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The dynamic motions of ion-acoustic (IA) waves in plasma are studied by considering the Kappa-Cairns electron distribution. The effect of external periodic force is also considered, here. Using the reductive perturbation technique, a Korteweg-de Vries (K-dV) equation is obtained. The quasiperiodic motions of IA waves are investigated by considering two-dimensional phase portraits and time-series analysis. The effects of non-thermal parameters (i.e., κ, α) and the strength (f0) of the external periodic force on nonlinear travelling wave structures are also discussed. [ABSTRACT FROM AUTHOR]

Local distribution and orientation of anisotropic nanoparticles in microphase-separated symmetric diblock copolymers has been simulated using dissipative particle dynamics and analyzed with a molecular theory. It has been demonstrated that nanoparticles are characterized by a non-trivial orientational ordering in the lamellar phase due to their anisotropic interactions with isotropic monomer units. In the simulations, the maximum concentration and degree of ordering are attained for nonselective nanorods near the domain boundary. In this case, the nanorods have a certain tendency to align parallel to the interface in the boundary region and perpendicular to it inside the domains. Similar orientation ordering of nanoparticles located at the lamellar interface is predicted by the molecular theory which takes into account that the nanoparticles interact with monomer units via both isotropic and anisotropic potentials. Computer simulations enable one to study the effects of the nanorod concentration, length, stiffness, and selectivity of their interactions with the copolymer components on the phase stability and orientational order of nanoparticles. If the volume fraction of the nanorods is lower than 0.1, they have no effect on the copolymer transition from the disordered state into a lamellar microstructure. Increasing nanorod concentration or nanorod length results in clustering of the nanorods and eventually leads to a macrophase separation, whereas the copolymer preserves its lamellar morphology. Segregated nanorods of length close to the width of the diblock copolymer domains are stacked side by side into smectic layers that fill the domain space. Thus, spontaneous organization and orientation of nanorods leads to a spatial modulation of anisotropic composite properties which may be important for various applications. [ABSTRACT FROM AUTHOR]

This work presents an extensive directional paleomagnetic database of the Kiaman reversed superchron. It is composed of 1,459 paleomagnetic directions from igneous rocks corresponding to 91 data sets (or paleomagnetic poles). An almost constant behavior of more concentrated and circular distributions for latitudes higher than 10° was found, which contrasts strongly with predictions of the representative models for the past few million years. We searched for simplified and spatially covariant Giant Gaussian Process (GGP) models that best explain the directional distribution of the Kiaman database. We used the mean strength based on the mean of virtual dipole moment (VDM) results for the period drawn from the available databases. Among the tested models, the one that best explains the directional paleosecular variation of the Kiaman database is the covariant type. According to this model, the correlations between the Gaussian coefficients are valid for the last 10 Myr and the Kiaman superchron. The resulting GGP models have β parameters similar to the 0–10 Ma models, which indicates that the relation between symmetric and antisymmetric families appears unchanged in the geological past. The relative variability of the Kiaman field, as inferred from the ratio αg10¯ from GGP models, is lower than for the past 10 Myr. Thus, as well as the paleointensity, αg10¯ seems to be a proxy that can be used for evaluating the geomagnetic development along the geological time. Key Points: A new database of paleomagnetic directions of igneous Kiaman sitesThe distributions of the Kiaman directions reveal low paleosecular variationThe first Giant Gaussian Process models for ages older than the last 10 million years are presented [ABSTRACT FROM AUTHOR]

The metal organic vapor deposition of MnP films on GaP (100) substrates is shown to have a substantial endotaxial component. A study of the growth time evolution of the endotaxial depths of MnP grains reveals a diffusion-controlled growth with a relatively large diffusion coefficient of Mn in GaP. The value (2.2±1.5)×10-15 (cm2/s) obtained at 650 °C is at least two orders of magnitude larger than the reported Mn diffusion in bulk GaP. GaP surface mounds provide further indirect evidence that this large diffusion coefficient is concurrent with the out-diffusion of Ga atoms at the growing MnP/GaP interface. No trace of dislocations could be observed at or near this interface, which strongly suggests that Mn diffusion occurs through vacant sites generated by the difference between the crystallographic structures of MnP and GaP. [ABSTRACT FROM AUTHOR]

The heat capacity of natural (Cu12-x (Fe, Zn, Ag)x(Sb, As)4S13) and synthetic (Cu12-xZn)xSb)4S)13 with x=0, 1, 2) tetrahedrite compounds was measured between 2K and 380 K. It was found that the temperature dependence of the heat capacity can be described using a Debye term and three Einstein oscillators with characteristic temperatures that correspond to energies of ∼1.0 meV, ∼2.8 meV, and ∼8.4 meV. The existence of localized vibrational modes, which are assigned to the displacements of the trigonally coordinated Cu atoms in the structure, is discussed in the context of anharmonicity and its effect on the low lattice thermal conductivity exhibited by these compounds. [ABSTRACT FROM AUTHOR]

The article focuses on theoretical physicist J. Robert Oppenheimer who was introduced to theoretician Paul Dirac and mentions invention of the atom bomb directed by Oppenheimer. Topics discussed include philosophies in the form of dialogue between discussing corpuscularian systems, abuses of rhetoric in learning and natural sciences, and laboratorial facilities not acquired by Royal Society.

We calculated the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between the magnetic impurities mediated by electrons in nanoribbons. It was shown that the RKKY interaction is strongly dependent on the width of the nanoribbon and the transverse positions of the impurities. The transverse confinement of electrons is responsible for the above size effect of the RKKY interaction. It provides a potential way to control the RKKY interaction by changing nanostructure geometry. [ABSTRACT FROM AUTHOR]

The current study investigates the magnetic properties of the Gdx Zn1-xO, with x=0.0625 and 0.0185, doped semiconductor using the full potential (linearized) augmented plane wave plus local orbital method. We show that in contrast to the findings of Shi et al. [J. Appl. Phys. 106, 023910 (2009)], the implementation of the Hubbard U parameter to the Gd f states favors an antiferromagnetic phase in both wurtzite GdO and Gdx Zn1-xO. Spin polarized calculations on Gdx Zn1-xO indicate that, even if a ferromagnetic ground state were favored, the magnetic influence of Gd in a perfect ZnO wurtzite lattice is highly localized and limited to the first three nearest neighboring O atoms. Increasing the supercell size and thus diluting the concentration of Gd within the ZnO matrix does not show any changes in the net magnetic moment between these three O atoms nor in the remaining lattice sites, indicating that sizing effects do not influence the range of matrix polarization. We conclude that the localized Gd induced polarization can not account for long range magnetic ordering in a defect-free ZnO wurtzite lattice. [ABSTRACT FROM AUTHOR]

The nature of electron and hole trapping in silicon ion-implanted silicon oxide (SiO2) with a dose of 1016 cm-2 were studied using photoluminescence and electron spin resonance (ESR) measurements. We observed an ESR signal with g = 2.006 after hole and electron injections. These results unambiguously imply that the Si nanoclusters created by the high-dose Si implants are both electron and hole traps in the SiO2 films. [ABSTRACT FROM AUTHOR]

In this paper, we demonstrate a phosphorescent organic light-emitting device (OLED) with enhanced current efficiency (in terms of cd/A) based on an oxadiazole (OXD) derivative as the electron-transporting host of the emitting layer (EML) doped with a phosphorescent dopant, iridium(III)bis[4,6-(di-fluorophenyl)-pyridinato-N, C2′] picolinate (FIrpic). The maximum current efficiency of OXD-based OLEDs was 13.0 cd/A. Compared to the phosphorescent OLED with a conventional hole-transporting host, 1,3-bis(carbazol-9-yl)benzene (mCP) with 11.1 cd/A in maximum current efficiency, 17.2% improvement was achieved. However, in terms of external quantum efficiency (EQE), the OXD- and mCP-based OLEDs were 4.01 and 4.66%, respectively, corresponding to a 13.9% decrease. Such a discrepancy can be understood from the electroluminescence (EL) variation. Contrary to the hole-transporting mCP, OXD exhibited electron transporting characteristics which shifted the recombination zone toward the anode. The optical interference effect result was that the relative intensity at long wavelengths (500-600 nm) was higher in the OXD-based OLED, which was more sensitive to the human eye and increased the current efficiency, even though the EQE was lower. Besides, in OXD-OLED, the recombination zone shifted toward the anode side with a high driving voltage, which was also deduced from the EL spectral variations. Under a high driving voltage, we observed the relative intensity of FIrpic emission ata longer wavelength increased which resulted from the optical interference effect, and emission from the hole-transporting layer increased. By using hole-transporting mCP and electron-transporting OXD as the hosts of double EML (DEML), the maximum current-efficiency and EQE of the optimized DEML-OLED further increases to 17.6 cd/A and 7.06%, which corresponds to improvements of 58.6% and 51.5%, compared to the single mCP-OLED, and by 35.4% and 76.1%, compared to the single OXD-OLED, respectively. This was a result of the better charge balance in DEML, and less quenching effects from transporting materials. [ABSTRACT FROM AUTHOR]

Neutron detection is an integral part of the global effort to prevent the proliferation of special nuclear material (SNM). Applications relying on neutron-detection technology range from traditional nuclear nonproliferation objectives, such as safeguarding material and verifying stockpile reductions, to the interdiction of SNM-a goal that has recently risen in priority to a level on par with traditional missions. Large multinational programs targeting interdiction and safeguards have deployed radiation-detection assets across the globe. In parallel with these deployments of commercially available technology, significant research and development has been directed toward the creation of next-generation assets. Neutron-detection technology plays a prominent role because of the capability of neutrons to penetrate materials that readily absorb gamma rays and the unique fission signatures neutrons possess. One particularly acute technology-development challenge results from dwindling supplies of 3He, partially triggered by widespread deployment of high-efficiency systems for portal monitoring. Other emerging missions, such as the desire to detect SNM at greater standoff distances, have also stimulated neutron-detection technology development. In light of these needs, this manuscript reviews the signatures of neutrons emitted by SNM, the principles of neutron detection, and various strategies under investigation for detection in the context of nuclear nonproliferation. [ABSTRACT FROM AUTHOR]

In this paper we report on the experimental demonstration of using relativistic electron diffraction and an radiofrequency deflecting cavity to capture in a single shot the entire time-history of the ultrafast laser-induced heating and melting of a single crystal gold sample. By recording the time variation in the Bragg peaks on the streak image of a 16 ps long electron beam it is possible to reconstruct with 400 fs temporal resolution the evolution of the sample structure induced by a 35 mJ/cm2 400 nm laser pump pulse. [ABSTRACT FROM AUTHOR]

Based on the linearly approximated F12 coupled cluster singles and doubles [CCSD(F12)] model, equation-of-motion [EOM-CCSD(F12)] methods for electron affinities (EAs) and excitation energies (EEs) have been formulated and implemented. Extended electron-attachment and excitation operators are introduced for balanced descriptions of both neutral and electron-attached (or excited) states. In our implementation the cusp conditions are used for the definition of extended electron-attachment (excitation) operators. It is shown that EA-EOM-CCSD(F12) provides vertical electron affinities accurate to 0.1 eV compared to those in the complete basis set limit. Numerical tests conducted on a selection of small molecules have also shown notable improvement in Rydberg excitation energies compared to valence ones by the use of correlation factors. [ABSTRACT FROM AUTHOR]

First-principles calculations are carried out to study the native defect properties in TlBr. Three important results emerge: (1) the native defects are benign in terms of electron trapping because the low-energy defects do not induce electron traps; (2) the dominant defects in nearly stoichiometric TlBr are Schottky defects that pin the Fermi level near the midgap, leading to high resistivity; and (3) the calculated low diffusion barriers for several native defects show that ionic conductivity can occur at room temperature. The important impacts of these material properties on the room-temperature radiation detection using TlBr are discussed. [ABSTRACT FROM AUTHOR]

This paper is devoted to the development and numerical modeling of a field emission cathode for low power Hall thrusters (100-300 W). Generally, Hall thrusters use hollow cathodes, which require a relatively large mass flow rate of xenon-gas to operate. For lower emission currents the cathode gas consumption is still substantial, which contributes to the drop in efficiency when operating a Hall thruster in a low power regime. Conventional field emission cathodes, which are considered as an alternative, do not provide the required low power consumption with an acceptable lifetime. In order to increase the efficiency of the field emission cathode while retaining an acceptable lifetime, an acceleration-deceleration electron optics is proposed. This system is used for the extraction of electrons from carbon nanotubes and the formation of the electron beam. Numerical modeling of the processes in the proposed cathode was carried out using a particle-in-cell approach. It has been shown that (1) it is possible to provide adequate transmission of the electron beam at the required deceleration voltage; (2) injecting xenon-gas at the outlet of the cathode for the creation of a 'plasma-bridge' between the cathode and ion beam does not lead to significant population of ions near the emitter surface. A cathode with electron current of 1 A was modeled using a particle-in-cell simulation. The cathode requires only ∼15 W of power and 0.11 mg/s of xenon mass flow rate. For the given current, these parameters are quite reasonable. [ABSTRACT FROM AUTHOR]

The formation and expansion of recombination-induced stacking faults (SFs) within 4H–SiC bipolar and unipolar devices is known to induce a drift in the forward voltage during forward bias operation. This drift renders devices unsuitable for commercial applications. While the expansion of SFs in 4H–SiC occurs by the recombination-enhanced dislocation glide mechanism, why SF expansion occurs, i.e., the energetic driving force, remains unclear. Recent experiments have revealed that SF contraction and a recovery of the forward voltage drift can be induced under many conditions, including forward bias operation. Such observations have enabled the identification of SF-related degradation in devices where imaging methods are not possible and are inconsistent with the previously reported energetic driving force models. We present a model that qualitatively explains these recent experimental observations, which is based on the quasi-Fermi energy of the electron population during forward bias operation. Device simulation results and further experiments are also reported in support of this model. [ABSTRACT FROM AUTHOR]

Recently, distinctive quantum features have been studied in the area of ridged quantum wells (RQWs). Periodic ridges on the surface of the quantum well layer impose additional boundary conditions on the electron wave function and reduce the quantum state density. Electrons, rejected from forbidden quantum states, have to occupy the states with higher energy. As a result, Fermi energy in RQW increases and work function (WF) decreases. We investigate low WF electrode, composed from a metal RQW layer and a base substrate. The substrate material was selected so that electrons were confined to the RQW. The WF value depends on ridge geometry and electron confinement. We calculate WF in the metal RQW films grown both on a semiconductor and metal substrates. In the case of semiconductor substrate, wide band gap materials are preferable as they allow more reduction in RQW WF. In the case of metal substrate, low Fermi energy materials are preferable. For most material pairs, the WF was reduced dramatically. Such structures, can serve as electrodes for room temperature thermionic and thermotunnel energy converters and coolers. [ABSTRACT FROM AUTHOR]

An interpretation of the first experiment on the observation of 400 GeV proton multiple volume reflection from different skew planes of one bent crystal (MVROC) is given. The possibilities of experimental observation of MVROC at lower particle energies are demonstrated. New features of the effect of particle capture into the channeling regime by bent skew planes are revealed as well as optimal choice of main crystal axis, crystal thickness, and beam orientation with respect to the crystal is discussed. [ABSTRACT FROM AUTHOR]

The microstructural properties of a GaN layer grown on a patterned sapphire substrate (PSS) were studied in detail using transmission electron microscope techniques to determine dislocation and growth behaviors. Regular and uniform recrystallized GaN islands were observed on the protruding pattern. On a flat sapphire surface, the crystallographic orientation relationship of [formula] and [formula] existed between the GaN and the substrate. On the other hand, the orientation relationship of [formula] and [formula] was confirmed among the GaN layer, the recrystallized GaN islands on an inclined sapphire surface and the PSS. The flat surface among the protruding patterns began to fill rapidly with GaN. Then, the GaN gradually overgrew the protruding pattern and coalesced near the summit as the growth time increased. The generation of threading dislocations was observed in the vicinity of the coalescence points near the top of the protruding patterns. [ABSTRACT FROM AUTHOR]

The bimolecular reaction O(1D)+N2O→NO+NO was photoinitiated in the (N2O)2 dimer at a wavelength of 193 nm and was investigated by three-dimensional (3D) velocity map imaging. State selective 3D momentum vector distributions were monitored and analyzed. For the first time, kinetic energy resolution and stereodynamic information about the reaction under constrained geometry conditions is available. Directly observable NO products exhibit moderate vibrational excitation and are rotationally and translationally cold. Speed and spatial distributions suggest a pronounced backward scattering of the observed products with respect to the direction of motion of the O(1D) atom. Forward scattered partner products, which are not directly detectable are also translationally cold, but carry very large internal energy as vibration or rotation. The results confirm and extend previous studies on the complex initiated reaction system. The restricted geometry of the van der Waals complex seems to favor an abstraction reaction of the terminal nitrogen atom by the O(1D) atom, which is in striking contrast to the behavior observed for the unrestricted gas phase reaction under bulk conditions. [ABSTRACT FROM AUTHOR]

Simple lattice model for self-associating molecules such as methanol or tert-butanol is proposed and studied in mean-field (MF) approximation in the case of methanol. In addition to the isotropic van der Waals interaction, the hydrogen bonding is present in this model when the neighboring alcohol molecules are in appropriate orientations. The orientation of the polar molecule is given by the unit vector n⁁ parallel to the vector connecting the center of the tail group with the center of the head group of the molecule. Stability region of the uniform fluid phase against gas-liquid separation and order-disorder transition is obtained for neat methanol in MF approximation. In order to describe the self-association patterns in the liquid, we consider the grand-canonical ensemble average of the scalar product of the orientations of the molecules ≤n⁁(x)·n⁁(x+Δx)≥ as a function of the vector Δx describing the separation between the centers of the molecules. For methanol we find in MF oscillatory decay of ≤n⁁(x)·n⁁(x+Δx)≥ for Δx⊥n⁁(x) and for Δx∥n⁁(x); the wavelength is somewhat less than two molecular diameters in both cases, and the decay length is larger in the perpendicular direction. This indicates that on average alternating antiparallel and parallel orientations of the second molecule are found for increasing separation from the first molecule in both directions. Such local orientational ordering of the molecules is consistent with association into zigzag chainlike clusters found in recent spectroscopic measurements and computer simulations. In Fourier representation the above structure function assumes maximum for the wave number that coincides with the prepeak position for site-site correlations found in simulations. We argue that ≤n⁁(x)·n⁁(x+Δx)> can provide a useful tool for discriminating between different local arrangements of any polar molecules. [ABSTRACT FROM AUTHOR]

Long-range-corrected hybrids, which incorporate all of the long-range exact exchange interaction, improve performance for a host of molecular properties. The long-range portion of exact exchange is both computationally and formally problematic in solids, and screened hybrids therefore eliminate it. While screened hybrids give similar results to their parent global hybrids for many molecular properties, one may worry that they perform poorly for those properties that are improved by the long-range-correction procedure. In this paper, we show that at least for the Heyd–Scuseria–Ernzerhof (HSE) screened hybrid, this is not the case; for many properties improved by long-range-correction, screened hybrids and global hybrids deliver essentially the same results. We suggest that this is because screened hybrids and global hybrids have fundamentally the same many-electron self-interaction error. We also introduce some small revisions to our computational implementation of the HSE screened hybrid, and we recommend these revisions for future applications of HSE. [ABSTRACT FROM AUTHOR]

Numerous studies have identified large quantum mechanical effects in the dynamics of liquid water. In this paper, we suggest that these effects may have been overestimated due to the use of rigid water models and flexible models in which the intramolecular interactions were described using simple harmonic functions. To demonstrate this, we introduce a new simple point charge model for liquid water, q-TIP4P/F, in which the O–H stretches are described by Morse-type functions. We have parametrized this model to give the correct liquid structure, diffusion coefficient, and infrared absorption frequencies in quantum (path integral-based) simulations. The model also reproduces the experimental temperature variation of the liquid density and affords reasonable agreement with the experimental melting temperature of hexagonal ice at atmospheric pressure. By comparing classical and quantum simulations of the liquid, we find that quantum mechanical fluctuations increase the rates of translational diffusion and orientational relaxation in our model by a factor of around 1.15. This effect is much smaller than that observed in all previous simulations of empirical water models, which have found a quantum effect of at least 1.4 regardless of the quantum simulation method or the water model employed. The small quantum effect in our model is a result of two competing phenomena. Intermolecular zero point energy and tunneling effects destabilize the hydrogen-bonding network, leading to a less viscous liquid with a larger diffusion coefficient. However, this is offset by intramolecular zero point motion, which changes the average water monomer geometry resulting in a larger dipole moment, stronger intermolecular interactions, and a slower diffusion. We end by suggesting, on the basis of simulations of other potential energy models, that the small quantum effect we find in the diffusion coefficient is associated with the ability of our model to produce a single broad O–H stretching band in the infrared absorption spectrum. [ABSTRACT FROM AUTHOR]

Fock space multireference coupled-cluster (FSMRCC) method emerged as an efficient tool to describe the electronic structure of nearly degenerate cases. Development of linear response has been one of the challenging problems in FSMRCC due to the multiple-root nature of the effective Hamiltonian. A response from any of the roots would span the space for getting the properties. Hence, all roots perturbed by the external field would proliferate the excited states. We recently developed the FSMRCC method for the efficient evaluation of analytic response properties using a constrained variation approach. In this paper, we present analytic dipole moments and polarizabilities of H2O, O3, and CH+ molecules in low-lying excited states along with brief discussion of singlet triplet decoupling of (1,1) sector of FSMRCC resulting from spin adaptation. [ABSTRACT FROM AUTHOR]

Annealing the Ru metal that typically contains residual carbon impurities offers a facile way to grow graphene on Ru(0001) at the macroscopic scale. Two superstructures of the graphene/Ru(0001) interface with periodicities of 3.0 and 2.7 nm, respectively, were previously observed by scanning tunneling microscopy. Using first principles density functional theory, we optimized the observed superstructures and found interfacial C–Ru bonding of C atoms atop Ru atoms for both superstructures, which causes the graphene sheet to buckle and form periodic humps of ∼1.7 Å in height within the graphene sheet. The flat region of the graphene sheet, which is 2.2–2.3 Å above the top Ru layer and has more C atoms occupying the atop sites, interacts more strongly with the substrate than does the hump region. We found that interfacial adhesion is much stronger for the 3.0 nm superstructure than for the 2.7 nm superstructure, suggesting that the former is the thermodynamically more stable phase. We explained the 3.0 nm superstructure’s stability in terms of the interplay between C–Ru bonding and lattice matching. [ABSTRACT FROM AUTHOR]

Snoek relaxation behavior of a Ti–25.2Nb–0.6O at. % alloy, which is processed to have an intensified normal plane∥{112} texture, is investigated on three specimens with specific crystalline orientations along the flexure vibration stressing direction. In the case of rolling direction specimen, the Snoek relaxation peak temperature is 473.5 K, the activation energy is 1.51 eV, and the peak broadness parameter r2(β) is 1.96. While all those factors show only slight variation with the change in specimen orientation, the deduced relaxation strength, QP-1/f2(0,β) changes linearly with the average orientation parameter Γ of the specimens. Considering that the observed Snoek relaxation peak can be modeled by a continuous spectrum, we get a dipole shape factor of 0.42 for the alloy with the average orientation parameters calculated by the optimized sampling grain approximation method. [ABSTRACT FROM AUTHOR]

The electronic excitation energies, molecular geometry, quadratic force fields, and vibrational frequencies in the ground 5Δg and low-lying excited 5Σg+ and 5Πg electronic states of iron difluoride are studied at sophisticated levels of theory. Two families of basis sets, nonrelativistic and Douglas–Kroll–Hess relativistic, are used that range in quality from triple-ζ to quintuple-ζ. These are augmented by additional diffuse functions (on fluorine atoms) and tight functions (on all atoms) for the description of core-valence correlation and utilized to determine complete basis set molecular properties. The quality of electron correlation treatment using conventional single reference coupled cluster methods CCSD and CCSD(T) is compared to that attained at the multiconfigurational quasidegenerate second-order perturbation theory (CASSCF+MCQDPT2) and the electron attachment equation-of-motion coupled cluster (EOMEA-CCSD) levels. Spin-orbit coupling effects are studied by the SO-MCQDPT2 method using the full Breit–Pauli spin-orbit operator. Effects of spin contamination in the coupled cluster molecular calculations are carefully analyzed. Results of the single reference CCSD(T) and multireference calculations are found to be in a remarkable agreement. The calculations indicate that the EOMEA-CC approach provides a suitable tool for an accurate treatment of FeF2 and other systems where delicate electron correlation effects have to be carefully dealt with. The inclusion of relativistic effects is shown to be necessary for an accurate description of the molecular geometry and excitation energies of FeF2. The results of calculations are in good agreement with the experimental data available. The predicted FeF2 molecular properties are compared to those of the related FeF3. [ABSTRACT FROM AUTHOR]

A spatial model of the electronic current density induced in the cubane molecule by applying an external magnetic-field has been constructed employing quantum mechanical methods at the Hartree–Fock level of accuracy. The topological features of the current density vector field are described via a stagnation graph that shows the isolated points and the lines at which the current vanishes. Shielding density maps based on the differential Biot–Savart law, along with a collection of current density maps, explain magnetic shielding at hydrogen and carbon nuclei, and virtual shielding at ring and cage centers. [ABSTRACT FROM AUTHOR]

We have studied the binding of NO to small Rh clusters, containing one to five atoms, using density functional theory in both spin-polarized and non-spin-polarized forms. We find that NO bonds more strongly to Rh clusters than it does to Rh(100) or Rh(111), suggesting that Rh clusters may be good catalysts for NO reduction. However, binding to NO also quenches the magnetism of the clusters. This (local) effect results in reducing the magnitude of the NO binding energy, and also washes out the clear size-dependent trend observed in the nonmagnetic case. Our results illustrate the competition present between the tendencies to bond and to magnetize, in small clusters. [ABSTRACT FROM AUTHOR]

In this study, we examine the possibility of electrode-barrier interactions in modifying the electrical characteristics and current switching behavior of a trilayer junction with silver sulfide as the barrier layer. A series of Al–Ag2S–Ag crossbar junction is fabricated by thermal evaporation technique varying the thickness (30–110 Å) of the sulfide layer. Current-voltage characteristics of the junctions are studied as a function of barrier layer thickness, which can suggest any role that electrode-barrier interaction may have in tuning their electrical behavior. To fully understand the performance of the barrier layer, structure and physical properties of Ag2S films are independently investigated. The microstructure of Ag2S films strongly depends on the deposition conditions that, in turn, affect their electrical and optical characteristics. The polarization of the lattice prevalent in Ag2S is shown to affect the charge carrier conduction in their films and dominates their electrical behavior and that of the junctions. [ABSTRACT FROM AUTHOR]

Microscopic structures of room temperature ionic liquid (IL) [bmim][PF6] on hydrophobic graphite surfaces have been studied in detail by molecular dynamics simulation. It is clearly shown that both the mass and electron densities of the surface adsorbed ionic liquid are oscillatory, and the first peak adjacent to the graphite surface is considerably higher than others, corresponding to a solidlike IL bottom layer of 6 Å thick. Three IL layers are indicated between the graphite surface and the inner bulk IL liquid. The individually simulated properties of single-, double-, and triple-IL layers on the graphite surface are very similar to those of the layers between the graphite surface and the bulk liquid, indicating an insignificant effect of vapor-IL interface on the ordered IL layers. The simulation also indicates that the imidazolium ring and butyl tail of the cation (bmim+) of the IL bottom layer lie flat on the graphite surface. [ABSTRACT FROM AUTHOR]

In the crystalline state, the OH- anion is shown to be capable of acting as a base or as an acid with respect to waters of crystallization to which it is linked by hydrogen bonds. We examined the OH- anion in three crystalline samples and studied its behavior using quantum mechanics. Four quantum mechanical approximations were employed (HF, B3LYP, SVWN, and MP2) to obtain the relative stability of isomers of the H3O2- molecule in the three crystals considered. In one crystal state (LICQIX), the H3O2- anion corresponds to a geometry in which OH- acts as an acid, but not so as a free molecule. The free anion H3O2- has two qualitatively different structures. In one structure, the hydrogen bond is long, while in the other structure, the hydrogen bond is shorter and the hydrogen atom lies at an equal distance between donor and acceptor oxygen atoms. [ABSTRACT FROM AUTHOR]

A systematic theoretical study of the PbnM (M=C, Al, In, Mg, Sr, Ba, and Pb; n=8, 10, 12, and 14) clusters have been investigated to explore the effect of impurity atoms on the structure and electronic properties of lead clusters. The calculations were carried out using the density functional theory with generalized gradient approximation for exchange-correlation potential. Extensive search based on large numbers of initial configurations has been carried out to locate the stable isomers of PbnM clusters. The results revealed that the location of the impurity atom depends on the nature of interaction between the impurity atom and the host cluster and the size of the impurity atom. Whereas, the impurity atoms smaller than Pb favor to occupy the endohedral position, the larger atoms form exohedral capping of the host cluster. The stability of these clusters has been analyzed based on the average binding energy, interaction energy of the impurity atoms, and the energy gap between the highest occupied and lowest unoccupied energy levels (HLG). Based on the energetics, it is found that p-p interaction dominates over the s-p interaction and smaller size atoms interact more strongly. The stability analysis of these clusters suggests that, while the substitution of Pb by C or Al enhances the stability of the Pbn clusters, Mg lowers the stability. Further investigations of the stability of PbnM clusters reveal that the interplay between the atomic and electronic structure is crucial to understand the stability of these clusters. The energy gap analysis reveals that, while the substitution of Mg atom widens the HLG, all other elements reduce the gap of the PbnM clusters. [ABSTRACT FROM AUTHOR]

A critical issue underlying the accuracy and applicability of the combined quantum mechanical/molecular mechanical (QM/MM) methods is how to describe the QM/MM boundary across covalent bonds. Inspired by the ab initio pseudopotential theory, here we introduce a novel design atom approach for a more fundamental and transparent treatment of this QM/MM covalent boundary problem. The main idea is to replace the boundary atom of the active part with a design atom, which has a different number of valence electrons but very similar atomic properties. By modifying the Troullier-Martins scheme, which has been widely employed to construct norm-conserving pseudopotentials for density functional calculations, we have successfully developed a design-carbon atom with five valence electrons. Tests on a series of molecules yield very good structural and energetic results and indicate its transferability in describing a variety of chemical bonds, including double and triple bonds. [ABSTRACT FROM AUTHOR]

High resolution optical spectroscopy has been used to study a molecular beam of molybdenum monocarbide (MoC). The Stark effect of the Re(0) and Qfe(1) branch features of the [18.6] 3Π1-X 3Σ-(0,0) band system of 98MoC were analyzed to determine the permanent electric dipole moments μe of 2.68(2) and 6.07(18) D for the [18.6] 3Π1(ν=0) and X 3Σ-(ν=0) states, respectively. The dipole moments are compared with the experimental value for ruthenium monocarbide [T. C. Steimle et al., J. Chem. Phys. 118, 2620 (2003)] and with theoretical predictions. A molecular orbital correlation diagram is used to interpret the observed and predicted trends of ground state μe values for the 4d-metal monocarbides series. [ABSTRACT FROM AUTHOR]

X-band and Q-band electron paramagnetic resonance (EPR) spectra of Cu2+ in BaF2 crystal were recorded in the temperature range of 4.2–200 K. Spin-Hamiltonian parameters of single Cu2+ complexes and of Cu2+–Cu2+ pairs were derived and discussed. A special attention was paid to the dimeric species. Their molecular ground state configuration was found as having antiferromagnetic intradimer coupling with the singlet-triplet splitting J=-35 cm-1. The zero-field splitting being D=0.0365 cm-1 at 4.2 K increases with temperature as an effect of thermal population of excited dimer configurations. Electron spin echo (ESE) method was used for measurements of electron spin lattice and phase relaxation. The spin-lattice relaxation data show that except for coupling to the host lattice phonons the Cu2+ ions are involved in local mode motions with energy of 82 cm-1. Phase relaxation (ESE dephasing) of single Cu2+ ions is due to spin diffusion at low temperatures. This relaxation is hampered for temperatures higher than 30 K due to the triplet state population of neighboring Cu2+–Cu2+ dimers, which disturb dipolar coupling between Cu2+ ions. For higher temperatures the relaxation is dominated by Raman T1 processes. Fourier transform ESE spectrum displays dipolar Cu–F splitting which allowed determination of the off-center shift of Cu2+ as δs=0.132 nm. The dynamical effects observed in EPR spectra and in electron spin relaxation both for single Cu2+ ions and Cu2+–Cu2+ pairs are discussed as due to jumps between six off-center positions in the crystal unit cell and jumps between various dimer configurations. [ABSTRACT FROM AUTHOR]

Neat methanol and tert-butanol are studied by molecular dynamics with the focus on the microstructure of these two alcohols. The site-site radial distribution functions, the corresponding structure factors, and an effective local one-body density function are shown to be the appropriate statistical quantities that point in a complementary manner towards the same microstructure for any given liquid. Methanol is found to be a weakly associated liquid forming various chainlike patterns (open and closed) while tert-butanol is almost entirely associated and forms micellelike primary pattern. The presence of stable local microheterogeneity within homogeneous disordered phase appears as a striking feature of these liquids. The absence of any such apparent clustering in water—a stronger hydrogen bonding liquid—through the same two statistical quantities is analyzed. [ABSTRACT FROM AUTHOR]

Local coupled cluster methods were applied for the automated generation of accurate multidimensional potential energy surfaces for a set of test molecules ranging from six to nine atoms. Based on these surfaces anharmonic fundamental frequencies were computed using vibrational self-consistent field and configuration interaction methods. The computed vibrational frequencies are compared to those obtained from similar calculations using conventional coupled cluster methods and to experimental values. The results from local and conventional methods are found to be of similar accuracy and in close agreement with experimental values. In addition, an efficient parallelization of the fully automated surface generation code is presented. [ABSTRACT FROM AUTHOR]

We report experiments on a pump-probe configuration to elucidate the formation of a plasma channel by the hydrodynamic evolution of air breakdown in laser focus. A stable air breakdown was produced by focusing a picosecond laser pulse to create a shock driven plasma channel in the laser focus for propagating a nanosecond pulse. A four fold increase in the lifetime of the channel estimated by monitoring the temporal evolution of the fluorescence of a spectral line at 504.5 nm of N+ transition 3p 3S-3s 3P0 is reported. Assuming plasma in local thermal equilibrium plasma temperature of ∼8.2 eV and an electron density of ∼1.4×1018 cm-3 were determined using a Stark broadening of 649.2 nm line of NII transition 3d 3D0-4p3D in the channel. An enhancement in the electron density of the plasma channel was observed at the 7 ns delay of the nanosecond laser pulse relative to the picosecond laser pulse. [ABSTRACT FROM AUTHOR]

Single crystal 6H-SiC wafers were fully amorphized at room temperature or 200 K using 1.0 or 2.0 MeV Au+ ion irradiation. The thickness of the amorphized layers has been determined using Rutherford backscattering spectrometry under ion channeling conditions. Microstructures of the irradiated SiC have been examined using cross-sectional transmission electron microscopy. The depth profiles of both the Si and C atoms have been studied using both x-ray photoelectron spectroscopy (XPS) and time-of-flight energy elastic recoil detection analysis. Neither Si nor C in the amorphized SiC exhibits a significant mass transport by diffusion during the irradiation and subsequent storage at room temperature. There is no observable phase segregation of either Si or C in the amorphized SiC. Ar+ ion sputtering leads to modifications of the composition, structure, and chemical bonding at the 6H-SiC surface. The Si–Si bonds at the sputtered surface (amorphized) do not appear, as suggested by the XPS; however, Raman backscattering data reveal the existence of the Si–Si bonds in the bulk amorphized SiC, in addition to the C–C and Si–C bonds that the XPS also identified. [ABSTRACT FROM AUTHOR]

We study the general energy and shape of the two-dimensional solid monolayer domains with the dipole-dipole interactions. Compared with the domain energy without tilted dipole moments [M. Iwamoto and Z. C. Ou-Yang, Phys. Rev. Lett. 93, 206101 (2004)], the general dipolar energy is not only shape and size but also boundary orientation dependent. The general shape equation derived by this energy using variational approach predicts a circular solution and an equilibrium shape grown from this circle. In particular, the latter is composed of two branches: a translation-induced growth of all odd harmonic modes and a pressure-induced cooperative deformation by all even harmonic modes. The good qualitative agreement between our prediction and the experimental observations shows the validity of the present theory. [ABSTRACT FROM AUTHOR]

Plastically deformed (hand-filed) Cu–Sn–5Zn ternary alloys with Sn concentrations 1, 2.5, and 5 wt % are investigated. Microstructural parameters are studied in terms of x-ray diffraction profile fitting analysis. It is observed by Dey et al. [Acta. Mater. 53, 4635 (2005)] that the change in stacking fault probability (α) with Sn concentration for ternary Cu–Sn–5Zn alloys is similar to Cu-based binary alloy (Cu–Sn) system but behaves in a different manner from Cu–1Sn–Zn ternary alloy systems. The crystallite size distribution is broader for alloy with 1 wt % Sn and becomes narrower with increasing Sn concentration. Value of dislocation density (ρ) is of the order of 1015 m-2 and shows a compositional dependence. Type of dislocation is found to be predominantly screw; <100>-type dipoles may also be present in the cold-worked alloys. The dislocation arrangement is found to be more correlated in case of 1 wt % Sn compared to other alloys of higher Sn concentration. The stacking fault energy (γ) is obtained from modified Reed-Schramm equation and is of the order of 20 mJ m-2 with no significant compositional dependence. [ABSTRACT FROM AUTHOR]

An intermetallic compound of nominal composition Tb0.3Dy0.7Fe1.95 was conventionally cast in the form of cylindrical rods of 8 mm diameter and directionally solidified by zoning at three different growth rates. Compounds of the same nominal composition were also directionally solidified in the form of cylindrical rods of 20 mm diameter by modified Bridgman technique, at two different growth rates. The microstructure of the directionally solidified compounds has been investigated as a function of solidification rate and compared with that of conventionally cast compound. The observed microstructural features of these samples have been correlated with the magnetostriction measured on the corresponding samples. Further, by examining the longitudinal sections cut along its cylindrical axis, a correlation of microstructure with magnetostriction has been brought out for each directionally solidified sample as a function of distance from the initially solidified end to the other. It has been observed that a large magnetostriction (at 4.5 kOe) and a high initial slope of magnetostriction versus applied magnetic field are realized in the directionally solidified samples as compared to the conventionally cast sample. An improvement in the property has been observed among the directionally solidified samples when solidified with higher growth rates. It is surmised on the basis of the observed microstructural features in this study that a large magnetostriction is realized if formation of (Tb,Dy)Fe2 occurs by congruent solidification rather than by the peritectic reaction L+(Tb,Dy)Fe3→(Tb,Dy)Fe2. [ABSTRACT FROM AUTHOR]

The atomic density function kinetic theory of a spontaneous atomic rearrangement at the atomic scale is developed. In this theory, the evolution of the system is described not by the individual movement of each atom in the same system but by the evolution of the atomic density functions, which are averages over the time-dependent ensemble. The temporal evolution of atomic densities is driven by the free energy relaxation. The thermodynamic aspects of the instability of the homogeneous liquid state and the formation of crystalline and amorphous state are discussed. Examples of self-assembling of atoms in the formation of a three-dimensional nanocrystal, and the two-dimensional diffusionless transformation in the crystalline and amorphous states are considered. The modeling reveals the special features of displacive phase transformations in the crystalline state through a transient stress-accommodating state. It also demonstrates that the thermodynamic relaxation of the liquid below its critical instability point produces a relaxed amorphous state that is practically a metastable phase. [ABSTRACT FROM AUTHOR]

Neutron reflection was used to measure the buildup of layers at a solid surface as the smectic phase is approached from higher temperatures in a nematic liquid crystal. The liquid crystal was 4-octyl-4′-cyanobiphenyl (8CB), and the solid was silicon with one of five different surface treatments that induce homeotropic alignment: (i) silicon oxide; (ii) a cetyltrimethylammonium bromide coating; (iii) an octadecyltrichlorosilane monolayer; (iv) an n-n-dimethyl-n-octadecyl-3- aminopropyltrimethyloxysilyl chloride monolayer; and (v) a lecithin coating. The development of surface smectic layers in the nematic phase of 8CB was followed by measuring specular reflectivity and monitoring the pseudo-Bragg peak from the layers. The scattering data were processed to remove the scattering from short-ranged smecticlike fluctuations in the bulk nematic phase from the specular reflection. The pseudo-Bragg peak at scattering vector Q∼0.2 Å-1 therefore corresponded to the formation of long-range smectic layers at the surface. The amplitude of the smectic density wave decayed with increasing distance from the surface, and the characteristic thickness of this smectic region diverged as the transition temperature was approached. It was found that the characteristic thickness for some of the surface treatments was greater than the correlation length in the bulk nematic. The different surfaces gave different values of the smectic order parameter at the surface. This suggests that the interaction with the surface is significantly different from a “hard wall” which would give the same values of the smectic order parameter and penetration depths similar to the bulk correlation length. Comparison of the different surfaces also suggested that the strength and range of the surface smectic ordering may be varied independently. [ABSTRACT FROM AUTHOR]

We consider a microscopic model of a polymer blend that is prone to phase separation. Permanent cross-links are introduced between randomly chosen pairs of monomers, drawn from the Deam-Edwards distribution. Thereby, not only density but also concentration fluctuations of the melt are quenched-in in the gel state, which emerge upon sufficient cross-linking. We derive a Landau expansion in terms of the order parameters for gelation and phase separation, and analyze it on the mean-field level, including Gaussian fluctuations. The mixed gel is characterized by thermal as well as time-persistent (glassy) concentration fluctuations. Whereas the former are independent of the preparation state, the latter reflect the concentration fluctuations at the instant of cross-linking, provided the mesh size is smaller than the correlation length of phase separation. The mixed gel becomes unstable to microphase separation upon lowering the temperature in the gel phase. Whereas the length scale of microphase separation is given by the mesh size, at least close to the transition, the emergent microstructure depends on the composition and compressibility of the melt. Hexagonal structures, as well as lamellas or random structures with a unique wavelength, can be energetically favorable. [ABSTRACT FROM AUTHOR]