different heating rates. Our results also show that preamorphization is not so effective in reducing the C54–TiSi2 grains when annealing temperature is raised to above 850 °C. However, preamorphization does improve the grain size uniformity. By optimizing the processing parameters such as peak annealing temperature, heating rates and preamorphization, we have demonstrated the formation of C54–TiSi2 on 0.12 μm polycrystalline silicon (polysilicon) lines with no agglomeration.
Field enhanced diffusion of nitrogen and boron in 4H–silicon carbide
Authors: Phelps G. J. , Chester E. G. , Johnson C. M. , Wright N. G.
Published: September 2003

A field enhanced diffusion (FED) model is proposed for the observed phenomenon of dopant migration within 4H silicon carbide (4H–SiC) during high temperature annealing. The proposed FED model is based on the combined effects of both dopant diffusion and the in-built p–n junction electric field gradients found within the doped substrate resulting from the presence of the substrate dopants. Measured as-implanted dopant concentration profile data prior to high temperature annealing are utilized as input data for the proposed FED model. The resultant FED profile predictions of the proposed model during annealing are shown to be in excellent agreement with experimental findings. Parameters are extracted using the FED model simulation for the high temperature ionic diffusivity and ionic field mobility for nitrogen dopant in boron co-doped 4H–SiC. The extracted values for the ionic diffusivity and the ionic mobility of the nitrogen donor in boron co-doped 4H–SiC found using the model are 7.2×10−15 cm2 s−1 and 1.6×10−13 cm2 V−1 s−1, respectively.
Crystalline orientation of polycrystalline silicon with disklike grains produced by silicide-mediated crystallization of amorphous silicon
Authors: Sohn Woo Sung , Choi Jong Hyun , Kim Kyung Ho , Oh Jae Hwan , Kim Seung Soo , Jang Jin
Published: September 2003

Crystalline orientations of disklike grains in polycrystalline silicon (poly-Si) formed by silicide-mediated crystallization (SMC) of amorphous silicon (a-Si) have been studied and compared with those of needlelike crystallites. The disklike grain can be obtained by SMC of a-Si with a silicon–nitride cap on it and its size is as large as 42 μm by using the average Ni area density of 2.43×1014 cm−2 on the cap. On the other hand, the poly-Si crystallized without the cap layer is composed of needlelike crystallites. The electron diffraction (ED) patterns of transmission electron microscope (TEM) remain unchanged for the selected area from 1 to 10 μm diameter inside of the disklike grain, but the ED patterns of SMC poly-Si composed of needlelike crystallites show a ring pattern for the area of 10 μm diameter due to many crystalline orientations. Electron backscattered diffraction and TEM analysis indicate that each disk-like grain has a single orientation, but its orientation is not always the same as that of neighboring grains.
Injection-limited electron current in a methanofullerene
Authors: van Duren J. K. J. , Mihailetchi V. D. , Blom P. W. M. , van Woudenbergh T. , Hummelen J. C. , Rispens M. T. , Janssen R. A. J. , Wienk M. M.
Published: September 2003

The dark current of bulk-heterojunction photodiodes consisting of a blend of a methanofullerene (PCBM) as n-type electron acceptor and a dialkoxy-(p-phenylene vinylene) (OC1C10−PPV) as a p-type electron donor sandwiched between electrodes with different work functions has been investigated. With ohmic contacts for hole and electron injection, the dark current appears completely dominated by the electron current in the PCBM, as a result of a much higher electron mobility. This electron current is bulk space-charge limited. With Au as a high work function metal, the electron current becomes injection limited. The injection-limited electron current from the Au electrode into PCBM is explained within a thermally assisted hopping model. In spite of the presence of an injection barrier of about 0.76 eV, the injection-limited electron current from a Au electrode into PCBM still exceeds the bulk-limited hole current in OC1C10−PPV.
Characterizing the interphase dielectric constant of polymer composite materials: Effect of chemical coupling agents
Authors: Todd Michael G. , Shi Frank G.
Published: September 2003

Recent research into the dielectric characteristics of polymer–ceramic composites has shown that the interphase region of the composite can have a dielectric constant significantly different from that of the polymer phase due to covalent bonding of the polymer molecules to the surface of the filler particles. Chemical coupling agents and surfactants such as functional silanes, organotitanates, organometallic chelating agents, phosphate esters, and various ionic and nonionic organic esters are commonly employed to enhance the compatibility between the polymer phase and dispersed filler phase of composite systems. Using experimental data and molecular dipole polarization calculations, we determine the effect of such coupling agents on the interphase dielectric constant. Our results show that the addition of functional silane coupling agents or nonionic surfactants at concentrations of 0.5 wt % or less of the total organics of a polymer–ceramic composite system has significant effects on the dielectric constant of the interphase region, yet has little or no effect on the dielectric constant values of the polymer phase. Furthermore, the chemical bonding of the coupling agents to the ceramic filler particles determine the dielectric constant of the interphase region as predicted by chemical polarization calculations. These results are fully consistent with experimental evidence and further validate the use of molecular polarization calculations of composite interphase regions to determine and predict the overall effective dielectric properties of packaging materials for a wide range of electrical, electronic, and rf applications.
Low-loss optical planar waveguides in YVO4 produced by silicon ion implantation at low doses
Authors: Chen Feng , Wang Xue-Lin , Li Shi-Ling , Fu Gang , Wang Ke-Ming , Lu Qing-Ming , Shen Ding-Yu , Nie Rui , Ma Hong-Ji
Published: September 2003

Planar waveguides in x-cut YVO4 crystals were produced by ion-implanted Si+ ions with energies from 2.6 to 3.0 MeV at doses of 1×1013−1.5×1014 ions/cm2. The number of propagation modes varied from 1 to 3 as the doses of the implanted ions increased. The effective refractive indices of all the observed waveguide modes were higher than the refractive index of the substrate, which meant an index-enhanced guiding layer with thickness of ∼2 μm formed to confine the light propagation. The minimum propagation loss of the measured YVO4 waveguide was 0.27 dB/cm after annealing under suitable conditions.
On crystallite size dependence of phase stability of nanocrystalline TiO2
Authors: Ghosh T. B. , Dhabal Sampa , Datta A. K.
Published: September 2003

Stability of anatase phase component present in commercial grade nanocrystalline powder of TiO2, synthesized from gas phase, is studied. Powders are heated at elevated temperatures under ambient conditions. X-ray and electron diffractometric studies are done to measure the microstructural parameters like crystallite size and rms strain. With annealing crystallite sizes of both anatase and rutile phases show a marginal increase up to 600 °C, beyond this temperature anatase phase transforms to rutile phase and the crystallite size of rutile phase increases rapidly. Annealing at 1000 °C resulted in the growth of hexagonal shaped crystallite of rutile phase oriented parallel to (110) plane. A detailed analysis of the dislocation arrangements in these nanosized crystallites is made from the x-ray diffraction data. The results of analysis show that at higher temperatures polygonization of the dislocations with lowering of dislocation density at the grain boundary possibly favors the growth of larger sized crystallites of rutile phase. High values of dislocation density obtained for anatase give evidence of piling up of dislocations and thereby increases the strain energy of the lattice. The increase in stored energy may be responsible for the observed changes in the lattice parameter for the anatase phase. Thus, it appears that the disordered lattice of anatase favors its transformfiation to rutile structure.
Near-absolute Hugoniot measurements in aluminum to 500 GPa using a magnetically accelerated flyer plate technique
Authors: Knudson M. D. , Lemke R. W. , Hayes D. B. , Hall C. A. , Deeney C. , Asay J. R.
Published: September 2003

Hugoniot measurements were performed on aluminum (6061-T6) in the stress range of 100–500 GPa (1–5 Mbar) using a magnetically accelerated flyer plate technique. This method of flyer plate launch utilizes the high currents, and resulting magnetic fields produced at the Sandia Z Accelerator to accelerate macroscopic aluminum flyer plates (approximately 12×25 mm in lateral dimension and ∼300 μm in thickness) to velocities in excess of 20 km/s. This technique was used to perform plate-impact shock-wave experiments on aluminum to determine the high-stress equation of state (EOS). Using a near-symmetric impact method, Hugoniot measurements were obtained in the stress range of 100–500 GPa. The results of these experiments are in excellent agreement with previously reported Hugoniot measurements of aluminum in this stress range. The agreement at lower stress, where highly accurate gas gun data exist, establishes the magnetically accelerated flyer plate technique as a suitable method for generating EOS data. Furthermore, the present results exhibit increased accuracy over the previous techniques used to obtain data in the higher-stress range. This improved accuracy enhances our understanding of the response of aluminum to 500 GPa, and lends increased confidence to the use of aluminum as a standard material in future impedance matching experiments.
Accurate prediction of excitation energies to high-lying Rydberg electronic states: Rydberg states of acetylene as a case study
Authors: Zyubin A. S. , Mebel A. M.
Published: September 2003

Ab initio outer-valence Green functions (OVGF), equation-of-motion coupled cluster (EOM-CCSD), and Hartree–Fock (HF) calculations with specially constructed basis sets have been carried out to predict excitation energies to Rydberg electronic states of acetylene with principal quantum numbers n up to 11. A comparison of calculated energies with experiment shows that the OVGF method gives accurate results for a broad energy range of 8–11.5 eV. The deviations from experimental data are as low as 0.03–0.04 eV for n=5–8 and are slightly larger for the higher states, ∼0.05 eV for n=9–11. The OVGF calculations are thus demonstrated to be able to reproduce excitation energies for high-lying Rydberg states with a good precision, higher than that provided by the EOM-CCSD method, if one uses experimental or high-level calculated values of the ionization potential. With an increase of the principal quantum number for the Rydberg state, the OVGF corrections to the HF-calculated energies decrease and one can use the HF approach to compute orbital energy levels and excitation energies for higher Rydberg states. The energy levels for high-lying Rydberg states are shown to be insensitive to the molecular geometry, so that the energy gaps between vertical absorption and vertical emission for these states should be similar and can be estimated by the relaxation energy of the neutral system starting from geometry of the positive ion.
Hydrogen bond dynamics in liquid methanol
Authors: Pagliai Marco , Cardini Gianni , Righini Roberto , Schettino Vincenzo
Published: September 2003

A Car–Parrinello molecular dynamics simulation has been performed on fully deuterated liquid methanol. The results are compared with the latest available experimental and theoretical data. It is shown that the liquid is aggregated in chains of hydrogen bonded molecules. The structure of the aggregates is characterized and it is found that the dynamics includes a fast and a slow regime. The weak H bond formed by the methyl group hydrogens and oxygen atom of surrounding molecules has been characterized. The importance of inductive effects is shown and discussed in terms of maximally localized Wannier function centers. Special attention is devoted to clarify how the molecular dipole moment depends on the number of H bonds formed by each molecule. The IR spectrum is computed and analyzed in terms of H-bond interactions. Insights on the short time dynamics and on the H-bond network are illustrated.
Stochastic unraveling of Redfield master equations and its application to electron transfer problems
Authors: Kondov Ivan , Kleinekathöfer Ulrich , Schreiber Michael
Published: September 2003

A method for stochastic unraveling of general time-local quantum master equations (QMEs) is proposed. The present kind of jump algorithm allows a numerically efficient treatment of QMEs which are not in Lindblad form, i.e., are not positive semidefinite by definition. The unraveling can be achieved by allowing for trajectories with negative weights. Such a property is necessary, e.g., to unravel the Redfield QME and to treat various related problems with high numerical efficiency. The method is successfully tested on the damped harmonic oscillator and on electron transfer models including one and two reaction coordinates. The obtained results are compared to those from a direct propagation of the reduced density matrix (RDM) as well as from the standard quantum jump method. Comparison of the numerical efficiency is performed considering both the population dynamics and the RDM in the Wigner phase space representation.
Conductivity and microviscosity of electrolyte solutions containing polyethylene glycols
Authors: Stojilkovic Kosta S. , Berezhkovskii Alexander M. , Zitserman Vladimir Yu. , Bezrukov Sergey M.
Published: September 2003

Electrical conductivity of potassium chloride solutions containing polyethylene glycol (PEG) of different molecular mass was measured in a wide range of the polymer concentration up to 33 wt. % for PEG 300, 600, 2000, 4600, and 10 000. The data were used to find the dependence of microviscosity, ηmicro, which characterizes the decrease of the ion mobility compared to that in the polymer-free solution, on the polymer volume fraction, φ. We find that the dependence is well approximated by a simple relation ηmicro/η0=exp[kφ/(1−φ)], where η0 is viscosity of the polymer-free solution and k is a fitting parameter. Parameter k weakly depends on the polymer molecular mass growing from 2.5 for PEG 300 to its limiting value close to 2.9 for long chains. Using the φ-dependence of microviscosity, we give a practical formula for the conductivity of PEG-containing electrolyte solutions.
Variations of long- and short-range-order structural and magnetic properties of thermally annealed Mn/GaAs digital alloys
Authors: Soo Y. L. , Wang S. , Kim S. , Kim G. , Cheon M. , Chen X. , Luo H. , Kao Y. H. , Sasaki Y. , Liu X. , Furdyna J. K.
Published: September 2003

The crystal structure and local environment surrounding Mn atoms in Mn/GaAs digital layers thermally annealed at different temperatures have been investigated using x-ray diffraction and extended x-ray absorption fine structure methods. As the annealing temperature is increased, a satellite peak near the GaAs (004) diffraction line systematically shifts towards higher angles, indicating an apparent decrease of lattice parameters in the Mn/GaAs layers. When the annealing temperature is increased to 550 °C the satellite peak position moves dramatically from below to above the GaAs (004) peak, accompanied by a corresponding increase of Mn–As bond length from 2.48 to 2.56 Å, suggesting that the local structure around Mn changes from Ga substitution in GaAs to that of MnAs-like phase. Variations of the long- and short-range-order structures are believed to be related to the observed changes of magnetic properties including the disappearance of ferromagnetism in the sample annealed at 550 °C.
Molecular Coulomb islands for single-electron tunneling in SiO2/molecular layer/SiO2 multilayers on Si(100)
Authors: Wakayama Y. , Kubota T. , Suzuki H. , Kamikado T. , Mashiko S.
Published: September 2003

We demonstrate the Coulomb blockade phenomenon by using organic molecules as Coulomb islands embedded in a metal–oxide–semiconductor structure. Staircases in current–voltage (I–V) curves are observed in an SiO2/fullerene(C60)/SiO2 multilayered structure on Si (100) substrates. The staircases in the I–V curves can be attributed to the Coulomb blockade. The fullerenes serve as Coulomb islands and the SiO2 layers serve as tunneling barriers. The same multilayered structure that includes a porphyrin derivative also exhibits similar staircases in an I–V curve. These results reveal the potential of molecules to act as Coulomb islands of single-electron tunneling devices.
Chemical-state analysis for low-dimensional Si and Ge films on graphite
Authors: Nath Krishna G. , Shimoyama I. , Sekiguchi T. , Baba Y.
Published: September 2003

Synchrotron radiation photoemission spectroscopy has been used to study the chemical interaction and the electronic structures of low-dimensional semiconducting Si and Ge on a graphite surface. The core level spectra, such as Si 1s, Ge 2p, and C 1s, show that no chemical interaction occurs between adatoms and the substrate atoms, and that the electronic structures of graphite remain intact upon deposition of the adatoms at room temperature. The Si 1s and Ge 2p photoemission spectra reveal that Si and Ge films on graphite remain in their elemental form. However, the electronic structures of both Si and Ge films are found to be thickness dependent. In the thicker films, such as Si 5.5 Å or Ge 4.2 Å, a single bulk-like component is detected. In the case of thinner films, such as Si 2.7 Å or Ge 0.3 Å, some additional components are observed at binding energies higher than the bulk-like component. We attribute these peaks to semiconductor nanostructures, for example, nanowires, and individual or polymerized chains consisting of nanoclusters.
Recovery of close Frenkel pairs produced by low energy recoils in SiC
Authors: Gao F. , Weber W. J.
Published: September 2003

Simulations of displacement cascades in silicon carbide (SiC) indicate that most defects created are point defects, where interstitials are displaced only a small distance from the nearest vacancies. Of the interstitial defects produced in SiC, about 40% survive as freely migrating interstitials that can contribute to microstructural evolution during irradiation of SiC. The stable defect configurations in this study were created with low-energy recoils using molecular dynamics (MD) simulation. These stable Frenkel pairs have been annealed at different temperatures, using MD methods, to determine the time required for interstitials to recombine with vacancies. The MD data have been analyzed using an Arrhenius relation, and the activation energies for defect recombination processes range from 0.22 to 1.6 eV for C Frenkel pairs and from 0.28 to 0.9 eV for Si Frenkel pairs. These low activation energies for spontaneous recovery of Frenkel pairs in SiC are qualitatively consistent with the recovery processes observed experimentally on both the Si and C sublattices below room temperature. Based on the data analysis, the spontaneous recombination distance is estimated to be 0.66 and 0.70 ao for the C and Si sublattices, respectively.
Investigation of antiphase domain annihilation mechanism in 3C–SiC on Si substrates
Authors: Ishida Y. , Takahashi T. , Okumura H. , Yoshida S.
Published: September 2003

We have proposed a model in order to explain antiphase domains (APDs) annihilation in 3C–SiC on Si (001) substrates. The models proposed so far have been classified by the planes in which antiphase boundaries (APBs) propagate. We examined these conventional models from the viewpoint of incorrect-bond sequence contained in APB and clarified the correlation between the elongated direction of APBs on initial (001) surfaces and the direction of APBs propagation along growth axis. And then, we tried to decide the suitable incorrect-bond sequence to explain the APDs annihilation in 3C–SiC on Si from the surface morphology at initial growth stage, and confirmed the validity of the model by simulation under the restricted condition of layer-by-layer growth. Next, in order to explain our experimental results, we extended the model by considering the existence of steps on the surfaces and the growth modes, and proposed a model. Finally, we showed the validity of our model by the simulation and demonstrated the capability of our model by applying other systems, e.g., GaAs on Si and GaAs on Ge.
Dynamics of the spontaneous formation of a planar phospholipid bilayer: A new approach by simultaneous electrical and optical measurements
Authors: Fujiwara Hisashi , Fujihara Masayuki , Ishiwata Takashi
Published: September 2003

An artificial lipid bilayer in planar form, well known as bilayer lipid membrane (BLM), spontaneously forms from a lipid droplet (L-α-phosphatidylcholine in n-decane and chloroform in this work) in an aperture of a thin partition in aqueous solution. The thinning dynamics of the lipid droplet or membrane has been studied by simultaneous capacitance and image recording, because the lipid membrane sandwiched by aqueous solutions can be considered as a parallel-plate capacitor. The simultaneous measurements have revealed the two-step thinning of the lipid membrane from its specific capacitance value: first, the initial droplet thins to yield a membrane of about 100 nm thickness (0.02 μF/cm2), and second, within this thin lipid membrane, a lipid bilayer of 4 nm thickness (0.42 μF/cm2) suddenly emerges and grows, keeping a bilayer structure. In addition, the simultaneous measurements have a time stamp, and thus can determine the trigger moment of the bilayer formation. The revealed dynamics provides the first quantitative support for a “zipper” mechanism [H. T. Tien and E. A. Dawidowicz, J. Colloid Interface Sci. 22, 438 (1966)]; in the mechanism, the first thinning results in a sandwich consisting of the organic solvent between two adsorbed lipid monolayers whose distance is the order of 100 nm, and then a chance contact of both monolayers initiates the formation and growth of a lipid bilayer in a zipper-like manner. However, because of the existence of the two solvent–water interfaces containing surface-active molecules, phospholipids, this work claims that the zipper mechanism should be modified in view of the Marangoni effect. The simultaneous measurements have also revealed the adjacency effect, namely, that only the prebilayer region just adjacent to the bilayer changes into a bilayer. The present formation and growth of a lipid bilayer, including the adjacency effect, can be explained by the classic nucleation theory of two-dimensional crystallization. BLM systems with the simultaneous measurements can be considered as a useful environment for the study of soft-matter chemical physics.
Nonequilibrium molecular dynamics simulations of transport and separation of supercritical fluid mixtures in nanoporous membranes. I. Results for a single carbon nanopore
Authors: Firouzi Mahnaz , Tsotsis Theodore T. , Sahimi Muhammad
Published: September 2003

The results of extensive nonequilibrium molecular dynamics simulations of transport of a supercritical fluid mixture through a carbon nanopore, in the presence of an external pressure gradient, are reported. If the downstream pressure is large enough, a pore packing phenomenon occurs in which several layers of the fluids fill the pore. We also find that the fluid molecules form dynamic clusters that travel the length of the pore, with cluster sizes that oscillate with the time. In addition, the simulations indicate that there may be an optimal pore size and an optimal applied pressure gradient for maximum separation of a supercritical fluid mixture into its components.