bluuebunny/crossref_metadata_2025 huggingface dataset:
Calorimetric studies of the heat capacity and relaxation of amorphous Si prepared by electron beam evaporation
Authors: Tsang K. H. , Kui H. W. , Chik K. P.
Published: July 2002

The heat capacity of a-Si thin film prepared by electron beam evaporation method was measured from 360 to 820 K by a differential scanning calorimeter. For the as-prepared a-Si specimen, two novel irreversible endothermic processes and one irreversible broad exothermic reaction were found. The origins of the endothermic reactions were not known. It is suggested that they may be caused by a change in the number and distribution of voids that occurs at approximately 465 K and the creation of dangling bonds at the higher temperature regime (≳620 K). The exothermic reaction is attributed to heat release during structural relaxation. When measuring the heat capacity of a-Si, Ca-Sip, these irreversible reactions were first eliminated by annealing the specimens at high temperatures. The heat capacity of crystalline Si, Cc-Sip, was also measured and the difference, ΔCSip=Ca-Sip −Cc-Sip, was used to evaluate the thermodynamic melting temperature of the a-Si, Tal, which is determined to be 1400 K.
Heavy ion elastic scattering cross sections for energies near the Coulomb barrier
Authors: Rauhala E. , Räisänen J.
Published: July 2002

Elastic scattering of 12C, 14N, and 16O ions by aluminum, silicon, and titanium has been investigated below 34 MeV at a backscattering angle of 170°. The excitation functions at energies near the Coulomb barrier and non-Rutherford threshold energies above which the Rutherford cross sections become invalid, are presented for direct scattering and the kinematically reversed reactions. Sharply decreasing cross sections above the threshold energies, but no broad resonance-like structures, were observed. The non-Rutherford threshold energies are clearly higher than the values predicted by a theoretical model based on nuclear perturbation and optical model calculations recently presented in the literature. A new unpublished version of the model gives better agreement.
Characterization of optical fibers using near-field scanning optical microscopy
Authors: Butler D. J. , Nugent K. A. , Roberts A.
Published: July 2002

A method of determining the field distribution within an optical fiber of arbitrary refractive index profile using near-field scanning optical microscopy is described. The 3D intensity distribution emerging from the end of a cleaved fiber is measured by scanning a subwavelength aperture through the distribution, and the field within the fiber is inferred from this information. In the case of a single-mode step-index fiber, results are found to agree with a Gaussian modal distribution.
Magnetism of ternary compounds RE6Fe13X; RE=Pr, Nd; X=Cu, Ag, Au, Zn, Cd, and Hg
Authors: Weitzer F. , Leithe-Jasper A. , Rogl P. , Hiebl K. , Rainbacher A. , Wiesinger G. , Steiner W. , Friedl J. , Wagner F. E.
Published: July 2002

Novel compounds RE6Fe13X (X=Cu, Ag, Au, Zn, Cd, Hg) have been synthesized by arc melting (Cu, Ag, Au) or by reaction sintering (Zn, Cd, Hg), followed by heat treatment at 550 or 600 °C up to 350 h in evacuated silica capsules. From room temperature x-ray powder diffraction analyses the alloys were found to crystallize with the ordered Nd6Fe13Si type. The Curie point of all the compounds is found to lie around room temperature. Due to their low magnetization values a ferrimagnetic coupling within the Fe sublattice is suggested. A canted RE-spin structure cannot be ruled out. 57Fe Mössbauer spectra were recorded from the samples with X=Ag and Au. Additionally, 197Au spectra were taken from the two Au-containing compounds. The 57Fe spectra are composed of various subpatterns, originating from the different Fe sites and were found to be essentially unaffected by the specific element RE and X. In all cases under investigation the spectra could only be fitted under the assumption of an easy axis of magnetization lying within the basal plane. The 197Au spectra reveal that the Au atoms predominantely occupy the 4a site.
Thermal magnetic noise due to eddy currents in a strip wound ferromagnetic core at 4.2 K (abstract)
Authors: Maslennikov Yu. V. , Snigirev O. , Cerdonio M. , Prodi G. A. , Vitale S.
Published: July 2002

Many applications involving high impedance signal sources and use of the low input impedance SQUID magnetometer would take advantage of the use of low noise ferromagnetic cores in matching transformers. Noise in ferromagnetic cores at liquid helium temperatures has been shown to be limited by thermal fluctuation of the magnetization for the two following cases: for bulk cores, where the skin effect sets a low frequency (<100 Hz) roll-off of the effective permeability, and the spectral density is white up to the roll-off, and for strip wound cores at frequencies lower than ≊100 Hz, where magnetic viscosity gives origin to a frequency independent imaginary permeability and correspondingly to a 1/ω shaped spectral density. Thanks to a high sensitivity dc SQUID magnetometer we have now been able to measure thermal noise due to the skin effect in a strip wound 15-μm-thick Ultraperm core where the roll-off frequency is ≊1 MHz. Data for the noise are in quite quantitative agreement with the prediction of the fluctuation dissipation formula and the measured values of μ(ω). At frequencies lower than a corner ≊1 kHz the noise is again dominated by the 1/ω tail due to magnetic viscosity. We discuss the relevance for applications.
Proximity effect in iridium-gold bilayers
Authors: Nagel U. , Nowak A. , Gebauer H.-J. , Colling P. , Cooper S. , Dummer D. , Ferger P. , Frank M. , Igalson J. , Nucciotti A. , Pröbst F. , Seidel W. , Kellner E. , Feilitzsch F.v. , Forster G.
Published: July 2002

We have studied the proximity effect in bilayers of thin films of iridium covered by gold. By varying the thicknesses of the iridium and gold layers, we achieved critical temperatures as low as 33 mK. The critical temperature of the bilayers is lower than predicted by the theory of de Gennes–Werthamer [J. J. Hauser, H. C. Treuerer, and N. R. Werthamer, Phys. Rev. 136, A637 (1964)], but adding a free parameter to the theory allows good agreement. The transitions of the bilayers typically had widths of a few mK, with the narrowest reaching 0.2 mK, and were always steeper than those of pure iridium films evaporated simultaneously. Such bilayers can be used as superconducting phase transition thermometers in cryogenic particle detectors.
Solid-state reaction to synthesize Ni-Mo metastable alloys
Authors: Zhang Z. J. , Liu B. X.
Published: July 2002

In the Ni-Mo system, amorphous alloys and three metastable crystalline (MX) phases, i.e., an hcp (hcp-I), an enlarged hcp (hcp-II), and a fcc phase, were synthesized by solid-state reaction in the Ni-rich multilayers. An interesting point is that with increasing the annealing temperature, the hcp-I phase was formed first in the Ni67Mo33 film and then turned amorphous, while in the Ni75Mo25 film, the amorphous phase was formed first and then changed into the hcp-I phase. The compositions of three MX phases were determined experimentally. A free-energy diagram of the Ni-Mo system was established by calculating the free energies of the amorphous phase and the three MX phases, as well as those of the solid solutions and the related compounds on the basis of the model of Niessen et al. [CALPHAD 7, 51 (1981)] and Alonso’s method [Solid State Commun. 46, 765 (1983)]. The calculated free-energy diagram can give relevant interpretation to the observed phase formation behaviors.
Band structure and electronic properties of FeSi and α-FeSi2
Authors: Girlanda R. , Piparo E. , Balzarotti A.
Published: July 2002

The results of self-consistent linear muffin-tin orbital calculations for the ordered end members of the FeSi1+x series, i.e., FeSi (x=0) with a CsCl structure and α-FeSi2 (x=1) with a tetragonal structure, predict semimetallic behavior for FeSi and metallic behavior and higher stability for α-FeSi2. The valence band density of state features of the α-FeSi2 differ slightly from those of the cubic fluorite (γ-FeSi2). The analysis of the valence band photoemission spectra taken on silicide thin films epitaxially grown on Si(111) substrates estimates the composition of the mono- and disilicide phases present in the annealed films but does not determine the proposed formation of the distorted fluorite structure (α-FeSi2) well below its bulk stabilization temperature of 950 °C.
Infrared spectroscopy study of initial stages of oxidation of hydrogen-terminated Si surfaces stored in air
Authors: Niwano Michio , Kageyama Jun-ichi , Kurita Kazunari , Kinashi Koji , Takahashi Isao , Miyamoto Nobuo
Published: July 2002

We have studied the initial stages of oxidation of the hydrogen-terminated Si(111) and (100) surfaces stored in air, using infrared spectroscopy in the multiple internal reflection geometry. We investigate the effect of surface roughness and humidity of air on the oxidation of the hydrogen-terminated Si surfaces. We suggest that surface roughness on a microscopic scale does not significantly affect the oxidation of the hydrogen-terminated Si surface and the oxidation occurs on the entire surface. It is demonstrated that water is predominantly involved in the oxidation of the surface Si—H bond, and that the surface Si—H bond is quite inert to the oxygen molecule.
Survey of the dependence on temperature of the coercivity of garnet films
Authors: Vértesy G. , Tomás̆ I.
Published: July 2002

The temperature dependence of the domain-wall coercive field of epitaxial magnetic garnets films has been investigated in the entire temperature range of the ferrimagnetic phase, and has been found to be described by a set of parametric exponents. In subsequent temperature regions different slopes were observed, with breaking points whose position was found to be sample dependent. A survey based on literature data as well as on a large number of our own samples shows the general existence of this piecewise exponential dependence and the presence of the breaking points. This type of domain-wall coercive field temperature dependence was found in all samples in the large family of the epitaxial garnets (about 30 specimens of more than ten chemical compositions) and also in another strongly anisotropic material (TbFeCo).
Strain effects in InGaSb/AlGaSb quantum wells grown by molecular beam epitaxy
Authors: Kuramochi Eiichi , Takanashi Yoshifumi
Published: July 2002

The dependence of photoluminescence spectra and surface morphology on misfit strain are investigated for highly strained InxGa1−xSb/Al0.35Ga0.65Sb quantum wells grown on GaSb(001) by molecular beam epitaxy. Infrared photoluminescence with a wavelength as long as 1.95 μm is observed at room temperature from a multiple-quantum well. The measured photoluminescence wavelength is in good agreement with that calculated by the theory in which strain effects are taken into account for a single-strained quantum well condition. The photoluminescence intensity decreases remarkably when width of strained quantum well exceeds the critical layer thickness (CLT) that depends on the misfit, i.e., indium content of the well layer. The mechanical-equilibrium theory of the generation of misfit dislocations predicts the CLT observed successfully.
He ion radiation effects in high electron mobility transistors
Authors: Papaioannou G. J. , Papastamatiou M. J. , Christou A.
Published: July 2002

The effects of He ion irradiation have been investigated in high electron mobility transistors. Heterojunctions have been also used to monitor the two-dimensional electron-gas degradation. Devices with different layer structures have been employed for the better understanding of failure mechanism sources. Finally, introduction of a charge control model allowed the determination of buffer layer degradation.
Structural and optical studies of InxGa1−xAs/GaAs multiple quantum wells
Authors: Di Dio M. , Lomascolo M. , Passaseo A. , Gerardi C. , Giannini C. , Quirini A. , Tapfer L. , Giugno P. V. , De Vittorio M. , Greco D. , Convertino A. L. , Vasanelli L. , Rinaldi R. , Cingolani R.
Published: July 2002

Strained multiple quantum wells of InxGa1−xAs/GaAs were grown by low pressure metalorganic chemical vapor deposition (LP-MOCVD) and characterized by secondary ion mass spectrometry, x-ray diffraction, and optical spectroscopy. The structural analysis demonstrates the excellent control of the interface morphology and composition achieved by MOCVD growth. Temperature dependent optical absorption, photoluminescence, and magnetotransmission were used to evaluate the well-width dependence of the major excitonic properties. The samples show sharp excitonic resonances with distinct excited states evolving into Landau-type excited states in high magnetic field. The well-width dependence of the excitonic eigenstates and of the exciton binding energy as well reproduced by envelope function and variational calculations, also in the presence of external electric field. Finally, nonlinear electro-optic modulation induced by the quantum confined Stark effect is demonstrated in a Schottky diode with extremely low switching threshold.
Empirical tight-binding calculation of the branch-point energy of the continuum of interface-induced gap states
Authors: Mönch Winfried
Published: July 2002

The band lineup at metal–semiconductor contacts as well as at semiconductor heterostructures may be described by one and the same physical concept, the continuum of interface-induced gap states. These intrinsic interface states derive from the virtual gap states (ViGS) of the complex semiconductor band structure and their character varies from predominantly donorlike closer to the valence band to mostly acceptorlike nearer to the conduction band. Calculations are presented of the respective branch points for elemental and binary as well as ternary compound semiconductors which make use of Baldereschi’s concept of mean-value points in the Brillouin zone [Phys. Rev. B 7, 5212 (1973)], Penn’s idea of dielectric band gaps [Phys. Rev. 128, 2093 (1962)], and the empirical tight-binding approximation (ETB). The results are as follows. First, at the mean-value point the band gaps calculated in the GW approximation have the same widths as the dielectric band gaps. Second, the ETB approximation reproduces the GW valence-band energies at the mean-value point. Third, the branch points of the ViGS are slightly below midgap at the mean-value point. The ETB branch-point energies excellently reproduce the barrier heights of gold Schottky contacts on 19 semiconductors and the valence-band offsets of Al1−xGaxAs/GaAs heterostructures.
Tensile and compressive strain relief in InxGa1−xAs epilayers grown on InP probed by Raman scattering
Authors: Groenen J. , Landa G. , Carles R. , Pizani P. S. , Gendry M.
Published: July 2002

Strain relaxation has been investigated by means of Raman scattering in strained InxGa1−xAs layers (with x ranging from 0 to 1) grown on In0.53Ga0.47As/InP (001). The epilayers are either under tensile (x<0.53) or compressive (x>0.53) strain. Relaxation coefficients have been deduced from the frequency shifts of the GaAs-like optical phonons. A marked dissymmetry in strain relief is found over the whole composition range between equivalent tensile and compressive misfits. Disorder activated Raman scattering features have been analyzed and correlated to the structural defects resulting from the strain relief in the two and three-dimensional growth modes. Strain inhomogeneities resulting from surface corrugation are evidenced by micro-Raman measurements on layers with tensile misfits.
Optimized process for the fabrication of mesoscopic magnetic structures
Authors: Adeyeye A. O. , Bland J. A. C. , Daboo C. , Hasko D. G. , Ahmed H.
Published: July 2002

We have used the advantage of the high etch selectivity between metals in a wet etching process to develop an optimized technique for engineering magnetic materials. This method is based on electron beam lithography and optimized pattern transfer by a combination of dry and wet etching. The technique has been used in fabricating mesoscopic Ni80Fe20 dots and wires with lateral dimensions down to 0.2 μm. We have used scanning electron microscopy to verify the lateral sizes and edge acuity of the structures. The magnetic properties were characterized using magneto-optic Kerr effect and magnetoresistance measurements. A marked increase in the coercive field and the saturation field is seen as the width of the wire is decreased. The magnetoresistance change (∂R) is found to increase significantly as the width of the wire is decreased.
High-field induced electrical aging in polypropylene films
Authors: Liufu D. , Wang X. S. , Tu D. M. , Kao K. C.
Published: July 2002

According to Kao’s model of electrical discharge and breakdown in condensed insulating materials, charge carrier injection from electrical contacts and subsequent dissociative trapping, and recombination play a decisive role in the breaking of polymer chains and the creation of free radicals or low-weight molecules, and hence traps. We believe that electrical aging is due to this gradual degradation process. The increase in structural degradation and trap concentration reflects the degree of electrical aging and hence the lifetime of the electrically stressed polymers. On the basis of this concept, we have derived a theoretical formula for the prediction of the lifetime of insulating polymers. We have also carried out experiments on polypropylene films at high electric fields. It is found that destructive breakdown occurs when the accumulation of field induced traps reaches a certain critical value. Experimental results are in good agreement with our theoretical model.
Terahertz radiation from coherent phonons excited in semiconductors
Authors: Tani M. , Fukasawa R. , Abe H. , Matsuura S. , Sakai K. , Nakashima S.
Published: July 2002

Terahertz radiation emitted by coherent phonons in Te, PbTe, and CdTe has been investigated by using an ultrafast photoconductive sampling detector. Pronounced coherent radiation originating from the longitudinal optical (LO) phonon oscillations of infrared-active modes was observed for all samples, irrespective of the different crystal structures. In addition, spectral dips at the transverse optical (TO) phonon frequencies, which could not be explained by absorption in the emitting volume, were observed for all samples. The model calculations indicate that the emission rate of the radiation into the air to that into the dielectric (semiconductor) side is scaled by 1/{1+(nd2+κd2)nd3} (nd and κd are the real and imaginary part of the complex refractive index, respectively). Thus, the enhanced emission of radiation by the coherent LO phonons and the spectral dips at the TO phonon frequencies can be explained by the respective increase and reduction of the emission efficiency of the radiation to the air due to the small and large value of the dielectric constant |εd(ω)|=nd2+κd2 near the LO and TO phonon frequencies, respectively.
Spectroscopic determination of the structure of amorphous nitrogenated carbon films
Authors: Bhattacharyya Somnath , Cardinaud C. , Turban G.
Published: July 2002

Studies on structure and electronic properties of amorphous nitrogenated carbon films prepared in dual electron cyclotron resonance–radio frequency plasma from a mixture of methane and nitrogen are presently reported. These films are characterized by Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, x-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), electrical conductivity measurement, and optical absorption spectroscopy. Symmetry breaking of aromatic rings are at a very small amount of nitrogen incorporation is understood from FTIR spectra. The relative contribution of C=N and C–N bonds is found to change with the variation of the nitrogen content in the samples, which shows a similar trend with the shift of the G peak to a higher wave number and the increase of the ID/IG ratio. From decomposition of XPS C 1s and N 1s peaks a three-phase model of CN bonds is proposed. UPS valence band spectra obtained by using a Helium II source, are decomposed into p-π, p-σ, 2s bands and a mixture of s-p band. The intensity of p-π band increases as a function of nitrogen concentration, confirming the increase of sp2 bonds in the samples. An enhancement of the room temperature electrical conductivity and a decrease of the optical gap are observed with the addition of nitrogen in the films. The effect of nitrogen doping in carbon films is also emphasized. Our analyses establish an interrelationship between the microstructure and electronic structure of nitrogenated carbon films, which helps to understand the change in electronic properties of the carbon films due to a low amount of nitrogen incorporation.
Alloy effects in boron doped Si-rich SiGe bulk crystals
Authors: Franz M. , Pressel K. , Gaworzewski P.
Published: July 2002

We have investigated boron doped Si1−xGex bulk crystals over the composition range 0⩽x⩽0.13 by photoconductivity, photoluminescence, and temperature-dependent Hall effect measurements. Hall effect measurements yield the relation EI=(44.4–108x) meV for the change of the boron ionization energy EI with alloy composition. The far-infrared photoconductivity onset caused by the shallow acceptor boron does not shift as much to lower energies with increasing Ge content as previously reported. Transitions that involve excited levels of the shallow acceptor boron are recognizable as a redshift of the photoconductivity onset with increasing temperature. Comparison of the Hall effect data with the shift of the photoconductivity spectra and the evaluation of photoluminescence linewidths give experimental evidence for an inhomogeneous distribution of Ge atoms in the alloy.
Particle size effects and surface anisotropy in Fe-based granular films
Authors: Chen Chen , Kitakami Osamu , Shimada Yutaka
Published: July 2002

Structure and magnetic properties of Fe/SiO2 and Fe/Al2O3 granular films were studied in the particle size (d) range of 1–50 nm. The coercivity (Hc) shows a maximum around 600 Oe at the particle size dm≈18 nm, and then decreases in the larger particle size range for both Fe/SiO2 and Fe/Al2O3 systems at room temperature. A relation of coercivity, particle size, and temperature was built by using the Néel–Brown theory and the Stoner–Wohlfarth model basically. The experimental results were successfully fitted by assuming that the α-Fe particles possess uniaxial surface anisotropy Ks≈0.4 erg/cm2.
Transient impact response of three polymers
Authors: Johnson J. N. , Dick J. J. , Hixson R. S.
Published: July 2002

Experimental data are presented for the transient response of three polymers, Estane, Estane with a nitro-plasticizer, and adiprene, subject to flat-plate impact. These data are analyzed in terms of a generalized Maxwell model and the theory of characteristics for the dynamical motion. Comparison of theory and experiment quantifies the high-frequency component in the Maxwell description; for Adiprene the high-frequency shear modulus and relaxation time are 300 MPa and approximately 50 ns, respectively. For estane the corresponding quantities are 300 MPa and approximately 200 ns. Low-pressure Hugoniot data are also presented for estane. An estimate of the low-pressure Hugoniot is given for Adiprene. Some experimental data are also presented for the Estane/nitro-plasticizer combination; this material is used as a binder in plastic-bonded explosives. Theory is presented for a thermodynamically consistent calculation of temperature in high-rate loading of polymers whose mechanical properties are represented by a generalized Maxwell model.
Paraconductivity in YBa2Cu3O7 and YBa2Cu3O7 thin film composites with cerium oxide and gold
Authors: Cukauskas E. J. , Allen Laura H.
Published: July 2002

The paraconductivity for thin films of YBa2Cu3O7 (YBCO) and composites of YBCO with CeO2 and gold was investigated and analyzed using the Aslamazov and Larkin theory. Plain YBCO thin films sputter deposited on MgO, SrTiO3, and yttrium stabilized zirconium typically showed no dimensional crossover, were described by three dimensional (3D) fluctuation conductivity, and had a critical slope just above the critical region in agreement with the prediction of theory. With few exceptions, the CeO2 and gold composite films showed a dimensional crossover and a reduced critical exponent. The change in dimensionality was typically from 3D to two dimensional (2D) fluctuation conductivity. These composite films all had reduced values for the critical exponent which we attribute to an enlarged or extended critical region. This is in agreement with the critical current response of these composites to small magnetic fields. Additionally, the composites showed a two step transition in the paraconductivity and a minimum below the critical region. The subsequent fall-off to the zero resistance state was characterized by two different slopes. Lastly, the paraconductivity was related to film degradation resulting from photoresist processing and time storage. Dimensionality changes were observed due to this degradation.
High-temperature tolerant diamond diode for carbon monoxide gas detection
Authors: Gurbuz Y. , Kang W. P. , Davidson J. L. , Kerns D. V.
Published: July 2002

A high-temperature tolerant microelectronic carbon monoxide gas sensor has been developed. This device is based on a diamond diode in the form of catalytic-metal/adsorptive-oxide/intrinsic-diamond/semiconductor-diamond structure. The gas sensing performance and detection mechanisms have been studied over a wide temperature range (50–500 °C). The gas sensitivity of the sensor is large, fast, repeatable, and reproducible. Gas detection mechanism is attributed to the modification of the oxygen vacancies in the SnOx layer of the sensor. This diamond diode sensor can be utilized in applications that require a sensor to operate at a wider operating temperature range.
Preparation, characterization and magnetic properties of an ordered FeCo single crystal
Authors: Chu S.-Y. , Kline C. , Huang M.-Q. , McHenry M. E. , Cross J. , Harris V. G.
Published: July 2002

Pristine FeCo single crystals with millimeter dimensions were prepared by annealing FeCo ingot samples. The composition and microstructure of a fine polished cylindrical single crystal specimen used in this work were characterized by energy dispersive x-ray analysis, scanning electron microscopy, back-reflection Laue diffraction photographs, and x-ray diffraction (XRD). The ordered α′ phase in the specimen was detected by XRD experiments using the synchrotron light source at Brookhaven National Laboratory. The full width at half maximum of the superlattice line, i.e., the (100) reflection, in phi scan was 2.5°. The magnetic field dependence of the magnetic moment was measured with the field oriented in different directions using a superconducting quantum interference device magnetometer and a vibrating sample magnetometer.
Structural and optical characterization of CdS films grown by photochemical deposition
Authors: Ichimura Masaya , Goto Fumitaka , Arai Eisuke
Published: July 2002

CdS thin films are grown by photochemical deposition from an aqueous solution and characterized by x-ray diffraction (XRD), Raman spectroscopy, photoluminescence measurement, and optical transmission spectroscopy. The films are deposited at room temperature and annealed at temperatures up to 500 °C. The as-deposited film is dominantly zinc blende cubic. The cubic phase remains dominant until the annealing temperature becomes higher than 400 °C. By the annealing at 450 °C, the XRD pattern turns to that of hexagonal phase. Moreover, its peak width decreases and the near-band-edge luminescence begins to be observed. The band gap is decreased by annealing below 400 °C and then abruptly increased by the annealing at 450 °C. This annealing behavior of the band gap is interpreted considering the quantum size effects, the band tail due to disorder, and the cubic-hexagonal transition.
Dependence of electrical properties on film thickness in lanthanum-doped lead zirconate titanate stannate antiferroelectric thin films
Authors: Xu Baomin , Ye Yaohong , Wang Qing-Ming , Cross L. Eric
Published: July 2002

Lanthanum-doped lead zirconate titanate stannate antiferroelectric thin films with compositions in antiferroelectric orthorhombic region have been prepared on Pt-buffered Si substrates by the sol–gel method, with the film thickness from 0.1 to 1.0 μm. The dependence of dielectric and ferroelectric properties on film thickness have been studied, with the emphasis on field-induced phase switching. The dielectric constant and maximum polarization decrease with the reduction of film thickness, which are similar to ferroelectric thin films. However, the decrease of film thickness also leads to the decrease of the phase switching fields, the appearance of remanent polarization in the hysteresis loops, and the smearing of antiferroelectric–ferroelectric phase switching. These phenomena are believed to be caused mainly by the stress effect between the film and substrate. Due to thermal mismatch, the films are under high tensile stress after annealing, and the stress effect becomes more significant with the decrease of film thickness. This makes the thinner films more preferable to the ferroelectric phase and causes the retention of ferroelectric phase in the films with thickness less than 0.4 μm.
The effect of frequency and duty cycle of a pulsed microwave plasma on the chemical vapor deposition of diamond
Authors: Khachan J. , Gardner D.
Published: July 2002

We examine the effect of a pulsed microwave discharge on the deposition rate of polycrystalline diamond by varying the pulse repetition rate and duty cycle. A simple model of the dynamic plasma chemistry is developed in order to explain the increase in growth rate with frequency for the same average power. Changing the duty cycle while keeping the total plasma on-time constant resulted in the same film thickness for all duty cycles. One possible implication of this is that growth takes place when the pulse is on.
Nanofabrication and electrostatic operation of single-crystal silicon paddle oscillators
Authors: Evoy S. , Carr D. W. , Sekaric L. , Olkhovets A. , Parpia J. M. , Craighead H. G.
Published: July 2002

We report the fabrication and characterization of paddle oscillators featuring nanometer-scale supporting rods. The devices show two resonances in the 1–10 MHz range, which we attribute to the translational and torsional modes of motion. While the frequency response of the translational motion shows evidence of nonlinear behavior, the torsional response remains symmetric throughout the range of excitation. We present a model for the electrostatic excitation of the two modes. Torsional motion is induced via asymmetries of the system, and amplified by a modulation of the effective torsional constant. The model of the translational motion predicts a nonlinear behavior for displacements as small as 15 nm. Analysis of both modes of motion consistently suggests structures softer than expected from bulk silicon. Quality factors approaching 103 are measured.
Theoretical modelling of phenomena in the pulsed-laser deposition process: Application to Ti targets ablation in low-pressure N2
Authors: Neamtu J. , Mihailescu I. N. , Ristoscu Carmen , Hermann J.
Published: July 2002

We present an analysis describing the phenomena that are involved in the interaction process between high intensity UV laser radiation and a solid target in a low-pressure gas environment. The laser action upon the solid surface is described by solving the one-dimensional (1D) heat-conduction equation. The 1D hydrodynamic model is further applied to describe the interaction between laser radiation and the plasma expanding in front of the target. The laser radiation absorption in the vapor plasma is considered using a multicomponent fluid model. The transit of ablated material from target to collector is treated by a Monte Carlo method. The time and space distributions of the main plasma parameters (e.g., atom density and plasma electron temperature and density) are calculated. Finally, the thickness profile of a film deposited on a collector parallel to the target is predicted. For comparison with available experimental data, the calculations were carried out for the case of a Ti target submitted to a multipulse UV laser irradiation in low-pressure nitrogen, when depending on gas pressure, stoichiometric TiN was deposited or mixtures of amorphous and crystalline Ti, TiN, Ti2N, and even Ti oxides were obtained.
Fabrication of refractive index distributions in polymer using a photochemical reaction
Authors: Kada Takeshi , Obara Atsushi , Watanabe Toshiyuki , Miyata Seizo , Liang Chuan Xin , Machida Hideaki , Kiso Koichi
Published: July 2002

We demonstrate that a photochemical reaction can create various distributions of refractive index in polymer. When the polymer containing a photochemically active material is irradiated by UV light, the photochemical reaction which breaks the π-conjugated system in the material and decreases its linear polarizability can reduce refractive index of the polymer. We prepared a PMMA film added DMAPN ((4-N,N-dimethylaminophenyl)-N′-phenylnitrone) with a rate of 23 wt % by use of spin coating. Electronic structural change of DMAPN and refractive indices of the film before and after UV irradiation were evaluated by UV absorption spectra and m-line method, respectively. The UV irradiation decreased λmax at 380 nm in the absorption spectra, which is attributed to nitrone, and the refractive indices exponentially with irradiation time. The change of refractive indices reached 0.028. The refractive index profile upon depth of the film was investigated by measuring refractive indices of stacked DMAPN/PMMA films. When UV with a power of 10.7 mW/cm2 irradiated upon three stacked DMAPN/PMMA films for 35 s, variation of the refractive index change showed a quadratic profile. The refractive index profile with various irradiation time can be accounted with the combination of the chemical kinetics with the steady state approximation and Lambert–Beer's law. Thus, the photochemical reaction can be used to control the refractive index distribution in polymer.
New opportunities in science, materials, and biological systems in the low-gravity (magnetic levitation) environment (invited)
Authors: Brooks J. S. , Reavis J. A. , Medwood R. A. , Stalcup T. F. , Meisel M. W. , Steinberg E. , Arnowitz L. , Stover C. C. , Perenboom J. A. A. J.
Published: July 2002

We discuss new opportunities that present themselves with the advent of very high magnetic field resistive magnets with appreciable central bore access. A detailed description of the parameters of the magnetic force environment for the case of diamagnetic materials in a water-cooled Bitter-type resistive magnet is provided for the reader who may have an interest in low-gravity experiments. We discuss emerging research activities involving novel uses of magnetic forces in high field resistive magnets at the National High Magnetic Field Laboratory. Particular attention is given to the area of diamagnetic materials that allow a low or “zero” gravity state, i.e., magnetic levitation. These include studies involving plant growth, protein crystallization, and dynamics of single particles and granular materials. In the latter case, unique aspects of the magnetic force environment allow low gravity experiments on particulates that cannot be performed on the Space Shuttle due to the lack of a weak confining potential in space.
FePt:SiO
2
granular thin film for high density magnetic recording
Authors: Luo C. P. , Liou S. H. , Sellmyer D. J.
Published: July 2002

Nanocomposite FePt:SiO2 thin films consisting of high anisotropy FePt particles embedded in a SiO2 matrix have been successfully fabricated by annealing the as-deposited FePt/SiO2 multilayers. By adjusting the annealing temperatures and compositions, films were obtained with coercivity of 3.8 kOe and grain size of 10 nm, which are suitable for high-density magnetic recording. Magnetic activation volumes were measured and thermal stability is discussed.
Laser pyrolysis fabrication of ferromagnetic γ′-Fe4N and FeC nanoparticles
Authors: Grimes C. A. , Qian D. , Dickey E. C. , Allen J. L. , Eklund P. C.
Published: July 2002

Using the laser pyrolysis method, single phase γ′-Fe4N nanoparticles were prepared by a two step method involving preparation of nanoscale iron oxide and a subsequent gas–solid nitridation reaction. Single phase Fe3C and Fe7C3 could be prepared by laser pyrolysis from Fe(CO)5 and 3C2H4 directly. Characterization techniques such as XRD, TEM and vibrating sample magnetometer were used to measure phase structure, particle size and magnetic properties of these nanoscale nitride and carbide particles.
Linear and nonlinear response functions for an exact state and for an MCSCF state
Authors: Olsen Jeppe , Jo/rgensen Poul
Published: July 2002

We have examined the response of an exact and an MCSCF reference state to a general time-dependent field. The time development of both the exact and the MCSCF reference state have been parametrized in terms of explicit exponential time-dependent transformations. The time development has been determined by requiring the Ehrenfest theorem to be satisfied through each order in the interaction between the molecular system and the field. The response of the exact and the MCSCF reference state has been used to evaluate linear, quadratic, and cubic response functions. It has been shown how a large variety of molecular properties may be expressed in terms of these response functions. It has also been demonstrated that molecular properties containing the electric dipole operator may be expressed in equivalent forms involving the momentum operator both for the exact and the MCSCF state. The MCSCF response functions have been transformed to computationally attractive expressions which do not contain summation indices over intermediate states and which allow direct techniques to be straightforwardly applied.
NH A 3Πi quenching at 1400 K
Authors: Garland Nancy L. , Jeffries Jay B. , Crosley David R. , Smith Gregory P. , Copeland Richard A.
Published: July 2002

We have measured electronic quenching rate constants, kQ, in the NH A 3Πi, v′=0 state for several quencher molecules at 1400 K. The NH radicals were produced in a laser pyrolysis apparatus when a mixture of SF6, CF4, NH3, and added quencher was irradiated by a CO2 laser. The ground state NH radicals were then excited to the A state by a pulsed tunable dye laser. The fluorescence decay rates as a function of added quencher pressure yielded the rate constants. The thermally averaged cross sections, σQ=kQ/〈v〉, for NH at 1400 K are all smaller than those observed for OH at 1200 K in this laboratory. In most cases, our high temperature σQ’s are the same as those observed elsewhere at room temperature. These experimental σQ’s are all less than those calculated with an attractive forces ‘‘collision complex’’ model.
EXAFS study of solute–solute and solute–solvent interactions in acetonitrile solutions of CuBr2+nLiBr (n=0, 1, and 2)
Authors: Sano Mitsuru , Maruo Tetsuya , Yamatera Hideo
Published: July 2002

The EXAFS studies of acetonitrile solutions of CuBr2, CuBr2+LiBr, and CuBr2+2LiBr have been carried out. The EXAFS spectra of Cu and Br edges were measured and transformed to radial distribution functions around Cu and Br ions, respectively. The curve-fitting analysis shows that the main species are [CuBr2(NCCH3)2], [CuBr3(NCCH3)]−, and [CuBr4]2− in the solutions containing Br− and Cu2+ in the Br−/Cu2+ ratio of 2, 3, and 4, respectively.
Nonequilibrium statistical mechanics of concentrated colloidal dispersions: Hard spheres in weak flows
Authors: Russel W. B. , Gast A. P.
Published: July 2002

We formulate a theory for the nonequilibrium structure and stresses in a sheared suspension with a fluid rest state. Many body interactions are handled exactly in the thermodynamics but truncated at the pair level for the hydrodynamics. Evaluation for hard spheres in weak flows demonstrates the importance of stresses arising from the nonequilibrium structure and explains the shear rate dependence observed at volume fractions greater than 0.25–0.30.
Lattice model of microemulsions
Authors: Widom B.
Published: July 2002

A lattice model of microemulsions is proposed. It proves to be equivalent to a spin-1/2 Ising model in a magnetic field, with ferromagnetic nearest-neighbor, antiferromagnetic next-nearest-neighbor (next-nearest defined as two lattice steps, regardless of the metrical distance), and three-spin interactions. The respective interaction constants H, J, M, and L in the Ising model are related to the ratios zBB/zAA and zAB/(zAAzBB)1/2 of the activities of the oil (AA), water (BB), and surfactant (AB), to the surfactant-film-curvature energy (surfactant–surfactant interaction energy) K, and to the curvature-bias parameter (Bancroft parameter) λ, in the microemulsion model. A table of translations is given. In mean-field approximation the symmetrical version of the model, in which H=L=0 (or zBB/zAA =1 and λ=0 in microemulsion language), is equivalent also to the ANNNI (anisotropic, or axial, next-nearest-neighbor Ising) model. The analog of the three-phase (Winsor III) equilibrium of surfactant solutions is identified in the ANNNI model’s phase diagram. States of vanishing tension of the microscopic surfactant film are identified in the symmetrical model. They prove to be the same as those in which the tension of the interface between (metastable) bulk oil and water phases (the ferromagnetic phases in the Ising model) vanishes. Those states are reflected in the phase diagram, and also in the ultralow tensions of the interfaces between stable phases in their neighborhood.
Simple model of induced smectic phases in binary mixtures of liquid crystals
Authors: de Jeu Wim H. , Longa Lech , Demus Dietrich
Published: July 2002

The origin of the occurrence of induced smectic phases (ISP) in binary mixtures of liquid crystals is discussed. First the experimental evidence is reviewed, which leads to a division in strong complexing (electron–donor–acceptor complex as indicated by a charge–transfer band) and weak complexing, where only indirect evidence is available. Within the mean field approximation a theoretical model is given in which the influence of complex formation on the nematic–smectic-A transition temperature is investigated for ideal orientational order. The ISP mechanism is attributed to the dispersive interactions between the complexes. However, in principle in a two-component mixture without complexing also ISP can be obtained due to interactions involving permanent dipoles. From fits to experimental phase diagrams and from estimates of parameters it follows that the first mechanism is more likely. Three nonstandard groups of phase diagrams with characteristic maxima of minima are predicted. The location of the extrema is calculated as a function of complex concentration and effective, intermolecular interactions. The model correctly describes presently existing, very rich, experimental data.
Temperature-dependent molecular motions of saturated acyl cholesteryl esters: A 13C NMR study
Authors: Croll David H. , Small Donald M. , Hamilton James A.
Published: July 2002

High resolution 13C NMR spectra were obtained vs temperature in the isotropic liquid and liquid-crystalline phases of four cholesteryl esters with saturated acyl chains [(6:0), (14:0), (16:0), and (18:0)] and a mixture rich in cholesteryl esters with unsaturated acyl chains [10% 16:0, 45% 18:1, 45% 18:2, w/w/w]. Except for peaks from carbons in the middle and/or distal part of the fatty acyl chain, the narrow peaks seen for the isotropic liquid broadened beyond detection below the isotropic→cholesteric phase transition, indicating a marked decrease in the rates and/or amplitudes of motions of most portions of the cholesteryl ester molecule. Rotational correlation times for the long molecular axis and the short nonunique axis (τrz and τrx, respectively), were calculated from the linewidth of the steroid ring methine C3 and C6 resonance for each ester system in the liquid phase. With decreasing temperature, τrx and τrz showed little or no increase for the saturated esters but a significant differential increase for the mixture. Because of stronger intermolecular interactions of neat saturated esters, molecular ordering near the liquid→liquid-crystalline phase transition may occur in such a narrow temperature range that the large pretransitional anisotropies observed for unsaturated esters are not observed for the saturated esters.
Electron affinities in the self-interaction-corrected local spin density approximation
Authors: Harrison J. G.
Published: July 2002

Electron affinities are calculated for first- and second-row atoms in the self-interaction-corrected local spin density approximation (SIC-LSDA). We compare results obtained by the orbital SIC method of Perdew and Zunger to those obtained by a new method which employs orbital SIC for exchange and Stoll’s spin-density SIC for correlation. The latter method exhibits exceptional accuracy in correlation and total energy for neutral atoms and negative ions. The orbital SIC method yields more accurate electron affinities overall. This represents a significant improvement over earlier results which employed a spherical approximation for the orbital densities. Both methods follow the experimental trends closely and yield results within 0.2 eV of experiment. This accuracy is shown to be the result of a striking cancellation of errors between exchange and correlation.
Helium atom differential cross sections for scattering from single adsorbed CO molecules on a Pt(111) surface
Authors: Lahee A. M. , Manson J. R. , Toennies J. P. , Wöll Ch.
Published: July 2002

Differential cross sections have been measured for the scattering of helium atoms from isolated CO molecules on a Pt(111) surface. The cross sections reveal an oscillatory structure as a function of scattering angle extending to large momentum transfer on both sides of the specular peak. Using a hard hemisphere model to approximate the interaction potential, the data can be well reproduced by a hard core of radius about 2.5 Å. This result is compared to the charge density profile of the adsorbed molecule and also to the gas phase interaction potential, and in both cases good agreement with the experimentally predicted classical turning points is found.
Physical interpretations of various dynamic light scattering data, their interconnections and relations to other relaxation data
Authors: Ngai K. L. , Wang C. H. , Fytas George , Plazek Daniel L. , Plazek D. J.
Published: July 2002

Dynamic light scattering has been proven to be a useful tool to study relaxations in polymeric and small molecular liquids, fused salts, ionic conductors, etc. We attempt to enhance the physical interpretations of the experimental data with the help of a general coupling theory of relaxations. It is demonstrated by many examples that the ideas and predictions of the coupling theory are matching very well to the experimental data. The relations between Brillouin scattering and photon correlation spectroscopy relaxation data are brought out. Physical meanings of the relaxation parameters become evident. Dynamic light scattering data then can be used to advantage in consideration of other physical phenomena and measurements.
Dipped adcluster model for chemisorptions and catalytic reactions on a metal surface
Authors: Nakatsuji Hiroshi
Published: July 2002

We propose a model for chemisorptions and surface reactions in which ‘‘adcluster’’ (admolecule+cluster) is dipped onto the electron ‘‘bath’’ of a solid metal and an equilibrium is established for the electron and/or spin transfer between them. The equilibrium condition is described with the use of the chemical potentials of the adcluster and the solid surface. Since the adcluster is a partial system, the number of the transferred electrons n is not necessarily an integer. Some typical behaviors of the energy E(n) of the adcluster are explained. A molecular orbital model is proposed to calculate the energy E(n) and the electronic structure of the adcluster. The electrostatic energy due to a charge polarization is also important though it is insensitive to the chemical structure of the adcluster. Sample application is given for the Pd–O2 system.