bluuebunny/crossref_metadata_2025 huggingface dataset:
Giant positive magnetoresistance in ultrathin films of mixed phase manganites
Authors: Yun Sung H. , Dhakal Tara , Goswami Devesh , Singh Guneeta , Herbard Arthur , Biswas Amlan
Published: March 2008

Magnetic tunnel junctions (MTJs) based on fully spin polarized ferromagnetic manganites have generated a lot of interest due to their enhanced field sensitivity at low temperatures. However, the tunneling magnetoresistance (TMR) drops rapidly with increasing temperature due to the reduction of spin polarization at the manganite-insulator interface. We have devised a method for creating intrinsic tunnel barriers by tuning the phase competition in manganites using substrate induced strain. Ultrathin films (7.5nm) of the mixed phase manganite (La0.5Pr0.5)0.67Ca0.33MnO3 (LPCMO) grown on the substrate (110) NdGaO3 using pulsed laser deposition show positive magnetoresistance (MR) of about 30% at magnetic fields less than 1T. Unlike the fabricated MTJ devices, this MR effect has its maximum value close to the insulator to metal transition temperature and reduces with decreasing temperature. To find out the mechanism leading to this positive MR, the effect of three orientations of the magnetic field on the LPCMO thin films were studied: (1) perpendicular to the plane of the film, (2) parallel to the plane of the film and applied current, and (3) parallel to the plane of the film but perpendicular to the applied current. The effect of field orientation suggests that a possible mechanism for the positive MR is tunneling magnetoresistance due to the spin conserving tunneling process across the insulating regions separating the ferromagnetic metallic regions. The voltage dependence of the MR also supports this mechanism. Our results suggest a novel method for obtaining enhanced TMR in manganite based MTJs by creating strain induced intrinsic tunnel barriers.
Bond frustration effect of Cr ions in magnetiochromite by Mössbauer spectroscopy
Authors: Choi Kang Ryong , Kim Sam Jin , Lee Bo Wha , Kim Chul Sung
Published: March 2008

Polycrystalline MgCr1.98Fe0.0257O4 compound was synthesized by sol-gel process. The crystal structure was found to be single-phase cubic spinel with space group of Fd3(−)m. The lattice constant a0 and the fractional coordinate (x) of the oxygen were determined to be 8.336Å and 0.260, respectively. The Cr–Cr linkages in ACr1.98Fe0.0257O4 (A=Mg,Zn) have bond lengths of 2.945 and 2.947Å, respectively. We have observed larger value of Néel temperature (TN=12–12.5K) in Mg, Zn chromite spinels than those of Cd, Hg-chromite spinel (TN=6–8K). Mössbauer spectra of MgCr1.98Fe0.0257O4 were taken from 4.2to295K using a Co57 source in a rhodium matrix. MgCr1.98Fe0.0257O4 Mössbauer spectra below TN show the line broadening due to bond frustration. Above the Néel temperature, paramagnetic doublet is observed. The magnetic properties and Mössbauer results can be explained by the B–B exchange interaction and bond frustration in MgCr1.98Fe0.0257O4.
Magnetism of NiMn2O4–Fe3O4 spinel interfaces
Authors: Nelson-Cheeseman B. B. , Chopdekar R. V. , Bettinger J. S. , Arenholz E. , Suzuki Y.
Published: March 2008

We investigate the magnetic properties of the isostructural spinel-spinel interface of NiMn2O4(NMO)–Fe3O4. Although the magnetic transition temperature of the NMO film is preserved, both bulk and interface sensitive measurements demonstrate that the interface exhibits strong interfacial magnetic coupling up to room temperature. While NMO thin films have a ferrimagnetic transition temperature of 60K, both NiFe2O4 and MnFe2O4 are ferrimagnetic at room temperature. Our experimental results suggest that these magnetic properties arise from a thin interdiffused region of (Fe,Mn,Ni)3O4 at the interface, leading to Mn and Ni magnetic properties similar to those of MnFe2O4 and NiFe2O4.
Benchmark density functional theory calculations for nanoscale conductance
Authors: Strange M. , Kristensen I. S. , Thygesen K. S. , Jacobsen K. W.
Published: March 2008

We present a set of benchmark calculations for the Kohn-Sham elastic transmission function of five representative single-molecule junctions. The transmission functions are calculated using two different density functional theory methods, namely an ultrasoft pseudopotential plane-wave code in combination with maximally localized Wannier functions and the norm-conserving pseudopotential code SIESTA which applies an atomic orbital basis set. All calculations have been converged with respect to the supercell size and the number of k∥ points in the surface plane. For all systems we find that the SIESTA transmission functions converge toward the plane-wave result as the SIESTA basis is enlarged. Overall, we find that an atomic basis with double zeta and polarization is sufficient (and in some cases, even necessary) to ensure quantitative agreement with the plane-wave calculation. We observe a systematic downshift of the SIESTA transmission functions relative to the plane-wave results. The effect diminishes as the atomic orbital basis is enlarged; however, the convergence can be rather slow.
Size effect of surface magnetic anisotropy in Cu2O nanoparticles
Authors: Shih Po-Hsun , Ji Jhong-Yi , Ma Yuan-Ron , Wu Sheng Yun
Published: March 2008

The thermal deviation of the saturation magnetization of Cu2O nanoparticles shows the size effect of a surface magnetic anisotropic energy gap, which is in contrast to the diamagnetism characteristic of bulk Cu2O. The thermal deviation of the saturation magnetization exhibits an exponential variation, which is a clear departure from the Bloch T3∕2 law expected for isotropic systems, signaling the onset of magnetic anisotropy, presumably due to the high surface-to-volume ratio of the nanoparticles and the surface atoms which gives rise to polarized moments. We assume a uniaxial anisotropy and adopt the Heisenberg model for the Ha-aligned moment-carrying Cu2O nanoparticles. It is known that the uniaxial anisotropy results in a gap in the energy dispersion. The surface magnetic anisotropic energy gap can be obtained from the fit and decreases with the reducing particle size.
Optical and microstructural studies of atomically flat ultrathin In-rich InGaN∕GaN multiple quantum wells
Authors: Kwon Soon-Yong , Kim Hee Jin , Yoon Euijoon , Jang Yudong , Yee Ki-Ju , Lee Donghan , Park Seoung-Hwan , Park Do-Young , Cheong Hyeonsik , Rol Fabian , Dang Le Si
Published: March 2008

Optical and microstructural properties of atomically flat ultrathin In-rich (UTIR) InGaN∕GaN multiple quantum well were investigated by means of photoluminescence (PL), time-resolved PL (TRPL), and cathodoluminescence (CL) experiments. The sample exhibits efficient trapping of the photoexcited carriers into quantum wells (QWs) and the effect of internal electric field in the QWs was found negligible by excitation power-dependent PL and TRPL. These phenomena were attributed to the nature of UTIR InGaN QWs, indicating the potential of this system for application in optoelectronic devices. Variation of TRPL lifetime across the PL band and spatially resolved monochromatic CL mapping images strongly suggest that there is micrometer-scale inhomogeneity in effective band gap in UTIR InGaN∕GaN QWs, which is originated from two types of localized areas.
Time-dependent imaging of space-charge dominated electron beams
Authors: Tian K. , Kishek R. A. , O’Shea P. G. , Fiorito R. B. , Feldman D. W. , Reiser M.
Published: March 2008

When considered as non-neutral plasmas, space-charge dominated charged particle beams have significantly smaller Debye lengths than their beam sizes. Therefore, collective effects due to space-charge forces are very important to the beam dynamics. As an example, density perturbations generated in a space-charge dominated charge particle beam will stimulate longitudinally space-charge waves. To study the modification of transverse beam distributions by longitudinal beam dynamics, experimental studies of low-energy electron beams, with and without longitudinal density perturbations, have been undertaken at the University of Maryland’s electron ring and the Long Solenoid Experiment (LSE). We have taken time-resolved beam images on these machines using two different diagnostics: Optical transition radiation, produced from an intercepting aluminized silicon screen, and a fast (<3ns decay time) phosphor screen. Results from both techniques show that both the transverse size and transverse particle distribution of a space-charge dominated beam are affected by the longitudinal dynamics of the beam. In addition to the fast imaging measurements, longitudinal mean energy profiles of different beams have also been measured at different locations in the LSE system.
Advanced parameter retrievals for metamaterial slabs using an inhomogeneous model
Authors: Li Hou Ling , Chin Jessie Yao , Yang Xin Mi , Lin Xian Qi , Liu Ruopeng , Xu Fu Yong , Cui Tie Jun
Published: March 2008

The S-parameter retrieval has proved to be an efficient approach to obtain electromagnetic parameters of metamaterials from reflection and transmission coefficients, where a slab of metamaterial with finite thickness is regarded as a homogeneous medium slab with the same thickness [D. R. Smith and S. Schultz, Phys. Rev. B 65, 195104 (2002)]. However, metamaterial structures composed of subwavelength unit cells are different from homogeneous materials, and the conventional retrieval method is, under certain circumstances, not accurate enough. In this paper, we propose an advanced parameter retrieval method for metamaterial slabs using an inhomogeneous model. Due to the coupling effects of unit cells in a metamaterial slab, the roles of edge and inner cells in the slab are different. Hence, the corresponding equivalent medium parameters are different, which results in the inhomogeneous property of the metamaterial slab. We propose the retrievals of medium parameters for edge and inner cells from S parameters by considering two- and three-cell metamaterial slabs, respectively. Then we set up an inhomogeneous three-layer model for arbitrary metamaterial slabs, which is much more accurate than the conventional homogeneous model. Numerical simulations verify the above conclusions.
Observability of nonlinear dynamics: Normalized results and a time-series approach
Authors: Aguirre Luis A. , Bastos Saulo B. , Alves Marcela A. , Letellier Christophe
Published: March 2008

This paper investigates the observability of nonlinear dynamical systems. Two difficulties associated with previous studies are dealt with. First, a normalized degree observability is defined. This permits the comparison of different systems, which was not generally possible before. Second, a time-series approach is proposed based on omnidirectional nonlinear correlation functions to rank a set of time series of a system in terms of their potential use to reconstruct the original dynamics without requiring the knowledge of the system equations. The two approaches proposed in this paper and a former method were applied to five benchmark systems and an overall agreement of over 92% was found.
A clamp-type pressure cell for high energy x-ray diffraction
Authors: Zimmermann M. v. , Nowak R. , Gu G. D. , Mennerich C. , Klauss H.-H. , Hücker M.
Published: March 2008

We present a clamp-type pressure cell for high energy x-ray diffraction. The pressure cell was specifically designed for studies of weak superstructure reflections at low temperatures in transition metal oxides, resulting from, e.g., charge density modulations. Using a photon energy of E=100keV, the bulk properties of single crystals with a volume of typically 2–5mm3 can be studied in transmission geometry. To demonstrate the performance of the pressure cell, we present data on the charge stripe order in the high-temperature superconductor La1.875Ba0.125CuO4.
Force balance of particles trapped at fluid interfaces
Authors: Domínguez Alvaro , Oettel Martin , Dietrich S.
Published: March 2008

We study the effective forces acting between colloidal particles trapped at a fluid interface which itself is exposed to a pressure field. To this end, we apply what we call the “force approach,” which relies solely on the condition of mechanical equilibrium and turns to be in a certain sense less restrictive than the more frequently used “energy approach,” which is based on the minimization of a free energy functional. The goals are (i) to elucidate the advantages and disadvantages of the force approach as compared to the energy approach, and (ii) to disentangle which features of the interfacial deformation and of the capillary-induced forces between the particles follow from the gross feature of mechanical equilibrium alone, as opposed to features which depend on the details of, e.g., the interaction of the interface with the particles or the boundaries of the system. First, we derive a general stress-tensor formulation of the forces at the interface. On that basis we work out a useful analogy with two-dimensional electrostatics in the particular case of small deformations of the interface relative to its flat configuration. We apply this analogy in order to compute the asymptotic decay of the effective force between particles trapped at a fluid interface, extending the validity of the previous results and revealing the advantages and limitations of the force approach compared to the energy approach. It follows the application of the force approach to the case of deformations of a nonflat interface. In this context, we first compute the deformation of a spherical droplet due to the electric field of a charged particle trapped at its surface and conclude that the interparticle capillary force is unlikely to explain certain recent experimental observations within such a configuration. We finally discuss the application of our approach to a generally curved interface and show as an illustrative example that a nonspherical particle deposited on an interface forming a minimal surface is pulled to regions of larger curvature.
Subwavelength imaging of light confinement in high-Q/small-V photonic crystal nanocavity
Authors: Lalouat Loic , Cluzel Benoit , de Fornel Frédérique , Velha Philippe , Lalanne Philippe , Peyrade David , Picard Emmanuel , Charvolin Thomas , Hadji Emmanuel
Published: March 2008

The optical near field of a high-Q and ultrasmall volume photonic crystal nanocavity is visualized with a subwavelength resolution by using a scanning near-field optical microscope (SNOM) operating at the same time in collection-scanning mode and in interaction-scanning mode. It is shown that the nanocavity resonant mode is selectively visualized by using the SNOM interaction-scanning mode while the whole electromagnetic field surrounding the nanocavity is probed using the SNOM collection-scanning mode. The different optical near-field images are compared in light of a three-dimensional numerical analysis and we demonstrate an unexpected mode coupling at the cavity resonance.
Optical absorption of carbon nanotube diodes: Strength of the electronic transitions and sensitivity to the electric field polarization
Authors: Mencarelli Davide , Pierantoni Luca , Rozzi Tullio
Published: March 2008

Aim of this work is to model electrostatically doped carbon nanotubes (CNT), which have recently proved to perform as ideal PN diodes, also showing photovoltaic properties. The new model is able to predict the optical absorption of semiconducting CNT as function of size and chirality. We justify theoretically, for the first time, the experimentally observed capability of CNTs to detect and select not only a well defined set of frequencies, as resulting from their discrete band structure, but also the polarization of the incident radiation. The analysis develops from an approach proposed in a recent contribution. The periodic structure of CNTs is formally modeled as a photonic crystal, that is characterized by means of numerical simulators. Longitudinal and transverse components of the electric field are shown to excite distinct interband transitions between well defined energy levels. Equivalently, for a given energy of the incident radiation, absorption may show polarization ratios strongly exceeding unity.
Local magnetic moment induced by Ga vacancy defect in GaN
Authors: Hong Jisang
Published: March 2008

Through the full potential linearized augmented plane wave method, we have explored the possibility of defect-induced magnetism in wurtize GaN. The N vacancy defect structure has no sign of a magnetic state. Nonetheless, very interestingly it has been found that the GaN with a Ga vacancy defect can show induced local magnetic moment in N atoms. The four N atoms in the tetrahedron sites neighboring the Ga vacancy have magnetic moments of 0.23 and 0.29μB depending on their positions. The spin-polarized N atoms have a metallic state. It has been observed that the px,y state mainly contributes to the spin polarization of N atoms in the base layer, while the pz state is important for the other N atoms. In addition, the theoretically calculated x-ray absorption spectroscopy and x-ray magnetic circular dichroism of the K edge have been presented.
A unified theory for charge-carrier transport in organic crystals
Authors: Cheng Yuan-Chung , Silbey Robert J.
Published: March 2008

To characterize the crossover from bandlike transport to hopping transport in molecular crystals, we study a microscopic model that treats electron-phonon interactions explicitly. A finite-temperature variational method combining Merrifield’s transformation with Bogoliubov’s theorem is developed to obtain the optimal basis for an interacting electron-phonon system, which is then used to calculate the bandlike and hopping mobilities for charge carriers. Our calculations on the one dimensional (1D) Holstein model at T=0K and finite temperatures show that the variational basis gives results that compared favorably to other analytical methods. We also study the structures of polaron states at a broad range of parameters including different temperatures. Furthermore, we calculate the bandlike and hopping mobilities of the 1D Holstein model in different parameters and show that our theory predicts universal power-law decay at low temperatures and an almost temperature independent behavior at higher temperatures, in agreement with experimental observations. In addition, we show that as the temperature increases, hopping transport can become dominant even before the polaron state changes its character. Thus, our result indicates that the self-trapping transition studied in conventional polaron theories does not necessarily correspond to the bandlike to hopping transition in the transport properties in organic molecular crystals. Finally, a comparison of our 1D results with experiments on ultrapure naphthalene crystals suggests that the theory can describe the charge-carrier mobilities quantitatively across the whole experimental temperature range.
First principles studies for formation mechanism and properties of ethylene molecule adsorbing on diamond (100) surface
Authors: Jin Hao , Dai Ying , Long Run , Guo Meng , Huang Baibiao , Zhang Ruiqin
Published: March 2008

We employed density functional theory within the generalized gradient approximation to investigate the diamond (100) surface, with hydrogen and ethylene terminations. The radical chain reaction is investigated by slab models, and two possible adsorption ways are found according to our calculations. In addition, the electron affinity of H-terminated diamond is also calculated, which indicates that the existence of negative electron affinity of H–C (100) surface provides a necessary condition for initiating radical chain reaction. Our results also imply that ethylene molecules can form strong C–C covalent bonds with diamond surface, which make it more resistant against degradation processes. Furthermore, according to the analysis of electronic structures, we have found localized gap state above the valence band, which is mainly contributed by the interaction between diamond surface and ethylene molecule and can weaken the surface conductivity of the adsorbed diamond.
Effects of fast monoenergetic electrons on the ion dynamics near the cathode in a pulsed direct current plasma sheath
Authors: Sharifian M. , Shokri B.
Published: March 2008

A detailed one-dimensional simulation of the ion dynamics of the plasma sheath near a substrate (cathode) in the presence of fast monoenergetic electrons has been carried out in this article. The sheath evolution is investigated by using a fluid model assuming that the ions, plasma electrons and monoenergetic, fast electrons act as three fluids (fluid approach). The effect of the density of fast electrons on the ion density, ion velocity, and ion energy near the cathode and the evolution of the sheath boundary in front of the cathode are separately explored. Also, the variation of the ion velocity and ion density at the vicinity of the cathode as a function of time is investigated in the absence and presence of the electron beam. Results indicate that the presence of fast electrons in the sheath causes significant change in the sheath thickness and therefore basically changes the ion velocity, ion density, and ion impact energy on the cathode compared to the absence of the electron beam case.
Signature of martensite transformation on conductivity noise in thin films of NiTi shape memory alloys
Authors: U Chandni , Ghosh Arindam , Vijaya H. S. , Mohan S.
Published: March 2008

Slow time-dependent fluctuations, or noise, in the electrical resistance of dc magnetron sputtered thin films of nickel-titanium shape memory alloys have been measured. Even in equilibrium, the noise was several orders of magnitude larger than that in simple diffusive metallic films and was found to be nonmonotonic around the martensitic transformation regime. The results are discussed in terms of the dynamics of structural defects, which also lay the foundation to a new noise-based characterization scheme of martensite transformation.
Circular polarization emission from an external cavity diode laser
Authors: Zhang Fan , Xu Jian , Lakhtakia Akhlesh , Zhu Ting , Pursel Sean M. , Horn Mark W.
Published: March 2008

An external cavity diode laser (ECDL) containing a chiral sculptured-thin-film (STF) mirror for very pure circularly polarized (CP) emission was fabricated and its single-mode, left-handed CP lasing performance was observed. The extinction ratio of the CP output was found to increase rapidly near the threshold of the injection current for the laser diode. The Jones-matrix representation of a chiral STF mirror was used to calculate the eigenstates in the external cavity of the ECDL with the transfer matrix method, the results suggesting that the only resonant mode in the external cavity of the laser is CP with the same handedness as the structural handedness of the chiral STF mirror.
Field and temperature dependent parameters of the dc field induced resonances in BaxSr1−xTiO3-based tunable thin film bulk acoustic resonators
Authors: Berge John , Norling Martin , Vorobiev Andrei , Gevorgian Spartak
Published: March 2008

Electrically tunable solidly mounted thin film bulk acoustic resonators based on Ba0.25Sr0.75TiO3 and BaTiO3 films are fabricated and measured in wide dc bias voltage and temperature ranges. At room temperature, the tunability of the series and parallel resonances for the Ba0.25Sr0.75TiO3 resonator are 1.7% and 0.3%, respectively, for 15V bias voltage applied over the 350nm thick ferroelectric film (43V∕μm). The electromechanical coupling coefficient increases with dc bias up to 3.7% at 15V. The measured tunability and coupling coefficient are limited partly by the quality of the used films. Potentially, they may be substantially increased for high quality films allowing application of higher dc fields. The resonator quality factor is approximately 100. The measured resonator response is in good agreement with available models based on the electromechanical equations describing the ferroelectric film under applied dc and ac electric fields. Measurements of the resonance frequencies of the Ba0.25Sr0.75TiO3 resonator as a function of temperature in the range 40–520K reveal a sharp step at 150K which is related to a structural phase transition of the ferroelectric material. The series resonance frequency is tuned to lower frequencies with increasing dc bias for all temperatures, while the parallel resonance frequency reveals a change in the sign of the tunability at approximately 150K, from being tuned to lower frequencies at high temperatures to being tuned to higher frequencies at low temperatures. Measurements of BaTiO3 resonators in the temperature range 300–520K are presented and compared to the Ba0.25Sr0.75TiO3 results.
Microstructure and ferromagnetic clusters in slightly doped LaxBa4−xMn3O10
Authors: Yao L. D. , Zhang W. , Li F. Y. , Jin C. Q. , Yu R. C.
Published: March 2008

The nominal composition LaxBa4−xMn3O10 (x=0–0.2) compounds were synthesized using solid-state reaction under high pressure. The structures of the two single-phase samples LaxBa4−xMn3O10 (x=0,0.05) have been studied by powder x-ray diffraction, electron diffraction, and high-resolution transmission electron microscopy (HRTEM). For the x=0.05 sample, some intergrowth structures and stacking faults were revealed by HRTEM observations. According to the results of electron energy-loss spectroscopy (EELS) analysis, the slight doping of La into the parent phase compound Ba4Mn3O10 was confirmed. Moreover, the L3∕L2 intensity ratio of Mn-L2,3 edges in the EELS spectra indicates the variation of Mn ionic valence state induced by the doping of La. Based on analysis of magnetic properties, the magnetization for the x=0 sample shows a broad peak centered around 170K and a sharp upturn at low temperature of 15K, which is due to a short-range low-dimensional antiferromagnetic ordering. For the x=0.05 sample, the slight doping of La leads to significant changes in magnetic properties. The zero-field cooled and field cooled magnetizations and hysteresis loop curves suggest that La doping induces ferromagnetic clusters in the antiferromagnetic matrix.
Origin of high piezoelectric activity in ferroelectric (K0.44Na0.52Li0.04)−(Nb0.84Ta0.1Sb0.06)O3 ceramics
Authors: Akdoğan E. K. , Kerman K. , Abazari M. , Safari A.
Published: March 2008

We study the temperature dependence of dielectric constant (K) and spontaneous polarization (Ps) in the range of −95–200°C. Cubic (C)-tetragonal (T) and T-orthorhombic (O) transitions are observed at 264 and 25°C, respectively. The Curie–Weiss temperature of C-T transition is 249°C, indicating it is first order. X-ray data indicate T-O phase coexistence at 25°C. A singularity in Ps at 25°C and a T-O phase coexistence spanning 25–31°C was observed, wherein Ps increases from 17×10−2C∕m2 at 31°Cto23×10−2C∕m2 25°C. The transition at 25°C appears diffusionless and polymorphic with martensite start and finish temperatures of 31 and 25°C, respectively. The maximum in d33 is 345pC∕N and is attributed to the instability at 25°C, where Ps and K show singularity.
Ga
N
∕
In
Ga
N
light emitting diodes with embedded photonic crystal obtained by lateral epitaxial overgrowth
Authors: David Aurélien , Moran Brendan , McGroddy Kelly , Matioli Elison , Hu Evelyn L. , DenBaars Steven P. , Nakamura Shuji , Weisbuch Claude
Published: March 2008

We introduce GaN∕InGaN light emitting diodes with a dielectric photonic crystal embedded in the epitaxial layer by lateral epitaxial overgrowth on a patterned GaN template. Overgrowth, coalescence, and epitaxial growth of the pn junction within a thickness of 500nm is obtained using metal-organic chemical vapor deposition. This design strongly modifies the distribution of guided modes, as confirmed by angle-resolved measurements. The regime of operation and potential efficiency of such structures are discussed.
Reversible magnetic domain-wall motion under an electric field in a magnetoelectric thin film
Authors: Chung Tien-Kan , Carman Gregory P. , Mohanchandra Kotekar P.
Published: March 2008

We report direct microscopic measurements that confirm the magnetic stripe-domain patterns can be reversibly changed under an electric field due to the converse magnetoelectric effect in a bilayer thin film ferromagnetic-Ni/ferroelectric-lead zirconate titanate (100nm∕1.28μm) heterostructure. Electric field-induced curving, bending, branching, and elongation of magnetic stripe-domain patterns in the Ni layer are observed with the use of magnetic force microscopy. Upon removal of the electric field, the magnetic stripe-domain patterns return to their original configuration, i.e., reversible.
Hydrogen: A metastable donor in TiO2 single crystals
Authors: Chen W. P. , Wang Y. , Chan H. L. W.
Published: March 2008

The properties of TiO2 (110) single crystals are greatly changed after hydrogen is doped into them through electrochemical hydrogen charging, in which the crystals have been placed in 0.01M NaOH solution to deposit hydrogen on their electrodes through the electrolysis of water. The changes in properties, including great increases in leakage current, strong dielectric dispersions over 102–106Hz, and a strong aging phenomenon suggest that hydrogen is a donor in TiO2 while ionized hydrogen is not bound to an oxygen site by a strong O–H bond. This behavior of hydrogen, which can be characterized as a metastable donor, is different from the usual donor behavior of hydrogen in oxides and should be interesting for a better understanding of the behavior of hydrogen in oxides.
Tracing information flow on a global scale using Internet chain-letter data
Authors: Liben-Nowell David , Kleinberg Jon
Published: March 2008

Although information, news, and opinions continuously circulate in the worldwide social network, the actual mechanics of how any single piece of information spreads on a global scale have been a mystery. Here, we trace such information-spreading processes at a person-by-person level using methods to reconstruct the propagation of massively circulated Internet chain letters. We find that rather than fanning out widely, reaching many people in very few steps according to “small-world” principles, the progress of these chain letters proceeds in a narrow but very deep tree-like pattern, continuing for several hundred steps. This suggests a new and more complex picture for the spread of information through a social network. We describe a probabilistic model based on network clustering and asynchronous response times that produces trees with this characteristic structure on social-network data.
Netrin-1 expression confers a selective advantage for tumor cell survival in metastatic breast cancer
Authors: Fitamant Julien , Guenebeaud Céline , Coissieux Marie-May , Guix Catherine , Treilleux Isabelle , Scoazec Jean-Yves , Bachelot Thomas , Bernet Agnès , Mehlen Patrick
Published: March 2008

Netrin-1, an axon navigation cue was proposed to play a crucial role during colorectal tumorigenesis by regulating apoptosis. The netrin-1 receptors DCC and UNC5H were shown to belong to the family of dependence receptors that share the ability to induce apoptosis in the absence of their ligands. Such a trait confers on these receptors a tumor suppressor activity. Expression of one of these dependence receptors at the surface of a tumor cell is indeed speculated to render this cell dependent on ligand availability for its survival, hence inhibiting uncontrolled cell proliferation or metastasis. Consequently, it is a selective advantage for a tumor cell to lose this dependence receptor activity, as previously described with losses of DCC and UNC5H expression in human cancers. However, the model predicts that a similar advantage may be obtained by gaining autocrine expression of the ligand. We describe here that, unlike human nonmetastatic breast tumors, a large fraction of metastatic breast cancers overexpress netrin-1. Moreover, we show that netrin-1-expressing mammary metastatic tumor cell lines undergo apoptosis when netrin-1 expression is experimentally decreased or when decoy soluble receptor ectodomains are added. Such treatments prevent metastasis formation both in a syngenic mouse model of lung colonization of a mammary cancer cell line and in a model of spontaneous lung metastasis of xenografted human breast tumor. Thus, netrin-1 expression observed in a large fraction of human metastatic breast tumors confers a selective advantage for tumor cell survival and potentially represents a promising target for alternative anticancer therapeutic strategies.
Unilamellar vesicle formation and encapsulation by microfluidic jetting
Authors: Stachowiak Jeanne C. , Richmond David L. , Li Thomas H. , Liu Allen P. , Parekh Sapun H. , Fletcher Daniel A.
Published: March 2008

Compartmentalization of biomolecules within lipid membranes is a fundamental requirement of living systems and an essential feature of many pharmaceutical therapies. However, applications of membrane-enclosed solutions of proteins, DNA, and other biologically active compounds have been limited by the difficulty of forming unilamellar vesicles with controlled contents in a repeatable manner. Here, we demonstrate a method for simultaneously creating and loading giant unilamellar vesicles (GUVs) using a pulsed microfluidic jet. Akin to blowing a bubble, the microfluidic jet deforms a planar lipid bilayer into a vesicle that is filled with solution from the jet and separates from the planar bilayer. In contrast with existing techniques, our method rapidly generates multiple monodisperse, unilamellar vesicles containing solutions of unrestricted composition and molecular weight. Using the microfluidic jetting technique, we demonstrate repeatable encapsulation of 500-nm particles into GUVs and show that functional pore proteins can be incorporated into the vesicle membrane to mediate transport. The ability of microfluidic jetting to controllably encapsulate solutions inside of GUVs creates new opportunities for the study and use of compartmentalized biomolecular systems in science, industry, and medicine.
Proper Cellular Reorganization duringDrosophilaSpermatid Individualization Depends on Actin Structures Composed of Two Domains, Bundles and Meshwork, That Are Differentially Regulated and Have Different Functions
Authors: Noguchi Tatsuhiko , Lenartowska Marta , Rogat Aaron D. , Frank Deborah J. , Miller Kathryn G.
Published: March 2008

During spermatid individualization in Drosophila, actin structures (cones) mediate cellular remodeling that separates the syncytial spermatids into individual cells. These actin cones are composed of two structural domains, a front meshwork and a rear region of parallel bundles. We show here that the two domains form separately in time, are regulated by different sets of actin-associated proteins, can be formed independently, and have different roles. Newly forming cones were composed only of bundles, whereas the meshwork formed later, coincident with the onset of cone movement. Polarized distributions of myosin VI, Arp2/3 complex, and the actin-bundling proteins, singed (fascin) and quail (villin), occurred when movement initiated. When the Arp2/3 complex was absent, meshwork formation was compromised, but surprisingly, the cones still moved. Despite the fact that the cones moved, membrane reorganization and cytoplasmic exclusion were abnormal and individualization failed. In contrast, when profilin, a regulator of actin assembly, was absent, bundle formation was greatly reduced. The meshwork still formed, but no movement occurred. Analysis of this actin structure's formation and participation in cellular reorganization provides insight into how the mechanisms used in cell motility are modified to mediate motile processes within specialized cells.
Claspin Promotes Normal Replication Fork Rates in Human Cells
Authors: Petermann Eva , Helleday Thomas , Caldecott Keith W.
Published: March 2008

The S phase-specific adaptor protein Claspin mediates the checkpoint response to replication stress by facilitating phosphorylation of Chk1 by ataxia-telangiectasia and Rad3-related (ATR). Evidence suggests that these components of the ATR pathway also play a critical role during physiological S phase. Chk1 is required for high rates of global replication fork progression, and Claspin interacts with the replication machinery and might therefore monitor normal DNA replication. Here, we have used DNA fiber labeling to investigate, for the first time, whether human Claspin is required for high rates of replication fork progression during normal S phase. We report that Claspin-depleted HeLa and HCT116 cells display levels of replication fork slowing similar to those observed in Chk1-depleted cells. This was also true in primary human 1BR3 fibroblasts, albeit to a lesser extent, suggesting that Claspin is a universal requirement for high replication fork rates in human cells. Interestingly, Claspin-depleted cells retained significant levels of Chk1 phosphorylation at both Ser317 and Ser345, raising the possibility that Claspin function during normal fork progression may extend beyond facilitating phosphorylation of either individual residue. Consistent with this possibility, depletion of Chk1 and Claspin together doubled the percentage of very slow forks, compared with depletion of either protein alone.
Distinct Angiogenic Mediators Are Required for Basic Fibroblast Growth Factor– and Vascular Endothelial Growth Factor–induced Angiogenesis: The Role of Cytoplasmic Tyrosine Kinase c-Abl in Tumor Angiogenesis
Authors: Yan Wei , Bentley Brooke , Shao Rong
Published: March 2008

Signaling pathways engaged by angiogenic factors bFGF and VEGF in tumor angiogenesis are not fully understood. The current study identifies cytoplasmic tyrosine kinase c-Abl as a key factor differentially mediating bFGF- and VEGF-induced angiogenesis in microvascular endothelial cells. STI571, a c-Abl kinase inhibitor, only inhibited bFGF- but not VEGF-induced angiogenesis. bFGF induced membrane receptor cooperation between integrin β3 and FGF receptor, and triggered a downstream cascade including FAK, c-Abl, and MAPK. This signaling pathway is different from one utilized by VEGF that includes integrin β5, VEGF receptor-2, Src, FAK, and MAPK. Ectopic expression of wild-type c-Abl sensitized angiogenic response to bFGF, but kinase dead mutant c-Abl abolished this activity. Furthermore, the wild-type c-Abl enhanced angiogenesis in both Matrigel implantation and tumor xenograft models. These data provide novel insights into c-Abl's differential functions in mediating bFGF- and VEGF-induced angiogenesis.
Src-mediated Cortactin Phosphorylation Regulates Actin Localization and Injurious Blebbing in Acinar Cells
Authors: Singh Vijay P. , McNiven Mark A.
Published: March 2008

Suprastimulation of pancreatic acini is a well-known model for pancreatitis, and it is characterized by actin reorganization and cell blebbing. Currently, however, the mechanisms underlying regulation of these aberrant cytoskeletal and membrane dynamics and how they contribute to cell injury are unclear. We observed that suprastimulation results in a rapid activation of Src and relocalization of the actin-binding protein cortactin from the apical to the basolateral domain at the necks of membrane blebs. Furthermore, Src-mediated cortactin tyrosine phosphorylation was markedly increased after suprastimulation. Pretreatment of acini with Src inhibitors or expression of a cortactin tyrosine phospho-inhibitory mutant reduced actin redistribution and bleb formation induced by suprastimulation in vitro. Importantly, inhibition of Src activity in rat models of suprastimulation-induced pancreatitis substantially reduced disease severity, as indicated by a reduction in serum amylase and pancreatic edema and a striking improvement in tissue histology. These findings indicate a novel, disease-relevant role for Src-mediated cortactin phosphorylation in aberrant reorganization of the actin cytoskeleton, a mechanism that is likely to have implications in other types of cell injury. In addition, they suggest a potential use for Src inhibitors as an approach to reduce cell injury.
Revealing the paradox of drug reward in human evolution
Authors: Sullivan Roger J , Hagen Edward H , Hammerstein Peter
Published: March 2008

Neurobiological models of drug abuse propose that drug use is initiated and maintained by rewarding feedback mechanisms. However, the most commonly used drugs are plant neurotoxins that evolved to punish, not reward, consumption by animal herbivores. Reward models therefore implicitly assume an evolutionarymismatchbetween recent drug-profligate environments and a relatively drug-free past in which a reward centre, incidentally vulnerable to neurotoxins, could evolve. By contrast, emerging insights from plant evolutionary ecology and the genetics of hepatic enzymes, particularly cytochrome P450, indicate that animal and hominid taxa have been exposed to plant toxins throughout their evolution. Specifically, evidence of conserved function, stabilizing selection, and population-specific selection of human cytochrome P450 genes indicate recent evolutionary exposure to plant toxins, including those that affect animal nervous systems. Thus, the human propensity to seek out and consume plant neurotoxins is a paradox with far-reaching implications for current drug-reward theory. We sketch some potential resolutions of the paradox, including the possibility that humans may have evolved to counter-exploit plant neurotoxins. Resolving the paradox of drug reward will require a synthesis of ecological and neurobiological perspectives of drug seeking and use.
On the inference of ancestries in admixed populations
Authors: Sankararaman Sriram , Kimmel Gad , Halperin Eran , Jordan Michael I.
Published: March 2008

Inference of ancestral information in recently admixed populations, in which every individual is composed of a mixed ancestry (e.g., African Americans in the United States), is a challenging problem. Several previous model-based approaches to admixture have been based on hidden Markov models (HMMs) and Markov hidden Markov models (MHMMs). We present an augmented form of these models that can be used to predict historical recombination events and can model background linkage disequilibrium (LD) more accurately. We also study some of the computational issues that arise in using such Markovian models on realistic data sets. In particular, we present an effective initialization procedure that, when combined with expectation-maximization (EM) algorithms for parameter estimation, yields high accuracy at significantly decreased computational cost relative to the Markov chain Monte Carlo (MCMC) algorithms that have generally been used in earlier studies. We present experiments exploring these modeling and algorithmic issues in two scenarios—the inference of locus-specific ancestries in a population that is assumed to originate from two unknown ancestral populations, and the inference of allele frequencies in one ancestral population given those in another.
Global Characterization of Cell-Specific Gene Expression through Fluorescence-Activated Sorting of Nuclei
Authors: Zhang Changqing , Barthelson Roger A. , Lambert Georgina M. , Galbraith David W.
Published: March 2008

We describe a simple and highly effective means for global identification of genes that are expressed within specific cell types within complex tissues. It involves transgenic expression of nuclear-targeted green fluorescent protein in a cell-type-specific manner. The fluorescent nuclei are then purified from homogenates by fluorescence-activated sorting, and the RNAs employed as targets for microarray hybridization. We demonstrate the validity of the approach through the identification of 12 genes that are selectively expressed in phloem.
Familial Cyclical Neutropenia
Authors: Morley A. A. , Carew J. P. , Baikie A. G.
Published: March 2008

The clinical and haematological features of twenty cases of cyclical neutropenia are described. Clinical manifestations commonly began in childhood and improved thereafter. The commonest were fever, oral ulceration and skin infections. Episodes of neutropenia occurred at intervals of 15–35 days and were often accompanied by monocytosis. Anaemia, eosinophilia or thrombocytopenia occurred in several patients.

The severity of the disease varied greatly. Some patients had grave infections and others were symptom‐free. The more serious infections occurred in severely neutropenic patients but others equally neutropenic were without infections.

The 20 cases came from five families. Thirteen presented on account of symptoms attributable to the disease and seven were found as the result of study of the families of index cases. The family studies suggest that the disease is transmitted as an autosomal dominant of high penetrance and variable expressivity.

It seems likely that many cases of cyclical neutropenia remain undetected and others are mistakenly considered to be non‐cyclical.
Absorption of Vitamin B12 in Patients with Anaemia after Polya Partial Gastrectomy
Authors: Turnbull A. L.
Published: March 2008

The absorption of radioactive vitamin B12 (cyanocobalamin) has been studied by the Schilling procedure in 27 patients with anaemia after Polya partial gastrectomy and in 10 patients with Addisonian pernicious anaemia.

In patients who have had Polya partial gastrectomy absorption of vitamin B12 was often increased when the dose was given with a meal containing only a small quantity of the vitamin, and when the dose was given after an injection of histamine.

In the patients with pernicious anaemia histamine did not enhance absorption of vitamin B12, and food did not alter the absorption of the vitamin when given with a small quantity of intrinsic factor.

Improved absorption of vitamin B12 with food or after histamine is likely to be due to stimulation of a small residual secretion of intrinsic factor.

The results of the standard fasting Schilling test appeared to correlate better with the development of deficiency of vitamin B12 than tests with food or after histamine.
Haematological Changes in Malignancy
Authors: Banerjee R. N. , Narang R. M.
Published: March 2008

1. Detailed haematological investigations were carried out in 65 cases of malignant disease of different types.

2. Anaemia was found to be the commonest haematological disorder (83 per cent).

3. Seventy per cent of the cases showed features of simple chronic anaemia while four out of 16 selected cases showed evidence of shortened red cell life span. Incidence of myelophthisic, aplastic and iron deficiency anaemias were found to be 8, 3 and 3 per cent, respectively.

4. The study of iron metabolism revealed a high incidence (71 per cent) of hypoferraemia with shortened plasma iron (59Fe) disappearance time, increased iron turnover rate but poor 59Fe localization and iron utilization by bone marrow.

5. Other significant haematological disorders noted were leucocytosis, eosinophilia, thrombocytopenia and raised erythrocyte sedimentation rate. The possible implications have been discussed.

6. Nearly 50 per cent of the cases, mostly of reticulosis and sarcomatous groups, showed a significant increase in the total blood volume and plasma volume.

7. Possible aetiological correlation between malignant disease and the dominant haematological disorders found in malignant disease have been discussed.