bluuebunny/crossref_metadata_2025 huggingface dataset:
Flow at interfaces: A new device for x-ray surface scattering investigations
Authors: Moulin J.-F. , Roth S. V. , Müller-Buschbaum P.
Published: January 2008

A fluidic cell based setup is described which allows for microbeam grazing incidence small angle x-ray scattering characterization of the interface between a solid substrate and a flowing liquid. This cell can potentially be used to study in situ a wide variety of systems ranging from synthetic and natural colloids to biological molecules. The selected channel geometry enables the characterization of the solid-liquid interface during mixing of different solutions. As a proof of concept, measurements on an aqueous gold nanoparticle solution in contact with a glass surface are presented that show that the structure at the interface can be probed during flow.
Creation of finely focused particle beams from single-component plasmas
Authors: Weber T. R. , Danielson J. R. , Surko C. M.
Published: January 2008

In a recent communication [Danielson et al., Appl. Phys. Lett. 90, 081503 (2007)], a nondestructive technique was described to create finely focused beams of electron-mass, charged particles (i.e., electrons or positrons) from single-component plasmas confined in a Penning–Malmberg trap. This paper amplifies and expands upon those results, providing a more complete study of this method of beam formation. A simple model for beam extraction is presented, and an expression for a Gaussian beam profile is derived when the number of extracted beam particles is small. This expression gives a minimum beam diameter of four Debye lengths (full width to 1/e) and is verified using electron plasmas over a broad range of plasma temperatures and densities. Numerical procedures are outlined to predict the profiles of beams with large numbers of extracted particles. Measured profiles of large beams are found in fair agreement with these predictions. The extraction of over 50% of a trapped plasma into a train of nearly identical beams is demonstrated. Applications and extensions of this technique to create state-of-the-art positron beams are discussed.
Phase behavior of a simple dipolar fluid under shear flow in an electric field
Authors: McWhirter J. Liam
Published: January 2008

Nonequilibrium molecular dynamics simulations are performed on a dense simple dipolar fluid under a planar Couette shear flow. Shear generates heat, which is removed by thermostatting terms added to the equations of motion of the fluid particles. The spatial structure of simple fluids at high shear rates is known to depend strongly on the thermostatting mechanism chosen. Kinetic thermostats are either biased or unbiased: biased thermostats neglect the existence of secondary flows that appear at high shear rates superimposed upon the linear velocity profile of the fluid. Simulations that employ a biased thermostat produce a string phase where particles align in strings with hexagonal symmetry along the direction of the flow. This phase is known to be a simulation artifact of biased thermostatting, and has not been observed by experiments on colloidal suspensions under shear flow. In this paper, we investigate the possibility of using a suitably directed electric field, which is coupled to the dipole moments of the fluid particles, to stabilize the string phase. We explore several thermostatting mechanisms where either the kinetic or configurational fluid degrees of freedom are thermostated. Some of these mechanisms do not yield a string phase, but rather a shear-thickening phase; in this case, we find the influence of the dipolar interactions and external field on the packing structure, and in turn their influence on the shear viscosity at the onset of this shear-thickening regime.
Stimulated emission and lasing from an Al0.13Ga0.87N∕GaN double heterostructure grown on a silicon substrate
Authors: Al-Ajmi F. S. , Kolbas R. M. , Roberts J. C. , Rajagopal P. , Cook J. W. , Piner E. L. , Linthicum K. J.
Published: January 2008

Stimulated emission and laser action with well developed longitudinal optical modes from an Al0.13Ga0.87N∕GaN double heterostructure with a 25nm GaN active layer grown on a silicon substrate by metal-organic chemical vapor deposition is demonstrated. Lasing was observed from cleaved platelets at room temperature with well resolved Fabry-Pérot modes at a wavelength as short as 368nm at room temperature. A clear threshold was observed in the plot of the emission intensity versus the pumping power at both 77K and room temperature. The effective index of refraction during laser operation was measured to be 2.65.
Lamellar morphology induced by two-step surface-directed spinodal decomposition in binary polymer mixture films
Authors: Yan Li-Tang , Xie Xu-Ming
Published: January 2008

A novel process for obtaining ordered morphology on the basis of two-step surface-directed spinodal decomposition is numerically investigated. The formation mechanism and evolution dynamics of this process are also discussed in detail. The calculated results of the chemical potential demonstrate that the equilibration state at the first quench affects the competition between the surface potential and the chemical potential in the bulk, leading to a surprising lamellar structure at the second further quench. It is also found that the lamella formation obeys the logarithmic growth. These results could provide a new approach for fabricating ordered structure of polymer materials and stimulate experimental studies based on this subject.
Quantum scattering of SiS with H2: Potential energy surface and rate coefficients at low temperature
Authors: Lique François , Kłos Jacek
Published: January 2008

Rotational excitation of the interstellar species SiS with H2 is investigated. We present a new four dimensional potential energy surface for the SiS–H2 system. Both molecules were treated as rigid rotors. Potential was obtained from the electronic structure calculations using a single- and double-excitation coupled cluster method with perturbative contributions from connected triple excitations [CCSD(T)]. The four atoms were described using the aug-cc-pVTZ basis sets. Bond functions were placed at mid-distance between the SiS center of mass and the center of mass of H2 for a better description of the van der Waals interaction. Additionally, at seven characteristic geometries, we calculated perturbational components of the interaction energy using symmetry-adapted perturbation theory approach to explain the anisotropy of the potential energy surface. Coupled-state calculations of the inelastic integral cross sections of SiS in collisions with para-H2 and ortho-H2 were calculated at low energies. After Boltzmann thermal averaging, rate coefficients were obtained for temperatures ranging from 5to50K. Significant differences exist between para- and ortho-H2 results. The strongest collision-induced rotational SiS transitions are the transitions with Δj=2 for collisions with para-H2 and the transitions with Δj=1 for collisions with ortho-H2.
Relative energy of the high-(T2g5) and low-(A1g1) spin states of the ferrous complexes [Fe(L)(NHS4)]: CASPT2 versus density functional theory
Authors: Pierloot Kristine , Vancoillie Steven
Published: January 2008

High-level ab initio calculations using multiconfigurational perturbation theory [complete active space with second-order perturbation theory (CASPT2)] were performed on the transition energy between the lowest high-spin (corresponding to (T2g5) in Oh) and low-spin (corresponding to A1g1 in Oh) states in the series of six-coordinated Fe(II) molecules [Fe(L)(NHS4)], where NHS4 is 2,2′-bis(2-mercaptophenylthio)diethylamine dianion and L=NH3, N2H4, PMe3, CO, and NO+. The results are compared to (previous and presently obtained) results from density functional theory (DFT) calculations with four functionals, which were already shown previously by Casida and co-workers [Fouqueau et al., J. Chem. Phys. 120, 9473 (2004); Ganzenmüller et al., ibid. 122, 234321 (2005); Fouqueau et al., ibid. 122, 044110 (2005); Lawson Daku et al., ChemPhysChem 6, 1393 (2005)] to perform well for the spin-pairing problem in these and other Fe(II) complexes, i.e., OLYP, PBE0, B3LYP, and B3LYP*. Very extended basis sets were used both for the DFT and CASPT2 calculations and were shown to be necessary to obtain quantitative results with both types of method. This work presents a sequel to a previous DFT/CASPT2 study of the same property in the complexes [Fe(H2O)6]2+, [Fe(NH3)6]2+, and [Fe(bpy)3]2+ [Pierloot et al., J. Chem. Phys. 125, 124303 (2006)]. The latter work was extended with new results obtained with larger basis sets and including the OLYP functional. For all considered complexes, the CASPT2 method predicts the correct ground state spin multiplicity. Since experimental data for the actual quintet-singlet (free) energy differences are not available, the performance of the different DFT functionals was judged based on the comparison between the DFT and CASPT2 results. From this, it was concluded that the generalized gradient OLYP functional performs remarkably well for the present series of ferrous compounds, whereas the success of the three hybrid functionals varies from case to case.
Elimination, in electronic structure calculations, of redundant orbital products
Authors: Foerster D.
Published: January 2008

We propose a direct method for reducing the dimension of the space of orbital products that occur, for example, in the calculation of time dependent density functional theory linear response and in Hedin’s GW approximation to the electron propagator. We do this by defining, within the linear space of orbital products, a subspace of dominant directions that are associated with a certain eigenvalue problem. These directions span the entire linear space of products with an error that decreases approximately exponentially with their number. Our procedure works best for atomic orbitals of finite range and it avoids the use of extra sets of auxiliary fit functions.
Electronic structure and bonding of the 3d transition metal borides, MB, M=Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu through all electron ab initio calculations
Authors: Tzeli Demeter , Mavridis Aristides
Published: January 2008

The electronic structure and bonding of the ground and some low-lying states of all first row transition metal borides (MB), ScB, TiB, VB, CrB, MnB, FeB, CoB, NiB, and CuB have been studied by multireference configuration interaction (MRCI) methods employing a correlation consistent basis set of quintuple cardinality (5Z). It should be stressed that for all the above nine molecules, experimental results are essentially absent, whereas with the exception of ScB and CuB the remaining seven species are studied theoretically for the first time. We have constructed full potential energy curves at the MRCI/5Z level for a total of 27 low-lying states, subsequently used to extract binding energies, spectroscopic parameters, and bonding schemes. In addition, some 20 or more states for every MB species have been examined at the MRCI/4Z level of theory. The ground state symmetries and corresponding binding energies (in kcal/mol) are Σ−5(ScB), 76; Δ6(TiB), 65; Σ+7(VB), 55; Σ+6(CrB), 31; Π5(MnB), 20; Σ−4(FeB), 54; Δ3(CoB), 66; Σ+2(NiB), 79; and Σ+1(CuB), 49.
The limits of local correlation theory: Electronic delocalization and chemically smooth potential energy surfaces
Authors: Subotnik Joseph E. , Sodt Alex , Head-Gordon Martin
Published: January 2008

Local coupled-cluster theory provides an algorithm for measuring electronic correlation quickly, using only the spatial locality of localized electronic orbitals. Previously, we showed [J. Subotnik et al., J. Chem. Phys. 125, 074116 (2006)] that one may construct a local coupled-cluster singles-doubles theory which (i) yields smooth potential energy surfaces and (ii) achieves near linear scaling. That theory selected which orbitals to correlate based only on the distances between the centers of different, localized orbitals, and the approximate potential energy surfaces were characterized as smooth using only visual identification. This paper now extends our previous algorithm in three important ways. First, locality is now based on both the distances between the centers of orbitals as well as the spatial extent of the orbitals. We find that, by accounting for the spatial extent of a delocalized orbital, one can account for electronic correlation in systems with some electronic delocalization using fast correlation methods designed around orbital locality. Second, we now enforce locality on not just the amplitudes (which measure the exact electron-electron correlation), but also on the two-electron integrals themselves (which measure the bare electron-electron interaction). Our conclusion is that we can bump integrals as well as amplitudes, thereby gaining a tremendous increase in speed and paradoxically increasing the accuracy of our LCCSD approach. Third and finally, we now make a rigorous definition of chemical smoothness as requiring that potential energy surfaces not support artificial maxima, minima, or inflection points. By looking at first and second derivatives from finite difference techniques, we demonstrate complete chemical smoothness of our potential energy surfaces (bumping both amplitudes and integrals). These results are significant both from a theoretical and from a computationally practical point of view.
The equation of state of hard hyperspheres in nine dimensions for low to moderate densities
Authors: Bishop Marvin , Clisby Nathan , Whitlock Paula A.
Published: January 2008

The equation of state of hard hyperspheres in nine dimensions is calculated both from the values of the first ten virial coefficients and from a Monte Carlo simulation of the pair correlation function at contact. The results are in excellent agreement. In addition, we find that the virial series appears to be dominated by an unphysical singularity or singularities on or near the negative density axis, in qualitative agreement with the recently solved Percus-Yevick equation of state in nine dimensions.
Mechanisms for O2 dissociation over the BaO (100) surface
Authors: Lu Nai-Xia , Fu Gang , Xu Xin , Wan Hui-Lin
Published: January 2008

We have investigated the atomic and molecular oxygen adsorptions on the various sites of the BaO (100) surface with both cluster models and the periodic slab models. We found that the atomic oxygen prefers to adsorb on the surface O2− to form the closed-shell peroxides with the binding energies of 83–88kcal∕mol. Such a high exothermicity provides a large driving force for the dissociation of molecular O2 on the BaO surfaces. As molecular oxygen approaches the BaO surfaces, the triplet ground state O2 molecule first binds electrostatically on top of the surface Ba2+ site. It further quenches to the singlet potential energy surface to form a covalently bonded O32− species. We proposed a plausible pathway in which the O32− species acts as the key precursor for further dissociation, leading eventually to the formation of surface peroxides O22−. This mechanism is helpful for the understanding of a series of related catalytic processes such as the oxidative coupling of methane, the NOx storage reduction, etc.
Ground state structures and excited state dynamics of pyrrole-water complexes:Ab initioexcited state molecular dynamics simulations
Authors: Kumar Anupriya , Kołaski Maciej , Kim Kwang S.
Published: January 2008

Structures of the ground state pyrrole-(H2O)n clusters are investigated using ab initio calculations. The charge-transfer driven femtosecond scale dynamics are studied with excited state ab initio molecular dynamics simulations employing the complete-active-space self-consistent-field method for pyrrole-(H2O)n clusters. Upon the excitation of these clusters, the charge density is located over the farthest water molecule which is repelled by the depleted π-electron cloud of pyrrole ring, resulting in a highly polarized complex. For pyrrole-(H2O), the charge transfer is maximized (up to 0.34a.u.) around ∼100fs and then oscillates. For pyrrole-(H2O)2, the initial charge transfer occurs through the space between the pyrrole and the π H-bonded water molecule and then the charge transfer takes place from this water molecule to the σ H-bonded water molecule. The total charge transfer from the pyrrole to the water molecules is maximized (up to 0.53a.u.) around ∼100fs.
Excited electronic state decomposition of furazan based energetic materials: 3,3′-diamino-4,4′-azoxyfurazan and its model systems, diaminofurazan and furazan
Authors: Guo Y. Q. , Bhattacharya A. , Bernstein E. R.
Published: January 2008

We report the first experimental and theoretical study of gas phase excited electronic state decomposition of a furazan based, high nitrogen content energetic material, 3,3′-diamino-4,4′-azoxyfurazan (DAAF), and its model systems, diaminofurazan (DAF) and furazan (C2H2N2O). DAAF has received major attention as an insensitive high energy explosive; however, the mechanism and dynamics of the decomposition of this material are not clear yet. In order to understand the initial decomposition mechanism of DAAF and those of its model systems, nanosecond energy resolved and femtosecond time resolved spectroscopies and complete active space self-consistent field (CASSCF) calculations have been employed to investigate the excited electronic state decomposition of these materials. The NO molecule is observed as an initial decomposition product from DAAF and its model systems at three UV excitation wavelengths (226, 236, and 248nm) with a pulse duration of 8ns. Energies of the three excitation wavelengths coincide with the (0-0), (0-1), and (0-2) vibronic bands of the NO A∑+2←X∏2 electronic transition, respectively. A unique excitation wavelength independent dissociation channel is observed for DAAF, which generates the NO product with a rotationally cold (20K) and a vibrationally hot (1265K) distribution. On the contrary, excitation wavelength dependent dissociation channels are observed for the model systems, which generate the NO product with both rotationally cold and hot distributions depending on the excitation wavelengths. Potential energy surface calculations at the CASSCF level of theory illustrates that two conical intersections between the excited and ground electronic states are involved in two different excitation wavelength dependent dissociation channels for the model systems. Femtosecond pump-probe experiments at 226nm reveal that the NO molecule is still the main observed decomposition product from the materials of interest and that the formation dynamics of the NO product is faster than 180fs. Two additional fragments are observed from furazan with mass of 40amu (C2H2N) and 28amu (CH2N) employing femtosecond laser ionization. This observation suggests a five-membered heterocyclic furazan ring opening mechanism with rupture of a CN and a NO bond, yielding NO as a major decomposition product. NH2 is not observed as a secondary decomposition product of DAAF and DAF.
Phase III of solid methane: The orientational potential and rotational tunneling
Authors: Hüller Alfred , Prager Michael , Press Werner , Seydel Tilo
Published: January 2008

After many unsuccessful efforts, the structure of solid CD4 III finally has been solved. In this paper, we examine if the known tunneling spectra are consistent with the orientational potentials at the two sites with different symmetries. To this end, we study the rotational kinetic energy of the molecules, construct appropriate pocket states for the tunneling problem, and set up a model potential. Approximate energy levels are obtained from the Ritz variational principle. The agreement between the experimentally determined tunneling frequencies and the calculations is rather good, corroborating the findings of the structural analysis. A continuation of this paper will deal with the partly deuterated methanes.
Atomic force microscope manipulation of gold nanoparticles for controlled Raman enhancement
Authors: Tong Lianming , Zhu Tao , Liu Zhongfan
Published: January 2008

We report on the controlled manipulation of two, three and four gold nanoparticles (AuNPs) using an atomic force microscope (AFM) for use as surface-enhanced Raman scattering substrates. For each arrangement, the interparticle electromagnetic (EM) coupling between adjacent AuNPs is studied at different polarization angles. It is found that the strength of EM coupling strongly depends on the arrangement of the AuNPs. In particular, the highest enhancement and the most pronounced polarization dependence is found for linear arrangement of closely spaced particles. Our results show that AFM manipulation has great potential for fabrication and investigation of controlled arrangements of nanoscale objects.
The sticking probability for H2 on some transition metals at a hydrogen pressure of 1bar
Authors: Johansson M. , Lytken O. , Chorkendorff I.
Published: January 2008

The sticking probability for hydrogen on films of Co, Ni, Cu, Ru, Rh, Pd, Ir, and Pt supported on graphite has been measured at a hydrogen pressure of 1bar in the temperature range 40–200°C. The sticking probability is found to increase in the order Ni, Co, Ir, Pd, Pt, Rh, and Ru at temperatures below 150°C, whereas at higher temperatures, the sticking probability for Pd is higher than for Pt. The sticking probability for Cu is below the detection limit of the measurement. The measured sticking probabilities are slightly lower than those obtained at high hydrogen coverage under ultrahigh vacuum conditions. This could be a consequence of the higher hydrogen pressure used here. The apparent desorption energies extracted from the steady-state desorption rate are found to agree reasonably well with published values for the heat of adsorption at high coverage. However, the sticking probability is not related in a simple way to published values for the heat of adsorption at low coverage, with Ru and Rh giving exceptionally high values for the sticking probability. It is suggested that this is due to the presence of adsorption sites with very low desorption energy on Ru and Rh.
Spectroscopically pure metal vapor source for highly charged ion spectroscopy and capillary discharge soft x-ray lasers
Authors: Tomasel F. G. , Shlyaptsev V. N. , Rocca J. J.
Published: January 2008

We describe a compact, pulsed metal vapor source used for the production of dense plasma columns of interest for both soft x-ray laser research and spectroscopy of highly ionized plasmas. The source generates spectroscopically pure cadmium vapor jets in a room-temperature environment by rapidly heating an electrode with a capacitive discharge. In the configuration described herein, the metal vapor jet produced by the source is axially injected into a fast (up to 15kA∕ns), high current (up to 200kA peak) capillary discharge to generate highly ionized cadmium plasma columns. Spectroscopic analysis of the discharge emission in the 12–25nm spectral range evidences the dominance of Cu-like (CdXX) and Ni-like (CdXXI) lines and shows strong line emission at 13.2nm from the 4dS01–4pP11 laser transition of Ni-like Cd. Hydrodynamic/atomic physics simulations performed to describe the dynamics of the plasma column and compute the optimum discharge conditions for laser amplification are discussed.
Defect structure of Sb2−xCrxTe3 single crystals
Authors: Horák J. , Quayle P. C. , Dyck J. S. , Drašar Č. , Lošt’ák P. , Uher C.
Published: January 2008

Single crystals of Sb2Te3 doped with Cr (cCr=0–6×1020 cm−3) were prepared by the Bridgman method. The measurements of the Hall coefficient reveal a nonmonotonous dependence of hole concentrations on the Cr content in the crystal. The hole concentration decreases at low content of Cr, while at higher content of Cr it increases again. However, according to magnetic measurements, Cr atoms enter the structure and form uncharged substitutional defects CrSb×, which cannot affect the free carrier concentration directly. The observed dependence can be elucidated by means of a point defect model. The model is based on an assumption that defect structure of Sb2Te3 can be treated as hybrid Schottky and antisite defect disorder. Thus, we assume an interaction of CrSb× with the most populated native defects in the structure—antisite defects SbTe−1 and vacancies in the Te sublattice VTe+2.
On the ternary complex analysis and its applications
Authors: Lipatov L. N. , Rausch de Traubenberg M. , Volkov G. G.
Published: January 2008

In previous works, a possible extension of the complex numbers together with its connected trigonometry was introduced. In the present paper, we focus on the simplest case of ternary complex numbers. Then, some types of holomorphy adapted to the ternary complex numbers and the corresponding results upon integration of differential forms are given. Several physical applications are discussed and, in particular, one type of holomorphic function gives rise to a new form of stationary magnetic field. The movement of a monopole-type object in this field is then studied and shown to be integrable. The monopole scattering in the ternary field is finally studied.
Measurement of temperature distributions across laser heated samples by multispectral imaging radiometry
Authors: Campbell Andrew J.
Published: January 2008

Two-dimensional temperature mapping of laser heated diamond anvil cell samples is performed by processing a set of four simultaneous images of the sample, each obtained at a narrow spectral range in the visible to near infrared. The images are correlated spatially, and each set of four points is fitted to the Planck radiation function to determine the temperature and the emissivity of the sample, using the gray body approximation. The method is tested by measuring the melting point of Pt at 1bar and measuring laser heated Fe at 20GPa in the diamond anvil cell. The accuracy and precision are shown to compare well to standard spectroradiometry, and the effect of imaging resolution on the measured distribution is evaluated. The principal advantages of the method are (1) the temperature and emissivity of the sample are mapped in two dimensions; (2) chromatic aberrations are practically eliminated by independent focusing of each spectral band; and (3) all of the spectral images are obtained simultaneously, allowing temporal variations to be studied. This method of measuring temperature distributions can be generalized to other hot objects besides laser heated spots.
Automating fruit fly Drosophila embryo injection for high throughput transgenic studies
Authors: Cornell E. , Fisher W. W. , Nordmeyer R. , Yegian D. , Dong M. , Biggin M. D. , Celniker S. E. , Jin J.
Published: January 2008

To decipher and manipulate the 14 000 identified Drosophila genes, there is a need to inject a large number of embryos with transgenes. We have developed an automated instrument for high throughput injection of Drosophila embryos. It was built on an inverted microscope, equipped with a motorized xy stage, autofocus, a charge coupled device camera, and an injection needle mounted on a high speed vertical stage. A novel, micromachined embryo alignment device was developed to facilitate the arrangement of a large number of eggs. The control system included intelligent and dynamic imaging and analysis software and an embryo injection algorithm imitating a human operator. Once the injection needle and embryo slide are loaded, the software automatically images and characterizes each embryo and subsequently injects DNA into all suitable embryos. The ability to program needle flushing and monitor needle status after each injection ensures reliable delivery of biomaterials. Using this instrument, we performed a set of transformation injection experiments. The robot achieved injection speeds and transformation efficiencies comparable to those of a skilled human injector. Because it can be programed to allow injection at various locations in the embryo, such as the anterior pole or along the dorsal or ventral axes, this system is also suitable for injection of general biochemicals, including drugs and RNAi.
Pinch current limitation effect in plasma focus
Authors: Lee S. , Saw S. H.
Published: January 2008

The Lee model couples the electrical circuit with plasma focus dynamics, thermodynamics, and radiation. It is used to design and simulate experiments. A beam-target mechanism is incorporated, resulting in realistic neutron yield scaling with pinch current and increasing its versatility for investigating all Mather-type machines. Recent runs indicate a previously unsuspected “pinch current limitation” effect. The pinch current does not increase beyond a certain value however low the static inductance is reduced to. The results indicate that decreasing the present static inductance of the PF1000 machine will neither increase the pinch current nor the neutron yield, contrary to expectations.
Modified Timoshenko formula for bending of ultrathin strained bilayer films
Authors: Zang Ji , Liu Feng
Published: January 2008

Mechanical bending of nanoscale thin films can be quite different from that of macroscopic thick films. However, current understanding of mechanical bending of nanoscale thin strained bilayer films is often limited within the Timoshenko model [Timoshenko, J. Opt. Soc. Am. 11, 233 (1925)], which was originally derived for macroscopic thick films. Here, we derive a modified Timoshenko formula by including the prominent effect of surface stress played in the nanofilms, which gives a much better agreement with the experiments than the classical formula.
Quantized electronic properties of diamond
Authors: Nebel Christoph E. , Yang Ninajun , Uetsuka Hiroshi , Yamada Takatoshi , Watanabe Hideyuki
Published: January 2008

Diamond is a promising semiconductor which shows some unique surface electronic features if grown optimized with low bulk and surface defect densities. The appearance of a highly conducting surface layer, if immersed into electrolyte solution, is maybe the most striking feature. Scanning tunneling microscopy experiments on diamond in electrolyte solutions or covered by adsorbates are applied to determine the electronic properties governing these transitions. These experiments reveal the formation of unoccupied quantized electronic states in the valence band close to the surface. A two-dimensional density-of-state distribution with three levels from light-, heavy-, and split-off-band holes is detected. The formation of this system is the origin for the transformation of insulating diamond into a highly conductive film. Removal of the electrolyte causes a reversible transition into the insulating state and vice versa. No transformation can be detected on oxidized diamond surfaces which remains insulating.
Fe-implanted ZnO: Magnetic precipitates versus dilution
Authors: Zhou Shengqiang , Potzger K. , Talut G. , Reuther H. , von Borany J. , Grötzschel R. , Skorupa W. , Helm M. , Fassbender J. , Volbers N. , Lorenz M. , Herrmannsdörfer T.
Published: January 2008

Nowadays ferromagnetism is often found in potential diluted magnetic semiconductor systems. However, many authors argue that the observed ferromagnetism stems from ferromagnetic precipitates or spinodal decomposition rather than from carrier-mediated magnetic impurities, as required for a diluted magnetic semiconductor. In the present article, we answer this question for Fe-implanted ZnO single crystals comprehensively. Different implantation fluences, temperatures, and post-implantation annealing temperatures have been chosen in order to evaluate the structural and magnetic properties over a wide range of parameters. Three different regimes with respect to Fe concentration and process temperature are found: (1) Disperse Fe2+ and Fe3+ at low Fe concentrations and low processing temperatures, (2) FeZn2O4 at very high processing temperatures, and (3) an intermediate regime with a coexistence of metallic Fe (Fe0) and ionic Fe (Fe2+ and Fe3+). Ferromagnetism is only observed in the latter two cases, where inverted spinel ZnFe2O4 and α-Fe nanocrystals are the origin of the observed ferromagnetic behavior, respectively. The ionic Fe in the last case could contribute to a carrier-mediated coupling. However, the separation between Fe ions is too large to couple ferromagnetically due to the lack of p-type carrier. For comparison, investigations of Fe-implanted epitaxial ZnO thin films are presented.
Spatial and temporal scaling and common apparent excitation temperature of laser-induced plasmas generated at constant irradiance with different pulse energies
Authors: Aragón C. , Aguilera J. A.
Published: January 2008

The characteristics of laser-induced plasmas generated with different pulse energies E in the range of 50–200mJ at constant irradiance (40GW∕cm2) have been investigated. The plasmas have been generated in air at atmospheric pressure from Fe–Ni samples using an infrared Nd:YAG laser. Plasma characterization has been carried out by emission spectroscopy with three types of measurements as follows. (1) Time-resolved, space-integrated measurements have shown that a temporal scaling exists between the curves of temporal evolution of the apparent (population-averaged) temperature and electron density of the plasmas, with scaling factors proportional to E0.4±0.1. (2) Time- and space-integrated measurements have led to a common value of the plasma apparent temperature (7200±200K) for all the pulse energies and to a scaling of the line intensities as E1.6±0.2. The coincidence of apparent temperatures is explained by the hypothesis that laser-induced plasmas generated with different pulse energies at constant irradiance show a space and time scaling. (3) Time- and space-resolved measurements of the plasma temperature have been carried out to verify the scaling properties. A qualitative interpretation of the experimental results is provided supposing that a laser-supported radiation wave is responsible for the absorption of the laser energy during the formation of the plasma.
Formation and control of magnetic vortex chirality in patterned micromagnet arrays
Authors: Konoto M. , Yamada T. , Koike K. , Akoh H. , Arima T. , Tokura Y.
Published: January 2008

We have succeeded in developing highly ordered magnetic vortices in dense arrays of microstructured magnets and in evaluating individual spin configuration by employing a high-precision spin-imaging technique. High-definition pictures reveal fundamental properties of the ordered vortices, such as stability and interaction of vortex chirality. We have also demonstrated control of the chirality with the use of a magnetic field gradient but without introducing geometric asymmetry in the magnet shape. A simple control mechanism is deduced from micromagnetic simulations.
Microstructure and recording mechanism of Bi–Fe–(N) layer for high-density write-once optical disk
Authors: Mai Hung-Chuan , Hsieh Tsung-Eong , Jeng Shiang-Yao , Chen Chong-Ming , Wang Jen-Long
Published: January 2008

Bi–Fe–(N) thin film as the recording layer of high density digital versatile disk recordable optical disk was developed. For the disk sample with optimized layer structure, maximum partial response signal to noise ratio of 21.1 and minimum simulated bit error rate of 5×10−7 were achieved at the write power=5.7mW. Transmission electron microscopy characterization illustrated that the separation of Bi and Fe elements to form the coarse granular structure in the mark regime during laser irradiation comprised of the recording mechanism.
Anomalous Hall effect of CeFe1.95Al0.05 in the vicinity of the metamagnetic transition
Authors: Zou J. D. , Shen B. G. , Sun J. R. , Shen J. , Rong C. B. , Li W.
Published: January 2008

The essence of the anomalous Hall effect (AHE) is not fully understood and it still attracts considerable attention. Magnetic and magnetotransport properties of CeFe1.95Al0.05 compound in diverse magnetic state are studied which is helpful to know the essence of AHE. The semiempirical relation ρxy=R0B+4πRsM, widely accepted, is valid in describing the behaviors of AHE in ferromagnetic and paramagnetic state, but invalid in antiferromagnetic state in CeFe1.95Al0.05 compound.
Optimal spin-current pulse of the Stoner-Wohlfarth problem
Authors: Wang X. R. , Sun Z. Z.
Published: January 2008

Fast magnetization reversal of uniaxial Stoner particles by spin-transfer torque generated by the spin-polarized electric current is investigated. It is found that the critical current required to reverse a magnetization can be dramatically reduced if a current with a properly designed time-dependent polarization direction is used. Under the condition that the magnitude of the current does not vary with time, the optimal time-dependent electron polarization is obtained such that the magnetization reversal is the fastest.
Identification of linker molecules suited for deposition and study of Mn12 single molecule magnets on Au surfaces
Authors: Voss S. , Herr S. , Fonin M. , Rüdiger U. , Burgert M. , Groth U.
Published: January 2008

The authors report on a scanning tunneling microscopy/spectroscopy investigation of the possibility to influence the assembly of monolayers of Mn12 single molecule magnets on a functionalized Au(111) surface by using flexible linker molecules. The results corroborate the general suitability of the deposition via ligand exchange reaction but, on the other hand, reveal the need for a compromise between conductivity and flexibility of the linker molecules. The results are discussed with respect to previous attempts [A. Naitabdi et al., Adv. Mater. (Weinheim, Ger.) 17, 1612 (2005)] to deposit ordered monolayers of Mn12 molecules on Au(111).
Magnetostriction and tunneling magnetoresistance of CoFeB∕AlOx∕Co∕IrMn junctions
Authors: Chen Yuan-Tsung , Jen S. U. , Tsai T. L. , Huang C. Y. , Yao Y. D.
Published: January 2008

Cross-strip magnetic tunnel junctions (MTJs) were fabricated following this sequence: Si(100)∕Ta(30Å)∕CoFeB(80Å)∕AlOx(to)∕Co(70Å)∕IrMn(90Å)∕Ta(100Å), under an in-plane deposition field (h)=500Oe, where to=12, 17, 22, and 26Å is the thickness of the AlOx layer. From the resistance (Ro) versus to plot, Ro increases from 255to1138Ω exponentially, as to increases from 12to26Å. This indicates that the quantum tunneling mechanism is valid in our case. Furthermore, the tunneling magnetoresistance (TMR) increases initially from 49% to 63% and then decreases to 33%, as to increases monotonically. This shows that with the increase of the barrier thickness, to⩾17Å, the spin polarization in the current after tunneling is weakened eventually. The microstructure of the to=17Å MTJ observed under high-resolution cross-sectional transmission electron microscope shows that the interfaces between CoFeB∕AlOx and AlOx∕Co are smooth, resulting in the high TMR ratio observed. Saturation magnetostriction (λs) of this series of CoFeB∕AlOx∕Co MTJs ranges from −5to−2ppm. The λs versus to plot also shows the concave-up feature. Finally, we find that the optimal MTJ has the following properties: TMR=63% and λs=−5ppm.
Use of microsecond current prepulse for dramatic improvements of wire array Z-pinch implosion
Authors: Calamy H. , Lassalle F. , Loyen A. , Zucchini F. , Chittenden J. P. , Hamann F. , Maury P. , Georges A. , Bedoch J. P. , Morell A.
Published: January 2008

The Sphinx machine [F. Lassalle et al., “Status on the SPHINX machine based on the 1microsecond LTD technology”] based on microsecond linear transformer driver (LTD) technology is used to implode an aluminium wire array with an outer diameter up to 140mm and maximum current from 3.5to5MA. 700to800ns implosion Z-pinch experiments are performed on this driver essentially with aluminium. Best results obtained before the improvement described in this paper were 1–3TW radial total power, 100–300kJ total yield, and 20–30kJ energy above 1keV. An auxiliary generator was added to the Sphinx machine in order to allow a multi microsecond current to be injected through the wire array load before the start of the main current. Amplitude and duration of this current prepulse are adjustable, with maxima ∼10kA and 50μs. This prepulse dramatically changes the ablation phase leading to an improvement of the axial homogeneity of both the implosion and the final radiating column. Total power was multiplied by a factor of 6, total yield by a factor of 2.5 with a reproducible behavior. This paper presents experimental results, magnetohydrodynamic simulations, and analysis of the effect of such a long current prepulse.
A hypersonic plasma bullet train traveling in an atmospheric dielectric-barrier discharge jet
Authors: Shi Jianjun , Zhong Fangchun , Zhang Jing , Liu D. W. , Kong M. G.
Published: January 2008

An experimental observation of fast-moving plasma bullets produced in an atmospheric dielectric-barrier discharge jet is reported in this paper. Nanosecond imaging suggests that the atmospheric discharge jet consists of a plasma bullet train traveling at a hypersonic speed from 7.0km∕sto43.1km∕s. Yet on a millisecond scale, the bullet train appears as a plasma jet of several centimeters long. The plasma bullets are produced through several possible mechanisms, the most likely of which is related to the ionization wave. Time and space resolved optical emission spectroscopy show that reactive plasma species can be delivered to different spatial sites with varying quantities.
Tuning the magnetization dynamics of silica-coated Fe3O4 core-shell nanoparticles by shell thickness control
Authors: Mohan Kant K. , Sethupathi K. , Ramachandra Rao M. S.
Published: January 2008

Core-shell silica (SiO2)-coated Fe3O4 nanoparticles of shell thickness between 2 and 52nm have been synthesized and characterized. The saturation magnetization scales with SiO2 wt % and these samples exhibit negligible hysteresis at room temperature, as compared to the bulk Fe3O4. The peak in the temperature-dependent ac susceptibility Tm shifts toward higher temperature with increasing frequency. Data of pristine to 40wt% SiO2-coated samples fit to critical slowing-down behavior model yielding relaxation time of ∼10−10s. For particles of higher shell thickness, smaller relaxation time of ∼10−13s is obtained. Temperature-dependent electron spin resonance measurements suggest overall weakening of magnetic interactions in Fe3O4 nanoparticles of thicker SiO2 shell. Variation of shell thickness in these core-shell nanoparticles could lead to diverse ground states ranging from interacting to noninteracting systems.
Magnetic and thermoelectric properties of quasi-one-dimensional oxides An+2CoBnO3n+3 (A=Ca,Sr, B=Co,Rh,Ir; n=1–3)
Authors: Takami T. , Ikuta H.
Published: January 2008

We studied the magnetic and thermoelectric properties of the homologous series An+2CoBnO3n+3 (A=Ca,Sr, B=Co,Rh,Ir; n=1–3), which possess one-dimensional chain structures consisting of one CoO6 trigonal prism and n BO6 octahedra. Both Ca3CoRhO6 and Ca3CoIrO6, the n=1 members, exhibited an abrupt drop in the magnetic susceptibility (χ) at around 35K with decreasing temperature. A broad peak was observed in the χ-T curves at 13 and 5K, respectively, for the n=2 and 3 samples of the B=Rh series. The change in the magnetic behavior of the B=Rh samples with increasing n is attributable to the increase of interchain distance, while the difference of Ca3CoRhO6 and Ca3CoIrO6, despite the valence and spin state are the same, may be due to a structural distortion of the latter compound. Thermopower (S) of all three n=1 samples was about 150μV∕K at around 1000K, and resistivity (ρ) showed semiconductive behavior, with Ca3CoRhO6 having the lowest resistivity. As a result, the power factor (=S2∕ρ) of Ca3CoRhO6 at 1073K was more than two times larger than that of Ca3Co2O6.
Preparation of Nd(Fe,Mo)12NX∕α-Fe nanocomposite magnets by the strip casting technique and subsequent mechanical alloying method
Authors: Han Jingzhi , Liu Shunquan , Zhang Xiaodong , Wang Changsheng , Du Honglin , Chen Haiying , Yang Yingchang
Published: January 2008

Preparation of Nd(Fe,Mo)12NX∕α-Fe nanocomposite magnets by the strip casting technique and subsequent mechanical alloying method are investigated. The result shows that the cooling rate of NdFe11Mo1 starting materials has great effects on the magnetic properties of the final Nd(Fe,Mo)12NX∕α-Fe nanocomposite magnets, and the rapid solidification is helpful for improving magnetic properties. The nanocomposite magnets starting from strip have a Br of 65emu∕g, an iHc of 8372Oe, and a (BH)max of 7.3MGOe, which is better than those of the corresponding materials starting from the conventional ingots. x-ray diffraction and scanning electron microscopy observations for the strips and the ingots show that not only the strip have much more 1:12 main phase and lower Fe-rich phase but also the distribution of 1:12 phase and Fe-rich phase in the strips are more uniform. The strips with the uniform microstructure are not only found to improve the efficiency of the high energy ball milling but also avoid the presence of excessive α-Fe nuclei. High resolution transmission electron microscopy observations show that the mechanically alloyed powders starting from the strips also have more homogeneous microstructure. In the end, the exchange coupling interactions are studied by the ratio of remanence coercivity Hr to the intrinsic coercivity Hc (Hr∕Hc). The Hr∕Hc of the nanocomposite magnets is found to be 1.25, while that of single phase Nd(Fe,Mo)12NX material is about 1.0. This shows that there exists strong exchange spring effect between the hard phase Nd(Fe,Mo)12NX and soft phase α-Fe.
High power solid state rf amplifier for proton accelerator
Authors: Jain Akhilesh , Sharma Deepak Kumar , Gupta Alok Kumar , Hannurkar P. R.
Published: January 2008

A 1.5kW solid state rf amplifier at 352MHz has been developed and tested at RRCAT. This rf source for cw operation will be used as a part of rf system of 100MeV proton linear accelerator. A rf power of 1.5kW has been achieved by combining output power from eight 220W rf amplifier modules. Amplifier modules, eight-way power combiner and divider, and directional coupler were designed indigenously for this development. High efficiency, ease of fabrication, and low cost are the main features of this design.
Current-induced motion of narrow domain walls and dissipation in ferromagnetic metals
Authors: Benakli M. , Hohlfeld J. , Rebei A.
Published: January 2008

Spin transport equations in a nonhomogeneous ferromagnet are derived in the limit where the sd exchange coupling between the electrons in the conduction band and those in the d band is dominant. It is shown that spin diffusion in ferromagnets assumes a tensor form. The diagonal terms are renormalized with respect to that in normal metals and enhance the dissipation in the magnetic system while the off-diagonal terms renormalize the precessional frequency of the conduction electrons and enhance the nonadiabatic spin torque. To demonstrate what additional physics is included in the theory, we show that self-consistent solutions of the spin diffusion equations and the Landau-Lifshitz equations in the presence of a current lead to an increase in the terminal velocity of a domain wall which becomes strongly dependent on its width. We also provide a simplified equation that predicts damping due to the conduction electrons.