bluuebunny/crossref_metadata_2025 huggingface dataset:
Synthesis, structures, and magnetic properties of novel Roddlesden–Popper homologous series Srn+1ConO3n+1 (n=1,2,3,4, and ∞)
Authors: Wang X. L. , Sakurai H. , Takayama-Muromachi E.
Published: May 2005

Roddlesden–Popper homologous series Srn+1ConO3n+1 (n=1,2,3,4, and ∞) compounds were successfully synthesized by a high pressure and high temperature technique. Structure refinement revealed that these compounds crystallize in tetragonal structures, while the compound n=∞ is cubic. These compounds are ferromagnetic with the Curie temperature decreasing from 255K for n=1 to about 200K for n=2–4 and down to 175K for SrCoO3. Co4+ ions present as intermediate spin states for n=1–4, but in the low spin state in SrCoO3. Negative magnetoresistance was observed for Sr2CoO4 and found to be larger than that for SrCoO3.
Magnetization and neutron diffraction studies on Dy5Si2Ge2
Authors: Nirmala R. , Sankaranarayanan V. , Sethupathi K. , Morozkin A. V. , Cai Q. , Chu Z. , Yang J. B. , Yelon W. B. , Malik S. K.
Published: May 2005

The compound Dy5Si2Ge2 crystallizes in an orthorhombic structure (Sm5Ge4 type, space group Pnma). Magnetization measurements performed in the temperature range of 2–300 K in applied fields up to 7 T reveal that this compound orders antiferromagnetically at 56 K (TN) but with a positive paramagnetic Curie temperature θP. Magnetization-field isotherms, obtained at 5 K and 20 K, display a field-induced antiferromagnetic to ferromagnetic transition. The magnetization approaches saturation in a field of 6 T with a moment value of ∼8μB∕Dy3+. Neutron diffraction measurements, carried out at 9.2 K, suggest that Dy moments arrange spirally along the a axis giving rise to a canted antiferromagnetic structure. The analysis of neutron diffraction data yields an ordered state magnetic moment of 7.63μBperDy3+ ion.
Quasi-two-dimensional anitiferromagnetic order in Ce2(Co1−xNix)7B3 compounds
Authors: Saito Y. , Ido H. , Yoshida H. , Sakon T. , Motokawa M.
Published: May 2005

Magnetic properties of Ce2(Co1−xNix)7B3 compounds with x⩽0.3 have been studied in the temperature region from 4.2Kto750K. It is proposed on the basis of experimental data that the compound Ce2Co7B3 has an antiferromagnetic order where the nearest two ferromagnetic layers separated by 12.9Å couple very weakly antiferromagnetically. Observed metamagnetic transition in Ce2Co7B3 compound is caused by the spin flip. In the compounds Ce2(Co1−xNix)7B3, Ni atoms substitute preferentially the 2c-site Co atoms, and all atoms in the crystals except for the 2c-site Co atoms [=Co(0) atoms] seem to be nonmagnetic. The compound Ce2(Co1−xNix)7B3 becomes Pauli paramagnetic for x larger than about 0.24.
Temperature dependence of the magnetic anisotropy and magnetostriction of Fe100−xGax (x=8.6, 16.6, 28.5)
Authors: Clark A. E. , Wun-Fogle M. , Restorff J. B. , Dennis K. W. , Lograsso T. A. , McCallum R. W.
Published: May 2005

The temperature dependence of the lowest order magnetic anisotropy constant K1 and the lowest order saturation magnetostriction constant, (3∕2)λ100, were measured from 4 K to 300 K for Fe91.4Ga8.6,Fe83.4Ga16.6, and Fe71.5Ga28.5 and were compared to the normalized magnetization power law, ml(l+1)∕2. Fe91.4Ga8.6 maintains the magnetostriction anomaly of Fe (dλ100∕dT>0) and K1 is a reasonable fit to the ml(l+1)∕2 power law with K1(0K)≅90kJ∕m3. Fe83.4Ga16.6 does not show a magnetostriction anomaly, but fits the power law remarkably well. Fe71.5Ga28.5 possesses a small K1(∼1kJ∕m3) at all temperatures and a large temperature dependent magnetostriction, reaching ∼800ppm at low temperature.
Magnetocaloric effect of La0.8Sr0.2MnO3 compound under pressure
Authors: Rocco Daniel L. , Silva R. Almeida , Carvalho A. Magnus G. , Coelho Adelino A. , Andreeta José P. , Gama Sergio
Published: May 2005

The La0.8Sr0.2MnO3 compound presents a ferromagnetic paramagnetic transition around room temperature to which a reasonably high magnetocaloric effect is associated, turning this material of interest for application in magnetic refrigeration. We synthesized this compound in fiber single crystalline form by the Laser Heated Pedestal Growth method. The sample was characterized by x-ray diffraction and magnetic measurements as a single phase and with the required magnetic properties. We measured the magnetic properties and the magnetocaloric effect under hydrostatic pressure for pressures up to 6 kbar as a function of temperature. Our results indicate that the Curie temperature increases with pressure while the low temperature transition from the orthorhombic to the rhombhoedral structures decreases as pressure increases. This is in close agreement with the literature. Measurement of the magnetocaloric effect at the high temperature transition indicates that the peak of the effect follows the trend of the Curie temperature, but its maximum value remains almost constant as a function of pressure.
Magnetically complexed tissue-mimicking peptides
Authors: Guertin R. P. , Valluzzi R. , Haas T. E. , Pochan D.
Published: May 2005

We have synthesized oligopeptides based on insect silks with added metal ion complexing groups at the ends of the oligopeptides. These oligopeptides can be complexed with magnetic ions and used to chemically deliver them into a pattern of nanoscale magnetic ion-rich layers without thin-film deposition. Gd-, Dy-, and Cr-complexed oligopeptide materials were studied and were found to be well-behaved paramagnets. With higher concentrations of magnetic ions, magnetic coupling interactions may be possible.
Electronic structure and magnetic anisotropy for nickel-based molecular magnets
Authors: Park Kyungwha , Yang En-Che , Hendrickson David N.
Published: May 2005

Recent magnetic measurements on molecular magnets [Ni(hmp)(ROH)Cl]4, where RCH3, CH2CH3, or (CH2)2C(CH3)3, and hmp is the monoanion of 2-hydroxymethylpyridine, revealed a strong exchange bias prior to the external magnetic-field reversal as well as anomalies in electron paramagnetic-resonance peaks at low temperatures. To understand the exchange bias and observed anomalies, we calculate the electronic structure and magnetic properties for the Ni4 molecules with the three different ligands, employing density-functional theory. Considering the optimized structure with possible collinear spin configurations, we find that the lowest-energy state has a total spin of S=0. The magnetic anisotropy barrier to reverse the magnetic moments of all four Ni ions simultaneously is calculated to be 4–6K.
Electron magnetic resonance of iron oxide nanoparticles mineralized in protein cages
Authors: Usselman Robert J. , Klem M. T. , Allen M. , Walter Eric D. , Gilmore K. , Douglas T. , Young M. , Idzerda Y. , Singel D. J.
Published: May 2005

Magnetic and structural properties determined by electron magnetic resonance (EMR) spectroscopy are reported for maghemite (γ-Fe2O3) nanoparticles formed through template-constrained mineralization within three protein cages with nominal diameters of 5, 8, and 24 nm. EMR spectra, obtained at 4.0, 9.2, 34.6, 94.9, and 130.0 GHz, and at ambient temperature, show dramatic frequency dependent effects in the line shapes, line-widths, and resonance-field shifts. Simulations of the spectra are used to obtain moment distribution parameters, which are consistent with size limitations imposed by the protein cages, but which reflect significant departures from bulk magnetization properties.
Magnetic properties of artificially synthesized ferritins
Authors: Kim B. J. , Lee H. I. , Cho S.-B. , Yoon S. , Suh B. J. , Jang Z. H. , St. Pierre T. G. , Kim S.-W. , Kim K.-S.
Published: May 2005

Human ferritin homopolymers with H or L subunits (rHF and rLF) were genetically engineered in E coli. Apoferritins were then reconstituted with 2000 Fe atoms. A big difference was observed in the rates of iron uptake, whereas the mean core size was similar in rHF and rLF. Magnetization of the recombinant human ferritins were measured as functions of temperature and field. The blocking temperature TB(H) at low fields is considerably higher in rLF than in rHF. From the fit of M(H) data to a modified Langevin function: M(H)=M0L(μpH∕kBT)+χaH, the effective magnetic moment μp is found to be much larger in rLF than in rHF. Experimental data demonstrate that the magnetic properties, in particular, the uncompensated spins of ferritin core are related to the biomineralization process in ferritins.
Theoretical study of photoinduced singlet and triplet excited states of 4-dimethylaminobenzonitrile and its derivatives
Authors: Xu Xuefei , Cao Zexing , Zhang Qianer
Published: May 2005

Singlet and triplet low-lying states of the 4-dimethylaminobenzonitrile and its derivatives have been studied by the density functional theory and ab initio methodologies. Calculations reveal that the existence of the methyl groups in the phenyl ring and the amino twisting significantly modify properties of their excited states. A twisted singlet intramolecular charge-transfer state can be accessed through decay of the second planar singlet excited state with charge-transfer character along the amino twisting coordinate or by an intramolecular charge-transfer reaction involved with a locally first excited singlet state. Plausible charge-transfer triplet states and intersystem crossing processes among singlet and triplet states have been explored by spin–orbit coupling calculations. The intersystem crossing process was predicted to be the dominant deactivation channel of the photoexcited 4-dimethylaminobenzonitrile.
Ground and valence-excited states of SF: A multireference configuration interaction with single and double excitations+Q study
Authors: Yang Xinzheng , Boggs James E.
Published: May 2005

Ab initio calculations on the ground and valence-excited states of the sulfur monofluoride radical have been performed using entirely uncontracted all-electron augmented correlation consistent polarized valence quintuple zeta basis sets and the internally contracted multireference configuration interaction with single and double excitations method and Davidson correction (+Q). Potential-energy curves of all valence electronic states and the spectroscopic constants of several bound states are fitted. It is the first time that the entire 27-Ω states generated from the 12 valence Λ-S states which come from the S(Pg3) and F(Pu2) atomic states of SF radical have been studied theoretically. The effects of spin-orbit coupling and the avoided crossing rule between Ω states of the same symmetry are analyzed. The calculated results reproduce well the available experimental values and predict the properties of several bound excited states that have never been observed in experiment. The transition properties of the dipole-allowed transitions from bound excited states to the ground state are predicted for the first time, including the transition dipole moments, the Franck–Condon factors, and the radiative lifetimes.
Theoretical studies of the potential surface and vibrational spectroscopy of CH3OH and its deuterated analogs
Authors: Sibert Edwin L. , Castillo-Chará Jairo
Published: May 2005

A potential-energy force field for methanol is calculated at the coupled-cluster singles doubles and noniterative triples correction level using the correlation-consistent polarized valence triple zeta basis set. The force field describes the coupled molecular vibrations, including the cubic and quartic anharmonicites, as a function of the torsional coordinate. The resulting molecular eigenstates are calculated using a combination of perturbative and variation calculations following the approach of Castillo-Chará and Sibert [J. Chem. Phys. 119, 11671 (2003)]. The energies, including torsional tunneling splittings, are compared with available spectroscopic data for all the fundamentals. Excellent agreement is found. Varying the torsional mass, correlation plots are constructed in order to elucidate the complex role of torsion–vibration coupling.
Long-range transition state theory
Authors: Georgievskii Yuri , Klippenstein Stephen J.
Published: May 2005

The implementation of variational transition state theory (VTST) for long-range asymptotic potential forms is considered, with particular emphasis on the energy and total angular momentum resolved (μJ-VTST) implementation. A long-range transition state approximation yields a remarkably simple and universal description of the kinetics of reactions governed by long-range interactions. The resulting (μJ-VTST) implementation is shown to yield capture-rate coefficients that compare favorably with those from trajectory simulations (deviating by less than 10%) for a wide variety of neutral and ionic long-range potential forms. Simple analytic results are derived for many of these cases. A brief comparison with a variety of low-temperature experimental studies illustrates the power of this approach as an analysis tool. The present VTST approach allows for a simple analysis of the applicability conditions for some related theoretical approaches. It also provides an estimate of the temperature or energy at which the “long-range transition state” moves to such short separations that short-range effects, such as chemical bonding, steric repulsion, and electronic state selectivity, must be considered.
Heat rectification in molecular junctions
Authors: Segal Dvira , Nitzan Abraham
Published: May 2005

Heat conduction through molecular chains connecting two reservoirs at different temperatures can be asymmetric for forward and reversed temperature biases. Based on analytically solvable models and on numerical simulations we show that molecules rectify heat when two conditions are satisfied simultaneously: the interactions governing the heat conduction are nonlinear, and the junction has some structural asymmetry. We consider several simplified models where a two-level system (TLS) simulates a highly anharmonic vibrational mode, and asymmetry is introduced either through different coupling of the molecule to the contacts, or by considering internal molecular asymmetry. In the first case, we present analytical results for the asymmetric heat current flowing through a single anharmonic mode using different forms for the TLS-reservoirs coupling. We also demonstrate numerically, studying a realistic molecular model, that a uniform anharmonic molecular chain connecting asymmetrically two thermal reservoirs rectifies heat. This effect is stronger for longer chains, where nonlinear interactions dominate the transfer process. When asymmetry is related to the internal level structure of the molecule, numerical simulations reveal a nontrivial rectification behavior. We could still explain this behavior in terms of an effective system-bath coupling. Our study suggests that heat rectification is a fundamental characteristic of asymmetric nonlinear thermal conductors. This phenomenon is important for heat control in nanodevices and for understanding of energy flow in biomolecules.
Relaxation response of polymers containing highly flexible side groups monitored by broadband dielectric spectroscopy
Authors: Alvarez Cristina , Lorenzo Vicente , Riande Evaristo
Published: May 2005

The relaxation behavior of poly(5-acryloxymethyl-5-methyl-1,3-dioxacyclohexane), a polymer containing highly flexible side groups, is studied by broadband dielectric spectroscopy in the frequency and temperature ranges 10−1–109Hz and 123–473 K, respectively. Above the glass transition temperature Tg the dielectric loss in the frequency domain exhibits a prominent α absorption, followed in increasing order of frequencies by two secondary absorptions called β and γ. At temperatures slightly higher than Tg, the α relaxation is well separated from the β, but as temperature increases overlapping between both relaxations augments forming an αβ absorption in the vicinity of 420 K. This latter absorption displays a shoulder on its high-frequency side corresponding to the γ relaxation. The strength of the α relaxation decreases with increasing temperature, eventually vanishing at the temperature at which the αβ absorption is formed. The time retardation spectra of the isotherms are calculated and further used to facilitate the deconvolution of the overlapping relaxations. The fact that the temperature dependence of the β relaxation also describes that of the αβ absorption suggests that both relaxations have the same nature. It seems that as temperature increases, the α relaxation feeds on the β absorption until its complete disappearance. The γ relaxation, in turn, seems to increase at the expense of the αβ process at high temperature.
Temperature and gas-phase composition measurements in supersonic flows using tunable diode laser absorption spectroscopy: The effect of condensation on the boundary-layer thickness
Authors: Tanimura Shinobu , Zvinevich Yury , Wyslouzil Barbara E. , Zahniser Mark , Shorter Joanne , Nelson David , McManus Barry
Published: May 2005

We used a tunable diode laser absorption spectrometer and a static-pressure probe to follow changes in temperature, vapor-phase concentration of D2O, and static pressure during condensation in a supersonic nozzle. Using the measured static-pressure ratio p∕p0 and the mass fraction of the condensate g as inputs to the diabatic flow equations, we determined the area ratio (A∕A*)Wet and the corresponding centerline temperature of the flow during condensation. From (A∕A*)Wet we determined the boundary-layer displacement thickness during condensation (δ#)Wet. We found that (δ#)Wet first increases relative to the value of δ# in a dry expansion (δ#)Dry before becoming distinctly smaller than (δ#)Dry downstream of the condensation region. After correcting the temperature gradient across the boundary layers, the temperature determined from p∕p0 and g agreed with the temperature determined by the laser-absorption measurements within our experimental error (±2 K), except when condensation occurred too close to the throat. The agreement between the two temperature measurements let us draw the following two conclusions. First, the differences in the temperature and mole fraction of D2O determined by the two experimental techniques, first observed in our previous study [P. Paci, Y. Zvinevich, S. Tanimura, B. E. Wyslouzil, M. Zahniser, J. Shorter, D. Nelson, and B. McManus, J. Chem. Phys. 121, 9964 (2004)], can be explained sufficiently by changes in δ# caused by the condensation of D2O, except when the phase transition occurs too close to the throat. Second, the extrapolation of the equation, which expresses the temperature dependence of the heat of vaporization of bulk D2O liquid, is a good estimate of the heat of condensation of supercooled D2O down to 210 K.
An interaction model for OH+H2O-mixed and pure H2O overlayers adsorbed on Pt(111)
Authors: Karlberg G. S. , Wahnström G.
Published: May 2005

A model potential for the adsorbate–adsorbate interaction among OH and H2O molecules adsorbed on a Pt(111) surface has been developed solely based on first-principle calculations. By combining this directional-dependent model potential for the lateral interaction with a lattice model of Ising type, large length scale structure calculations can be made. The strength of different hydrogen bonds can be analyzed in detail from this model potential. It is found that the hydrogen bond between OH and H2O molecules is stronger than that between two H2O molecules (0.4 eV per pair as compared to 0.2 eV per pair, respectively). Via the computed chemical potential for water in mixed OH+H2O overlayers the water uptake as a function of oxygen precoverage on Pt(111) has been determined. The results compare very well with recent experiments.
Water’s size-dependent freezing to cubic ice
Authors: Johari G. P.
Published: May 2005

Water has been occasionally found to freeze to cubic ice. To investigate this occurrence thermodynamically, we use the known enthalpy and interfacial energy of hexagonal and cubic ices and calculate a critical radius rc of ∼15nm for a water droplet and a critical thickness δc of ∼10nm for water’s flat film. Accordingly, water droplets smaller than 15 nm radius and films thinner than 10 nm would freeze to cubic ice in the 160–220 K range and bigger droplets and thicker films would freeze to hexagonal ice. This provides a thermodynamic basis for the occasionally found presence of cubic ice in the atmosphere, and explains why water’s nanometer-sized clusters and water confined to nanometer-sized pores freeze to cubic ice. Conditions for cubic ice–hexagonal ice phase inversion have been discussed. Impurities in water and different extents of proton ordering in the crystallites of cubic and hexagonal ices would have a significant effect on rc and δc.
Application of variational reduced-density-matrix theory to the potential energy surfaces of the nitrogen and carbon dimers
Authors: Gidofalvi Gergely , Mazziotti David A.
Published: May 2005

The acceleration of the variational two-electron reduced-density-matrix (2-RDM) method, using a new first-order algorithm [D. A. Mazziotti, Phys. Rev. Lett. 93, 213001 (2004)], has shown its usefulness in the accurate description of potential energy surfaces in nontrivial basis sets. Here we apply the first-order 2-RDM method to the potential energy surfaces of the nitrogen and carbon dimers in polarized valence double-ζ basis sets for which benchmark full-configuration-interaction calculations exist. In a wave function formalism accurately stretching the triple bond of the nitrogen dimer requires at least six-particle excitations from the Hartree–Fock reference. Furthermore, cleaving the double bond of C2 should produce a “non-Morse”-like potential curve because the ground state near equilibrium (XΣg+1) has an avoided crossing with the second excited state (B′Σg+1) and a level crossing with the first excited state (BΔg1). Because the 2-RDM method variationally optimizes the energy over correlated 2-RDMs on the two-electron space without parametrization of the many-electron wave function, it captures multireference correlations that are difficult to describe with approximate wave functions. The 2-RDM method yields for N2 a potential energy surface with features and spectroscopic constants that are more accurate than those from single-reference methods and similar in accuracy to multireference techniques, and it describes the non-Morse-like behavior of C2 which is not captured by single-reference methods.
Small angle x-ray scattering of a supercritical electrolyte solution: The effect of density fluctuations on the hydration of ions
Authors: Testemale Denis , Coulet Marie Vanessa , Hazemann Jean Louis , Simon Jean Paul , Bley Françoise , Geaymond Olivier , Argoud Roger
Published: May 2005

Synchrotron small angle x-ray scattering measurements on water and zinc bromide ZnBr2 aqueous solutions were carried out from ambient to supercritical conditions. For both systems several isobars (between 285 and 600bars) were followed beyond the critical isochore. The data were analyzed through an Ornstein–Zernike formalism in terms of correlation length and null angle structure factor. The results for pure water are in agreement with previously published values. Solutions of different electrolyte concentrations were studied. In each case, the values of the correlation length and null angle structure factor are larger than those of pure water. This effect is more pronounced for higher concentrations and/or for pressure closer to the critical point of pure water. This is in agreement with the shift of the critical point determined in the literature for NaCl solutions. Comparing these results to previous x-ray absorption measurements carried out on identical samples we propose the following two step sequence for ionic hydration up to supercritical conditions: (1) from ambient to about 300°C, an increase of ion pairing and formation of multi-ionic complexes which can be correlated to the decrease of the dielectric constant; (2) an enhancement of the local solvation shell of ions due to the onset of the thermal density fluctuations at high temperature, leading to a screening effect between ions and inhibiting the ion pairing processes.
Monitoring and modeling silicon homoepitaxy breakdown with real-time spectroscopic ellipsometry
Authors: Teplin Charles W. , Levi Dean H. , Iwaniczko Eugene , Jones Kim M. , Perkins John D. , Branz Howard M.
Published: May 2005

Real-time spectroscopic ellipsometry (RTSE) is used to monitor the breakdown of low-temperature homoepitaxial growth of silicon on silicon wafers in a hot-wire chemical-vapor deposition reactor. We develop and evaluate two optical models to interpret the RTSE data, revealing the progression of epitaxy and its eventual breakdown into amorphous silicon growth. Comparison of the RTSE analysis with cross-sectional transmission electron microscopy, ex situ variable-angle spectroscopic ellipsometry, and Raman spectroscopy measurements shows that RTSE provides accurate and fast quantitative feedback about the progression of epitaxy.
The shape of a flexible polymer in a cylindrical pore
Authors: Morrison G. , Thirumalai D.
Published: May 2005

We calculate the mean end-to-end distance R of a self-avoiding polymer encapsulated in an infinitely long cylinder with radius D. A self-consistent perturbation theory is used to calculate R as a function of D for impenetrable hard walls and soft walls. In both cases, R obeys the predicted scaling behavior in the limit of large and small D. The crossover from the three-dimensional behavior (D→∞) to the fully stretched one-dimensional case (D→0) is nonmonotonic. The minimum value of R is found at D∼0.46RF, where RF is the Flory radius of R at D→∞. The results for soft walls map onto the hard wall case with a larger cylinder radius.
A slope-theory approach to electrical probe recording on phase-change media
Authors: Aziz Mustafa M. , Wright C. David
Published: May 2005

A theoretical approach to predicting the spatial extent of the amorphous to crystalline transition region during the probe recording process on phase-change storage media is presented. The extent of this transition region determines the ultimate achievable linear density for data storage using phase-change materials. The approach has parallels with the slope theory used to find magnetic transition lengths in magnetic recording, and shows that the amorphous to crystalline transition length can be minimized by reducing the thickness of the phase-change layer, by minimizing lateral heat flow, and by maximizing the ratio of the activation energy for crystallization to the transition temperature Ec∕Tt.
Behavior of SiO2 nanostructures under intense extreme ultraviolet illumination
Authors: Heun S. , Kremmer S. , Ercolani D. , Wurmbauer H. , Teichert C.
Published: May 2005

The stability of conducting atomic force microscope (C-AFM)-induced surface modifications of thermally grown SiO2 under intense illumination in the extreme ultraviolet (EUV) is investigated with low-energy electron microscopy (LEEM) and x-ray photoemission electron microscopy (XPEEM). With LEEM we find that the protrusions are heavily charged after their formation, but this charge is annihilated after exposure of the sample to short pulses of EUV radiation. The spectra obtained from XPEEM reveal that the stripes formed by C-AFM consist of SiO2. After extended EUV exposure, a radiation-induced desorption of the stripes as well as a desorption of the thermal oxide is observed.
High-resolution elemental profiles of the silicon dioxide∕4H-silicon carbide interface
Authors: Chang K.-C. , Cao Y. , Porter L. M. , Bentley J. , Dhar S. , Feldman L. C. , Williams J. R.
Published: May 2005

High-resolution elemental profiles were obtained from SiO2(N)∕4H-SiC structures by spatially resolved electron energy-loss spectroscopy (EELS) performed in the scanning transmission electron microscopy mode. The results show that following annealing in NO, N was exclusively incorporated within ∼1nm of the SiO2(N)∕4H-SiC interface. Mean interfacial nitrogen areal densities measured by EELS were ∼(1.0±0.2)×1015cm−2 in carbon-face samples and (0.35±0.13)×1015cm−2 in Si-face samples; these results are consistent with nuclear reaction analysis measurements. Some of the interface regions in the C-face samples also showed excess carbon that was not removed by the NO annealing process, in contrast with previous results on Si-face samples.
Electrical and optical characterization of 4H-SiC diodes for particle detection
Authors: Schifano Ramón , Vinattieri Anna , Bruzzi Mara , Miglio Stefania , Lagomarsino Stefano , Sciortino Silvio , Nava Filippo
Published: May 2005

The electronic and optical properties of several (medium to high quality) 4H-SiC epitaxial sensors for particle detection have been studied. The samples are n-doped Schottky diodes with different nitrogen concentrations (6×1013cm−3–5×1015cm−3) and thicknesses (20–40μm). A full electrical and optical characterization has been performed by capacitance versus voltage measurements and near-band-edge low-temperature photoluminescence. The effective doping along the epilayer and the depletion width have been determined and data are consistent with the charge collection efficiency characterization performed with a minimum ionizing β-source. All the investigated samples exhibit a 100% collection efficiency. In particular, the best samples yield a highly reproducible signal, well separated from the pedestal. Photoluminescence results show a linear relationship between the effective doping and the ratio of nitrogen-bound excitonic emission (Q0) and free excitonic line (I76), in agreement with a previous work on 4H-SiC with a higher doping concentration [I. G. Ivanov, C. Hallin, A. Henry, O. Kordina, and E. Janzén, J. Appl. Phys. 80, 3504 (1996)]. Moreover we show that the dependence of the major spectral features as a function of the penetration depth of the exciting laser beam can quantitatively provide information on substrate contribution to the photoluminescence. In conclusion, we bring evidence that a detailed characterization of SiC-based detectors, by all optical techniques, yields an accurate value for the net doping and gives a qualitative information on the epilayer thickness prior to any electrical wafer tests.
Production of carbon nanotubes by microwave plasma torch at atmospheric pressure
Authors: Hong Yong Cheol , Uhm Han Sup
Published: May 2005

The key requirements of nanotube formation are an atomic carbon source and a source of nanometal particles. Carbon nanotubes (CNTs) have been synthesized by an argon/nitrogen microwave plasma torch using a mixture of acetylene and vapor-phase iron pentacarbonyl at the atmospheric pressure. The synthesized CNTs have been analyzed by scanning electron microscopy, field-emission transmission electron microscopy, and Raman spectroscopy, and are shown to be multiwalled and have a bamboo-shaped structure. The synthesized CNTs in some areas are well aligned. It is also found that the higher the content of nitrogen gas used, the higher the number of rough and wavy surfaces and the inner intersecting layers.
Experimental studies of anode sheath phenomena in a Hall thruster discharge
Authors: Dorf L. , Raitses Y. , Fisch N. J.
Published: May 2005

Both electron-repelling and electron-attracting anode sheaths in a Hall thruster were characterized by measuring the plasma potential with biased and emissive probes [L. Dorf, Y. Raitses, V. Semenov, and N. J. Fisch, Appl. Phys. Lett. 84, 1070 (2004)]. In the present work, two-dimensional structures of the plasma potential, electron temperature, and plasma density in the near-anode region of a Hall thruster with clean and dielectrically coated anodes are identified. Possible mechanisms of anode sheath formation in a Hall thruster are analyzed. The path for current closure to the anode appears to be the determining factor in the anode sheath formation process. The main conclusion of this work is that the anode sheath formation in Hall thrusters differs essentially from that in the other gas discharge devices, such as a glow discharge or a hollow anode, because the Hall thruster utilizes long electron residence times to ionize rather than high neutral pressures.
Macroscopic quantum tunneling of the magnetization in the single crystal of [(Mn0.96Cr0.04)12O12(CH3COO)16(H2O)4]∙2CH3COOH∙4H2O
Authors: He L. H. , Yan Q. W. , Wang F. W.
Published: May 2005

Large-size single crystals of the single-molecule magnet [(Mn0.96Cr0.04)12O12 (CH3COO)16(H2O)4]∙2CH3COOH∙4H2O ((Mn0.96Cr0.04)12-Ac) were grown, and the effect of Cr substitution on quantum tunneling of the magnetization (QTM) was investigated. In comparison with its parent Mn12-Ac {abbreviated for [Mn12O12(CH3COO)16(H2O)4]∙2CH3COOH∙4H2O}, a faster relaxation, i.e., a higher QTM probability, has been observed in (Mn0.96Cr0.04)12-Ac. This modulation of the relaxation time due to the Cr doping could be attributed to additional intrinsic, but distributed, transverse and longitudinal anisotropies raised by a subtle change of the local environment in the magnetic Mn12 core.
Microlens arrays with integrated pores as a multipattern photomask
Authors: Yang Shu , Ullal Chaitanya K. , Thomas Edwin L. , Chen Gang , Aizenberg Joanna
Published: May 2005

Photolithographic masks are key components in the fabrication process of patterned substrates for various applications. Different patterns generally require different photomasks, whose total cost is high for the multilevel fabrication of three-dimensional microstructures. We developed a photomask that combines two imaging elements—microlens arrays and clear windows—in one structure. Such structures can be produced using multibeam interference lithography. We demonstrate their application as multipattern photomasks; that is, by using the same photomask and simply adjusting (i) the illumination dose, (ii) the distance between the mask and the photoresist film, and (iii) the tone of photoresist, we are able to create a variety of different microscale patterns with controlled sizes, geometries, and symmetries that originate from the lenses, clear windows, or their combination. The experimental results agree well with the light field calculations.
Radiation resistance of electro-optic polymer-based modulators
Authors: Taylor Edward W. , Nichter James E. , Nash Fazio D. , Haas Franz , Szep Attila A. , Michalak Richard J. , Flusche Brian M. , Cook Paul R. , McEwen Tom A. , McKeon Brian F. , Payson Paul M. , Brost George A. , Pirich Andrew R. , Castaneda Carlos , Tsap Boris , Fetterman Harold R.
Published: May 2005

Mach–Zehnder interferometric electro-optic polymer modulators composed of highly nonlinear phenyltetraene bridge-type chromophores within an amorphous polycarbonate host matrix were investigated for their resistance to gamma rays and 25.6 MeV protons. No device failures were observed and the majority of irradiated modulators exhibited decreases in half-wave voltage and optical insertion losses compared to nonirradiated control samples undergoing aging processes. Irradiated device responses were attributed to scission, cross-linking, and free volume processes. The data suggests that strongly poled devices are less likely to de-pole under the influence of ionizing radiation.
Ge–Si system nanoclusters in Si matrix formed by solid-phase epitaxy
Authors: Xiao Qinghua , Tu Hailing
Published: May 2005

Solid-phase epitaxy combined with Ge implantation has been employed to prepare Ge–Si system nanoclusters. The nanoclusters coherently embedded in the single-crystalline Si matrix are, on average, controlled within the size range of a few nanometers. The distribution and the number of these nanoclusters are closely correlated with the implant energy and dose as well as the surface oxidation. Within the nanoclusters, the lattice is modulated with a period of four times of the (111) plane spacing in the regular Si lattice. It is argued that the lattice modulated characteristics should result from Ge ordering instead of moiré interference.
Monte Carlo simulation of double gate silicon on insulator devices operated as velocity modulation transistors
Authors: Sampedro C. , Gamiz F. , Godoy A. , Prunnila M. , Ahopelto J.
Published: May 2005

We used an ensemble Monte Carlo simulator to study both the dc and transient behavior of a double gate silicon-on-insulator transistor (DGSOI) operated as a velocity modulation transistor (VMT) and as a conventional field effect transistor (FET). Operated as a VMT, the DGSOI transistor provides switching times shorter than 1ps regardless of the channel length, with a significant current modulation factor at room temperature. The same device operated as a FET provides much longer switching times which, in addition, increase with the channel length.
Highly unidirectional InAs∕InGaAs∕GaAs quantum-dot ring lasers
Authors: Cao Hongjun , Deng Hui , Ling Hai , Liu Chiyu , Smagley Vladimir A. , Caldwell Robert B. , Smolyakov Gennady A. , Gray Allen L. , Lester Luke F. , Eliseev Petr G. , Osiński Marek
Published: May 2005

We report fabrication and characterization of semiconductor ring lasers with quantum-dot active region. The InAs∕InGaAs∕GaAs “dots-in-a-well” ridge-waveguide ring lasers are monolithically integrated with coupling waveguides and monitoring quantum-dot photodetectors. The lowest threshold current density for semiconductor ring lasers is demonstrated. When enhanced by a forward biased S-section waveguide, stable unidirectional operation with record suppression ratio of counterpropagating waves exceeding 30dB is achieved.
Kinetic study for hopping conduction through deoxyribonucleic acid molecules
Authors: Yang Yong-Gang , Yin Peng-Gang , Li Xin-Qi , Yan YiJing
Published: May 2005

Recent experiments indicated that disorder effect in deoxyribonucleic acid (DNA) may lead to a transition of the electronic hole transport mechanism from band resonant tunneling to thermally activated hopping. In this letter, based on Mott’s variable-range hopping theory, we present a kinetic study for the hole transport properties of DNA molecules. Beyond the conventional argument in large-scale systems, our numerical study for finite-size DNA molecules reveals a number of unique features for: (i) the current-voltage characteristics, (ii) the temperature and length dependence, and (iii) the transition from conducting to insulating behaviors.
Impact of the top-electrode material on the permittivity of single-crystalline Ba0.7Sr0.3TiO3 thin films
Authors: Plonka R. , Dittmann R. , Pertsev N. A. , Vasco E. , Waser R.
Published: May 2005

We observed significant influence of the top-electrode material on the thickness and temperature dependences of the dielectric response of single-crystalline Ba0.7Sr0.3TiO3 thin-film capacitors. For SrRuO3∕Ba0.7Sr0.3TiO3∕SrRuO3 samples, the position of dielectric maximum shifts to lower temperatures with decreasing film thickness, whereas the samples with Pt top electrodes exhibit an opposite trend. Moreover, the apparent “interfacial” capacitance, extracted from the film-thickness dependence of dielectric response, is very different for these two types of samples and strongly depends on temperature. Experimental results are analyzed theoretically in light of the depolarizing-field and strain effects on the transition temperature and permittivity of ferroelectric films.
Continuous-wave terahertz system with a 60 dB dynamic range
Authors: Gregory I. S. , Tribe W. R. , Baker C. , Cole B. E. , Evans M. J. , Spencer L. , Pepper M. , Missous M.
Published: May 2005

We have developed a high-performance continuous-wave terahertz imaging system based on photomixing. The emitter and detector are driven by compact, unstabilized, single-mode diode lasers. The all-optoelectronic, homodyne detection scheme yields both amplitude and phase information, and with careful optimization and matching of both emitter and receiver, a 60 dB dynamic range, at 0.53 THz, can be routinely achieved. This replicates the performance of established pulsed THz imagers at this frequency.
Interferometric determination of the anisotropic refractive index dispersion of poly-(p-phenylene-vinylene)
Authors: Galli M. , Marabelli F. , Comoretto D.
Published: May 2005

The anisotropic refractive index of highly stretch-oriented free-standing films of poly(p-phenylene-vinylene) has been determined with high accuracy over a broad spectral range by using an interferometric method which directly measures the phase shift of a light-beam passing through the sample. The comparison with transmittance data allows one to evaluate the sample thickness with high precision, avoiding ambiguities in the extraction of the absolute value of the refractive index. The use of polarized light allows evaluation of the strong birefringence of the material.
Effects of Sr doping on the magnetic properties of SrxBa1−xCoO3
Authors: Pardo V. , Rivas J. , Baldomir D.
Published: May 2005

Magnetic properties of the Sr-doped BaCoO3 are explained on the basis of ab initio calculations and the analysis of experimental literature. Formation of magnetic clusters bigger than in the parent compound and an increase in the blocking temperature are observed. Superparamagnetism remains at room temperature. The possibility of tuning the size and properties of the magnetic clusters with doping is explored.
Cross-reactivity and conformational multiplicity of an anti-polycyclic aromatic hydrocarbon mAb
Authors: Grubor Nenad M. , Hayes John , Small Gerald J. , Jankowiak Ryszard
Published: May 2005

Cross-reactivity and multispecific functionality of antibodies play a central role in the immune system. The Ab's promiscuity is attributed to structural flexibility and conformational multiplicity of its binding sites governed by the rearrangement of hydrogen bonding centers. However, antibodies whose recognition and binding rely on less directional hydrophobic interactions might follow different interaction pathways. We investigated interaction of anti-polycyclic aromatic hydrocarbon mAb with two biologically important cross-reactants, pyrene and benzo(a)pyrene. Complex formation was characterized by means of low-temperature laser-induced fluorescence spectroscopy in both low- and high-resolution fluorescence line-narrowing (FLN) modes. It is shown that the FLN spectroscopy can identify various haptens cross-reacted with an Ab, as well as simultaneously differentiate between free and immunocomplexed haptens. In addition, our results suggest an interesting case of an Ab binding a particular cross-reactant by adopting two distinct conformations of its binding sites. The existence of the multiple conformations for anti-polycyclic aromatic hydrocarbon mAb that are trapped at low temperature can be rationalized through the existing models for Ab binding. Finally, as revealed by FLN spectra of immunocomplexed chromophores, π–π interactions, rather than hydrogen bonding, play the central role in complex formation.
The Poggendorff illusion explained by natural scene geometry
Authors: Howe Catherine Q. , Yang Zhiyong , Purves Dale
Published: May 2005

One of the most intriguing of the many discrepancies between perceived spatial relationships and the physical structure of visual stimuli is the Poggendorff illusion, when an obliquely oriented line that is interrupted no longer appears collinear. Although many different theories have been proposed to explain this effect, there has been no consensus about its cause. Here, we use a database of range images (i.e., images that include the distance from the image plane of every pixel in the scene) to show that the probability distribution of the possible locations of line segments across an interval in natural environments can fully account for all of the behavior of this otherwise puzzling phenomenon.
The effect of herbal supplementation on the severity of exercise-induced pulmonary haemorrhage
Authors: Epp TS , McDonough P , Padilla DJ , Cox JH , Poole DC , Erickson HH
Published: May 2005

Exercise-induced pulmonary haemorrhage (EIPH) is a serious condition that affects the health and possibly the performance of all racehorses. However, only two treatments, furosemide and the Flair™ equine nasal strip, both of which reduce capillary transmural pressure, have been successful in reducing EIPH. Alternatively, transient impairment of platelet function and coagulation during exercise has been considered an additional contributor to EIPH. Consequently, herbal formulations designed to enhance platelet function, and hence coagulation, are hypothesized to reduce EIPH. To investigate the validity of this hypothesis, five Thoroughbred horses completed three maximal incremental exercise tests on a 10% inclined treadmill in a randomized cross-over design experiment. Treatments included twice daily oral administration (for 3 days) of a placebo (PL; cornstarch) and two herbal formulas,Yunnan Paiyao(YP) orSingle Immortal(SI). Blood samples for coagulation profiles, complete blood counts and biochemistry profiles were collected before each exercise test. During each test, pulmonary arterial pressure, oxygen uptake, arterial blood gases, plasma lactate and time-to-fatigue were measured. Severity of EIPH was quantified via bronchoalveolar lavage (BAL) at 30–60 min post-exercise. The herbal formulations were not effective in decreasing EIPH (×106red blood cells ml−1BAL fluid: PL, 27.1±11.6; YP, 33.2±23.4; SI, 35.3±15.4,P>0.05) or in changing any of the other variables measured with the exception of time-to-fatigue, which was slightly but significantly prolonged bySingle Immortalcompared with placebo andYunnan Paiyao(PL, 670±9.6 s; YP, 665±5.5 s; SI, 685±7.9 s,P<0.05). Thus, these results do not support the use of these herbal formulations in the prevention of EIPH.
Oxygen uptake (VO2) kinetics in different species: a brief review
Authors: Poole David C , Kindig Casey A , Behnke Brad J , Jones Andrew M
Published: May 2005

When a human begins to move or locomote, the energetic demands of its skeletal muscles increase abruptly and the oxygen (O2) transport system responds to deliver increased amounts of O2to the respiring mitochondria. It is intuitively reasonable that the rapidity with which O2transport can be increased to and utilized by (VO2) the contracting muscles would be greater in those species with a higher maximal VO2capacity (i.e., VO2max). This review explores the relationship between VO2maxand VO2dynamics or kinetics at across a range of species selected, in part, for their disparate VO2maxcapacities. In healthy humans there is compelling evidence that the speed of the VO2kinetics at the onset of exercise is limited by an oxidative enzyme inertia within the exercising muscles rather than by VO2delivery to those muscles. This appears true also for the horse and dog but possibly not for a certain species of frog. Whereas there is a significant correlation between VO2maxand the speed of VO2kinetics among different species, it is possible to identify species or individuals within a species that exhibit widely disparate mass-specific VO2maxcapacities but similar VO2kinetics (i.e., superlative human athlete and horse).