bluuebunny/crossref_metadata_2025 huggingface dataset:
Synthesis and optical properties of II-O-VI highly mismatched alloys
Authors: Yu K. M. , Walukiewicz W. , Shan W. , Wu J. , Beeman J. W. , Scarpulla M. A. , Dubon O. D. , Becla P.
Published: May 2004

We have synthesized ternary and quaternary diluted II-VI oxides using the combination of O ion implantation and pulsed laser melting. CdOxTe1−x thin films with x up to 0.015, and the energy gap reduced by 150 meV were formed by O+-implantation in CdTe followed by pulsed laser melting. Quaternary Cd0.6Mn0.4OxTe1−x and Zn0.88Mn0.12OxTe1−x with mole fraction of incorporated O as high as 0.03 were also formed. The enhanced O incorporation in Mn-containing alloys is believed to be due to the formation of relatively strong Mn-O bonds. Optical transitions associated with the lower (E−) and upper (E+) conduction subbands resulting from the anticrossing interaction between the localized O states and the extended conduction states of the host are clearly observed in these quaternary diluted II-VI oxides. These alloys fulfill the criteria for a multiband semiconductor that has been proposed as a material for making high efficiency, single-junction solar cells.
Exchange bias of MnPt/CoFe films prepared by ion beam deposition
Authors: Rickart M. , Freitas P. P. , Trindade I. G. , Barradas N. P. , Alves E. , Salgueiro M. , Muga N. , Ventura J. , Sousa J. B. , Proudfoot G. , Pearson D. , Davis M.
Published: May 2004

We report on exchange bias of Mn100−xPtx/Co90Fe10 bottom-pinned bilayers prepared by ion beam deposition. The Pt content in the film was varied 6 at. % with x between 46 and 52 at. % by changing the angle of substrate relative to the target in a range of 40°. Exchange coupling for a Mn100−xPtx(20 nm)/Co90Fe10(5 nm) bilayer was found to be maximum (Jex=0.34 erg/cm2) at a composition with x=50 at. %. Structure and magnetic properties of the bilayers deposited on Ta, Ta/Ru, and Ta/Ni81Fe19 seed layers were studied by vibrating sample magnetometry, x-ray diffraction, Rutherford backscattering spectroscopy, and atomic force microscopy. Exchange bias and coercive field can be tuned as functions of the seed layer. The effect of different annealing conditions on exchange bias is discussed to improve thermal stability of the bilayers. Maximum exchange coupling is obtained at an annealing temperature TA⩾280 °C, while the blocking temperature TB keeps increasing with annealing up to TA=420 °C.
Radio-frequency-induced transport of Cooper pairs in superconducting single electron transistors in a dissipative environment
Authors: Lotkhov S. V. , Bogoslovsky S. A. , Zorin A. B. , Niemeyer J.
Published: May 2004

We investigate low-temperature and low-voltage-bias charge transport in a superconducting Al single electron transistor in a dissipating environment, realized as on-chip high-ohmic Cr microstrips. In samples with relatively large charging energy Ec>EJ, where EJ is the Josephson coupling energy, two transport mechanisms were found to be dominant, both based on discrete tunneling of individual Cooper pairs: Depending on the gate voltage Vg, either sequential tunneling of pairs via the transistor island [in the conducting state of the transistor around the points Qg≡CgVg=e mod (2e), where Cg is the gate capacitance] or their cotunneling through the transistor (for Qg away of these points) was found to prevail in the net current. As the conducting state of our transistors had been found to be subject to quasiparticle poisoning, high-frequency gate cycling (at f∼1 MHz) was applied to study the sequential tunneling mechanism. A simple model based on the master equation was found to be in a good agreement with the experimental data.
Local conductivity and surface photovoltage variations due to magnesium segregation in p-type GaN
Authors: Simpkins B. S. , Yu E. T. , Chowdhury U. , Wong M. M. , Zhu T. G. , Yoo D. W. , Dupuis R. D.
Published: May 2004

Conductive atomic force microscopy (C-AFM) and surface photovoltage (SPV) microscopy were used to investigate local electronic structure in p-type GaN. C-AFM imaging revealed locally reduced forward- and reverse-bias conductivity near threading dislocations. In addition, regions near threading dislocations demonstrated significantly enhanced surface photovoltage response when compared to regions away from dislocations. Analytical treatment of the surface photovoltage as a function of pertinent material properties indicated that reduced background dopant concentration is the most likely cause for the increased SPV. Both reduced conductivity and enhanced surface photovoltage are shown to be consistent with Mg segregation to dislocation cores that results in regions of locally decreased electrically active Mg concentration surrounding the dislocations.
Evaluation of ytterbium doped strontium barium niobate as a potential tunable laser crystal in the visible
Authors: Ramı́rez M. O. , Jaque D. , Ivleva L. , Bausá L. E.
Published: May 2004

A first spectroscopic characterization of Yb3+ ions in Sr0.6Ba0.4(NbO3)2 nonlinear crystal is presented. From low temperature emission and absorption spectra the energy level diagram of Yb3+ ions in this crystal has been built up. The ground state splitting has been found to be as high as 940 cm−1 allowing for potential quasi-four-level laser operation. In order to assess the ability of this system as a tunable solid state laser the main parameters of relevance in laser dynamics have been obtained and compared to those reported for other Yb3+ doped lasers. Optical microscopy has been used to study the ferroelectric domains appearing in the as-grown crystals. A relatively broad distribution of domain sizes (around 3 μm) will, in principle, allow for broadband noncritical second-harmonic generation of fundamental radiation by quasiphase matching. The obtained results make this crystal a promising candidate for tunable laser generation in the visible domain.
Bragg attenuation length in metallo-dielectric photonic band gap materials
Authors: Neve-Oz Y. , Golosovsky M. , Davidov D. , Frenkel A.
Published: May 2004

We analyze the ability of the Bragg reflector model to account for the properties of metallo-dielectric photonic crystals. We perform mm-wave transmission measurements on photonic structure consisting of conducting spheres in air and compare our results to the model predictions. In particular, we verify the universal relation between the Bragg attenuation length and the width of the photonic stopband, which is predicted by the Bragg reflector model.
Control of conductivity type in undoped ZnO thin films grown by metalorganic vapor phase epitaxy
Authors: Ma Y. , Du G. T. , Yang S. R. , Li Z. T. , Zhao B. J. , Yang X. T. , Yang T. P. , Zhang Y. T. , Liu D. L.
Published: May 2004

The properties of the ZnO thin films prepared by metalorganic vapor phase epitaxy under various oxygen partial pressures were thoroughly studied. It was found that the conduction type in undoped ZnO epilayers could be controlled by adjusting the family VI precursor, oxygen partial pressure during growth. The films were characteristic of n-type conductivity under oxygen partial pressure lower than 45 Pa. With the increase of oxygen content, the crystallinity of the ZnO thin films was degraded to polycrystalline with additional (10–12) orientation and the intrinsic p-type ZnO was produced as the oxygen partial pressure was larger than 55 Pa. The hole concentration and mobility could reach to 1.59×1016 cm−3 and 9.23 cm2 V−1 s−1, and the resistivity was 42.7 Ω cm. The near-band-edge emission and the deep level emission in photoluminescence (PL) spectra at room temperature were influenced strongly by the oxygen partial pressure. Temperature-dependent PL spectra in n-type ZnO films showed a dominant neutral-donor bound exciton emission, while p-ZnO was dominated by neutral-acceptor bound exciton emission. Both peaks increased in intensity with the decrease of the temperature and shifted to the short-wavelength side. The band that originated from zinc vacancies emerged at a temperature lower than 155 K only in the p-type films. The origin of intrinsic p-type conductivity in ZnO thin films might be related to zinc vacancy.
Li insertion into V6O13 battery cathodes studied by soft x-ray spectroscopies
Authors: Schmitt T. , Augustsson A. , Duda L.-C. , Nordgren J. , Höwing J. , Gustafsson T.
Published: May 2004

Changes in the electronic structure of V6O13 on lithium-ion insertion into battery cathodes were studied by soft x-ray absorption (SXA) spectroscopy and resonant soft x-ray emission (SXE) spectroscopy. SXA and resonant SXE spectra were recorded ex situ for cycled battery cathodes discharged to different potentials corresponding closely to distinct lithiated stages (LixV6O13,x=0,1,…,6). Large systematic changes were observed in the vanadium and oxygen x-ray spectra, reflecting the effects of electrochemical reduction associated with the Li-ion insertion. Spectral shape analysis indicates that a large fraction of the vanadium ions have been reduced to V3+ ions for the highest degree of lithiation, x=6. Nevertheless, further lithiation may be possible, in view of the linear development of the vanadium and oxygen bands on charge uptake.
Effective-susceptibility tensor for a composite with ferromagnetic inclusions: Enhancement of effective-media theory and alternative ferromagnetic approach
Authors: Bregar V. B. , Pavlin M.
Published: May 2004

For calculating magnetic properties of a composite usually effective-medium theories are used. However, we show that for a composite with ferromagnetic inclusions such theories, in particular, Maxwell-Garnett equation, give peculiar and unphysical results, such as significant shift of ferromagnetic-resonance frequency with diminishing volume fraction of ferromagnetic inclusions. Starting from ferromagnetic theory we derive a simple expression for the calculation of the effective magnetic susceptibility of a composite and follow with detailed magnetostatic derivation of tensor equivalent of Maxwell-Garnett equation. By demonstrating the equivalence of both derivations we confirm the validity of the expression which we obtained from the ferromagnetic theory. Furthermore, we identify errors leading to unphysical results of effective-medium theories and show the correct application of these theories.
Full-zone k⋅p method of band structure calculation for wurtzite semiconductors
Authors: Beresford R.
Published: May 2004

The full-zone k⋅p method of band structure calculation is extended to crystals of wurtzite structure, point group C6v. The form of the momentum matrix elements as allowed by symmetry is deduced and used to model the dispersion relations throughout the Brillouin zone for AlN, GaN, and InN. By means of a fitting procedure that refers to an empirical pseudopotential calculation of the eigenvalues at selected high-symmetry points, a truncated basis set of 23 states is derived that enables realistic representation of the first ten states, with the remainder serving to encompass all of the deleted remote-band interactions. The result is a relatively small-dimension effective Hamiltonian suitable for full-zone envelope-function applications.
Widely tunable terahertz-wave generation in an organic crystal and its spectroscopic application
Authors: Taniuchi T. , Okada S. , Nakanishi H.
Published: May 2004

Terahertz (THz)-wave generation using difference-frequency mixing in an organic 4-N, N-dimethylamino-4′-N′-methyl-stilbazolium tosylate (DAST) crystal was investigated theoretically and experimentally. We developed a dual-wavelength optical parametric oscillator (OPO) with two KTiOPO4 crystals as the input light source. This oscillator has a tunable range from 1300–1450 nm and is pumped by a diode-pumped, frequency-doubled, Q-switched Nd:YAG laser with a pulse duration of 10 ns. Widely tunable THz waves ranging from 2 to 20 THz were generated from DAST crystals with 0.5- and 1-mm thickness using the OPO. THz output energies of 82 nJ (peak power of 10.3 W) at 11.6 THz and 110 nJ (peak power of 13.8 W) at 19 THz were obtained with a 1-mm-thick DAST crystal. Using a tunable THz-wave system consisting of the THz source and a pyroelectric detector, THz spectroscopic data for a polytetrafluoroethylene sheet were obtained and were comparable to data obtained from a conventional far-infrared spectrometer.
Studies of strong magnetic field produced by permanent magnet array for magnetic refrigeration
Authors: Xu X. N. , Lu D. W. , Yuan G. Q. , Han Y. S. , Jin X.
Published: May 2004

The success of a room temperature magnetic refrigerator (RTMR) depends critically on two essential parts: a high magnetic field and a magnetic refrigerant material with large magnetocaloric effect. A carefully designed hollow cylindrical permanent magnet array (HCPMA) can be used to provide strong magnetic field in the cavity, the magnitude of the resulting static field can be even greater than the remanence magnetization of the magnets comprising a HCPMA. A thorough understanding of the magnetic field distribution will provide an invaluable insight into the design and optimization of HCPMA in the reciprocating and rotary RTMR systems. Here, we show a construction of a 16 piece HCPMA with realistic dimensions and we illustrate the mechanism of generating a high magnetic field in such device. We present an effective way to calculate the field distribution of a permanent magnet array with finite size and an unsymmetrical geometry. Furthermore, detailed numerical results of the magnetic field distribution and its dependence on device dimensions are presented.
Divergence-narrowed external-cavity broad-area laser-diode array
Authors: Zheng Yujin , Gao Xin , Miyajima Hirofumi , Kan Hirofumi
Published: May 2004

We demonstrate a divergence-narrowed broad-area laser-diode array (LDA) with an external-cavity configuration that consists of a beam-transformation system and an off-axis reflecting mirror. Sixteen off-axis external-cavity laser diodes are formed for the high-power LDA. At a drive current of 21.2 A (four times the threshold current), the divergence angle full width at half maximum of the slow axis is reduced from 7.26° without the external cavity to 1.45° with the external cavity. The external-cavity LDA achieves a cw power output of 9.8 W, which is 60% of the free-running LDA.
Magnetic and structural properties of nickel zinc ferrite nanoparticles synthesized at room temperature
Authors: Morrison Shannon A. , Cahill Christopher L. , Carpenter Everett E. , Calvin Scott , Swaminathan Raja , McHenry Michael E. , Harris Vincent G.
Published: May 2004

Nickel zinc ferrite nanoparticles (Ni0.20Zn0.44Fe2.36O4) have been produced at room temperature, without calcination, using a reverse micelle process. Particle size is approximately 7 nm as determined by x-ray powder diffraction and transmission electron microscopy. Saturation magnetization values are lower than anticipated, but are explained by elemental analysis, particle size, and cation occupancy within the spinel lattice. Extended x-ray absorption fine structure analysis suggests that a significant amount of Zn2+, which normally occupies tetrahedral sites, actually resides in octahedral coordination in a zinc-enriched outer layer of the particles. This “excess” of diamagnetic Zn can thus contribute to the overall decrease in magnetism. Further, this model can also be used to suggest a formation mechanism in which Zn2+ is incorporated at a later stage in the particle growth process.
Computer-aided simulation of a rotary sputtering magnetron
Authors: Fan Qi Hua , Gracio J. J. , Zhou Li Qin
Published: May 2004

In the past, computer-aided simulation of sputtering magnetron has been applied mainly to planar cathodes with flat target surfaces. In this work, we have simulated the target erosion profile of a cylindrical rotary magnetron by tracing electron trajectories and predicting ionization distribution. The electric potential is prescribed as a radial function. A fourth-order Runge–Kutta method is used to solve the electron movement equations, and a Monte Carlo method is employed to predict electron/Ar collision. It is shown that the simulation can predict the target erosion with reasonable accuracy.
The high-explosive channel effect: Influence of boundary layers on the precursor shock wave in air
Authors: Tanguay Vincent , Higgins Andrew J.
Published: May 2004

The high-explosive channel effect is investigated to study the dynamics of the precursor shock wave in air when there is no coupling of the precursor with the detonation. This is achieved experimentally by using Detasheet in square channels. It is found that the precursor shock wave initially propagates at the velocity dictated by one-dimensional gasdynamics, but then slows down from its theoretical velocity. In fact, the precursor eventually (after hundreds of channel diameters) reaches a terminal velocity equal to the detonation velocity. It is found that boundary layers are responsible for this effect: shocked gas leaks past the detonation products through the boundary layer and, as a result, the precursor shock slows down. This phenomenon is modeled analytically and the results are found to agree well with experiments.
On the nature of quantum dash structures
Authors: Dery H. , Benisty E. , Epstein A. , Alizon R. , Mikhelashvili V. , Eisenstein G. , Schwertberger R. , Gold D. , Reithmaier J. P. , Forchel A.
Published: May 2004

We describe a theoretical model for the linear optical gain properties of a quantum wire assembly and compare it to the well known case of a quantum dot assembly. We also present a technique to analyze the gain of an optical amplifier using bias dependent room temperature amplified spontaneous emission spectra. Employing this procedure in conjunction with the theoretical gain model, we demonstrate that InAs/InP quantum dash structures have quantum-wire-like characteristics. The procedure was used to extract the net gain coefficient, the differential gain, and the relative current component contributing to radiative recombination.
Dielectric properties of lead lanthanum zirconate stanate titanate antiferroelectric thin films prepared by pulsed laser deposition
Authors: Yao Yingbang , Zhai Jiwei , Chen Haydn
Published: May 2004

Lead lanthanum zirconate stanate titanate (PLZST) thin films of approximately 200 nm in thickness were deposited on Pt-buffered silicon substrates using pulsed laser deposition method. The samples were prepared under different substrate temperatures but with the same oxygen partial pressure of 0.2 mbar and laser fluence of approximately 4 J/cm2. We found that the Pb loss during the deposition process plays an important role in the formation of perovskite phase. Postdeposition annealing under different oxygen pressure, ranging from 0.2 mbar to atmospheric oxygen pressure but for the same annealing duration (15 min), was carried out in order to clarify the effects of the annealing ambient, from the viewpoint of crystallographic structure as well as electrical and dielectric properties. The physical origin is discussed.
Optical and electrical characteristics of p-GaSe doped with Te
Authors: Shigetomi S. , Ikari T.
Published: May 2004

Measurements of the Hall-effect, photoluminescence (PL), and photocurrent (PC) have been made on Te-doped GaSe. The carrier transport for the Hall-effect measurement is dominated by the two acceptor levels at 0.08 and 0.02 eV above the valence band. The two acceptor levels have the same energy positions as the impurity levels in the PC spectra. By comparison with the results of GaSe grown with excess Se atoms, it was found that these acceptor levels are caused by the interstitial Te atoms. Moreover, the PC and PL spectra are dominated by the transition related to the indirect band and show the relaxation of the selection rule.
Polymerization of nitrogen in sodium azide
Authors: Eremets M. I. , Popov M. Yu. , Trojan I. A. , Denisov V. N. , Boehler R. , Hemley R. J.
Published: May 2004

The high-pressure behavior of nitrogen in NaN3 was studied to 160 GPa at 120–3300 K using Raman spectroscopy, electrical conductivity, laser heating, and shear deformation methods. Nitrogen in sodium azide is in a molecularlike form; azide ions N3− are straight chains of three atoms linked with covalent bonds and weakly interact with each other. By application of high pressures we strongly increased interaction between ions. We found that at pressures above 19 GPa a new phase appeared, indicating a strong coupling between the azide ions. Another transformation occurs at about 50 GPa, accompanied by the appearance of new Raman peaks and a darkening of the sample. With increasing pressure, the sample becomes completely opaque above 120 GPa, and the azide molecular vibron disappears, evidencing completion of the transformation to a nonmolecular nitrogen state with amorphouslike structure which crystallizes after laser heating up to 3300 K. Laser heating and the application of shear stress accelerates the transformation and causes the transformations to occur at lower pressures. These changes can be interpreted in terms of a transformation of the azide ions to larger nitrogen clusters and then polymeric nitrogen net. The polymeric forms can be preserved on decompression in the diamond anvil cell but transform back to the starting azide and other new phases under ambient conditions.
Simulation of hot electron and quantum effects in AlGaN/GaN heterostructure field effect transistors
Authors: Braga N. , Mickevicius R. , Gaska R. , Hu X. , Shur M. S. , Khan M. Asif , Simin G. , Yang J.
Published: May 2004

We report on simulations of electrical characteristics of AlGaN/(InGaN)/GaN heterostructure field effect transistors with quantum and hot electron effects taken into account. Polarization charges lead to quantum confinement of electrons in the channel and to the formation of two-dimensional electron gas. The electron quantization leads to the spread of the electron wave function into the barrier and bulk but does not have significant impact on dc electrical characteristics. Hot electrons play an important part in the charge transport by spilling over into the bulk GaN where they are captured by traps. This leads to negative differential conductivity, which is also observed experimentally. The simulation results are in good agreement with measured dc characteristics.
Conductance quantization in deep mesa-etched gate-controlled ballistic electron waveguides
Authors: Raichev O. E. , Debray P.
Published: May 2004

We present a theoretical study of the ballistic conductance in electron waveguides created by deep mesa etching from quantum-well structures with a two-dimensional electron gas (2DEG) in the well. The widths of the waveguides are controlled by gate bias voltages. We consider three different cases: the etched waveguide is completely covered by a gate [continuous gate (CG)], the gate is deposited on top of the waveguide [top gate (TG)], and when the gates are located on the etched side walls [side gate (SG)]. The number and periodicity of the quantized conductance steps, as well as the energy separation of the one-dimensional subbands near the Fermi level are determined as functions of the parameters of the device. The CG device provides a fairly periodic quantized conductance staircase. The highest subband separation is achieved for the TG device etched well below the 2DEG layer and for the SG device etched slightly below it.
Poole-Frenkel electron emission from the traps in AlGaN/GaN transistors
Authors: Mitrofanov Oleg , Manfra Michael
Published: May 2004

Defect-related localized electronic states in AlGaN/GaN transistors give rise to commonly observed charge trapping phenomena. Electron dynamics through the trapping centers is strongly affected by electric fields, which can exceed values of 106 V/cm during device operation. The field-assisted emission characteristics provide a unique way to determine the physical properties of the trapping centers. We present a detailed study of the effects of electric field and temperature on the rate of electron emission from the barrier traps in AlGaN/GaN high-electron-mobility transistors. We demonstrate that for temperatures above 250 K, the Poole-Frenkel (PF) emission is the dominant mechanism for electrons to escape from the trapping centers. The emission rate increases exponentially with the square root of the applied field consistent with the decrease of the apparent activation energy predicted by the PF model. We find that the observed trapping center is described by the attractive long-range Coulomb potential with the zero-field binding energy of ∼0.5 eV.
Modeling of avalanche multiplication and noise in heterojunction avalanche photodiodes
Authors: Groves C. , David J. P. R. , Rees G. J. , Ong D. S.
Published: May 2004

A simple Monte Carlo model is used to simulate the avalanche process in a multiplication region which incorporates a heterojunction, intended to introduce localization into the ionization process and reduce excess avalanche noise. The results are compared with those of models where the ionization path length distribution is represented by an exponential decay, displaced from the origin by a ballistic dead space. While the latter results depend sensitively on the arbitrary choice of scheme used to evaluate the model parameters, they agree remarkably well with Monte Carlo, considering the simplicity of the model.
Thermodynamic forces in highly curved fluid interfaces
Authors: Lovett Ronald , Baus Marc
Published: May 2004

The identification of the force distribution in curved interfaces as a thermodynamic force [Baus and Lovett, J. Chem. Phys. 101, 377 (1995)] can be interpreted as a relation between the force distribution and the grand canonical free energy difference between two distinct systems. Using this interpretation, molecular expressions are developed for the force distribution in cylindrical and spherical interfaces that remain valid for very highly curved interfaces.
Rotational state-dependent mixings between resonance states of vibrationally highly excited DCO (X̃ 2A′)
Authors: Wei J. , Tröllsch A. , Tesch C. , Temps F.
Published: May 2004

Rotational state-dependent mixings between highly excited resonance states of DCO (X̃ 2A′) were investigated by stimulated emission pumping spectroscopy via a series of intermediate rotational levels in the B̃ 2A′ electronic state of the radical. Two examples for such interactions, between pairs of accidentally nearly degenerate vibrational states at energies of Ev≈6450 and Ev≈10 060 cm−1, respectively, were analyzed in detail. Deperturbations of the measured spectra provided the zeroth-order vibration–rotation term energies, widths, and rotational constants of the states and the absolute values of the vibrational coupling matrix elements. The coupled states turned out to have very different A rotational constants so that their mixings switch on or off as they are tuned relative to each other as function of the Ka rotational quantum number. The respective zeroth-order states could be assigned to different interlaced vibrational polyads. Thus, when two states belonging to different polyads are accidentally nearly isoenergetic, even very weak interpolyad interactions may start to play important roles. The derived interpolyad coupling elements are small compared to the typical intrapolyad coupling terms so that their influences on the vibrational term energies are small. However, large effects on the widths (i.e., decay rates) of the states can be observed even from weak coupling terms when a narrow, long-lived state is perturbed by a broad, highly dissociative state. This influence contributes to the previously observed strong state-to-state fluctuations of the unimolecular decay rates of the DCO radical as function of vibrational excitation. Similar mechanisms are likely to promote the transition to “statistical” rates in many larger molecules.
The nucleation and growth of silicon thin films on silicate glasses of variable composition using supersonic gas source molecular beam deposition
Authors: Schroeder T. W. , Engstrom J. R.
Published: May 2004

Supersonic molecular beam techniques have been used to study the nucleation and growth of Si thin films on glass surfaces of variable composition using Si2H6 as the precursor to film growth. We have examined, in particular, the early stages of growth using scanning electron microscopy. Making use of molecular beam techniques to control accurately the precursor exposure we have examined trends in the evolution of the Si island density as a function of the composition of the glass, x, in (2⋅SiO2)1−x(Al2O3⋅CaO)x. The silica composition (1−x) for these samples was varied between 0.25 and 0.75, and comparisons were also made to the nucleation of Si on SiO2 thin films made by thermal oxidation and Corning 1737 display glass. We have found that the incubation time τinc varies only weakly with substrate composition, increasing by only a factor of 3 over the range 1−x=0.25–1.0. Examination of a later stage of nucleation and growth, the time for coalescence, τcoal, indicated a stronger dependence on composition, and this metric varied by a factor of 8 over the same range of composition. These results indicate that the intrinsic reactivity of the surface scales with the silica content of the surface. The maximum island density shows a much stronger, superlinear dependence on silica content, increasing by a factor of 15 as 1−x increased from 0.25 to 1.0. For the silica rich compositions, i.e., SiO2 and 1737, Nmax is essentially independent of substrate temperature and the results can be interpreted by a model for nucleation that is purely heterogeneous, and where surface diffusion plays a minimal role. In contrast, on the most silica dilute glass surface (1−x=0.25), Nmax exhibits an Arrhenius temperature dependence with an apparent activation energy of 1.1 eV. Coupled with the observation of a broader island size distribution on this surface, we conclude that surface diffusion plays a role in nucleation and growth on this silica dilute surface, possibly via Ostwald ripening.
The role of the catalytic particle in the growth of carbon nanotubes by plasma enhanced chemical vapor deposition
Authors: Ducati C. , Alexandrou I. , Chhowalla M. , Robertson J. , Amaratunga G. A. J.
Published: May 2004

Vertically aligned carbon nanotubes were synthesized by plasma enhanced chemical vapor deposition using nickel as a metal catalyst. High resolution transmission electron microscopy analysis of the particle found at the tip of the tubes reveals the presence of a metastable carbide Ni3C. Since the carbide is found to decompose upon annealing at 600 °C, we suggest that Ni3C is formed after the growth is stopped due to the rapid cooling of the Ni-C interstitial solid solution. A detailed description of the tip growth mechanism is given, that accounts for the composite structure of the tube walls. The shape and size of the catalytic particle determine the concentration gradient that drives the diffusion of C atoms across and though the metal.
Effect of alkaline-earth and transition metals on the electrical transport of double perovskites
Authors: Poddar A. , Das S. , Chattopadhyay B.
Published: May 2004

The transport properties of A2BMoO6 (A=Ca, Sr, and Ba; B=Cr, Mn, and Fe) crystals are investigated from studies of x-ray diffraction, thermoelectric power (TEP) S, and resistivity ρ measurements over a wide temperature range (20–900 K). The resistivity of A2FeMoO6 (A=Ca, Sr, and Ba) crystals shows a metallic character and its analysis suggests that the electron-electron scattering is one of the contributions governing the conduction mechanism of these compounds. On the other hand, Ca2MnMoO6 shows an insulator-metal transition similar to that reported in rare earth nickelate [Phys. Rev. B 45, 8209 (1992)]. The conductivity σ(T) data of Sr2CrMoO6 can be best fitted in terms of simultaneous contributions to σ with an activated-type process and variable range hopping (VRH) mechanism. In the case of Sr2MnMoO6, ρ(T) behavior follows an activated-type hopping for T>400 K whereas for T<85 K it can be described satisfactorily by VRH mechanism. For all the compounds, TEP exhibits negative values (n type) except for Ca2FeMoO6 which shows a crossover from positive (p type) to negative values (n type) around T=147 K. From the metallic diffusion part of TEP, Fermi energy EF is estimated to be 0.86 eV, 1.2 eV, and 0.8 eV for Sr2FeMoO6, Ca2FeMoO6, and Ca2MnMoO6, respectively.
The effect of grain size on the sensitivity of nanocrystalline metal-oxide gas sensors
Authors: Rothschild Avner , Komem Yigal
Published: May 2004

The effect of grain size on the sensitivity of chemoresistive nanocrystalline metal-oxide gas sensors was evaluated by calculating the effective carrier concentration as a function of the surface state density for a typical sensing material, SnO2, with different grain sizes between 5 and 80 nm. This involved numerical computation of the charge balance equation (the electroneutrality condition) using approximated analytical solutions of Poisson’s equation for small spherical crystallites. The calculations demonstrate a steep decrease in the carrier concentration when the surface state density reaches a critical value that corresponds to a condition of fully depleted grains, namely, when nearly all the electrons are trapped at the surface. Assuming that the variations in the surface state density are induced by surface interactions with ambient gas molecules, these calculations enable us to simulate the response curves of nanocrystalline gas sensors. The simulations show that the conductivity increases linearly with decreasing trapped charge densities, and that the sensitivity to the gas-induced variations in the trapped charge density is proportional to 1/D, where D is the average grain size.
Molecular simulation study of miscibility of ternary and quaternary InGaAlN alloys
Authors: Adhikari Jhumpa , Kofke David A.
Published: May 2004

Molecular simulations are conducted to determine the limits of miscibility of a valence force field model for zinc-blende-structured In1−x−yGaxAlyN semiconductor alloys. The transition matrix Monte Carlo method is used to calculate the free energy of the model alloys as a function of temperature and alloy composition (considering both x and y ranging from zero to unity). Analysis of the free-energy surface provides values for the upper critical solution temperature of the ternary alloys: InGaN (1550 K), InAlN (2700 K), and GaAlN (195 K). The miscibility envelope of the quaternary alloy is determined at 773 K and 1273 K. The excess properties of the mixtures are calculated, and it is found that the excess entropy is negligible, and the excess enthalpy is nearly independent of temperature. Consequently, regular-solution theory provides a good description of the thermodynamic properties of the alloys, and comparison of the simulation results with the phase behavior previously reported using regular-solution theory finds good agreement. Structural properties of the ternary compounds are examined in terms of the local compositions. For InGaN it is found (surprisingly) that there is a slight preference for In atoms to have Ga atoms rather than other In atoms as neighbors, in comparison to a random mixture. The two other ternary compounds exhibit the expected behavior, in which the (small) deviations from random mixing tend to favor segregation of like atoms. Among the ternaries, GaAlN is found to show the greatest deviations from random mixing.
An exact reformulation of the diagonalization step in electronic structure calculations as a set of second order nonlinear equations
Authors: Liang WanZhen , Head-Gordon Martin
Published: May 2004

A new formulation of the diagonalization step in self-consistent-field (SCF) electronic structure calculations is presented. It exactly replaces the diagonalization of the effective Hamiltonian with the solution of a set of second order nonlinear equations. The density matrix and/or the new set of occupied orbitals can be directly obtained from the resulting solution. This formulation may offer interesting possibilities for new approaches to efficient SCF calculations. The working equations can be derived either from energy minimization with respect to a Cayley-type parametrization of a unitary matrix, or from a similarity transformation approach.
Density inhomogeneity and diffusion behavior of fluids in micropores by molecular-dynamics simulation
Authors: Liu Ying-Chun , Wang Qi , Lu Ling-Hong
Published: May 2004

The density profiles and the diffusion behavior of fluid argon confined in micropores were studied by molecular-dynamics simulations. The effects of pore size (width), temperature and number density on the density profiles and the self-diffusion coefficients in micropores were simulated with pore widths from 0.6 to 4.0 nm. The density profiles are greatly affected by the pore size. Strong inhomogeneities in the channel direction and vapor–liquid phase separation in the micropores were observed when initial conditions were chosen in the coexistence region of the fluid. The self-diffusion coefficient in the channel direction in the pores was found to be much lower than in the bulk, and decreasing with decreasing pore size, decreasing temperature, and increasing density.
Analysis of the vibronic fine structure in circularly polarized emission spectra from chiral molecular aggregates
Authors: Spano Frank C. , Zhao Zhen , Meskers Stefan C. J.
Published: May 2004

Using a Frenkel-exciton model, the degree of circular polarization of the luminescence (glum) from one-dimensional, helical aggregates of chromophoric molecules is investigated theoretically. The coupling between the electronic excitation and a local, intramolecular vibrational mode is taken into account. Analytical expressions for the fluorescence band shape and glum are presented for the case of strong and weak electronic coupling between the chromophoric units. Results are compared to those from numerical calculations obtained using the three particle approximation. glum for the 0-0 vibronic band is found to be independent of the relative strength of electronic coupling between chromophores and excitation-vibration coupling. It depends solely on the number of coherently coupled molecules. In contrast, for the higher vibronic transitions |glum| decreases with decreasing strength of the electronic coupling. In the limit of strong electronic coupling, |glum| is almost constant throughout the series of vibronic transitions but for weak coupling |glum| becomes vanishingly small for all vibronic transitions except for the 0-0 transition. The results are interpreted in terms of dynamic localization of the excitation during the zero point vibrational motion in the excited state of the aggregate. It is concluded that circular polarization measurements provide an independent way to determine the coherence size and bandwidth of the lowest exciton state for chiral aggregates.
Centroid molecular dynamics approach to the transport properties of liquid para-hydrogen over the wide temperature range
Authors: Yonetani Yoshiteru , Kinugawa Kenichi
Published: May 2004

Fundamental transport properties of liquid para-hydrogen (p-H2), i.e., diffusion coefficients, thermal conductivity, shear viscosity, and bulk viscosity, have been evaluated by means of the path integral centroid molecular dynamics (CMD) calculations. These transport properties have been obtained over the wide temperature range, 14–32 K. Calculated values of the diffusion coefficients and the shear viscosity are in good agreement with the experimental values at all the investigated temperatures. Although a relatively large deviation is found for the thermal conductivity, the calculated values are less than three times the amount of the experimental values at any temperature. On the other hand, the classical molecular dynamics has led all the transport properties to much larger deviation. For the bulk viscosity of liquid p-H2, which was never known from experiments, the present CMD has given a clear temperature dependence. In addition, from the comparison based on the principle of corresponding states, it has been shown that the marked deviation of the transport properties of liquid p-H2 from the feature which is expected from the molecular parameters is due to the quantum effect.
Colossal magnetoresistance effect of electron-doped manganese oxide thin film La1−xTexMnO3 (x=0.1,0.15)
Authors: Tan Guotai , Zhang X. , Chen Zhenghao
Published: May 2004

In this study, electron-doped manganese oxide thin films La1−xTexMnO3 (x=0.1,0.15) have been fabricated and their magnetic properties and colossal magnetoresistance effect have been investigated. The experimental results show that the thin films have a high metal–insulator transition temperature TMI of ∼300 K and a large magnetoresistance ratio of ∼80% at 4 T. In addition, the resistivity of thin films has a similar change of an on–off function near room temperature and this change might be advantageous in technological application of La1−xTexMnO3 materials.
Brownian dynamics simulation to determine the effective thermal conductivity of nanofluids
Authors: Bhattacharya P. , Saha S. K. , Yadav A. , Phelan P. E. , Prasher R. S.
Published: May 2004

A nanofluid is a fluid containing suspended solid particles, with sizes on the order of nanometers. Normally, nanofluids have higher thermal conductivities than their base fluids. Therefore, it is of interest to predict the effective thermal conductivity of such a nanofluid under different conditions, especially since only limited experimental data are available. We have developed a technique to compute the effective thermal conductivity of a nanofluid using Brownian dynamics simulation, which has the advantage of being computationally less expensive than molecular dynamics, and have coupled that with the equilibrium Green–Kubo method. By comparing the results of our calculation with the available experimental data, we show that our technique predicts the thermal conductivity of nanofluids to a good level of accuracy.
Half-metallic CrO2 thin films on Pt/TiO2/SiO2/Si substrates by chemical vapor deposition
Authors: Son J. Y. , Bang S. H. , Park J. W. , Jang Y. H. , Cho J. H.
Published: May 2004

We report difference of growth mechanism of half-metallic CrO2 thin films deposited by chemical vapor deposition method on preferentially oriented (111) and (200) Pt/TiO2/SiO2/Si substrates. The CrO2 thin films grown on (111) Pt/TiO2/SiO2/Si substrate were (200) oriented, and highly crystalline rectangular grains were observed. On the other hand, grown on (200) Pt/TiO2/SiO2/Si substrate, the CrO2 thin films were (110) oriented, and wedge-shaped grains were observed. To study the orientational dependence of magnetic properties of CrO2 thin films, ferromagnetic domains were observed with a magnetic force microscope at room temperature and orientational dependences of magnetization were observed using a superconducting quantum interference device magnetometer. While the ferromagnetic domains of both films showed in-plane ferromagnetic easy-axis resulting from shape anisotropy as well as orientation of an easy axis along the in-plane film direction, magnetic domains showed a marked contrast difference at domain boundaries resulting from the different surface topographies.
A magnetic and Mössbauer spectral study of the spin reorientation in NdFe11Ti and NdFe11TiH
Authors: Piquer Cristina , Grandjean Fernande , Isnard Olivier , Pop Viorel , Long Gary J.
Published: May 2004

The ac susceptibilities of NdFe11Ti and NdFe11TiH, both of which crystallize with the I4/mmm tetragonal ThMn12-type structure, have been measured between 20 and 300 K and the results reveal spin reorientations at ∼185 K and at 100±1 K, respectively. The Mössbauer spectra of NdFe11Ti and NdFe11TiH have also been measured between 4.2 and 295 K and fits of the observed line shape profiles indicate that the spin-reorientations in both compounds correspond to a rotation from an axial orientation to a canted or basal orientation upon cooling. Both the low temperature canted or basal magnetic structures and the lowering of the spin-reorientation temperature upon hydrogenation result from the competing neodymium and iron magnetic anisotropies and differing relative second-, fourth-, and sixth-order contributions as a function of temperature and hydrogen content of the neodymium to the magnetic anisotropy energy.
Landscapes and fragilities
Authors: Ruocco G. , Sciortino F. , Zamponi F. , De Michele C. , Scopigno T.
Published: May 2004

The concept of fragility provides a possibility to rank different supercooled liquids on the basis of the temperature dependence of dynamic and/or thermodynamic quantities. We recall here the definitions of kinetic and thermodynamic fragility proposed in the last years and discuss their interrelations. At the same time we analyze some recently introduced models for the statistical properties of the potential energy landscape. Building on the Adam–Gibbs relation, which connects structural relaxation times to configurational entropy, we analyze the relation between statistical properties of the landscape and fragility. We call attention to the fact that the knowledge of number, energy depth, and shape of the basins of the potential energy landscape may not be sufficient for predicting fragility. Finally, we discuss two different possibilities for generating strong behavior.
Intermolecular double-quantum coherences in two-dimensional spectra of binary mixtures in solution. The role of diffusion
Authors: Engelsberg M. , Barros Wilson , Hallwass Fernando
Published: May 2004

Proton NMR two-dimensional 2-D spectra of binary mixtures, obtained with the correlation spectroscopy revamped by asymmetric Z gradient echo detection pulse sequence, were employed to test various assumptions usually adopted to describe the role of diffusion in intermolecular double quantum coherences. When two molecular species, with significantly different diffusivities, are considered, the relative amplitudes of the peaks, and their widths, furnish a stringent test that unveils some inadequacies in standard approximations.
Near-infrared spectra and rovibrational dynamics on a four-dimensional ab initio potential energy surface of (HBr)2
Authors: Castillo-Chará J. , McIntosh A. L. , Wang Z. , Lucchese R. R. , Bevan J. W.
Published: May 2004

Supersonic jet investigations of the (HBr)2 dimer have been carried out using a tunable diode laser spectrometer to provide accurate data for comparison with results from a four-dimensional (4-D) ab initio potential energy surface (PES). The near-infrared ν1±, ν2±, and (ν1+ν4)− bands of (H 79Br)2, (H 79Br–H 81Br), and (H 81Br)2 isotopomers have been recorded in the range 2500–2600 cm−1 using a CW slit jet expansion with an upgraded near-infrared diode laser spectrometer. The 4-D PES has been calculated for (HBr)2 using second-order Møller–Plesset perturbation theory with an augmented and polarized 6-311G basis set. The potential is characterized by a global minimum occurring at the H bond structure with the distance between the center of masses (CM) of the monomer being RCM=4.10 Å with angles θA=10°, θB=100° and a well depth of 692.2 cm−1, θA is the angle the HBr bond of monomer A makes with the vector from the CM of A to the CM of B, and θB is the corresponding angle monomer B makes with the same CM–CM vector. The barrier for the H interchange occurs at the closed C2h structure for which RCM=4.07 Å, θA=45°, θB=135°, and the barrier height is 73.9 cm−1. The PES was fitted using a linear-least squares method and the rovibrational energy levels of the complex were calculated by a split pseudospectral method. The spectroscopic data provide accurate molecular parameters for the dimer that are then compared with the results predicted on the basis of the 4-D ab initio PES.