bluuebunny/crossref_metadata_2025 huggingface dataset:
High spatial resolution subsurface microscopy
Authors: Ippolito S. B. , Goldberg B. B. , Ünlü M. S.
Published: July 2002

We present a high-spatial-resolution subsurface microscopy technique that significantly increases the numerical aperture of a microscope without introducing an additional spherical aberration. Consequently, the diffraction-limited spatial resolution is improved beyond the limit of standard subsurface microscopy. By realizing a numerical aperture of 3.4, we experimentally demonstrate a lateral spatial resolution of better than 0.23 μm in subsurface inspection of Si integrated circuits at near infrared wavelengths.
Thermal diffusivity in amorphous silicon carbon nitride thin films by the traveling wave technique
Authors: Chattopadhyay S. , Chen L. C. , Wu C. T. , Chen K. H. , Wu J. S. , Chen Y. F. , Lehmann G. , Hess P.
Published: July 2002

Thermal diffusivity (α) of amorphous silicon carbon nitride (a-SiCxNy) thin films on crystalline silicon, prepared by ion beam sputtering, has been studied using the traveling wave technique. The variation of thermal diffusivity with carbon content in amorphous silicon carbon nitride samples are reported. Thermal diffusivity decreased from ∼0.35 cm2/s for samples with carbon contents of less than 30 at. %, to about 0.15 cm2/s for a-SiCxNy films with a carbon content of ∼70 at. %. A similar variation was found for the film density as measured by surface acoustic wave spectroscopy as a function of the carbon content. The results on a-SiCxNy, elucidate the relation between thermal diffusivity and the bonding configuration, density and microstructure of the network.
Thermodynamic and structural properties of Yukawa hard chains
Authors: Wang Xiao-Yan , Chiew Y. C.
Published: July 2002

The thermodynamic and structural properties of Yukawa hard chains are considered in this work. We specifically examined the influence of the Yukawa potential range parameter λ on the thermodynamic properties, inter- and intrachain correlation functions of the chain fluid. The compressibility factor or pressure of Yukawa chain fluids was calculated via a first-order perturbation theory using hard-sphere chain fluid as a reference. Monte Carlo simulations were performed to calculate the thermodynamic properties and inter- and intrachain correlation functions of the fluid for λ=1.8 and 3.0. Simulation results for the compressibility were compared with calculations from the first-order perturbation theory and the Statistical Associating Fluid Theory-Variable Range (SAFT-VR) model. We found that both theories represent the simulated data very well. In particular, the SAFT-VR equation provides an excellent estimate of the properties of Yukawa hard chains over wide ranges of temperature and density. Simulated data for interchain and intrachain correlation functions reveal that the local structure of the Yukawa hard chains is strongly effected by the range parameter of the potential.
Experimental study and simulations of electronegative discharges at low pressure
Authors: Riccardi C. , Barni R. , Fontanesi M.
Published: July 2002

We have performed an experimental and theoretical analysis of the rf discharges at low pressure in a small reactor for industrial applications. In particular, we focused on the plasma produced from electronegative gases (air and SF6) in the low pressure range between 0.05 and 1 mbar. The device pumping system sustains a flow velocity of about 50 cm/s, with residence time in the discharge of about 0.2 s, at P=0.1 mbar. The discharge parameters have been measured by means of movable electrostatic probes and lies in the range ne≅107–109 cm−3, ni≅1010–1012 cm−3, and Te≅5–10 eV. Particular care in the analysis of the data proved necessary due to the presence of a substantial amount of negative ions. On the other hand, we have performed simulations of the discharge composition through the implementation of a suitable numerical model of the chemical kinetics in the device. On the basis of the experiments, a comparison of charged species production with those predicted by the theory was performed and a more realistic description of the involved phenomena was obtained. In particular, several assumptions concerning the ion diffusion coefficient were tested, allowing one to pin down the relative importance of transport and bulk processes in the discharge state.
An algorithm for computing linear four-point probe thickness correction factors
Authors: Weller Robert A.
Published: July 2002

An accurate algorithm is described for the computation of the theoretical values of the linear four-point probe thickness correction factors for point injection of current and insulating and conducting substrates. Three expressions are presented for the insulating substrate case that yield maximum fractional errors of 0.03%, 10−6, and 10−16, respectively. Additional expressions are presented for the case of a film on a conducting substrate and a critical comparison of the two cases is made. The theoretical basis of the algorithms, a Euler–Maclaurin expansion, is described.
Experimental and simulation studies on the transport of gaseous diatomic molecules in polycarbonate membranes
Authors: López-González Mar , Saiz Enrique , Guzmán Julio , Riande Evaristo
Published: July 2002

Measurements on the pure transport of oxygen, nitrogen, and carbon monoxide in membranes prepared from poly[bisphenol A carbonate-co-4,4′-(3,3,5-trimethylcyclohexylidene) diphenol carbonate] are reported. The isotherms representing the variation of the permeability coefficient with the upstream pressure can be described by assuming that the diffusants are retained in the matrix by Henry and Langmuir mode sorptions. The values of the permeability coefficient of oxygen, nitrogen and carbon monoxide at 35 °C and p0=1 atm are 8.75, 1.87, and 2.91 barrers, respectively. The values of the diffusion coefficient in 10−8 cm2/s, measured in the same conditions, are 8.26, 2.81, and 3.08, respectively. These membranes exhibit higher permeability than polycarbonate membranes with the two methyl groups of bisphenol A replaced by other different molecular groups, without a substantial diminution of permselectivity. The permeability and the diffusion coefficients obey Arrhenius behavior. The application of the transition state approach to the simulation of the transport of diatomic molecules in the membranes is described. Good agreement between simulated and experimental results is found by assuming that the polymer chains of the membrane fluctuate around equilibrium positions with a root mean-square displacement Δ≈0.3 Å.
Efficient electronic cooling in heavily doped silicon by quasiparticle tunneling
Authors: Savin A. M. , Prunnila M. , Kivinen P. P. , Pekola J. P. , Ahopelto J. , Manninen A. J.
Published: July 2002

Cooling of electrons in a heavily doped silicon by quasiparticle tunneling using a superconductor–semiconductor–superconductor double-Schottky-junction structure is demonstrated at low temperatures. In this work, we use Al as the superconductor and thin silicon-on-insulator (SOI) film as the semiconductor. The electron–phonon coupling is measured for the SOI film and the low value of the coupling is shown to be the origin of the observed significant cooling effect.
Degradation mechanism of ferroelectric properties in Pt/Bi4-xLaxTi3O12/Pt capacitors during forming gas annealing
Authors: Chon Uong , Kim Ki-Bum , Jang Hyun M.
Published: July 2002

Degradation mechanism of ferroelectric properties in the Pt/Bi4-xLaxTi3O12/Pt (Pt/BLT/Pt) capacitors during forming gas annealing (FGA) was systematically investigated by examining ferroelectric responses and spatial distributions of relevant species using secondary ion mass spectrometry. It was shown that the degradation of ferroelectric properties during FGA was not originated from the oxygen loss induced by a reducing atmosphere but was mainly caused by protons catalytically dissociated from molecular hydrogen (H2) by the top Pt electrode. The following sequential mechanism has been identified from the present study: (i) the adsorption and dissociation of H2 to produce protons and electrons by the top Pt electrode, (ii) the columnar penetration of protons into the BLT film, accelerated by the region of negatively charged Bi-site vacancies near the bottom electrode, and (iii) the decomposition of perovskite phase after FGA at 400 °C.
Effect of silicate layer anisotropy on cylindrical and spherical microdomain ordering in block copolymer nanocomposites
Authors: Krishnamoorti Ramanan , Silva Adriana S. , Mitchell Cynthia A.
Published: July 2002

The influence of the addition of small quantities of anisotropic layered silicates on the ordering of block copolymers is studied by a combination of linear viscoelasticity and small angle neutron scattering. Specifically, we examine the influence of varying the lateral dimensions of thermodynamically roughly equivalent layered silicates on the development of cylindrical and spherical microdomain order in a blend of a matched diblock and triblock copolymer. The kinetics for the development of spheres arranged on a bcc lattice from an initial disordered state are dramatically accelerated by the two larger layered silicates with equivalent diameters of ∼1 and 10 μm, while the incorporation of an organically modified laponite, with an equivalent diameter of ∼30 nm, has no influence on these kinetics. On the other hand, the addition of layered silicates, irrespective of layer dimensions, has no influence on the development of cylindrical ordered microdomains and the epitaxial transformation of shear-aligned cylindrical microdomains to spherical microdomains.
Photodissociation of methyl nitrite on Ag(111): Simulation
Authors: Kim Seong Kyu , White J. M. , Agrawal Paras M. , Thompson Donald L.
Published: July 2002

The photodissociation dynamics of methyl nitrite, CH3ONO, on Ag(111) have been simulated using a description that models 61 cis–methyl nitrite molecules adsorbed on a three-layer block of Ag(111). Based on classical intra- and intermolecular potentials and periodic boundary conditions, molecular dynamics (MD) simulation led to two domain structures at 100 K: those with CONO planes oriented nearly parallel and nearly perpendicular to the Ag(111) surface. To simulate photodissociation dynamics of NO, many NO trajectories were determined, each carried out as follows. At some instant of the MD simulation, a CH3ONO molecule was randomly selected from within the group of 61 and its internal CH3O–NO bond was stretched to a defined dissociation transition state. The nascent NO was given momentum along the direction of the bond broken and NO translational and internal energies were chosen to match those determined experimentally in collision-free gas phase photodissociation. The motion of the whole adsorbate–substrate system was then calculated while following the trajectory of NO. Analyzing the ensemble of NO trajectories, we conclude that, while the initial orientation of the dissociating CH3ONO influences the number of subsequent collisions, the exit direction, and the final translational and internal energy of NO, it does not fully account for the properties of ejected NO. Furthermore, for those molecules lying nearly parallel to the surface, a transition state prepared by simply stretching the O–N bond is often located away from the lowest potential energy exit path due to interactions with nearest neighbor species. As a result, coordinates, e.g., internal twisting, other than the internal CH3O–NO stretching mode are intimately involved in the dissociation channel.
The competition between chemical bonding and magnetism in the adsorption of atomic Ni on MgO(100)
Authors: Markovits A. , Skalli M. K. , Minot C. , Pacchioni G. , López N. , Illas F.
Published: July 2002

The present study is devoted to atomic adsorption of Ni on MgO (100) surface and focuses on the magnetic moment of adsorbed atomic Ni in the initial phase of metal deposition. Both cluster and periodic slab models lead to the same description, i.e., they show an almost degeneracy of the magnetic and nonmagnetic states. However, the total energy related properties are strongly dependent on the computational method chosen. At variance with the large difference for the calculated heats of adsorption, the optimized values for the Ni–O distances are remarkably close: 1.8–1.9 Å for the singlet state and larger than 2 Å for the triplet. The paper also presents a detailed analysis of the problems connected to the definition of a proper reference atomic energy in density functional theory approaches.
Emission lifetime of polarizable charge stored in nano-crystalline Si based single-electron memory
Authors: Hinds Bruce J. , Yamanaka Takayuki , Oda Shunri
Published: July 2002

The lifetime of the emission of a single electron stored in a nanocrystalline Si (nc-Si) dot has been studied in order to understand the physical processes for memory applications. A small active area field effect transistor channel (50×25 nm) is defined by electron-beam lithography on a thin (20 nm) silicon-on-insulator channel and allows for the electrical isolation of a single nc-Si dot. Remote plasma enhanced chemical vapor deposition is used to form 8±1 nm diameter nc-Si dots in the gas phase from a pulsed SiH4 source. Electrons stored in a dot results in an observed discrete threshold shift of 90 mV. Analysis of lifetime as a function of applied potential and temperature show the dot to be an acceptor site with nearly Poisson time distributions. An observed 1/T2 dependence of lifetime is consistent with a direct tunneling process, and interface states are not the dominant mechanism for electron storage in this device structure. Median emission lifetimes as a function of applied gate bias are readily modeled by the polarizability of an electron in a delocalized bound state over the entire semiconducting dot.
Pure and alkali–ion-doped droplets of He4
Authors: Galli D. E. , Buzzacchi M. , Reatto L.
Published: July 2002

We present the results of variational Monte Carlo calculations of clusters of He4 systems: We study the pure He4 case and the case of a cluster doped with a single alkali–ion impurity. The results are compared with similar calculations in bulk He4. Our trial wave function is a glue-shadow wave function that can describe successfully self-binding and localization in space. The local Bose–Einstein condensate in the pure clusters is calculated. We give the results on the microscopic structure of the doped cluster in terms of the radial density profile around the ion and of angular correlations. We have studied the case of Na+ and K+. In both cases the local order of the atoms in the first shell around the ion is quite distinct compared to that present in the bulk liquid.
The Morse potential with angular momentum
Authors: Killingbeck John P. , Grosjean Alain , Jolicard Georges
Published: July 2002

Following the attempts by previous authors to construct an effective Morse potential to treat the problem of the Morse potential plus an r−2 term, it is shown that the hypervirial perturbation method, which gives exact results for the Morse potential, also gives results for the perturbed problem which are much more accurate than those of previous authors.
Influence of palmitic acid and hexadecanol on the phase transition temperature and molecular packing of dipalmitoylphosphatidyl-choline monolayers at the air–water interface
Authors: Lee Ka Yee C. , Gopal Ajaykumar , von Nahmen Anja , Zasadzinski Joseph A. , Majewski Jaroslaw , Smith Gregory S. , Howes Paul B. , Kjaer Kristian
Published: July 2002

Palmitic acid (PA) and 1-hexadecanol (HD) strongly affect the phase transition temperature and molecular packing of dipalmitoylphosphatidylcholine (DPPC) monolayers at the air–water interface. The phase behavior and morphology of mixed DPPC/PA as well as DPPC/HD monolayers were determined by pressure-area-isotherms and fluorescence microscopy. The molecular organization was probed by synchrotron grazing incidence x-ray diffraction using a liquid surface diffractometer. Addition of PA or HD to DPPC monolayers increases the temperature of the liquid-expanded to condensed phase transition. X-ray diffraction shows that DPPC forms mixed crystals both with PA and HD over a wide range of mixing ratios. At a surface pressure (π) of 40 mN/m, increasing the amount of the single chain surfactant leads to a reduction in tilt angle of the aliphatic chains from nearly 30° for pure DPPC to almost 0° in a 1:1 molar ratio of DPPC and PA or HD. At this composition we also find closest packing of the aliphatic chains. Further increase of the amount of PA or HD does not change the lattice or the tilt.
Oxygen tracer diffusion in IrO2 barrier films
Authors: Pinnow C. U. , Kasko I. , Nagel N. , Mikolajick T. , Dehm C. , Jahnel F. , Seibt M. , Geyer U. , Samwer K.
Published: July 2002

The O18 tracer diffusion method was used to investigate oxygen diffusion in reactively dc-sputtered IrO2 films. The profile measurements were done by secondary ion mass spectrometry. For the investigation of the oxygen diffusivity in the samples a temperature range from 600 to 765 °C was chosen. The oxygen tracer diffusion in IrO2 films was found to be described by an Arrhenius law with D0=(2.8±2.5)10−6 m2 s−1 and an activation energy of Ea=(2.73±0.07) eV. It was also shown that the extrinsic oxygen diffusion is strongly influenced by the film preparation conditions, which is especially important for the application of IrO2 films as an oxygen barrier in future memory device applications.
Electrical and optical properties of modulation-doped p-AlGaN/GaN superlattices
Authors: Polyakov A. Y. , Smirnov N. B. , Govorkov A. V. , Osinsky A. V. , Norris P. E. , Pearton S. J. , Van Hove J. , Wowchak A. , Chow P.
Published: July 2002

Electrical and optical properties of modulation-doped p-AlGaN/GaN superlattices are compared to those of similarly doped p-GaN films. It is shown that modulation doping increases the sheet hole concentration by several times. In p-AlGaN/GaN superlattices grown on GaN underlayers, this increase is accompanied by a significant increase in hole mobility which results in a remarkable decrease in sheet resistivity of the structure compared to p-GaN films and this decrease in sheet resistivity should hold up to temperatures exceeding 350 °C. For superlattices prepared on AlGaN underlayers, the mobility decreases compared to p-GaN. In such superlattices, one also observes a strong redshift and a strong broadening of the band edge luminescence peak coming most probably from increased mosaicity and strain which would also explain the observed deterioration of mobility. The magnitude of the redshift in the position of the band edge luminescence band slightly increased upon application of reverse bias which is interpreted as a manifestation of quantum-confined Stark effect.
Space-charge lens for focusing negative ion beam
Authors: Goretsky V. P. , Soloshenko I. A. , Shchedrin A. I.
Published: July 2002

The idea of a space-charge lens for focusing intensive negative hydrogen ion beams is presented. Estimations of focal length of the lens are performed independent of parameters of the beam and gaseous medium. The effectiveness of the focusing H− ion beam with current up to ∼30 mA and energy of ∼10 keV by means of such a lens is demonstrated experimentally. Measured values of focal length are in good agreement with calculated ones.
The wormhole move: A new algorithm for polymer simulations
Authors: Houdayer J.
Published: July 2002

A new Monte Carlo move for polymer simulations is presented. The “wormhole” move is build out of reptation steps and allows a polymer to reptate through a hole in space; it is able to completely displace a polymer in time N2 (with N the polymer length) even at high density. This move can be used in a similar way as configurational bias; in particular, it allows grand canonical moves, it is applicable to copolymers, and can be extended to branched polymers. The main advantage is speed, since it is exponentially faster in N than configurational bias, but is also easier to program.
Fast pulsed electroluminescence from polymer light-emitting diodes
Authors: Wang J. , Sun R. G. , Yu G. , Heeger A. J.
Published: July 2002

Transient electroluminescence (EL) from polymer light emitting diodes is investigated by measurements of the response to short voltage pulses. The carrier mobility is derived from the delay time between the electrical pulse and the onset of EL, μ≈3×10−4 cm2/V s. Bilayer devices with a polyethylene-dioxythiophene (PEDOT), hole injection layer are also studied. The delay time between the electrical pulse and the onset of EL is independent of the thickness of the injection layer, implying that the conducting PEDOT functions as a part of the electrode. When a dc forward bias is applied to the device, the delay time decreases, probably as a result of the shift of the emission zone towards the anode. The EL turn-on depends on the amplitude of the voltage pulse. The data are modeled by an equivalent circuit with a fixed capacitance connected in parallel with a nonlinear resistance. The solution of the differential equation depends on the exact form of the device’s I–V curve. Two analytical solutions are provided, and an analysis based on space-charge-limited current is presented. By applying a dc forward bias in advance to precharge the space-charge capacitance, the turn-on response time is reduced to 12 ns. The EL decay consists of two components with time constants of 15 ns and 1 μs. The decay does not depend on either the amplitude of the voltage pulse or the prebias.
Computer studies on the spontaneous fast reconnection evolution in a force-free current sheet system
Authors: Ugai M. , Shimizu T.
Published: July 2002

Two-dimensional magnetohydrodynamic simulations study the spontaneous fast reconnection evolution in a force-free current sheet system where the magnetic field simply rotates by 180 deg across the central current sheet without changing its magnitude. It is demonstrated that, as in the conventional coplanar case, the fast reconnection mechanism drastically evolves because of the positive feedback between (current-driven) anomalous resistivity and global reconnection flow; also, the fast reconnection evolution becomes more drastic for the lower plasma β. Once an anomalous resistivity is ignited and a sufficient amount of the sheared field component Bz is ejected from near the X reconnection point, the ambient magnetic field collapses into the X point, giving rise to the drastic buildup of the fast reconnection mechanism. On the nonlinear saturation phase, the Bz field is completely swept away from the reconnection region, so that coplanar slow shocks extend outward, and a large-scale plasmoid swells and propagates. The resulting plasmoid has a double structure that is quite different from the well-known coplanar one or from the so-called flux rope. In the backward half of the plasmoid, the plasma pressure is enhanced in the butterfly-shaped region, and (coplanar) slow shocks stand along the plasmoid boundary. On the other hand, in the forward half of the plasmoid a finite-amplitude intermediate wave stands along the plasmoid boundary; hence, across the plasmoid boundary, the magnetic field simply rotates without changing plasma quantities nor magnetic field magnitude.
Optimization of configuration interaction coefficients in multideterminant Jastrow–Slater wave functions
Authors: Schautz Friedemann , Fahy Stephen
Published: July 2002

A quantum Monte Carlo method for obtaining multideterminant Jastrow–Slater wave functions for which the energy is stationary with respect to variations of CI coefficients is presented. It is a generalization of a recently developed approach to the optimization of single particle functions [C. Filippi and S. Fahy, J. Chem. Phys. 112, 3523 (2000)]. Using ground state calculations of the atoms Be, C, and Ne and the dimer Si2 as illustrative examples, the method is shown to converge rapidly and to significantly lower the energy in most cases.
Origin of the bimodal distribution of low-pressure metal-organic-vapor-phase-epitaxy grown InGaAs/GaAs quantum dots
Authors: Saint-Girons G. , Patriarche G. , Mereuta A. , Sagnes I.
Published: July 2002

The formation process of a bimodal distribution of low-pressure metal-organic-vapor-phase-epitaxy (LP–MOVPE) grown InGaAs/GaAs quantum dots (QDs) is studied by transmission electronic microscopy. We demonstrate that in our growth conditions, the deposition of an InGaAs layer on an already existing array of InAs formed QDs leads to the nucleation of a second dots population. The InAs QDs nucleation is diffusion limited, inducing a low dots density due to the high In-atoms diffusion length typical of the MOVPE. On the contrary, the InGaAs QDs nucleation is enhanced by the roughness of the highly strained wetting layer of the InAs QDs, leading to higher density. The study of the photoluminescence spectra shows that the nucleation of InGaAs only occurs when the deposited InAs thickness exceeds about 1.4 monolayers, i.e., after the formation of the InAs QDs.
Excess current investigations of silicon p+–i–n+ diodes
Authors: Reitemann G. , Kasper E.
Published: July 2002

Si p+–i–n+ junctions, as used in modern devices, are investigated for excess current contributions in their forward characteristics. The excess current is caused by tunneling through band-gap states. The current characteristics of epitaxially grown junctions are studied experimentally at a temperature range from −60 to 100 °C, thereby the temperature dependence of the excess current is described and explained by the ratio of built-in voltage divided by the tunneling reference voltage Vt0. The voltage dependence is investigated by the curvature coefficient, the value of which is described by a modified Chynoweth theory. [A. G. Chynoweth, W. L. Feldmann, and R. A. Logan, Phys. Rev. 121, 684 (1961)].
Influence of interstitial nitrogen on the structural and magnetic properties of FeCoV/TiNx multilayers
Authors: Kumar M. Senthil , Böni P.
Published: July 2002

FeCoV/TiN x and FeCoV/Ti multilayers having tFeCoV=30–700 Å prepared by dc magnetron sputtering are investigated by x-ray diffraction, stress, and magnetization measurements. The x-ray diffraction data of the FeCoV/TiNx system show the presence of interstitial N atoms in the FeCoV layers due to reactive sputtering of Ti with nitrogen. The interstitial N causes an expansion of the FeCoV lattice in FeCoV/TiNx for small tFeCoV. However, for the samples with large tFeCoV, no lattice expansion is observed. In addition to the lattice expansion caused by the intake of N atoms, a change in the crystalline texture of FeCoV layers is also observed as indicated by the enhancement of the FeCoV(200) peaks. The magnetic hysteresis measurements on the samples show that the easy direction of magnetization lies in the plane of the layers. They further show that there are easy and hard axes of magnetization within the plane of the FeCoV layers. The stress anisotropy present in the plane of the samples induces a magnetic anisotropy through magnetostrictive effects leading to the formation of the in- plane easy axis. The hysteresis and stress measurements carried out on these samples clearly show the influence of N on the in-plane magnetic anisotropy. The magnetoelastic energy in the case of the FeCoV/TiNx system, calculated from the stress data and from the magnetization measurements as a function of tFeCoV is found to agree over a large range of thickness, whereas the curves deviate significantly for small layer thickness. This deviation may be due to the role of the FeCoVNx phase. Hysteresis measurements also show that the remanence is about 95% for all the samples of the FeCoV/TiNx system. In contrast, the coercivity increases linearly with increasing tFeCoV in this system. The coercivity of the FeCoV/Ti system is larger and increases more rapidly with tFeCoV, as compared with the FeCoV/TiNx system. This behavior is attributed to a smaller grain size in the FeCoV/TiNx system due to the reactive sputtering of the Ti layers. However, there is no significant influence of N on the saturation magnetization of both systems.
Controlling the magnetic properties of CoCrPt thin films by means of thin hexagonal-close-packed intermediate layers
Authors: Lee Hwan-Soo , Laughlin David E. , Bain James A.
Published: July 2002

CoCrPt films for longitudinal recording media were investigated as a function of substrate bias applied during the deposition. CoCrPt films were deposited on an underlayer stack consisting of glass/NiAl/CrMn with bias applied only during deposition of the CoCrPt layer. A strong dependence of the CoCrPt coercivity on the substrate bias was observed when a thin hexagonal-close-packed CoCrTa template was added as an intermediate layer between the CoCrPt and NiAl/CrMn underlayers. This effect was not observed in the absence of the CoCrTa layer. Microstructural investigation of CoCrPt thin films using grazing incidence and conventional x-ray diffraction indicated that the lattice of the CoCrPt increased in both the in-plane and normal directions as the bias was increased. A compositional analysis from both inductively coupled plasma and energy dispersive x-ray fluorescence measurements revealed that negative substrate bias applied during CoCrPt deposition changed the Pt content from 12 to 37 at. % in the CoCrPt films. The compositional change greatly improved the coercivity of the cobalt films up to 4300 Oe. At these large Pt concentrations, the CoCrTa intermediate layer was needed to reduce the lattice mismatch between the CoCrPt magnetic layer and NiAl/CrMn underlayers. This resulted in suppression of the Co (00.2) out-of-plane texture, and allowed high coercivity to be achieved.
Hydrogen bond-induced nanosegregation in the thermotropic cubic phase forming molecule, BABH8
Authors: Yoneya Makoto , Nishikawa Etsushi , Yokoyama Hiroshi
Published: July 2002

Local-structures of a thermotropic cubic phase forming molecule, 1,2-bis-[4-n-octyloxy benzoyl]-hydrazine (BABH8) were studied by molecular dynamics simulations. Spontaneous formation of hydrogen bond networks and smectic ordered clusters were obtained in these simulations. Furthermore, spontaneous segregation was observed in the simulations with the artificial binary system which was composed separately of BABH8 mesogen cores and alkyl tails. These results indicate that the BABH8 core and tails have a strong tendency to segregate and the smectic order formation is promoted by intermolecular hydrogen-bonding of the core hydrazine group. Connections between this segregation character and cubic phase formation were discussed.
Calorimetric and neutron scattering studies on the boson peak of lithium chloride aqueous solution glasses
Authors: Madokoro Yasushi , Yamamuro Osamu , Yamasaki Hirotaka , Matsuo Takasuke , Tsukushi Itaru , Kamiyama Takashi , Ikeda Susumu
Published: July 2002

The heat capacities of the glassy and liquid states of three lithium chloride aqueous solutions (LiCl⋅4H2O,LiCl⋅6H2O,LiCl⋅7.5H2O) were measured with an adiabatic calorimeter in the temperature range 5–340 K; the crystalline state was also measured for LiCl⋅4H2O. For all of the glassy samples, a large non-Debye excess heat capacity appeared below 30 K. The neutron scattering spectra of glassy LiCl⋅4H2O and LiCl⋅6H2O were measured with two spectrometers operating with different energy windows (E<20 meV and E<100 meV). A boson peak, corresponding to the excess heat capacity, appeared at around 4 meV in the S(Q,E) spectra of both glassy samples. From the analysis combining the heat capacity and neutron scattering data, the excess density of states due to the boson peak (compared with hexagonal ice) was found to be 0.10 degree of freedom per water molecule for both glassy samples. A librational peak of water molecules appeared at around 60 meV in glassy samples. This energy is about 10 meV lower than that of hexagonal and amorphous ices, indicating that the hydrogen bonds connecting water molecules are broken due to the strong coordination of water molecules to lithium ions.
New magnetic dipole transition of the oxygen molecule: B′ 3Πg←X 3Σg−(0,0)
Authors: Roberts E. H. , Nixon K. L. , Dedman C. J. , Gibson S. T. , Lewis B. R.
Published: July 2002

Through the use of isotopically pure gas at a temperature of 77 K, a weak photoabsorption band of O216 is found near 1856 Å, underlying the stronger Schumann–Runge (SR) band B 3Σu−←X 3Σg−(8,0). The location, structure, and intensity of this new band are consistent with expectation for the magnetic dipole transition B′ 3Πg←X 3Σg−(0,0), where the designation B′ is chosen to represent the II 3Πg valence state. This electronic transition contributes to the “excess absorption” underlying the SR bands [B. R. Lewis, S. T. Gibson, and E. H. Roberts, J. Chem. Phys. 115, 245 (2001)].
Density functional theory of spin-coupled models for diiron-oxo proteins: Effects of oxo and hydroxo bridging on geometry, electronic structure, and magnetism
Authors: Rodriguez Jorge H. , McCusker James K.
Published: July 2002

We have performed a comprehensive study of the electronic structure and magnetic properties of structurally characterized models for diiron-oxo proteins. Results from Kohn–Sham density functional theory show that two complexes, with formula Fe2(μ-O)(μ-O2CCH3)2(HBpz3)2 and [Fe2(μ-OH)(μ-O2CCH3)2(HBpz3)2]+, are strongly and weakly antiferromagnetically coupled, respectively, in agreement with experiment. The physical origin of the stronger and weaker exchange typically measured for oxo- and hydroxo-bridged diiron complexes, respectively, has been elucidated. The main superexchange pathways giving rise to molecular antiferromagnetism in both complexes have been identified. The dominant pathway in the oxo-bridged complex, Fe1(dxz):μ-O(px):Fe2(dxz), was formed by π interactions whereas that of the hydroxo-bridged, Fe1(dz2):μ-OH(p∥):Fe2(dz2), was formed by σ interactions. We also found a pathway mediated by the bridging acetates, Fe1(dx2−y2):bis(μ-acetato):Fe2(dx2−y2), which induces weak antiferromagnetism in the oxo-bridged complex but is significantly more important in the hydroxo-bridged complex. The antiferromagnetic exchange constants that parameterize the Heisenberg Hamiltonian H=JS1⋅S2 have been predicted for both, strongly and weakly, coupled complexes. Overall, the signs, trends, and magnitudes of the theoretical values (Jμ-Ocalc=+152.7 cm−1, Jμ-OHcalc=+23.3 cm−1) were in excellent agreement with experiment. The geometries of the complete molecular structures have been optimized in C2v symmetry and used to calculate molecular properties such as atomic charges and spin densities. The electronic configurations (Fe:4s0.293d5.93,μ-O:2s1.922p4.99;Fe:4s0.303d5.82,μ-OH:2s1.822p5.25,H:1s0.51) of the respective binuclear cores revealed relatively high occupancies for the nominally ferric ions, thus reflecting a donating character of their immediate N3O3 coordination. In addition, the diiron-oxo protein hemerythrin has been discussed. Theoretical and structural considerations indicated that the oxo-bridged diferric complex considered herein models extremely well the antiferromagnetic behavior of azidomet- and azidometmyo-hemerythrin. Finally, the magnetic behavior of closely related oxo-bridged diferric and hydroxo-bridged diferrous complexes containing Me3TACN capping ligands has been explained in light of the results presented in this work.
Optical control of molecular dynamics in a liquid
Authors: Demirplak Mustafa , Rice Stuart A.
Published: July 2002

We report the results of a study of the influence of solvent fluctuations on the efficiency of selective population transfer from an initial state to a designated target state of a solute molecule. Our model of the influence of liquid fluctuations on the states of the solute assumes that dephasing is the dominant relaxation process, and utilizes an analog of the Kubo stochastic theory of line shape. The solvent fluctuations are represented as a Gaussian random process that independently modulates each of the energy levels of the solute molecule. For typical liquid densities the maximum amplitude of these fluctuations is taken to be of the order of 150 cm−1, and the correlation time of the fluctuations is taken to be of the order of a few hundred femtoseconds, but we have also explored the effects of varying the fluctuation frequency and correlation time. It is shown that STIRAP (stimulated Raman adiabatic passage) generated population transfer to a designated target state of the solute remains efficient when the frequency of the solvent fluctuations is large or small relative to the inverse of the widths of the pump and Stokes pulses. It is further shown that extended STIRAP generated selective transfer to one of a pair of degenerate states of the solute remains efficient under the same conditions. These results suggest, subject to the accuracy of the representation of the influence of the solvent on the solute, that it should be possible, using coherent superpositions of states generated with picosecond excitation, to control population transfer, hence reactivity, for a class of reactions carried out in the liquid phase.
Exchange-coupled FePt nanoparticle assembly
Authors: Zeng Hao , Sun Shouheng , Vedantam T. S. , Liu J. P. , Dai Z.-R. , Wang Z.-L.
Published: July 2002

We have produced exchange-coupled FePt nanoparticle assemblies by chemical synthesis and subsequent thermal annealing. As the interparticle distances decrease by tuning the annealing conditions, interparticle interactions change from dipolar type to exchange type, and the magnetization reversal mechanism switches from rotation controlled to domain-nucleation controlled. With increasing annealing temperature, the coercivity first increases due to improved chemical ordering, and then drops significantly, resulting from excessive interparticle exchange coupling. For the samples exhibiting exchange coupling, both the remanence ratio and coercive squareness increase.
Josephson effect in Nb/Al2O3/Al/MgB2 large-area thin-film heterostructures
Authors: Carapella G. , Martucciello N. , Costabile G. , Ferdeghini C. , Ferrando V. , Grassano G.
Published: July 2002

We report the demonstration of dc and ac Josephson effects in Nb/Al2O3/Al/MgB2 thin-film heterostructure. The heterostructure exhibits moderately hysteretic current–voltage characteristic with a dc Josephson current branch and regular microwave-induced Shapiro steps. From conductance spectrum, a gap of about 2 meV at 7.7 K is estimated for the proximized surface of MgB2 electrode.
Temperature dependence of the fluorescence of Eu3+-ion doped in various silicate glasses
Authors: Wang Jiwei , Song Hongwei , Kong Xianggui , Xu Wu , Xia Haiping
Published: July 2002

The temperature dependence of emission intensity of the Eu3+ ion in several silicate glasses was investigated between 77 and 670 K, under the excitation of a 488-nm laser. It was observed that in some glasses the total emission intensity of the Eu3+ ion had a maximum at a certain temperature below 670 K, while in the other glasses increased monotonically with the increasing temperature. A unified theoretical model was given considering thermal activation, phonon-assisted absorption and nonradiative energy transfer processes. Based on the model, all of the experimental data were well fitted. The temperature-quenching channels were discussed, and the temperature-quenching rates were determined.
Optical properties of a laser dye in a solid-state polymeric host
Authors: Sardar Dhiraj K. , Salinas Felipe S.
Published: July 2002

The optical properties of C28H20N2O2B2F4H2O, an organic laser dye embedded in a solid plastic host, have been characterized for a number of laser wavelengths in the visible region. The index of refraction of the dye in a plastic host is measured by the conventional method of minimum deviation at these wavelengths. The one-dimensional two flux Kubelka–Munk method based on the diffusion approximation has been employed to determine the absorption, scattering, and scattering anisotropy coefficients of the dye in a plastic host from the measurements of total diffuse transmittance, total diffuse reflectance, and collimated transmittance using double-integrating spheres. The total attenuation coefficients obtained by this method have been compared with those determined by the collimated transmission and from the total attenuation measurement on a Cary-14 spectrophotmeter.
Coexistence of ferromagnetic and glassy behavior in semiconducting LaNi0.2Co0.8O3
Authors: Androulakis J. , Katsarakis N. , Giapintzakis J.
Published: July 2002

Systematic dc and ac magnetic susceptibility studies have been performed on single-phase LaNi0.2Co0.8O3 powder samples. Evidence of hysteresis loops supports the existence of ferromagnetic correlations that lead to a rapid increase of the dc susceptibility at T⩽55 K. The frequency dependence of the ac susceptibility is suggestive of a spin-glass-like phase. The temperature dependence of the nonlinear susceptibility components, χ2 and χ3, in zero dc magnetic field demonstrates the existence of spontaneous magnetization, appears to exclude the occurrence of a canonical spin-glass phase and instead indicates that a cluster-glass phase occurs in LaNi0.2Co0.8O3. It is suggested that the spontaneous magnetization in this system is due to the existence of Ni-based clusters with strong intracluster ferromagnetic correlations, embedded randomly in a paramagnetic matrix.
Surface dependent electron and negative ion density in SF6/argon gas mixtures
Authors: Hebner G. A. , Abraham I. C.
Published: July 2002

Electron and negative ion densities were measured in an inductively driven plasma containing mixtures of SF6 and Argon. The electron and negative ion density were measured as functions of the induction coil power, pressure, bias power, and SF6/argon ratio. To investigate the influence of surface material, the rf biased electrode was covered with a silicon wafer or a fused silica (SiO2) wafer. Line integrated electron density was determined using a microwave interferometer, and absolute negative ion densities in the center of plasma were inferred using laser photodetachment spectroscopy. Voltage and current at the induction coil and rf biased electrode were also measured for both surfaces as functions of induction coil power, pressure, rf bias, and SF6/argon ratio. For the range of induction powers, pressures, and bias powers investigated, the electron density had a maximum of 5×1012 cm−2 (line-integrated) or approximately 5×1011 cm−3. Over this same range the negative ion density had a maximum of 2×1011 cm−3, and was always less than the electron density. For most conditions, the negative ion density above the oxide surface was a factor of 5 to 10 larger than the density above the silicon surface. In contrast, the electron density above the oxide surface was equal to or slightly higher than the density above the silicon surface. Surface dependent changes in the induction coil and rf bias voltage and current were also observed.
Effect of intrinsic defects on the electron mobility of gallium arsenide grown by molecular beam epitaxy and metal organic chemical vapor deposition
Authors: Jorio Anouar , Sellami Lamia , Aubin Marcel , Carlone Cosmo
Published: July 2002

Temperature dependent electron mobility measurements are reported for lightly doped n-type gallium arsenide (GaAs) grown by metal organic chemical vapor deposition (MOCVD GaAs). Using the Brooks–Herring model, the charge state of the impurity scattering centers is deduced to be 1. The same measurements are reported for irradiated MOCVD GaAs, and it is deduced that the charge state of the defects introduced is also 1. These observations are different from previously reported mobility measurements on irradiated gallium arsenide grown by molecular beam epitaxy (MBE GaAs), in which it was deduced that multiply charged scattering centers are introduced by the irradiation. Photoluminescence and deep level transient spectroscopy are used to identify the defects present in the as-grown and in irradiated GaAs. Through the observation of the antisites, it is concluded that the MOCVD GaAs was grown under arsenic rich conditions and the MBE GaAs under gallium rich conditions. Moreover, the gallium vacancy defect (VGa) is observed in irradiated MOCVD GaAs, but not in irradiated MBE samples. The mobility measurements on the MOCVD samples imply that the VGa charge state defect is −1. It is speculated that the arsenic vacancy defect is introduced in irradiated MBE GaAs and its charge state is −2.
Structure and characteristics of large Fe–O/AlOx/Fe–O tunnel junctions
Authors: Pan Liqing , Qiu Hong , Wang Fengping , Wu Ping , Tian Yue , Luo Sheng , Shi Ji , Hashimoto Mituru
Published: July 2002

The Fe–O/AlOx/Fe–O tunnel junctions with an area of 1 cm2 were fabricated by reactive magnetron sputtering. The structure and properties of the junctions were studied by scanning electron microscopy, scanning Auger microprobe, x-ray diffraction (XRD), cross-sectional transmission electron diffraction (XTED), atomic force microscopy, vibrating specimen magnetometer, and I–V characteristic. The as-deposited and annealed Fe–O layers have disparate crystalline grain structure. Only bcc–Fe structure is observed in the Fe–O layers through XRD and XTED measurement. Most of the oxygen atoms exist as impurity in the films, and large compressive stress (σ≈−7.9×109 Pa) is induced in the Fe–O layers. The insulating AlOx layer formed by radio-frequency sputtering directly from the alumina target is dense and shows good insulating property.
Bright and efficient exciplex emission from light-emitting diodes based on hole-transporting amine derivatives and electron-transporting polyfluorenes
Authors: Jiang Xuezhong , Liu Michelle S. , Jen Alex K.-Y.
Published: July 2002

We report highly efficient and bright emission from exciplexes generated between a series of hole-transporting amine derivatives and two electron-transporting fluorene–dicyanophenyl (FCNP) copolymers. These exciplexes were formed at either the interface between tetraphenyldiamine-containing perfluorocyclobutane polymers and the FCNP copolymers, or in the blends of the FCNP copolymers with small molecule amine derivatives such as triphenylamine, N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine, and N,N′-diphenyl-N,N′-bis(1-naphthyl)-[1,1′-biphenyl]-4,4′-diamine. The exciplex emission is largely dependent on the composition of the hole-transporting materials. The best device derived from these exciplexes demonstrated a very low turn-on voltage (2.8 V), a high external quantum efficiency (0.91%), and a high brightness of 3370 cd/m2. The desirable properties of these devices were attributed to the excellent electron transport ability of the FCNP copolymers.
Metalorganic chemical vapor deposition of aluminum oxide on Si: Evidence of interface SiO2 formation
Authors: Roy Chowdhuri A. , Takoudis C. G. , Klie R. F. , Browning N. D.
Published: July 2002

Thin films of aluminum oxide were deposited on H-passivated Si(100) substrate using trimethylaluminum and oxygen at 0.5 Torr and 300 °C. Fourier transform infrared (FTIR) and x-ray photoelectron spectroscopic analyses of these films showed no aluminum silicate phase at the film–substrate interface. The O/Al ratio in the deposited film was found to be higher than that in stoichiometric Al2O3. On annealing the as-deposited samples in Ar at 900 °C, an absorption peak due to the transverse optical phonon for the Si–O–Si stretching mode appeared in the FTIR spectra. A combination of Z-contrast imaging and electron energy-loss spectroscopy in the scanning transmission electron microscope confirmed that the annealed samples developed a layer of silicon dioxide at the aluminum oxide–Si interface. Our results suggest that excess oxygen present in the deposited film reacts with the underlying Si substrate and forms silicon oxide.