bluuebunny/crossref_metadata_2025_split huggingface dataset:
Pressure Induced Phase Transformations in Silica
Authors: Çağin Tahir , Demiralp Ersan , Goddard William A. 
Published: April 2011

ABSTRACTSilica, SiO2, is one of the most widely studied substance because of its complex and unusual properties. We have used a recently developed 2-body interaction force field [1] to study the structural phase transformations in silica under various pressure loading conditions. The specific transformations we studied are the a-quartz to stishovite, coesite to stishovite and fused glass to a dominantly six-coordinated dense glassy phase. Molecular dynamics simulations are performed under constant loading rates ranging from 0.1 GPa/ps to 1.0 GPa/ps, with final pressures upto 100 GPa and at temperatures of 300, 500, and 700 K. We observe the crystal to crystal transformations to occur reconstructively, whereas it occurs in a smooth and displacive manner from glass to a stishovite-like phase confirming earlier conjectures.[2] We studied the dependence of transition pressure on the loading rate and the temperature to elucidate the shock loading experiments. We also studied the unloading behavior of each transformation to assess the hysterisis effect.
Theoretical Studies of the Structures of the Liquid-Vapor Interfaces of Metals and Binary Alloys
Authors: Rice Stuart A. , Zhao Meishan , Chekmarev Dmitriy 
Published: April 2011

AbstractWe report the results of self-consistent quantum Monte Carlo simulations of the structure of the liquid-vapor interfaces of Ga and of alloys of Bi in Ga and In in Ga. The single particle density distribution along the normal to the interface is predicted to display stratification, with a spacing of about an atomic diameter, and the pair structure function in the plane of the liquid-vapor interface is predicted to be essentially the same as that in the bulk liquid. In all of the cases considered the qualitative and quantitative character of these predictions are in good agreement with the results of experimental studies.
Spin-Polarized Density of States and Electron Tunnelling from the CO/Al2O3 Interface
Authors: Nguyen-Manh D. , Tsymbal E.Yu. , Pettifor D. G. , Arcangeli C. , Tank R. , Andersen O. K. , Pasturel A. 
Published: April 2011

ABSTRACTIn order to elucidate the mechanism of spin-polarized electron tunneling in thin-film ferromagnet-insulator junctions, self-consistent band structure calculations of the CO/Al2O3 interface have been performed using a new LMTO technique. Since the results of the calculations are very sensitive to the distance between the Co and Al planes, we have minimised the total energy with respect to this distance. Our calculations show that at the Fermi energy a strong bonding between the 3d-electrons of Co with the sp-electrons of Al at the interface can have an important influence on the spin polarization of the layer-projected density of states (LPDOS) of inner Al and O layers. Since the Fermi energy lies within the minority-spin d-band of Co but above the majority-spin d-band, the sp-d bonding results in a smaller LPDOS of the minority-spin electrons of the interfacial Al layers in comparison to that of the majority-spin electrons. This asymmetry in the LPDOS extends to the inner Al2O3 layers implying a positive spin polarization of the tunneling density of states. The result is consistent with experimental observations on tunnelling from cobalt through alumina where positive values of the spin polarization of the tunnelling current were measured.
Comparison Between Atomistic and Continuum-Mechanics Modelling of Grain-Boundary Fracture
Authors: Cleri F. , Phillpot S. R. , Wolf D. 
Published: April 2011

ABSTRACTWe use the Peierls-Nabarro continuum mechanics model of dislocation nucleation, modified according to the results of atomistic simulations, to interpret the experimental results of fracture response in symmetric-tilt grain boundaries in Cu. We then directly perform Molecular Dynamics simulations of fracture propagation and dislocation emission from a microcrack placed in the interface plane of the symmetric-tilt (221)(221) grain boundary in fee Cu. Direction-dependent fracture response is observed in agreement with experiments, namely the microcrack advancing by brittle fracture along the [114] direction and being blunted by dislocation emission along the opposite [114] direction. Moreover, we are able to quantify important differences with respect to the continuum model due to the shielding of the stress field at the crack-tip and to the presence of the excess stress at the interface.
Negative Magnetoresistance in (Bi,Pb)2Sr3Co2O9 Layered Cobalt Oxides
Authors: Tsukada I. , Yamamoto T. , Takagi M. , Tsubone T. , Uchinokura K. 
Published: April 2011

ABSTRACTTransport and magnetic properties of layered cobalt oxide (BiPb)2Sr3Co2O9 are investigated in detail under magnetic field up to 8 T. Parent compound, Bi2Sr3Co2O9, is a typical band insulator with Co ions being in a low-spin 3+ state because of the well-separated dε and dγ levels possibly due to a strong crystal field. We have tried to introduce holes mainly by Pb substitution for Bi. The hole-doped sample shows metallic behavior in a resistivity measurement between 300 and 30 K. Below 30 K, however, the resisitivity increases. Under the magnetic field the resistivity is strongly suppressed in this region. We observed more than 30% resistivity drop at 2 K under H = 8 T, which is comparable to insulating (La,Sr)CoO3 system. We discuss the mechanism of hole doping and the origin of negative magnetoresistance with tranport and magnetic properties, and point out that the conventional double-exchange mechanism cannot be applied to this system. This means that some new mechanism is necessary to explain this phenomenon.
Surface Morphology and Lattice Misfit in Yig and La: Yig Films Grown by Lpe Method on GGG Substrate
Authors: Choi Duk-yong , Chung Su-jin 
Published: April 2011

ABSTRACTY3Fe5O12(YIG) and La-doped YIG films were grown on the {111} GGG substrate using the PbO-B2O3 flux system. Pb, La incorporation and lattice misfit and annealing behaviors were studied. In the case of LPE growth of YIG film, lead ions from flux are substituted inevitably, and they play an important role in controlling film misfit. For a complete lattice matching, high supercooling is necessary in pure YIG growth, but this induces high defect concentration. In this experiment, La ions were added in the solution to sufficiently increase lattice parameter of the film grown under low supercooling. The concentration of substituted Pb and La were increased as the growth temperature was lowered and growth rate increased. The effective distribution coefficient of La was about 0.2 at a supercooling of 30 °C. The optimum growth conditions which bring about very small misfit were determined by measuring the misfit by double crystal diffractometer. Strain distributions of pre-annealed and annealed samples were investigated by triple crystal diffractometer.
Room Temperature Magnetoresistive Response in CMR Perovskite Manganite Thin Films
Authors: Todd Michael A. , Seegel Charles , Baum Thomas H. 
Published: April 2011

ABSTRACTPerovskite-structured LaxSryMnO3 thin-films have been deposited onto LaAlO3 substrates via liquid delivery chemical vapor deposition (LD-CVD) using metal(β-diketonato) precursors, M(thd)x [where M= Ca, Sr, La and Mn, thd = 2,2,6,6-tetramethyl-3,5-heptanedionato and x = 2–3]. Thin films were deposited at temperatures between 500 and 700 °C and subsequently annealed at 1000 °C under O2. These films possess stoichiometries that are: i) vastly different from the La0.67Sr0.33MnO3 compositions commonly reported in the literature and ii) display high temperature, low field responses that may be technologically important. Resistance versus temperature measurements revealed a metal to semiconductor transition at room temperature and above. Hall measurements on a film of La0.35Sr0.24MnO3 displayed a magnetoresistive response (MR) of -10% at 57 °C in a fixed magnetic field of 780 Oe. Based upon our research, the observed film properties are directly related to the deposited film stoichiometry and the best results were observed at Sr / La ratios between 0.30 and 1.0 for A-site deficient LaxSryMnO3 thin-films after thermal annealing.
Spin Tunneling in Conducting Oxides
Authors: Bratkovsky Alexander 
Published: April 2011

ABSTRACTDifferent tunneling mechanisms in conventional and half-metallic ferromagnetic tunnel junctions are analyzed within the same general method. Direct tunneling is compared with impurity-assisted, surface state assisted, and inelastic contributions to a tunneling magnetoresistance (TMR). Theoretically calculated direct tunneling in iron group systems leads to about a 30% change in resistance, which is close to experimentally observed values. It is shown that the larger observed values of the TMR might be a result of tunneling involving surface polarized states. We find that tunneling via resonant defect states in the barrier radically decreases the TMR (down to 4% with Fe-based electrodes), and a resonant tunnel diode structure would give a TMR of about 8%. With regards to inelastic tunneling, magnons and phonons exhibit opposite effects: one-magnon emission generally results in spin mixing and, consequently, reduces the TMR, whereas phonons are shown to enhance the TMR. The inclusion of both magnons and phonons reasonably explains an unusual bias dependence of the TMR.The model presented here is applied qualitatively to half-metallics with 100% spin polarization, where one-magnon processes are suppressed and the change in resistance in the absence of spin-mixing on impurities may be arbitrarily large. Even in the case of imperfect magnetic configurations, the resistance change can be a few 1000 percent. Examples of half-metallic systems are CrO2/TiO2 and CrO2/RuO2, and an account of their peculiar band structures is presented. The implications and relation of these systems to CMR materials, which are nearly half-metallic, are discussed.
Fabrication Of La0.7Sr0.3MnO3/La0.5Sr0.5CoO3/La0.7Sr0.3MnO3 Heterostructures for Spin Valve Applications
Authors: Robson M. C. , Ogale S. B. , Godfrey R. , Venkatesan T. , Johnson M. , Ramesh R. 
Published: April 2011

ABSTRACTEpitaxial growth of oxide heterostructures, which may be utilized in spin valve applications, has been demonstrated. The heterostructures consist of two ferromagnetic layers separated by a nonmagnetic metallic interlayer. The ferromagnetic material used is the manganese perovskite oxide, La0.7Sr0.3MnO3, while the metallic oxide interlayer is La0.5Sr0.5CoO3. X-ray diffraction spectra demonstrate the high structural quality of the heterostructures. The magnetization of the heterostructure as a function of magnetic field measured at room temperature yields a double hysteresis loop that is characteristic of this type of spin valve structure. The behavior of this double hysteresis loop is also examined as a function of the metallic interlayer thickness.
Fabrication of High Temperature Superconductor-Colossal Magnetoresistor Spin Injection Devices
Authors: Kim J. , Stroud R. M. , Auyeung R. C Y. , Eddy C. R. , Koller D. , Osofsky M. S. , Soulen R. J. , Horwitz J. S. , Chrisey D. B. 
Published: April 2011

ABSTRACTTrilayer YBa2Cu3O7-δ/(SrTiO3, CeO2)/La0.67Sr0.33MnO3-δ devices have been fabricated for the study of supercurrent suppression due to the injection of spin-polarized quasiparticle current. The critical current for a YBa2Cu3O7-δ/100 Å SrTiO3/La0.67Sr0.33MnO3-δ device was found to decrease from 118 mA to 12.6 mA, for an injection current of 60 mA. The effect of film microstructure on the critical current suppression was investigated. Defects in the SrTiO3 and CeO2 layers were found to control the device properties.
Volume-Based Considerations for the Metal-Insulator Transition of CMR Oxides
Authors: Neumeier J. J. , Cornelius A. L. , Hundley M. F. , Andres K. , McClellan K. J. 
Published: April 2011

ABSTRACTThe sensitivity of ρ to changes in volume which occur through: (1) applied pressure, (2) variations in temperature, and (3) phase transitions, is evaluated for some selected CMR oxides. It is argued that the changes in volume associated with (2) and (3), which are referred to as self pressures, are equivalent in magnitude to changes in volume resulting from pressures in the range of 0.18 to 0.45 GPa. Through consideration of thermal expansion and electrical resistivity data, it is shown that these self pressures are responsible for large features in the electrical resistivity and are an important component for the occurrence of the metallicity below Tc. It is suggested that this is a manifestation of a strong volume dependence of the electron phonon coupling in the CMR oxides.
Raman Investigation of the Layered Manganese Perovskite La1.2Sr1.8Mn2O7
Authors: Romero D. B. , Podobedov V. B. , Weber A. , Rice J. P. , Mitchell J. F. , Sharma R. P. , Drew H. D. 
Published: April 2011

ABSTRACTWe report the results of a detailed polarization and temperature dependence study of the Raman scattering from La1.2Sr1.8Mn2O7. The Raman spectra reveal three general spectral features. First, there are sharp peaks due to long-wavelength optic phonons. Phonons, attributed to the distortion of the MnO6 octahedra, reveal an anomalous behavior which correlates with the transition from a paramagnetic-insulating (PI) to a ferromagnetic-metallic (FM) phase at a critical temperature Tc. Second, there is an electronic continuum that is suppressed for ω < 500 cm−1 at T ≪ Tc. Third, broad peaks between 400 cm1 to 800 cm−1 seen in the PI state, surprisingly, disappear in the FM state. The implications of these results are discussed.
X-Ray Induced Insulator-Metal Transitions in CMR Manganites
Authors: Kiryukhin V. , Casa D. , Keimer B. , Hill J. P. , Vigliante A. , Tomioka Y. , Tokura Y. 
Published: April 2011

ABSTRACTIn this work we report a study of the photoinduced insulator-to-metal transition in manganese oxide perovskites of the formula Pr1-xCaxMnO3. The transition is closely related to the magnetic field induced insulator-to-metal transition (CMR effect) observed in these materials. It is accompanied by a dramatic change in the magnetic properties and lattice structure: the material changes from an insulating charge-ordered canted antiferromagnet to a ferromagnetic metal. We present an investigation of the transport and structural properties of these materials over the course of the transition (which usually takes about an hour to complete). The current-voltage characteristics exhibited by the material during the transition are highly nonlinear, indicating a large inhomogeneity of the transitional state. Possible practical applications of this novel type of transition are briefly discussed. We also report a high-resolution x-ray diffraction study of the charge-ordering in these materials. The temperature dependent charge ordering structure observed in these compounds is more complex than previously reported.
Paramagnetic Susceptibility of the CMR Compound La1-xCaxMnO3
Authors: Goodwin D. H. , Neumeier J. J. , Lacerda A. H. , Torikachvili M. S. 
Published: April 2011

ABSTRACTMeasurements and analysis of the paramagnetic susceptibility of La1-xCaxMnO3+y (0 ≤ x ≤ 1) are reported. The magnetization was also measured in large fields (H ≤ 18 tesla) for a specimen of La0.79Ca0.21MnO3+y. The paramagnetic susceptibility is observed to be strongly enhanced for most compositions studied, including a number of antiferromagnetic compositions. This behavior is attributed to the existence of magnetic polarons.
Molecular Dynamics Study of Lithium Diffusion in Lithium-Manganese Spinel Cathode Materials
Authors: Cygan Randall T. , Westrich Henry R. , Doughty Daniel H. 
Published: April 2011

ABSTRACTA series of molecular dynamics computer simulations of the self-diffusion of lithium in pure and several doped lithium-manganese spinel materials has been completed. The theoretical approach is part of an effort to understand the mechanisms and rates of lithium diffusion, and to evaluate the structural control of the cathode materials upon lithium intercalation (charge-discharge) process. The molecular dynamics approach employs a fully ionic forcefield that accounts for electrostatic, repulsive, and dispersion interactions among all ions. A reference unit cell comprised of 56 ions (Li8Mn3+8Mn4+8O32) is used to perform the simulations under constant volume and constant pressure constraints. All atomic positions are allowed to vary during the simulation. Simulations were completed for the undoped and doped LiMn2O4 at various levels of lithium content (based on the number of lithium ions per unit cell and manganese oxidation state). The molecular dynamics results indicate an activation energy of approximately 97 kJ/mole for self-diffusion of lithium in the undoped material. Lithium ion trajectories from the simulations provide diffusion coefficients that decrease by a factor of ten as the cathode accumulates lithium ions during discharge. Molecular dynamics results for the doped spinel suggest a decrease in the diffusion rate with increasing dopant ion.
Structure and Electrochemical Potential Simulation for the Cathode Material Li1+xV3O8
Authors: Benedek R. , Thackeray M. M. , Yang L. H. 
Published: April 2011

ABSTRACTThe structure and electrochemical potential of monoclinic Li1+xV3O8 were calculated within the local-density-functional-theory framework by use of plane-wave-pseudopotential methods. Special attention was given to the compositions 1+x=1.2 and 1+x=4, for which x-ray diffraction structure refinements are available. The calculated low-energy configuration for 1+x=4 is consistent with the three Li sites identified in x-ray diffraction measurements and predicts the position of the unobserved Li. The location of the tetrahedrally coordinated Li in the calculated low-energy configuration for 1+x=1.5 is consistent with the structure measured by x-ray diffraction for Li1.2V3O8. Calculations were also performed for the two monoclinic phases at intermediate Li compositions, for which no structural information is available. Calculations at these compositions are based on hypothetical Li configurations suggested by the ordering of vacancy energies for Li4V3O8 and tetrahedral site energies in Li1.5V3O8. The internal energy curves for the two phases- cross near 1+x=3. Predicted electrochemical potential curves agree well with experiment.
Ab INITIO CALCULATION OF THE LixCoO2 PHASE DIAGRAM
Authors: Der Ven Anton Van , Aydinol Mehmet K , Ceder Gerbrand 
Published: April 2011

ABSTRACTThe electrochemical properties of the layered intercalation compound LiCoO2 used as a cathode in Li batteries have been investigated extensively in the past 15 years. Despite this research, little is known about the nature and thermodynamic driving forces for the phase transformations that occur as the Li concentration is varied. In this work, the phase diagram of LixCoO2 is calculated from first principles for x ranging from 0 to 1. Our calculations indicate that there is a tendency for Li ordering at x = 1/2 in agreement with experiment [1]. At low Li concentration, we find that a staged compound is stable in which the Li ions selectively segregate to every other Li plane leaving the remaining Li planes vacant. We find that the two phase region observed at high Li concentration is not due to Li ordering and speculate that it is driven by a metal-insulator transition which occurs at concentrations slightly below x < 1.
Thin Film Synthesis of Novel Electrode Materials for Solid-Oxide Fuel Cells
Authors: Jankowski Alan F. , Morse Jeffrey D. 
Published: April 2011

ABSTRACTElectrode materials for solid-oxide fuel cells are developed using sputter deposition. A thin film anode is formed by co-deposition of nickel and yttria-stabilized zirconia. This approach is suitable for composition grading and the provision of a mixed-conducting interfacial layer to the electrolyte layer. Similarly, synthesis of a thin film cathode proceeds by co-deposition of silver and yttria-stabilized zirconia. The sputter deposition of a thin film solid-oxide fuel cell is next demonstrated. The thin film fuel cell microstructure is examined using scanning electron microscopy whereas the cell perfomance is characterized through current-voltage measurement and corresponding impedance spectroscopy.
Novel Proton Exchange Membrane for High Temperature Fuel Cells
Authors: Bhamidipati M. , Lazaro E. , Lyons F. , Morris R. S. 
Published: April 2011

ABSTRACTThis research effort sought to demonstrate that combining select phosphonic acid additives with Nafion could improve Nafion's high temperature electrochemical performance. A 1:1 mixture of the additive with Nafion, resulted in a film that demonstrated 30% higher conductivity than a phosphoric acid equilibrated Nafion control at 175°C. This improvement to the high temperature conductivity of the proton exchange membrane Nafion is without precedent. In addition, thermal analysis data of the test films suggested that the additives did not compromise the thermal stability of Nafion. The results suggest that the improved Nafion proton exchange membranes could offer superior electrochemical performance, but would retain the same degree of thermal stability as Nafion. This research could eventually lead to portable fuel cells that could oxidize unrefined hydrocarbon fuels, resulting in wider proliferation of fuel cells for portable power.
Effects of Microstructure on Oxygen Permeation in Some Perovskite Oxides
Authors: Zhang K. , Yang Y. L. , Jacobson A. J. , Salama K. 
Published: April 2011

ABSTRACTThe effects of microstructure on the oxygen permeation in SrCo0.8Fe0.2O3-δ(SCFO) and La0.2 Sr0.8 Fe0.8Cr0.2 O3 (LSFCO) was investigated using disc samples fabricated under different processing conditions. The microstructure of LSFCO remained unchanged when the sintering temperature was increased from 1300 to 1450 °C, but the average grain size of SCFO increased considerably when the sintering temperature was increased from 930 to 1200 °C. The change in grain size was found to have a strong effect on the oxygen permeation flux in SCFO, which increased considerably as the grain size was decreased. This indicates that the contribution of the grain boundary diffusion to the steady state oxygen flux in SCFO is substantial and grain boundaries provide faster diffusion paths in oxygen permeation through the samples.
Synthesis and Preliminary Electrochemical Characterization of LiNi0.5Co0.5O2 Powders Obtained by the Complex Sol-Gel Process (CSGP)
Authors: Deptula A. , Lada W. , Olczak T. , Croce F. , Ronci F. , Ciancia A. , Giorgi L. , Brignocchi A. , Di Bartolomeo A. 
Published: April 2011

ABSTRACTComplex Sol-Gel Process (CSGP) was applied to the preparation of LiNi0.5Co0.5O2. Starting sol-solutions were prepared in two different ways: I, in which aqueous ammonia was added to a starting solution of Li+-Ni2+-Co2+ acetate-ascorbate, and II in which LiOH was added to a solution of Ni2+- Co2+ and NH4+ acetate-ascorbate. It was found that in the absence of ascorbic acid, or at its lower content (≤0.2 M on 1M σ Li+- Ni2+- Co2+) precipitation of Ni hydroxides occurred. Regular sols were concentrated ∼3 times, gelled and dried at 140°C. Intensive foaming was observed for samples during further heating. Consequently for scaling up to 200g in a run a preliminary long drying procedure followed by self-ignition step (∼400°C) was introduced. Thermal transformation of the gel to solid was studied by TG, DTA, XRD and IR. The main feature of this step is carbonate formation. The final structure LiNi0.5Co0.5O2 is observed after heating for Ih at 800°C. For larger scale production the extension of firing time was necessary. Electrochemical properties of the LiNi0.5Co0.5O2 compound, prepared by the CSGP were evaluated and considered satisfactory.