bluuebunny/crossref_metadata_2025 huggingface dataset:
Growth-induced uniaxial anisotropy in grazing-incidence deposited magnetic films
Authors: van Dijken Sebastiaan , Di Santo Giovanni , Poelsema Bene
Published: July 2002

The deposition angle dependence of the in-plane magnetic anisotropy in ultrathin Co films on Cu(001) has been investigated by using the magneto-optic Kerr effect. Grazing-incidence molecular beam epitaxy of Co has been found to result in a uniaxial magnetic anisotropy, whose strength increases with increasing deposition angle. The measured magnetic anisotropy is related to the formation of elongated adatom structures during growth. Calculations of the deposition dynamics reveal that the uniaxial magnetic anisotropy and surface morphology originate from a redistribution of incident atom flux. This redistribution is caused by long-range attractive forces between incident atoms and substrate atoms.
Efficient free energy calculations by variationally optimized metric scaling: Concepts and applications to the volume dependence of cluster free energies and to solid–solid phase transitions
Authors: Miller Mark A. , Reinhardt William P.
Published: July 2002

Finite-time variational switching is an efficient method for obtaining converging upper and lower bounds to free energy changes by computer simulation. Over the course of the simulation, the Hamiltonian is changed continuously between the system of interest and a reference system for which the partition function has an analytic form. The bounds converge most rapidly when the system is kept close to equilibrium throughout the switching. In this paper we introduce the technique of metric scaling to improve adherence to equilibrium and thereby obtain more rapid convergence of the free energy bounds. The method involves scaling the coordinates of the particles, perhaps in a nonuniform way, so as to assist their natural characteristic evolution over the course of the switching. The scaling schedule can be variationally optimized to produce the best convergence of the bounds for a given Hamiltonian switching path. A correction due to the intrinsic work of scaling is made at the end of the calculation. The method is illustrated in a pedagogical one-dimensional example, and is then applied to the volume dependence of cluster free energies, a property of direct relevance to vapor–liquid nucleation theory. Order-of-magnitude improvements in efficiency are obtained in these simple examples. As a contrasting application, we use metric scaling to calculate directly the free energy difference between face-centered-cubic and body-centered-cubic Yukawa crystals. A continuous distortion is applied to the lattice, avoiding the need for separate comparison of the two phases with an independent reference system.
Excitation energies of dissociating H2: A problematic case for the adiabatic approximation of time-dependent density functional theory
Authors: Gritsenko O. V. , van Gisbergen S. J. A. , Görling A. , Baerends E. J.
Published: July 2002

Time-dependent density functional theory (TDDFT) is applied for calculation of the excitation energies of the dissociating H2 molecule. The standard TDDFT method of adiabatic local density approximation (ALDA) totally fails to reproduce the potential curve for the lowest excited singlet Σu+1 state of H2. Analysis of the eigenvalue problem for the excitation energies as well as direct derivation of the exchange-correlation (xc) kernel fxc(r,r′,ω) shows that ALDA fails due to breakdown of its simple spatially local approximation for the kernel. The analysis indicates a complex structure of the function fxc(r,r′,ω), which is revealed in a different behavior of the various matrix elements K1c,1cxc (between the highest occupied Kohn–Sham molecular orbital ψ1 and virtual MOs ψc) as a function of the bond distance R(H–H). The effect of nonlocality of fxc(r,r′) is modeled by using different expressions for the corresponding matrix elements of different orbitals. Asymptotically corrected ALDA (ALDA-AC) expressions for the matrix elements K12,12xc(στ) are proposed, while for other matrix elements the standard ALDA expressions are retained. This approach provides substantial improvement over the standard ALDA. In particular, the ALDA-AC curve for the lowest singlet excitation qualitatively reproduces the shape of the exact curve. It displays a minimum and approaches a relatively large positive energy at large R(H–H). ALDA-AC also produces a substantial improvement for the calculated lowest triplet excitation, which is known to suffer from the triplet instability problem of the restricted KS ground state. Failure of the ALDA for the excitation energies is related to the failure of the local density as well as generalized gradient approximations to reproduce correctly the polarizability of dissociating H2. The expression for the response function χ is derived to show the origin of the field-counteracting term in the xc potential, which is lacking in the local density and generalized gradient approximations and which is required to obtain a correct polarizability.
Nonlinear wavepacket interferometry for polyatomic molecules
Authors: Cina Jeffrey A.
Published: July 2002

We investigate the application of a previously considered nonlinear wavepacket interferometry scheme to molecules with a single stable conformation in the electronic ground state. It is shown that interference experiments with pairs of phase-locked ultrashort pulse-pairs can be used to determine the complex overlaps of a nonstationary nuclear wavefunction evolving in an excited electronic state with a collection of compact displaced wavepackets moving in specified ways in the ground-state potential.
Atomic beam deposition of lanthanum- and yttrium-based oxide thin films for gate dielectrics
Authors: Guha S. , Cartier E. , Gribelyuk M. A. , Bojarczuk N. A. , Copel M. C.
Published: July 2002

We report on the electrical and microstructural characteristics of La- and Y-based oxides grown on silicon substrates by ultrahigh vacuum atomic beam deposition, in order to examine their potential as alternate gate dielectrics for Si complementary metal oxide semiconductor technology. We have examined the issues of polycrystallinity and interfacial silicon oxide formation in these films and their effect on the leakage currents and the ability to deposit films with low electrical thickness. We observe that polycrystallinity in the films does not result in unacceptably high leakage currents. We show significant Si penetration in both types of films. We find that the interfacial SiO2 is much thicker at ∼1.5 nm for the Y-based oxide compared to the La-based oxide where the thickness is <0.5 nm. We also show that while the Y-based oxide films show excellent electrical properties, the La based films exhibit a large flat band voltage shift indicative of positive charge in the films.
Multiple channel dynamics of the O+CH3F reaction
Authors: Shu Jinian , Lin Jim J. , Lee Yuan T. , Yang Xueming
Published: July 2002

The O(1D,3P)+CH3F reaction has been investigated using universal crossed molecular beam methods. The following reaction channels: HF+H2CO, H+CHFOH/CFH2O, and H2+CFOH/CHFO, have been observed in a single set of experiments. Time-of-flight spectra have been measured for products from these channels. Product angular distributions have been determined for these product channels, indicating that the dominant dynamical pathway for the reaction is the insertion mechanism. Relative branching ratios of the observed reaction channels have also been estimated. Experimental results in this work indicate that the HF formation channel is the dominant reaction pathway. This study provides a rather complete dynamical picture of the multiple channel O+CH3F reaction. It is believed that the O(1D) reaction is playing a major role in this experiment since it is energetically more favorable.
Discovery of benzene cation in a very long-lived excited electronic state
Authors: Kim Myung Soo , Kwon Chan Ho , Choe Joong Chul
Published: July 2002

Presence of benzene cation in a long-lived excited electronic state at ∼2.3 eV above the ground state was found through photodissociation kinetics and charge exchange ionization mass spectrometry. The lifetime of this state seems to be longer than 10 μs, maybe much longer. The experimental energy level of this state suggests that B̃ 2E2g is the best candidate. The radiative transition from the latter state to the ground state, B̃ 2E2g→X̃ 2E1g, is electric dipole forbidden. Then, a very long lifetime requires an inefficient radiationless decay of the state. This is in contrast with the very fast decay proposed previously to account for the lack of fluorescence from the B̃ 2E2g state. The present finding opens the possibility of studying a new excited state chemistry.
Potato, banana, local, and nonlocal transport
Authors: Shaing K. C.
Published: July 2002

The relationship between potato and banana transport theories is addressed using the solution of the drift kinetic equation. It is shown that they are two limits of a complete theory. The potato theory is the ψ→0 limit and the banana theory is the ψ→∞ limit. Here, ψ is the poloidal flux function. These local transport theories are valid even in the steep gradient situations because the real orbit width is usually smaller than the gradient scale length when the appropriate orbit squeezing effects are taken into account. The characteristic feature of a nonlocal theory is also discussed. It is shown that the equilibrium distribution in such a theory must be non-Maxwellian and nonexpandable.
Spectra of jet-cooled SO232 and SO234 in systems ã 3B1 and b̃ 3A2–X̃ 1A1: Rotational structure of perturbed b̃ 3A2
Authors: Huang Cheng-Liang , Chen I-Chia , Merer Anthony J. , Ni Chi-Kung , Kung A. H.
Published: July 2002

Spectra of ã 3B1(ν1ν2ν3)=(110) and (200)–X̃ 1A1(000) of SO232 and SO234 are obtained in a supersonic jet at a resolution 0.015 cm−1. The rotational structures are analyzed for both isotopic species to obtain precise rotational and spin constants. Two new bands at 27 032.222(1) and 27 515.41(1) cm−1 arising from vibronic interaction separately with vibrational state (110) and (200) of ã 3B1 are assigned to transitions to b̃ 3A2(011) and (101). The rotational structure of b̃ 3A2 is observed and analyzed for the first time. The two vibrational states are analyzed to yield effective rotational constants A=1.4348(3), 1.89(3), B=0.3610(3), 0.412(3), and C=0.2689(3), 0.239(3) cm−1, respectively. Then the difference ν1−ν2 of vibrational frequency of b̃ 3A2 is obtained to be 483.19 cm−1. Because of interaction to b̃ 3A2(011) the observed spacing of K stack and spin constants α and β for ã 3B1(110) are smaller whereas the state ã 3B1(200) is relatively unaffected and only spin constants vary significantly.
Formation mechanism of interfacial Si–oxide layers during postannealing of Ta2O5/Si
Authors: Ono Haruhiko , Hosokawa Yumiko , Ikarashi Taeko , Shinoda Keisuke , Ikarashi Nobuyuki , Koyanagi Ken-ichi , Yamaguchi Hiromu
Published: July 2002

The Si–O–Si bonds formed at the Ta2O5/Si interface by annealing were investigated by using Fourier transform infrared absorption spectroscopy. The Ta2O5 thin films deposited on Si substrates were annealed in different ambient (H2O, O2, and N2) at temperatures between 500 and 800 °C. When annealing is done in H2O, the interfacial silicon–oxide grows very rapidly, because the oxidation species can easily diffuse through Ta2O5 films, and because the Si–O formation is controlled by the diffusion of H2O in the interfacial layer. When annealing is done in O2, the oxidation species can also easily diffuse through Ta2O5, but not through the interfacial layer. The interfacial layer is formed by a reaction between Ta2O5 and Si even if the annealing ambient does not contain oxidation species, as is the case when annealing is done in N2. We conclude that the Si–O formation during postannealing in O2 and N2 is controlled by the diffusion of the Si from the substrate through the interfacial layer with an activation energy of 0.7 to 0.8 eV, and that new Si–O bonds are formed at the interface between the Ta2O5 and interfacial layer. Oxidation species from the annealing ambient enhance the frequency factor of the reaction, but do not control Si–O formation.
Symbolic dynamics and computation in model gene networks
Authors: Edwards R. , Siegelmann H. T. , Aziza K. , Glass L.
Published: July 2002

We analyze a class of ordinary differential equations representing a simplified model of a genetic network. In this network, the model genes control the production rates of other genes by a logical function. The dynamics in these equations are represented by a directed graph on an n-dimensional hypercube (n-cube) in which each edge is directed in a unique orientation. The vertices of the n-cube correspond to orthants of state space, and the edges correspond to boundaries between adjacent orthants. The dynamics in these equations can be represented symbolically. Starting from a point on the boundary between neighboring orthants, the equation is integrated until the boundary is crossed for a second time. Each different cycle, corresponding to a different sequence of orthants that are traversed during the integration of the equation always starting on a boundary and ending the first time that same boundary is reached, generates a different letter of the alphabet. A word consists of a sequence of letters corresponding to a possible sequence of orthants that arise from integration of the equation starting and ending on the same boundary. The union of the words defines the language. Letters and words correspond to analytically computable Poincaré maps of the equation. This formalism allows us to define bifurcations of chaotic dynamics of the differential equation that correspond to changes in the associated language. Qualitative knowledge about the dynamics found by integrating the equation can be used to help solve the inverse problem of determining the underlying network generating the dynamics. This work places the study of dynamics in genetic networks in a context comprising both nonlinear dynamics and the theory of computation.
Reconstruction of roughness profile of fractal surface from scattering measurement at grazing incidence
Authors: Jin Ya-Qiu , Li Zhongxin
Published: July 2002

As a scalar wave is incident upon a rough surface at grazing angle, the Helmholtz wave equation can be approximated by the parabolic equation. Inverse scattering has been formulated in the form of a pair of coupled integral equations in two unknown functions, i.e., surface current and surface roughness. The aim of this article is to implement this approach for profile reconstruction of complex fractal rough surface. The fractal rough surfaces described by a band-limited Weieztrass–Mandelbrot function are numerically realized using the Monte Carlo method. As the scattering fields along a line parallel to the mean surface are measured or solved with a scalar wave incidence at low grazing angle, the rough surface profile is progressively inverted. Numerical examples show that the reconstructed profile and its parameters such as the fractal dimension, roughness variance, and correlation length are well matched with the simulated surfaces.
Electron-impact vibrational excitation of polyatomic gases: Exploratory calculations
Authors: Cascella M. , Curik R. , Gianturco F. A. , Sanna N.
Published: July 2002

We present model calculations for the inelastic cross sections of electron collisions with tetrahedral molecules XH4 (X=C, Si, Ge) when only the molecular “breathing” mode, ν1, is being excited. The collision energy range is well above the excited thresholds and up to 12 eV, where the adiabatic approximation for the inelastic T matrix is expected to hold. The results show the efficiency of the t2 shape resonance in enhancing the inelastic process and the appearance, in the two heavier targets, of a further a1 resonance in the inelastic channels of both molecules. The corresponding excitation rates are also computed together with estimates of the vibrational excitation functions.
Non-isothermal modification of purely elastic flow instabilities in torsional flows of polymeric fluids
Authors: Rothstein Jonathan P. , McKinley Gareth H.
Published: July 2002

Previous experimental measurements and linear stability analyses of curvilinear shearing flows of viscoelastic fluids have shown that the combination of streamwise curvature and elastic normal stresses can lead to flow destabilization. Torsional shear flows of highly elastic fluids with closed streamlines can also accumulate heat from viscous dissipation resulting in nonuniformity in the temperature profile within the flow and nonlinearity in the viscometric properties of the fluid. Recently, it has been shown by Al-Mubaiyedh et al. [Phys. Fluids 11, 3217 (1999)] that the inclusion of energetics in the linear stability analysis of viscoelastic Taylor–Couette flow can change the dominant mode of the purely elastic instability from a nonaxisymmetric and time-dependent secondary flow to an axisymmetric stationary Taylor-type toroidal vortex that more closely agrees with the stability characteristics observed experimentally. In this work, we present a detailed experimental study of the effect of viscous heating on the torsional steady shearing of elastic fluids between a rotating cone and plate and between two rotating coaxial parallel plates. Elastic effects in the flow are characterized by the Deborah number, De, while the magnitude of the viscous heating is characterized by the Nahme–Griffith number, Na. We show that the relative importance of these two competing effects can be quantified by a new dimensionless thermoelastic parameter, Θ=Na1/2/De, which is a material property of a given viscoelastic fluid independent of the rate of deformation. By utilizing this thermoelastic number, experimental observations of viscoelastic flow stability in three different fluids and two different geometries over a range of temperatures can be rationalized and the critical conditions unified into a single flow stability diagram. The thermoelastic number is a function of the molecular weight of the polymer, the flow geometry, and the temperature of the test fluid. The experiments presented here were performed using test fluids consisting of three different high molecular weight monodisperse polystyrene solutions in various flow geometries and over a large range of temperatures. By systematically varying the temperature of the test fluid or the configuration of the test geometry, the thermoelastic number can be adjusted appreciably. When the characteristic time scale for viscous heating is much longer than the relaxation time of the test fluid (Θ≪1) the critical conditions for the onset of the elastic instability are in good agreement with the predictions of isothermal linear stability analyses. As the thermoelastic number approaches a critical value, the strong temperature gradients induced by viscous heating reduce the elasticity of the test fluid and delay the onset of the instability. At even larger values of the thermoelastic parameter, viscous heating stabilizes the flow completely.
Spontaneous thermocapillary interaction of drops: Unsteady convective effects at high Peclet numbers
Authors: Lavrenteva O. M. , Nir A.
Published: July 2002

Initial nonequilibrium distribution of temperature or surfactant concentration between suspended drops and the continuous fluid in a suspension results in an unsteady-state heat/mass transfer between the phases. Nonuniformities of temperature or solute concentration, which arise as a natural result of local geometrical inhomogeneities in the suspension, produce temperature/concentration gradients along the interfaces that, in turn, generate thermocapillary fluid motion along the interfaces and migration of drops toward or apart from each other. Asymptotic analysis of the process is carried out for large Peclet numbers of the dispersed phase. The dynamics of drops is studied and the approach time is estimated for the limiting cases of small and large Peclet numbers of the continuous phase.
Measurement of shear-induced dispersion in a dilute emulsion
Authors: King Michael R. , Leighton David T.
Published: July 2002

The time-dependent drop distribution of a dilute, polydisperse emulsion is measured in a simple shear flow. The suspending fluid is much more viscous than the dispersed phase (1:1000). Drops are found to drift away from either bounding wall and accumulate near the center of the gap, due to the anisotropy of droplet–plane interactions. An expression for this drift velocity has been derived for single drops by Chan and Leal [J. Fluid Mech. 92, 131 (1979)] and was in agreement with isolated drop migration observed in our work. Eventually the inward drift is balanced by a shear-induced gradient diffusivity, and a steady-state concentration distribution is reached. When the drops are sufficiently far from either wall a self-similar, parabolic concentration profile is predicted at all times. Droplet diffusivities were determined for capillary numbers Ca=γ̇āμ/σ between 0.17 and 0.92, where γ̇ is the shear rate, ā is the mean drop radius, μ is the viscosity of the suspending fluid, and σ the interfacial tension. The values obtained are an order of magnitude lower than theoretical predictions of Loewenberg made in the limit of small deformation.
The effects of the presence of an alkaline atomic cation in a molecular hydrogen environment
Authors: Barbatti M. , Jalbert Ginette , Nascimento M. A. C.
Published: July 2002

Ab initio calculations were performed for X+(H2)n clusters [X=Li, Na (n=1–7) and K (n=1–3)]. For n=1–6, the equilibrium geometries correspond to spherically symmetrical distributions of H2 units around the X+. The binding energies and the geometric parameters indicate that the seventh H2 unit opens a new shell of ligands for the cluster with X=Li but not for X=Na.
Cavity ring down study of the densities and kinetics of Si and SiH in a remote Ar-H2-SiH4 plasma
Authors: Kessels W. M. M. , Hoefnagels J. P. M. , Boogaarts M. G. H. , Schram D. C. , van de Sanden M. C. M.
Published: July 2002

Cavity ring down absorption spectroscopy is applied for the detection of Si and SiH radicals in a remote Ar-H2-SiH4 plasma used for high rate deposition of device quality hydrogenated amorphous silicon (a-Si:H). The formation and loss mechanisms of SiH in the plasma are investigated and the relevant plasma chemistry is discussed using a simple one-dimensional model. From the rotational temperature of SiH typical gas temperatures of ∼1500 K are deduced for the plasma, whereas total ground state densities in the range of 1015–1016 m−3 for Si and 1016–1017 m−3 for SiH are observed. It is demonstrated that both Si and SiH have only a minor contribution to a-Si:H film growth of ∼0.2% and ∼2%, respectively. From the reaction mechanisms in combination with optical emission spectroscopy data, it is concluded that Si and SiH radicals initiate the formation of hydrogen deficient polysilane radicals. In this respect, Si and SiH can still have an important effect on the a-Si:H film quality under certain circumstances.
Core performance and mix in direct-drive spherical implosions with high uniformity
Authors: Meyerhofer D. D. , Delettrez J. A. , Epstein R. , Glebov V. Yu. , Goncharov V. N. , Keck R. L. , McCrory R. L. , McKenty P. W. , Marshall F. J. , Radha P. B. , Regan S. P. , Roberts S. , Seka W. , Skupsky S. , Smalyuk V. A. , Sorce C. , Stoeckl C. , Soures J. M. , Town R. P. J. , Yaakobi B. , Zuegel J. D. , Frenje J. , Li C. K. , Petrasso R. D. , Séguin F. H. , Fletcher K. , Padalino S. , Freeman C. , Izumi N. , Lerche R. , Phillips T. W. , Sangster T. C.
Published: July 2002

The performance of gas-filled, plastic-shell implosions has significantly improved with advances in on-target uniformity on the 60-beam OMEGA laser system [T. R. Boehly, D. L. Brown, R. S. Craxton et al., Opt. Commun. 133, 495 (1997)]. Polarization smoothing (PS) with birefringent wedges and 1-THz-bandwidth smoothing by spectral dispersion (SSD) have been installed on OMEGA. The beam-to-beam power imbalance is ⩽5% rms. Implosions of 20-μm-thick CH shells (15 atm fill) using full beam smoothing (1-THz SSD and PS) have primary neutron yields and fuel areal densities that are ∼70% larger than those driven with 0.35-THz SSD without PS. They also produce ∼35% of the predicted one-dimensional neutron yield. The results described here suggest that individual-beam nonuniformity is no longer the primary cause of nonideal target performance. A highly constrained model of the core conditions and fuel–shell mix has been developed. It suggests that there is a “clean” fuel region, surrounded by a mixed region, that accounts for half of the fuel areal density.
Time-domain simulation of two electrons in a quantum dot
Authors: Sullivan Dennis , Citrin D. S.
Published: July 2002

A time-domain simulation method is presented that utilizes the Hartree–Fock formulation to characterize two particles in a quantum dot. The basis of the simulation is the finite-difference time-domain method. The computation is made tractable by formulating the Coulomb and exchange terms as digital filtering problems, and utilizing two-dimensional fast Fourier transforms. Two-electron wave packet dynamics are calculated.
In-plane spin reorientation transitions in epitaxial Fe(110)/GaAs(110) films
Authors: Höllinger R. , Zölfl M. , Moosbühler R. , Bayreuther G.
Published: July 2002

Epitaxial Fe films in a thickness range from 4 to 64 monolayers (ML) were grown by molecular beam epitaxy on GaAs(110) at room temperature. The growth was characterized by reflection high energy electron diffraction. The magnetic in-plane anisotropy was investigated by alternating gradient magnetometry in a temperature range from 150 to 295 K. For a 64 ML thick Fe(110) film the [001] axis is the easy axis, the [−110] the intermediate axis, and the hard axis is between [−110] and [−111]. For Fe films with a thickness below 24.2±1.2 ML the [−110] becomes an easy axis at room temperature. A 24 ML Fe film shows a reorientation of the easy axis with decreasing temperature: Above the critical temperature of (251±3) K [−110] is the easy axis, for lower temperatures it becomes an intermediate axis.
Thermal resistance of bridged cracks in fiber-reinforced ceramic composites
Authors: Dryden John , Zok Frank
Published: July 2002

The thermal resistance of a bridged matrix crack in a fiber-reinforced ceramic composite is analyzed. The problem is cast in terms of a unit cell comprising an infinitely long composite cylinder with a single matrix crack perpendicular to the fiber axis. At the outset, it is demonstrated that the thermal resistance of such a crack can be represented by a simple circuit consisting of two parallel resistors; one resistor represents the thermal resistance of the gas phase Rg within the matrix crack, and the other resistor represents the constriction resistance Rc of the bridging fiber. The main focus of the article is on determination of Rc and bounds on this resistance are obtained by the use of variational calculus. The analogy between problems involving steady-state heat flow and elasticity in multiphase materials is emphasized. The results for the constriction resistance are compared with the predictions of an approximate analytical model presented by [T. J. Lu and J. W. Hutchinson, Philos. Trans. R. Soc. London, Ser. A 351, 595 (1995)]. In their model the radial temperature variation within the matrix is neglected. The domain in which such variations can be justifiably neglected is found.
Inverse magnetoresistance in chromium-dioxide-based magnetic tunnel junctions
Authors: Gupta A. , Li X. W. , Xiao Gang
Published: July 2002

Magnetic tunnel junctions have been fabricated using half-metallic chromium-dioxide (CrO2) epitaxial film with a Co counterelectrode. The native insulating layer formed on the surface of CrO2 after air exposure is used as the tunneling barrier. These junctions exhibit nonlinear current–voltage characteristics, and the changes in junction resistance with applied field correspond to the coercivities of the two magnetic layers. The maximum observed magnetoresistance (MR) is about 8% at 4.2 K and has a negative sign, i.e., the resistance of the junction with parallel alignment of the electrodes is higher than with antiparallel alignment. This is opposite of what is normally observed with transition-metal electrodes and an Al2O3 barrier. Possible reasons for the inverse MR are discussed based on previous results on manganite/Co junctions.
Soft x-ray-excited luminescence and optical x-ray absorption fine structures of tris (8-hydroxyquinoline) aluminum
Authors: Naftel S. J. , Zhang P. , Kim P.-S. , Sham T. K. , Coulthard I. , Antel W. J. , Freeland J. W. , Frigo S. P. , Fung M.-K. , Lee S. T. , Hu Y. F. , Yates B. W.
Published: July 2002

Photoluminescence from tris (8-hydroxyquinoline) aluminum (Alq3) films has been observed using tunable soft x rays as an excitation source. The photons were tuned to energies above and below the K absorption edges of C, N, O, and Al. The luminescence was in turn used to monitor the absorption. It was found that the luminescence induced by soft x ray exhibits additional emission bands at shorter wavelengths compared to ultraviolet excitation. While all K edges exhibit optical x-ray absorption fine structures (XAFS) similar to those of total electron and fluorescence yield, the optical XAFS at the C K-edge resonance are enhanced for the C1s to π* transitions, indicating site specificity. These observations are attributed to the energetics of the process and the local electronic structure.
Optical temperature probe
Authors: Hooper Brett A. , Domankevitz Yacov , Anderson R. Rox , Lin Charles
Published: July 2002

In order to quantify the energy deposition at a dielectric-tissue interface, a simple optical temperature probe to measure the transient temperature rise of the laser-heated layer at this interface is presented. The optical temperature probe (1) is noninvasive, so as not to alter the optical, thermal, and mechanical properties of the sample, (2) has a fast time response (nanoseconds), in order to measure the temperature increase during a short laser pulse, and (3) has high spatial resolution (micrometers), in order to probe only the heated layer. The probe is also capable of detecting ablation at the optic-tissue interface.
Giant magnetroresistance properties of specular spin valve films in a current perpendicular to plane structure
Authors: Nagasaka K. , Seyama Y. , Varga L. , Shimizu Y. , Tanaka A.
Published: July 2002

Conventional and specular spin valve films in a current perpendicular to plane (CPP) structure have been investigated. The specular spin valve film with bottom type structure had two oxidized layers: one in the pinned layer, which was oxidized during an in situ deposition process, and the other in the free layer, which was a naturally oxidized Cu/Ta cap. Both films had increasing resistance, R, and resistance change, ΔR, with decreasing element size. The conventional spin valve film showed a resistance times area product, RA, of 144 mΩ μm2 and a resistance change area product, ΔRA, of 0.7 mΩ μm2 while the specular spin valve film showed RA of 1120 mΩ μm2 and ΔRA of 23 mΩ μm2. The ΔRA of the specular spin valve film was about 33 times larger than that of the conventional spin valve film. The calculated magnetoresistance (MR) ratios, MRSV, of each spin valve film were 1.9% and 2.3%, respectively. We think oxidized layers in the spin valve film caused the specular electron scattering and this lengthened the path of the conduction electrons, enhancing the interfacial and bulk spin dependent scattering. We estimated the output voltage change of the 0.01 μm2 element, the size required for 150 Gb/in.2 recording density, of the specular spin valve film in CPP mode to be 5.3 mV. It was almost six times larger than that of the conventional spin valve film at constant power consumption. Specular spin valve film are advantageous for the CPP structure element for future giant MR sensors.
Analysis of the scattered light distribution of a tightly focused laser beam by a particle near a substrate
Authors: Inami Wataru , Kawata Yoshimasa
Published: July 2002

We present a calculation method to analyze the scattered electromagnetic field by a particle near a substrate. The method is based on the iterative calculations of the extended Mie scattering theory [J. P. Barton, D. R. Alexander, and S. A. Schaub, J. Appl. Phys. 64, 1632 (1988)] and of plane-wave decompositions in order to include the multi-scattering between the particle and the substrate. The method is applicable to an arbitrary incident beam and an arbitrary distance between the particle and the substrate. It is possible to analyze the interaction between a particle and a multilayered substrate. We present the electromagnetic field distribution when the particle near the substrate is illuminated with focused laser beam.
Possible giant magnetoelectric effect of ferromagnetic rare-earth–iron-alloys-filled ferroelectric polymers
Authors: Nan Ce Wen , Li Ming , Feng Xiqiao , Yu Shouwen
Published: July 2002

Coupled magnetic–mechanical–electric effects in a composite with ferromagnetic rare-earth–iron alloys (e.g., Tb1−xDyxFe2) filled in ferroelectric polymers [e.g, poly(vinylidene-fluoride–trifluoroethylene) copolymer] are studied by using the Green’s function technique. Numerical results suggest a possible giant linear magnetoelectric effect in the ferroic polymer–matrix composite, which is markedly larger than that in the best-known magnetoelectric materials. In addition, the mechanically flexible composite exhibits large magnetostriction. The present results may stimulate further interest in the area of magnetoelectric materials for technological applications.
Temperature dependence of electrical properties of gallium-nitride bulk single crystals doped with Mg and their evolution with annealing
Authors: Litwin-Staszewska E. , Suski T. , Piotrzkowski R. , Grzegory I. , Bockowski M. , Robert J. L. , Kończewicz L. , Wasik D. , Kamińska E. , Cote D. , Clerjaud B.
Published: July 2002

Comprehensive studies of the electrical properties of Mg-doped bulk GaN crystals, grown by high-pressure synthesis, were performed as a function of temperature up to 750 °C. Annealing of the samples in nitrogen ambient modifies qualitatively their resistivity values ρ and the ρ(T) variation. It was found that our material is characterized by a high concentration of oxygen-related donors and that the charge transport in the studied samples is determined by two types of states, one of shallow character (Mg-related state, EA≈0.15 eV), and the second one much more deep, E2≈0.95 eV (above the valence band). Depending on the effective concentration of either states, different resistivities ρ can be observed: lower resistivity (ρ<104 Ω cm at ambient temperature) in samples with dominant EA states and very high resistivity (ρ>106 Ω cm at ambient temperature) in samples with dominant E2 states. For the first type of samples, annealing at Tann<500 °C leads to a decrease of their resistivity and is associated with an increase of the effective concentration of the shallow Mg acceptors. Annealing of both types of samples at temperatures between 600 and 750 °C leads to an increase of the deep state concentration. The presence of hydrogen ambient during annealing of the low-resistivity samples strongly influences their properties. The increase of the sample resistivity and an appearance of a local vibrational mode of hydrogen at 3125 cm−1 were observed. These effects can be removed by annealing in hydrogen-free ambient.
Rotationally resolved photoelectron spectroscopy of hot N2 formed in the photofragmentation of N2O
Authors: Rijs Anouk M. , Backus Ellen H. G. , de Lange Cornelis A. , Janssen Maurice H. M. , Wang Kwanghsi , McKoy Vincent
Published: July 2002

The photoionization dynamics of rotationally hot molecular nitrogen are studied employing resonance enhanced multiphoton ionization in combination with photoelectron spectroscopy. Photodissociation of N2O at ∼203 nm results in highly rotationally excited N2 fragments in X 1∑g+(N″,v″=0,1) states and O atoms in the excited D21 state. Photoelectron detection of the rotationally hot N2 states is performed by a two-photon excitation to the lowest a″ 1∑g+ Rydberg state followed by one-photon ionization. The large number of observed rotational levels, from N′=49 up to N′=94, results in improved rotational parameters for a″ 1∑g+ (v′=0). In addition, experimental and theoretical rotationally resolved photoelectron spectra of the a″ 1∑g+(v′=0,1;N′) state are presented. In these spectra only ΔN=N+−N′=even transitions are observed, with a dominant ΔN=0 peak and rather weak ΔN=±2 peaks. The one-photon ionization is dominated by ejection of electrons in p and f partial waves. The agreement between experimental and calculated spectra is excellent.
A direct approach to one photon interference contributions in the coherent control of photodissociation
Authors: Batista Victor S. , Brumer Paul
Published: July 2002

Formally exact quantum mechanical expressions for cumulative transition matrix elements μjk(ξ,E)=∑n〈j|με|E,ξ,n−〉〈E,ξ,n−|με|k〉, central to one photon coherent control scenarios of photodissociation, are derived. The resultant approach bypasses the need for solving the complete state-to-state quantum mechanical reactive scattering problem to obtain control results. These exact expressions are implemented both quantum mechanically and via a semiclassical initial value representation method to investigate coherent control in the generic photodissociation of a triatomic into more than one product. The semiclassical approach is shown to provide an accurate description of bimolecular control in this system.
Self-consistent solution of Dyson’s equation up to second order for atomic systems
Authors: Van Neck D. , Peirs K. , Waroquier M.
Published: July 2002

In this paper, the single-particle Green’s function approach is applied to the atomic many-body problem. We present the self-consistent solution of the Dyson equation up to second order in the self-energy for nonrelativistic spin-compensated atoms. This Dyson second-order scheme requires the solution of the Hartree–Fock integro-differential equations as a preliminary step, which is performed in coordinate space (i.e., without an expansion in a basis set). To cope with the huge amount of poles generated in the iterative approach to tackle Dyson’s equation in second order, the BAGEL (BAsis GEnerated by Lanczos) algorithm is employed. The self-consistent scheme is tested on the atomic systems He, Be, Ne, Mg, and Ar with spin-saturated ground state S01. Predictions of the total binding energy, ionization energy, and single-particle levels are compared with those of other computational schemes [density functional theory, Hartree–Fock (HF), post-HF, and configuration interaction] and with experiment. The correlations included in the Dyson second-order algorithm produce a shift of the Hartree–Fock single-particle energies that allow for a close agreement with experiment.
Thermodynamics of the Mg–B system: Implications for the deposition of MgB2 thin films
Authors: Liu Zi-Kui , Schlom D. G. , Li Qi , Xi X. X.
Published: July 2002

We have studied the thermodynamics of the Mg–B system with the calculation of phase diagrams modeling technique using a computerized optimization procedure. Temperature–composition, pressure–composition, and pressure–temperature phase diagrams under different conditions are obtained. The results provide helpful insights into appropriate processing conditions for thin films of the superconducting phase, MgB2, including the identification of the pressure–temperature region for adsorption-controlled growth. Due to the high volatility of Mg, MgB2 is thermodynamically stable only under fairly high-Mg overpressures for likely growth temperatures. This places severe temperature constraints on deposition techniques employing high-vacuum conditions.
Fundamental strings in open string theory at the tachyonic vacuum
Authors: Sen Ashoke
Published: July 2002

We show that the world-volume theory on a D-p-brane at the tachyonic vacuum has solitonic string solutions whose dynamics is governed by the Nambu–Goto action of a string moving in (25+1) dimensional space–time. This provides strong evidence for the conjecture that at this vacuum the full (25+1) dimensional Poincaré invariance is restored. We also use this result to argue that the open string field theory at the tachyonic vacuum must contain closed string excitations.
Si–N linkage in ultrabright, ultrasmall Si nanoparticles
Authors: Rogozhina E. , Belomoin G. , Smith A. , Abuhassan L. , Barry N. , Akcakir O. , Braun P. V. , Nayfeh M. H.
Published: July 2002

Ultrabright ultrasmall (∼1 nm) blue luminescent Si29 nanoparticles are chlorinated by reaction with Cl2 gas. A Si–N linkage is formed by the reaction of the chlorinated particles with the functional amine group in butylamine. Fourier transform infrared spectroscopy and x-ray photospectroscopy measurements confirm the N linkage and the presence of the butyl group, while emission, excitation, and autocorrelation femtosecond optical spectroscopy show that, after the linkage formation, the particles with the ultrabright blue luminescent remain, but with a redshift of 40 nm.
Theoretical study of the electronic states of small cationic niobium clusters, Nbn+ (n=3–5)
Authors: Majumdar D. , Balasubramanian K.
Published: July 2002

Geometries and energy separations of the various low-lying electronic states of Nbn+ (n=3–5) clusters with different structural arrangements have been investigated. The complete active space multiconfiguration self-consistent field (CASMCSCF) method followed by the multireference singles plus doubles configuration interaction (MRSDCI) that included up to 13 million configuration spin functions have been used to compute several electronic states of these clusters. A A25 isosceles triangle geometry in C2v symmetry and a A′2 pyramid structure in Cs symmetry are computed as the ground states of Nb3+ and Nb4+ clusters, respectively. In the case of Nb5+, a A′1 state of distorted edge-capped tetrahedral structure (in Cs symmetry) was found to be the ground state. We also compared our MRSDCI results with density functional calculations. The dissociation and atomization energies have been calculated at the MRSDCI level and the results have been found to be in agreement with experimental findings.
The translational and rotational drag on Langmuir monolayer domains
Authors: Steffen P. , Heinig P. , Wurlitzer S. , Khattari Z. , Fischer Th. M.
Published: July 2002

The ratio of the rotational and translational drag coefficient of a circular liquid condensed Langmuir monolayer domain moved in different phases is measured. A single domain is fixed at its boundary and forced to undergo combined translational and rotational motion. It is observed, that the drag force is dominated by the viscous dissipation of the three dimensional subphase and affected by the elasticity of the surrounding monolayer phase.
Abnormal behavior in the three-photon resonance enhanced multiphoton ionization spectroscopy of CO E 1Π←X 1Σ+(0,0)
Authors: Chen Xiangling , Sha Guohe , Jiang Bo , Xie Jinchun , Zhang Cunhao
Published: July 2002

The abnormal behavior of CO E 1Π(v=0)←X 1Σ+(v=0) three-photon resonance enhanced multiphoton ionization (REMPI) spectroscopy is studied, including the anomalous band structures and adverse pressure effects on the spectral intensity. One- and two-color experiments with laser beams in co- and counterpropagations have been performed to elucidate the mechanisms. It is demonstrated that the abnormalities originate from two mechanisms, i.e., the reabsorption of the resultant third harmonic generation (THG) in the one-color case or four wave mixing sum frequency generation (FWSFG) in the two-color case, and the interference cancellation between the one-photon excitation channel induced by THG/FWSFG reabsorption and the three-photon direct excitation channel. The former mechanism dominates in the wing region of a resonant transition profile, while the latter is restricted within the kernel portion of the transition.
Preparation of bulk melt chain configurations of polycyclic polymers
Authors: Neyertz Sylvie , Brown David
Published: July 2002

The configurations of oligomers of polyimide and polyetherketone polycyclic polymers in the melt are predicted by a new hybrid pivot Monte Carlo (PMC)/molecular dynamics (MD) single-chain sampling technique restricted to a limited number of near-neighbor interactions. These are then compared to configurations obtained for the same models by running MD simulations on the corresponding multichain systems in the bulk melt. A new phantom-atom technique is introduced which avoids interlocking rings during construction of the bulk melt samples. Similar to earlier work carried out on polyethylene, polyvinylchloride and uncharged polyethylene oxide, both theoretical and bulk melt sampled conformational and configurational properties are found to be in very good agreement. This confirms that the new hybrid PMC/MD sampling is a promising and cost-effective technique for preparing polymer samples prior to subsequent MD simulations of the bulk amorphous phase.
A mixed order density functional theory for adhesive hard sphere fluid confined between two hard walls
Authors: Zhou Shiqi , Sun Xingming , Chen Hong , Li Haoran
Published: July 2002

The second order direct correlation function of uniform adhesive hard sphere fluid was divided into a hard sphere like part and tail part. Then the density functional formalism based on the universality of the free energy density functional was employed to treat the nonuniform first order direct correlation function for the hard sphere like part, the second order perturbative expansion approximation was employed to treat the tail part for the nonuniform case. The two parts were added together to construct the nonuniform first order direct correlation function of the adhesive hard sphere fluid. Then the ensued result was substituted into the density profile equation in the density functional theory, the density distribution profile predictions from the present method were in good agreement with the corresponding computer simulation data.
Structural properties of the perovskite manganites
Authors: Li J. Q.
Published: July 2002

The structural distortions arising from the condensations of two essential kinds of phonon modes: the triply degenerated rotational modes (φx,φy,φz) of MnO6 and the doubly degenerated Jahn–Teller (JT) active modes (Q1,Q2), have been systematically investigated in the perovskite manganites. The microstructure features of several typical structural phases have been characterized by transmission-electron microscopy investigations. Due to the strong coupling among crystal lattice, carriers, and spins, the condensations of the rotational modes evidently impact the fundamental behaviors of the low-temperature JT polarization and magnetic orders. In the A-type antiferromagnetic Pr(Sr)MnO3 and Nd(Sr)MnO3 materials, the properties of charge-ordering transitions and structural distortions have been extensively investigated. Several typical kinds of defect structures, including the antiphase boundaries and the 90°-twin domains in the charge-order states, have been analyzed.