bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural and magnetic study of submicronic single crystal cobalt box arrays
Authors: Hehn M. , Ounadjela K. , Padovani S. , Bucher J. P. , Arabski J. , Bardou N. , Bartenlian B. , Chappert C. , Rousseaux F. , Decanini D. , Carcenac F. , Cambril E. , Ravet M. F.
Published: July 2002

We present a study of magnetic dot arrays fabricated in Ru(5 nm)/Co/Ru(32 nm) films with cobalt thickness varying from 10 to 500 nm. Large array areas up to 5×5 mm2 have been obtained with lateral dot size and spacing of 0.5 μm. The magnetization curves are characteristic of a perpendicular magnetized multidomain structure. Lateral size reduction does not drasti- cally change the domain structure but has immediate consequences on the nucleation and saturation fields. A magnetic force microscopy study confirms the perpendicular multidomain struc- ture and provides intrinsic magnetic characteristics.
Host-dependent optical transitions of Er3+ ions in lead–germanate and lead-tellurium-germanate glasses
Authors: Pan Zhengda , Morgan Steven H. , Dyer Keith , Ueda Akira , Liu Huimin
Published: July 2002

Differential scanning calorimetry, Raman scattering, optical absorption, and upconversion of infrared to green luminescence have been studied for Er3+-doped lead-germanate glass 55GeO2⋅20PbO⋅10BaO⋅10ZnO⋅5K2O (GPBZK) and lead-tellurium-germanate glass 30GeO2⋅30PbO⋅30TeO2⋅10CaO (GPTC). Judd–Ofelt intensity parameters of Er3+ in the two host glasses were determined and used to calculate radiative transition rates and lifetimes. Values of the radiative quantum yield of the 4S3/2→4I15/2 transition and the infrared (797 nm) to green (547 nm) upconversion efficiency of Er3+ were obtained. It has been found that the 4S3/2→4I15/2 radiative transition rate of Er3+ in GPTC glass is about twice that in the GPBZK glass and the upconversion efficiency in the GPTC glass is about four times larger than that in the GPBZK glass. These host-dependent properties are mainly attributed to the enhanced local field and the reduced multiphonon rate in lead-tellurium-germanate glass compared to lead-germanate glass.
A Raman scattering study of the ferroelectric ordering in rhombohedral and tetragonal La-modified lead zirconate titanate ceramics
Authors: El Marssi M. , Farhi R. , Dai X. , Morell A. , Viehland D.
Published: July 2002

A Raman study of La-doped lead zirconate titanate ceramics with La/Zr/Ti ratios of 9/65/35 and 12/40/60 has been undertaken in order to compare the onset of ferroelectric order in these two compositions. Some features, such as a splitting of a threefold degenerate T1u mode of the cubic structure into at least two lines or an enhancement of the T2u mode, can be considered as the signature of the occurrence of some ferroelectric order below Tc≂363 K in the 12/40/60 compound. Nevertheless, the Raman spectra obtained for the two ceramics are very similar, and no significant narrowing of the lines was observed in the ferroelectric phase of 12/40/60. It is thus suggested that the onset of long range order in this compound is not developed homogeneously in every direction of space, in agreement with previous transmission electron microscopy observations. The Raman scattering was dominately second order, with the broadening of the lines reflecting a distribution of the correlation function of the quasistatic polarization in reciprocal space, which was not sharply peaked at q=0 in all directions.
Deep center luminescence in p-type CdTe
Authors: Krustok J. , Valdna V. , Hjelt K. , Collan H.
Published: July 2002

The deep-level photoluminescence (PL) emission in the 1.1 eV spectral region in p-type CdTe:Cl polycrystalline samples was studied as a function of the excitation laser power and temperature. The relatively broad 1.1 eV PL band has a nonsymmetrical shape which can be easily changed by varying the excitation laser power. Detailed analysis of the line shape shows that the 1.1 eV emission contains two distinct separate bands. These two bands have their zero-phonon peaks located at 1.08 and 1.17 eV, respectively, and they have quite different half-widths. A donor–acceptor (DA) pair model with a deep donor and a deep acceptor with ED and EA both ≳0.5 eV is proposed to explain the observed experimental findings. In this model the 1.08 and 1.17 eV bands are formed as a DA recombination between pairs of the nearest neighbors, and between pairs of the next-nearest neighbors, respectively. It is concluded that the acceptor in these pairs must be an interstitial atom. One possible realization for this kind of a DA pair is the VTe–Tei complex, where VTe is acting as a donor and Tei as an acceptor.
Stress distribution in heteroepitaxial chemical vapor deposited diamond films
Authors: von Kaenel Y. , Stiegler J. , Michler J. , Blank E.
Published: July 2002

The stress distribution in heteroepitaxial chemical vapor deposited diamond films has been investigated by Raman spectroscopy. A new method for stress determination based on polarized confocal micro-Raman is presented and used for the measurement of the stress evolution across the film thickness in the center of the sample. The presence of highly inhomogeneous stresses at a microscopic scale is first demonstrated. The interface appears to be under compressive stress which rapidly decreases and then stabilizes, but remains compressive. The strain tensor is also shown to vary. Near the interface, the common assumption of biaxial stress in the plane of the film has been confirmed. Near the growth surface, the stress tensor appears to be more complicated. Grain boundaries are suggested to be mainly responsible for the intrinsic stress generation when the grain boundary density is high. Inhomogeneous impurity distribution could be related to stress inhomogeneities near the growth surface, where the grain boundary density becomes small. Agreement has been obtained between micro- and macro-Raman stress measurements. The average stress (over film thickness) as determined by macro-Raman is shown to increase by 30%–40% from the sample center to the edge.
Total electron yield method in x-ray absorption spectroscopy: A closer look at the saturation/self-absorption effects (abstract)
Authors: Chakarian V. , Idzerda Y. U.
Published: July 2002

Total electron yield (TEY) is a common method by which photoabsorption cross sections are measured in x-ray absorption spectroscopy (XAS). Since various material parameters are extracted from these XAS spectra, how well TEY spectra mimic the absorption cross section is a major concern. In cases in which the photoabsorption cross sections are large (e.g., at the absorption edge white lines), saturation and self-absorption effects can play a detrimental role, significantly altering the spectral weights in the near-edge region. In this presentation, we show that at the L edges of transition metals (as well as M edges of rare earths), the XAS spectra measured via TEY do not represent the true absorption cross sections. As a case in point, we discuss the ramifications of this saturation/self-absorption effects on the magnetic circular dichroism measurements, which often rely on TEY data. We evaluate the changes in the spectral weights of the near-edge features and determine the resulting errors in the extracted material quantities, i.e., orbital and spin moments, as a function of film thickness and photon incidence angle.
Properties of amorphous and crystalline Ta2O5 thin films deposited on Si from a Ta(OC2H5)5 precursor
Authors: Chaneliere C. , Four S. , Autran J. L. , Devine R. A. B. , Sandler N. P.
Published: July 2002

In this work, the structural and electrical properties of amorphous and crystalline Ta2O5 thin films deposited on p-type Si substrates by low-pressure chemical vapor deposition from a Ta(OC2H5)5 precursor have been investigated. The as-deposited layers are amorphous, whereas crystalline Ta2O5 was obtained after postdeposition O2 treatment at 800 °C. As evidenced by x-ray diffraction, a hexagonal structure was obtained in the latter case. Physicochemical analysis of our layers shows that the O2-annealing step leads to the growth of a thin (∼1 nm) interfacial SiO2 layer but was not sufficient to reduce the level of hydrocarbon contamination. The dominant conduction mechanism in amorphous Ta2O5 is clearly due to the Poole–Frenkel effect, whereas the situation remains unclear for crystalline Ta2O5 for which no simple law can be invoked to correctly describe its conduction properties. From capacitance–voltage measurements, the dielectric constant was found to be ∼25 for amorphous samples, but values ranging from 56–59 were found for crystalline layers, suggesting a particularly high anisotropic character of the crystalline phase. Finally, the effects of postdeposition annealing in N2 and forming gas at 425 °C have been investigated for both types of films.
Streamer mechanism for negative corona current pulses
Authors: Černák Mirko , Hosokawa Tatsuzo , Kobayashi Shigeo , Kaneda Teruo
Published: July 2002

Current wave forms of initial stages of breakdown and corona formation in a short negative point-to-plane gap have been measured with a nanosecond time resolution in air, O2, and N2 at pressures 13.33–100 kPa, and at various overvoltages. The experiments revealed the existence of a stepped form of negative corona Trichel pulses in O2 at atmospheric pressure. To test existing models for the negative corona pulse formation, effects of changing cathode secondary electron emission were studied using a brass cathode coated by CuI and graphite. It is concluded that a negative corona (Trichel) pulse is associated with the ignition of a cathode-directed streamer in the immediate vicinity of the cathode and the subsequent formation of a glow-discharge-type cathode region at the streamer arrival to the cathode. The implications of these results to negative corona applications are discussed.
Dependence of exchange coupling on antiferromagnetic layer thickness in NiFe/CoO bilayers
Authors: Ambrose T. , Chien C. L.
Published: July 2002

A systematic study of the dependence of exchange coupling in NiFe/CoO bilayers on CoO layer thickness tAF from 5 to 500 Å has been made. For large CoO thicknesses (tAF>100 Å), the exchange field varies as 1/tAF, whereas for small CoO thicknesses (tAF<100 Å), finite-size scaling of the Néel temperature TN and also the blocking temperature TB dominate.
Noise in optical measurements of cantilever deflections
Authors: Garcı́a-Valenzuela Augusto , Villatoro Joel
Published: July 2002

The optical beam deflection method using the knife edge detection technique is one of the simplest optical sensing methods. We investigate theoretically the limit to the resolution when sensing the displacements of a mechanical cantilever using the knife edge detection technique implemented with a quasimonochromatic thermal-like optical source. Simple formulas for the minimum detectable displacement of the cantilever’s front end are derived assuming all nonoptical noises negligible. It is shown that superluminiscent diodes can yield a minimum detectable displacement of the cantilever about 5 dBs above the shot noise for 1 mW of optical power. It is also shown that in many cases of practical interest the source induced noise is smaller than the thermal-vibration noise of the cantilever. Thus, in many cases, using an inexpensive thermal-like source with the knife edge detection technique can be as effective as using a high-coherence shot-noise limited source.
Nonvolatile memory effects in nitrogen doped tetrahedral amorphous carbon thin films
Authors: Gerstner E. G. , McKenzie D. R.
Published: July 2002

Electrical measurements of nitrogen doped tetrahedral amorphous carbon (ta-C:N) thin films have revealed a reversible nonvolatile memory effect, related to the excitation and de-excitation of electrons between deep acceptor states and shallow donor states within the mobility gap. This effect is characterized by changes in the small signal film conductivity of up to 10 times, and has been used to fabricate 1-bit memory cells with effective memory retention times in the order of several months.
Hexagonal pore arrays with a 50–420 nm interpore distance formed by self-organization in anodic alumina
Authors: Li A. P. , Müller F. , Birner A. , Nielsch K. , Gösele U.
Published: July 2002

Self-organized hexagonal pore arrays with a 50–420 nm interpore distance in anodic alumina have been obtained by anodizing aluminum in oxalic, sulfuric, and phosphoric acid solutions. Hexagonally ordered pore arrays with distances as large as 420 nm were obtained under a constant anodic potential in phosphoric acid. By comparison of the ordered pore formation in the three types of electrolyte, it was found that the ordered pore arrays show a polycrystalline structure of a few micrometers in size. The interpore distance increases linearly with anodic potential, and the relationship obtained from disordered porous anodic alumina also fits for periodic pore arrangements. The best ordered periodic arrangements are observed when the volume expansion of the aluminum during oxidation is about 1.4 which is independent of the electrolyte. The formation mechanism of ordered arrays is consistent with a previously proposed mechanical stress model, i.e., the repulsive forces between neighboring pores at the metal/oxide interface promote the formation of hexagonally ordered pores during the oxidation process.
Photoluminescence properties of Zn1-xMgxSe on misoriented GaAs substrates by molecular beam epitaxy
Authors: Tu R. C. , Su Y. K. , Chou S. T.
Published: July 2002

We have investigated the photoluminescence (PL) properties of Zn1−xMgxSe epilayers grown on GaAs substrates with different misorientation angles by molecular beam epitaxy. According to the data measured by PL and by electron-probe microanalysis, the Mg incorporation in the Zn1−xMgxSe epilayer decreases with increasing misorientation angle. In addition, the PL spectra showed that the full width at half maxima of the band-edge excitonic emission and the intensity of the defect-related donor-acceptor emission in Zn1−xMgxSe epilayers decreased appreciably when a substrate with a misorientation angle of 15° was used.
Characterization of surface structure in sputtered Al films: Correlation to microstructure evolution
Authors: Lita Adriana E. , Sanchez John E.
Published: July 2002

Quantitative roughness and microstructural analysis of as-deposited Al films, 0.1–1.0 μm thick, were performed by atomic force microscopy (AFM), one-dimensional power spectral density analysis (1DPSD), transmission electron microscopy, and x-ray pole figure methods. The variation of grain size (d) with thickness (h) in the columnar grained film was d∝h0.9. The initial crystallographic texture was nearly random, with a strong Al (111) fiber texture evolving by ≈0.2 μm in deposited thickness. AFM imaging revealed a surface structure with hillocks, grains, and grain boundary grooves, and periodic within-grain ridges extending over entire grains. The root-mean-square surface height variation (RRMS) initially decreased during deposition but increased as RRMS∝h0.55 from 0.3 to 1.0 μm thickness. The 1DPSD analysis revealed three spatially resolved regimes of roughness evolution; a frequency independent regime at low frequency attributed to hillock growth, an intermediate frequency self-similar regime attributed to grains and grain boundary grooves, and a high frequency self-similar regime attributed to within-grain ridges. Two characteristic dimensions (CD) were defined at the inverse frequencies of transition between each 1DPSD roughness regime. CDI at high frequency was identified as the peak-to-peak ridge spacing which remained independent of film thickness. The ridge spacing is proposed to represent the upper limit of an effective surface diffusion length (λ0) due to the effects of surface diffusion and flux shadowing. The CDII at lower frequency was identified as the grain size which increased with thickness. The evolution and interactions of roughness and microstructural features are discussed in terms of surface diffusion, grain boundary motion, and flux shadowing mechanisms.
Calculation of the hole concentration in boron-doped diamond
Authors: Fontaine Frédéric
Published: July 2002

Calculations of the hole concentration and its activation energy in boron-doped diamond are presented. The detailed structure of the valence band is taken into account by using a temperature-dependent degeneracy factor. The calculations indicate that the customary disregard of the spin-orbit band leads to the overestimation of the compensation ratio. The model developed by Lee and McGill for silicon [J. Appl. Phys. 46, 373 (1975)] is then applied to p-type diamond. The activation energy of the hole concentration is compared with experimental data. When valence-band-edge shift, valence-band-edge tailing and broadening of the acceptor level are concurrently taken into consideration, the decrease in the experimental activation energy observed with increasing boron concentration can be satisfactorily described. The model presented here significantly improves the understanding of the temperature dependence of the hole concentration in boron-doped diamond.
Internal photoemission characteristics of metal–insulator–semiconductor structures at low electric fields in the insulator
Authors: Przewlocki H. M.
Published: July 2002

This article completes the presentation of a model of photoelectric phenomena taking place in metal–insulator–semiconductor (MIS) structures at low electric fields, which was introduced in Ref. [H. M. Przewlocki, J. Appl. Phys. 78, 2550 (1995)]. A solution and analysis of model’s equations is given for the general case of nonzero net photoelectric current (J≠0) in the external circuit. This solution allows calculation of the photocurrent–voltage characteristics of MIS structures illuminated with ultraviolet radiation of different wavelengths λ. To verify these results, experimental characteristics were taken and are shown in comparison with the calculated ones. Excellent agreement between calculated and experimental characteristics, obtained for a wide range of different MIS structures, strongly supports the validity of the model. Limits of applicability of this model are illustrated and discussed. The model finds applications in developing new, highly precise, photoelectric measurement methods of MIS structures. Principles of three such methods are outlined. One of these methods has already been fully verified and has been successfully used in a number of investigations, while the other two, are currently being verified and optimized.
A study of low temperature crystallization of amorphous thin film indium–tin–oxide
Authors: Paine David C. , Whitson T. , Janiac D. , Beresford R. , Yang Cleva Ow , Lewis Brian
Published: July 2002

Deposition of tin-doped–indium-oxide (ITO) on unheated substrates via low energy processes such as electron-beam deposition can result in the formation of amorphous films. The amorphous-to-crystalline transformation was studied in this system using in situ resistivity, time resolved reflectivity, glancing incidence angle x-ray diffraction, and transmission electron microscopy. The resistivity of 180 nm thick In2O3(9.9 wt. %SnO2) was monitored during isothermal anneals at 125, 135, 145, and 165 °C. The dependence of the resistance on the volume fraction of crystalline phase was established using glancing incidence angle x-ray diffraction and a general two phase resistivity model for this system was developed. These studies show that, upon annealing, as-deposited amorphous ITO undergoes both a structural relaxation and crystallization. Structural relaxation of the amorphous material includes local ordering that increases the ionized vacancy concentration which, in turn, increases the carrier density in the material. Kinetic growth parameters were extracted from the data, which reveal that the relaxation of the amorphous structure occurs via a process that obeys a first order reaction rate law, while crystallization occurs via classical nucleation and growth with a growth mode parameter that is consistent with two- to three-dimensional transformation geometry. Both the relaxation and crystallization processes have an activation energy of approximately 1.3±0.2 eV. Time resolved reflectivity analysis of the electron beam deposited ITO reveals that there is a sharp and monotonic decrease in reflectivity during the anneal of the sample which is associated with the amorphous relaxation process.
Photocapacitance study of bulk deep levels in ZnSe grown by molecular-beam epitaxy
Authors: Hierro A. , Kwon D. , Ringel S. A. , Rubini S. , Pelucchi E. , Franciosi A.
Published: July 2002

The influence of initial growth conditions and lattice matching on the deep level spectrum of n-ZnSe grown on GaAs by molecular-beam epitaxy is investigated by means of deep level optical spectroscopy. A detailed study of both the steady-state and transient photocapacitance allows us to measure optical threshold energies, concentrations, and emission rates of electronically active defects in the ZnSe layer. Several deep levels are found in the ZnSe layer at Ec−Et=1.15, 1.46, 1.90, and 2.25 eV with concentrations in the 1012–1014 cm−3 range. When a 2-nm-thick composition controlled interface layer is grown at different beam pressure ratios prior to the ZnSe growth, a distinct decrease in the 1.46 eV level concentration with increasing Se content is found. Deposition of a lattice-matched InxGa1−xAs buffer layer prior to the ZnSe growth reduces the concentration of both the 1.15 and 1.46 eV levels by over an order of magnitude, indicating the role of lattice matching in the ZnSe overlayer. We also perform depth profiling of the defect distributions within the ZnSe overlayer to see the effect of the ZnSe thickness on the concentration of these levels as well as their possible association to the ZnSe/GaAs interface. We find that only the 2.25 eV level concentration shows a dependence on depth, increasing as the II–VI/III–V interface is approached.
Structural analysis of Sr8Ga16Ge30 clathrate compound
Authors: Schujman S. B. , Nolas G. S. , Young R. A. , Lind C. , Wilkinson A. P. , Slack G. A. , Patschke R. , Kanatzidis M. G. , Ulutagay M. , Hwu S.-J.
Published: July 2002

Crystal structural characterization of the Sr8Ga16Ge30 compound was carried out with the Rietveld refinement method using x-ray powder diffraction data. The structure corresponds to that of the type-I clathrate hydrate, with the Sr atoms filling the “cages” formed by the Ga and Ge atoms. Anisotropic displacement parameters were refined. The limiting figure for rms values of the Sr(1) atoms enclosed in the dodecahedral cages is a sphere, while that for the Sr(2) atoms in the tetrakaidecahedral cages move is an oblate ellipsoid. A comparison of the sizes of the cages in this structure with the ones in other M8GaxGe46−x (M=K, Ba, or void) structures shows that once the structure is filled (i) there is very little variation in size of the tetrakaidecahedral cages and (ii) the dodecahedral cages expand only slightly (measurably) as a function of the filler size. This is an important result from the thermoelectric point of view because it means that once the structure is full, the cages will not “close over” the guests, limiting their movement and, thus, raising the thermal conductivity. The melting point of Sr8Ga16Ge30 is approximately 765 °C and the apparent optical gap is 0.05 eV.
Thermal and nonthermal regimes of gliding arc discharge in air flow
Authors: Mutaf-Yardimci Ozlem , Saveliev Alexei V. , Fridman Alexander A. , Kennedy Lawrence A.
Published: July 2002

Gliding discharges comprising both equilibrium and nonequilibrium plasma conditions offer high energy efficiency and selectivity for chemical processes. Prevailing parameters satisfying nonequilibrium plasma conditions at relatively high power levels should be well understood and characterized. In the present work, gliding discharges formed between diverging electrodes in air flow were studied experimentally over a wide range of gas velocities and power levels. Depending on the system parameters the following discharge regimes were observed: low power nonequilibrium discharge; thermal quasiequilibrium discharge; and gliding discharge with equilibrium to nonequilibrium transition. The effect of system parameters on discharge characteristics is analyzed. The equilibrium to nonequilibrium transition was experimentally observed as a change of voltage increase rate with discharge length growth. The local electric field, defined as dV/dl, increased up to three times, indicating the change of plasma conditions. However, previously reported phenomenon of length explosion was not supported by our experimental data. The co-existence of equilibrium and nonequilibrium phases is also discussed in the frame of phenomenological theory, assuming formation of a growing nonequilibrium fragment inside the gliding discharge channel. It was found that high flow velocities provide intensive cooling, an increase of electric field, and a decrease of gas temperature, promoting equilibrium to nonequilibrium transition at high power levels.
Semitransparent cathodes for organic light emitting devices
Authors: Burrows P. E. , Gu G. , Forrest S. R. , Vicenzi E. P. , Zhou T. X.
Published: July 2002

We optimize transparent organic light emitting devices (TOLEDs) using compound cathodes consisting of a thermally evaporated metal contact layer capped with indium–tin–oxide (ITO). The ITO is sputtered at rates of up to 1.6 Å/s using a high power radio frequency magnetron process. With a Mg:Ag contact layer, we demonstrate a TOLED with 50% transparency and an operating voltage within 0.3 V of a device with identical organic layers and a conventional Mg:Ag cathode. The operational lifetime of the TOLED is shown to be equal to that of a similar, nontransparent device. We also study the effects of using different contact metals, including Ca, Al and LiF, on the operating characteristics of the TOLEDs. With a thin Ca contact layer, undoped TOLEDs with >80% peak transparency operating at (5.9±0.1) V at a brightness of >100 cd/m2 are demonstrated. These devices have application to transparent, head-up displays and to full color, stacked organic light emitting devices.
X-ray magnetic circular dichroism temperature dependent study of RCo2 compounds
Authors: Garcia F. , Sampaio L. C. , Takeuchi A. Y. , Tolentino H. , Fontaine A.
Published: July 2002

We have performed a temperature-dependent study of the rare earth L2,3 and Co K edges x-ray magnetic circular dichroism (XMCD) in HoCo2 and PrCo2. We have found in both samples that the L2/L3 ratio is temperature independent. This result contradicts previous findings that indicated a possible different behavior for L2 and L3 edges owing to modifications in the degree of 3d–5d hybridization. A further interesting result is that the temperature dependence of ratio E1/E2 for the L3 edge — where E1 and E2 correspond to the two area under the dichroic signal separated in two peaks, positive dipolar dominant, negative with significant contributions of both dipolar and quadrupolar channels — is also constant, suggesting that the dipolar and quadrupolar channels follow the same temperature dependence. This effect might be interpreted as the dominant 4f contribution to the 5d polarization (much larger than the 3d-induced contribution) to create the L-edge XMCD signal. The microscopic origin of the metamagnetic Co subsystem is also addressed.
Contact stiffness of layered materials for ultrasonic atomic force microscopy
Authors: Yaralioglu G. G. , Degertekin F. L. , Crozier K. B. , Quate C. F.
Published: July 2002

A method to calculate the contact stiffness between a layered material and an ultrasonic atomic force microscope (UAFM) tip is proposed. The radiation impedance method is used to determine the ratio of the applied force to the average displacement within the contact area. This information is used in an iterative algorithm based on Hertzian theory to obtain the contact stiffness. The algorithm converges into a couple of iterations and does not suffer from numerical convergence difficulties as does finite element analysis (FEA). In the ultrasonic frequency range, comparisons with Hertzian theory and FEA show the validity of the results in a quasistatic case. Definitions of the minimum detectable layer thickness and the penetration depth of the UAFM are given and evaluated for several thin film–substrate pairs. These results also show the potential of the method for modeling defects and power loss due to radiation in layered materials.
Changes in the magnetoresistive properties of first AFM Co/Cu multilayers upon annealing
Authors: Kubinski D. , Parsons M.
Published: July 2002

We have studied the irreversible changes in the room temperature magnetoresistive properties of [Co(15Å)/Cu(9Å)]20 multilayer films after annealing at temperatures up to 300 °C. The maximum magnetoresistance was found to degrade with increasing annealing temperature and time, even at temperatures as low as 150 °C. The sensitivity to changes in the magnetic field also degraded upon annealing, but by a lesser amount due to the accompanying narrowing of the magnetoresistance versus field curves. The resistivity was estimated for the Co/Cu ML stack and the underlying Cu buffer layer before and after annealing at 240 °C. At the sacrifice of the maximum magnetoresistance and sensitivity, we find that we can improve the long-term stability of the magnetoresistive properties to long-term exposure at 150 °C (the upper operating temperature of a device) by annealing at elevated temperatures.
Dry etch damage in n-type GaN and its recovery by treatment with an N2 plasma
Authors: Lee Ji-Myon , Chang Ki-Myung , Kim Sang-Woo , Huh Chul , Lee In-Hwan , Park Seong-Ju
Published: July 2002

We report on etch-induced damage in n-type GaN caused by an inductively coupled plasma, and damage recovery by means of treatment with an N2 plasma. As the plasma dc bias was increased by increasing the rf table power during etching, the optical and electrical properties of the etched GaN films deteriorated as the result of etch-induced damage. However, an N2 plasma treatment for the etched samples effectively removed the etch-induced defects and damage on the surface, leading to improved surface morphology, photoluminescence, and ohmic contact in n-type GaN.
Far-infrared laser magnetic resonance detection and microwave spectroscopy of the PO2 radical
Authors: Kawaguchi Kentarou , Saito Shuji , Hirota Eizi , Ohashi Nobukimi
Published: July 2002

Pure rotational transitions of the PO2 radical have been detected for the first time by far-infrared laser magnetic resonance with a CO2 laser pumped laser as a source. The radical was generated by the reaction of red phosphorus with mirowave discharge products of an H2/O2 mixture. Microwave spectroscopy was subsequently applied to a similar reaction system to observe rotational transitions in the region of 82–161 GHz. The microwave and far-infrared spectra thus observed were simultaneously analyzed using an asymmetric-rotor Hamiltonian which includes the spin-rotation and hyperfine interactions, to determine the rotational, spin rotation interaction, and magnetic hyperfine interaction constants. The observed rotational constants yield the r0 structure: r0 (P–O)=1.4665(41) Å and θ0(OPO)=135°17′(50), and the centrifugal distortion constants obtained give estimates for the vibrational frequencies: ω1=1090 cm−1, ω2=377 cm−1, and ω3=1278 cm−1.
Mass spectra of Si, Ge, and Sn clusters
Authors: Martin T. P. , Schaber H.
Published: July 2002

Clusters of group IV B elements have been produced and observed in a mass spectrometer. Irregularities in the ion intensities identify clusters with particularly high or low stability. For example, Si+x,Ge+x, and Sn+x show an enhanced stability for x=6, 10. However, Sn+14 is unstable, whereas Ge+14 is particularly stable.
Alkali halides in water: Ion–solvent correlations and ion–ion potentials of mean force at infinite dilution
Authors: Pettitt B. Montgomery , Rossky Peter J.
Published: July 2002

Using the specialization of the extended RISM equation to infinitely dilute systems, we have calculated correlation functions and interionic potentials of mean force for a set of models corresponding to the first few alkali halides in water. From the results obtained at infinite dilution we calculate the lowest order corrections to the solution properties of the ions. Higher concentrations are explored by using the interionic potentials of mean force at infinite dilution as effective solvent mediated pair potentials. Our results indicate that certain thermodynamic properties, such as the mean activity coefficients and osmotic pressures, are quite sensitive to the details of both the theory and the potential models.
Boron nuclear quadrupole coupling in o- and m-carborane (B1C2H12)
Authors: Lötz Albert , Voitländer Jürgen
Published: July 2002

The 11B nuclear quadrupole coupling in the title compounds is reported. The experimental technique applied was double resonance with level crossing. The values for (e2qQ/h)(1+η2/3)1/2 are (in MHz, errors of last digits and assignments in brackets): o-carborane: 1.10(6) (B8,B10) (tentative), 1.22(6) (B9,B12), 1.60(4) (B4,B5,B7,B11), 1.86(4) (B3,B6); m-carborane: 1.22(4) (B9,B10), 1.56(12) (all other borons, unresolved).
Monolayer adsorption on fractal surfaces: A simple two-dimensional simulation
Authors: Van Damme H. , Levitz P. , Bergaya F. , Alcover J. F. , Gatineau L. , Fripiat J. J.
Published: July 2002

We have simulated the adsorption of homologous series of ‘‘molecules’’ on deterministic fractal curves in the whole range 1≤D<2. Adsorption differs from a regular Hausdorf dimension determination in the sense that in the latter process the molecules are put with their center on the curve in order to cover it, whereas in the former process the molecules have to reach the curve, coming from one side only, and are not allowed to cross it. Three series of molecules were used: (i) circles of increasing diameter, r; (ii) rectangles of increasing height, t; (iii) rectangles of high aspect ratio (almost lines) and increasing length, l. The fractal exponents Dr, Dt, and Dl were derived from ln Nm vs ln r, ln t, and ln l plots, respectively, Nm being the number of molecules necessary to reach ‘‘monolayer’’ coverage. With linear molecules, we found Dl=1 in whole range 1≤D<2. With the circular and rectangular molecules, two regimes were observed. In the range 1≤D<1.5, Dr=D and Dt=D−1. These results are consistent with each other and suggest that, in the range 1≤D<1.5, the ln Nm vs ln l surface is a plane of equation ln Nm=α ln l+β ln t+γ, with α=−1 and β=1−D. This provides the basis for predicting the size dependence of Nm for molecules of any aspect ratio. An entirely different regime

was observed in the range 1.5<D<2. Instead of increasing with D, Dr and Dt were found to decrease according to Dr=3−D and Dt=2−D, respectively. This can be assigned to the generation of pore necks as the fractal dimension and the local concavity of the curve increases, and can be rationalized in terms of the rules governing the intersection of fractals. The model has been extended to adsorption on fractal surfaces. The prediction is that the maximum fractal dimension which could be measured without error is D=2.5, with molecules with constant aspect ratio. Using molecules of different shape would lead to considerable errors. However, this conclusion only applies to the particular class of fractals that we have considered, i.e., those for which the fractal character is restricted to their boundary. Fractal particles in which the fractal character stems from a particular internal morphology (i.e., from the distribution of mass, like in sponges or aggregates) may lead to an entirely different behavior.
Kinetics of activated processes from nonstationary solutions of the Fokker–Planck equation for a bistable potential
Authors: Cartling Bo
Published: July 2002

The kinetics of thermally activated processes are studied by the nonstationary solutions of the Fokker–Planck equation, or Kramers’ equation, for a particle moving in a bistable potential and coupled to a heat bath. An alternate direction implicit method is formulated and used to determine the time evolution of the probability density function and probability density current in the phase space for a large range of the strength of coupling to the heat bath. In addition to the rate constant in a first-order rate equation, corresponding to the lowest real eigenvalue in an eigenfunction expansion, transient processes corresponding to higher and complex eigenvalues are also studied. The results for strong coupling to the heat bath are compared to those obtained from the Smoluchowski equation. The present results are also used to evaluate the accuracy of previous treatments.
Ground and excited states of group IVA diatomics from local-spin-density calculations: Model potentials for Si, Ge, and Sn
Authors: Andzelm Jan , Russo Nino , Salahub Dennis R.
Published: July 2002

LCGTO-MP-LSD results are reported for the spectroscopic constants and electronic structure of the diatomic molecules Si2, Ge2, Sn2, SiGe, SiSn, and GeSn in their low-lying electronic states. For the homonuclear molecules we found that the ground state is 3Σ−g with the most important lower-lying excited states being 3Πu, 1Πu, and 1Σ+g, respectively. Our results are in good agreement with the available experimental data and also in qualitative agreement with other theoretical studies. We present here the first theoretical study on the heteronuclear molecules, for which experimental data are not available. We found the 3Σ− state to be the lowest, followed by 3Π and 1Σ+ states. Model potentials (MP) are reported for the Si, Ge, and Sn atoms. The reliable results for molecules complement those for the atoms and show that the LSD model potentials presented here allow for an accurate description of chemical bonding and spectroscopic properties in the title molecules.
Effect of vibrational energy on the dissociative chemisorption of N2 on Fe(111)
Authors: Rettner C. T. , Stein H.
Published: July 2002

Molecular beam techniques have been employed to determine the effect of vibrational energy on the dissociative chemisorption of N2 on Fe(111). We find that for kinetic energies in the range 0.3 to 0.6 eV that vibrational energy is an average only about half as effective as translational energy. Results are discussed in terms of a precursor mechanism, where access to the precursor state is blocked by a potential barrier that is more easily overcome with translational than with vibrational energy.
Vibrationally mediated photodissociation of hydrogen peroxide
Authors: Ticich T. M. , Likar M. D. , Dübal H.-R. , Butler L. J. , Crim F. F.
Published: July 2002

Vibrationally mediated photodissociation is a means of studying the spectroscopy of bound vibrational overtone states and of probing the electronic photodissociation dynamics of highly vibrationally excited molecules. In these experiments, a highly vibrationally excited hydrogen peroxide molecule prepared by initial excitation in the region of the third (4νOH) or fourth (5νOH) overtone of the OH stretching vibration absorbs an additional photon to dissociate to OH fragments whose individual quantum state populations are measured by laser induced fluorescence. This technique is a means of obtaining excitation spectra for bound highly vibrationally excited states and confirms the accuracy of a model that incorporates the role of the torsional vibration in the vibrational overtone spectroscopy. The photodissociation dynamics of highly vibrationally excited molecules are substantially different from those observed for dissociation by single photons of comparable or greater energy. Approximately 11% of the OH fragments formed in the vibrationally mediated photodissociation through 4νOH are vibrationally excited as compared to an unobservable amount (≤2%) in the single photon ultraviolet dissociation.
The forces between surfaces of mobile, orientable dipoles. The method of reflection coefficients
Authors: Attard Phil , Mitchell D. John
Published: July 2002

The interaction free energy between two planar bodies interacting across a uniform dielectric, expressed as a sum over allowed electrodynamic modes, is interpreted in terms of reflection coefficients. It is proved that, within the approximations implied by this modal approach, the force between identical bodies interacting across a uniform dielectric is always attractive. Explicit electrostatic results for interacting dipolar surfaces are obtained and these are applicable to the measured long range attractive forces between polar lipid bilayers.
A molecular dynamics simulation of the Lennard-Jones liquid–vapor interface
Authors: Nijmeijer M. J. P. , Bakker A. F. , Bruin C. , Sikkenk J. H.
Published: July 2002

The surface tension of a Lennard-Jones liquid–vapor interface has been determined accurately in a molecular dynamics simulation. Our values tend to be smaller than those from previous simulations. It is shown that the usually truncated tail of the potential strongly increases the surface tension if taken into account.
Radiolysis of liquids with high static dielectric constant: An estimate of the total ionization yield, electron thermalization distance, and contribution of heterogeneous reactions
Authors: Ferradini C. , Jay-Gerin J.-P.
Published: July 2002

In a previous study, we found an exponential dependence of the free-ion yield (Gfi) on the static dielectric constant (εs) for a number of irradiated liquids with εs>10. On the basis of this study, we develop here a simple model by which we quantitatively estimate the total ionization yield (Gtot), the most probable electron thermalization distance (b), and the yield of solvated electrons that are removed by diffusion-controlled reactions during spur expansion (Gdif). Using solvated electron yields available in the literature, we get Gtot≂6.6 mol/100 eV (value nearly independent of the nature of the liquid) and b≂29 Å at 298 K. b is found not to depend appreciably on εs which indicates that the Coulomb attractive force between the ion and secondary electron is practically inefficient before electron thermalization occurs. The evaluation of Gdif teaches us that spur reactions have a profound influence in the fate of ion pairs formed during radiolysis of liquids of high εs values.
An explanation of the 1D2 anomalous crystal field splitting in PrCl3
Authors: Garcia D. , Faucher M.
Published: July 2002

Taking into account configuration interaction between 4f2 and 4f15d1 via the odd rank crystal field parameters eliminates most of the well-known discrepancy of the 1D2 multiplet of trivalent praseodymium in PrCl3, found when ‘‘standard’’ crystal field treatment is applied to experimental data. Ab initio values of crystal field parameters agree with the new parameters including the configuration interaction.
Thermal metal cluster anion reactions: Behavior of aluminum clusters with oxygen
Authors: Leuchtner R. E. , Harms A. C. , Castleman A. W.
Published: July 2002

Aluminum anions, (Al−n,n=5–>40), were generated using laser vaporization and equilibrated to room temperature in a flow tube apparatus. The behavior of these species, in general, show a propensity to lose the neutral oxides. In sharp contrast, several anions, Al−13, Al−23, and Al−37, show special inertness to these etching reactions and indeed, the etching reactions even contribute to their further production. The jellium model predicts these species to have closed shells. There are some exceptions in reactivity, however, which indicate the likelihood that the local structure may also play a role. An odd/even alternation, where even atom clusters react more quickly than successive odd ones, is also apparent. This is attributed to electron pairing within the cluster.
Generalized Brownian dynamics. I. Numerical integration of the generalized Langevin equation through autoregressive modeling of the memory function
Authors: Smith D. E. , Harris C. B.
Published: July 2002

A method is presented for numerical integration of the generalized Langevin equation (GLE) based on modeling of the ‘‘random force’’ as a discrete autoregressive process. This modeling procedure, drawn from digital signal processing and spectral estimation methods which have been used extensively in electrical engineering applications, provides for efficient evaluation of the friction integral in the GLE as well as for generation of a random force process with the desired spectrum. The method is shown, through comparison with molecular dynamics results, to be effective in simulating the force autocorrelation function of an iodine atom dissolved in Lennard-Jones (LJ) xenon. In a companion paper this method is applied in a simulation of the vibrational relaxation of I2 in LJ xenon at two very different densities and found to perform well.
Theoretical studies of clustering reactions. Sequential reactions of SiH+3 with silane
Authors: Raghavachari Krishnan
Published: July 2002

The mechanisms for the sequential clustering reactions of SiH+3 with SiH4 have been studied by means of accurate quantum chemical techniques using polarized basis sets and including zero-point corrections and the effects of electron correlation. Detailed reaction pathways including the necessary transition structures have been investigated to understand the structures and stabilities of the reaction intermediates and products. In accordance with the experimental observations of Mandich, Reents, and Kolenbrander, the reactions lead to the formation of Si2H+5 and Si3H+7 without significant overall barriers. In addition, bimolecular attachment complexes (Si2H+7,Si3H+9 and Si4H+11) bound by bridging hydrogen atoms are involved as reaction intermediates and have also been observed by Mandich et al. as collisionally stabilized products. Unusual hydrogen bridged complexes play a key role in the termination of the growth sequence. The first bottleneck in the sequence of growth reactions is due to the formation of the particularly stable hydrogen bridged complex Si3H+7 containing a four-membered Si–Si–Si–H ring. Further growth beyond Si3H+7 other than bimolecular attachment involves transition states which have significant barriers and could only happen in the presence of excess energy. Isotopic exchange reactions which have been seen experimentally are also rationalized by this mechanism. Quantitative comparisons are made between our calculated transition state barriers and complexation energies and those derived by Mandich et al. from phase space calculations using the experimental reaction rates. The mean absolute deviation between the two results for five different energetic quantities is only ≂3 kcal/mol.
Dynamics in regular solutions: Concentration and viscosity dependence of orientational correlation time of a benzene molecule
Authors: Matsubayashi N. , Nakahara M.
Published: July 2002

The conformal solution theory by Longuet-Higgins was extended to the dynamic problem through the first-order perturbation theory for the interpretation of the concentration dependence of dynamic solution properties; molecular similarity and dissimilarity are reflected by the linear ideal terms and the quadratic regular terms, respectively. To test the theory, the orientational correlation times τ2R were determined for benzene-d6 in the three regular solutions of benzene (B) with toluene (T), carbon tetrachloride (C), and cyclohexane (H) as a function of the concentration (mole fraction x, 0≤x≤1) at 25 °C by measuring the spin-lattice relaxation time of the quadrupole nucleus 2H. The temperature effect was also studied in the BC solution. The τ2R–x curve is linear in BT and BC whose excess enthalpies HE are small, but it is quadratic in BH with a larger HE. The quadratic concentration dependence indicates the important effect of liquid structure on the orientation dynamics as well as the viscosity. The validity of the Stokes–Einstein–Debye (SED) law was tested with respect to the linear relation between τ2R and η/T (T the temperature, η the solution viscosity). The SED law holds when η is varied only by T or x under some limited conditions. The conditions for this simple relation have been clarified using the extended conformal solution theory.