bluuebunny/crossref_metadata_2025 huggingface dataset:
Determination of precipitate nucleation and growth rates from ultrasonic harmonic generation
Authors: Cantrell John H. , Yost William T.
Published: July 2002

An analytical model based on precipitate-dislocation interactions is presented that predicts a decrease in ultrasonic harmonic generation during the nucleation of precipitates and an increase during precipitate growth up to the point of precipitate-matrix coherency loss. The opposing effects allow independent, indirect assessments of nucleation and growth rates from a curve fit of the model equation to experimental data. Application to the assessment of S′ precipitation during the artificial aging of AA2024 yields a constant nucleation rate of 5.4×1015 m−3 s−1 and a growth rate increasing as the 1.2 power of time in the range 72–432 min during heat treatment at 463 K.
Influence of sp2 clusters on the field emission properties of amorphous carbon thin films
Authors: Carey J. D. , Forrest R. D. , Khan R. U. A. , Silva S. R. P.
Published: July 2002

The influence of the concentration and size of sp2 carbon clusters on the field emission properties of hydrogenated amorphous carbon thin films is investigated. In combination with electron paramagnetic resonance and optical measurements, it is shown that the trend in the threshold field for emission for films deposited under certain conditions can be explained in terms of improvements in the connectivity between sp2 clusters. These clusters are believed to be located near the Fermi level, and the connectivity is primarily determined by the cluster size and concentration, which in turn is determined by the choice of deposition conditions. Details of the appropriate emission mechanisms for different types of deposited carbon films are discussed.
High resolution x-ray photoemission study of plasma oxidation of indium–tin–oxide thin film surfaces
Authors: Christou V. , Etchells M. , Renault O. , Dobson P. J. , Salata O. V. , Beamson G. , Egdell R. G.
Published: July 2002

The influence of plasma oxidation and other surface pretreatments on the electronic structure of indium–tin–oxide (ITO) thin films has been studied by high resolution x-ray photoemission spectroscopy. Plasma oxidation compensates n-type doping in the near surface region and leads to a reduction in the energy of plasmon satellite structure observed in In 3d core level spectra. In parallel, the Fermi level moves down within the conduction band, leading to a shift to low binding energy for both core and valence band photoemission features; and the work function increases by a value that corresponds roughly to the core and valence band binding energy shifts. These observations suggest that the conduction band of ITO is fixed relative to the vacuum level and that changes of work function are dominated by shifts of the Fermi level within the conduction band.
Self-propagating exothermic reactions between silicon and transition metals of groups IVA–VIA induced by mechanical alloying
Authors: Yen Bing K.
Published: July 2002

Chemical reactions between Si and transition metals of groups IVA–VIA induced by mechanical alloying have been investigated. Depending on the exothermicity, reactions could proceed via the solid-state diffusion mechanism or the mechanically induced self-propagating reaction (MSR) mechanism, which is analogous to that of the combustion synthesis. For reactant systems with high adiabatic temperatures (Tad), such as silicides of Zr and Nb, Ti5Si3, TiSi, and MoSi2, spontaneous reactions proceeded via the MSR mechanism. There existed a threshold Tad of 1850 K, below which silicon–metal systems reacted via the gradual solid-state diffusion mechanism during mechanical alloying. Since morphological observations of the MSR product indicated the occurrence of self-propagating reactions in the presence of a liquid phase, the effect of reactant melting temperatures on the threshold Tad was elucidated with additional experiments involving carbon–metal reactant systems.
Structure genesis and magnetic orderings in compounds of the ThCr2Si2 type
Authors: Gadjiev B. R.
Published: July 2002

It is shown in a phenomenological approach that the symmetry space group I4/mmm of the paramagnetic phase in compounds of the ThCr2Si2 type arises as a result of a structural phase transition from a close-packed paraphase with space group Im3m. It is found that the real magnetic orderings in compounds of the ThCr2Si2 type is described by transition parameters belonging to a single direction, along the line joining the points of maximum symmetry in the Brillouin zone of the I4/mmm group. It is shown that the variations of the modulus of the wave vector are a consequence of a change in the dopant concentration. The spatial dependence of the order parameter in the incommensurate phases is obtained for the corresponding universality classes.
Charging current measurements and charging synchronization in continuous jets
Authors: Nilsson J. , Palm L.
Published: July 2002

The phase relationship between droplet cutoff and stimulation of a continuous jet can be determined by pulsing the charging voltage during a time considerably shorter than the droplet formation period. The resulting charging current varies depending on when the pulse is applied in relation to droplet cutoff. By stepping the charging pulse over the complete droplet formation period the position of the droplet cutoff can be determined. The principle of the method is discussed and a high speed ac method is proposed for performing the measurements. The jets used in the experiments were 9.5 μm in diameter, traveling at around 53 m/s. The droplet formation frequency was around 1 MHz.
Raman scattering studies on single-crystalline bulk AlN under high pressures
Authors: Kuball M. , Hayes J. M. , Prins A. D. , van Uden N. W. A. , Dunstan D. J. , Shi Ying , Edgar J. H.
Published: July 2002

We report on the Raman analysis of wurtzite single-crystalline bulk AlN under hydrostatic pressures up to 10 GPa. The pressure dependence of the AlN phonon frequencies was investigated. Mode Grüneisen parameters of 1.39, 1.57, 1.71, 0.93, and 1.26 were determined for the A1 (TO), E1 (TO), E2 (high), A1 (LO), and the quasi-longitudinal optical phonons, respectively. Recent theoretical calculations underestimate the pressure-induced frequency shift of the AlN phonons by about 20%–30%. Mode Grüneisen parameters of AlN were compared to those of GaN.
Scaling properties and electromigration resistance of sputtered Ag metallization lines
Authors: Hauder M. , Gstöttner J. , Hansch W. , Schmitt-Landsiedel D.
Published: July 2002

Resistivity and electromigration were investigated for thin sputtered Ag films and microstructured Ag lines. Resistivities of thin films were found to be lower compared to copper and follow the prediction of the size effect. Microstructured Ag lines show a high electromigration resistance at accelerated stress measurements. Considering joule heating of the lines, the activation energy for electromigration was found to be 0.58 eV. The extrapolated median lifetime of Ag lines was found to be similar or higher than for Cu lines.
An apparatus for the measurement of internal stress and thermal expansion coefficient of metal oxide films
Authors: Lee Cheng-Chung , Tien Chuen-Lin , Sheu Wean-Shyang , Jaing Cheng-Chung
Published: July 2002

A measuring apparatus based on a phase shifting interferometry technique to determine the mechanical properties of metal oxide films was presented. Thin films were prepared by ion-beam sputter deposition at low substrate temperature. Quantitative determination of the mechanical properties such as the internal stress, biaxial elastic modulus, and thermal expansion coefficient of metal oxide films were investigated. A phase shifting Twyman–Green interferometer with the phase reduction algorithm was setup to measure the temperature-dependent stress in thin films. Two types of circular glass plates, with known Young’s moduli, Poisson’s ratios, and thermal expansion coefficients, were used as coating substrate. The temperature-dependent stress behavior of the metal oxide films was obtained by heating samples in the range from room temperature to 70 °C. The stresses of thin films deposited on two different substrates were plotted against the stress measurement temperature, showing a linear dependence. Four oxide films were reported for their film stresses and thermal expansion coefficients.
k-cosymplectic manifolds and Lagrangian field theories
Authors: de León Manuel , Merino Eugenio , Salgado Modesto
Published: July 2002

A geometrical description of classical field theories of first order is given. The underlying k-cosymplectic structure permits to derive the corresponding field equations.
Fourth order gradient symplectic integrator methods for solving the time-dependent Schrödinger equation
Authors: Chin Siu A. , Chen C. R.
Published: July 2002

We show that the method of splitting the operator eε(T+V) to fourth order with purely positive coefficients produces excellent algorithms for solving the time-dependent Schrödinger equation. These algorithms require knowing the potential and the gradient of the potential. One fourth order algorithm only requires four fast Fourier transformations per iteration. In a one dimensional scattering problem, the fourth order error coefficients of these new algorithms are roughly 500 times smaller than fourth order algorithms with negative coefficient, such as those based on the traditional Forest–Ruth symplectic integrator. These algorithms can produce converged results of conventional second or fourth order algorithms using time steps 5 to 10 times as large. Iterating these positive coefficient algorithms to sixth order also produced better converged algorithms than iterating the Forest–Ruth algorithm to sixth order or using Yoshida’s sixth order algorithm A directly.
Measurement of the energy decay rate for the first vibrational-excited-state in solid parahydrogen
Authors: Li J. Z. , Suzuki M. , Katsuragawa M. , Hakuta K.
Published: July 2002

Energy decay rate of the first vibrational excited state for solid parahydrogen is measured from 4.8 K to 12 K. Measurements are carried out using the transient optothermal lensing spectroscopy with the vibrational excitations via the stimulated Raman scattering. The decay rate is measured to be 2.3×104 s−1 at 4.8 K, and it increases to 6.1×104 s−1 by raising the temperature to 12 K.
Molecular modeling of electron traps in polymer insulators: Chemical defects and impurities
Authors: Meunier M. , Quirke N. , Aslanides A.
Published: July 2002

The presence of space charge in the polymeric insulation of high-voltage cables is thought to be correlated with electric breakdown. However, a direct link between molecular properties, space charge formation and eventual breakdown has still to be established. It is clear that both physical (e.g., conformational disorder) and chemical defects (e.g., broken bonds and impurities) are present in insulating materials and that both may trap electrons. We have shown that by defining the defect energy in terms of the molecular electron affinity, a relationship is established between the electron trap and the molecular properties of the material. In a recent paper [M. Meunier and N. Quirke, J. Chem. Phys. 113, 369 (2000)] we proposed methods that have made it possible to provide estimates of the energy, number and residence times of electrons in conformational traps in polyethylene. Typical physical trap energies are of the order of 0.15 eV and all are less than 0.3 eV. In the present paper we focus on the role of chemical defects, where we expect much deeper traps but at very low concentrations. Following the methodology used in our previous paper we have used density-functional theory to calculate trap energies for a set of chemical impurities and additives commonly found in polyethylene used for high-voltage cable insulation. In an extension of our approach we have estimated the effect of neighboring molecules on the trap energies of such defects. The resulting trap energy-trap density distribution reveals some very deep (>1 eV) traps presumably implicated in the formation of long-lived space charge in polymeric insulators and consequently to changes in the dielectric properties of the material.
Bloch oscillations of magnetic solitons as an example of dynamical localization of quasiparticles in a uniform external field (Review)
Authors: Kosevich A. M.
Published: July 2002

The theory of the oscillatory motion of a band particle or particlelike excitation in a uniform field—the so-called Bloch oscillations—is reviewed. It is explained that this unusual motion is contingent on two circumstances: the time dependence of the motion of the particle under the influence of the external fields is governed by a classical equation of motion (dp/dt=F), and the energy spectrum of the particle is of a band nature, which presupposes a periodic dependence of the energy of the particle on its momentum (quasimomentum) ε=ε(p)=ε(p+p0), where p0, the period in p space, arises in a natural way in the description of the motion in a spatially periodic structure (lattice). Quasiclassical and quantum descriptions of the Bloch oscillations are given. Since a systematic exposition of the theory of this phenomenon has not been set forth in any monographs, the first part of this review gives a rather detailed presentation (with all the basic calculations) of the results on the oscillatory dynamics of an elementary excitation of a one-dimensional discrete chain, including the theory of the motion both in a uniform static field and in a uniform field with a harmonic time dependence. An interpretation is offered for the relationship of the frequency of quasiclassical Bloch oscillations and the equidistant spectrum of energy levels in the so-called Wannier–Stark ladder. An explanation of the physical nature of the phenomenon of dynamical localization of a band particle in a spatially uniform alternating field is given. It is shown that the basic results of such a dynamics carry over to the motion of a dynamical soliton of the discrete nonlinear Schrödinger equation. The second part of this review describes the Bloch oscillations of topological and dynamical magnetic solitons. It is shown that the phenomenological Landau–Lifshitz equations for the magnetization field in a magnetically ordered medium have surprising soliton solutions. The energy of a soliton is a periodic function of its momentum, even though its motion occurs in a continuous medium. The presence of this periodicity is sufficient to explain the Bloch oscillations of magnetic solitons. The quantum oscillatory dynamics of a soliton in a discrete spin chain is described. The review concludes with a discussion of the conditions for this oscillatory motion and the possibilities for its experimental observation.
Heat capacity of methane–krypton solid solutions. Conversion effect
Authors: Minchina I. Ya. , Manzhelii V. G. , Bagatskii M. I. , Sklyar O. V. , Mashchenko D. A. , Pokhodenko M. A.
Published: July 2002

The heat capacity of Kr–nCH4 solid solutions with the concentrations n=1; 5; 10% and of the solid solution Kr–1% CH4–0.2% O2 is studied at 0.7–8 K. The contributions Crot to the heat capacity of the solutions due to the rotation of the CH4 molecules are estimated. The deviations of the measured Crot from the values corresponding to the equilibrium distribution of the CH4 nuclear spin modifications are dependent on the correlation between the characteristic times of conversion and of the calorimetric experiment. The effects of temperature, O2 impurities, and CH4 clusters upon the conversion rate are studied. It is shown that the hybrid mechanism of conversion proposed by Berlinsky and Nijman, which takes into account both intramolecular and intermolecular interactions of the proton spins, is predominant.
Crystal size and temperature measurements in nanostructured silicon using Raman spectroscopy
Authors: Viera G. , Huet S. , Boufendi L.
Published: July 2002

In this work we present a detailed structural characterization by Raman spectroscopy of hydrogenated amorphous silicon (a-Si:H) and of nanostructured silicon (ns-Si:H) thin films grown in radio-frequency plasma. The ns-Si:H thin films, also called polymorphous Si thin films, consist of a two-phase mixture of amorphous and ordered Si. The Raman spectra were measured at increasing laser intensities. Very low laser power densities (∼1 kW/cm2) were used to thoroughly analyze the structure of as-deposited thin films. Higher Raman laser powers were found to induce the crystallization of the films, which was characterized by the appearance of a sharp peak around 500 cm−1. This was attained faster in the ns-Si:H than in the conventional a-Si:H thin films because the silicon-ordered particles cause a heterogeneous nucleation process in which they act as seeds for crystallization. The laser power densities for film crystallization, crystal size, and surface temperature were determined from this Raman analysis. The validity and application ranges of the different models that can be used to calculate these parameters are critically discussed.
Calculating free energies using average force
Authors: Darve Eric , Pohorille Andrew
Published: July 2002

A new, general formula that connects the derivatives of the free energy along the selected, generalized coordinates of the system with the instantaneous force acting on these coordinates is derived. The instantaneous force is defined as the force acting on the coordinate of interest so that when it is subtracted from the equations of motion the acceleration along this coordinate is zero. The formula applies to simulations in which the selected coordinates are either unconstrained or constrained to fixed values. It is shown that in the latter case the formula reduces to the expression previously derived by den Otter and Briels [Mol. Phys. 98, 773 (2000)]. If simulations are carried out without constraining the coordinates of interest, the formula leads to a new method for calculating the free energy changes along these coordinates. This method is tested in two examples — rotation around the C–C bond of 1,2-dichloroethane immersed in water and transfer of fluoromethane across the water-hexane interface. The calculated free energies are compared with those obtained by two commonly used methods. One of them relies on determining the probability density function of finding the system at different values of the selected coordinate and the other requires calculating the average force at discrete locations along this coordinate in a series of constrained simulations. The free energies calculated by these three methods are in excellent agreement. The relative advantages of each method are discussed.
Search for the minimum thermal conductivity in mixed cryocrystals (CH4)1−ξKrξ
Authors: Konstantinov V. A. , Manzhelii V. G. , Revyakin V. P. , Pohl R. O.
Published: July 2002

The isochoric thermal conductivity of (CH4)1−ξKrξ solid solutions is studied between 40 K and ∼150 K over a wide range of concentrations (ξ=0.013, 0.032, 0.07, 0.115, 0.34, 0.71, 0.855, 0.937, and 0.97). A gradual transition from the thermal conductivity of a highly perfect crystal to the minimum thermal conductivity is observed as the crystal becomes increasingly more disordered. A qualitative description is given in the framework of Debye model of thermal conductivity, which takes into consideration the fact that phonon mean free path cannot decrease indefinitely.
Agglomeration and percolation conductivity
Authors: Sieradzki K. , Bailey K. , Alford T. L.
Published: July 2002

We report on results of agglomeration experiments for the Ag/SiO2/Si(100) system. Thin silver films, 100 nm in thickness, were annealed, and their electrical resistance was continuously monitored using a four-point probe technique. Scanning electron microscopy and digital image analysis were used to correlate the time-dependent agglomeration morphology to the sheet resistance of the Ag thin film. Our results indicated that the area fraction of the surface uncovered during agglomeration scaled linearly in time. We found that at fixed annealing temperature, the normalized sheet resistance followed power-law scaling in time, i.e., R∼|t−τ|−μ, (μ=1.25±0.1) where τ corresponds to the time it takes to reach electrical failure of the sample.
FAR-IR/THz radiation from the Jefferson Laboratory, energy recovered linac, free electron laser
Authors: Williams G. P.
Published: July 2002

The free electron laser at Jefferson Laboratory is the first of a new generation of light sources based on a photoinjected energy recovered linac. The present machine has a 40 MeV electron beam and an average current of 5 mA. The electron bunches are extremely short with full width at half maximum values that are in the few hundred femtosecond regime. These electron bunches pass a chicane around the optical cavity, and therefore, emit synchrotron radiation. In the far-IR region, the wavelength of the light being emitted approaches that of the electron bunch length, giving rise to multiparticle coherent enhancement. The result is a broadband spectrum whose average brightness is more than five orders of magnitude higher than can be obtained from conventional incoherent synchrotron IR sources. We will discuss preliminary measurements of this radiation, and applications to spectroscopy and imaging.
Low-temperature thermal expansion of fullerite C60 alloyed with argon and neon
Authors: Aleksandrovskii A. N. , Gavrilko V. G. , Esel’son V. B. , Manzhelii V. G. , Udovidchenko B. G. , Maletskiy V. P. , Sundqvist B.
Published: July 2002

The linear thermal expansion of compacted fullerite C60 alloyed with argon (ArxC60) and neon (NexC60) are investigated by a dilatometric method. The experimental temperature is 2–12 K. In the same temperature interval the thermal expansion of ArxC60 and NexC60 are examined after partial desaturation of the gases from fullerite. It is found that Ar and Ne alloying affects the temperature dependence of the thermal expansion coefficient of C60 quite appreciably. The libration and translation contributions to the thermal expansion of pure C60 are separated. The experimental results on the thermal expansion are used to obtain the Debye temperature of pure C60. The effects observed are tentatively interpreted.
Apparent fluid slip at hydrophobic microchannel walls
Authors: Tretheway Derek C. , Meinhart Carl D.
Published: July 2002

Micron-resolution particle image velocimetry is used to measure the velocity profiles of water flowing through 30×300 μm channels. The velocity profiles are measured to within 450 nm of the microchannel surface. When the surface is hydrophilic (uncoated glass), the measured velocity profiles are consistent with solutions of Stokes’ equation and the well-accepted no-slip boundary condition. However, when the microchannel surface is coated with a 2.3 nm thick monolayer of hydrophobic octadecyltrichlorosilane, an apparent velocity slip is measured just above the solid surface. This velocity is approximately 10% of the free-stream velocity and yields a slip length of approximately 1 μm. For this slip length, slip flow is negligible for length scales greater than 1 mm, but must be considered at the micro- and nano scales.
Optical dispersion, two-photon absorption and self-phase modulation in silicon waveguides at 1.5 μm wavelength
Authors: Tsang H. K. , Wong C. S. , Liang T. K. , Day I. E. , Roberts S. W. , Harpin A. , Drake J. , Asghari M.
Published: July 2002

We measure linear optical dispersion, nonlinear refraction and two-photon absorption in a silicon waveguide at 1.54 μm wavelength. The total dispersion in the silicon waveguide was found to be −9.1 fs/(nm cm). At 1.54 μm wavelength, the two-photon absorption coefficient was found to be 0.45 cm/GW and a π phase shift from self-phase modulation was observed in optical pulses of 60 W peak-coupled power, generated from an amplified gain-switched laser diode.
Full characterization of the interface between the organic semiconductor copper phthalocyanine and gold
Authors: Peisert H. , Knupfer M. , Schwieger T. , Auerhammer J. M. , Golden M. S. , Fink J.
Published: July 2002

We present a study of the interface properties of the molecular organic semiconductor copper phthalocyanine (CuPC) on single crystalline as well as polycrystalline Au using photoemission spectroscopy. Despite the different orientation of the molecules on the two substrates, the observed energy level alignment is identical. We observe the formation of an interface dipole while band bending is very small. In addition, we have carried out complementary studies of the CuPC/GeS(001) interface to pin down photoemission final state effect contributions to the observed energy level shifts.
Assessment of GaN metal–semiconductor–metal photodiodes for high-energy ultraviolet photodetection
Authors: Monroy E. , Palacios T. , Hainaut O. , Omnès F. , Calle F. , Hochedez J.-F.
Published: July 2002

We report on the fabrication and characterization of low dark-current GaN metal–semiconductor–metal (MSM) photodiodes. Their quantum efficiency in the vacuum-ultraviolet range has been analyzed, demonstrating that these devices are an excellent choice for high-energy photodetection. Models to explain and control the performance as a function of residual doping and geometry are applied to GaN-based MSM photodiodes.
Two-section InGaN multiple-quantum-well laser diode with integrated electroabsorption modulator
Authors: Kneissl Michael , Paoli Thomas L. , Kiesel Peter , Treat David W. , Teepe Mark , Miyashita Naoko , Johnson Noble M.
Published: July 2002

Q -switching is demonstrated in a two-section InGaN multiple-quantum-well (MQW) laser diode consisting of an electroabsorption modulator and amplifier (gain) section. The modulator and gain sections are optically coupled and share the same InGaN MQW active region, but they are electrically separated by a narrow dry-etched trench. Applying a reverse bias voltage to the modulator section controls the absorption in the modulator portion of the device by compensating the piezoelectric field in the InGaN quantum wells. Changes in the absorption coefficient as large as 5000 cm−1 were realized with a moderate reverse bias of 7.2 V. By forward biasing, the amplifier section at a constant current of 225 mA and by controlling the reverse bias modulator voltage, the output power of the two-section laser diode could be switched between <0.5 mW (off state) and more than 3 mW (on state) with a laser emission wavelength near 401 nm.
Comparison of heat-affected zones due to nanosecond and femtosecond laser pulses using transmission electronic microscopy
Authors: Le Harzic R. , Huot N. , Audouard E. , Jonin C. , Laporte P. , Valette S. , Fraczkiewicz A. , Fortunier R.
Published: July 2002

This letter presents a method aimed at quantifying the dimensions of the heat-affected zone (HAZ), produced during nanosecond and femtosecond laser–matter interactions. According to this method, 0.1 μm thick Al samples were microdrilled and observed by a transmission electronic microscopy technique. The holes were produced at laser fluences above the ablation threshold in both nanosecond and femtosecond regimes (i.e., 5 and 2 J/cm2, respectively). The grain size in the samples was observed near the microholes. The main conclusion is that a 40 μm wide HAZ is induced by the nanosecond pulses, whereas the femtosecond regime does not produce any observable HAZ. It turns out that the width of the femtosecond HAZ is less than 2 μm, which is our observation limit.
Growth of aligned carbon nanotubes with controlled site density
Authors: Tu Y. , Huang Z. P. , Wang D. Z. , Wen J. G. , Ren Z. F.
Published: July 2002

Pulse-current electrochemical deposition has been used to prepare Ni nanoparticles that are used as the catalysts for the growth of aligned carbon nanotubes. The nucleation site density of the Ni nanoparticles was controlled by changing the magnitude and duration of the pulse current. The aligned carbon nanotubes from the nickel nanoparticles were grown by plasma enhanced hot filament chemical vapor deposition. The site density of the aligned carbon nanotubes varied from 105 to 108 cm−2. The achievement of controlling the site density is significant for applications of carbon nanotubes as field emitters, nanoelectrodes array, etc.
On the grain-boundary phase in iron rare-earth boron magnets
Authors: Ramesh R. , Chen J. K. , Thomas G.
Published: July 2002

Results of structural and microanalytical studies on sintered iron rare-earth boron permanent magnets are presented in this paper. It is shown that the structure of the phase present at the two-grain boundaries is the same as that of the phase at the three-grain junctions; both are fcc with a lattice parameter of 5.24 Å. It is also shown that this fcc phase is stabilized by the presence of significant quantities of oxygen. The possible effects of thin foil preparation by ion milling techniques in producing the observed bcc phase are also briefly discussed.
Growth and magnetic characterization of single-crystal Ni films on ZnSe(001)
Authors: Jonker B. T. , Krebs J. J. , Prinz G. A.
Published: July 2002

We have grown single-crystal films of Ni by molecular-beam epitaxy on ZnSe(001) epilayers which were grown in turn on GaAs(001) substrates. The surface structure and composition were determined by reflection high-energy electron diffraction and Auger electron spectroscopy. X-ray θ-2θ and oscillation/Weissenberg measurements show that the films retain the normal fcc lattice structure and grow in the (001) orientation, despite the large misfit to the substrate. Ferromagnetic resonance and vibrating sample magnetometry were used to obtain the magnetic properties of the samples. The films exhibit a reduced magnetization relative to bulk Ni, a perpendicular component to the anisotropy, and an in-plane uniaxial anisotropy such that the hard in-plane 〈100〉 axes are inequivalent.
Temperature-dependent thermoelectric power of semiconducting bismuth-vanadate glass
Authors: Ghosh Aswini
Published: July 2002

The temperature dependence of the thermoelectric power of the semiconducting bismuth-vanadate glasses was presented in a range of compositions. The high-temperature thermoelectric power was satisfactorily explained by Heikes’ relation [R. R. Heikes and R. W. Ure, Eds., Thermoelectricity (Interscience, New York, 1961), p. 81]. The thermoelectric power data also showed evidence of small polaron formation in the glass and revealed that the disorder energy happened to increase with the increase of V2O5 content in the glass. An estimate of the disorder energy was made from the low-temperature thermoelectric power data.
Freezing of the polarization fluctuations in lead magnesium niobate relaxors
Authors: Viehland Dwight , Jang S. J. , Cross L. Eric , Wuttig Manfred
Published: July 2002

The dielectric relaxation of a solid solution of 10-mol % lead titanate in lead magnesium niobate is found to be similar to the magnetic relaxation in spin-glass systems.1–3 Based on this analogy, it is proposed that the relaxor ferroelectric is a polar-glassy system which has thermally activated polarization fluctuations above a static freezing temperature. An activation energy and freezing temperature of 0.0407 eV and 291.5 K, respectively, were found by analyzing the frequency dependence of the temperature of the dielectric maximum using the Vogel–Fulcher relationship.4,5 It has also been shown that a macroscopic polarization is sustained on heating up to this freezing temperature. A coupling between nanometer scale clusters is believed to control the kinetics of the fluctuations and the development of a frustration as the system freezes into states of local equilibrium. The possibility of an orientational freezing associated with the ferroelastic nature of the nanosized polar regions in the rhombohedral relaxor families as well as a polar freezing is discussed. A diffuse phase transformation is believed to arise due to a dispersion in the fluctuation frequency of the polarization. A qualitative model for the relaxation time spectrum is also proposed in which the width of the spectrum broadens strongly near the freezing temperature.
Formation of R2Fe14C compounds (R=Y, Ce) by rapid solidification
Authors: Fuerst C. D. , Herbst J. F.
Published: July 2002

We report formation of the Nd2Fe14B-type compounds Y2Fe14C and Ce2Fe14C via heat treatment of melt-spun ribbons. The existence of several R2Fe14C (R≡rare earth) phases has been documented in the literature, but the light R compounds are extremely difficult to prepare by conventional procedures. In particular, the yttrium compound was not found in a study of the equilibrium Y-Fe-C phase diagram, and previously available information on Ce2Fe14C was obtained only by extrapolation of data for Mn-substituted materials. We have prepared the Pr, Nd, Sm, Gd, Dy, Er, and Tm carbides as well, and we analyze the behavior of the Curie temperature through the series using a simple mean-field model. The analysis indicates that the R-Fe exchange interaction energy is significantly smaller for the heavy-R, as compared with the light-R, members of the series.
Magnetotransport properties of magnetically soft spin-valve structures (invited)
Authors: Dieny B. , Speriosu V. S. , Metin S. , Parkin S. S. P. , Gurney B. A. , Baumgart P. , Wilhoit D. R.
Published: July 2002

The magnetic and magnetotransport properties of several series of sandwiches consisting of two ferromagnetic layers (Ni, Co, Ni80Fe20) separated by a noble metal (Cu, Ag, Au) are described. In order to vary the relative orientation of the magnetizations of the two ferromagnets, one of them was constrained by exchange anisotropy (e.g., NiFe/Fe50Mn50). The ferromagnetic layers are magnetically soft and not coupled antiparallel, giving very large changes of resistance at low fields. At room temperature relative changes ΔR/R of 4.1% in 10 Oe for Si/Ta 50 Å/NiFe 62 Å/Cu 22 Å/NiFe 40 Å/FeMn 70 Å/Ta 50 Å and 8.7% in 20 Oe has been obtained for a structure based on Co/Cu/Co layers. The magnetoresistance versus the thickness of the ferromagnetic layer shows a broad peak near 80 Å for Ni, Co and NiFe, demonstrating the importance of bulk rather than interfacial spin-dependent scattering, in contrast to Fe/Cr multilayers. The magnetoresistance decreases exponentially with increasing interlayer (Cu and Au) thickness, indicating that the magnetoresistance is due to the exchange of polarized electrons from one ferromagnetic layer to the other. The variation with Ag interlayer thickness is different for structural reasons.
Anisotropic optical properties of YBa2Cu3O7
Authors: Nee Tsu-Wei
Published: July 2002

Based upon the published results of YBa2Cu3O7 single-crystal reflectance spectra measured at room temperature, the empirical anisotropic spectral density functions and quasidielectric functions ε1j+iε2j [j=a,b,c] are determined in the 0.05 to 6 eV spectral range. These functions are an excellent fit to the reflectance spectra of samples in single-crystal, twin-crystal, and polycrystalline forms; they agree well with the published anisotropic transport and ellipsometry data. For polarizations along the a and b axes, the numerically determined empirical spectral density functions have shown two interband absorption peaks at 2.5 and 5 eV. For energy above 3 eV, the spectral density functions do not agree with the published theoretical interband conductivities that are derived from the band structure calculations.
Thin films of Pd/Ni alloys for detection of high hydrogen concentrations
Authors: Hughes R. C. , Schubert W. K.
Published: July 2002

Thin alloy films of Pd and Ni (8%<Ni<20% in at. %) formed by dual-electron-beam evaporation techniques have been found to give durable and quickly reversible detectors of high H2 concentrations (pH2 0.1%–100%, 0.7–700 Torr) near 1 atm and 300 K, including accurate determinations of pH2 around the lower explosive limit of 4% in air. The addition of Ni suppresses the α to β phase transition found in pure Pd under these conditions. The measurement of resistivity changes in the thin films along with flatband shifts of metal-oxide-semiconductor capacitors on the same Si wafer gives accurate values of pH2 over more than six decades.
Models for numerical treatment of multicomponent diffusion in simple phases
Authors: Andersson Jan-Olof , Ågren John
Published: July 2002

A general formalism for multicomponent diffusion in simple phases is presented in some detail. The formalism is mainly based on previous work by various authors. The purpose of the present work is to develop the formalism to such an extent that it is suitable for implementation on a computer. Expressions for the multicomponent diffusion-coefficient matrix is given. The concentration dependence of the kinetic coefficients is discussed in terms of simple models.
Nitrogen donors in 4H-silicon carbide
Authors: Götz W. , Schöner A. , Pensl G. , Suttrop W. , Choyke W. J. , Stein R. , Leibenzeder S.
Published: July 2002

Hall-effect and infrared-absorption measurements are performed on n-type 4H-SiC samples to investigate the energy positions of the ground state and the excited states of the nitrogen donor in the 4H polytype of silicon carbide. Two electrically active levels (Hall effect) and three series of absorption lines (infrared spectra) are assigned to two nitrogen donor species which substitute on the two inequivalent lattice sites (h,k) in 4H-SiC. Valley-orbit splitting of the ground-state level of the nitrogen donors on hexagonal sites (h) is found to be equal to ΔEvo(h)=7.6 meV. It is shown that the energy position of excited states of both nitrogen donors can be calculated by the effective-mass approximation by assuming anisotropic effective masses m⊥=0.18m0 and m∥=0.22m0. The influence of the two inequivalent lattice sites on the values of ionization energy and valley orbit splitting of the nitrogen donor ground-state levels is discussed.
Electrical and optical properties of TiO2 anatase thin films
Authors: Tang H. , Prasad K. , Sanjinès R. , Schmid P. E. , Lévy F.
Published: July 2002

Electrical and optical spectroscopic studies of TiO2 anatase thin films deposited by sputtering show that the metastable phase anatase differs in electronic properties from the well-known, stable phase rutile. Resistivity and Hall-effect measurements reveal an insulator–metal transition in a donor band in anatase thin films with high donor concentrations. Such a transition is not observed in rutile thin films with similar donor concentrations. This indicates a larger effective Bohr radius of donor electrons in anatase than in rutile, which in turn suggests a smaller electron effective mass in anatase. The smaller effective mass in anatase is consistent with the high mobility, bandlike conduction observed in anatase crystals. It is also responsible for the very shallow donor energies in anatase. Luminescence of self-trapped excitons is observed in anatase thin films, which implies a strong lattice relaxation and a small exciton bandwidth in anatase. Optical absorption and photoconductivity spectra show that anatase thin films have a wider optical absorption gap than rutile thin films.
1/f noise in giant magnetoresistive materials
Authors: Hardner H. T. , Parkin S. S. P. , Weissman M. B. , Salamon M. B. , Kita E.
Published: July 2002

The main source of 1/f resistance noise in materials exhibiting giant magnetoresistance (GMR) is the GMR effect itself. Large 1/f noise from fluctuations in the parallel vs antiparallel alignment appears when ‖dR/dH‖ is large. We have studied this noise as a function of the number of Co/Cu bilayers in multilayer samples and discuss the implications for devices. Barkhausen noise in the resistance as the field is swept through the GMR transition is used to estimate coherent domain sizes between 0.1 and 10 μm2 for a single Co layer in the multilayers. GMR noise is also seen in a Co/Ag granular system.
Laser ablation synthesis of lanthanide oxide clusters: Mechanisms and chemistry
Authors: Gibson John K.
Published: July 2002

Excimer laser ablation into vacuum of hydrated lanthanide oxalates has produced new lanthanide (Ln) oxide cluster ions which were identified by time-of-flight mass spectrometry. In addition to binary oxide clusters (LnmO+n), mixed lanthanide oxide clusters [Lnm1Lnm2′O+n with (m1+m2)≤9] were discerned for the following Ln-Ln′: La-Tb, La-Ho, La-Lu, and Ho-Lu. The observed cluster ion stoichiometries, abundance distributions, and hydration systematics provide insights into cluster formation mechanisms and chemistries. Time-variable ion sampling revealed cluster enhancement in the tail of the ablation plume. The body of experimental results support cluster formation by aggregation of small ablated species.
The electromigration short-length effect in Ti-AlCu-Ti metallization with tungsten studs
Authors: Filippi R. G. , Biery G. A. , Wachnik R. A.
Published: July 2002

The electromigration short-length effect has been investigated by testing a two-level structure with Ti-AlCu-Ti stripes and interlevel tungsten (W) stud vias. This investigation represents a complete study of the short-length effect using a technologically realistic test structure. Lifetime measurements and resistance changes as a function of time were used to describe this phenomenon, where the latter approach provides new insights into the electromigration behavior of multilayered metallizations. A linear increase in resistance was followed by a resistance change with time that approached zero. For the same product of current density and stripe length, longer stripes increased in resistance to higher values than shorter stripes. The sigma of the lognormal distribution increased as the current density decreased and/or as the maximum allowed resistance change increased. The lifetime, or t50, at relatively small current densities did not obey Black’s empirical equation. Rather, the lifetime data obeyed a modified version of this equation that includes a critical current density jc as a new parameter. As an alternate approach to quantifying the short-length effect, we propose a novel and practical model for determining jc that focuses on the apparent saturation of the resistance increase with time of the W stud chains. Unlike the modified Black’s model, the resistance saturation approach allows one to estimate lower bound limits of jc. The threshold product, (jL)c, is determined from the modified Black’s model and from the resistance saturation model for stripe lengths of 50, 70, and 100 μm. Both models indicate that jc strongly depends on the fail criterion, or the magnitude of the resistance change, but is independent of temperature in the range 175–250 °C. No evidence was found of a (jL)c below which no electromigration-induced damage occurs.
Overview of Nd–Fe–B magnets and coercivity (invited)
Authors: Fidler J. , Schrefl T.
Published: July 2002

High performance Nd2Fe14B-based permanent magnets are produced with different composition and various processing techniques. The composition and the processing route influence the complex, multiphase microstructure of the magnets, such as grain size, alignment, and distribution of phases. Grain sizes in the range between 10 and 500 nm are obtained by melt spinning, mechanical alloying, and the HDDR process. Sintered and hot worked magnets exhibit grain sizes above 1 μm. The coercive field is determined by the high uniaxial magnetocrystalline anisotropy as well as the magnetostatic and exchange interactions between neighboring hard magnetic grains. The dipolar interactions between misaligned grains are more pronounced in large-grained magnets, whereas exchange coupling reduces the coercive field in small grained magnets. Transmission electron microscopy has been used to study the influence of substituent and dopant elements on microstructure, coercivity, and corrosion resistance of advanced (Nd,S1)–(Fe,S2)–B:(M1,M2) magnets. The replacement of the Nd-rich intergranular phase by secondary phases formed after doping by M1 and M2 type elements improves the corrosion resistance, especially in large-grained magnets. Secondary, nonmagnetic phases reduce the remanence and the energy product. In addition to the characterization of the microstructure, special attention has been paid to the computer modeling of the interaction between microstructure and coercivity. The simulation of the magnetization reversal process based on the real microstructure reveals a good agreement with experimental values. It is shown that the coercive field depends on grain size, distribution, and misorientation of grains. A strong exchange coupling between hard magnetic grains is desired in nanostructured magnets in order to improve the remanence. This effect is further increased by secondary, soft magnetic phases. Nanocrystalline, composite Nd–Fe–B based magnets show a remanence enhancement, both in experiments and in model calculations.
Pulse phase infrared thermography
Authors: Maldague X. , Marinetti S.
Published: July 2002

An approach is proposed which combines simultaneously advantages both of pulse (PT) and modulated infrared thermography. In a nondestructive evaluation perspective, the specimen is pulse heated as in PT and the mix of frequencies of the thermal waves launched into the specimen is unscrambled by performing the Fourier transform of the temperature evolution over the field of view. Of interest is the maximum phase image with many attractive features: deeper probing, less influence of surface infrared and optical characteristics, rapid image recording (pulse heating, surface-wide inspection), and the possibility to inspect high thermal conductivity specimens. Several results are presented and the theory is discussed as well.
Metamagnetic transition in Nd6Fe14−xAlx compounds
Authors: Jonen S. , Rechenberg H. R.
Published: July 2002

Ferrimagnetic Nd6Fe14−xAlx compounds with x=3, 3.5, and 4 have been studied by magnetization measurements in fields up to 9 T. Néel temperatures were determined from Mössbauer spectra to be 308, 286, and 239 K, respectively. The net magnetization at T=4.2 K is 3.6 μB/f.u.(x=3), 2.2 μB/f.u.(x=3.5), and zero for x=4. Metamagnetic transitions were observed for all samples, at decreasing critical fields with increasing x. Both the metamagnetic transition hysteresis and the net magnetization decrease with increasing temperature and become negligible near 40 K. The initial susceptibility of Nd6Fe10Al4 shows an anomalous temperature dependence near T=25 K, which might be associated with a spin reorientation. The electrical resistance of the latter compound, measured as a function of applied field, exhibits an abrupt 3% increase at the metamagnetic transition.
Cleaning of AlN and GaN surfaces
Authors: King S. W. , Barnak J. P. , Bremser M. D. , Tracy K. M. , Ronning C. , Davis R. F. , Nemanich R. J.
Published: July 2002

Successful ex situ and in situ cleaning procedures for AlN and GaN surfaces have been investigated and achieved. Exposure to HF and HCl solutions produced the lowest coverages of oxygen on AlN and GaN surfaces, respectively. However, significant amounts of residual F and Cl were detected. These halogens tie up dangling bonds at the nitride surfaces hindering reoxidation. The desorption of F required temperatures >850 °C. Remote H plasma exposure was effective for removing halogens and hydrocarbons from the surfaces of both nitrides at 450 °C, but was not efficient for oxide removal. Annealing GaN in NH3 at 700–800 °C produced atomically clean as well as stoichiometric GaN surfaces.