bluuebunny/crossref_metadata_2025 huggingface dataset:
Nanoscale phase separation in Fe3O4(111) films on sapphire(0001) and phase stability of Fe3O4(001) films on MgO(001) grown by oxygen-plasma-assisted molecular beam epitaxy
Authors: Farrow R. F. C. , Rice P. M. , Toney M. F. , Marks R. F. , Hedstrom J. A. , Stephenson R. , Carey M. J. , Kellock A. J.
Published: April 2003

We report a phase instability in oxygen-plasma-assisted molecular beam epitaxy of Fe3O4 films on sapphire (0001) substrates. Under a wide range of growth conditions, Fe3O4(111) films phase separate, on a nanometer length scale, into Fe3O4, FeO and metallic Fe, which is attributed to formation of the thermodynamically unstable phase FeO in the initial stages of (111) growth. In contrast, Fe3O4(001) films, grown simultaneously on MgO(001) substrates, do not exhibit this phase instability. We specify growth conditions for which single-phase, epitaxial Fe3O4(111) films can be grown by plasma-assisted molecular beam epitaxy or by reactive evaporation of Fe in molecular oxygen. Film orientation and phase separation strongly influence magnetic properties. Single-phase Fe3O4(111) films are much more difficult to magnetize than Fe3O4(001) films and phase separation makes the films even more difficult to magnetize.
Morphology and interdiffusion behavior of evaporated metal films on crystalline diindenoperylene thin films
Authors: Dürr A. C. , Schreiber F. , Kelsch M. , Carstanjen H. D. , Dosch H. , Seeck O. H.
Published: April 2003

We present a transmission electron microscopy (TEM)/Rutherford backscattering spectrometry (RBS)/x-ray-diffraction (XRD) study of Au evaporated on crystalline organic thin films of diindenoperylene (DIP). Cross-sectional TEM shows that the preparation conditions of the Au film (evaporation rate and substrate temperature) strongly determine the interfacial morphology. In situ XRD during annealing reveals that the organic layer is thermally stable up to about 150 °C, a temperature sufficient for most electronic applications. The x-ray measurements show that the “as-grown” Au layer exhibits a large mosaicity of around 10°. Upon annealing above ≈120 °C the Au film starts to reorder and shows sharp (111)-diffraction features. In addition, temperature dependent RBS measurements indicate that the Au/DIP interface is thermally essentially stable against diffusion of Au in the DIP layer up to ≈100 °C on the time scale of hours, dependent on the Au thickness.
Tails of near-edges x-ray absorption spectra as a fingerprint of magnetic and structural phase transitions. Application to metallic 3d ultra thin films
Authors: Marangolo M. , Ohresser P. , Brooks N. B. , Cherifi S. , Boeglin C. , Eddrief M. , Etgens V. H.
Published: April 2003

Tails of x-ray absorption near-edge structure (XANES)/x-ray magnetic circular dichroism spectroscopy (XMCD) spectra (50–100 eV above the edges) are characterized by the beginning of the extended x-ray absorption fine-structure/magnetic extended x-ray absorption fine-structure oscillations. We present a systematic study of these tails for three different thin films: Fe and Fe0.65Ni0.35 on stepped Cu(111) and Fe/ZnSe(001). The dependance of the detected oscillations on the film thickness leads us to conclude that XANES/XMCD tails are a useful in situ probe of the structural and magnetic phase of the sample.
Rapid analysis of phase behavior with density functional theory. I. Novel numerical methods
Authors: Salinger Andrew G. , Douglas Frink Laura J.
Published: April 2003

The phase behavior of confined fluids is rich even for simple models of fluids and simple confining geometries. There has been a great deal of work to understand these systems, and density functional theories (DFT) of inhomogeneous fluids are often applied to determine phase diagrams quickly for these simple systems where symmetry in the physical problem reduces the computational problem to a one-dimensional calculation. More recently, there has been interest in developing DFT algorithms for treating fluids in complex confining geometries or at chemically heterogeneous surfaces where two- or three-dimensional calculations are required. In this paper we present three algorithms for the rapid and robust study of phase behavior in DFT models of inhomogeneous fluids and demonstrate their utility by analyzing capillary condensation in slit pores and ordered two-dimensional arrays of cylindrical fibers. The three algorithms are arclength continuation algorithms for tracing connected stable, metastable, and unstable branches, a phase transition tracking algorithm that allows for rapid computation of phase envelopes, and a spinodal tracking algorithm that allows one to assess the limits of metastability of a given state. In Paper II of this series, we apply these algorithms in a detailed investigation of capillary condensation in disordered porous media.
Rapid analysis of phase behavior with density functional theory. II. Capillary condensation in disordered porous media
Authors: Douglas Frink Laura J. , Salinger Andrew G.
Published: April 2003

For some time, there has been interest in understanding adsorption and capillary condensation in disordered porous media from a molecular perspective. It has been documented that the free energy landscape in these systems is complex with many metastable states. In this paper we explore the complexity of adsorption and capillary condensation in several simple models of disordered porous media constructed with parallel cylindrical fibers. We present nonlocal density functional theory calculations on a Lennard-Jones model fluid adsorbing in these porous materials coupled with the arclength continuation and phase transition tracking algorithms we presented in Paper I of this series. The arclength continuation algorithm allows us to trace out all the possible states between vapor-filled and liquid-filled pores. We find that capillary condensation is likely to occur in stages at high temperatures and strong wall–fluid interactions while the condensation occurs as a single transition at low temperatures and weak wall–fluid interactions. This paper also compares the extent of hysteresis on adsorption and desorption, discusses the validity of the Gibbs adsorption equation, and considers application of simple pore models in predicting the complexity of phase diagrams in disordered porous media.
Quasiclassical trajectory study of the dynamics of the H+N2O reaction on a new potential energy surface
Authors: Castillo J. F. , Collins M. A. , Aoiz F. J. , Bañares L.
Published: April 2003

A new ab initio potential energy surface (PES) for the H+N2O→OH+N2 reaction has been constructed using the GROW package of Collins and co-workers. The ab initio calculations have been done using the Becke three-parameter nonlocal exchange functional with the nonlocal correlation of Lee, Yang, and Parr density functional theory. A detailed quasiclassical trajectory study of integral and differential cross sections, product rovibrational populations, and internal energy distributions on the new PES is presented. The theoretical integral cross sections as a function of collision energy are in qualitative agreement with the experimental measurements. A good correspondence is found between the calculated OH(v′=0,1) rovibrational populations and the recent measurements of Brouard and co-workers at 1.48 eV collision energy. In particular, the calculated kinetic energy release distributions for state resolved OH(v′,N′) products predict a substantial fraction of total energy going into rotational excitation of the N2 co-product, in good agreement with the experimental findings.
Relationship between charge distribution and its image by electrostatic force microscopy
Authors: Lambert Jérôme , Guthmann Claudine , Saint-Jean Michel
Published: April 2003

We shall demonstrate in this article that characterization of the charge distribution by electrostatic force microscopy is not straightforward: we will stress the important role played by the electrostatic images of the scanned charge distribution in the tip and by the operating mode in the formation of extra features in images obtained with this instrument. To illustrate this, we will describe two models that correspond to the scanning of small and extended charge distributions. These models will be compared with experimental images.
Electronic structure and magnetic properties of Ni2MnGa alloy films with different structural orders
Authors: Rhee J. Y. , Kudryavtsev Y. V. , Dubowik J. , Lee Y. P.
Published: April 2003

The structural dependence of magnetic and transport properties of nearly stoichiometric Ni2MnGa alloy films prepared by flash evaporation is investigated. The films deposited at 720 K have the L21 Heusler structure and behave nearly the same as the bulk Ni2MnGa alloy, while the films deposited at 150 K have an almost amorphous structure. Disordered Ni2MnGa alloy films do not exhibit magnetic ordering down to 4 K, but rather have the characteristics of highly resistive Pauli paramagnets. Annealing of disordered films restores the L21 structure. The magnetic and transport properties of the annealed films are similar to those of the films deposited at 720 K. A possible explanation of the loss of magnetic moment in the disordered film is discussed with the aid of first-principles electronic-structure calculations.
Influence of the Ar-ion irradiation on the giant magnetoresistance in Fe/Cr multilayers
Authors: Kopcewicz M. , Stobiecki F. , Jagielski J. , Szymański B. , Schmidt M. , Dubowik J. , Kalinowska J.
Published: April 2003

The influence of 200 keV Ar-ion irradiation on the interlayer coupling in Fe/Cr multilayers exhibiting the giant magnetoresistance (GMR) effect is studied by the conversion electron Mössbauer spectroscopy (CEMS), vibrating sample magnetometer hysteresis loops, magnetoresistivity, and electric resistivity measurements and supplemented by the small-angle x-ray diffraction. The increase of Ar-ion dose causes an increase of interface roughness, as evidenced by the increase of the Fe step sites detected by CEMS. The modification of microstructure induces changes in magnetization reversal indicating a gradual loss of antiferromagnetic (AF) coupling correlated with the degradation of the GMR effect. Distinctly weaker degradation of AF coupling and the GMR effect observed for irradiated samples with a thicker Cr layer thickness suggest that observed effects are caused by pinholes creation. The measurements of temperature dependence of remanence magnetization confirm increase of pinhole density and sizes during implantation. Other effects which can influence spin dependent contribution to the resistance, such as interface roughness as well as shortening of mean-free path of conduction electrons, are also discussed.
Structural characterization using spectroscopic techniques of Yb films grown on W(110) under ultrahigh vacuum conditions
Authors: Dávila M. E. , Molodtsov S. L. , Avila J. , Laubschat C. , Asensio M. C.
Published: April 2003

Forward scattering photoelectron diffraction has been used to study the structure of Yb(111) films epitaxially deposited onto W(110) single crystals. The measurements have been carried out recording the intensity modulations of the Yb 4f photoemission core levels throughout a complete set of polar and azimuthal scans. We observe the coexistence of two domains with a characteristic face-centered-cubic (fcc) structure. Both domains appear to be rotated 60° toward each other. The possible presence of a hexagonal-close-packed (hcp) phase or a mixture of fcc and hcp phases, which could be stabilized by the presence of oxygen, has been particularly investigated. However, these two possibilities have been disregarded due to the clear disagreement observed between the scattering simulations and the experimental data.
Comparative study of the magnetic and magnetotransport properties of a metallic and a semiconducting member of the solid solution LaNixCo1−xO3
Authors: Androulakis J. , Katsarakis N. , Viskadourakis Z. , Giapintzakis J.
Published: April 2003

We present a comparative study of both the magnetic and magnetotransport properties for two members of the perovskite solid solution LaNixCo1−xO3 (x=0.2, 0.6) located on opposite sides of the chemically induced metal-to-insulator transition. LaNi0.6Co0.4O3 exhibits metallic behavior and small but negative magnetoresistance, whereas LaNi0.2Co0.8O3 exhibits semiconducting behavior and giant negative magnetoresistance at low temperatures. On the other hand, we observe pronounced similarities in the magnetic properties of both compounds. We provide a consistent explanation regarding the origin of the magnetoresistance in the two members of the solid solution.
Anisotropic coarse-grained statistical potentials improve the ability to identify nativelike protein structures
Authors: Buchete N.-V. , Straub J. E. , Thirumalai D.
Published: April 2003

We present a new method to extract distance and orientation dependent potentials between amino acid side chains using a database of protein structures and the standard Boltzmann device. The importance of orientation dependent interactions is first established by computing orientational order parameters for proteins with α-helical and β-sheet architecture. Extraction of the anisotropic interactions requires defining local reference frames for each amino acid that uniquely determine the coordinates of the neighboring residues. Using the local reference frames and histograms of the radial and angular correlation functions for a standard set of nonhomologue protein structures, we construct the anisotropic pair potentials. The performance of the orientation dependent potentials was studied using a large database of decoy proteins. The results demonstrate that the new distance and orientation dependent residue–residue potentials present a significantly improved ability to recognize native folds from a set of native and decoy protein structures.
Calculated paramagnetic resonance parameters (g,Ahfi) of the Re6S8Br63−, Re6S8I63−, and Re6Se8I63− cluster ions
Authors: Arratia-Pérez Ramiro , Hernández-Acevedo Lucı́a
Published: April 2003

Dirac molecular orbital calculations on the octahedral paramagnetic Re6S8Br63−, Re6S8I63−, and Re6Se8I63− 23e cluster ions, are reported. As the parent diamagnetic Re6Q8X64− (Q=S, Se, X=halide, etc.) 24e clusters, the calculated relativistic molecular orbitals indicate that the manifold of closely spaced unoccupied energy levels are mainly localized on the octahedral [Re6Q8]3+ core, while the cluster highest occupied molecular orbitals and the manifold below it are largely centered on the terminal halide ligands. These results suggest that the title 23e cluster ions could also be luminescent. The current calculations predict an isotropic Zeeman interaction for each cluster, which is in good agreement with the available data from electron paramagnetic resonance experiments. We also calculated the paramagnetic hyperfine interactions (Ahfi) of the Rre187, CL35, Br79, I127, S33, and Se77 nuclei, allowing us to describe that the metal and apical ligand magnetic hyperfine tensors are anisotropic, while the magnetic hyperfine tensors of the capping (S, Se) ligands are small and isotropic. These reversible redox (24e/23e) couples [Re6Q8X64−/Re6Q8X63−] could become part of molecular nanocells for applications in molecular electronics, and, also as part of nanosensor devices.
Nuclear spin-spin coupling density in molecules
Authors: Soncini A. , Lazzeretti P.
Published: April 2003

It is shown that nuclear spin-spin coupling in a molecule can be rationalized in terms of property density functions which depend on the position in three-dimensional space. The spin-spin coupling density surface, calculated as a table of values for a grid of coordinates on a plane through the molecular domain, yields a direct physical picture and offers a physical interpretation of the phenomenology, by showing the path whereby coupling takes place. The different role and the relative importance of the Fermi contact, spin-dipolar, and diamagnetic and paramagnetic spin-orbit mechanisms is readily assessed. The display of the spin-spin density reveals that the major contribution comes from the electrons close to the coupled nuclei. The economy of thinking achieved by the use of functions of three coordinates in real space, instead of n-electron wave functions depending on 3n coordinates in Hilbert space, is evident in the present case. The utility of spin-spin coupling densities has been discussed for the molecules of hydrogen fluoride, water, ammonia, and methane.
Analysis of multijunction solar cell degradation in space and irradiation induced recombination centers
Authors: Zazoui M. , Mbarki M. , Aldin A. Zin , Bourgoin J. C. , Gilard O. , Strobl G.
Published: April 2003

We demonstrate how, using electroluminescence, the parameters characterizing the recombination center induced by irradiation in a solar cell can be measured. Because electroluminescence is able to provide information on an individual cell in a multijunction (MJ) cell device, independently of the others, we apply this technique to measure these parameters in InGaP/GaAs/Ge MJ cells. We then calculate the variations of the open-circuit voltage and short-circuit current of such cells versus fluence. The results are compared with experimental data obtained for 1 MeV electron irradiations.
Infrared spectroscopy of the isomers of magnesium–HCN formed in helium nanodroplets: Comparisons with ab initio calculations
Authors: Stiles P. L. , Moore D. T. , Miller R. E.
Published: April 2003

High-resolution infrared spectra are reported for two isomers of the magnesium–HCN binary complex, corresponding to the magnesium bonding at the nitrogen and hydrogen ends of the molecule. Stark spectra are also reported for these complexes, from which the corresponding dipole moments are determined. Ab initio calculations confirm that the potential energy surface has two minima, consistent with the experimentally determined structures. The wave functions of the two dimensional intermolecular coupled cluster singles doubles (triples) potential energy surface, calculated with the collocation method, are also reported for the magnesium–HCN system and are used to calculate the vibrationally averaged dipole moment for both isomers, which are then compared to experiment.
Conformational disorder and energy migration in MEH-PPV with partially broken conjugation
Authors: Grage M. M.-L. , Wood P. W. , Ruseckas A. , Pullerits T. , Mitchell W. , Burn P. L. , Samuel I. D. W. , Sundström V.
Published: April 2003

In order to obtain a better understanding of the role of conformational disorder in the photophysics of conjugated polymers the ultrafast transient absorption anisotropy of partially deconjugated MEH-PPV has been measured. These data have been compared to the corresponding kinetics of Monte Carlo–simulated polymer chains, and estimates of the energy hopping time and energy migration distances for the polymers have been obtained. We find that the energy migration in the investigated MEH-PPV is approximately 3 times faster than in previously studied polythiophenes. We attribute this to a more disordered chain conformation in MEH-PPV.
Diffusion-controlled kinetics of carbon nanotube forest growth by chemical vapor deposition
Authors: Louchev Oleg A. , Laude Thomas , Sato Yoichiro , Kanda Hisao
Published: April 2003

A detailed theoretical study of carbon nanotube (NT) forest growth by chemical vapor deposition is given, including (i) ballistic mode of carbon species impingement into the NT surface, (ii) the carbon diffusion over NT surface and through the metal nanoparticle, and (iii) the temperature drop at the NT tip occurring with increase in NT length. For typical NT forest growth parameters the ballistic flux of carbon species impinging into the NT surface decays quasiexponentially within several microns from the top. A variety of feasible growth modes, ranging from linear to exponential versus time, is predicted agreeing well with reported experiments. The presence of a metal nanoparticle is shown to shift NT growth from being surface diffusion controlled to being controlled by bulk diffusion through the nanoparticle. For typical growth conditions the growth rate is shown to be controlled simultaneously by surface diffusion over NT surface and bulk diffusion of carbon through metal nanoparticle. However, even in specific cases where NT growth rate is controlled by bulk diffusion through the nanoparticle the initial stage may be controlled by surface diffusion, as revealed by the exponential change in NT length with time. A parametric study of the growth rate of NT forest with metal nanoparticles held at the NT tips as a function of temperature reveals the existence of a maximum near 1050–1100 K, agreeing with reported experimental data. A thermal analysis based upon the heat conductance equation shows that with NT forest growth the temperature of the NT tips decreases, leading to growth deceleration and termination. Our study shows that the larger the pressure the smaller the NT forest height that may be grown. In particular, for pressures ≈105 Pa the NT tips should be “frozen” even at a length of a few microns, disabling further NT growth. In contrast, under low pressures of ≈103 Pa NT forest of several dozens of microns may be successfully grown without significant growth deceleration.
On the electric field gradient induced birefringence and electric quadrupole moment of CO, N2O, and OCS
Authors: Coriani Sonia , Halkier Asger , Jonsson Dan , Gauss Jürgen , Rizzo Antonio , Christiansen Ove
Published: April 2003

An ab initio coupled cluster investigation of the electric field gradient induced birefringence of three dipolar linear molecules—CO, N2O, and OCS—is presented. Special emphasis is given to the effect of triple excitations in the cluster operator on the so-called effective quadrupole center as well as to the accurate determination of the molecular electric quadrupole moment relative to this origin. Triple excitations are found to be essential in the case of CO, with reference in particular to the existing disagreement between two semiclassical theories for the interpretation of the experimentally observed birefringence. The present results favor the older theory due to Buckingham and Longuet-Higgins.
Detection of acoustic solitary waves in nonlinear lithium niobate crystals
Authors: Stevenson Adrian C. , Mehta Hiren , Sethi Rajinder S. , Lowe Christopher R.
Published: April 2003

Pulses of acoustic energy that are propagating in a normal linear material have a tendency for their profile to flatten and broaden, becoming depleted and ill-defined in form; in contrast, nonlinear materials narrow the profile of the pulse envelope. However, provided there is sufficient energy in the pulse, propagation in a nonlinear and dispersive material allows these opposing wave characteristics to cancel each other out, so that a persistent packet of energy known as a solitary wave or soliton can form. Here, we provide experimental evidence for the existence of acoustic solitons and describe the instrumentation, transducers, material conditions, and appropriate solitary wave test for acoustic pulses. Our key contribution was the use of acoustic chirp profiling for verifying the formation of acoustic solitary waves over the short path lengths necessary for a practical demonstration.
Vibrational energy relaxation rate constants from linear response theory
Authors: Shi Qiang , Geva Eitan
Published: April 2003

A new approach for the calculation of vibrational energy relaxation rate constants is introduced. The new approach is based on linear response theory, and is shown to have several distinct advantages over the standard Landau–Teller formula, which is based on the Bloch–Redfield theory, namely: (1) weak system–bath coupling is not assumed; (2) selectivity in choosing the vibrational energy relaxation pathway, including non-Landau–Teller pathways, is possible; (3) the validity of rate kinetics can be explicitly verified; (4) direct extraction of the high-frequency tail of the force–force correlation function is avoided. A detailed analysis of the conditions under which the new expression reduces into the Landau–Teller formula, and an application in the case of bilinear coupling to a harmonic bath are provided.
Density functional studies of solvation forces in hard sphere polymer solutions confined between adsorbing walls. I. Solvent effects and dependence on surface potential range
Authors: Forsman Jan , Woodward Clifford E. , Freasier Ben C.
Published: April 2003

Solvation forces between large surfaces in athermal polymer solutions, in which both solvent particles and polymers are adsorbed at the surfaces, are studied with density functional theory. We investigate how the range of the surface potential affects the net interaction between the surfaces. Predictions from treatments in which the solvent is explicitly induced are compared with those obtained with more approximate models, where the solvent is either neglected, or enters the description implicitly. The results are interpreted via comparisons with simpler model systems. It is shown that a long-ranged surface potential, acting equally on monomers and solvent, leads to a solvent dominated repulsive solvation force, while polymer specific contributions dominate the net interactions when the adsorption potential has a short range. Effects of preferential polymer adsorption are also investigated.
Diabatic intermolecular potentials and bound states of open-shell atom–molecule dimers: Application to the F(2P)–H2 complex
Authors: Zeimen W. B. , Kłos J. , Groenenboom G. C. , van der Avoird A.
Published: April 2003

We present a general derivation of the expansion of diabatic intermolecular potentials for an open-shell atom interacting with a closed-shell molecule and the multipolar expansion of these potentials in the long range. It is outlined how to compute bound states of the open-shell atom–molecule complex from the set of asymptotically degenerate diabatic potentials in a body-fixed basis of rovibrational wave functions with the inclusion of spin–orbit coupling. This method is applied to produce all the bound energy levels of the F(2P)–H2 van der Waals complex with recent diabatic potentials obtained from ab initio calculations by Kłos et al. [Int. J. Quantum Chem. 90, 1038 (2002)]. The binding energy D0 is 14.6 cm−1 for the para-H2 complex and 19.3 cm−1 for the ortho-H2 complex. The para-H2–F complex does not possess any bound states for rotational quantum numbers J larger than 92, the ortho-H2–F complex has a maximum J of 112.
Change of geometry by vibrational excitation: The vHF=3 spectrum and structure of HF–CO2
Authors: Yu Zhenhong , Stone Todd A. , Chuang Cheng-Chi , Drisdell Walter , Klemperer William
Published: April 2003

We have observed the vHF=3 levels of HF–CO2 in the region 11 150 to 11 210 cm−1 using intracavity Ti-sapphire laser induced fluorescence. The complex shown to be quasilinear at v1=vHF=0 and 1 becomes a semirigid bent species at v1=3 with the CO2 submolecule oriented at an angle near 40° with respect to the connector of the CO2 and HF centers of mass. Transitions to the K=0 and 2 levels of the vHF=3 (3 000 000) from the (0 000 000) ground state and to K=1 of (3 000 000) from the (0 000 001) level are observed, showing a 198.36(5) cm−1 HF vibrational redshift. The rotational constants of the (3 000 000) state are A=2.96(2) cm−1, (B+C)/2=0.0742(10) cm−1, 0.0717(10) and 0.0696(10) cm−1 for the K=0, 1, and 2 levels, respectively, and the centrifugal distortion DK=0.270(5) cm−1, which is large but in agreement with mechanical expectation. The observed transition intensities are a consequence of an appreciable rotation of the inertial axes in the transition. The spectral lines are Lorentzian with ΓL, full width at half-maximum equal 9.0(9), 7.2(6), and 4.5(6) GHz for the K=0, 1, and 2 levels of the (3 000 000) state. In addition, the soft mode CO2 bending combination transitions to the K=0 and 2 level of (3 000 001) are also observed and fit with (B+C)/2=0.0760(10) and 0.0714(10) cm−1, ΓL=10.5(9) and 6.0(9) GHz, respectively, and A=2.35(2) cm−1. The CO2 bending frequency, v7, is 24.75(10) cm−1. The HF bending level (3 000 010) has been observed at 11 538.92(10) cm−1 by the perpendicular transition from the ground state, indicating a HF bending frequency of 362.47(15) cm−1. The spectroscopic constants of (3 000 010) are (B+C)/2=0.0668(10) cm−1 and ΓL=4.2(6) GHz.
Kerr observations of asymmetric magnetization reversal processes in CoFe/IrMn bilayer systems
Authors: McCord J. , Schäfer R. , Mattheis R. , Barholz K.-U.
Published: April 2003

The magnetization reversal process in the ferromagnetic layer of an exchange-biased Co90Fe10(20 nm)/Ir23Mn77(10 nm) film structure, deposited by dc-magnetron sputtering, is imaged by high-resolution Kerr microscopy. Additionally, high-resolution magnetization loops are measured by deriving the magnetization signal from the average image intensity. The magnetization reversal occurs first by magnetization rotation under the development of ripple-like structures. The modulated structures then partially switch, generating complicated multidomain configurations, which finally annihilate by large angle domain wall movement. The amount of magnetization rotation at different field directions is quantified by measuring the transversal magnetization components during reversal. A strong asymmetry, both in domain behavior and magnetization loop, between the forward and recoil branch of the magnetization reversal is found. The magnitude of asymmetry strongly depends on small angle misalignments between the direction of exchange-bias and the external magnetic field. The observed domain behavior is explained by anisotropy dispersion in the ferro- and antiferromagnetic layer. The observed differences for both branches of the hysteresis loop are described in terms of domain nucleation mechanisms due to changes in the antiferromagnetic layer leading to an effectively wider anisotropy distribution.
Structure and mechanical properties of thin films deposited from 1,3,5-trimethyl-1,3,5-trivinylcyclotrisiloxane and water
Authors: Burkey Daniel D. , Gleason Karen K.
Published: April 2003

Pulsed-plasma chemical vapor deposition of 1,3,5-trimethyl-1,3,5-trivinylcyclotrisiloxane (V3D3) and water produced thin films with significant Si–OH content. Subsequent annealing of the films resulted in condensation of proximal Si–OH groups, further generating a Si–O–Si network and strengthening the film. Fourier-transform infrared spectroscopy analysis showed increasing OH content with increasing plasma duty cycle, and nanoindentation results confirmed increasing hardness with duty cycle, with the 10–40 duty cycle annealed sample having a hardness value of 0.527 GPa. These results were explained within the context of the continuous random network theory and percolation of rigidity arguments. Thermal stability was excellent, with a best-case thickness retention of 99.25% after a 2 h anneal at 400 °C under N2. Dielectric constants for the annealed films ranged between 2.55 and 2.9. The moderate power involved (200 W peak) is amenable to inclusion of a porogen species, opening the possibility of using this methodology to generate a porous thin film with adequate mechanical properties via chemical vapor deposition.
Influence of a pre-deposited carbon submonolayer on the Ge island nucleation on Si(001)
Authors: Dentel D. , Bischoff J. L. , Kubler L. , Stoffel M. , Castelein G.
Published: April 2003

Si surfaces manipulated by a carbon (C) pre-deposition have been used to modify the growth morphology of Ge islands. In situ reflection high-energy electron diffraction and x-ray photoelectron diffraction and ex situ atomic force microscopy studies have been conducted for constant C seeding and varying growth temperatures and Ge coverages, with the aim of deepening the understanding of the relevant Ge quantum dot formation. With temperatures ranging from 400 to 600 °C, well structured Ge islands grow in a Volmer–Weber mode as soon as 0.4 ML of C and 1 ML of Ge are deposited. Strongly modified behaviors are nevertheless observed by changing the Ge growth temperature from 500 to 600 °C. By increasing the Ge coverage from 1 to 6 ML at 500 °C, the island height increases at constant density, whereas, at 600 °C, a strong reduction of the density is observed, with a three-dimensional-two-dimensional transition probably due to a partial Ge intermixing in the Si matrix. These different nucleation schemes are connected with varying evolutions of the initial C-related c(4×4) reconstruction.
Decay mechanism of fracture induced electromagnetic pulses
Authors: Rabinovitch A. , Frid V. , Bahat D. , Goldbaum J.
Published: April 2003

In this article we consider the rise and fall time (which were earlier shown experimentally to be the same) of electromagnetic radiation (EMR) from propagating cracks. This feature is shown theoretically to be inversely proportional to the pulse frequency ω and to the fourth degree of the absolute temperature. It is shown experimentally that in glass and in glass ceramics, which are not porous, and in granite, whose porosity is of the order of 5%, τ is indeed inversely proportional to ω. In chalk, whose porosity is as high as 40%, however, this relation is not observed. We argue that the latter result is due to the interaction between the cracks which emit the EMR and the pores of the material and specifically to the spread of ensuing temperatures of the cracks caused by this interaction.
Structural, electric and magnetic properties of the electron-doped manganese oxide: La1−xTexMnO3 (x=0.1, 0.15)
Authors: Tan G. T. , Dai S. Y. , Duan P. , Zhou Y. L. , Lu H. B. , Chen Z. H.
Published: April 2003

In this study, the electrical and magnetic properties of La1−xTexMnO3 (x=0.1, 0.15), which is a new material and shows good colossal magnetoresistance behavior, have been investigated. These compounds have rhombohedral structure. In this material, Te replaced a part of La ions, which induced the lattice cell constriction and Mn–O–Mn bond angle widening. X-ray photoemission spectroscopy measurement revealed that the Te ions were in the tetravalent state and the manganese ions could be considered as in a mixture state of Mn2+ and Mn3+. Thus the material could be viewed as an electron-doped compound. The Curie temperature (Tc) was about 240 and 255 K for x=0.1, 0.15, respectively. The maximum magnetoresistance ratio MR=[ρ(0)−ρ(H)]/ρ(0) was about 51% at 200 K and in the applied magnetic field of 40 kOe.
The He–CaH(2Σ+) interaction. II. Collisions at cold and ultracold temperatures
Authors: Balakrishnan N. , Groenenboom G. C. , Krems R. V. , Dalgarno A.
Published: April 2003

We present cross sections for rotational, vibrational, and fine-structure transitions in He–CaH(2Σ) collisions at cold and ultracold temperatures calculated using the ab initio potential energy surface reported in the preceding paper. Rotational quenching is fast, vibrational quenching is slow. The spin-rotational interaction, although small and having no influence at temperatures above 10 K, changes significantly the rate coefficients for rotational quenching at lower temperatures. The theoretical rotational, vibrational, and elastic cross sections are compared with the results of a buffer gas cooling experiment carried out at a temperature of about 0.4 K. The theoretical predictions for the vibrational and elastic cross sections are larger than the measured values. The sensitivity to the potential energy surface is explored. A modified surface diminishes but does not remove the differences between theory and experiment.
Comparison of luminescent efficiency of InGaAs quantum well structures grown on Si, GaAs, Ge, and SiGe virtual substrate
Authors: Yang V. K. , Ting S. M. , Groenert M. E. , Bulsara M. T. , Currie M. T. , Leitz C. W. , Fitzgerald E. A.
Published: April 2003

In order to study the luminescent efficiency of InGaAs quantum wells on Si via SiGe interlayers, identical In0.2Ga0.8As quantum well structures with GaAs and Al0.25Ga0.75As cladding layers were grown on several substrates by an atmospheric metalorganic vapor deposition system. The substrates used include GaAs, Si, Ge, and SiGe virtual substrates. The SiGe virtual substrates were graded from Si substrates to 100% Ge content. Because of the small lattice mismatch between GaAs and Ge (0.07%), high-quality GaAs-based thin films with threading dislocation densities <3×106 cm−2 were realized on these SiGe substrates. Quantitative cathodoluminescence was used to compare the luminescent efficiency of the quantum well structure on the different substrates and cross-sectional transmission electron microscopy was used to characterize dislocation densities. Our results show that the InGaAs quantum wells grown on the GaAs substrates have the highest luminescent efficiencies due to the lowest dislocation densities. Interestingly, InGaAs quantum wells grown on the SiGe virtual substrates outperform those on Ge substrates, both in terms of luminescent efficiency and dislocation density. This difference is attributed to the variation in thermal expansion coefficient (α) and its impact on defect structure during the process cycle. The SiGe virtual substrate has a smaller α compared to a Ge substrate because of the smaller α of the Si substrate, which helps minimize compressive strain in the quantum well layer during the temperature decrease from the growth temperature. Consequently, fewer misfit dislocations are created between the quantum well and cladding interfaces. These misfits can greatly affect the luminescent efficiency since they can act as recombination sites. In general, the efficiencies of the quantum wells on the SiGe and Ge substrates were affected only by higher misfit dislocation densities, whereas the quantum wells on the Si substrate had low efficiency due to high threading dislocation density.
Susceptibility functions for slow relaxation processes in supercooled liquids and the search for universal relaxation patterns
Authors: Blochowicz Th. , Tschirwitz Ch. , Benkhof St. , Rössler E. A.
Published: April 2003

In order to describe the slow response of a glass former, we discuss some distributions of correlation times, e.g., a generalized gamma distribution (GG) and an extension thereof (GGE), the latter allowing us to reproduce a simple peak susceptibility such as of the Cole–Davidson type as well as a susceptibility exhibiting an additional high-frequency power law contribution (excess wing). Applying the GGE distribution to the dielectric spectra of glass formers exhibiting no β process peak (glycerol, propylene carbonate, and picoline) we are able to reproduce the salient features of the slow response (10−6–109 Hz). A line shape analysis is carried out either in the time or frequency domain and in both cases an excess wing can be identified. The latter evolves in a universal way while cooling and shows up for correlation times τα>10−8 s. It appears that its first emergence marks the breakdown of the high-temperature scenario of mode coupling theory. In order to describe a glass former exhibiting a β process peak we have introduced a distribution function which is compatible with assuming a thermally activated process in contrast to some commonly used fit functions. Together with the GGE distribution this function allows in the frame of the Williams–Watts approach to completely interpolate the spectra, e.g., of fluoro aniline (10−6 Hz–109 Hz). The parameters obtained indicate an emergence of both the excess wing and the β process again at τα>10−8 s.
Neutron and x-ray scattering study of fully deuterated poly(3-n.decylpyrrole)
Authors: Orecchini A. , Petrillo C. , Ruggeri G. , Cagnolati R.
Published: April 2003

The structure of a fully deuterated sample of the conducting polymer poly(3-n.decylpyrrole) was investigated by neutron and x-ray diffraction. The different sensitivity of the two probes to deuterium atoms allowed us to obtain novel insights about the intermolecular organization of the polymer, with special focus on the alkyl substituent. The polymer turns out to be arranged in parallel layers. Within each layer, neighboring pyrrole rings maintain a coplanar disposition, whereas the alkyl decane tails are kinked out of the pyrrole plane. Monomers belonging to different adjacent layers display a high degree of superposition, due to a considerable overlap of both pyrrole rings and decane tails. Thus, despite its folded conformation, the alkyl substituent does not affect the short-range order of the compound, but rather contributes to determine its layered structure.
Effect of hydrogen-related impurities on the thermal behavior of mechanical stress in silicon oxides suitable for integrated optics
Authors: Domı́nguez C. , Rodrı́guez J. A. , Riera M. , Llobera A. , Dı́az B.
Published: April 2003

Silicon oxide films were deposited by plasma enhanced chemical vapor deposition on crystalline silicon substrates from a mixture of silane and nitrous oxide. Substrate temperature and [N2O]/[SiH4] precursor gas flow ratio were varied between 200–350 °C and 5.5–110, respectively. Rapid thermal annealing at 900 °C to achieve impurity effusion led to significant changes in the dependence of the mechanical stress with the gas flow ratio R. In situ stress measurements during thermal cycles up to 300 °C as well as measurements of transmission infrared spectra for the as-deposited and annealed samples were performed. The changes in the stress behavior were explained considering the layer shrinkage resulting from the effusion of the different impurities present and the network rearrangement taking place during rapid thermal annealing, for different ranges of flow ratios.
Dependence of ablation parameters on the temperature and phase of ablated material
Authors: Franklin Sushmita R. , Thareja R. K.
Published: April 2003

A simple theoretical model for simulating the interaction of a laser radiation with a material is presented. A heat conduction equation that includes the temperature and phase dependence of material is numerically solved for various pulse shapes of the incident radiation. Results show a marked difference in ablation characteristics like threshold energy, evaporation onset time, and shielding, etc., when the temperature and phase dependence of the material is included. The temperature and density profiles inside the target and ablated vapors are also generated. The model can be approximated to give results already reported in the literature.
Precipitation-induced photostimulated luminescence in CsBr:Eu2+
Authors: Hackenschmied P. , Schierning G. , Batentschuk M. , Winnacker A.
Published: April 2003

This article concerns the unusual storage mechanism in the x-ray storage phosphor CsBr:Eu2+. Data are presented and discussed regarding the identification of x-ray diffraction patterns and the photostimulated luminescence (PSL) intensity as a function of the annealing temperature for various Eu concentrations of CsBr:Eu2+ powder crystals and the spatial correlation of trapped electrons and holes. These results together with an analysis of previous literature data enable us to propose a model explaining the formation of phases which are active in the PSL-process of the CsBr:Eu2+ storage phosphor.
Electrical characterization of metal/AlN/Si thin film hydrogen sensors with Pd and Al gates
Authors: McCullen E. F. , Prakasam H. E. , Mo Wenjun , Naik R. , Ng K. Y. S. , Rimai L. , Auner G. W.
Published: April 2003

The electrical characteristics of a hydrogen sensor utilizing a metal/AlN/Si(111) structure, which behaves similarly to a metal-insulator-semiconductor-type capacitor, have been investigated. The AlN layer, which acts as the insulator, was deposited by plasma source molecular-beam epitaxy. The device employed alternating gates of Pd and Al, and an Al back contact, all grown by magnetron sputtering. The Pd gate structures were found to be sensitive to hydrogen, down to parts per million concentrations in the surrounding flow, while the Al gate structures showed no response to hydrogen. Both structures show similar capacitance versus voltage as well as conductance versus voltage curves. Therefore the combination of Al and Pd gates can be utilized in a balanced hydrogen sensor structure. Furthermore, the frequency dependence of the capacitance and conductance curves clearly indicated the presence of series resistance effects, but with multiple time constants, requiring a complex equivalent circuit representation. Evidence was found for the presence of carrier traps at the AlN/Si interface, with energy levels distributed throughout the silicon band gap, and with an estimated surface density ranging between 8×1014 and 1.5×1015 m−2 eV−1.
Different thermodynamic pathways to the solvation free energy of a spherical cavity in a hard sphere fluid
Authors: Chen Yng-gwei , Weeks John D.
Published: April 2003

This paper determines the excess free energy associated with the formation of a spherical cavity in a hard sphere fluid. The solvation free energy can be calculated by integration of the structural changes induced by inserting the cavity using a number of different exact thermodynamic pathways. We consider three such pathways, including a new density route derived here. Structural information about the nonuniform hard sphere fluid in the presence of a general external field is given by the recently developed hydrostatic linear response (HLR) integral equation. Use of the HLR results in the different pathways gives a generally accurate determination of the solvation free energy for cavities over a wide range of sizes, from zero to infinity. Results for a related method, the Gaussian field model, are also discussed.
The structure of lithium–ammonia and sodium–ammonia solutions by neutron diffraction
Authors: Wasse Jonathan C. , Hayama Shusaku , Masmanidis Sotiris , Stebbings Sarah L. , Skipper Neal T.
Published: April 2003

The microscopic structures of lithium–ammonia and sodium–ammonia solutions have been measured by the technique of isotopic labeling in neutron diffraction, at and above the metal–nonmetal transition that occurs in the range 2–8 mole percent metal (MPM). Substitution of Li* by Li6 has been used to obtain the lithium-centered first-order difference function at 8 MPM and 230 K. This function shows us that the lithium cations are strongly solvated by 4 ammonia molecules. Substitution of N* by N15 has then been used to probe the nitrogen-centered structure in lithium–ammonia solutions at 4, 8, and 12.5 MPM and sodium–ammonia at 12.5 MPM. These functions give us new insight into both the disruption of hydrogen bonding as alkali metal is added to ammonia, and the solvation structure of the sodium cations. The former manifests itself through a progressive loss of the hydrogen-bonded N–D peak at ∼2.4 Å. The latter appears as an N–Na shoulder at ∼2.5 Å, and shows us that sodium is solvated by ∼5.5 ammonia molecules. In contrast to previous data for saturated (∼21 MPM) metal–ammonia solutions, we do not observe intermediate-range ordering of the solvated cations at the concentrations studied here.
Monte Carlo simulations for the phase behavior of symmetric nonadditive hard sphere mixtures
Authors: Jagannathan Kamakshi , Yethiraj Arun
Published: April 2003

Computer simulation results are presented for the phase behavior of a symmetric binary mixture of nonadditive hard spheres. In this model, the hard sphere diameters are given by σAA=σBB=λd and σAB=d. At high densities and for small enough λ, this hard sphere mixture exhibits a fluid–fluid phase separation into an A-rich and a B-rich phase. Semigrand ensemble simulations are performed for the critical point and the phase behavior of this model for various values of λ. The results for the critical density are significantly different from previous simulation estimates. A comparison of our simulation results to existing theories shows that none of the theories are accurate for the location of the critical point, over the entire range of λ.