bluuebunny/crossref_metadata_2025 huggingface dataset:
Broadening of Er3+ electronic states in phosphate glasses
Authors: Francini R. , Grassano U. M. , Tarasov G. G.
Published: July 2002

Absorption and emission spectra of Er3+-doped and Er3+–Yb3+-codoped phosphate glasses have been studied within the temperature interval from 10 to 300 K. The analysis of Er3+ Stark splitting for the excited state I13/24 and the ground-state I15/24 multiplets is performed. Simulation of inhomogeneous broadening of Stark levels has shown that all (2J+1) components of both multiplets are split by random distortions of the initial symmetric polyhedron, consisting of the neighboring oxygen ions in phosphate glasses. Inhomogeneous broadening of Stark levels is different for each energy level and reflects the sensitivity of a particular energy level to the variation of crystal field. At least three energy levels participate in shaping the low-energy side of the emission spectra, caused by the I13/24→I15/24 transition. The emission linewidth corresponding to the transition to the highest by energy Stark component is assumed to derive from comparable contributions of inhomogeneous and homogeneous broadening, the latter caused by intra-multiplet transitions. The presence of the highest Stark level in the crystal-field splitting of the ground-state I15/24 manifold with a high rate of nonradiative relaxation is essential for writing the rate equations which describe the laser operation of erbium glass lasers.
Short wave phase shifts by large free surface solitary waves: Experiments and models
Authors: Guizien Katell , Barthélemy Eric
Published: July 2002

In this paper, we compare experiments on short gravity wave phase shifting by surface solitary waves to a Wentzel–Kramers–Brillouin–Jeffreys (WKBJ) refraction theory. Both weak interactions (head-on interaction) and strong interactions (overtaking interaction) are examined. We derive a dispersion relation and wave action conservation relation which are similar to the ones obtained for short waves refraction on slowly varying media. The model requires an exact solitary wave solution. To this end, a steady wave solution is numerically computed using the algorithm devised by Byatt-Smith [Proc. R. Soc. London, Ser. A 315, 405 (1970)]. However, two other solitary wave solutions are incorporated in the model, namely the classical Korteweg and De Vries (KdV) [Phil. Mag. 39, 422 (1895)] solution (weakly nonlinear/small amplitude solitary wave) and the Rayleigh [Phil. Mag. 1, 257 (1876)] solution (strongly nonlinear/large amplitude solitary wave). Measurements of the short wave phase shift show better agreement with the theoretical predictions based on the Byatt-Smith numerical solution and the Rayleigh solution rather than the Korteweg and De Vries one for large amplitude solitary waves. Theoretical phase shifts predictions based on Rayleigh and Byatt-Smith numerical solutions agree with the experiments for A/h0⩽0.5. A new heuristic formula for the phase shift allowing for large amplitude solitary waves is proposed as a limiting case when the short wave wave number increases.
Quantum waveguide array generator for performing Fourier transforms: Alternate route to quantum computing
Authors: Akis R. , Ferry D. K.
Published: July 2002

Recently, quantum computing has gained attention as a possible means of greatly increasing the speed of certain calculations when compared with traditional, binary computing. A key part of many quantum computing algorithms is the performance of a Fourier transform. In this letter, we propose a quantum waveguide array device that can perform such operations quickly with a parallel approach that uses the analog properties of waves and rather than the qubit concept.
Photonic band-gap guidance in high-porosity luminescent porous silicon
Authors: Ferrand P. , Loi D. , Romestain R.
Published: July 2002

We demonstrate a nanoporous silicon (nPS) multilayered structure, offering an efficient photonic band-gap (PBG) single-mode guidance in a high-porosity (i.e., low index) layer in the near-infrared. We show the good agreement between the calculated band structure and the measured spectral range of efficient guiding. We discuss the different kinds of guided modes that exist within the structure, and verify that only one PBG mode can be guided efficiently. Quantitative measurements of the losses are performed using the room-temperature photoluminescence of nPS.
Slow flow across macroscopically rectangular fiber lattices and an open region: Visualization by magnetic resonance imaging
Authors: Bijeljic B. , Mantle M. D. , Sederman A. J. , Gladden L. F. , Papathanasiou T. D.
Published: July 2002

Creeping flow of a Newtonian fluid across aligned and staggered rows of cylinders (fiber lattices) bounded by an open region is studied experimentally by magnetic resonance imaging (MRI) velocimetry. The model systems are formed by circular cylindrical rods, macroscopically arranged in rectangular fashion and confined inside a Hele–Shaw cell. The thus formed fiber arrays are bounded by the open region from one side and the wall of the cell on the other side, thus forming a heterogeneous fibrous medium of dual porosity. The influence of the fiber lattice volume fraction and lattice unit-cell geometry on the local aspects of the flow in the interior of and exterior to the fiber arrays are investigated. The steady-state velocity maps of the longitudinal and, in particular, transverse velocity components are shown to be advantageous in studying the local aspects of the flow field in such a heterogeneous porous medium. The most important feature of local velocity distributions in the regions ahead of and behind the lattice-channel arrangements is evidenced as substantial transverse velocities. This local flow aspect is termed edge effect and found to be dependent on lattice porosity. Local flow disturbances are present on either side of the open channel–fiber lattice interfaces, at the length-scale corresponding to the size of unit cells of the fiber lattices. Regions with regular patterns of very low fluid velocities are identified throughout the fiber lattices. The local values for the velocity vector at the entrance/exit of the fiber lattices are considerably higher than the average values within the fiber arrangements. These local flow enhancements, which are caused by the proximity of velocity gradients in the adjoining free flow region, are termed entrance/exit effects.
Influence of inertia on the transient axisymmetric free-surface flow inside thin cavities of arbitrary shape
Authors: Khayat Roger E.
Published: July 2002

The influence of inertia is examined for transient axisymmetric free surface flow inside a thin cavity of arbitrary shape. The flow field is obtained by solving the lubrication equations, which are averaged over the cavity gap by expanding the velocity in terms of Chandrasekhar functions and using the Galerkin projection method. The formulation accounts for the transverse flow, as well as nonlinearities stemming from inertia and front location. Both flows under an imposed flow rate, and an imposed pressure at the cavity entrance are examined. The influence of inertia, aspect ratio, gravity, and cavity geometry on the evolution of the front, flow rate, and pressure is assessed particularly in the early stage of flow. Comparison with existing results shows full qualitative agreement for cavities of various geometries and flow conditions. Inertia is found to have a significant influence on early transient behavior, leading to the development of a flow of the “boundary-layer” type upon inception. The effect of inertia is further explored by developing a multiple-scale analysis to obtain an approximate solution at small Reynolds number, Re. Comparison with the exact (numerical) solution indicates a wide range of validity for the multiple-scale approach, even in the moderately small Re range.
Structural, magnetic, and transport properties in a Cu-doped La0.7Ca0.3MnO3 system
Authors: Wang K. Y. , Song W. H. , Dai J. M. , Ye S. L. , Wang S. G. , Fang J. , Chen J. L. , Gao B. J. , Du J. J. , Sun Y. P.
Published: July 2002

The structural, magnetic, and transport properties of La0.7Ca0.3Mn1−xCuxO3 (0⩽x⩽0.2) have been investigated. The maximum magnetoresistance (MR) is about 4×106% for the sample with x=0.15, which is about ∼104 times the maximum MR for samples with x⩽0.1. A change in the structural parameters and magnetic and resistivity properties of the samples shows that Cu exists in Cu3+ ionic form in the sample with x⩽0.05. However, Cu2+ ions begin to appear and then they increase with further Cu doping.
Towards low-temperature electrostatic accelerometry
Authors: Lafargue Laurent , Rodrigues Manuel , Touboul Pierre
Published: July 2002

Ultrasensitive accelerometers with electrostatic suspension use a technology allowing operation at ambient temperature. However, their resolution is theoretically limited to around 10−12 m s−2/Hz1/2 by thermal noise. To overcome this limit, and in the perspective of future space missions, an electrostatic sensor compatible with operation at cryogenic temperatures is currently under development. It should ultimately reduce thermodynamic noise and improve the instrument characteristics. The perspectives for a rectangular proof mass electrostatically suspended with six degrees of freedom at liquid-helium temperature are presented and discussed, as are tests and test data.
Design and evaluation of a simple ultralow vibration vacuum environment
Authors: Lawall John , Kessler Ernest
Published: July 2002

A vibration-isolated platform in a high-vacuum environment designed for prototyping optical interferometers is described. The system provides isolation from both seismic and acoustic disturbances with a residual acceleration of 2.9×10−6 m/s2 rms (root-mean-square) in the vertical direction and 6.3×10−6 m/s2 rms in the horizontal in the bandwidth from 5 to 400 Hz. The environmental decoupling provided by the spring suspension, air-spring table legs, and vacuum are evaluated sequentially to illustrate the effects of each. The vibration contributed by the turbomolecular pump is measured and shown to fall into a low-frequency zone from 26 to 76 Hz as well as peaks at the pump rotation frequency of 820 Hz and its harmonic at 1.64 kHz. The low-frequency noise arising from the pump is less than 2% of the total residual acceleration in the bandwidth from 0.25 to 400 Hz. The platform is used to construct a high-finesse open-cavity Fabry–Perot interferometer, using standard commercial mounts, with relative motion below 2 pm rms in a 3 kHz bandwidth. Of this motion, the contribution due to the pump at 820 Hz is only 0.3 pm. The acoustic decoupling provided by the vacuum environment allows experiments that are prohibitive at atmospheric pressure even in a tightly sealed enclosure.
Dipolar eddies in a stratified shear flow
Authors: Voropayev S. I. , Smirnov S. A. , Brandt A.
Published: July 2002

The aim of this contribution is to present the results of experiments conducted to elucidate the conditions of formation and evolution of planar dipolar eddies in a stratified fluid in the presence of a vertical background shear. The flow regime diagram is derived and conditions under which the formation of dipolar eddies is possible are found and explained. The estimates for the dipole lifetime are given. Possible applications include oceanic mushroom-like currents, late stratified wakes and other cases when dipolar or quasi-dipolar eddies are observed in a stratified shear flow.
Transient ship waves on an incompressible fluid of infinite depth
Authors: Liu Minjia , Tao Mingde
Published: July 2002

In this paper, we focus our attention on transient ship waves induced by a submerged moving body in an incompressible fluid assumed to be inviscid or viscous. The Oseen equations are extended to the transient case for the viscous fluid and solved by employing a combinative model. Transient ship waves from calmness to the generation of steady-state ship waves are described. When the viscosity vanishes, the above results are degenerated into those for the inviscid fluid.
Electron mobility in amorphous silicon thin-film transistors under compressive strain
Authors: Gleskova H. , Wagner S.
Published: July 2002

We evaluated amorphous silicon thin-film transistors under uniaxial compressive strain of up to 1%. The on-current and hence the electron linear mobility decrease. The off-current, leakage current, and the threshold voltage do not change. The mobility decreases linearly with applied compressive strain. Upon the application of stress for up to 40 h the mobility drops “instantly” and then remains unchanged. We conclude that compressive strain broadens both the valence and conduction band tails of the a-Si:H channel material, and thus reduces the effective electron mobility.
Doppler-free two-photon absorption spectroscopy and the Zeeman effect of the A 1B2u←X 1A1g14111 band of benzene
Authors: Misono Masatoshi , Wang Jinguo , Ushino Miho , Okubo Mitsushi , Katô Hajime , Baba Masaaki , Nagakura Saburo
Published: July 2002

A Doppler-free absorption spectrum and the Zeeman effect of the A 1B2u(v14=1,v1=1)←X 1A1g(v=0) transition of benzene have been measured by means of two-photon absorption spectroscopy with counterpropagating light beams of identical wavelength within an external cavity. Rotational lines were fully resolved, and 647 QQ lines of J=0–43, K=0–43 have been assigned. The molecular constants of the A 1B2u(v14=1,v1=1) state have been determined as A=B=0.181 046 1, C=0.090 548 9, DJ=0.544×10−7, DJK=−1.093×10−7, DK=0.587×10−7, and T0=40 578.2672 cm−1. The Zeeman splittings for lines of a given J were observed to increase regularly with K and reach a maximum at K=J. This demonstrates that the magnetic moment lies along the c axis (perpendicular to the molecular plane). The magnetic moment of the A 1B2u(v14=1,v1=1,J=43,K=43) level was determined to be 0.005μB. The Zeeman splittings of the K=J levels were observed to increase with increasing J. Via analysis of the rotationally resolved Zeeman spectra, it is concluded that the A 1B2u state is mixed with the E2u3 state. This new finding suggests that vibronic interactions between E2u3 and B1u3(T1) and between E2u3 and E1u3(T2) through the mixed E2u3 component, contribute to the B2u1(S1)→3B1u(T1) and B2u1(S1)→3E1u(T2) intersystem crossings, respectively.
Optoelectronic behavior in a double-barrier-emitter triangular barrier switch
Authors: Guo Der-Feng
Published: July 2002

In this study, a triangular-barrier and a double-barrier structure were integrated to form a bi-directional switching device. In the center of the triangular-barrier structure, a delta-doped (δ-doped) quantum well was inserted to enhance the carrier accumulation. Owing to the resonant tunneling through the double barrier and avalanche multiplication in the reverse-biased junction, N-shaped and S-shaped negative-differential-resistance phenomena occurred in the current–voltage (I–V) characteristics under normal and reverse operation modes, respectively. The device characteristics also showed variations from dark to illumination conditions.
Parasitic conduction phenomena in modulation doped CdTe/CdMgTe:I heterostructures grown on GaAs substrates
Authors: Wasik D. , Baj M. , Siwiec-Matuszyk J. , Wojtowicz T. , Janik E. , Karczewski G.
Published: July 2002

We have shown how various parasitic phenomena can influence electrical properties of low dimensional CdTe/Cd1−xMgxTe heterostructures, and how they can be minimized or even eliminated. We have found that: (1) a parallel conduction caused by spontaneous formation of a potential valley in the region of the doping with iodine donors is eliminated if a spacer separating QW from the doping region is not thicker than 20 nm and the quantum well depth is greater than about 240 meV; (2) the cap layer at least 150 nm wide protects free electrons in the quantum well and/or the doping layer from trapping by surface defects; (3) the use of thick buffer layer between the substrate and the structure leads to a significant improvement of the sample stability under hydrostatic pressure making a degradation process, observed even without pressure, much less essential than in samples with thin buffer layers.
Finite element analysis of epitaxial lateral overgrown GaN: Voids at the coalescence boundary
Authors: Benyoucef M. , Kuball M. , Hill G. , Wisnom M. , Beaumont B. , Gibart P.
Published: July 2002

We report on the finite element analysis of stress distribution at the coalescence boundary in epitaxial lateral overgrown (ELO) GaN related to voids. Different void geometries were considered in our model to investigate the influence of their size/shape on the stress distribution. Large compressive stress is localized in the vicinity of the voids, also an increased tensile stress is present at the corners of the SiN mask. Confocal micro-Raman mapping experiments confirm the presence of increased stress at the coalescence boundary of ELO GaN.
Measurement of shadow mask vibration and Q value using two position-sensitive detectors
Authors: Bae Jong Han , Kim Jung-Ki , Kang Sung Gu , Nam Eun Hee , Kwon Jin Hyuk , Jeong Tae Young , Shin Ki Mog
Published: July 2002

A noncontact optical system that can measure both the vibration characteristics of the shadow mask and the relative distance (Q value) between the shadow mask and the phosphor surface of the glass panel of a cathode ray tube (CRT) is developed. Two position-sensitive detectors (PSDs) were employed to detect the positions of two beams that were reflected by the shadow mask surface and the inner surface of the glass panel. The relative distance between them is critical for the performance of the CRT. The vibration signals obtained from the PSDs were displayed in both temporal domain and spectral domain by using LabView software. The resonance frequencies of 20, 75, 400, and 490 Hz were identified and the low 20 Hz component was responsible for generating discomfort to the human eye.
Dustbuster: a compact impact-ionization time-of-flight mass spectrometer for in situ analysis of cosmic dust
Authors: Austin Daniel E. , Ahrens Thomas J. , Beauchamp J. L.
Published: July 2002

We report on the design and testing of a compact impact-ionization time-of-flight mass spectrometer for analysis of cosmic dust, suitable for use on deep space missions. The instrument, Dustbuster, incorporates a large target area with a reflectron, simultaneously optimizing mass resolution, particle detection, and ion collection. Dust particles hit the 65-cm2 target plate and are partially ionized by the impact. The resulting ions, with broad energy and angular distributions, are accelerated through a modified reflectron, focusing ions of specific m/z in space and time to produce high-resolution mass spectra. The cylindrically symmetric instrument is 10 cm in diameter and 20 cm in length, considerably smaller than previous in situ dust analyzers, and can be easily scaled as needed for specific mission requirements. Laser desorption ionization of metal and mineral samples embedded in the impact plate simulated particle impacts for evaluations of instrument performance. Mass resolution in these experiments ranged from 60–180, permitting resolution of isotopes. The mass spectrometer can be combined with other instrument components to determine dust particle trajectories and sizes.
Improvements to the design of an electrostatic toroidal backscattered electron spectrometer for the scanning electron microscope
Authors: Rau E. I. , Khursheed A. , Gostev A. V. , Osterberg M.
Published: July 2002

Design improvements of a toroidal sectoral backscattered electron energy spectrometer adapted for application in the scanning electron microscope were made. Various geometrical parameters in the design of the spectrometer were optimized on the basis of simulation and improvement in the energy resolution of the spectrometer by more than a factor of 2 was confirmed by experiment.
Fast switchable nematic emulsions with twisted director
Authors: Di Profio G. , Lanzo J. , Nicoletta F. P. , De Filpo G. , Chidichimo G.
Published: July 2002

Switchable nematic emulsions, obtained by a fast cooling of isotropic mixtures of nematic liquid crystal and monomers, show very low saturation fields but rather long relaxation times because of the bipolar configuration adopted by nematic directors. Such inconvenience has prevented up to now use of switchable nematic emulsions as electro-optical devices. In order to obtain a good compromise between low saturation fields and fast relaxation times, we have experimentally studied the effect of the addition of a chiral dopant on the electro-optical properties of switchable nematic emulsions. The addition of chiral molecules results in a significant decrease of decay times, while the saturation fields are slightly affected. Results are in a rather good agreement with a phenomenological model, which takes into account the increase of elastic energy due to the induced twist deformation.
Micromagnetic modeling of overlaid exchange-biased giant magnetoresistance head
Authors: Zheng Yuankai , You Dan , Wu Yihong
Published: July 2002

Overlaid exchange-biased structures for giant magnetoresistance head have been proposed and investigated. A home-developed three-dimensional micromagnetic modeling tool has been used to simulate synthetic antiferromagnetic spin valves of this type of biased structure with dimensions of 100 nm in width and 80 nm in height. Simulation results showed that, with a properly chosen antiferromagnetic material and structure, the exchange-biasing field could be made sufficient to suppress the noise without severely reducing the sensitivity. The sensitivity of overlaid exchange-biased spin valves is 1.73 times that of the abutted hard biased ones. Microtrack profiles showed that side reading effect could be suppressed effectively with an effective exchange-biasing field over 600 Oe.
Micromagnetic simulations of antiferromagnetically coupled media
Authors: Greaves S. J. , Tham K. K. , Miura M. , Goodman A. M.
Published: July 2002

Antiferromagnetically coupled media were simulated using a micromagnetic model. Magnetic parameters, initially determined by fitting hysteresis loops to experimental media, were varied to determine their effect upon recording performance. The signal-to-noise ratio can be enhanced by tailoring the magnetic properties of the bias layer to an optimum Ms, introducing a small amount of exchange coupling and orienting the media.
Electronic structure and bonding in unligated and ligated FeII porphyrins
Authors: Liao Meng-Sheng , Scheiner Steve
Published: July 2002

The electronic structure and bonding in a series of unligated and ligated FeII porphyrins (FeP) are investigated by density functional theory (DFT). All the unligated four-coordinate iron porphyrins have a A2g3 ground state that arises from the (dxy)2(dz2)2(dπ)2 configuration. The calculations confirm experimental results on Fe tetraphenylporphine but do not support the resonance Raman assignment of Fe octaethylporphine as Eg3, nor the early assignment of Fe octamethyltetrabenzporphine as B2g5. For the six-coordinate Fe–P(L)2 (L=HCN, pyridine, CO), the strong-field axial ligands raise the energy of the Fe dz2 orbital, thereby making the iron porphyrin diamagnetic. The calculated redox properties of Fe–P(L)2 are in agreement with experiment. As models for deoxyheme, the energetics of all possible low-lying states of FeP(pyridine) and FeP(2-methylimidazole) have been studied in detail. The groundstate configuration of FeP(2-methylimidazole) was confirmed to be high-spin (dxy)2(dz2)1(dπ)2(dx2−y2)1; FeP (pyridine) is shown to be a poor model for high-spin deoxyheme.
Isotope effects in liquid water by infrared spectroscopy
Authors: Max Jean-Joseph , Chapados Camille
Published: July 2002

The light and heavy liquid water (H2O–D2O) mixtures in the 0–1 molar fraction were studied in the mid-infrared by Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy. Five principal factors were retrieved by factor analysis (FA). When D2O is mixed with H2O, the HDO formed because of the hopping nature of the proton (H or D) results in three types of molecules in equilibrium. Because of the nearest-neighbor interactions, the three molecules give rise to nine species. Some of the species evolve concomitantly with other species giving the five principal factors observed. We present the spectra of these factors with their abundances. The calculated probability of the species present at different molar fractions which when the concomitant species are combined gives the observed abundances. To appreciate clearly the difference between the principal spectra, a Gaussian simulation of the bands was made. Because of the numerous components that make up the stretch bands, they are not very sensitive to changes in composition of the solutions; nevertheless, they do indicate the presence of new entities other than the pure species. The deformation bands, more sensitive to such changes than the stretch bands, clearly indicate the presence of the three types of molecules as well as of intermediate species. These bands are sensitive to the two hydrogen bonds on the oxygen atom that a reference molecule makes with its nearest-neighbors, but not to the hydrogen bonds that the nearest-neighbors make with the next nearest neighbors.
Turbulence statistics in an open rotor–stator configuration
Authors: Lygren M. , Andersson H. I.
Published: July 2002

Turbulence statistics, including complete budget data for the six individual components of the Reynolds stress tensor, have been compiled from a direct numerical simulation of the three-dimensional flow in an open rotor–stator configuration. Comparisons between turbulence statistics near the two disks and in plane Couette flow showed that the stator-side data closely resemble those found in Couette flow. At the rotor side rotational production terms due to the Coriolis force made only modest contributions to the budget equations for the Reynolds stresses, but the subtle influence of system rotation was by no means negligible.
Temperature dependence of the magnetization of transition-metal thin films: Fe- monolayers and bilayers
Authors: Garibay-Alonso R. , Dorantes-Dávila J. , Pastor G. M.
Published: July 2002

The functional integral theory of bulk itinerant magnetism is extended to be applied to low-dimensional transition-metal systems. Using a real-space recursive expansion of the local Green’s function we calculate the magnetization curves and the Curie temperature TC for the bulk, monolayer, and bilayer of Fe. Disorder is treated by using the virtual crystal approximation. The local-environment dependence of TC is analyzed by comparing the results for bulk and the (001) and (110) oriented films of Fe. Finally, the applicability of the theory is discussed.
Thermodynamic stability and diffusivity of near-equiatomic Ni–Mn alloys
Authors: Ding Ling , Ladwig Peter F. , Yan Xinyan , Chang Y. Austin
Published: July 2002

The equilibrium phase diagram of the nickel-manganese system is determined between 500 and 850 °C, in the composition range between 25 and 70 at. % Ni. A combination of electron probe microanalysis, x-ray diffraction, and optical microscopy was employed to analyze 47 samples that were annealed anywhere from three to seven months. The equiatomic, antiferromagnetic, L10-NiMn phase that is of considerable technological interest was found to exist continuously between 500 and 700 °C. No other intermediate phases were found in this study at low temperatures. These results are in contrast to the currently accepted phase diagram published in most handbooks. A hot-isobaric-pressing method was used to initially bond samples that were subsequently used to determine interdiffusion coefficients in the Ni–Mn system at 650 °C. The Boltzmann–Matano method [T. Heumann, Z. Phys. Chem. 201, 168 (1952)] allowed the calculation of these interdiffusion coefficients across the α-Mn, β-Mn, γ-Mn, L10-NiMn, and γ-Ni phases.
Short-range impurities in d-dimensional uniaxial dipolar magnets
Authors: Nicolaides Demetris
Published: July 2002

Dipole–dipole interactions affected by a Gaussian random field destroy long-range order in three dimensions for uniaxial magnetic systems with low critical temperature. Specifically, it is derived that the critical behavior of these magnets occurs in a new spatial range, 3<d<5.
Spin glass properties of Y0.7Ca0.3MnO3 and Th0.35Ba0.37Ca0.28MnO3
Authors: Sedmidubský D. , Hejtmánek J. , Maryško M. , Jirák Z. , Hardy V. , Martin C.
Published: July 2002

Two perovskite manganites with the same nominal concentration of Mn4+ have been studied by the magnetic susceptibility and specific heat measurements: the Y0.7Ca0.3MnO3 single crystal (YCa) with low tolerance factor and negligible ionic radii variance and the Th0.35Ba0.37Ca0.28MnO3 (ThBaCa) ceramics with favorable tolerance factor but huge substitution induced disorder. Both systems were found to possess an insulating ground state with activated conductivity. The ac susceptibility data exhibit a frequency dependent peak at Tsg=32 K for YCa and Tsg=43 K for ThBaCa. The Cole–Cole analysis revealed a sharper than exponential increase of the relaxation time for the former sample while almost linear slowly growing characteristics below Tsg were observed for the latter one. Using the heat capacity curves, the magnetic contribution was separated and magnetic peaks with typical linear on-set tails were obtained. This observation and the abovementioned characteristics are all indicative of spin or cluster glass character. The necessary magnetic frustration is here induced by a considerable tilting of MnO6 octahedra and deformation of Mn–O bonds which occur in a cooperative manner for YCa and locally for ThBaCa.
Low-temperature Raman spectra of nitromethane single crystals. Lattice dynamics and Davydov splittings
Authors: Ouillon R. , Pinan-Lucarré J.-P. , Ranson P. , Baranovic G.
Published: July 2002

By means of Raman scattering experiment performed on single crystals of solid nitromethane in various polarization geometries at normal pressure and low temperatures, the Davydov splitting of the vibrational levels in the solid phase of hydrogenated CH3NO2 and perdeuterated CD3NO2 nitromethane is reported for the first time. A re-examination of the lattice vibrations in their various symmetry species is also given. These experimental data are discussed on the basis of calculations using harmonic models of stretching, bending, and torsional motions of an isolated molecule and using density functional theory and second-order Møller–Plesset perturbation theory. Then, following Sorescu et al. [J. Phys. Chem. B 104, 8406 (2000).], the intermolecular interactions (Buckingham 6-exp form plus charge–charge Coulombic interactions) were added to the intramolecular potential. The frequencies of the q≈0 phonons or vibrons of different symmetry species were calculated paying a special attention to the symmetric and antisymmetric NO2 stretching modes whose wave numbers and bandwidths show specific behavior in solid phase transition induced by pressure at ambient temperature. The calculated Davydov splittings are in reasonable agreement with the observed ones with the largest deviation found just for the NO2 stretching vibrations. Finally, it emerges from the calculations that there is no pure torsion in the lattice region and that the mixed character of phonons (libration+translation+torsion) prevents the use of simple formulas for determination of the isotopic shifts.
Segmental dynamics of disordered styrene–isoprene tetrablock copolymers
Authors: Doxastakis M. , Chrissopoulou K. , Aouadi A. , Frick B. , Lodge T. P. , Fytas G.
Published: July 2002

The local segmental dynamics of four styrene-b-isoprene-b-styrene-b-isoprene (SISI) tetrablock copolymers with different styrene composition fs and constant total degree of polymerization N≈120 has been studied in the disordered state in the nano-picosecond time scale, by incoherent quasielastic neutron (QENS), and Brillouin (BS) and depolarized Rayleigh (DRS) light scattering. Far above the glass transition temperature, all three techniques demonstrate the presence of two distinct time scales from which the fast segmental relaxation was quantitatively resolved. This process is associated with the mobility of the polyisoprene (PI) component, and is moderately slower and possesses a broader distribution of relaxation times than in bulk PI. The comparison between the correlation times of DRS and the characteristic times of QENS suggest that segment (hydrogen nucleus) diffusion over a distance of ≈0.8 nm suffices for the loss of local orientation correlations. The faster times of the BS experiment correspond to shorter displacements, ≈0.3 nm. These results demonstrate that the segmental dynamics of the PI are much faster than would be inferred from the monomeric friction factor of PI previously extracted by diffusion and viscosity measurements in the same tetrablock matrices. This, in turn, indicates a substantial local spatial heterogeneity in the segmental dynamics. The slow process is due to the PS segments, which do not relax, appreciably on the time scales accessible here.
Study of magnetic field determined from vanishing rotational hysteresis loss in CoCrPtB perpendicular thin film media
Authors: Saito Shin , Sato Yoshihiro , Hoshi Fumikazu , Djayaprawira David D. , Takahashi Migaku
Published: July 2002

The magnetic field determined from vanishing rotational hysteresis loss of CoCrPtB perpendicular thin film media was experimentally investigated. It was clarified that (1) in Co72Cr16Pt8B4 media with dmag of less than 200 nm and Co64Cr24Pt8B4 media with dmag of less than 100 nm, a homogeneous structure was realized, except for the existence of an initial growth region. (2) For both media, the perpendicular rotational hysteresis loss, Wr, the maximum field of Wr, Hp, and the field where Wr=0, HWr=0, increased with an increase in dmag. (3) For both media, HWr=0/Hkgrain increased with an increase in dmag and saturated at a magnitude of about 1. (4) For the medium with thin magnetic film thickness, at applied field of HWr=0<H<Hkgrain, a multidomain state was realized in the torque measurement in spite of no observation of loss of rotational hysteresis. This is considered to be due to the existence of thermal agitation between the multidomains.
HITPERM soft magnetic underlayers for perpendicular thin film media
Authors: Kumar S. , Ohkubo T. , Laughlin D. E.
Published: July 2002

In this work, a class of nanocrystalline alloys, HITPERM (Fe, Co)–M–B–Cu (M=Zr, Hf, Nb, and etc.) found to exhibit excellent soft–magnetic properties in bulk were used as soft–magnetic underlayers for perpendicular thin film media. A Ti intermediate layer was used to promote a (00⋅2) texture and exchange de-couple the magnetic layer (CoCrPt) from the soft-magnetic underlayer. Specimens were deposited at both room and elevated temperature (∼ 250 °C). The results of x-ray diffraction and transmission electron microscope structural studies, along with magnetic properties are presented.
A model of pole tip recession growth in magnetic heads
Authors: Scott William W. , Bhushan Bharat
Published: July 2002

Pole tip recession (PTR), which is caused by differences in wear resistance of the various phases in magnetic disk and tape heads, must be reduced because it is probably the main impediment to reduction of spacing. Here, a numerical model of PTR is presented. Experimental results suggest that three-body abrasion, which leads to primarily plastic wear, is the mode responsible for PTR. In the model, the authors assume this wear mode, and assume that wear is a function of load, sliding distance, hardness of the worn surface, and dimensionless wear coefficients. As sliding proceeds, PTR increases continuously, which leads to a continuous change in contact between three-body abrasives and poles; the load carried by the poles decreases as PTR increases. This is modeled by assuming that asperities of the head and medium deform and wear plastically to accommodate passage of particles through the interface. The pole wears less as PTR increases since the opening for sliding particles increases. This accounts for the decreasing rate of change of PTR with sliding distance found for disk and tape heads numerically and experimentally. For tape heads, the model predicts that each of the following leads to higher PTR: increasing the thickness of three-body particles, increasing tape tension, decreasing pole hardness, and increasing the pole wear coefficient.
Two dimensional metallic conductivity in Pr1−xSrxMnO3 antiferromagnets
Authors: Hejtmánek J. , Jirák Z. , Pollert E. , Sedmidubský D. , Strejc A. , Martin C. , Maignan A. , Hardy V.
Published: July 2002

A complex magnetic, structural, transport and thermal investigation has been performed using a series of well sintered ceramics in the x=0.50–0.60 range. Fine steps of the substitution of Sr2+ for Pr3+ enabled us to evidence the evolution of magnetic transtions with x and to exemplify the impact of concentration of eg electrons on charge carrier and thermal transport. The lowering of x from 0.56 down to 0.50 led to the concomitant decrease of the low temperature electrical conductivity and thermoelectric power pointing thus to impeded charge carrier transport for samples with x∼0.5. Nonetheless, the band-like character of eg carriers for these compounds was independently confirmed by low temperature specific heat measurement.
Maximal accelerations for charged drops in an electric field
Authors: Mestel A. J.
Published: July 2002

The equilibrium shapes of highly conducting, charged drops accelerating in an electric field are found. The maximum possible charge for a given field stength and surface tension is calculated. A spheroidal approximation often used for uncharged drops is generalized to include charge and is found to agree very well with the numerical solution when account is taken of the asymmetric surface stress. The charge and field values giving rise to the maximum possible acceleration of the drop are found with a view to optimizing the efficiency of certain electrospraying devices. The effects of air resistance are considered, at both low and high Reynolds numbers. In the former case, internal motion of the drop contributes to the shape, but the results are broadly similar to those for the accelerating drop. At high Reynolds number potential air flow with a constant pressure wake is assumed. It is found that the drop shape can be oblate in this case and may be stable for higher charge and field values.
Magnetic domain wall transitions based on chirality change and vortex position in thin Permalloy™ films
Authors: Redjdal M. , Kákay A. , Ruane M. F. , Humphrey F. B.
Published: July 2002

The three possible transitions of a wall involving a change of chirality and position of a vortex were previously identified in Permalloy™ using the Kerr effect. These transitions have now been simulated using direct integration of the Landau–Lifshitz–Gilbert equation in a 1 300 000 point Cartesian lattice. One transition is between two C-shaped, same chirality walls whose vortices are on opposite sides. The transition is done via a 32-nm-long pi-VBL structure at the surfaces and at the center via a shape transition 117 nm long. The pi-vertical Bloch line (pi-VBL), which changes chirality along the wall, conducts the flux between the walls of opposite chirality via a vortex structure by letting the magnetization rotate out of the plane of the walls at the center of this vortex. The Néel caps switch chirality via an antivortex flux at one surface and a vortex flux at the other surface. Another transition is a pi-VBL that takes place between two C-shaped, opposite chirality walls whose vortices are on opposite sides of the walls. The transition between the Néel caps of same chirality is a jog at the top surface, while it is a vortex–antivortex pair at the bottom surface.
Three dimensional eddy current calculation using magnetic scalar potential in conducting regions
Authors: Hur Jin , Hyun Dong-Seok , Kim Soo-Seok , Kang Gyu-Hong , Hong Jung-Pyo
Published: July 2002

A computational method for analysis of the 3D eddy current fields has been developed. The method is capable of modeling eddy currents and analyzing their characteristics using only a scalar potential at each node with less computational time than other 3D analysis techniques. A benchmark model of an asymmetrical conductor with a hole is analyzed to verify the application of the developed method. Finally, the validity of the developed method is confirmed by comparing the calculated values with both the measured values and 3D finite element method results.
Fabrication, magnetic properties, and electronic structures of nanoscale zinc-blende MnAs dots (invited)
Authors: Ono Kanta , Okabayashi Jun , Mizuguchi Masaki , Oshima Masaharu , Fujimori Atsushi , Akinaga Hiro
Published: July 2002

Ferromagnetic nanoscale zinc-blende MnAs dots were successfully fabricated on a sulfur-passivated GaAs (001) surface by molecular-beam epitaxy. Transmission electron microscopy and selected area electron diffraction showed that the crystalline structure was not the same as that of bulk MnAs with NiAs-type hexagonal crystalline structure, but of zinc-blende type. In in situ photoemission spectroscopy of the zinc-blende MnAs dots, the Fermi edge was not clearly observed and the Mn 3d partial density of states was similar to that of the diluted ferromagnetic semiconductor Ga1−xMnxAs, which also supports the fabrication of zinc-blende MnAs in the nanoscale.
Charge stacking in the half-doped manganites
Authors: Popović Z. , Satpathy S.
Published: July 2002

The stability of the charge-stacked structure vis-à-vis the charge-alternate structure in the half-doped manganites is studied with a model that includes electronic kinetic energy, onsite and intersite Coulomb interactions, the Jahn–Teller energy, and the antiferromagnetic superexchange between the manganese core spins. It is shown that for a single zigzag chain, the electronic kinetic energy stabilizes the standard chain, with Mn3+ at the bridge site and Mn4+ at the corner site, over the “reversed” zigzag chain with the two Mn valences interchanged. The electronic kinetic energy and magnetic interactions stabilize the three-dimensional charge-stacked structure, while a large intersite Coulomb interaction V⩾Vc would stabilize the charge-alternate structure. It is argued that the magnitude of V is small enough that the charge-stacked structure is stabilized in the half-doped manganites such as La1/2Ca1/2MnO3.
Investigation of the magnetic properties in strontium–borate vanadate glasses
Authors: Chinkhota Manford , Fodor Petru S. , Khattak G. D. , Wenger L. E.
Published: July 2002

To further elucidate the nature of the valence state of V ions in vanadate glasses, magnetic susceptibility measurements in the temperature range of 5 to 300 K have been performed on a series of vanadium–strontium–borate (V2O5+SrO+B2O3) oxide glasses with V2O5 concentrations greater than 50 mol %. The magnetic susceptibility for these oxide glasses is found to consist of a temperature-independent paramagnetic contribution arising from V2O5 and a Curie–Weiss temperature-dependent contribution associated with magnetic V4+ ions being present in concentrations between 2% and 10% of the total V concentration. The negative Curie–Weiss temperatures in the range of 0 to −2.8 K indicate a weak antiferromagnetic interaction between the V4+ ions. These results are consistent with a glass network structure consisting of VO5 polyhedra in which the V4+ would be predominantly isolated species, and any interactions between the V4+ ions would result from superexchange interactions through V–O–V bonds.
Critical exponents of inhomogeneous ferromagnets
Authors: Berger A. , Campillo G. , Vivas P. , Pearson J. E. , Bader S. D. , Baca E. , Prieto P.
Published: July 2002

A method for the analysis of temperature and field dependent magnetization data near the Curie temperature TC is devised, which allows the reliable determination of critical exponents even in strongly inhomogeneous ferromagnets. It is shown, both experimentally and theoretically, that the field-induced broadening of the ferromagnetic phase transition follows a power law with a characteristic critical exponent β⋅δ, where β and δ are the conventional temperature and field dependent critical exponents of the magnetization. Experimental results on inhomogeneous La2/3Ca1/3MnO3 films yield critical exponents β=0.3682±0.0033 and δ=4.76±0.32, even though the intrinsic TC distribution width is larger than the field-induced effect.
Effect of impurities on quasi-two-dimensional quantum antiferromagnet
Authors: Chernyshev A. L. , Chen Y. C. , Castro Neto A. H.
Published: July 2002

We have studied the static and dynamic properties of quasi-two-dimensional (2D) quantum antiferromagnets (AFs) diluted with spinless impurities using spin-wave theory and T-matrix approximation. We show that the spectrum of a 2D AF at long wavelengths is overdamped at an arbitrary concentration of spinless impurities. The scattering leads to a length scale l/a∼eπ/4x, x being impurity concentration and a the lattice spacing, beyond which the influence of impurities on the spectrum is dominant. Although the dynamical properties are significantly modified we show that 2D is not the lower critical dimension for this problem. Thus, in low-dimensional systems with disorder the connection between static and dynamic quantities is not straightforward. Our results are in quantitative agreement with the recent Monte Carlo simulations and experimental data for S=1/2, S=1, and S=5/2. We have also proposed experiments which can further test the results of our theory.
On the spectral problem for boundary-layer flows
Authors: Ng B. S. , Reid W. H.
Published: July 2002

In this article, we derive an asymptotic approximation to the eigenvalue relation for boundary-layer flows with a relative error of order (αRe)−1/2 using only first-order outer (WKB) approximations which are valid in the “complete” sense. This approximation involves only elementary transcendental functions and it appears to be capable of providing an accurate global description of both the temporal and spatial spectra for flows of boundary-layer type.
A parametric study of self-similar collapsing shock waves in radiating gas
Authors: Hirschler T.
Published: July 2002

For a collapsing shock wave in a thermal radiating thick gas the temperature gradient at the shock front is a free parameter in the framework of similarity. It must be adjusted to a particular converging shock-broadening problem to achieve information about the collapse of this shock wave. In a previous work a constraint was put on this temperature gradient to show only the influence of less radiation upon the similarity solution in the thick limit. In this paper a variation of this temperature gradient is permitted. In the framework of a parametric study the whole variety of possible similarity solutions is shown. Special attention is paid to those solutions showing the influence of much radiation. For the sake of a better overview, the parametric study first deals with a shock front where no radiative heat flux is exchanged between the shock front and the flow field, but the absorptivity of the gas is altered. Then the absorptivity of the gas is kept constant, but a heat flux exchange between the shock front and the flow field is permitted. In the framework of this parametric study, the isothermal solution plays the role of a limiting solution for a decreasing absorptivity. Solutions are shown up to this extreme and the isothermal solution is treated separately.