bluuebunny/crossref_metadata_2025 huggingface dataset:
Depth resolved x-ray absorption fine structure study in magnetic multilayers using x-ray standing waves
Authors: Gupta Ajay , Kumar Dileep , Meneghini Carlo , Zegenhagen Jorg
Published: May 2007

X-ray standing waves generated using total external reflection from Au buffer layer has been used to get information about the structure of the interfaces in a Tb∕Fe∕Tb trilayer. X-ray absorption fine structure (XAFS) measurements done under standing wave conditions have sufficient depth resolution in order to differentiate the center of the Fe layer from the interfacial region. XAFS measurements are done in two geometries, with plane of polarization of the incident x-ray lying either in the film plane or perpendicular to it. In these two geometries information is obtained about the bonds lying in the film plane or perpendicular to it, respectively. These measurements provide clear evidence that Fe–Tb bonds in the interfacial region lie preferentially normal to the film plane. This asymmetry of Fe–Tb bonds is the cause of perpendicular magnetic anisotropy in the system. Present studies demonstrate that use of x-ray standing waves can provide sufficient depth resolution in order to elucidate the interface structure in magnetic multilayers.
Magnetocaloric effect in layer structural Gd5(SixGe1−x)4∕Gd composite material
Authors: Yue M. , Zhang J. X. , Zeng H. , Chen H. L. , Liu X. B.
Published: May 2007

A series of layer structural composite magnetic refrigerant materials with composition of (Gd5Si2Ge2)x(Gd)y(Gd5Si1.85Ge2.15)z was synthesized via spark plasma sintering technique. The adiabatic temperature change (ΔTad) of the composites and of the three individual components was measured directly from 245to315K with a magnetic field change of 1.5T using a homemade magnetocaloric effect measuring apparatus. For all composites with different x:y:z ratios, their resultant ΔTad uniformly peaks at 246, 276, and 296K, respectively, corresponding to the magnetic transition temperature of the three components. In addition, proper x:y:z ratio can improve both the value and the constancy of ΔTad of the composite. Compared with the three components, the value of ΔTad of the composites exhibits a more constant tendency, which is more suitable for practical application in room temperature refrigeration, when compared to those of the three components.
Proof of principle experiments that demonstrate utility of cocktail hohlraums for indirect drive ignition
Authors: Jones O. S. , Schein J. , Rosen M. D. , Suter L. J. , Wallace R. J. , Dewald E. L. , Glenzer S. H. , Campbell K. M. , Gunther J. , Hammel B. A. , Landen O. L. , Sorce C. M. , Olson R. E. , Rochau G. A. , Wilkens H. L. , Kaae J. L. , Kilkenny J. D. , Nikroo A. , Regan S. P.
Published: May 2007

This work is a summary of experiments, numerical simulations, and analytic modeling that demonstrate improved radiation confinement when changing from a hohlraum made from gold to one made from a mixture of high Z materials (“cocktail”). First, the results from several previous planar sample experiments are described that demonstrated the potential of cocktail wall materials. Then a series of more recent experiments are described in which the radiation temperatures of hohlraums made from uranium-based cocktails were directly measured and compared with a gold reference hohlraum. Cocktail hohlraums meeting the oxygen specification (<5% atomic fraction oxygen) demonstrated an increase in radiation of up to 7 eV, agreeing well with modeling. When applied to an indirectly driven fusion capsule absorbing ∼160kJ of x-ray energy, these data suggest that a hohlraum made from a suitably chosen uranium-based cocktail would have about 17% less wall losses and require about 10% less laser energy than a gold hohlraum of the same size.
Structure and internal dynamics of a side chain liquid crystalline polymer in various phases by molecular dynamics simulations: A step towards coarse graining
Authors: Ilnytskyi Jaroslav M. , Neher Dieter
Published: May 2007

Side chain liquid crystalline polymer with relatively long spacer was modeled on a semiatomistic level and studied in different liquid crystalline phases with the aid of molecular dynamics simulations. Well equilibrated isotropic, polydomain smectic and monodomain smectic phases were studied for their structural and dynamic properties. Particular emphasis was given to the analysis on a coarse-grained level, where backbones, side chains, and mesogens were considered in terms of their equivalent ellipsoids. The authors found that the liquid crystalline phase had a minor influence on the metrics of these objects but affected essentially their translational and orientational order. In the monodomain smectic phase, mesogens, backbones, and side chains are confined spatially. Their diffusion and shape dynamics are frozen along the mesogen director (the one-dimensional solidification) and the reorientation times increase by one to one-and-half orders of magnitude. In this phase, besides obvious orientational order of mesogens and side chains, a stable detectable order of the backbones was also observed. The backbone director is confined in the plane perpendicular to the mesogen director and constantly changes its orientation within this plane. The backbone diffusion in these planes is of the same range as in the polydomain smectic phase at the same temperature. A detailed analysis of the process of field-induced growth of the smectic phase was performed. The study revealed properties of liquid crystalline polymers that may enable their future fully coarse-grained modeling
High planarity x-ray drive for ultrafast shockless-compression experiments
Authors: Smith Raymond F. , Pollaine Stephen M. , Moon Stephen J. , Lorenz K. Thomas , Celliers Peter M. , Eggert Jon H. , Park Hye-Sook , Collins Gilbert W.
Published: May 2007

A spatially planar (Δtime∕time∼0.2%) longitudinal stress drive extending over millimeter scale lengths is used to shocklessly compress an aluminum sample to a peak stress of 210GPa over nanosecond time scales. Direct laser irradiation onto the inner wall of an Au halfraum creates an x ray distribution with a near-uniform blackbody temperature of up to 137eV. The x rays ablate material from a low-Z foil in a region of planarity closely matched to the diameter of the halfraum. The resultant ablatively driven shock is converted into a ramp-stress-wave in a secondary aluminum target through unloading across an intermediate vacuum gap. Higher peak stresses and shorter associated risetimes result from increasing input laser energy. Ramp-compression experiments can provide single shot equation-of-state data close to the isentrope, information on the kinetics of phase transformations, and material strength at high pressures.
Preparation method of rare earth bonded magnets with continuously controlled anisotropy directions
Authors: Yamashita F. , Kawamura K. , Okada Y. , Murakami H. , Ogushi M. , Nakano M. , Fukunaga H.
Published: May 2007

An advanced preparation method was carried out to obtain a highly dense radially anisotropic rare earth bonded magnet with continuously controlled anisotropy directions. An 8-pole/12-slot surface permanent magnet synchronous motor with a rotor 40.9mm in outer diameter, which comprised the above-mentioned magnet, was prepared, and it was found that the cogging torque decreases by approximately 20% and the back emf increases by 134% compared with those of a conventional isotropic Nd2Fe14B one with the same dimensions.
High temperature scanning Hall probe microscopy using AlGaN∕GaN two dimensional electron gas micro-Hall probes
Authors: Primadani Zaki , Osawa Hirotaka , Sandhu Adarsh
Published: May 2007

Hall sensors fabricated using conventional narrow band gap semiconductors such as InSb and GaAs are unstable above room temperature due to the onset of intrinsic conduction and physical degradation of the semiconductor materials. Gallium nitride (GaN) based wide band gap semiconductors are stable at elevated temperatures and show potential for fabrication of high temperature electronic devices. Here, we incorporated high sensitivity AlGaN∕GaN two dimensional electron gas heterostructure micro-Hall probes (HPs) into a high temperature scanning Hall probe microscope and for magnetic imaging of domains in crystalline iron garnet thin films from room temperature to 100°C. The active area and Hall coefficient the HPs were 2×2μm2 and 0.01Ω∕G at 100°C, respectively. The evolution of the size of magnetic domains with increasing temperature under external magnetic fields is described.
Enhancement of magneto-optical properties of a magnetic grating
Authors: Kim J. B. , Lee G. J. , Lee Y. P. , Rhee J. Y. , Yoon C. S.
Published: May 2007

The magneto-optical properties of one-dimensional magnetic grating structure of Co2MnSi film were investigated. By using the interference pattern of two femtosecond laser beams, a selective-area annealing of as-deposited Co2MnSi film was achieved and one-dimensional magnetic grating structures were fabricated. The longitudinal Kerr rotations of the zeroth- and the first-order diffracted beams were measured. The longitudinal Kerr rotation of the first-order diffracted beam is nearly 18 times larger than that of the zeroth-order beam.
Field-flow fractionation of nucleic acids and proteins under large-scale gradient magnetic fields
Authors: Iwasaka M.
Published: May 2007

For the purpose of developing techniques for separating biological macromolecules, the present study reports a magnetic chromatography system employing high performance liquid chromatography and superconducting magnets of 14 and 5T. We observed chromatograms of catalase and albumin, which were eluded from columns that were exposed to magnetic fields of up to 14T with a maximum gradient of 90T∕m. Without the magnetic fields, the chromatograms of the macromolecules showed a clear peak, while the chromatograms changed to have separated peaks for the same molecules after exposure to gradient magnetic fields. When the chromatocolumn was placed so the magnetic forces were opposite to the direction of flow, the albumin molecules separated into two groups. In addition, the chromatograms of catalase exposed to the magnetic fields indicated that the retention times of the two kinds of magnetically separated catalase were relatively changed if the column-field configuration was changed. Probably, the balance of paramagnetism in the heme and diamagnetism in the protein controlled the transport velocity under the influence of the gradient magnetic fields. In addition, the transport velocity of DNA molecules in the flow with a high gradient magnetic field was observed using a time-resolved spectrophotometric system.
Magneto-optical properties of ionic magnetic fluids
Authors: Benicio G. D. , Pelegrini F. , Bakuzis A. F. , Miranda K. L. C. , Sartoratto P. P. C.
Published: May 2007

Maghemite magnetic fluids were investigated by x-ray diffraction, revealing a particle size of 4.8nm with static magnetic birefringence. The effect of nanoparticle concentration on the magneto-optical properties of the system was investigated under low and high laser power conditions. Our results suggest that low power experiments should be used to obtain the correct particle size dispersity. In addition, we found that the mean agglomerate size increases along with particle volume fraction. However, unexpectedly, the mean agglomerate size only agrees qualitatively with the theoretical model, suggesting that other interaction terms, besides the dipolar one, should be included in the theoretical calculation.
Valence states of transition-metal ions in cubic perovskites SrMn1−xFexO3
Authors: Lee H. J. , Kim G. , Kang J.-S. , Dabrowski B. , Han S. W. , Lee S. S. , Hwang C. , Jung M. C. , Shin H. J. , Lee H. G. , Kim J.-Y. , Min B. I.
Published: May 2007

The electronic structures of SrMn1−xFexO3 (0⩽x⩽1) have been investigated by using photoemission spectroscopy (PES) and soft x-ray absorption spectroscopy (XAS). Mn ions in SrMnO3 are found to be in the nearly tetravalent (Mn4+) states, but with a small mixture of the Mn2+ configuration. With increasing x in SrMn1−xFexO3, Mn ions remain in the 4+ states. Fe ions in SrMn1−xFexO3 are also in the nearly tetravalent (Fe4+) states for the whole range of x. Valence-band PES reveals that the occupied Mn t2g3 states are located about 2.5eV below EF, while the occupied Fe t2g3eg1 states are broader than the Mn 3d occupied states, and are located between 3 and 6eV below EF. The large overlap between Fe 3d and the O 2p states is found in SrFeO3, reflecting the strong hybridization between the Fe 3d states and the O 2p states, in agreement with the metallic nature of SrFeO3.
Magnetic properties of iron oxalatophosphates with layer and framework structures
Authors: Lee S. F. , Tsai T. C. , Chang G. S. , Sheu C. Y. , Lii K. H.
Published: May 2007

The magnetic properties of two iron(III) oxalatophosphates, Cs2Fe(C2O4)0.5(HPO4)2 and CsFe(C2O4)0.5(H2PO4)(HPO4), were studied. The zero-field-cooled and field-cooled magnetizations versus temperature curves showed Curie-Weiss behavior from room temperature down to about 70K. Antiferromagnetic ordering and frustration behavior were observed in distinct temperature ranges at lower temperature. The frustration behavior can be attributed to the Dzyaloshinsky-Moriya interaction of the dimerlike magnetic structure, which causes spin canting states.
Effect of ac on current-induced domain wall motion
Authors: Kim W. J. , Lee T. D. , Choa S. H. , Seo S. M. , Lee K. J.
Published: May 2007

Saitoh et al. [Nature (London) 432, 203 (2004)] have reported the experimental result showing the interplay of a transverse domain wall with an electrical ac of megahertz-range frequencies. They observed a single peak of resistance in the frequency range and interpreted it with a nonadiabatic spin torque. It was argued that an ac current can induce a micrometer-range displacement of domain wall. We reconstructed the experiment in micromagnetic simulations considering the local nonzero nonadiabatic spin torque. We could not observe either an explicit single peak in the frequency-dependent resistance or an eventual displacement of domain wall by use of an ac. It indicates the local nonadiabatic torque is inappropriate to explain the experimental results of ac-induced domain wall motion. Other approaches such as the nonlocal nonadiabatic spin torque may be needed.
Thermal effect and current pulse durations on spin-transfer magnetization switching in MgO-based tunnel junctions
Authors: Lee J. M. , Lee Ching-Ming , Ye L. X. , Weng M. C. , Chen Y. C. , Su J. P. , Wu Te-ho
Published: May 2007

We have measured current-induced magnetization switching for MgO-based tunnel junctions with different current amplitudes and pulse durations at room temperature. The pulse widths ranged from 50μsto1ms and the current ratios J∕JC0 ranged from 0.4 to 0.97 (JC0 is the intrinsic critical current density). The results showed that the model of thermal assisted spin-transfer switching works well for the range of low to medium current ratio. However, for high current ratio, the switching probability did not fit the simple exponential function but grew with the current ratio linearly. The corresponding values of energy barrier as the current ratio approaches to unity indicated that thermal activation may still play an important role in magnetization switching.
Ordering in diluted magnetic semiconductors: A magnetic percolation phenomenon (invited)
Authors: Eriksson O. , Iuşan D. , Knut R. , Sanyal B.
Published: May 2007

We report on first principles calculations that describe the electronic structure, magnetic exchange interactions, and ordering temperatures of diluted magnetic semiconductors. The calculated interatomic exchange couplings are used in a Heisenberg Hamiltonian, and ordering temperatures are calculated with the use of Monte Carlo simulations. The accuracy of the method is analyzed by comparing observed and calculated ordering temperatures of several Mn and Cr doped III-V and II-VI semiconductors. The effect of magnetic percolation is discussed along with clustering phenomena and the effect of strong electron-electron interaction.
Noncollinear magnetism, magnetocrystalline anisotropy, and spin-spiral structures in Fe∕W(110)
Authors: Nakamura Kohji , Mizuno Naoki , Akiyama Toru , Ito Tomonori , Freeman A. J.
Published: May 2007

Spin-spiral structures in the Fe monolayer on a W(110) substrate are determined by means of the first principles film full-potential linearized augmented plane-wave method including full-noncollinear magnetism. The results obtained predict that spin-spiral structures with a wave vector of 0.05a−1–0.1a−1, where a is the lattice constant of bulk W, are energetically favorable over the ferromagnetic (FM) state. When compared with the calculated magnetocrystalline anisotropy (MCA) energy, however, the formation of the spin-spiral structures may be suppressed due to the large MCA that arises from the strong spin-orbit coupling at the Fe∕W(110) interface, and so the system appears to be the FM state—as observed in experiments.
Magnetic impurities in magic-number clusters
Authors: Skomski Ralph , Sellmyer D. J.
Published: May 2007

It is investigated how magnetic impurities modify the behavior of metallic clusters. Two complementary models are used, an s-d exchange model with a stable magnetic moment and a Hubbard-type Kondo model. The s-d and s-f interactions are modeled by a pointlike potential, as in the Ruderman-Kittel-Kasuya-Yosida approximation, but the perturbed levels are obtained by diagonalizing the interaction matrix rather than using perturbation theory. The spin polarization of the conduction electrons due to the magnetic impurities leads to a reduction of the highest occupied molecular orbital–lowest unoccupied molecular orbital splitting. A particularly interesting case is Pt, which is used in catalysis and whose well-delocalized 5d electrons are easily spin polarized by conduction electrons. Strikingly, the simplest realization of the Kondo effect is reproduced by an unrestricted Hartree-Fock approximation.
Correlation effect and magnetic moments in Cr2Te3
Authors: Youn S. J. , Kwon S. K. , Min B. I.
Published: May 2007

The electronic and magnetic structures of Cr2Te3 have been studied theoretically using the linearized muffin-tin orbitals band method. Experimental photoemission spectra and magnetic moments can be described better when the on-site Coulomb correlation U of Cr 3d electrons is considered using the local spin-density approximation+U method. The proper size of U is found to be U∼1.7eV. The complex magnetic behaviors of Cr2Te3 come from the degeneracy of parallel and antiparallel alignments of CrI spin to CrII and CrIII spins.
Power spectrum of current-induced magnetization dynamics in uniaxial nanomagnets
Authors: Serpico Claudio , Bertotti Giorgio , Bonin Roberto , d’Aquino Massimiliano , Mayergoyz Isaak D.
Published: May 2007

Nanomagnets with uniaxial symmetry driven by spin polarized currents are considered, in which anisotropy, applied field, and spin polarization are all aligned along the symmetry axis. Thermal fluctuations are taken into account by adding a Gaussian white noise stochastic term to the equation for the deterministic dynamics. The corresponding Fokker-Planck equation is derived and it is solved by using separation of variable and eigenfunction expansion techniques. From the solution of the Fokker-Planck equation, the autocorrelation function and the power spectral density of magnetization are computed. The position and the linewidth of the peaks in the power spectral density are discussed in detail.
Visualization of magnetization processes of soft magnetic composites by the magneto-optical imaging technique
Authors: Lin Zhi Wei , Zhu Jian Guo , Zhong Jin Jiang , Johansen Tom H. , Wang Xiao Lin , Yu Wei Yang
Published: May 2007

Attractive features of soft magnetic composites (SMCs) are high resistivity, isotropic three-dimensional flux behavior, and easy compression into the complicated shapes required in electromagnetic devices. Comprehensive understanding of the materials will help optimize design of electromagnetic devices. This paper presents the magnetization processes in a SMC sample in micron scale by means of the magneto-optical imaging technique. The sample was magnetized by magnetic fields tangential or perpendicular to the observation surface. It is observed that the flux density is higher at the particle region but lower at the interparticle space. When a tangential field is applied, the stray fields change polarization at the particle boundaries. Both results suggest that the magnetized sample behaves as a collection of individual magnetized particles rather than as a uniform and continuous magnetic substance such as soft iron although there are some interactions between neighboring particles.
Current-induced switching study of the canting model for ferrimagnet thin films
Authors: Canizo-Cabrera A. , Wang Yi-Rung , Chang Yao-Jen , Garcia-Vazquez Valentin , Wu Te-ho
Published: May 2007

We present a study on the influence of the canting angle between the magnetization vectors of a two-subnetwork system on the magnetization switching exerted by a spin-polarized induced current. Numerical calculations are performed on current-induced magnetization switching in a magnetic system with a canting state. The modified Landau-Lifshitz-Gilbert equation is used to produce quantitative predictions for the critical current as a function of the exchange coupling constant (which characterizes the canting strength) at several applied magnetic fields. The results indicate that the critical current reduces its value to one third when the coupling constant is varied.
Josephson-vortex flow resistance in Bi2Sr2Ca2Cu3Oy single crystals and its possible application in the manipulation of spin and charge textures in diluted magnetic semiconductors
Authors: Wang X. L. , Lin C. T. , Liang B. , Yu S. , Ooi S. , Hirata K. , Ding S. Y. , Shi D. Q. , Dou S. X. , Lin Z. W. , Zhu J. G.
Published: May 2007

In this work, the flow of the Josephson vortices (JVs) has been studied for the highly anisotropic Bi2Sr2Ca2Cu3Oy (Bi2223) single crystals. A giant flow of JVs or giant positive magnetoresistance (MR) of over 500%–2000% was obtained in fields of 0.1–5T and remained almost constant over a wide temperature range from 110 down to 4K, in contrast to superconducting vortices (SVs), which only produced MR in the vicinity of Tc. The flow of the JVs is expected to be much faster than that of SVs. It is proposed that the Josephson vortices could be used to manipulate the spin and charge in magnetic semiconductors in the same way as SVs [M. Berciu, T. G. Rappoport, and B. Jankó, Nature (London) 435, 71 (2005)]. Hybrid systems consisting of layered superconductors with Josephson junctions and magnetic semiconductors will be discussed.
The complementary nature of coercivity enhancement and exchange bias in a general ferro-antiferromagnet exchange coupled system
Authors: Liu Congxiao , Fujiwara Hideo , Sun Min
Published: May 2007

Complementary nature of coercivity enhancement and exchange bias in a ferro-antiferromagnet (F-AF) exchange coupled system with arbitrary configuration refers to the relation Hsw0−=Ms∕χin,max, with Ms being the saturation magnetization of the F part, Hsw0− being the field at which the F magnetization starts to be unstable, applied opposite to the stable direction of the F magnetization in the absence of external field, and χin,max being the maximal initial susceptibility, for which the field is applied perpendicular to that stable direction. This relation is a special case of a more general result for a single domain particle, proved in this paper. In a F-AF system, when the F magnetization behaves coherently, the effect of F-AF coupling can be absorbed into the anisotropy of the F part and the system can be treated as a single domain in an external field.
Mössbauer study on magnetite nanochains synthesized by chemical self-assembly in magnetic field
Authors: Zhang Y. , Sun L. , Zhai Y. , Huang H. B. , Huang R. S. , Lu H. X. , Zhai H. R.
Published: May 2007

Solid and hollow nanospherical chains of magnetite with different diameters and diameter/length aspect ratios were prepared by chemical precipitation method in magnetic field. By transmission electron microscopy, x-ray diffraction, magnetometry, and Mössbauer spectroscopy, it was found that the application of magnetic field during precipitation induced the formation of spherical chain structures and simultaneously caused the change of hyperfine field and isomer shift of the octahedral sublattice spectra, implying that the nanospherical chains were not chains of nanoparticles formed simply by magnetostatic attraction. In addition, the solid and hollow nanospherical chains with the same diameter of spheres exhibited slight variation of isomer shift and sublattice hyperfine field, while there were the dramatic changes when the diameter of hollow nanospherical chains is down to 100nm.
Generalized path dependent representations for gauge theories
Authors: Reyes Marat C.
Published: May 2007

A set of differential operators acting by continuous deformations on path dependent functionals of open and closed curves is introduced. Geometrically, these path operators are interpreted as infinitesimal generators of curves in the base manifold of the gauge theory. They furnish a representation with the action of the group of loops having a fundamental role. We show that the path derivative, which is covariant by construction, satisfies the Ricci and Bianchi identities. Also, we provide a geometrical derivation of covariant Taylor expansions based on particular deformations of open curves. The formalism includes, as special cases, other path dependent operators such as end point derivatives and area derivatives.
Developing depleted uranium and gold cocktail hohlraums for the National Ignition Facility
Authors: Wilkens H. L. , Nikroo A. , Wall D. R. , Wall J. R.
Published: May 2007

Fusion ignition experiments are planned to begin at the National Ignition Facility (NIF) [J. A. Paisner, E. M. Campbell, and W. J. Hogan, Fusion Technol. 26, 755 (1994)] using the indirect drive configuration [J. D. Lindl, P. Amendt, R. L. Berger, S. G. Glendinning, S. H. Glenzer, S. W. Haan, R. L, Kauffman, O. L. Landen, and L. J. Suter, Phys. Plasmas 11, 339 (2004)]. Although the x-ray drive in this configuration is highly symmetric, energy is lost in the conversion process due to x-ray penetration into the hohlraum wall. To mitigate this loss, depleted uranium is incorporated into the traditional gold hohlraum to increase the efficiency of the laser to x-ray energy conversion by making the wall more opaque to the x rays [H. Nishumura, T. Endo, H. Shiraga, U. Kato, and S. Nakai, Appl. Phys. Lett. 62, 1344 (1993)]. Multilayered depleted uranium (DU) and gold hohlraums are deposited by sputtering by alternately rotating a hohlraum mold in front of separate DU and Au sources to build up multilayers to the desired wall thickness. This mold is removed to leave a freestanding hohlraum half; two halves are used to assemble the complete NIF hohlraum to the design specifications. In practice, exposed DU oxidizes in air and other chemicals necessary to hohlraum production, so research has focused on developing a fabrication process that protects the U from damaging environments. This paper reports on the most current depleted uranium and gold cocktail hohlraum fabrication techniques, including characterization by Auger electron spectroscopy, which is used to verify sample composition and the amount of oxygen uptake over time.
Effects of deep defect concentration on junction space charge capacitance measurements
Authors: Buchwald Walter R. , Morath Christian P. , Drevinsky Peter J.
Published: May 2007

Expressions for the small signal capacitance and pulse bias-induced large signal capacitance transient of an abrupt n+-p junction in the presence of one or more deep traps of arbitrary concentrations are presented. The expressions show that a simple inverse relationship between the measured small signal capacitance and the junction’s space charge depletion width is not expected. Unexpected effects on defect capture cross section and defect concentration determination, when applied to deep level transient spectroscopy simulations, are also reported. Experimental results of both temperature dependent capacitance and transient capacitance measurements performed on electron irradiated Si n+-p junctions are presented and analyzed in the context of these expressions. Modeling of the measured signals using these expressions is shown to be in good agreement with the experiment.
Andreev reflection study of the new magnetic superconductor Mo3Sb7: evidence of gap anisotropy
Authors: Dmitriev V. M. , Rybaltchenko L. F. , Khristenko E. V. , Ishchenko L. A. , Bukowski Z. , Troć R.
Published: May 2007

Point-contact (PC) Andreev-reflection experiments on the new paramagnetic superconductor Mo3Sb7 are reported for which we have observed strong distinctions in the PC spectra measured for different contact axis orientations. Analysis of the PC spectra in the framework of the Blonder–Tinkham–Klapwijk theory has shown that the gap parameter Δ is strongly anisotropic, varying over a very wide range depending on the contact orientation: the maximum Δmax can exceed the minimum Δmin by at least 40 times. For these and other reasons we suggest that Mo3Sb7 is not a trivial BCS (s-wave) superconductor but rather has (s+g)-wave or another unconventional pairing symmetry.
Transfer operators for hard-rod-type models
Authors: Rilke Florian
Published: May 2007

We describe a new approach to the hard rod model and construct a Ruelle-Mayer-type transfer operator which satisfies a dynamical trace formula. This yields a meromorphic continuation of the dynamical zeta function in two variables among them the inverse temperature.
High P-T transformations of nitrogen to 170GPa
Authors: Gregoryanz Eugene , Goncharov Alexander F. , Sanloup Chrystele , Somayazulu Maddury , Mao Ho-kwang , Hemley Russell J.
Published: May 2007

X-ray diffraction and optical spectroscopy techniques are used to characterize stable and metastable transformations of nitrogen compressed up to 170GPa and heated above 2500K. X-ray diffraction data show that ϵ-N2 undergoes two successive structural changes to complex molecular phases ζ at 62GPa and a newly discovered κ at 110GPa. The latter becomes an amorphous narrow gap semiconductor on further compression and if subjected to very high temperatures (∼2000K) crystallizes to the crystalline cubic-gauche-N structure (cg-N) above 150GPa. The diffraction data show that the transition to cg-N is accompanied by 15% volume reduction.
Amorphous gallium indium zinc oxide thin film transistors: Sensitive to oxygen molecules
Authors: Kang Donghun , Lim Hyuck , Kim Changjung , Song Ihun , Park Jaechoel , Park Youngsoo , Chung JaeGwan
Published: May 2007

In this study, the authors report characteristic of indium gallium zinc oxides (GIZOs) which is strongly associated with the film surface. In ambient air, turn-on voltage of GIZO thin film transistors is approximately −7V. However, at the pressure of 8×10−6Torr, the turn-on voltage dramatically shifts to nearly −47V of the negative gate bias direction. When the oxygen is introduced in the chamber, the turn-on voltage returns to the normal value, that of air. It is believed that the adsorbed oxygen forms depletion layer below the surface, resulting in Von shifts. The carrier concentration of the channel varies from 1×1019to1×1020cm−3 due to oxygen adsorption.
Electrical characteristics and reliability properties of metal-oxide-semiconductor field-effect transistors with ZrO2 gate dielectric
Authors: Hsu De-Cheng , Wang Ming-Tsong , Lee Joseph Ya-min , Juan Pi-Chun
Published: May 2007

Metal-oxide-semiconductor capacitors and metal-oxide-semiconductor field-effect transistors with ZrO2 gate dielectric were fabricated. The time dependent dielectric breakdown (TDDB) of ZrO2 capacitors was studied. It was observed that the Weibull slopes were independent of the capacitor area. The Weibull slopes had no clear dependence on ZrO2 thickness. The TDDB of ZrO2 follows the E model. The activation energy Ea was linearly dependent on the electric field and the field acceleration parameter γ is independent of temperature.
Simulation of thermal-field directed self-assembly of epitaxial quantum dots
Authors: Kumar Chandan , Friedman Lawrence H.
Published: May 2007

Thermal-field directed self-assembly (TDSA) of epitaxial self-assembled quantum dots (SAQDs) is the method of using spatially varying temperature patterns to influence SAQD growth kinetics and ultimately the order and positions of SAQDs. The effectiveness of TDSA in enhancing the size and spatial order or precision placement of single dot or dot cluster is investigated via a two dimensional (one dimensional surface) finite element based model of Ge dots grown on Si. Three different cases of heating are studied, namely, spatially constant heating, spatially periodic heating, and a Gaussian shaped hot spot. Statistical measures are used to study the evolution of quantum dot heights and spacings between the quantum dots for different temperatures and heating cases. Spatially periodic heating is investigated for a wavelength of about 185nm corresponding to the wavelength of an excimer laser. In order to investigate the effectiveness of spatially periodic heating in enhancing the spatial and size order of the SAQD array, the results are compared with those obtained from spatially uniform heating. Simulations using Gaussian shaped hot spot are performed to demonstrate its effectiveness in placing a distinct dot at a desired location. Results from the simulations indicate that spatially periodic heating proves to be an effective means for producing an array of uniformly sized and spaced quantum dots for a broad range of temperatures. Also, the results from the simulations using Gaussian shaped hot spots show that such a form of heating can effectively place a distinct quantum dot near a desired location for a broad range of temperature values.
Persistent photoinduced changes in charge states of transition-metal donors in hydrothermally grown ZnO crystals
Authors: Jiang Yongquan , Giles N. C. , Halliburton L. E.
Published: May 2007

Electron paramagnetic resonance (EPR) has been used to monitor photoinduced changes in the charge states of deep transition-metal donors (Mn, Fe, Co, and Ni), shallow donors (Al and Ga), and lithium acceptors in a hydrothermally grown ZnO crystal. All of these impurities except the lithium were unintentionally present. Changes in charge states were photoinduced at low temperature using 325nm light. Before illumination, the transition-metal donors were present in divalent and trivalent charge states, and all the lithium acceptors were in the singly ionized charge state. During illumination, neutral lithium acceptors were formed, the Fe3+, Ni3+, and Mn3+ spectra disappeared (i.e., they converted to divalent states), and the Mn2+ and Co2+ spectra increased. When the light was removed, these changes in charge states were persistent and stable at low temperature. The thermal stabilities of the changes were monitored during a pulsed anneal (the sample was warmed in steps to sequentially higher temperatures and returned each time to a lower monitoring temperature). Electrons were released from Fe2+ donors in the 90–120K range, from Ni2+ and Co2+ donors in the 110–140K range, and from Mn2+ donors in the 220–300K range. Thermal activation energies for Fe2+ and Ni2+ are estimated to be 240 and 280meV, respectively.
Effect of magnetic polarons on the magnetic, magnetocaloric, and magnetoresistance properties of the intermetallic compound HoNiAl
Authors: Singh Niraj K. , Suresh K. G. , Nirmala R. , Nigam A. K. , Malik S. K.
Published: May 2007

The magnetic, magnetocaloric, and magnetoresistive properties of the polycrystalline compound HoNiAl have been studied. The temperature variations of magnetization and heat capacity show that the compound undergoes two magnetic transitions, one at 14K and the other at 5K. The former is due to the paramagnetic-ferromagnetic transition, while the latter is attributed to the onset of an antiferromagnetic ordering, as the temperature is lowered. The M-H isotherm obtained at 2K shows a metamagnetic transition with a critical filed of about 13kOe. The maximum values of isothermal magnetic entropy change and adiabatic temperature change, for a field change of 50kOe, are estimated to be 23.6J∕kgK and 8.7K, respectively. The relative cooling power is found to be about 500J∕kg for a field change of 50kOe. A large magnetoresistance of about 16%, near the ordering temperature of 14K, is observed for a field of 50kOe. The magnetic, magnetocaloric, and magnetoresistance data seem to suggest the presence of magnetic polarons in this compound.
Structure and dynamics of short chain molecules in disordered porous materials: A molecular dynamics simulation study
Authors: Chang Rakwoo , Yethiraj Arun
Published: May 2007

The static and dynamic properties of short polymer chains in disordered materials are studied using discontinuous molecular dynamics simulations. The polymers are modeled as chains of hard spheres and the matrix is a collection of fixed hard spheres. The simulations show that the chain size is a nonmonotonic function of the matrix concentration for all polymer concentrations. The dependence of polymer diffusion D on the degree of polymerization N becomes stronger as the matrix concentration is increased. At high matrix concentrations we observe a decoupling between translational and rotational diffusion, i.e., the rotational relaxation time becomes very large but the translational diffusion is not affected significantly. We attribute this to the trapping of a small number of polymers. Under these conditions the polymer chains diffuse via a hopping mechanism.
Classical interaction model for the water molecule
Authors: Baranyai András , Bartók Albert
Published: May 2007

The authors propose a new classical model for the water molecule. The geometry of the molecule is built on the rigid TIP5P model and has the experimental gas phase dipole moment of water created by four equal point charges. The model preserves its rigidity but the size of the charges increases or decreases following the electric field created by the rest of the molecules. The polarization is expressed by an electric field dependent nonlinear polarization function. The increasing dipole of the molecule slightly increases the size of the water molecule expressed by the oxygen-centered σ parameter of the Lennard-Jones interaction. After refining the adjustable parameters, the authors performed Monte Carlo simulations to check the ability of the new model in the ice, liquid, and gas phases. They determined the density and internal energy of several ice polymorphs, liquid water, and gaseous water and calculated the heat capacity, the isothermal compressibility, the isobar heat expansion coefficients, and the dielectric constant of ambient water. They also determined the pair-correlation functions of ambient water and calculated the energy of the water dimer. The accuracy of theirs results was satisfactory.
Electric and hydrodynamic stretching of DNA-polymer conjugates in free-solution electrophoresis
Authors: Nedelcu S. , Meagher R. J. , Barron A. E. , Slater G. W.
Published: May 2007

The conjugation of an uncharged polymer to DNA fragments makes it possible to separate DNA by free-solution electrophoresis. This end-labeled free-solution electrophoresis method has been shown to successfully separate ssDNA with single monomer resolution up to about 110 bases. It is the aim of this paper to investigate in more detail the coupled hydrodynamic and electrophoretic deformation of the ssDNA-label conjugate at fields below 400V∕cm. Our model is an extension of the theoretical approach originally developed by Stigter and Bustamante [Biophys. J. 75, 1197 (1998)] to investigate the problems of a tethered chain stretching in a hydrodynamic flow and of the electrophoretic stretch of a tethered polyelectrolyte. These two separate models are now used together since the charged DNA is “tethered” to the uncharged polymer (and vice versa), and the resulting self-consistent model is used to predict the deformation and the electrophoretic velocity for the hybrid molecule. Our theoretical and experimental results are in good qualitative agreement.
Wigner crystals of Na+ ions at the surface of a silica hydrosol
Authors: Tikhonov Aleksey M.
Published: May 2007

The symmetry of the surface of an electrolyte solution can be anisotropic, regardless of the bulk’s isotropic symmetry, because of spatial correlations between adsorbed ions. The author used x-ray grazing-incidence diffraction to measure the spatial correlations between sodium ions in “classical one-component plasma” adsorbed with Bjerrum’s density at the surface of a monodispersed 22nm particle colloidal silica solution stabilized by NaOH with a total bulk concentration ∼0.05mol∕L. The authors findings show that the surface compact layer is in a two-dimensional crystalline state (symmetry p2), with four sodium ions forming the unit cell and a ∼30Å translational correlation length between the ions.
Characterization of a heat flux sensor using short pulse laser calibration
Authors: Löhle Stefan , Battaglia Jean-Luc , Batsale Jean-Christophe , Enouf Olivier , Dubard Jimmy , Filtz Jean-Remy
Published: May 2007

A method to calibrate classical heat flux sensors is presented. The classical approach to measure the temperature inside a known material by using a thermocouple fails when the measurement time is very short. In this work the surface heat flux is determined by solving the inverse heat conduction problem using a noninteger identified system as a direct model for the estimation process. Using short pulse laser calibration measurements the crucial design aspects of the sensor that play a significant role when assuming one-dimensional, semi-infinite heat transfer have been accounted for. The theoretical approach as well as the calibration results are presented and comparisons to the classical approach and results from finite element modeling are shown. It is concluded that the new method ameliorate the heat flux sensor significantly and extend its application to very short measurement times.
Defect-induced electrical conductivity increase in individual multiwalled carbon nanotubes
Authors: Agrawal S. , Raghuveer M. S. , Li H. , Ramanath G.
Published: May 2007

The authors demonstrate that ozone exposure of individual multiwalled carbon nanotubes (CNTs) results in up to threefold increase in CNT conductivity and 50% decrease in carrier transport activation energy. Ozone exposure induces bond breaking in the individual shells and promotes cross-shell bridging via sp3 bond formation. Intershell bridging facilitates charge carrier hopping to inner shells, which can serve as additional charge carrier transport pathways, offsetting the effect of defect-scattering-induced conductivity decrease. The CNT etch rate systematically increases with decreasing initial outer diameter and decreases with incremental ozone exposure, which could provide means to controllably tailor the CNT conductance. The results suggest that controlled defect creation could be an attractive strategy to induce electrical conductivity increase in multiwalled CNTs for use in nanodevice wiring and related applications.
Epitaxially stabilized growth of orthorhombic LuScO3 thin films
Authors: Heeg T. , Roeckerath M. , Schubert J. , Zander W. , Buchal Ch. , Chen H. Y. , Jia C. L. , Jia Y. , Adamo C. , Schlom D. G.
Published: May 2007

Metastable lutetium scandate (LuScO3) thin films with an orthorhombic perovskite structure have been prepared by molecular-beam epitaxy and pulsed-laser deposition on NdGaO3(110) and DyScO3(110) substrates. Stoichiometry and crystallinity were investigated using Rutherford backscattering spectrometry/channeling, x-ray diffraction, and transmission electron microscopy. The results indicate that LuScO3, which normally only exists as a solid solution of Sc2O3 and Lu2O3 with the cubic bixbyite structure, can be grown in the orthorhombically distorted perovskite structure. Rocking curves as narrow as 0.05° were achieved. A critical film thickness of approximately 200nm for the epitaxially stabilized perovskite polymorph of LuScO3 on NdGaO3(110) substrates was determined.
Two-photon absorption and Kerr coefficients of silicon for 850–2200nm
Authors: Bristow Alan D. , Rotenberg Nir , van Driel Henry M.
Published: May 2007

The degenerate two-photon absorption coefficient β and Kerr nonlinearity n2 are measured for bulk Si at 300K using 200fs pulses with carrier wavelength of 850<λ<2200nm for which indirect gap transitions occur. With a broad peak near the indirect gap and maximum value of 2±0.5cm∕GW, the dispersion of β compares favorably with theoretical calculations of Garcia and Kalyanaraman [J. Phys. B 39, 2737 (2006)]. Within our wavelength range, n2 varies by a factor of 4 with a peak value of 1.2×10−13cm2∕W at λ=1800nm.
Soft-mold-induced self-construction of polymer patterns under microwave irradiation
Authors: Ko Fu-Hsiang , Wu Chia-Tien , Chen Mei-Fen , Chen Jem-Kun , Chu Tieh-Chi
Published: May 2007

In this study, the authors used a soft-mold-induced self-construction method to fabricate three-dimensional patterns under microwave irradiation for 1min. The authors estimated the actual pattern growth temperature using a fluorescence probe technique. The temperature at which pattern growth originated was, by necessity, higher than the glass transition temperature of the novolak resist. Electrostatic forces and surface tension effects under the electromagnetic field contributed significantly to the pattern growth, and the use of an antisticking agent allowed easy demolding.
Galvanic corrosion induced degredation of tensile properties in micromachined polycrystalline silicon
Authors: Miller David C. , Boyce Brad L. , Gall Ken , Stoldt Conrad R.
Published: May 2007

Immersion of polycrystalline silicon in hydrofluoric acid-based solutions is often utilized in microsystem fabrication to liberate mechanical structures. The authors demonstrate, using microfabricated tensile specimens, that such etching can cause a catastrophic reduction in tensile strength and elastic modulus in silicon galvanically coupled to a metallic layer, such as commonly used gold. Galvanically corroded silicon exhibits grain-boundary attack leading to intergranular fracture and/or generalized material removal. The severity of damage and corresponding losses in strength and modulus depend on etch duration and etch chemistry. In contrast, without a metallic layer, uncorroded silicon fails transgranularly and independent of etch duration or chemistry.
Plasmon excitations in scanning tunneling microscopy: Simultaneous imaging of modes with different localizations coupled at the tip
Authors: Romero Manuel J. , van de Lagemaat Jao , Rumbles Garry , Al-Jassim Mowafak M.
Published: May 2007

The authors investigate the localization of photons emitted at the tip during scanning tunneling microscopy measurements on atomically flat gold substrates. Emission patterns of the plasmon-mediated luminescence exhibit distinct features that are assigned to the localized modes of the surface plasmon (LSP) confined to the tunneling gap and propagating modes (PSP) coupled to the LSP by the optical cavity beneath the tip. Tunneling luminescence spectroscopy reveals that the plasmon localization at the tip increases when modes of higher energy are excited. Acquisition of local emission patterns allows us for the simultaneous imaging of LSP and PSP modes.
One-dimensional electron transport in Cu-tetracyanoquinodimethane organic nanowires
Authors: Zhou Zhixian , Xiao Kai , Jin R. , Mandrus D. , Tao Jing , Geohegan D. B. , Pennycook Stephen
Published: May 2007

The temperature and bias voltage dependent electrical transport properties of in situ fabricated Cu-tetracyanoquinodimethane organic nanowire devices are investigated. The low bias conductance and current exhibit a power-law dependence on temperature and bias voltage, respectively. The overall behavior of these nanowires can be well described by a theoretical model of nearly independent parallel chains of quantum dots created by randomly distributed defects.