bluuebunny/crossref_metadata_2025 huggingface dataset:
Photodissociation of C6H5I, C6F5I, and related iodides in the ultraviolet
Authors: Kavita K. , Das Puspendu K.
Published: July 2002

The quantum yields of I*(2P1/2) production from iodobenzene and pentafluoroiodobenzene at five different dissociation wavelengths of 222, 236, 266, 280, and ∼305 nm are presented and compared with those obtained from nonaromatic cyclic iodides (i.e., cyclohexyl iodide and adamantyl iodide). The I(2P3/2) and I*(2P1/2) atoms generated in the photolysis of the above iodides were monitored using a two-photon laser-induced fluorescence technique. From the measured I* quantum yields, two general observations are made for aryl iodides. They are that (i) the I* yield is influenced by the σ*←n as well as π*←π transitions at all photolysis wavelengths within the A band and (ii) there is a clear indication of a fluorine substitution effect on the dynamics of I* production. The contribution from the benzene type π*←π transition varies with excitation wavelength. Fluorine substitution in aryl iodides is found to increase the I* quantum yield similar to what is reported in alkyl iodides. The effect of fluorine substitution is more pronounced at the red edge of the A-band excitation than at any other wavelengths. This is explained by invoking the presence of a charge-transfer band arising due to the transition of a 5pπ nonbonding iodine electron to the π* molecular orbital near the red edge of the A band. This charge-transfer state is coupled more strongly to the Q13 state of the σ*←n transition in pentafluoroiodobenzene than in iodobenzene. The dynamics of I* formation is found to be unaltered by ring strain in cyclic iodides except at the blue wing excitation. At the blue wing, B-band transitions affect the dynamics of I* production in cyclic iodides, leading to the formation of more I* from adamantyl iodide.
Sol-gel Er-doped SiO2–HfO2 planar waveguides: A viable system for 1.5 μm application
Authors: Gonçalves R. R. , Carturan G. , Zampedri L. , Ferrari M. , Montagna M. , Chiasera A. , Righini G. C. , Pelli S. , Ribeiro S. J. L. , Messaddeq Y.
Published: July 2002

70SiO 2 – 30HfO 2 planar waveguides, doped with Er3+ concentrations ranging from 0.3 to 1 mol %, were prepared by sol-gel route, using dip-coating deposition on silica glass substrates. The waveguides show high densification degree, effective intermingling of the two components of the film, and uniform surface morphology. Propagation losses of about 1 dB/cm were measured at 632.8 nm. When pumped with 987 or 514.5 nm continuous-wave laser light, the waveguides show the I413/2→I415/2 emission band with a bandwidth of 48 nm. The spectral features are found independent both on erbium content and excitation wavelength. The I413/2 level decay curves presented a single-exponential profile, with a lifetime between 2.9 and 5.0 ms, depending on the erbium concentration.
Unusual size effect on the polarization patterns in micron-size Pb(Zr,Ti)O3 film capacitors
Authors: Stolichnov Igor , Colla Enrico , Tagantsev Alexander , Bharadwaja S. S. N. , Hong Seungbum , Setter Nava , Cross Jeffrey S. , Tsukada Mineharu
Published: July 2002

Direct observation of polarization distribution at nanoscale by scanning force microscopy across partially etched Pb(Zr,Ti)O3 film ferroelectric capacitors with size 2×3 μm and preferential orientation (111) has revealed an anomalous polarization pattern characterized by polarization inversion in the center of the capacitor. This self-structured pattern is reproducible and independent on the spontaneous polarization orientation, but strongly influenced by the film texture and capacitor size. In particular, capacitors of the same texture with size 0.5×0.5 μm as well as capacitors with (100) preferential orientation of any size did not exhibit such anomalous polarization distribution. A tentative explanation consistent with the essential features of the observed polarization behavior in terms of strain-induced phase transition is discussed.
Sensitivity of reflection transducers
Authors: Alcock R. D. , Emmony D. C.
Published: July 2002

This article discusses the sensitivity of new high-speed optical transducers for measuring acoustic transients propagating in water. These devices are based upon the measurement of reflectance at a solid–water interface using a probe beam, perturbations in reflected intensity being brought about by the changes in pressure from an acoustic transient traversing the boundary. It is found that the sensitivity of such transducers is highly dependent not only upon the angle of incidence of the probe beam used to measure reflectance, but also the divergence of the probe beam and the acoustic properties of the transducer material. This results in a trade off between obtaining high spatial resolution and high sensitivity with such a transducer, and higher sensitivities give rise to a more nonlinear response. The fabrication and calibration of a robust prototype transducer made out of perspex is discussed and initial experimental results measuring acoustic transients associated with laser cavitation are shown. High spatial resolution and sensitivity were achieved by focusing the probe beam onto the interface at an angle which was slightly smaller than the critical angle. Finally, a transducer with yet higher sensitivity fabricated out of glass is demonstrated, allowing the study of smaller amplitude rarefaction waves.
Compression of piezoelectric ceramic at constant electric field: Energy absorption through non-180° domain-wall motion
Authors: Chaplya Pavel M. , Carman Gregory P.
Published: July 2002

The effect of bias electric field on the nonlinear stress–strain response of a lead zirconate–lead titanate piezoelectric ceramic is studied. The material is subjected to various compressive stress amplitudes (25–300 MPa) under constant electric field (from −0.5 to 2.0 MV/m) along the original poling direction. Application of a positive electric-field bias results in closed stress–strain hysteresis loops absorbing significant amounts of mechanical energy. Increasing the positive electric field increases the specific damping and decreases the elastic modulus. The trend is reversed when the electric field becomes sufficiently high to inhibit the domain-wall motion by the mechanical stresses. Measured specific damping values vary from 0.18 to 0.46 depending on the stress amplitude and bias electric field. The corresponding secant modulus varies from 79 to 24 GPa. The coercive stress is found to approach zero as the negative electric-field bias approaches the coercive field value. The coercive stress increases with increasing positive electric field as expected from the balance of mechanical and electrical energies. The physics of the observed phenomena is explained in terms of non-180° domain-wall motion.
Studies of electronic structure of ZnO grain boundary and its proximity by using spatially resolved electron energy loss spectroscopy
Authors: Ong H. C. , Dai J. Y. , Du G. T.
Published: July 2002

The low electron energy loss and complex dielectric functions of an arbitrary grain boundary and its proximity in ZnO thin films have been studied by using the spatially resolved electron energy loss spectroscopy. The critical point parameters have been determined by fitting the dielectric functions simultaneously with analytical line shape model. Gradual changes have been observed in the dielectric functions spectra. The critical points are found to redshift and then blueshift when the electron beam scanned across the grain boundary, which suggest the distinctive electronic structure not only of the grain boundary but also of the depletion region. In addition, comparison has been made between the experiment and the recent theoretical studies to account for the interband transitions that occur in the grain boundaries. Several features predicted by the theory are qualitatively found to be consistent with our results. The presence of dangling bonds instead of bond distortion is attributed to be the major cause of defects in the grain boundaries of ZnO.
What can be learned from the schematic mode-coupling approach to experimental data?
Authors: Krakoviack V. , Alba-Simionesco C.
Published: July 2002

We propose a detailed investigation of the schematic mode-coupling approach to experimental data, a method based on the use of simple mode-coupling equations to analyze the dynamics of supercooled liquids. Our aim here is to clarify different aspects of this approach that appeared so far uncontrolled or arbitrary, and to validate the results obtained from previous works. Analyzing the theoretical foundations of the approach, we first identify the parameters of the theory playing a key role and obtain simple requirements to be met by a schematic model for its use in this context. Then we compare the results obtained from the schematic analysis of a given set of experimental data with a variety of models and show that they are all perfectly consistent. A number of potential biases in the method are identified and ruled out by the choice of appropriate models. Finally, reference spectra computed from the mode-coupling theory for a model simple liquid are analyzed along the same lines as experimental data, allowing us to show that, despite the strong simplification in the description of the dynamics it involves, the method is free from spurious artifacts and provides accurate estimates of important parameters of the theory. The only exception is the exponent parameter, the evaluation of which is hindered, as for other methods, by corrections to the asymptotic laws of the theory present when the dynamics is known only in a limited time or frequency range.
Solvation dynamics following electron photodetachment from I− in aqueous clusters
Authors: Elola M. Dolores , Laria Daniel
Published: July 2002

Equilibrium and dynamical aspects of the earliest stages of solvation following the electronic excitation of I− dissolved in aqueous clusters were investigated, using a mixed quantum-classical molecular dynamics algorithm. The systems were modeled as composed by an excess quantum charge coupled to a classical bath that includes a neutral iodine and Nw=6, 50, and 100 water molecules. In small clusters, the equilibrium solvation structures of the iodine are characterized by surface states that gradually turn into interior states as one considers larger aggregates. Electronic properties of the ground and first excited states are described. In small aggregates, both states are characterized by highly diffuse density profiles that extend well beyond the spatial extent of the aggregates. In larger clusters, the confining potential provided by the classical bath localizes the electron within the aggregates. Dynamical aspects of the solvation relaxation following a vertical excitation of the solute were also examined. In all cases, the relaxation mechanisms involve a global reorganization of the original solvation structure characterized by a gradual stabilization of the solvent-solvent interactions in detriment of a less favorable electron solvation. The overall characteristic time for the solvation is in the order of τs≈0.05 ps for Nw=6 and attains τs≈0.60 ps for Nw=100.
A molecular dynamics simulation study of the dimethyl sulfoxide liquid–vapor interface
Authors: Senapati Sanjib
Published: July 2002

In this study, a fully flexible, nonpolarizable model potential of dimethyl sulfoxide (DMSO) has been used to investigate the DMSO liquid–vapor interface, based on classical molecular dynamics simulation techniques. A series of four simulations in the temperature range of 298–373 K is carried out to examine the temperature dependence of the structural, thermodynamic, and dynamical properties. The full Ewald summation technique is employed to account for the long-range electrostatic interactions. Computed bulk properties of the liquid such as density, diffusion are found to be in good agreement with experimental values. Self-diffusion coefficient of bulk DMSO molecules is computed to be smaller than at the interface. The study demonstrates the importance of inclusion of flexibility in the model and the use of Ewald sums, which have an influence on dynamics.
Evolution of populations in a multi-collision environment: Towards a quantum state resolved model of dis-equilibrium
Authors: Marsh Richard J. , McCaffery Anthony J.
Published: July 2002

We exploit the accuracy and computational speed of the angular momentum model of inelastic transfer to follow changes in quantum state populations as a gas ensemble evolves from an initial state of dis-equilibrium. Results on two prototype systems in specific initial states are presented and the manner by which these approach equilibrium is discussed. There are wide differences in the rates at which different internal modes equilibrate and although Boltzmann-type distributions are found within a mode, individual modes may not be in equilibrium with one another. These findings have relevance, e.g., to upper atmosphere modeling where the rapid establishment of local thermodynamic equilibrium is often assumed.
Green emission from B2N2CO thin films doped with Tb
Authors: Liu Q. L. , Zhang F. X. , Tanaka T. , Aizawa T.
Published: July 2002

Visible light emission has been obtained at room temperature by photoluminescence (PL) and cathodoluminescence (CL) from Tb-doped B2N2CO thin films prepared by radio-frequency magnetron sputtering. The films were deposited by sputtering TbBn (n=4, 6, 12, 25, and 50) targets in an atmosphere of Ar and N2 gas mixture. The films are uniformly composed of very small particles with particle size of about 20–40 nm, which consist of turbostratic BN phase and amorphous phase. The characteristic PL peaks are observed in the spectral range from 300 to 800 nm, and correspond to the two groups of transitions of Tb3+ ions: D35→7FJ (J=6, 5, 3) and D45→7FJ (J=6, 5, 4, 3). The strongest PL peak at ∼547.2 nm is about four times greater than the other signals in the investigated range, indicating that the films have strong green light emission. CL intensity is very weakly dependent on the temperature.
Direct imaging of reverse-bias leakage through pure screw dislocations in GaN films grown by molecular beam epitaxy on GaN templates
Authors: Hsu J. W. P. , Manfra M. J. , Molnar R. J. , Heying B. , Speck J. S.
Published: July 2002

Excess reverse-bias leakage in GaN films grown by molecular beam epitaxy on GaN templates is correlated with the presence of pure screw dislocations. A scanning current–voltage microscope was used to map the spatial locations of leakage current on high quality GaN films under reverse bias. Two samples with similar total dislocation density (∼109 cm−2) but with pure screw dislocation density differing by an order of magnitude were compared. We found that the density of reverse-bias leakage spots correlates well with pure screw dislocation density, not with mixed dislocation density. Thus, pure screw dislocations have a far more detrimental impact on gate leakage than edge or mixed dislocations.
Magnetic switching field and giant magnetoresistance effect of multilayers with synthetic antiferromagnet free layers
Authors: Inomata K. , Nozaki T. , Tezuka N. , Sugimoto S.
Published: July 2002

Synthetic antiferromagnets (SyAF) made of Co90Fe10(t1)/Ru(d)/Co90Fe10(t2) trilayers and spin-valve type multilayers using a SyAF free layer were deposited on a buffer layer of Cu(2.5 nm)/Ru(10 nm) on a SiO2/Si substrate by using an ultrahigh vacuum sputtering system and a metal mask. The magnetization switching field of the SyAF in both trilayers and multilayers was independent of the Ru thickness, suggesting that the switching field is independent of the magnitude of the antiferromagnetic interlayer exchange coupling, and was proportional to t1/Δt (Δt=|t1−t2|), which is in good agreement with the model of coherent rotation with perfect antiparallel alignment during magnetization reversal of the SyAF. Giant magnetoresistance of the multilayers exhibited two types of curves depending on t1>t2 or t1<t2.
Estimating electron correlation energies of A2 from A− and A+ ionic units with a simple scaling approach
Authors: Zhuo Shuping , Wei Jichong , Ju Guanzhi , Wei Zhaorong
Published: July 2002

Electron pair correlation contribution from individual electron pairs of the molecular orbitals is defined and used in analyzing the correlation patterns of F2, O22−, and CH3CH3 isoelectronic covalent systems. Based on our simple strategy so-called “separating large system into smaller ones” for estimating the correlation energies by investigating both the ionic and the diradical partitioning schemes for covalent systems, a simple scaling scheme is presented for estimating the total correlation energy. It is achieved by summing the scaled correlation energies of its ionic fragments derived from the ionic partitioning scheme rather than from the diradical partitioning scheme. Of the three estimated results, the absolute deviations are less than ±0.29 kcal/mol, however, using this simple scaling approach, at least 90% of computational work can be saved. At the present condition with the computational demand for calculating the electron correlation energies of large covalent molecules, it is hopeful that this simple scaling approach could be useful to estimate the correlation energies of large CH-containing alkaline compounds.
Three-dimensional carrier profiling of InP-based devices using scanning spreading resistance microscopy
Authors: Xu M. W. , Hantschel T. , Vandervorst W.
Published: July 2002

Scanning spreading resistance microscopy (SSRM) is a carrier profiling method based on atomic force microscopy (AFM), which has proven its power for two-dimensional semiconductor device analysis in the last few years. A three-dimensional approach is presented for depth profiling on the nm scale and for device inspection from the top surface. A procedure for SSRM on the InP structure is developed, where layers a few nm thick are continuously scratched away while scanning the region of interest with a highly doped diamond tip. This extends the SSRM to the third dimension, as SSRM images taken at different depths can be combined to construct a three-dimensional image of the sample. Based on the same technology, a “depth profiling” mode is developed whereby a well-defined etched pit is made. SSRM measurements taken on the side made of the etched pit provide a carrier profile to overcome the limitations of standard cross-sectional profiling and enable profiling of the layer with nm resolution.
Quantum chemical study of the elementary reactions in zirconium oxide atomic layer deposition
Authors: Widjaja Yuniarto , Musgrave Charles B.
Published: July 2002

Elementary reactions in atomic layer deposition of zirconia using zirconium tetrachloride and water are investigated using the density functional theory. The atomistic mechanisms of the two deposition half cycles on the Zr–OH and Zr–Cl surface sites are investigated. Both half reactions proceed through the formation of stable intermediates, resulting in high barriers for HCl formation. We find that the intermediate stability is lowered as the surface temperature is raised. However, increasing temperature also increases the dissociation free-energy barrier, which in turn results in increased desorption of adsorbed precursors.
Stability of magnetic vortices in flat submicron permalloy cylinders
Authors: Schneider M. , Hoffmann H. , Otto S. , Haug Th. , Zweck J.
Published: July 2002

We have investigated the magnetic properties of flat permalloy cylinders by Lorentz transmission electron microscopy and micromagnetic simulations. The magnetization patterns during in situ magnetizing experiments have been imaged and they revealed that the magnetization reversal of the cylindrically shaped dots investigated is determined by the formation and annihilation of magnetic vortices. Furthermore, the experiments and micromagnetic simulations showed a dependence of the vortex annihilation field not only on the aspect ratio but also on the absolute thickness of the cylinders. The diameter of the cylindrical dots was varied between 150 and 1000 nm, and the thicknesses were 3, 5.5, 8.3, 15, and 20 nm, respectively. The formation of inhomogeneous magnetization patterns prior to vortex evolution was observed and by a comparison of the experimental to simulated Fresnel images these patterns can be identified as S- and C-like states.
Terahertz intersubband emission in strong magnetic fields
Authors: Blaser Stéphane , Rochat Michel , Beck Mattias , Hofstetter Daniel , Faist Jérôme
Published: July 2002

Electroluminescence in quantum-cascade structures based on vertical transitions is studied in a strong perpendicular magnetic field in the limit in which the cyclotron energy is larger than the intersubband transition energy. Cyclotron emission and a luminescence intensity enhancement up to a factor of 6 is observed in GaAs/AlGaAs and InGaAs/InAlAs vertical transition-based quantum-cascade structures.
Coalescence and electron activation energy in CdTe/ZnTe nanostructures
Authors: Kim T. W. , Choo D. C. , Lee D. U. , Lee H. S. , Jang M. S. , Park H. L.
Published: July 2002

Atomic force microscopy (AFM) and photoluminescence (PL) measurements were carried out to investigate the coalescence and electron activation energy in CdTe/ZnTe nanostructures. The results of the AFM images show that uniform CdTe quantum dots (QDs) are formed and that the transformation from CdTe QDs to CdTe quantum wires is caused by the coalescence. The excitonic peaks corresponding to the transition from the ground electronic subband to the ground heavy-hole band in the CdTe/ZnTe QDs shifted to higher energy in comparison with those of the CdTe/ZnTe quantum wires. The activation energy of the electrons confined in the CdTe QDs, as obtained from the temperature-dependent PL spectra, was higher than those in CdTe quantum wells and quantum wires. The present results can help to improve the understanding of coalescence and electron activation energy in CdTe/ZnTe nanostructures.
Potential energy surface for the CH3+HBr→CH4+Br hydrogen abstraction reaction: Thermal and state-selected rate constants, and kinetic isotope effects
Authors: Espinosa-Garcı́a J.
Published: July 2002

The gas-phase hydrogen abstraction title reaction was carefully investigated. First, ab initio molecular orbital theory was used to study the stationary points along the reaction path: reactants, hydrogen-bonded complex, saddle point, and products. Optimized geometries and harmonic vibrational frequencies were calculated at the second-order Møller–Plesset perturbation theory level, and then single-point calculations were performed at a higher level of calculation: coupled-cluster with triple-zeta basis set. The effects of the level of calculation, zero-point energy (ZPE), thermal corrections [TC (298.15 K)], spin–orbit coupling, and basis set superposition error (BSSE) on the energy changes were analyzed. It was concluded that at room temperature (i.e., with ZPE and TC), when the BSSE was included, the complex disappears and the activation enthalpy is +0.39 kcal mol−1 above the reactants. Second, an analytical potential energy surface was constructed with suitable functional forms to represent vibrational modes, and was calibrated by using experimental and theoretical stationary point properties and the tendency of the kinetic isotope effects. On this surface, the forward and reverse thermal rate constants were calculated using variational transition state theory with semiclassical transmission coefficients over a wide temperature range. In both cases, we found a direct dependence on temperature and, therefore, positive activation energies. The influence of the tunneling factor was very small due to the flattening of the surface in the entrance valley. This surface was also used to analyze dynamical features, such as reaction-path curvature, the coupling between the reaction coordinate and vibrational modes, and the effect of vibrational excitation on the rate constants. It was found that excitation of the BrH stretching mode enhances the forward reaction, whereas the excitation of the CH3 umbrella mode has the opposite effect.
Quantum dynamics study on predissociation of H3 Rydberg states: Importance of indirect mechanism
Authors: Tashiro Motomichi , Kato Shigeki
Published: July 2002

Predissociation of H3 Rydberg states was investigated using the effective Hamiltonian which describes the vibronic transitions among the Rydberg states as well as the predissociation through the vibrationally excited 2s states. The motion of a Rydberg electron and the vibrations of the H3+ ion core was treated simultaneously without assuming the Born–Oppenheimer approximation. We developed the effective potential for a Rydberg electron, which contains the Coulomb potential and the exchange effect. The energies and predissociation lifetimes of H3 Rydberg states were obtained by analyzing the effective Hamiltonian and compared with the available experimental values. The s and p Rydberg states with lower vibrational excitation have lifetimes between a few ps to 1 ns and show an irregular lifetime distribution with respect to the principal quantum number. In contrast, d and f Rydberg states have longer lifetime, 10 ns for example. The energy level spacings of the Rydberg states obey the distribution close to the Poisson one and thus indicates these states being regular. The route of predissociation was investigated by propagating a wave packet as well as analyzing the eigenvectors of the effective Hamiltonian. We found that the energy level matching between nearby states play an important role for efficient predissociation. The present results suggest that the predissociation of the H3 molecule and the dissociative recombination of the H3+ ion might be efficient under rotational excitation through inclusion of additional energy levels.
Magnetoelectric coupling in Terfenol-D/polyvinylidenedifluoride composites
Authors: Mori Kiyotake , Wuttig Manfred
Published: July 2002

The elastic coupling in the ferromagnetic/ferroelectric composite Terfenol-D/polyvinylidenefluoride (PVDF) leads to a magnetic field induced magnetoelectric (ME) effect: an applied magnetic field creates a piezomagnetic strain in the Terfenol-D that in turn induces a piezoelectric charge or voltage in the PVDF. This letter reports a MEH coefficient of 1.43 V/(cm Oe) in agreement with an analytical calculation based on the properties of the components. The ME coupling coefficient of this composite is 11%.
Electrical spin injection in a ferromagnet/tunnel barrier/semiconductor heterostructure
Authors: Motsnyi V. F. , De Boeck J. , Das J. , Van Roy W. , Borghs G. , Goovaerts E. , Safarov V. I.
Published: July 2002

We demonstrate experimentally the electrical electron spin injection from a ferromagnetic metal/tunnel barrier contact into a III–V semiconductor light emitting diode (LED). The injected electrons have in-plane spin orientation. By applying a relatively small oblique external magnetic field this spin orientation within the semiconductor can be manipulated to have a nonzero out-of-plane component. By measuring the resulting circular polarization of the emitted light, we observe injected spin polarization in excess of 9% at 80 K in a CoFe/AlOx/(Al,Ga)As/GaAs surface-emitting spin-LED.
Metal-dependent conductance quantization of nanocontacts in solution
Authors: Li Jingze , Kanzaki Taisuke , Murakoshi Kei , Nakato Yoshihiro
Published: July 2002

Metal nanocontacts of Cu, Ag, Ni, Pd, and Pb were fabricated via the electrochemical etching or deposition method at a gap of sustainable thin Au wires in solution. Relatively stable behaviors of the conductance quantization exhibiting a unit value were apparent at the contacts of Cu, Ag, Ni, and Pd at room temperature. Conductance quantization behavior of the Pb contact was rather unstable in comparison with other metals.
Medium-range order in pyrolyzed carbon films: Structural evidence related to metal–insulator transition
Authors: Meenakshi V. , Subramanyam S. V.
Published: July 2002

Structural properties of carbon films prepared by the pyrolysis of tetra chloro phthalic anhydride are investigated in an attempt to correlate it with the metal–insulator transition observed earlier as a function of deposition temperature, from electrical transport studies. Anomalous features are observed which evolve as the pyrolysis temperature is varied from 700 to 900 °C. This includes prepeaks in the x-ray diffraction pattern in the region less than 1 Å−1 providing evidence for medium range order, and a broad shoulder around 1200 cm−1 in the Raman spectra, which are seen in addition to the usually observed peaks in amorphous carbon films. A simple model is proposed according to which presence of “polymeric domains” containing chains of carbon atoms with single and double bond alternation (–C=C–), in a predominantly aromatic ring structure, could give rise to the anomalous features.
Large enhancement in room-temperature magnetoresistance and dramatic decrease in resistivity in La0.7Ca0.3MnO3–Ag composites
Authors: Huang Yun-Hui , Yan Chun-Hua , Luo Feng , Song Wei , Wang Zhe-Ming , Liao Chun-Sheng
Published: July 2002

A ferromagnet-metal-type composite, La0.7Ca0.3MnO3 (LCMO)–Ag, was fabricated by a two-step chemical route which can avoid the doping of Ag into the lattice of LCMO. The grain size of Ag can be reduced by increasing calcination temperature, which favors the penetration of Ag into LCMO matrices. A large enhancement in magnetoresistance (MR) near room temperature and a dramatic decrease in resistivity are observed for the samples calcined at above the melting temperature of Ag. We suggest that the shift of metal–insulator transition up to Curie temperature in melted-Ag-added LCMO and magnetic inhomogeneity are responsible for the enhanced MR.
Inelastic electron tunneling across magnetically active interfaces in cuprate and manganite heterostructures modified by electromigration processes
Authors: Belogolovskiı̆ M. A. , Revenko Yu. F. , Gerasimenko A. Yu. , Svistunov V. M. , Hatta E. , Plitnik G. , Shaternik V. E. , Rudenko E. M.
Published: July 2002

We report a study of the electron tunneling transport in point-contact junctions formed by a sharp Ag tip and two different highly correlated oxides, namely, a magnetoresistive manganite La0.66Ca0.34MnO3 and a superconducting cuprate LaBa2Cu3O7−x. Strong chemical modifications of the oxide surface (supposedly, oxygen ion displacements) caused by applying high voltages to the junctions have been observed. This effect is believed to be responsible for an enormous growth of inelastic tunneling processes across a transition region that reveals itself in an overall V-shaped conductance background, with a strong temperature impact. The mechanism of the inelastic scattering is ascribed to charge transmission across magnetically active interfaces between two electrodes forming the junction. To support the latter statement, we have fabricated planar junctions between Cr and Ag films with an antiferromagnetic chromium oxide Cr2O3 as a potential barrier and at high-bias voltages have found an identical conductance trend with a similar temperature effect.
Measured cross sections and ion energies for a CHF3 discharge
Authors: Peko B. L. , Champion R. L. , Rao M. V. V. S. , Olthoff J. K.
Published: July 2002

Trifluoromethane (CHF3) is used in semiconductor plasma processing chambers to achieve high-etch selectivity of an oxide layer over a silicon substrate. Such surface etching is governed by the ion and molecule fluxes near the surface, the concentrations of which are dependent upon species interactions in and their transport through the plasma. In order to assist in the interpretation of ion flux measurements and to provide fundamental data required for plasma modeling, we report the first total cross sections for significant ion-molecule reactions occurring in CHF3 discharges. The reactions studied include collision-induced dissociation for CF3+ on CHF3, dissociative charge transfer for CF3+ and F+ on CHF3, and electron detachment from F− on CHF3. Collision energies range from a few to a few hundred electron volts. In addition, ion-flux energy distributions and relative ion intensities have been measured and are presented for dc townsend discharges with E/N values ranging from 5×10−18 to 25×10−18 V m2 [5 to 25 kTd]. The townsend discharge results are qualitatively interpreted using the cross-section measurements.
Coercivity enhancement above the Néel temperature of an antiferromagnet/ferromagnet bilayer
Authors: Leighton C. , Suhl H. , Pechan Michael J. , Compton R. , Nogués J. , Schuller Ivan K.
Published: July 2002

Single-crystal thin films of the antiferromagnet FeF2 have been used to exchange bias overlayers of Fe. An unexpected coercivity enhancement is observed at temperatures above the Néel temperature of the FeF2. This coercivity reaches a peak value of over 600 Oe close to the Néel temperature and persists to above 300 K. The coercivity is correlated with the growth of an anisotropy in the ferromagnet, the increase of the antiferromagnetic susceptibility and the increase of the ferromagnetic resonance linewidth. We argue that the growth of spin fluctuations in the antiferromagnet leads to an enhanced ferromagnetic anisotropy, and therefore coercivity, above the Néel temperature.
Wide bandwidth, large, and tunable polarization mode dispersions in multilayered omnidirectional reflectors
Authors: Wang Zheng , Miller David A. B. , Fan Shanhui
Published: July 2002

We numerically demonstrate the generation of large, broadband, and tunable polarization mode dispersion (PMD) with omnidirectionally reflecting dielectric stacks with embedded cavities. Both first-order and second-order polarization mode dispersion can be generated by similar configurations. Our examples show that the first-order PMD can be tuned from 0 to 30 ps over an 80 GHz bandwidth and second-order PMD from 0 to 50 ps/nm can be generated over a 100 GHz bandwidth. We also show that in our configuration there is no significant modal distortion of the light beams when the beam diameters exceed 3.5 mm.
Systematic examination of carrier polarity in composition spread ZnO thin films codoped with Ga and N
Authors: Tsukazaki A. , Saito H. , Tamura K. , Ohtani M. , Koinuma H. , Sumiya M. , Fuke S. , Fukumura T. , Kawasaki M.
Published: July 2002

We have grown high-crystallinity ZnO thin films on lattice-matched ScAlMgO4 substrates by pulsed-laser deposition with doping donor (Ga) and acceptor (N) simultaneously. Alternating ablation of ceramics with concentrated Ga addition and highly pure single crystal targets yielded in a controlled Ga concentration (CGa) in a wide range of 1018–1020 cm−3 with minimal contamination of undesired impurities such as Al and Si. The use of the originally developed temperature-gradient method, where controlled and continuous gradient of the growth temperature is given to the single substrate with a range of about 50–200 °C, results in a continuous spread of N concentration (CN) in a controlled fashion. Therefore, the ratio of CN/CGa can be varied continuously in a wide range for each film, assuring that a region satisfying p-type codoping condition predicted by T. Yamamoto and H. K. Yoshida [Jpn. J. Appl. Phys., Part 2 38, L166 (1999)] is included in the sample. The electrical properties were measured for over thousand specimens of lithographically patterned Hall bars without observing any sign of p-type conduction.
Thermal conductance and electron-phonon coupling in mechanically suspended nanostructures
Authors: Yung C. S. , Schmidt D. R. , Cleland A. N.
Published: July 2002

We have fabricated and characterized the principal thermal properties of a mechanically suspended nanostructure, consisting of a micron-scale suspended GaAs island, upon which we have defined superconductor-insulator-normal metal tunnel junctions. The tunnel junctions allow for sensitive thermometry and heating of the electrons in a thermally isolated normal metal element, permitting the determination of the low-temperature thermal conductance of the legs that support the GaAs island, as well as the low-temperature electron-phonon coupling. This device forms the basis of a nanoscale bolometric detector, whose optical performance can be estimated from these measurements.
Electrical conductivity studies of chemical vapor deposited sulfur-incorporated nanocomposite carbon thin films
Authors: Gupta S. , Martı́nez A. , Weiner B. R. , Morell G.
Published: July 2002

The room-temperature electrical conductivity of sulfur-incorporated nanocomposite carbon (n-C:S) thin films synthesized by hot-filament chemical vapor deposition was investigated as a function of the sulfur concentration. The films were prepared using a 2% CH4/H2 gas mixture and H2S as the dopant source. The n-C:S films exhibited an increase in conductivity by four orders of magnitude (up to 160 S cm−1) with an increase in sulfur content compared with those grown without sulfur (n-C). The films grown at the highest [H2S] possess the highest carrier concentration (∼5×1019/cm3) and the lowest carrier mobility (0.005 cm2 V−1 s−1). These findings are discussed in terms of the multiple roles of sulfur atoms in the films: (i) induction of structural defects and their corresponding midgap states, (ii) enhancement of the spatial connectivity of the sp2-bonded carbon network, and (iii) electrical activity of only a small fraction of the sulfur atoms.
Energy gap and band alignment for (HfO2)x(Al2O3)1−x on (100) Si
Authors: Yu H. Y. , Li M. F. , Cho B. J. , Yeo C. C. , Joo M. S. , Kwong D.-L. , Pan J. S. , Ang C. H. , Zheng J. Z. , Ramanathan S.
Published: July 2002

High-resolution x-ray photoelectron spectroscopy (XPS) was applied to characterize the electronic structures for a series of high-k materials (HfO2)x(Al2O3)1−x grown on (100) Si substrate with different HfO2 mole fraction x. Al 2p, Hf 4f, O 1s core levels spectra, valence band spectra, and O 1s energy loss all show continuous changes with x in (HfO2)x(Al2O3)1−x. These data are used to estimate the energy gap (Eg) for (HfO2)x(Al2O3)1−x, the valence band offset (ΔEν) and the conduction band offset (ΔEc) between (HfO2)x(Al2O3)1−x and the (100) Si substrate. Our XPS results demonstrate that the values of Eg, ΔEν, and ΔEc for (HfO2)x(Al2O3)1−x change linearly with x.
Aharonov-Bohm oscillation of resistance observed in a ferromagnetic Fe-Ni nanoring
Authors: Kasai S. , Niiyama T. , Saitoh E. , Miyajima H.
Published: July 2002

The magnetoresistance of a ferromagnetic Fe–Ni nanoring with 420 nm in inner diameter and 500 nm in outer diameter was measured at very low temperatures below 100 mK. When a magnetic field is applied to the ring, the magnetoresistance exhibits oscillations whose period varies according to the field direction. This demonstrates the Aharonov–Bohm effect of the conduction electrons in the ferromagnetic nanoring. The amplitude of the oscillations can be regarded from the viewpoint of competition between the system size and the phase coherence length of the conduction electrons.
Generation of continuous and pulsed diagnostic imaging x-ray radiation using a carbon-nanotube-based field-emission cathode
Authors: Yue G. Z. , Qiu Q. , Gao Bo , Cheng Y. , Zhang J. , Shimoda H. , Chang S. , Lu J. P. , Zhou O.
Published: July 2002

X-ray radiation is widely used in medical and industrial applications. The basic design of the x-ray tube has not changed significantly in the last century. In this paper, we demonstrate that medical diagnostic x-ray radiation can be generated using a carbon nanotube (CNT)-based field-emission cathode. The device can readily produce both continuous and pulsed x-rays with a programmable wave form and repetition rate. A total emission current of 28 mA was obtained from a 0.2 cm2 area CNT cathode. The x-ray intensity is sufficient to image a human extremity at 14 kVp and 180 mAs. Pulsed x-ray with a repetition rate greater than 100 kHz was readily achieved by programming the gate voltage. The CNT-based cold-cathode x-ray technology can potentially lead to portable and miniature x-ray sources for industrial and medical applications.
Photoluminescence stabilization of anodically-oxidized porous silicon layers by chemical functionalization
Authors: Boukherroub Rabah , Wayner Danial D. M. , Lockwood David J.
Published: July 2002

Electrochemical oxidation of porous silicon (PSi) produces a surface that is covered with native silicon–hydrogen (Si–Hx) bonds and regions with oxidized Si–Si back-bonds (OSi–Hx). Such anodically oxidized PSi layers were chemically modified using 1-decene under thermal conditions. The hydrosilylation reaction consumes mainly the nonoxidized Si–Hx bonds and yields a surface with oxidized and alkylated regions that were characterized using transmission IR and Raman spectroscopies. The brightest photoluminescence (PL) was obtained when the PSi sample was anodized in 1 M sulfuric acid (H2SO4) at 3 mA/cm2 for 5 min. The chemical process preserves the PL and the physical properties of the porous layer. The derivatized PSi surfaces are stable in boiling CCl4 and in water.
Chemical interaction between atomic-layer-deposited HfO2 thin films and the Si substrate
Authors: Cho Moonju , Park Jaehoo , Park Hong Bae , Hwang Cheol Seong , Jeong Jaehack , Hyun Kwang Soo
Published: July 2002

HfO 2 thin films were deposited on Si wafers using an atomic layer deposition technique at temperatures ranging from 200 to 400 °C with HfCl4 as the precursor and H2O as the oxidant. The time-dependent interfacial-layer growth behavior was dependent on the deposition temperature. The interfacial layer grew with increasing deposition time at 200 °C. However, the film thicknesses decreased with increasing deposition time after reaching a certain maximum value at 300 and 400 °C due to the enhanced dissolution of SiOx into the growing films at these temperatures. Post-annealing at 800 °C under a N2 atmosphere resulted in the precipitation of a Si-rich interfacial layer even for the initially interfacial layer free films. This had the effect of reducing the capacitance density of the films.
Dark decay of holograms in photorefractive polymers
Authors: Bittner Reinhard , Meerholz Klaus , Steckman Gregory , Psaltis Demetri
Published: July 2002

The decay of holograms stored in photorefractive polymer composites based on poly(N-vinyl-carbazole) with and without extrinsic deep traps is investigated. The photorefractive phase shift is identified as one of the key parameters determining the dark decay dynamics. This has important implications for all kinds of photorefractive imaging applications including holographic data storage. A trade off will be required between accepting a certain degree of hologram distortion due to two-beam coupling on the one hand and achieving high hologram stability during idle periods in the dark with the external field applied on the other.
Single-electron transistor as a radio-frequency mixer
Authors: Knobel R. , Yung C. S. , Cleland A. N.
Published: July 2002

We demonstrate the use of the single-electron transistor as a radio-frequency mixer, based on the nonlinear dependence of current on gate charge. This mixer can be used for high-frequency, ultrasensitive charge measurements over a broad and tunable range of frequencies. We demonstrate operation of the mixer, using a lithographically defined thin-film aluminum transistor, in both the superconducting and normal states of aluminum, over frequencies from 10 to 300 MHz. We have operated the device both as a homodyne detector and as a phase-sensitive heterodyne mixer. We demonstrate a charge sensitivity of <4×10−3 e/Hz, limited by room-temperature electronics. An optimized mixer has a theoretical charge sensitivity of ≲1.5×10−5 e/Hz.
Enhancement of the optical nonlinearity in pseudoisocyanine J aggregates embedded in distributed feedback microcavities
Authors: Sasaki F. , Kobayashi S. , Haraichi S.
Published: July 2002

Coherent transients of pseudoisocyanine J aggregates in distributed feedback microcavities are reported. Under the resonant excitation of the cavity-mode observed at the low-energy side of the excitonic resonance, the response time is about 0.5 ps, approximately equal to the pump–pulse duration. We find the enhancement of the optical nonlinearity by one order of magnitude in the microcavity. The excitation of the dielectric-band mode is effective to enhance the nonlinearity with the ultrafast response.
Effects of absorption on coherence domain path length resolved dynamic light scattering in the diffuse regime
Authors: Petoukhova Anna L. , Steenbergen Wiendelt , van Leeuwen Ton G. , de Mul Frits F. M.
Published: July 2002

A low coherence Mach–Zehnder interferometer is developed for path length resolved dynamic light scattering in highly turbid media. The path length distribution of multiply scattered photons in Intralipid is changed by the addition of absorbing dyes. Path length distributions obtained for various absorption levels are shown to be mutually related by Lambert–Beer’s law. Furthermore, the broadening of path length resolved heterodyne light beating spectra of media with identical scattering properties is shown to be independent of the absorption coefficient.
Metal–insulator transition at room temperature and infrared properties of Nd0.7Eu0.3NiO3 thin films
Authors: Capon F. , Laffez P. , Bardeau J.-F. , Simon P. , Lacorre P. , Zaghrioui M.
Published: July 2002

Nd 0.7 Eu 0.3 NiO 3 thin films are deposited by rf sputtering and subsequent oxygen pressure annealing on (100) oriented silicon substrate. We characterize the thermochromic properties of films by measuring electrical transition, infrared transmittance, and reflectance. The thermochromic effect at room temperature is observed. Resistivity measurements exhibit a sharper hysteresis loop than is usually observed in NdNiO3 thin films. Infrared properties in the 8–14 μm wavelength range spectra reveal a contrast of 30% in reflectance and 55% in transmittance.
Visible photoluminescence in amorphous ABO3 perovskites
Authors: Pizani P. S. , Basso H. C. , Lanciotti F. , Boschi T. M. , Pontes F. M. , Longo E. , Leite E. R.
Published: July 2002

The photoluminescence observed in ABO3 type perovskite in their highly structural disordered state can be explained by a model in which is assumed a distribution of electronic states localized within the energy band gap coupled to lattice local vibrational states. The model fits very well the experimental results and indicates that photoluminescence in the visible region can be considered as a general behavior of disordered solids.
Tunneling magnetoresistance of Co clusters in MgF2
Authors: Hackenbroich B. , Zare-Kolsaraki H. , Micklitz H.
Published: July 2002

Samples of Co clusters embedded in a MgF2 matrix have been prepared by the codeposition of well-defined Co clusters (mean diameter L≃4.5 nm) and MgF2 molecules onto a cold substrate. This system shows a temperature-dependent tunneling magnetoresistance (TMR) which has a value of 34.7(2)% at 2.5 K, the largest TMR value reported for a granular Co system. This large TMR value either is due to an antiferromagnetic coupling of those Co clusters between which the tunneling occurs or is caused by an enhanced spin-polarization of the tunneling electrons. The latter may be caused by a cluster–surface/matrix interaction leading to dominated d-electron tunneling due to Co(3d)–F(2p) hybridization.
Effects of nonequilibrium atmospheric pressure plasmas on the heterotrophic pathways of bacteria and on their cell morphology
Authors: Laroussi Mounir , Richardson J. Paul , Dobbs Fred C.
Published: July 2002

To date, most research on the interaction of nonequilibrium, atmospheric pressure plasma discharges with bacteria has concentrated on the germicidal effects. Therefore, published results deal mainly with killing efficacy and little attention is given to physical mechanisms and biochemical pathways and their potential alterations when cells of microorganisms are exposed to the plasma. In this letter, an attempt to investigate the effects of plasma exposure on the biochemical pathways of bacteria is presented. In addition, using electron microscopy, we investigate if any gross morphological changes take place when cells are exposed to a lethal dose of plasma. We are testing the hypothesis that disruption of the cell membrane, sometimes to the point of cell lysis, is the mechanism whereby plasma kills cells.