bluuebunny/crossref_metadata_2025 huggingface dataset:
Photoelectrochemical cell using dye sensitized zinc oxide nanowires grown on carbon fibers
Authors: Unalan Husnu Emrah , Wei Di , Suzuki Kenichi , Dalal Sharvari , Hiralal Pritesh , Matsumoto Hidetoshi , Imaizumi Shinji , Minagawa Mie , Tanioka Akihiko , Flewitt Andrew J. , Milne William I. , Amaratunga Gehan A. J.
Published: October 2008

Zinc oxide (ZnO) nanowires (NWs) grown on carbon fibers using a vapor transport and condensation approach are used as the cathode of a photoelectrochemical cell. The carbon fibers were obtained by electrospray deposition and take the form of a flexible carbon fabric. The ZnO NW on carbon fiber anode is combined with a “black dye” photoabsorber, an electrolyte, and a platinum (Pt) counterelectrode to complete the cell. The results show that ZnO NW and carbon fibers can be used for photoinduced charge separation/charge transport and current collection, respectively, in a photoelectrochemical cell.
Finite size effects on locating conformational transitions for macromolecules
Authors: Sharma Sumit , Kumar Sanat K.
Published: October 2008

It has been shown from simulation and experiment that locations of peaks in structural and thermodynamic quantities accompanying “phase” transitions of a single macromolecule (collapse or crystallization/melting) do not coincide. Thus, for chains with finite lengths these different measures yield apparently different results for transition temperatures. To resolve this issue we use scaling, verified by computer simulations, to conclusively show that these different locations for peak positions are simply a consequence of the finite chain length, as has been conjectured previously.
Direct probe of the built-in electric field of Mg-doped a-plane wurtzite InN surfaces with time-resolved electric-field-induced second harmonic generation
Authors: Chang Y.-M. , Hong Y.-L. , Gwo S.
Published: October 2008

The exceptionally large electron affinity of InN causes the pinning of surface Fermi level well above the conduction band minimum. This unique electronic property leads to the electron accumulation at InN surfaces and a large built-in electric field in the topmost few nanometers of InN surfaces. In this letter, we demonstrate that this surface electric field can be unambiguously determined and monitored in a-plane wurtzite InN surface via time-resolved electric-field-induced second harmonic generation. This finding makes it possible to directly probe and characterize the surface electronic properties of Mg-doped InN with an all-optical technique in ambient environment.
Electronic and thermal properties of Ti3Al(C0.5,N0.5)2, Ti2Al(C0.5,N0.5) and Ti2AlN
Authors: Scabarozi T. , Ganguly A. , Hettinger J. D. , Lofland S. E. , Amini S. , Finkel P. , El-Raghy T. , Barsoum M. W.
Published: October 2008

In this paper we report on the electronic, magnetotransport, thermoelectric, and thermal properties of Ti3Al(C0.5,N0.5)2, Ti2Al(C0.5,N0.5), and Ti2AlN. The electrical conductivities, Hall coefficients, and magnetoresistances are analyzed within a two-band framework and compared with the end members, Ti2AlC and Ti3AlC2. The analysis shows that all compounds are compensated conductors with hole and electron carrier densities of about 1.5×1027 m−3. The room temperature thermal conductivities of the carbonitrides are both over ≈50 W/mK, with the phonon contribution of Ti3Al(C0.5,N0.5)2 particularly large at over 35 W/mK. The low-temperature specific heat of Ti3Al(C0.5,N0.5)2 and Ti2Al(C0.5,N0.5) yield Debye temperatures of 685 and 724 K, respectively, comparable to those of the end members.
Lamb wave attenuation in a rough plate. II. Analytical and numerical results in a fluid plate
Authors: Potel Catherine , Leduc Damien , Morvan Bruno , Depollier Claude , Hladky-Hennion Anne-Christine , Izbicki Jean-Louis , Pareige Pascal , Bruneau Michel
Published: October 2008

This paper aims at providing analytical and numerical approaches to analyze the behavior of guided waves (Lamb-type waves) propagating inside plates with a rough surface considered here as distributed small perturbations. A physical interpretation of the attenuation phenomenon of the propagating mode generated by the source (mode coupling) is provided for an ideal rough plate (fluid plate) using an analytical model available in the literature. This analytical model is validated using a numerical method. The obtained results on periodically corrugated surfaces of fluid plates emphasize the role played by the spatial period and the modal coupling. They serve also as results for interpreting the effect due to randomly rough surfaces in solid plates, notably the effect of the power spectrum density of the rough profile (including the effect of the statistical roughness parameters mentioned in Part I). The results obtained experimentally and theoretically, for the attenuation factor of the main Lamb wave, are found close to one another.
Laser induced plane acoustic wave generation, propagation, and interaction with rigid structures in water
Authors: Ko Seung H. , Ryu Sang G. , Misra Nipun , Pan Heng , Grigoropoulos Costas P. , Kladias Nick , Panides Elias , Domoto Gerald A.
Published: October 2008

Short pulsed laser induced single acoustic wave generation, propagation, interaction with rigid structures, and focusing in water are experimentally and numerically studied. A large area short duration single plane acoustic wave was generated by the thermoelastic interaction of a homogenized nanosecond pulsed laser beam with a liquid-solid interface and propagated at the speed of sound in water. Laser flash schlieren photography was used to visualize the transient interaction of the plane acoustic wave with various submerged rigid structures [(a) a single block, (b) double blocks, (c) 33° tilted single block, and (d) concave cylindrical acoustic lens configurations]. Excellent agreement between the experimental results and numerical simulation is observed. Our simulation results demonstrate that the laser induced planar acoustic wave can be focused down to several tens of micron size and several bars in pressure.
Hydrodynamic interactions in dissipative particle dynamics
Authors: Li Zhigang , Drazer German
Published: October 2008

Dissipative particle dynamics (DPD) has recently attracted great interest due to its potential to simulate the dynamics of colloidal particles in fluidic devices. In this work, we explore the validity of DPD to reproduce the hydrodynamic interaction between a suspended particle and confining solid walls. We first show that a relatively large Schmidt number of the DPD fluid can be obtained by increasing the ratio between the strength of the dissipative force and the kinetic energy of the particles. We then measure the mobility and diffusion coefficient of the colloidal particles and show good agreement with the predicted results. We then focus on the particle-solid interactions and measure the force on a colloidal particle moving both parallel and perpendicular to two parallel walls. In both cases we found good agreement with the theoretical predictions based on Stokes flows for separations as small as one-tenth of the particle radius.
Magnetotransport properties of an individual single-crystalline Bi nanowire grown by a stress induced method
Authors: Shim Wooyoung , Kim Dohun , Lee Kyoung-il , Jeon Kye Jin , Ham Jinhee , Chang Joonyeon , Han Suk-Hee , Jeung Won Young , Johnson Mark , Lee Wooyoung
Published: October 2008

The magnetotransport properties of an individual crystalline Bi nanowire have been investigated in the range of 2–300 K using four-point measurements. I-V measurements show that the contacts were Ohmic at both 2 and 300 K, corresponding to resistivities of 1.0×10−4 and 8.2×10−5 Ω cm, respectively. The transverse magnetoresistance (MR) (2496% at 110 K) and longitudinal MR (−38% at 2 K) for the Bi nanowire were found to be larger than any values reported in the literature, demonstrating that the Bi nanowires grown by a stress induced method are high-quality single crystalline. The observed transverse and longitudinal MR behaviors in the Bi nanowire are consistent with variations in carrier concentrations as well as electronic structures, such as Fermi level and band overlap, based on simple two band model.
Laser-induced light emission from carbon nanoparticles
Authors: Osswald S. , Behler K. , Gogotsi Y.
Published: October 2008

Strong absorption of light in a broad wavelength range and poor thermal conductance between particles of carbon nanomaterials, such as nanotubes, onions, nanodiamond, and carbon black, lead to strong thermal emission (blackbody radiation) upon laser excitation, even at a very low (milliwatts) power. The lasers commonly used during Raman spectroscopy characterization of carbon can cause sample heating to very high temperatures. While conventional thermometry is difficult in the case of nanomaterials, Raman spectral features, such as the G band of graphitic carbon and thermal emission spectra were used to estimate the temperature during light emission that led to extensive graphitization and evaporation of carbon nanomaterials, indicating local temperatures exceeding 3500 °C.
Optical properties of Si- and Mg-doped gallium nitride nanowires grown by plasma-assisted molecular beam epitaxy
Authors: Furtmayr Florian , Vielemeyer Martin , Stutzmann Martin , Laufer Andreas , Meyer Bruno K. , Eickhoff Martin
Published: October 2008

The optical properties of GaN nanowires grown by catalyst free plasma-assisted molecular beam epitaxy on Si (111) are investigated by photoluminescence (PL) spectroscopy. The influence of the Si- and Mg-flux as well as the III-V ratio during growth on the PL properties is discussed. The Mg concentration as determined by secondary ion mass spectroscopy ranges from 5×1018 to 1×1020 cm−3. Raman scattering reveals that the nanowires are strain-free, irrespective of Si- or Mg-doping. The near band-edge emission of undoped or slightly Si-doped material is dominated by the narrow D0X recombination at 3.4715 eV with a full width at half maximum of 1.5 meV at 4 K. For high Si-fluxes, a blueshift of the D0X peak by 1 meV is found, which is attributed to band-filling effects. For moderate Mg-fluxes the acceptor-bound exciton recombination was detected at 3.4665 eV. Point defects due to the N-rich growth conditions are discussed as the origin of the emission band at 3.45 eV. Recombination at coalescence boundaries were identified as the origin of an emission band at 3.21 eV. The luminescence properties below 3.27 eV in highly Mg-doped samples are shown to be affected by the presence of cubic inclusions in the otherwise wurtzite nanowires.
The glass transition and the Jarzynski equality
Authors: Williams Stephen R. , Searles Debra J. , Evans Denis J.
Published: October 2008

A simple model featuring a double well potential is used to represent a liquid that is quenched from an ergodic state into a history-dependent glassy state. Issues surrounding the application of the Jarzynski equality to glass formation are investigated. We demonstrate that the Jarzynski equality gives the free energy difference between the initial state and the state we would obtain if the glass relaxed to true thermodynamic equilibrium. We derive new variations of the Jarzynski equality which are relevant to the history-dependent glassy state rather than the underlying equilibrium state. It is shown how to compute the free energy differences for the nonequilibrium history-dependent glassy state such that it remains consistent with the standard expression for the entropy and with the second law inequality.
Room temperature ferromagnetism in Mn-doped CdS nanorods
Authors: Delikanli Savas , He Shuli , Qin Yueling , Zhang Peihong , Zeng Hao , Zhang Hongwang , Swihart Mark
Published: October 2008

Mn-doped CdS nanorods synthesized by solution phase chemistry demonstrate robust ferromagnetic properties at and above room temperature. The nanorods show large coercivity, possibly originating from the shape anisotropy. The hysteresis measurements reveal strong temperature dependence. Possible origin of the ferromagnetism is discussed.
Millimeter-wave spectroscopy of CoNO Produced by UV laser photolysis of Co(CO)3NO
Authors: Sakamoto Ai , Hayashi Masato , Harada Kensuke , Tanaka Takehiko , Tanaka Keiichi
Published: October 2008

The rotational spectrum of cobalt mononitrosyl (CoNO) produced by ultraviolet photolysis of Co(CO)3NO was observed in the millimeter-wave region. Seven rotational transitions in the ground state ranging from J=6−5 to 12−11, with hyperfine splittings due to the Co nucleus (I=7/2), were detected in a supersonic jet environment, while higher-frequency transitions in the range from J=29−28 to 35−34 were measured in the ground, ν1, ν2, ν3, and 2ν2 vibrational states using a free-space absorption cell. It was confirmed from the observed spectral pattern that the CoNO molecule has a linear structure with the electronic ground state of Σ1+ symmetry. The rotational lines in the 2ν2(Σ) and ν3 states were observed to be perturbed by Fermi resonance. The equilibrium rotational constant Be is determined to be 4682.207(15) MHz. The CoN bond length is derived to be 1.5842 Å assuming the NO bond length of 1.1823 Å. A large nuclear spin-rotation interaction constant, CI=123.8(11) kHz, was determined, suggesting a Π1 electronic excited state lying close to the ground state.
A family of model Kohn–Sham potentials for exact exchange
Authors: Staroverov Viktor N.
Published: October 2008

The exact-exchange Kohn–Sham potential is partitioned into Slater’s averaged exchange charge potential and a correction. A family of nonempirical approximations to the correction term is proposed based on the known second-order gradient expansion of the exact potential. By taking the uniform electron gas limit of the correction term and using alternative definitions of the average relative electron momentum that are motivated by analysis of the Negele–Vautherin density matrix expansion, we recover the “modified Slater potential” of Harbola and Sen and the much more accurate Becke–Johnson approximation [J. Chem. Phys. 124, 221101 (2006)]. Inclusion of an explicit gradient-dependent term in the Becke–Johnson model yields an even more realistic approximation, as demonstrated by comparing the shapes of these potentials and integrated exchange energies for a series of atoms.
Nonadditive interactions in ns2 and spin-polarized ns metal atom trimers
Authors: Kłos Jacek , Żuchowski Piotr S. , Rajchel Łukasz , Chałasiński Grzegorz , Szczęśniak Małgorzata M.
Published: October 2008

The origins of nonadditivity in the following groups of metal trimers are examined: alkali earth metals of the IIA group (Be, Mg, and Ca), Zn as a transition metal analog of this group, spin-polarized alkali metals from IA group (Li, Na, K), and the spin-polarized Cu as its transition metal analog. The nonadditive interactions in these trimers are analyzed using the following hierarchy of approximations: the Heitler–London, self-consistent field (SCF), and correlated levels of theory. The exchange nonadditivity, which is included at the Heitler–London level, constitutes a bulk of nonadditive interactions in these systems in their equilibrium structures. The SCF treatment reveals some unphysical characteristics. At the post-SCF levels of theory the multireference character of the wave function increases from atom to dimer to trimer. The role of configurations involving excitations ns-np increases in this sequence and it is the genuine nonadditive effect. There is also a dramatic change in the characteristics of the excited states upon formation of clusters. We use the parameters of these excited states to predict which complexes are bound by the unusually strong nonadditive interactions and which are not.
Spectra of atomic sulfur D1 in transitions to autoionizing Rydberg states in the region of 75 800–89 500 cm−1
Authors: Pan Wan-Chun , Chen I-Chia , Huang Tzu-Ping , Yuh Jih-Young , Lee Yin-Yu
Published: October 2008

We recorded photoionization spectra of sulfur atoms in transitions from state D1 in the range of 75 800–89 500 cm−1. Dissociation of CS2 after photolysis at 193 nm produced these sulfur atoms in a singlet excited state; they were then ionized with synchrotron radiation (NSRRC, beamline U9CGM) at resolution of up to 3 cm−1 and detected with a quadruple mass filter. Rydberg series 3s23p3(D23/20)nd[3/2] and 3s23p3(D25/20)ns[5/2] with n extending to 16 and 32, respectively, to limit D20 are assigned. New Rydberg series 3s23p3(D23/20)nd[1/2]1, (D25/20)nd[5/2], and (D23/20)nd[5/2] with n from 5–9 for the former two series and 7–13 for the latter are assigned. A new Rydberg line at 85 335 cm−1 is assigned to 3s23p3(D23/20)6d P1.
Deep/shallow levels in arsenic-doped HgCdTe determined by modulated photoluminescence spectra
Authors: Yue Fangyu , Wu Jun , Chu Junhao
Published: October 2008

The modulated photoluminescence spectra have been performed on as-grown and in situ annealed arsenic-doped Hg1−xCdxTe grown by molecular beam epitaxy. Besides the discussions about shallow levels including VHg, AsHg, VHg–AsHg complex, and TeHg, two deep levels have been observed in as-grown with an ionization energy of ∼77.0 and ∼95.0 meV, respectively, which can be completely eliminated by annealing and temporarily ascribed to As-related clusters or interstitials.
Modeling molecular and ionic absolute solvation free energies with quasichemical theory bounds
Authors: Rogers David M. , Beck Thomas L.
Published: October 2008

A recently developed statistical mechanical quasichemical theory (QCT) has led to significant insights into solvation phenomena for both hydrophilic and hydrophobic solutes. The QCT exactly partitions solvation free energies into three components: (1) Inner-shell chemical, (2) outer-shell packing, and (3) outer-shell long-ranged contributions. In this paper, we discuss efficient methods for computing each of the three parts of the free energy. A Bayesian estimation approach is developed to compute the inner-shell chemical and outer-shell packing contributions. We derive upper and lower bounds on the outer-shell long-ranged portion of the free energy by expressing this component in two equivalent ways. Local, high-energy contacts between the solute and solvent are eliminated by spatial conditioning in this free energy piece, leading to near-Gaussian distributions of solute-solvent interaction energies. Thus, the average of the two mean-field bounds yields an accurate and efficient free energy estimate. Aqueous solvation free energy results are presented for several solutes, including methane, perfluoromethane, water, and sodium and chloride ions. The results demonstrate the accuracy and efficiency of the methods. The approach should prove useful in computing solvation free energies in inhomogeneous, restricted environments.
Robustness and dissipation of mitogen-activated protein kinases signal transduction network: Underlying funneled landscape against stochastic fluctuations
Authors: Wang Jin , Zhang Kun , Wang Erkwang
Published: October 2008

We uncovered the underlying energy landscape of the mitogen-activated protein kinases signal transduction cellular network by exploring the statistical natures of the Brownian dynamical trajectories. We introduce a dimensionless quantity: The robustness ratio of energy gap versus local roughness to measure the global topography of the underlying landscape. A high robustness ratio implies funneled landscape. The landscape is quite robust against environmental fluctuations and variants of the intrinsic chemical reaction rates. As the environmental fluctuations or the variances of the inherent chemical reaction rates increase further more, the landscape becomes less robust and more flatter. We also show that more robust network has less dissipation costs. Our approach is quite general and can be applied to other cellular networks.
Quantum Drude friction for time-dependent density functional theory
Authors: Neuhauser Daniel , Lopata Kenneth
Published: October 2008

Friction is a desired property in quantum dynamics as it allows for localization, prevents backscattering, and is essential in the description of multistage transfer. Practical approaches for friction generally involve memory functionals or interactions with system baths. Here, we start by requiring that a friction term will always reduce the energy of the system; we show that this is automatically true once the Hamiltonian is augmented by a term of the form ∫a(q;n0)[∂j(q,t)∕∂t]⋅J(q)dq, which includes the current operator times the derivative of its expectation value with respect to time, times a local coefficient; the local coefficient will be fitted to experiment, to more sophisticated theories of electron-electron interaction and interaction with nuclear vibrations and the nuclear background, or alternately, will be artificially constructed to prevent backscattering of energy. We relate this term to previous results and to optimal control studies, and generalize it to further operators, i.e., any operator of the form ∫a(q;n0)[∂c(q,t)∕∂t]⋅C(q)dq (or a discrete sum) will yield friction. Simulations of a small jellium cluster, both in the linear and highly nonlinear excitation regime, demonstrate that the friction always reduces energy. The energy damping is essentially double exponential; the long-time decay is almost an order of magnitude slower than the rapid short-time decay. The friction term stabilizes the propagation (split-operator propagator here), therefore increasing the time-step needed for convergence, i.e., reducing the overall computational cost. The local friction also allows the simulation of a metal cluster in a uniform jellium as the energy loss in the excitation due to the underlying corrugation is accounted for by the friction. We also relate the friction to models of coupling to damped harmonic oscillators, which can be used for a more sophisticated description of the coupling, and to memory functionals. Our results open the way to very simple finite grid description of scattering and multistage conductance using time-dependent density functional theory away from the linear regime, just as absorbing potentials and self-energies are useful for noninteracting systems and leads.
Re-entrant phase diagram and pH effects in cross-linked gelatin gels
Authors: Abete T. , Del Gado E. , de Arcangelis L. , Serughetti D. Hellio , Djabourov M.
Published: October 2008

Experimental results have shown that the kinetics of bond formation in chemical cross-linking of gelatin solutions are strongly affected not only by gelatin and reactant concentrations but also by the solution pH. We present an extended numerical investigation of the phase diagram and of the kinetics of bond formation as a function of the pH, via Monte Carlo simulations of a lattice model for gelatin chains and reactant agent in solution. We find a re-entrant phase diagram, namely, gelation can be hindered either by loop formation at low reactant concentrations, or by saturation of chain active sites via formation of single bonds with a cross-linker at high reactant concentrations. The ratio of the characteristic times for the formation of the first and the second bond between the cross-linker and an active site of a chain is found to depend on the reactant reactivity, in good agreement with experimental data.
Two-phase magnetoelectric nanopowder/polyurethane composites
Authors: Guyomar Daniel , Guiffard Benoit , Belouadah Rabah , Petit Lionel
Published: October 2008

This study deals with the observation of magnetoelectric (ME) effect in nanocomposite films constituted of semicrystalline polyurethane matrix filled with magnetic Fe3O4 (hard) or Ni (soft) nanoparticles. The measurement of the magnetic field-induced ground current of the so-called particulate composites enabled the precise determination of true ME current after extraction of the part corresponding to the inductively coupled (loop) current. Experimental ME current could be successfully simulated considering coexistence of both true linear and quadratic ME effects and by taking into account the nonlinear variation of applied dc bias and ac field magnitude due to the magnetization saturation of the magnetic field generator. Although linear ME coefficients of particulate composites are lower than those of laminate composites, they are of the same order of magnitude than that of reference ME material Cr2O3 (up to 18 mV/cm Oe). Besides, nanocomposites are simple to prepare, flexible, easily integrable, and sensitive to magnetic field anomaly at room temperature. At last, they present a linear ME coupling apparently not triggered by magnetostrictive phenomenon.
Negative charge and charging dynamics in Al2O3 films on Si characterized by second-harmonic generation
Authors: Gielis J. J. H. , Hoex B. , van de Sanden M. C. M. , Kessels W. M. M.
Published: October 2008

Thin films of Al2O3 synthesized by atomic layer deposition provide an excellent level of interface passivation of crystalline silicon (c-Si) after a postdeposition anneal. The Al2O3 passivation mechanism has been elucidated by contactless characterization of c-Si/Al2O3 interfaces by optical second-harmonic generation (SHG). SHG has revealed a negative fixed charge density in as-deposited Al2O3 on the order of 1011 cm−2 that increased to 1012–1013 cm−2 upon anneal, causing effective field-effect passivation. In addition, multiple photon induced charge trapping dynamics suggest a reduction in recombination channels after anneal and indicate a c-Si/Al2O3 conduction band offset of 2.02±0.04 eV.
Enhanced thermal stability of carbon nanotubes by plasma surface modification in Al2O3 composites
Authors: Cho Hoonsung , Shi Donglu , Guo Yan , Lian Jie , Ren Zhifeng , Poudel Bed , Song Yi , Abot Jandro L. , Singh Dileep , Routbort Jules , Wang Lumin , Ewing Rodney C.
Published: October 2008

A plasma polymerization method was employed to deposit an ultrathin pyrrole film of 3 nm onto the surfaces of single wall carbon nanotubes (SWCNTs) and Al2O3 nanoparticles for developing high-strength nanocomposites. The surfaces of plasma coated SWCNTs and Al2O3 nanoparticles were studied by high resolution transmission electron microscopy (TEM) and time-of-flight secondary ion mass spectroscopy. After sintering the SWCNTs-Al2O3 composites at different temperatures (maximum of 1200 °C), the thermal stability of plasma-coated SWCNTs was significantly increased, compared to their uncoated counterparts. After hot-press sintering, the SWCNTs without plasma coating were essentially decomposed into amorphous clusters in the composites, leading to degraded mechanical properties. However, under the same sintering conditions, the plasma surface modified SWCNTs were well preserved and distributed in the composite matrices. The effects of plasma surface coating on the thermal stability of SWCNTs and mechanical behavior of the nanocomposites are discussed.
Evidence of compositional inhomogeneity in InxGa1−xN alloys using ultraviolet and visible Raman spectroscopy.
Authors: Kar Ayan , Alexson Dimitri , Dutta Mitra , Stroscio Michael. A.
Published: October 2008

In this paper we report a study of phase separation in bulk InxGa1−xN films grown by metal organic chemical-vapor deposition using mid-UV Raman spectroscopy. Evidence of phase separation is observed by the occurrence of low frequency shoulders identified as minority phase in the A1(LO) Raman mode. A phase transition in the alloy from the metastable to unstable region was found to be occurring at an indium concentration of about 25%. Raman spectroscopic results also indicate that the compositional inhomogenity in our samples increase, as would be expected, with depth in the film. A direct correspondence is also found between the percentage of indium concentration in the film and the amount of compositional inhomogenity.
Electrical measurements of a VO2 thin film under high pressure of 25 GPa generated by a load-controllable point-contact structure
Authors: Sakai Joe
Published: October 2008

Utilizing a load-controllable point-contact structure with a steel probe as a high-pressure generator, pressure effects on electrical properties were investigated on a VO2 thin film grown on a metallic Ti substrate. Through comparisons at 300 and 348 K of the resistance of a VO2/Ti sample under the probe and the resistivity of VO2 under hydrostatic pressure that was previously reported, the pressure corresponding to a probe load of 25 gf was estimated to be as high as 25.5 GPa, and the contact area was supposed to be unchanged up to this probe load. The combination of a point-contact system and a film deposited on a metal substrate has enabled the observation of a switching induced by an electric field perpendicular to the plane of a VO2 film. The transition voltage was drastically suppressed by an application of about 10 GPa with the probe. The load-controllable point-contact system was demonstrated to be a simple but usable tool for studying the physical properties of thin films under high pressures.
First-principles study of energetic and electronic properties of A2Ti2O7 (A=Sm, Gd, Er) pyrochlore
Authors: Xiao H. Y. , Zu X. T. , Gao Fei , Weber W. J.
Published: October 2008

First-principles calculations have been carried out to study the electronic properties of A2Ti2O7 (A=Sm, Gd, Er) pyrochlores. It was found that f electrons have negligible effect on the structural and energetic properties, but have significant effect on the electronic properties. Density of state analysis shows that A-site 4f electrons do take part in the chemical bonding. Also, we found that ⟨Gd-O48f⟩ bond is less covalent than ⟨Sm-O48f⟩ and ⟨Er-O48f⟩ bonds, while ⟨Ti-O48f⟩ bond in Gd2Ti2O7 is more covalent. It was proposed that for A2Ti2O7 (A=Sm, Gd, Er) pyrochlores, ⟨Ti-O48f⟩ bonds may play more significant role in determining their radiation resistance to amorphization.
Impact of incorporated Al on the TiN/HfO2 interface effective work function
Authors: Xiong Ka , Robertson John , Pourtois Geoffrey , Pétry Jasmine , Müller Markus
Published: October 2008

First principles calculations of the impact of Al incorporation on the effective work function of a TiN/HfO2 interface are presented. The undoped interface has a midgap effective work function. We find that Al in the metal and Al substituting for O in the dielectric make the effective work function more n-type. More importantly, Al substituting for Hf in the oxide near the interface—the energetically stable position for most growth conditions—increases the effective work function, making it more p-type. Furthermore, the shift of the work function increases with increasing the Al concentration at the interface. The calculated results are consistent with experimental data.
X-ray diffraction study of the damage induced in yttria-stabilized zirconia by swift heavy ion irradiations
Authors: Costantini Jean-Marc , Guillet François , Lambert Sébastien , Grébille Dominique , Beuneu François , Trautmann Christina
Published: October 2008

The lattice damage was investigated by x-ray diffraction techniques in yttria-stabilized zirconia single crystals with the (100) or (110) orientation upon irradiation with swift heavy ions (from 100-MeV C to 2.6-GeV U) in a broad electronic stopping power range (from about 0.3 to 48 keV nm−1). The θ-2θ scans show that no amorphization or change to a new crystalline phase occurs regardless of the ion and crystal features. However, the rocking curves (ω scans) and reciprocal space mappings show evidence of the mosaicity of the crystals, which is produced above a threshold electronic stopping power between 18 and 27 keV nm−1. This threshold is in agreement with our previous Rutherford backscattering spectroscopy/channeling spectroscopy data. Two kinds of damage phenomena are found: (i) nuclear-collision induced clusters of point defects which generate Bragg peak shifts and broadening in the 2θ-ω and θ-2θ scans, and (ii) electronic-excitation induced lattice damage yielding broad peaks in the ω scans above the stopping power threshold at high fluences.
A route to tunable direct band-gap diamond devices: Electronic structures of nanodiamond crystals
Authors: Sasagawa Takao , Shen Zhi-xun
Published: October 2008

Electronic structures of nanodiamond crystals are elucidated by means of first principles calculations. It is found that the lowest unoccupied molecular orbital of diamond molecules forms the conduction band with a parabolic dispersion centered at the momentum origin. This results in a direct band gap in the nanodiamond crystal, which contrasts sharply with the indirect gap in bulk diamond. The corresponding light absorption/emission energy can be tuned within the ultraviolet wavelength range by choosing different nanodiamonds, which provides significant potential for their optoelectronic applications.
Carbon nanofiber growth in plasma-enhanced chemical vapor deposition
Authors: Denysenko I. , Ostrikov K. , Cvelbar U. , Mozetic M. , Azarenkov N. A.
Published: October 2008

A theoretical model to describe the plasma-assisted growth of carbon nanofibers (CNFs) is proposed. Using the model, the plasma-related effects on the nanofiber growth parameters, such as the growth rate due to surface and bulk diffusion, the effective carbon flux to the catalyst surface, the characteristic residence time and diffusion length of carbon atoms on the catalyst surface, and the surface coverages, have been studied. The dependence of these parameters on the catalyst surface temperature and ion and etching gas fluxes to the catalyst surface is quantified. The optimum conditions under which a low-temperature plasma environment can benefit the CNF growth are formulated. These results are in good agreement with the available experimental data on CNF growth and can be used for optimizing synthesis of related nanoassemblies in low-temperature plasma-assisted nanofabrication.
Effect of strained-Si layer thickness on dislocation distribution and SiGe relaxation in strained-Si/SiGe heterostructures
Authors: Lu Jinggang , Rozgonyi George , Seacrist Mike , Chaumont Michelle , Campion Alan
Published: October 2008

Eight strained-Si (sSi) on SiGe heterostructures with 8, 13, 25, or 40 nm sSi on top of 300 or 600 nm Si0.77Ge0.23 buffer have been prepared by chemical vapor deposition and examined by preferential defect etching and Raman spectroscopy. Defect etching revealed that threading dislocations (TDs) in the supercritical thickness sSi samples are more evenly distributed, while they are severely trapped inside TD pileups in the subcritical thickness sSi samples. It is proposed that relaxation of the supercritical sSi layer, which is realized by TD gliding under a tensile stress, helps to breakup those pileups formed under a compressive stress. Defect etching revealed a TD density of (3–5)×106 cm−2, and no dependence on the sSi or SiGe thickness was observed. Raman spectroscopy revealed that the relaxation degree of the 300 nm SiGe layer decreases from 80% to 67% with the sSi layer increasing from 8 to 40 nm. This suggests a continuous relaxation of the highly compressively strained, thin SiGe buffer during or even after sSi growth, and its gradual suppression by the presence of a tensile strained sSi layer. The 600 nm SiGe buffer has an ∼82% relaxation for all sSi thickness, suggesting that its relatively small residual strain cannot support any further relaxation after switching to sSi growth and consequently the absence of any dependence on sSi thickness. Based on these observations, we suggest that an in situ thermal annealing prior to the sSi growth will help to enhance the strain relaxation of thin SiGe buffers.
Simultaneous ultrasonic and synchrotron x-ray studies on pressure induced α-ω phase transition in zirconium
Authors: Liu Wei , Li Baosheng , Wang Liping , Zhang Jianzhong , Zhao Yusheng
Published: October 2008

Compressional and shear wave velocities as well as unit-cell volumes across the α-ω transition of Zr have been measured up to 10.5 GPa at room temperature using ultrasonic interferometry in conjunction with synchrotron x radiation. In the pressure range of 5.0 to 6.2 GPa, a mixture of α and ω phase is clearly identified and the α phase is completely transformed to ω phase at 6.8 GPa. The measured velocity jumps across the α-ω transition are 9.6% and 13.1% for P and S waves, respectively. Negative pressure dependence for the shear modulus of α-Zr has been found at pressures from 1.4 to 2.9 GPa, which may be ascribed to the shear instability of the α phase prior to transition. According to velocity data, the onset pressure of the α-ω transition is determined to be ∼4.0 GPa.
Transverse magnetic field polarity effects on the terahertz radiation from GaAs/AlGaAs modulation-doped heterostructures with varying AlGaAs spacer-layer thickness
Authors: Estacio Elmer , Sarukura Nobuhiko , Ponseca Carlito , Somintac Armando , Bailon-Somintac Michelle , Garcia Alipio , Salvador Arnel
Published: October 2008

We report the effect of changing the polarity of a magnetic field parallel to the surface plane of GaAs/AlGaAs modulation-doped heterostructures (MDHs) with various spacer thicknesses on the terahertz radiation power and its spectral characteristics. Results show that flipping the direction of the transverse 1 T magnetic field modifies the extent of field-induced terahertz radiation enhancement. The observations are analyzed in the context of junction electric field strength, carrier confinement and mobility, and the AlGaAs/GaAs interface roughness. This terahertz method may prove as an efficient tool to qualitatively evaluate the characteristics of MDH layers.
Henkel analysis of Fe/Er multilayers prepared by electron-beam evaporation
Authors: Geng K. W. , Pan F. , Yao R. H.
Published: October 2008

Fe/Er multilayers were prepared by high-vacuum electron-beam evaporation system and their microstructures have been investigated. Henkel analysis was performed via isothermal remanent magnetization curve and direct current demagnetization curve. Magnetic measurement indicates that Fe/Er multilayers have an easy axis in film plane and a small value of positive magnetoresistance. It is showed that the Er layer will change to an amorphous structure when its thickness is below 3.5 nm. Hence, Δm plots were established according to Henkel analysis data. The results show that Δm plot has a strong dependence on the thickness of Fe and Er layers. When the thickness of Fe layer remained unchanged, Δm plot indicated that the magnetic coupling was oscillating with the layer thickness; and when the thickness of Er layer was 8.8 nm, Δm plot showed a positive-negative overturn of magnetic interaction, which revealed the interplay between the demagnetization and exchange coupling effect in multilayer. The explanation of Δm plot was also discussed via a simplified model, i.e., a competition between demagnetizing interaction and magnetizing interaction.
Polar domain fluctuations in doped liquid nitrobenzene
Authors: Shelton David P.
Published: October 2008

Improved spectral measurements of the narrow spike in the high resolution hyper-Rayleigh light scattering spectrum for deuterated liquid nitrobenzene (C6D5NO2) doped with triflic acid (CF3SO3H) determined that the reorientation time for the dopant-induced polar domains is 2.9 μs in the dilute limit. Two models based on ions dissolved in nitrobenzene fit the measured spectral broadening function.
Phase-space averaging and natural branching of nuclear paths for nonadiabatic electron wavepacket dynamics
Authors: Yonehara Takehiro , Takatsuka Kazuo
Published: October 2008

We propose a simple and tractable method to treat quantum electron wavepacket dynamics that nonadiabatically couples with “classical” nuclear motions in mixed quantum-classical representation. The electron wavepacket is propagated inducing electronic-state mixing along our proposed paths. It has been shown in our previous studies that classical force working on nuclei in a nonadiabatic region is represented in a matrix form (called the force matrix), and the solutions of the Hamilton canonical equations of motion for nuclei based on this force matrix give rise to a cascade of infinitely many branching paths when solved simultaneously with electronic-state mixing. As a tractable approximation to these rigorous solutions, we here devise a method to provide much simpler nonadiabatic paths: (i) extract one or a few number of representative paths by taking an average over the paths in phase space (not averaging over the forces) that should be otherwise undergo the fine branching. (ii) After the nonadiabatic coupling becomes sufficiently small, let these paths naturally branch by running them with their own individual eigenforces (the eigenvalues of the force matrix). Since the eigenforces coincide with the forces of adiabatic potential energy surfaces in the limit of zero nonadiabatic coupling, these branching paths eventually run on one of possible adiabatic potential energy surfaces, converging to a classical path (Born–Oppenheimer path). The paths thus created are theoretically satisfactory in that they realize the coherent mixing of electronic states in the manner of quantum entanglement and yet eventually become consistent with the Born–Oppenheimer classical trajectories. We test the present method numerically with the use of two- and three-state systems that are extracted from ab initio calculations for the excited states of LiH molecule.
Tendency toward crossover of the effective susceptibility exponent from its doubled Ising value to its doubled mean-field value near a double critical point
Authors: Pradeep U. K.
Published: October 2008

The critical behavior of osmotic susceptibility in an aqueous electrolyte mixture 1-propanol (1P)+water (W)+potassium chloride is reported. This mixture exhibits re-entrant phase transitions and has a nearly parabolic critical line with its apex representing a double critical point (DCP). The behavior of the susceptibility exponent is deduced from static light-scattering measurements, on approaching the lower critical solution temperatures (TL’s) along different experimental paths (by varying t) in the one-phase region. The light-scattering data analysis substantiates the existence of a nonmonotonic crossover behavior of the susceptibility exponent in this mixture. For the TL far away from the DCP, the effective susceptibility exponent γeff as a function of t displays a nonmonotonic crossover from its single limit three-dimensional (3D)-Ising value (∼1.24) toward its mean-field value with increase in t. While for that closest to the DCP, γeff displays a sharp, nonmonotonic crossover from its nearly doubled 3D-Ising value toward its nearly doubled mean-field value with increase in t. The renormalized Ising regime extends over a relatively larger t range for the TL closest to the DCP, and a trend toward shrinkage in the renormalized Ising regime is observed as TL shifts away from the DCP. Nevertheless, the crossover to the mean-field limit extends well beyond t>10−2 for the TL’s studied. The observed crossover behavior is attributed to the presence of strong ion-induced clustering in this mixture, as revealed by various structure probing techniques. As far as the critical behavior in complex or associating mixtures with special critical points (like the DCP) is concerned, our results indicate that the influence of the DCP on the critical behavior must be taken into account not only on the renormalization of the critical exponent but also on the range of the Ising regime, which can shrink with decrease in the influence of the DCP and with the extent of structuring in the system. The utility of the field variable tUL in analyzing re-entrant phase transitions is demonstrated. The effective susceptibility exponent as a function of tUL displays a nonmonotonic crossover from its asymptotic 3D-Ising value toward a value slightly lower than its nonasymptotic mean-field value of 1. This behavior in the nonasymptotic, high tUL region is interpreted in terms of the possibility of a nonmonotonic crossover to the mean-field value from lower values, as foreseen earlier in micellar systems.
A computational analysis of the interaction of lattice and intramolecular vibrational modes in crystalline α-RDX
Authors: Boyd Sylke G. , Boyd Kevin J.
Published: October 2008

The vibrational spectrum of a computer model of crystalline RDX was studied using a 216-molecule periodic supercell, allowing for intra- and intermolecular degrees of freedom using the force field by Boyd et al. [J. Chem. Phys. 124, 104508 (2006)]. The normal modes were analyzed with regard to their activity involving molecule center-of-mass translations and rotations, as well as 15 intramolecular degrees of freedom, including bond stretches, bend and dihedral angle variations, and out-of-plane motions of the nitro groups. We correlate center-of-mass motions with the occupation of internal degrees of freedom for all of the normal modes in the model with particular attention to correlations between nitro rotations and lattice modes. Transfer of lattice energy to internal degrees of freedom can occur through doorway modes and is significant for the initiation of detonation. Several clusters of potential doorway modes are found which involve significant lattice motion as well as nitro rotations. Such groups of modes have been found in the ranges between 186 and 220 and between 420 and 434cm−1. Symmetry properties and details of the involved molecular motions are described.
Real time analysis of self-assembled InAs/GaAs quantum dot growth by probing reflection high-energy electron diffraction chevron image
Authors: Kudo Takuya , Inoue Tomoya , Kita Takashi , Wada Osamu
Published: October 2008

Self-assembling process of InAs/GaAs quantum dots has been investigated by analyzing reflection high-energy electron diffraction chevron images reflecting the crystal facet structure surrounding the island. The chevron image shows dramatic changes during the island formation. From the temporal evolution of the chevron tail structure, the self-assembling process has been found to consist of four steps. The initial islands do not show distinct facet structures. Then, the island surface is covered by high-index facets, and this is followed by the formation of stable low-index facets. Finally, the flow of In atoms from the islands occurs, which contributes to flatten the wetting layer. Furthermore, we have investigated the island shape evolution during the GaAs capping layer growth by using the same real-time analysis technique.
Refractive index of Si-doped n-InGaAs
Authors: Gozu Shin-ichiro , Mozume Teruo , Ishikawa Hiroshi
Published: October 2008

The dependence of the refractive index of Si-doped n-InGaAs on carrier density was investigated. The shift in the refractive index from that of undoped InGaAs was found to be mainly caused by the band-filling effect, the band-gap shrinkage effect, and the plasma effect. Model calculations agreed better with the measured data when the conduction band nonparabolicity effect was taken into account. Nevertheless, the calculated refractive index values were slightly lower than the experimental values. Because the difference was larger for photon energies close to the band-gap energy of InGaAs, the shift in the refractive index is attributed to the absorption tail effect that appears near the band-gap energy due to Si doping.
Faraday effect and magnetic susceptibility analyses in TbAlO3
Authors: Valiev Uygun V. , Uzokov Abdulla A. , Rakhimov Sharof A. , Gruber John B. , Nash Kelly L. , Sardar Dhiraj K. , Burdick Gary W.
Published: October 2008

The magnetic susceptibility χ and the Verdet constant V obtained along the crystallographic axis [110] of the rhombic TbAlO3 crystal have been investigated experimentally and theoretically in the temperature range between 78 and 300 K. We find that the Verdet constant V is not a linear function based on the magnetic susceptibility measured along the [110] axis over the same temperature range. It is shown that the temperature dependence of the Verdet constant measured along the [110] axis is associated with the absence of a contribution to the Faraday rotation of the Van Vleck “mixing” of the electronic states of the rare-earth ion Tb3+ that takes place in the presence of an external magnetic field.