bluuebunny/crossref_metadata_2025 huggingface dataset:
Low temperature magnetic force microscopy with enhanced sensitivity based on piezoresistive detection
Authors: Volodin A. , Temst K. , Van Haesendonck C. , Bruynseraede Y.
Published: July 2002

We describe the design and performance of a low temperature magnetic force microscope (MFM) based on commercially available piezoresistive cantilevers. The sensitivity has been increased by exciting the cantilever at a higher (second or third) flexural mode. The operation at higher mechanical resonances allows to improve the signal-to-noise ratio by a factor of 3. Our MFM is particularly advantageous for studying magnetic vortices on the surface of superconductors. The magnetic tip coating was optimized by relying on Co/Au multilayers grown by molecular beam epitaxy. This allows one to keep the interaction with the vortices small, and it becomes possible to observe a stable vortex lattice on the surface of a cleaved NbSe2 crystal. From our measurements of the disordered vortex state in thin Nb films we infer that the magnetic stray field induced by the tip is in the range 0.3–0.5 mT.
LiF/Al cathodes and the effect of LiF thickness on the device characteristics and built-in potential of polymer light-emitting diodes
Authors: Brown T. M. , Friend R. H. , Millard I. S. , Lacey D. J. , Burroughes J. H. , Cacialli F.
Published: July 2002

We report the characteristics of a series of polymer light-emitting diodes, fabricated with LiF/Al cathodes and differing only by the thickness of the LiF interlayer (0 nm⩽d⩽11 nm). Electroabsorption studies of the internal electrostatic potential give direct evidence of a sizable reduction of the cathodic barrier height brought about by the LiF films. These results also correlate with photoemission experiments [S. E. Shaheen, G. E. Jabbour, M. M. Morrell, Y. Kawabe, B. Kippelen, N. Peyghambarian, M. F. Nabor, R. Schlaf, E. A. Mash, and N. R. Armstrong, J. Appl. Phys. 84, 2324 (1998)] and with the electroluminescence performance of the diodes.
Laser-induced modification of carbon nitride thin films
Authors: Badi N. , Bensaoula A. , Ageev V. , Karabutov A. , Ugarov M. , Loubnin E.
Published: July 2002

The results of experiments on local pulsed ultraviolet laser annealing of carbon nitride (CNx) thin films are reported. It is shown that laser radiation can be used for efficient graphitization of CNx thin films. The degree of local transformation of diamond-like sp3 bonded CNx compound films into graphite-like ones is dependent upon the radiation energy density. It is also shown that the electron field emission properties of the thin films can be modified by the laser treatment.
Correlation between structural and optical properties of Si nanocrystals embedded in SiO2: The mechanism of visible light emission
Authors: Garrido B. , López M. , González O. , Pérez-Rodrı́guez A. , Morante J. R. , Bonafos C.
Published: July 2002

The size distribution, band gap energy, and photoluminescence of silicon nanocrystals embedded in SiO2 have been measured by direct and independent methods. The size distribution is measured by coupling high-resolution and conventional electron microscopy in special imaging conditions. The band gap is calculated from photoluminescence excitation measurements and agrees with theoretical predictions. Their correlation allows us to report the experimental Stokes shift between absorption and emission, which is 0.26±0.03 eV, independent of average size. This is almost exactly twice the energy of the Si–O vibration (0.134 eV). These results suggest that the dominant emission is a fundamental transition spatially located at the Si–SiO2 interface with the assistance of a local Si–O vibration.
Magnetic and structural properties of hexagonal-close-packed-CoCrPt–C granular media for high areal density recording
Authors: Xu Yingfan , Wang Jian Ping , Shan Z. S. , Jiang Hua , Chong Chong Tow
Published: July 2002

Magnetic and structural properties of hexagonal-close-packed-(CoCrPt)100−xCx thin films for potential applications in magnetic recording media have been investigated. CrTi and Cr underlayers were used to improve the in-plane preferred orientation. High coercivity of 4.4 kOe and anisotropy of 3.6×106 erg/cm3 have been achieved in films with a CrTi underlayer after postdeposition annealing. The addition of Ti to the Cr underlayer results in lower annealing temperature and shorter annealing time, while retaining higher coercivity. Vibrating sample magnetometer measurements showed that the magnetic moment lies fairly well in the film plane under proper preparation conditions; while x-ray diffraction measurements showed that the CrTi underlayer promoted Co (10.0) in-plane preferred orientation. Grain isolation in the magnetic layer was improved by the segregated C and further enhanced by postdeposition annealing. The peak value of δM curves decreased upon annealing and changed to a negative value when higher annealing temperatures were applied, indicating that strong exchange coupling in as-deposited film decreased and changed to dipolar interaction dominance upon annealing.
Evolution of the dielectric breakdown in Co/Al2O3/Co junctions by annealing
Authors: Schmalhorst J. , Brückl H. , Justus M. , Thomas A. , Reiss G. , Vieth M. , Gieres G. , Wecker J.
Published: July 2002

The temperature and dielectric stability of magnetic tunnel junctions are important requirements for magnetic memory devices and their integration in the semiconductor process technology. We have investigated the changes of the tunneling magnetoresistance (TMR), the barrier properties (height, thickness, and asymmetry) and the dielectric stability upon isochronal annealing up to 410 °C in Co/Al2O3/Co junctions with an artificial antiferromagnet as a pinning layer. Besides a small decrease of the TMR signal after annealing up to 230 °C, a strong decrease between 300 and 350 °C is found. According to Auger and transmission electron microscopy investigations, this decrease is mainly due to interdiffusion of the metallic layers. The dielectric breakdown is characterized by voltage ramp experiments. The size-averaged breakdown voltage improves from 1.35 V for the as prepared junctions to 1.55 V by annealing at 300 °C. At higher temperatures the breakdown voltage decreases strongly to 0.8 V (at 380 °C). Simultaneously, the typical breakdown process changes from few sudden current jumps to a large number of small steps. The breakdown properties are discussed within a statistical model and related to structural changes of the barrier.
Spin-angle topography of hexagonal manganites by magnetic second-harmonic generation
Authors: Fiebig M. , Fröhlich D. , Lottermoser Th. , Kohn K.
Published: July 2002

Magnetic second-harmonic generation is used for imaging of the orientation of the spins in antiferromagnetic ScMnO3 and HoMnO3 with 10 μm resolution and an accuracy of about 1°. This allows the observation of sample inhomogeneities and the coexistence of magnetic phases. The method is based on simple geometric arguments which should be applicable to a large variety of magnetic materials.
Conductance of polymer nanowires fabricated by a combined electrodeposition and mechanical break junction method
Authors: He H. X. , Li C. Z. , Tao N. J.
Published: July 2002

We electrochemically deposit conducting polymer to bridge two closely placed electrodes, and then form a polymer nanowire by stretching the polymer bridge with the electrodes. During stretching, the conductance increases initially as the polymer chains are aligned in parallel, and then decreases in a stepwise fashion, due to abrupt changes in the nanowire thickness. We study the current–voltage (I–V) characteristics of the nanowire as a function of its electrochemical potential in an analogous fashion to the control of the gate voltage in semiconductor devices. Depending on the potential, the I–V curves vary from ohmic to rectifying characteristics.
Comment on “Surplus function variational quantum Monte Carlo approach: Excited state processing” [J. Chem. Phys. 112, 5257 (2000)]
Authors: Marmorino M. G.
Published: July 2002

The surplus function approach to calculating ground-state energies and wave functions of molecules has recently been extended to excited states. We show that the original iterative procedure, designed for ground state use only, is equivalent to a matrix eigenvalue problem using Krylov subspaces, which is the Lanczos method. Excited-state energies and eigenfunctions are easily accessible in both procedures.
Site-selective x-ray absorption fine structure analysis of an optically active center in Er-doped semiconductor thin film using x-ray-excited optical luminescence
Authors: Ishii Masashi , Tanaka Yoshihito , Ishikawa Tetsuya , Komuro Shuji , Morikawa Takitaro , Aoyagi Yoshinobu
Published: July 2002

In order to discuss the local structure of an optically active center in Er-doped Si thin film, site-selective x-ray absorption fine structure (XAFS) analysis using x-ray-excited optical luminescence was performed. The XAFS spectrum at the Er LIII edge was obtained from the x-ray photon energy dependence of the peak intensity of infrared luminescence due to Er intra-4f transition. Although conventional XAFS measurement analyzes the average structure of all of the Er, this method intrinsically provides structural information for only optically active Er. A broad 2p–5d resonant peak in the site-selective XAFS spectrum is reproduced by a density-of-state calculation of a distorted ErO6 cluster, assuming an Er transformation from an octahedral center of 0.25 Å.
Light scattering from (K0.5Na0.5)0.2(Sr0.75Ba0.25)0.9Nb2O6 with the tungsten bronze structure: An analogy with relaxor ferroelectrics
Authors: Siny I. G. , Lushnikov S. G. , Siny S. I. , Schmidt V. H. , Savvinov A. A. , Katiyar R. S.
Published: July 2002

The Brillouin and Raman scattering from a complex single crystal from the tungsten–bronze family, (K0.5Na0.5)0.2(Sr0.75Ba0.25)0.9Nb2O6 doped with Cu2+ (KNSBN:Cu), have been comparatively studied in a wide temperature range around the ferroelectric transition. Step-like anomalies in hypersonic velocity and damping confirm the first-order structural transition. These anomalies look like some perturbations on the high-temperature slopes of both a broad dip in sound velocity and a broad maximum in damping that develop in a wide temperature range. The acoustic behavior of KNSBN:Cu does not simply follow the Landau theory prediction valid for many ferroelectrics. Instead it resembles that of relaxors, to which the KNSBN:Cu behavior is analogous intrinsically. The total intensity of the Raman spectra as well as the intensity of separate internal and external vibrations and their width correlate with acoustic anomalies, namely there are step-like drops at the same temperature as the first-order transition and a broad range where the intensity is drastically increased. All these broad anomalies imply the existence of a wide preceding phase in respect to the relaxor ferroelectric state. Unusual properties of this preceding phase are discussed as well as the phase diagram relation to the dynamical evolution of other relaxors from the perovskite family, such as PbMg1/3Nb2/3O3 and Na1/2Ba1/2TiO3.
Gel–gel adhesion by tethered polymers
Authors: Huang Yanbin , Szleifer Igal , Peppas Nikolaos A.
Published: July 2002

The behavior of tethered polymers on gel/gel adhesion is studied with the single-chain mean-field (SCMF) theory. It is shown that the gel surface structure, the gel/gel adhesion strength, the equilibrium gel/gel distance, and the detailed interface structures can be tailored by specifically designed tethered layers on gel surfaces. The SCMF theory allows to study the effect of various variables of tethered layers, such as the surface coverage, the attraction between polymers and gels, and the composition of block copolymers. These theoretical results provide guidelines for experimental designs of novel gel materials with tethered layers.
Electron propagator method with a multiconfigurational second-order perturbation theory wave function as the initial state in the fermion operator block
Authors: Heryadi Dodi , Yeager Danny L.
Published: July 2002

We have developed an electron propagator method using a multiconfigurational second-order perturbation theory (CASPT2) wave function as the initial state [electron propagator CASPT2 (EPCASPT2)] in the fermion operator block (block 1). In the other blocks a multiconfigurational self-consistent field wave function is the initial state. We apply our new method to directly determine the low-lying vertical ionization potentials of Be, CH2, NH2, and H2O. We compare our results with the results of the calculations using multiconfigurational spin tensor electron propagator (MCSTEP), full configuration interaction (FCI), and multireference configuration interaction (MRCI) methods with the same geometries and basis sets. The calculations are performed using complete active space (CAS) choices that are usually excellent for MCSTEP ionization potential (IP) calculations and also for CAS choices that are inadequate for MCSTEP IP calculations. We show that EPCASPT2 generally improves MCSTEP IPs compared to ΔFCI when the MCSTEP IPs are in very good to excellent agreement with ΔFCI IPs and that EPCASPT2 can effectively mimic ΔFCI even when the CAS choice for the initial state is inadequate for MCSTEP.
Electron stimulated desorption of H− from thin films of thymine and uracil
Authors: Hervé du Penhoat Marie-Anne , Huels Michael A. , Cloutier Pierre , Jay-Gerin Jean-Paul , Sanche Léon
Published: July 2002

We present measurements of low energy (0–38 eV) electron stimulated desorption of H− from room temperature thin films of pure thymine (T) and uracil (U), condensed on polycrystalline Pt, and describe in detail the experimental methods required for such studies. The nominal film thicknesses are estimated to range from 0.08 to 2.7 monolayers; sublimation of the films at 69 °C (T) and 82 °C (U) onto the room temperature Pt substrate leads to nonuniform film growth, i.e., volumetric clustering, particularly in the submonolayer regime. H− formation by electron impact occurs via dissociative electron attachment (DEA) to the molecules, and results in strong desorption peaks near 8.6 eV for either molecule, whereas above 12–13 eV nonresonant dipolar dissociation dominates the desorption yields. Comparison of the present condensed phase results with gas phase measurements suggests that the desorbing H− produced at the DEA peak are mainly the result of CH bond cleavage, while near the desorption threshold of about 5 eV NH bond cleavage via DEA may also contribute to the H− signal. The present measurements suggest that localized resonances involving DNA bases, leading to the formation of anions and their associated neutral radical moieties, contribute to the resonant signature observed recently in the strand break yields of double stranded DNA irradiated with 3–20 eV electrons.
Reverse Monte Carlo study of structural relaxation in vitreous selenium
Authors: Brüning Ralf , Irving Edward , LeBlanc Gilles
Published: July 2002

Vitreous selenium is used extensively for light detection. Its photoconducting properties are altered by structural relaxation near room temperature. X-ray diffraction, densiometry, and calorimetry are used to determine the change of the glass state during relaxation. Annealing for 24 h at 300 K increases the density by 0.14% and lowers the enthalpy by 105 J/mol. The structure factor of cast samples was measured before and after annealing, and reverse Monte Carlo models of the structure of selenium glass were generated to match the data for the quenched and annealed states. Atoms in amorphous selenium are arranged in randomly oriented chains. In the model clusters annealing effects are found by analyzing inter- and intrachain distances, bond angle distributions, and dihedral angle distributions. The average bond length remains unchanged upon annealing, while the distribution of bond lengths becomes narrower by 1.2%. Distances between atoms in adjacent chains decrease by about 0.03%, i.e., the selenium chains move closer together, and the distribution of interchain distances becomes narrower. Bond angles within the chains are affected slightly. The mean bond angle decreases by 0.016°, which indicates a greater folding of the selenium chains and represents an evolution towards the angles found in crystalline selenium phases. The bond angle distribution for the annealed state is 0.006° narrower.
Intrinsic and extrinsic precursors to adsorption: Coverage and temperature dependence of Kr adsorption on Pt(111)
Authors: Carlsson A. F. , Madix R. J.
Published: July 2002

The kinetics of krypton adsorption on Pt(111) were investigated using supersonic molecular beam techniques. Krypton adsorbs at defects via an intrinsic precursor below a surface temperature of 85 K. The difference in activation energies for desorption and migration of a Kr atom on the terrace seeking a defect site is 10.7 kJ/mol, indicating that at 80 K, a Kr atom makes about 107 site hops before desorbing or finding a binding site. Below 60 K stable adsorption occurs on terraces, where the initial adsorption probability is independent of surface temperature. The activation energy for zero-order desorption from Pt(111) terraces is 12.9 kJ/mol; the activation energy for Kr migration on the terraces is then calculated to be ⩽2.2 kJ/mol. Krypton adsorption proceeds at nonzero coverages via an extrinsic precursor. The adsorption probability of Kr increases with self-coverage, and is described by the modified Kisliuk model [H. C. Kang, C. B. Mullins, and W. H. Weinberg, J. Chem. Phys. 92, 1397 (1990); C. R. Arumainayagam, M. C. McMaster, and R. J. Madix, J. Phys. Chem. 95, 2461 (1991)]. The Kr overlayer on terraces compresses between 60 and 42 K, then forms a second state before forming stable multilayers at 34 K.
High frequency dielectric properties of A5B4O15 microwave ceramics
Authors: Kamba S. , Petzelt J. , Buixaderas E. , Haubrich D. , Vaněk P. , Kužel P. , Jawahar I. N. , Sebastian M. T. , Mohanan P.
Published: July 2002

High-frequency dielectric properties of A5B4O15 (A=Ba, Sr, Mg, Zn, Ca; B=Nb, Ta) dielectric ceramics are studied by means of the microwave cavity technique, a combination of far-infrared reflection and transmission spectroscopy and time-resolved terahertz transmission spectroscopy. Microwave permittivity ε′ and Q×f factor vary, depending on the chemical composition, between 11 and 51, and 2.4 and 88 THz, respectively. The temperature coefficient τf varies between −73 and 232 ppm/°C, and in two samples |τf| is less than 15 ppm/°C. It is shown that the microwave permittivity ε′ of the ceramics studied is determined by the polar phonon contributions and that linear extrapolation of the submillimeter dielectric loss ε″ down to the microwave region is in agreement with the microwave data of single phase samples. The relationship among phonon spectra, the crystal structure, and the unit cell volume is discussed.
Calculation of heat capacities and entropies of metal halides with quantum chemical methods
Authors: Jug Karl , Janetzko Florian , Köster Andreas M.
Published: July 2002

Statistical thermodynamics and quantum chemistry are combined to evaluate the temperature dependence of heat capacities Cp and entropies S for metal halides. Density functional calculations are used to supply the structural data and vibrational frequencies. The accuracy of these data is established by comparison with experimental data. It is shown that they provide a reliable basis for the calculation of Cp and S. Discrepancies between calculated values and experimental literature values can mainly be attributed to vibrational anharmonicities and contributions from excited electronic states. It is demonstrated that quantum chemical calculations can provide a powerful tool for systems where experimental data are not available.
Simultaneous operation of two adjacent double dots in silicon
Authors: Single C. , Prins F. E. , Kern D. P
Published: July 2002

We have studied the electrical transport through two adjacent double dot structures realized in Si-on-insulator material. The small dimensions of dots and the distance between the centers of the dots, here 100 nm, allow for dc measurements at 4.2 K. We demonstrate that although the structures are entirely doped, we are able to operate both double dot structures simultaneously in the double dot regime. Clear Coulomb blockade characteristics with a Coulomb blockade region of 20 mV are observed in both structures. Using gates, the number of electrons on each individual dot can be manipulated simultaneously. From the measured charging diagrams the capacitances between the gates and the dots are determined.
Influence of orientational ordering transition on diffusion of carbon dioxide in carbon nanopores
Authors: Papadopoulos George K.
Published: July 2002

Simulation studies predicted an orientational ordering transition of the molecular axis of supercritical CO2, inside individual graphite pores, with size of the order of few molecular diameters. Study of the diffusion equation over a real porous medium, showed significant influence of the transition on the adsorbate diffusivity. Measured permeability of CO2 adsorbed in a nanoporous carbon membrane, confirmed the simulation results by exhibiting unusually high permeability values in the vicinity of the transition, and elucidated the significance of the thermodynamics in the study of the kinetic properties of the adsorbed phase.
Simulation of three-dimensional nonperiodic structures of π-vertical Bloch line (pi-VBL) and 2π-VBL (2pi-VBL) in Permalloy films
Authors: Redjdal M. , Kakay A. , Trunk T. , Ruane M. F. , Humphrey F. B.
Published: July 2002

Three-dimensional magnetic structures of pi-vertical-Bloch line (VBL) and 2pi-VBL are investigated in 80–320-nm-thick Permalloy films using direct integration of the Landau–Lifshitz–Gilbert equation in an 128×128×80 point Cartesian lattice. A pi-VBL reflects the shapes of its adjacent walls, which change with film thickness. The pi-VBL conducts the flux between walls of opposite chirality by letting the magnetization rotate out of the plane of the walls via a vortex structure. The Néel caps switch chirality via a “converging point,” or cross-tie, flux at one surface and a vortex flux, or a swirl, at the other surface. The pi-VBL energy per unit area is 0.44, 0.20, and 0.30 erg/cm2 in 80, 160, and 320 nm films, respectively. The corresponding pi-VBL widths are 88, 60, and 86 nm. A stable winding 2pi-VBL structure was also computed by combining two pi-VBL structures of appropriate chirality. The Néel caps intersect via the pair-(swirl, converging point) like flux at both surfaces. The width of the 2pi-VBL is 140 nm and its energy per unit area is 0.57 erg/cm2.
Structure of a two-component mixture of sticky hard-sphere fluids in a planar gap
Authors: Jamnik A.
Published: July 2002

The structural properties of nonuniform, binary, sticky hard-sphere (SHS) fluid mixture (the particular components being denoted by SHS1 and SHS2) within a planar pore, maintaining the equilibrium with the bulk phase, were investigated. The wall–SHS1 and wall–SHS2 correlations of the SHS mixture confined in the pore, and its distribution between the pore and the bulk phase were studied on the basis of the solution to the Percus–Yevick/Ornstein–Zernike equation, obtained by using the analytic results for the direct correlation functions of the bulk SHS mixture. The density profiles of particular components with the hard core diameters σ1 and σ2 revealed distinct layering, a characteristic feature being the discontinuities in the slopes of the profiles at the distances n⋅σ1+m⋅σ2 (n=0,1,…;m=0,1,…) from the wall–SHS1 and wall–SHS2 contact planes. It was shown that narrowing of the pore considerably affects the partitioning of both components between the bulk phase and the interior of the pore, the effects being more pronounced in the case of a more adhesive component. As the individual components of the binary mixture are selectively partitioned between the bulk phase and narrow pore, the porous material, i.e., a network of narrow pores, makes possible a partial separation of both components from the bulk SHS mixture.
Magnetic tunnel junctions on magnetic shield smoothed by gas cluster ion beam
Authors: Sun J. J. , Shimazawa K. , Kasahara N. , Sato K. , Kagami T. , Saruki S. , Araki S. , Matsuzaki M.
Published: July 2002

In this work, a technique, gas cluster ion beam (GCIB), was introduced to smooth the bottom NiFe magnetic shield for magnetic tunnel junction (MTJ) read heads. The GCIB treatment can bring the surface roughness of the shield from 15 to 20 Å to around 5 Å, and the most of scratch marks can be removed. The efficiency of the GCIB process is dependent on the initial surface morphology. The MTJs grown on the magnetic shield smoothed by the GCIB show that the resistance area product RA is increased from 60 to ∼100 Ω μm2 with the GCIB dose up to 1×1016 ions/cm2, arising from a smooth insulating layer, meanwhile, the tunneling magnetoresistance (TMR) is almost constant or slightly decreases. This GCIB process can also improve breakdown voltage (approximately 0.019 V per 1015 ions/cm2) of the MTJs, and slightly increase the ferromagnetic coupling mainly due to the change of the surface morphology. Using this technology, an RA as low as 3.5–6.5 Ω μm2 together with a TMR of 14%–18% can be obtained for MTJs grown on the GCIB treated NiFe magnetic shield.
The heat capacity of the restricted primitive model electrolyte
Authors: Luijten Erik , Fisher Michael E. , Panagiotopoulos Athanassios Z.
Published: July 2002

The constant-volume heat capacity, CV(T,ρ), of the restricted primitive model (RPM) electrolyte is considered in the vicinity of its critical point. It is demonstrated that, despite claims, recent simulations for finite systems do not convincingly indicate the absence of a divergence in CV(T,ρ)—which would point to non-Ising-type criticality. The strong qualitative difference between CV for the RPM and for a Lennard-Jones fluid is shown to result from the low critical density of the former. If one considers the theoretically preferable configurational heat-capacity density, CV/V, the finite-size results for the two systems display qualitatively similar behavior on near-critical isotherms.
Relabeling and classification of the Rydberg states
Authors: Li Jian , Liu Yaoming , Dai Xingcan , Li Li , Field Robert W.
Published: July 2002

All 84 electronic states of Na2 that correlate adiabatically to separated atom limits up to Na(3s)+Na(5p) have been relabeled according to the dominant hydrogenic nlλ characters of the Rydberg orbital and classified as core-penetrating vs core-nonpenetrating states. This labeling and classification scheme establishes a unique correspondence between experimentally observed and theoretically predicted electronic states, which provides insights into the electronic structure of Na2. Hydrogenic nlλ labels for Rydberg states are of particular value for predicting and interpreting transition strengths, sensitivity to external electric fields, and surprisingly disparate collisional properties for nearly isoenergetic states.
Predicted predissociation linewidths in the Schumann–Runge bands of O2 compared with recent high resolution measurements
Authors: Tong G. S. M. , Cheung A. S-C. , Jamieson M. J.
Published: July 2002

We derive and use new parameters to quantify the semiempirical expressions of Julienne and Krauss [J. Mol. Spectrosc. 56, 270 (1975)] for the Πu5, Σu+3, Πu3, and Πu1 potentials that predissociate the rovibrational levels of the B 3Σu− state of O2. Using the new parameters in the model of Julienne and Krauss we evaluate fine-structure predissociation linewidths for the Schumann–Runge bands that terminate on the v=13,14 rovibrational levels of the B 3Σu− state. We compare these linewidths and those calculated with several existing sets of parameters with the measurements of Dooley et al. [J. Chem. Phys. 109, 3856 (1998)]. We also show the deperturbing effect of the level shifts, calculated with the new parameters, on the energies of the rotationless v=0–17 vibrational levels of the B 3Σu− state.
Predissociation of Rydberg states of CO investigated by the detection of atomic fragments
Authors: Okazaki Akihiro , Ebata Takayuki , Mikami Naohiko
Published: July 2002

Predissociation of Rydberg states of CO has been investigated by the C (3P and D1) and O(3P) photofragment measurements in the region of 103 000–114 000 cm−1 (88–97 nm). The simulations of the rotational structures of np and nf Rydberg states were also carried out by using the l-uncoupling Hamiltonian model. The photofragment yield spectra were compared with the ion-dip spectra which correspond to the absorption spectra, indicating that all the Rydberg states, ns, np, nd, and nf converging to the X 2Σ+ CO+ ion, were subject to the predissociation. It was found that the lower member npπ and ndπ states exhibit two dissociation path ways, that is the C(3P)+O(3P) and the C(1D)+O(3P) channels. Especially, for the 4pπ L 1Π(v=0) state the two channels were found to be competitive with respect to parity as well as rotational quantum number J. At higher np series, such a parity and J-dependence of the predissociation disappeared, and the observed rotational structure was simulated very well by the model with no parity nor J dependence. For the ndσ states, the spectra of the 3dσ and 5dσ(v=0) states were diffuse, while the 4dσ(v=0) state showed a rotationally resolved photofragment yield spectrum. All the photofragment yield spectra of the nf(v=0) states exhibited sharp structures compared with those of other Rydberg states with a small l value. From a comparison between the photofragment yield spectrum and the ion-dip spectrum, it was found that the predissociation rate of the e-symmetry component is larger than that of the f-symmetry component. It was suggested that the e-symmetry levels predissociate through D′ 1Σ+ valence states, while the f-symmetry levels predissociate through the 2 1Π state.
The Cotton–Mouton effect of gaseous CO2, N2O, OCS, and CS2. A cubic response multiconfigurational self-consistent field study
Authors: Jonsson Dan , Norman Patrick , Ågren Hans , Rizzo Antonio , Coriani Sonia , Ruud Kenneth
Published: July 2002

The hypermagnetizability and the hypermagnetizability anisotropy of CO2, N2O, OCS, and CS2 are computed at a wavelength of 632.8 nm using cubic response theory with multiconfigurational self-consistent field wave functions. The anisotropies of the electric dipole polarizability and of the magnetizability are also obtained. This allows us to study the temperature dependence of the Cotton–Mouton constant for all four molecules and thus to compare to the results of the experimental study by Kling and Hüttner [Chem. Phys. Lett. 90, 207 (1984)]. We also assess the importance of pure and zero-point vibrational effects on the relevant molecular properties. In particular, we show that for CO2, OCS, and CS2, the pure vibrational effects to the hypermagnetizability anisotropy can be even more important than the electronic contribution.
Phase-transition-induced residual strain in ferromagnetic MnAs films epitaxially grown on GaAs(001)
Authors: Trampert A. , Schippan F. , Däweritz L. , Ploog K. H.
Published: July 2002

We investigate the atomic interface structure and the residual strain state of ferromagnetic α (hexagonal) MnAs layers on cubic GaAs(001) by means of high-resolution transmission electron microscopy and electron diffraction. Despite the different symmetries of the adjacent planes at the heterointerface and the large and orientation-dependent lattice mismatch, the hexagonal MnAs grows epitaxially on GaAs(001) with the (11̄.0) prism plane parallel to the cubic substrate. The atomic arrangement at the interface, which is defined by the accommodation of the large lattice mismatch, explains this extreme case of heteroepitaxial alignment. The anisotropic residual strain distribution is discussed with respect to the particular process of lattice misfit relaxation in the presence of the ferromagnetic phase transition.
Description of tunneling with the help of coupled frozen Gaussians
Authors: Shalashilin Dmitrii V. , Child Mark S.
Published: July 2002

A previous initial value coherent state representation is applied to Gaussian wave packet propagation on one-dimensional and two-dimensional double well potentials. The method uses a basis of trajectory guided frozen Gaussian coherent states, sampled from a Monte Carlo ensemble, for numerical solutions of the quantum time dependent integro-differential Schrödinger equation. Accurate tunneling splittings are obtained. Comparisons are also made with corresponding results obtained by the Herman–Kluk semiclassical initial value representation wave packet method.
On the computer simulation of silicate glass surfaces
Authors: Bakaev V. A. , Steele W. A. , Pantano C. G.
Published: July 2002

The surface layer of xNa2O(1−x)SiO2 (x=0.245) was simulated by the molecular dynamics. The glass transition temperatures Tg (for the molecular dynamics time scale) in the bulk and for the surface layer were determined by the method of inherent structures. In the bulk Tg is larger than at the surface. The isotherm of adsorption of CO2 was also simulated on that model surface by grand canonical Monte Carlo. The isotherm for the unannealed surface lies higher than that on the annealed one but the difference between them is considerably smaller than that for the corresponding surfaces of silica. The correlation between the stability (chemical durability) of the surface and physical adsorption is discussed. It is shown that molecules of CO2 can penetrate more than 2 nm under the surface of the silicate.
Nitrogen deactivation by implantation-induced defects in 4H–SiC epitaxial layers
Authors: Åberg D. , Hallén A. , Pellegrino P. , Svensson B. G.
Published: July 2002

Ion implantation causes free charge carrier reduction due to damage in the crystalline structure. Here, nitrogen-doped 4H silicon carbide (n type) epitaxial layers have been investigated using low ion doses in order to resolve the initial stage of the charge carrier reduction. It was found that the reduction of free carriers per ion-induced vacancy increases with increasing nitrogen content. Nitrogen is suggested to be deactivated through reaction with migrating point defects, and silicon vacancies or alternatively interstitials are proposed as the most likely candidates.
Analysis on the Cina–Harris proposal for the preparation and detection of chiral superposition states
Authors: Duarte-Zamorano Roberto P. , Romero-Rochı́n Vı́ctor
Published: July 2002

We study the preparation and measurement of superpositions of molecular chiral states with sequences of phase-locked ultrashort laser pulses following the scheme proposed by Cina and Harris [J. Chem. Phys. 100, 2531 (1994)]. The model is a parity-invariant molecule with a handed electronic ground state and a harmonic electronic excited state. We analyze the problem by numerically solving the Schrödinger equation. We find that the process of preparation of the superposition of chiral states works very well, in agreement with the analytic approximate solution of Cina and Harris. The detection part, in terms of a fluorescence interferogram, however, turns out to be more delicate. That is, in order to reproduce the main features in the interferogram that reveal the superposition of the chiral states, very stringent conditions on the involved time scale must be satisfied. The most unrealistic characteristic is that very high vibrational states of the (harmonic) excited electronic states are involved. Nevertheless, the proposal is quite feasible and we discuss that the detection restrictions may be softened.
Rapid estimation of electronic degrees of freedom in Monte Carlo calculations for polarizable models of liquid water
Authors: Mahoney Michael W. , Jorgensen William L.
Published: July 2002

The inclusion of electronic polarization within Monte Carlo calculations of simple models of molecular liquids is hampered, relative to its inclusion within molecular dynamics calculations, by the need to fully determine the variables that specify the electronic configuration every time each molecule is moved, i.e., N times per cycle, rather than once per cycle. Classical statistical mechanical Monte Carlo calculations on two models of liquid water have been performed. For each of the models, electronic degrees of freedom are modeled by polarizable sites; thus it is the components of the induced dipole vector that must be determined at each step. Commonly used approximation methods have been characterized and found to be inadequate. Efficient procedures have been devised to estimate the dipole vector and have been tested on reproducing electronic, thermodynamic, and structural properties of the two polarizable water models. The most promising procedure, considering both computational time saved and accuracy at reproducing pure liquid properties, involves approximating the induced dipoles at each step by an initial perturbative modification of the dipoles from the previous step, followed by an iteration of the induced dipoles on only the moved molecule. With this procedure, the CPU time is dramatically reduced, and the thermodynamic and structural properties are estimated correctly to within a few percent. They are reproduced more rapidly and with greater accuracy than in calculations in which the dipole vector is estimated by a single iterative cycle starting with the dipoles from the previous Monte Carlo step.
Perturbation of a lattice spectral band by a nearby resonance
Authors: Motovilov A. K. , Sandhas W. , Belyaev V. B.
Published: July 2002

A soluble model of weakly coupled “molecular” and “nuclear” Hamiltonians is studied in order to exhibit explicitly the mechanism leading to the enhancement of fusion probability in case of a narrow near-threshold nuclear resonance. We, further, consider molecular cells of this type being arranged in lattice structures. It is shown that if the real part of the narrow nuclear resonance lies within the molecular band generated by the intercellular interaction, an enhancement, proportional to the inverse width of the nuclear resonance, is to be expected.
Solvent effects on ground and excited electronic state structures of p-nitroaniline
Authors: Moran Andrew M. , Kelley Anne Myers
Published: July 2002

Resonance Raman intensities of p-nitroaniline, a prototypical “push–pull” chromophore with a large first hyperpolarizability (β), have been measured in dilute solution in five solvents having a wide range of polarities (cyclohexane, 1,4-dioxane, dichloromethane, acetonitrile, and methanol) at excitation wavelengths spanning the strong near-ultraviolet charge-transfer absorption band. The absolute Raman excitation profiles and absorption spectra are simulated using time-dependent wave packet propagation techniques to determine the excited-state geometry changes along the five or six principal Raman-active vibrations as well as estimates of the solvent reorganization energies. The total vibrational reorganization energy decreases and the solvent reorganization energy increases with increasing solvent polarity in all solvents except methanol, where specific hydrogen-bonding interactions may be important. The dimensionless normal coordinate geometry changes obtained from the resonance Raman analysis are converted to actual bond length and bond angle changes with the aid of normal mode coefficients from a ground-state density functional theory calculation. The geometry changes upon electronic excitation involve predominantly the Cphenyl–Nnitro, N–O, and phenyl C2–C3 bond lengths, with little involvement of the amino group. Nonresonant Raman spectra in 1,4-dioxane, dichloromethane, ethyl acetate, acetone, acetonitrile, and methanol show only a very small solvent dependence of the vibrational frequencies. This suggests that changing the solvent affects the excited state more than the ground state, calling into question two-state models that treat the ground and charge-transfer excited states as linear combinations of neutral and zwitterionic basis states with solvent dependent coefficients.
Ab initio Hartree–Fock study of electron transfer in organic molecules
Authors: Pati Ranjit , Karna Shashi P.
Published: July 2002

Electron transfer (ET) in σ-bonded organic cage structures (bicyclo[1.1.1]pentane, cubane, and bicyclo[2.2.2]octane) has been studied with the help of ab initio Hartree–Fock calculations in the framework of a two-state model. The calculated values of the ET coupling matrix element VAB exhibit strong dependence on the basis set employed. A minimal basis set underestimates the value of VAB with respect to an extended (double-zeta and polarization) basis set. The ET shows correlation with the electronic and geometrical structure of the molecules studied. It is found that the more strained the chemical bonds in the cage structure are, the stronger is the coupling between the two states participating in ET. Furthermore, the ET matrix element VAB is calculated to have its maximum value when the two end groups attached to the cage structures are coplanar, and its minimum value when two end π groups are perpendicular to each other. However, for coplanar end-groups, minimal changes are noted in the value of VAB with respect to the rotation of the σ-bonded cage. The dependence of ET on the relative orientation of the planes of the end groups offers a mechanism for designing molecular switches.
Portable long trace profiler: Concept and solution
Authors: Qian Shinan , Takacs Peter , Sostero Giovanni , Cocco Daniele
Published: July 2002

Since the early development of the penta-prism long trace profiler (LTP) and the in situ LTP, and following the completion of the first in situ distortion profile measurements at Sincrotrone Trieste (ELETTRA) in Italy in 1995, a concept was developed for a compact, portable LTP with the following characteristics: easily installed on synchrotron radiation beam lines, easily carried to different laboratories around the world for measurements and calibration, convenient for use in evaluating the LTP as an in-process tool in the optical workshop, and convenient for use in temporarily installation as required by other special applications. The initial design of a compact LTP optical head was made at ELETTRA in 1995. Since 1997 further efforts to reduce the optical head size and weight, and to improve measurement stability have been made at Brookhaven National Laboratory. This article introduces the following solutions and accomplishments for the portable LTP: (1) a new design for a compact and very stable optical head, (2) the use of a small detector connected to a laptop computer directly via an enhanced parallel port, and there is no extra frame grabber interface and control box, (3) a customized small mechanical slide that uses a compact motor with a connector-sized motor controller, and (4) the use of a laptop computer system. These solutions make the portable LTP able to be packed into two laptop-size cases: one for the computer and one for the rest of the system.
Photolysis of intrasilicalite I2: Spectroscopy and dynamics
Authors: Pedersen David B. , Weitz Eric
Published: July 2002

The 308 nm photoexcitation of intrasilicalite I2 results in emission in the near-infrared that is characteristic of caged electronic states of I2. This is the first observation of such emission in a zeolite environment. A first order model is developed which adequately describes the repulsive interactions between iodine and the zeolite cage that leads to “caging.” However, state specific interactions between the intrazeolite species and the zeolite can occur. States with vacant bonding orbitals can be stabilized as a result of donation of electron density from the zeolite cage into these vacancies. These state-specific chemical interactions between I2 and the zeolite cage can lead to a shift in the energies of the relevant states and correspondingly a shift in the energy of optical transitions involving these states. The magnitude of the change in energy of the affected state correlates with the electron donating ability of the caging environment. Within the context of this correlation, silicalite has an effective ionization energy of approximately 11.8 eV. The absorption band of intrasilicalite I2, which has been excited in this study at 308 nm, is attributed to promotion of the system to an ion pair state. An ion pair model of the excited state potential qualitatively describes the observed ultraviolet-visible emission from this system.
A magnifying lens for velocity map imaging of electrons and ions
Authors: Offerhaus H. L. , Nicole C. , Lépine F. , Bordas C. , Rosca-Pruna F. , Vrakking M. J. J.
Published: July 2002

We have designed and implemented an electrostatic lens that magnifies the images of an existing velocity map imaging apparatus up to a factor of 20. The lens can be used to vary the magnification while keeping the field strength in the interaction region constant. For the region of interest where magnification is required (low energy ions or electrons, in a high external field) the lens does not add any observable aberrations to the imaging. We have characterized the performance of the lens using the imaging of slow photoelectrons.
Efficient polymer light emitting diodes with metal fluoride/Al cathodes
Authors: Yang Xiaohui , Mo Yueqi , Yang Wei , Yu Gang , Cao Yong
Published: July 2002

We reported efficient polymer light emitting diodes with alkali and alkaline earth metal fluoride Al cathodes. The quantum efficiencies of these devices are more than two orders of magnitude higher than those in devices without fluoride layer, comparable to the best data reported with similar electroluminescent polymers. The mechanism of the metal fluoride/Al bilayer cathodes was also discussed based on photovoltaic, ac impedance, and photoluminescence efficiency measurement.
Optical and photovoltaic properties of indium selenide thin films prepared by van der Waals epitaxy
Authors: Sánchez-Royo J. F. , Segura A. , Lang O. , Schaar E. , Pettenkofer C. , Jaegermann W. , Roa L. , Chevy A.
Published: July 2002

Indium selenide thin films have been grown on p-type gallium selenide single crystal substrates by van der Waals epitaxy. The use of two crucibles in the growth process has resulted in indium selenide films with physical properties closer to these of bulk indium selenide than those prepared by other techniques. The optical properties of the films have been studied by electroabsorption measurements. The band gap and its temperature dependence are very close to those of indium selenide single crystals. The width of the fundamental transition, even if larger than that of the pure single crystal material, decreases monotonously with temperature. Exciton peaks are not observed even at low temperature, which reveals that these layers still contain a large defect concentration. The current–voltage characteristic of indium selenide thin film devices was measured under simulated AM2 conditions. The solar conversion efficiency of these devices is lower than 0.6%. The high concentration of defects reduces the diffusion length of minority carriers down to values round to 0.2 μm.
Hysteresis losses in a thin high-temperature superconductor strip exposed to ac transport currents and magnetic fields
Authors: Schönborg N.
Published: July 2002

In many prospective power applications based on high-temperature superconductors, the hysteresis losses will be an important parameter. To obtain an optimized device, models of the hysteresis losses valid at applicationlike conditions are needed. In this article the superconductor is modeled as a thin strip with negligible thickness, a geometry which probably is especially suitable for the new wires of the coated-conductor type. The strip is assumed to carry a transport current in phase with an externally applied magnetic field perpendicular to the strip. Calculations of the hysteresis losses, based on the critical state theory, are performed resulting in exact analytical equations.