bluuebunny/crossref_metadata_2025 huggingface dataset:
Photoluminescence properties of partially phase separated silicon nitride films
Authors: Mercaldo Lucia V. , Esposito Emilia M. , Veneri Paola Delli , Rezgui Béchir , Sibai Abel , Bremond Georges
Published: May 2011

Photoluminescence properties of partially phase separated amorphous silicon nitride films, lying in between homogeneous materials and composites embedding pure Si nanoparticles, have been investigated. With excitation energy above the Tauc gap, the emission band systematically blue-shifts and broadens with decreasing silicon content, suggesting tail to tail recombination as the prevailing luminescence mechanism. With subgap excitation, the response is instead peaked at 1.7 eV independently of the stoichiometry, likely as an effect of the spatial fluctuations of the gap. In this case, excitation and emission selectively occur in the Si rich domains within the material, and direct band to band recombination is proposed as the possible dominant process. Another effect of the nonuniform material composition is likely the unusual S-shaped evolution of the integrated photoluminescence intensity versus temperature observed for the sample richest in Si. This behavior has been described with a phenomenological model including two thermally activated nonradiative processes and one electron trap energy level.
Collisional transfer of population and orientation in NaK
Authors: Wolfe C. M. , Ashman S. , Bai J. , Beser B. , Ahmed E. H. , Lyyra A. M. , Huennekens J.
Published: May 2011

Collisional satellite lines with |ΔJ| ≤ 58 have been identified in recent polarization spectroscopy V-type optical–optical double resonance (OODR) excitation spectra of the Rb2 molecule [H. Salami et al., Phys. Rev. A 80, 022515 (2009)]. Observation of these satellite lines clearly requires a transfer of population from the rotational level directly excited by the pump laser to a neighboring level in a collision of the molecule with an atomic perturber. However to be observed in polarization spectroscopy, the collision must also partially preserve the angular momentum orientation, which is at least somewhat surprising given the extremely large values of ΔJ that were observed. In the present work, we used the two-step OODR fluorescence and polarization spectroscopy techniques to obtain quantitative information on the transfer of population and orientation in rotationally inelastic collisions of the NaK molecules prepared in the 2(A)1Σ+(v′ = 16, J′ = 30) rovibrational level with argon and potassium perturbers. A rate equation model was used to study the intensities of these satellite lines as a function of argon pressure and heat pipe oven temperature, in order to separate the collisional effects of argon and potassium atoms. Using a fit of this rate equation model to the data, we found that collisions of NaK molecules with potassium atoms are more likely to transfer population and destroy orientation than collisions with argon atoms. Collisions with argon atoms show a strong propensity for population transfer with ΔJ = even. Conversely, collisions with potassium atoms do not show this ΔJ = even propensity, but do show a propensity for ΔJ = positive compared to ΔJ = negative, for this particular initial state. The density matrix equations of motion have also been solved numerically in order to test the approximations used in the rate equation model and to calculate fluorescence and polarization spectroscopy line shapes. In addition, we have measured rate coefficients for broadening of NaK 31Π ← 2(A)1Σ+spectral lines due to collisions with argon and potassium atoms. Additional broadening, due to velocity changes occurring in rotationally inelastic collisions, has also been observed.
Studies on mechanism of reaction and density behavior during anhydrous D-fructose mutarotation in the supercooled liquid state
Authors: Wlodarczyk P. , Paluch M. , Hawelek L. , Kaminski K. , Pionteck J.
Published: May 2011

Recently, we have studied the mutarotation kinetics in D-fructose by means of dielectric spectroscopy. In the present work we investigate density behavior of D-fructose during mutarotation process. By performing volume measurements at temperature T = 303 K and pressure p = 10 MPa we are able to monitor kinetics of this process. As a result we found nearly the same value of the rate constant as previously determined from dielectric measurements. However, these two experimental methods monitor different molecular aspects of mutarotation phenomenon in D-fructose. Dielectric spectroscopy is sensitive to the decay of former ring as well as to the forming of another, while specific volume measurements are sensitive to the forming of new tautomers only. Calculations of activation energy of mutarotation in D-fructose led us to the conclusion, that mechanism of this reaction in amorphous phase could be based on internal proton transfer. Moreover it was found that the main mutarotation path in quenched D-fructose melt is transformation of α,β-furanose to β-pyranose.
Crystallization of tetrahedral patchy particles in silico
Authors: Romano Flavio , Sanz Eduardo , Sciortino Francesco
Published: May 2011

We investigate the competition between glass formation and crystallization of open tetrahedral structures for particles with tetrahedral patchy interactions. We analyze the outcome of such competition as a function of the potential parameters. Specifically, we focus on the separate roles played by the interaction range and the angular width of the patches, and show that open crystal structures (cubic and hexagonal diamond and their stacking hybrids) spontaneously form when the angular width is smaller than about 30°. Evaluating the temperature and density dependence of the chemical potential of the fluid and of the crystal phases, we find that adjusting the patch width affects the fluid and crystal in different ways. As a result of the different scaling, the driving force for spontaneous self-assembly rapidly grows as the fluid is undercooled for small-width patches, while it only grows slowly for large-width patches, in which case crystallization is pre-empted by dynamic arrest into a network glass.
Cutoff radius effect of the isotropic periodic sum and Wolf method in liquid–vapor interfaces of water
Authors: Takahashi Kazuaki Z. , Narumi Tetsu , Yasuoka Kenji
Published: May 2011

As a more economical but similarly accurate computation method than the Ewald sum, the isotropic periodic sum (IPS) method for nonpolar molecules (IPSn) and polar molecules (IPSp), along with the Wolf method are of interest, but the cutoff radius dependence is an important issue. To evaluate the cutoff radius effect of the three methods, a water-vapor interfacial system has been studied by molecular dynamics. The Wolf method can produce adequate results for surface tension compared to that of the Ewald sum (within 2.9%) at a long enough cutoff radius, rc. However, the estimation of the electrostatic potential profile and dipole orientational function is poor. The Wolf method cannot estimate electrostatic configuration at rc ⩽ Lz/2 (Lz is the longest lattice of the system). We have found that the convergence of the surface tension and the electrostatic configuration of the IPSn method is faster than that of the IPSp method. Moreover, the IPSn method is most accurate among the three methods for the same cutoff radius. Furthermore, the behavior of the surface tension against the cutoff radius shows a greater difference for the IPSn and IPSp method. The surface tension of the IPSp method fluctuates and presents a similar result to that of the Ewald sum, but the surface tension for the IPSn method greatly deviates near rc = Lz/3. The cause of this deviation is the difference between the interfacial configuration of the water surface and the cutoff treatment of the IPS method. The deviation becomes insignificant far from rc = Lz/3. In spite of this shortcoming, the IPSn method gives the most accurate result in estimating the surface tension at rc = Lz/2. From all the results in this work, the IPSn and IPSp method have been found to be more accurate than the Wolf method. In conclusion, the surface tension and structure of water-vapor interface can be calculated by the IPSn method when rc is greater than or equal to the longest lattice of the system. The IPSp method and the Wolf method require a longer cutoff radius than the longest lattice of the system to estimate interfacial properties.
Synthesis of quantum dot doped chalcogenide glasses via sol-gel processing
Authors: Mashford Benjamin , Baldauf Julia , Nguyen Tich-Lam , Funston Alison M. , Mulvaney Paul
Published: May 2011

Semiconductor quantum dots (QDs) are used to dope wide-bandgap chalogenide glasses via sol-gel processing. Such chalcogenides enhance surface passivation of the quantum dots, as evidenced by the increased PL emissions of both core and core shell species used, while a ZnO glass leads to irreversible oxidation of the embedded quantum dots. The embedded QDs are photostable.
Precision, all-optical measurement of external quantum efficiency in semiconductors
Authors: Wang Chengao , Li Chia-Yeh , Hasselbeck Michael P. , Imangholi Babak , Sheik-Bahae Mansoor
Published: May 2011

External quantum efficiency of semiconductor photonic devices is directly measured by wavelength-dependent laser-induced temperature change (scanning laser calorimetry) with very high accuracy. Maximum efficiency is attained at an optimum photo-excitation level that can be determined with an independent measurement of power-dependent temperature or power-dependent photoluminescence. Time-resolved photoluminescence lifetime and power-dependent photoluminescence measurements are used to evaluate unprocessed heterostructures for critical performance parameters. The crucial importance of parasitic background absorption is discussed.
Octahedral tilt transitions in relaxed epitaxial Pb(Zr1−xTix)O3 films
Authors: Tinberg Daniel S. , Johnson-Wilke Raegan L. , Fong Dillon D. , Fister Timothy T. , Streiffer Stephen K. , Han Yisong , Reaney Ian M. , Trolier-McKinstry Susan
Published: May 2011

Relaxed epitaxial {100}pc and {111}pc oriented films (350 nm) of Pb(Zr1−xTix)O3 (0.2 ≤ x ≤ 0.4) on SrRuO3/SrTiO3 substrates were grown by pulsed laser deposition and studied using high resolution synchrotron X-ray diffraction and transmission electron microscopy. The dielectric behavior and ferroelectric phase transition temperatures of the films were consistent with bulk PZT. However, weak 1/2{311}pc reflections in x-ray diffraction profiles were recorded above bulk TTilt (as indicated in the Jaffe, Cooke, and Jaffe phase diagram, where pc denotes pseudocubic indices). Moreover, anomalies in the dielectric and ferroelectric response were detected above TTilt which are explained by coupling of short coherence or weakly tilted regions to the ferroelectric polarization.
Anion vacancy-driven magnetism in incipient ferroelectric SrTiO3 and KTaO3 nanoparticles
Authors: Eliseev E. A. , Morozovska A. N. , Glinchuk M. D. , Blinc R.
Published: May 2011

We predict that undoped nanoparticles (size ≤ 10–100 nm) of incipient ferroelectrics without any magnetic ions can become ferromagnetic even at room temperatures due to the inherent presence of a new type of magnetic defects with spin S = 1, namely oxygen vacancies, where the magnetic triplet state is the ground state in the vicinity of the surface (magnetic shell), while the nonmagnetic singlet is the ground state in the bulk material (nonmagnetic core). In particular, for a particle radii R less than the critical radius Rc(T) ferromagnetic long-range order appears in a shell region of thickness 5–50 nm once the concentration of magnetic defects N exceeds the magnetic percolation threshold Np. At vacancy concentrations N < Np and radii R < Rc(T), short-range ferromagnetic order and consequently a spin glass state may appear. For particle radii R > Rc(T), only the paramagnetic phase is possible.
Numerical model of a single nanocrystal devoted to the study of disordered nanocrystal floating gates of new flash memories
Authors: Leroy Yann , Armeanu Dumitru , Cordan Anne-Sophie
Published: May 2011

The improvement of our model concerning a single nanocrystal that belongs to a nanocrystal floating gate of a flash memory is presented. In order to extend the gate voltage range applicability of the model, the 3D continuum of states of either metallic or semiconducting electrodes is discretized into 2D subbands. Such an approach gives precise information about the mechanisms behind the charging or release processes of the nanocrystal. Then, the self-energy and screening effects of an electron within the nanocrystal are evaluated and introduced in the model. This enables a better determination of the operating point of the nanocrystal memory. The impact of those improvements on the charging or release time of the nanocrystal is discussed.
A poly(vinylidene fluoride) composite with added self-passivated microaluminum and nanoaluminum particles for enhanced thermal conductivity
Authors: Zhou Yongcun , Wang Hong , Xiang Feng , Zhang Hu , Yu Ke , Chen Liang
Published: May 2011

A polymer composite was prepared by embedding fillers made of self-passivated aluminum particles in two kind of sizes, micrometer size and nanometer size with different volume proportions into polyvinylidene fluoride matrix. The thermal conductivity and dielectric properties of the composite were studied. The results showed that the thermal conductivity of composites was significantly increased to 3.258 W/mK when the volume proportion of micrometer size Al particles to nanometer size Al particles is at 20:1, also the relative permittivity was about 75.8 at 1 MHz. The effective simulation model values were in good accordance with experimental results.
Correlation effects in molecular conductors
Authors: Goyer Francois , Ernzerhof Matthias
Published: May 2011

The source-sink potential (SSP) model introduced previously [F. Goyer, M. Ernzerhof, and M. Zhuang, J. Chem. Phys. 126, 144104 (2007)10.1063/1.2715932] enables one to eliminate the semi-infinite contacts in molecular electronic devices (MEDs) in favor of complex potentials. SSP has originally been derived for independent electrons and extended to interacting two-electron systems subsequently [A. Goker, F. Goyer, and M. Ernzerhof, J. Chem. Phys. 129, 194901 (2008)10.1063/1.3013815]. Here we generalize SSP to N-electron systems and consider the impact of electron correlation on the transmission probability. In our correlated method for molecular conductors, the molecular part of the Hückel Hamiltonian of the original SSP is replaced by the Hubbard Hamiltonian. For the contacts, however, the single-electron picture is retained and they are assumed to be spin polarized. Using our method, we study electron transmission in molecular wires, cross-conjugated chains, as well as aromatic systems. We find that, for realistic values of the electron–electron repulsion parameter, correlation effects modify the transmission probability quantitatively, the qualitative features remain. However, we find subtle new effects in correlated MEDs, such as Coulomb drag, that are absent in uncorrelated systems.
Low energy (e,2e) studies from CH4: Results from symmetric coplanar experiments and molecular three-body distorted wave theory
Authors: Nixon K. L. , Murray Andrew J. , Chaluvadi Hari , Ning Chuangang , Madison D. H.
Published: May 2011

Low energy experimental and theoretical triply differential cross sections are presented for electron impact ionization of methane (CH4) for both the highest occupied molecular orbital (HOMO) and next highest occupied molecular orbital (NHOMO). The HOMO is a predominantly p-type orbital which is labeled 1t2 and the NHOMO is predominantly s-type labeled 2a1. Coplanar symmetric (symmetric both in final state electron energies and observation angles) are presented for final state electron energies ranging from 2.5 to 20 eV. The theoretical M3DW (molecular three-body distorted wave) results are in surprisingly good agreement with experiment for the HOMO state and less satisfactory agreement for the NHOMO state. The molecular NHOMO results are also compared with the ionization of the 2s shell of neon which is the isoelectronic atom.
Communication: The electrostatic polarization is essential to differentiate the helical propensity in polyalanine mutants
Authors: Wei Caiyi , Tung Dickson , Yip Yew Mun , Mei Ye , Zhang Dawei
Published: May 2011

The folding processes of three polyalanine peptides with composition of Ac-(AAXAA)2-GY-NH2 (where X is chosen to be Q, K, and D) are studied by molecular dynamics simulation in solvent of 40% trifluoroethanol using both polarized and unpolarized force fields. The simulations reveal the critical role of polarization effect for quantitative description of helix formation. When polarized force field is used, peptides with distinctive helical propensity are correctly differentiated and the calculated helical contents are in close agreement with experimental measurement, indicating that consideration of polarization effect can correctly predict the effect of sequence variation on helix formation.
Communication: Bubbles, crystals, and laser-induced nucleation
Authors: Knott Brandon C. , LaRue Jerry L. , Wodtke Alec M. , Doherty Michael F. , Peters Baron
Published: May 2011

Short intense laser pulses of visible and infrared light can dramatically accelerate crystal nucleation from transparent solutions; previous studies invoke mechanisms that are only applicable for nucleation of ordered phases or high dielectric phases. However, we show that similar laser pulses induce CO2 bubble nucleation in carbonated water. Additionally, in water that is cosupersaturated with argon and glycine, argon bubbles escaping from the water can induce crystal nucleation without a laser. Our findings suggest a possible link between laser-induced nucleation of bubbles and crystals.
Photo- and field-induced charge-separation and phosphorescence quenching in organometallic complex Ir(ppy)3
Authors: Mehata Mohan Singh , Ohta Nobuhiro
Published: May 2011

Electric field effects on absorption and photoluminescence (PL) spectra of organometallic phosphorescent emitter Ir(ppy)3, {tris[2-phenylpyridinato-C2,N] iridium (III)} doped in a film of polymethyl methacrylate (PMMA) have been confirmed at temperatures in the range of 40–295 K. Field-induced quenching of PL observed for Ir(ppy)3 is attributed to the decrease both of emitting state population and of the lifetime of PL. The quenching is independent of excitation energy as well as temperature. Field-assisted charge separation or dissociation of electron-hole (e-h) pair produced by photoexcitation may decrease the population of the emitting state. The Stark shifts on absorption and PL spectra have also been analyzed.
Solvation dynamics through Raman spectroscopy: Hydration of Br2 and Br3−, and solvation of Br2 in liquid bromine
Authors: Branigan Edward T. , Halberstadt N. , Apkarian V. A.
Published: May 2011

Raman spectroscopy of bromine in the liquid phase and in water illustrates uncommon principles and yields insights regarding hydration. In liquid Br2, resonant excitation over the B(3Π0u+) ← X(1Σg+) valence transition at 532 nm produces a weak resonant Raman (RR) progression accompanied by a five-fold stronger non-resonant (NR) scattering. The latter is assigned to pre-resonance with the C-state, which in turn must be strongly mixed with inter-molecular charge transfer states. Despite the electronic resonance, RR of Br2 in water is quenched. At 532 nm, the homogeneously broadened fundamental is observed, as in the NR case at 785 nm. The implications of the quenching of RR scattering are analyzed in a simple, semi-quantitative model, to conclude that the inertial evolution of the Raman packet in aqueous Br2 occurs along multiple equivalent water–Br2 coordinates. In distinct contrast with hydrophilic hydration in small clusters and hydrophobic hydration in clathrates, it is concluded that the hydration shell of bromine in water consists of dynamically equivalent fluxional water molecules. At 405 nm, the RR progression of Br3− is observed, accompanied by difference transitions between the breathing of the hydration shell and the symmetric stretch of the ion. The RR scattering process in this case can be regarded as the coherent photo-induced electron transfer to the solvent and its radiative back-transfer.
Spectroscopic investigation of fluoroiodomethane, CH2FI: Fourier-transform microwave and millimeter-/submillimeter-wave spectroscopy and quantum-chemical calculations
Authors: Puzzarini Cristina , Cazzoli Gabriele , López Juan Carlos , Alonso José Luis , Baldacci Agostino , Baldan Alessandro , Stopkowicz Stella , Cheng Lan , Gauss Jürgen
Published: May 2011

Guided by theoretical predictions, the rotational spectrum of fluoroiodomethane, CH2FI, has been recorded and assigned. Accurate values are reported for the ground-state rotational constants, all quartic, sextic, and two octic centrifugal-distortion constants. The hyperfine structure of the rotational spectrum was thoroughly investigated using a Fourier-transform microwave spectrometer and the Lamb-dip technique in the millimeter-/submillimeter-wave region, thus allowing the accurate determination of the complete iodine quadrupole-coupling tensor and of the diagonal elements of the iodine spin-rotation tensor. Relativistic effects turned out to be essential for the accurate theoretical prediction of the dipole moment and quadrupole-coupling constants and were accounted for by direct perturbation theory and a spin-free four-component treatment based on the Dirac-Coulomb Hamiltonian. The relativistic corrections to the dipole moment amount to up to 34% and to the iodine quadrupole-coupling tensor to about 15–16% of the total values.
Anisotropic magnetoplasmon spectrum of two-dimensional electron gas systems with the Rashba and Dresselhaus spin-orbit interactions
Authors: Li Chao , Zhai Feng
Published: May 2011

The magnetoplasmon spectrum of two-dimensional electron gas systems with both Rashba and Dresselhaus spin-orbit interactions is investigated theoretically. The coexistence of the two types of spin-orbit interactions leads to resonant splittings of the collective excitation modes and makes the magnetoplasmon spectrum anisotropic with respect to the direction of the wave vector. The anticrossing gaps in the magnetoplasmon spectrum vary with the angle of the wave vector and reach their extrema at several fixed directions. Such anisotropic splittings could be used to evaluate the relative strength of the Rashba and Dresselhaus spin-orbit interactions in two-dimensional electron systems.
Calculation of the structure, potential energy surface, vibrational dynamics, and electric dipole properties for the Xe:HI van der Waals complex
Authors: Preller M. , Grunenberg J. , Bulychev V. P. , Bulanin M. O.
Published: May 2011

We report the structure and spectroscopic characteristics for the Xe:HI van der Waals binary isomers determined from variational solutions of two-dimensional and three-dimensional (3D) vibrational Schrödinger equations. The solutions are based on a potential energy surface computed at the coupled-cluster level of theory including single and double excitations and a non-iterative perturbation treatment of triple excitations [CCSD(T)]. The dipole moment surface was calculated using quadratic configuration interaction (QCISD). The global potential minimum is shown to be located at the anti-hydrogen-bonded Xe−IH isomer, 21 cm−1 below the secondary local minimum associated with the hydrogen-bonded Xe−HI isomeric form. The dissociation energy from the global minimum is 245.9 cm−1. 3D Schrödinger equations are solved for the rotational quantum numbers J = k = 0, 1, and 2, without invoking an adiabatic separation of high- and low-frequency degrees of freedom. The vibrational ground state resides in the Xe−HI potential well, while the first excited state, 8.59 cm−1 above the ground, occupies the Xe−IH well. We find that intra-complex dynamics exhibits a sudden transformation upon increase of the r(HI) bond length, accompanied by abrupt changes in the geometric and dipole parameters. A similar chaotic behavior is predicted to occur for Xe:DI at a shorter r(DI) bond length, which implies stronger coupling between low- and high-frequency motions in the heavier complex. Our calculations confirm a strong enhancement for the r(HI) stretch fundamental and a significant weakening for the first overtone vibrational transitions in Xe:HI, as compared to those in the free HI molecule. A qualitative explanation of this, earlier experimentally detected effect is suggested.
Monte Carlo simulation of magnetoelectric coupling in multiferroic BiFeO3
Authors: Yang Y. , Li L. J. , Li J. Y.
Published: May 2011

A Monte Carlo method is developed to simulate the magnetoelectric coupling in multiferroic BiFeO3 with coexistence of ferroelastic, ferroelectric, and antiferromagnetic order parameters. Long-range interactions among dipoles and spins are summed through cutoff, while elastic interactions among transformation strain are evaluated using Green’s function method. Short-range interactions among ferroelectric dipoles and antiferromagnetic spins are also considered. Through the Monte Carlo simulation, coupled ferroelastic, ferroelectric, and antiferromagnetic domains are demonstrated, the switching of magnetic spins by electric field is verified, and mechanical switching of spins is also predicted.
The flexible Ca-test: An improved performance in a gas permeability measurement system
Authors: Song Eun Ho , Park Young Wook , Choi Jin Hwan , Park Tae Hyun , Jeong Jin Wook , Choi Hyun Ju , Ju Byeong Kwon
Published: May 2011

A flexible performance permeability measuring test for flexible organic light-emitting diodes is described in this paper. A single thin film layer of gas barriers is constructed on polyethersulfone (PES). The barrier coats the upper and lower surfaces of the PES layer. Two PES samples, one coated with Al2O3 on both surfaces and the other coated on a single surface, were made for comparison. According to this test, the time-dependent transmission curve of the one sided barrier sample had a linear slope which measured 1.65 g/m2/day at room temperature at a 50% relative humidity. This result shows that the measurement time is about 182% faster than has been achieved with the conventional test structure that uses a glass substrate. In addition, this measurement structure not only reduces the inevitable electrical noise which occurs during measurement but also increases the water vapor permeation signal. These effects improve the sensing reliability of the test. In addition, this structure is flexible, so one can instantly detect barrier performance changes when applying external stress.
Theoretical study of the photodissociation of Li2+ in one-color intense laser fields
Authors: Li Yuanjun , Jiang Wanyi , Khait Yuriy G. , Hoffmann Mark R.
Published: May 2011

A theoretical treatment of the photodissociation of the molecular ion Li2+ in one-color intense laser fields, using the time-dependent wave packet approach in a Floquet Born–Oppenheimer representation, is presented. Six electronic states 1,2 2Σg+, 1,2 2Σu+, 1 2Πg, and 1 2Πu are of relevance in this simulation and have been included. The dependences of the fragmental dissociation probabilities and kinetic energy release (KER) spectra on pulse width, peak intensity, polarization angle, wavelength, and initial vibrational level are analyzed to interpret the influence of control parameters of the external field. Three main dissociation channels, 1 2Σg+ (m = −1), 2 2Σg+ (m = −2), and 2 2Σu+ (m = −3), are seen to dominate the dissociation processes under a wide variety of laser conditions and give rise to well separated groups of KER features. Different dissociation mechanisms for the involved Floquet channels are discussed.
Thermal Gaussian molecular dynamics for quantum dynamics simulations of many-body systems: Application to liquid para-hydrogen
Authors: Georgescu Ionuţ , Deckman Jason , Fredrickson Laura J. , Mandelshtam Vladimir A.
Published: May 2011

A new method, here called thermal Gaussian molecular dynamics (TGMD), for simulating the dynamics of quantum many-body systems has recently been introduced [I. Georgescu and V. A. Mandelshtam, Phys. Rev. B 82, 094305 (2010)]. As in the centroid molecular dynamics (CMD), in TGMD the N-body quantum system is mapped to an N-body classical system. The associated both effective Hamiltonian and effective force are computed within the variational Gaussian wave-packet approximation. The TGMD is exact for the high-temperature limit, accurate for short times, and preserves the quantum canonical distribution. For a harmonic potential and any form of operator \documentclass[12pt]{minimal}\begin{document}$\hat{A}$\end{document}Â, it provides exact time correlation functions CAB(t) at least for the case of \documentclass[12pt]{minimal}\begin{document}$\hat{B}$\end{document}B̂, a linear combination of the position, \documentclass[12pt]{minimal}\begin{document}$\hat{x}$\end{document}x̂, and momentum, \documentclass[12pt]{minimal}\begin{document}$\hat{p}$\end{document}p̂, operators. While conceptually similar to CMD and other quantum molecular dynamics approaches, the great advantage of TGMD is its computational efficiency. We introduce the many-body implementation and demonstrate it on the benchmark problem of calculating the velocity time auto-correlation function for liquid para-hydrogen, using a system of up to N = 2592 particles.
Collisional stabilization of van der Waals states of ozone
Authors: Ivanov Mikhail V. , Babikov Dmitri
Published: May 2011

The mixed quantum-classical theory developed earlier [M. Ivanov and D. Babikov, J. Chem. Phys. 134, 144107 (2011)] is employed to treat the collisional energy transfer and the ro-vibrational energy flow in a recombination reaction that forms ozone. Assumption is that the van der Waals states of ozone are formed in the O + O2 collisions, and then stabilized into the states of covalent well by collisions with bath gas. Cross sections for collision induced dissociation of van der Waals states of ozone, for their stabilization into the covalent well, and for their survival in the van der Waals well are computed. The role these states may play in the kinetics of ozone formation is discussed.
Balancing single- and multi-reference correlation in the chemiluminescent reaction of dioxetanone using the anti-Hermitian contracted Schrödinger equation
Authors: Greenman Loren , Mazziotti David A.
Published: May 2011

Direct computation of energies and two-electron reduced density matrices (2-RDMs) from the anti-Hermitian contracted Schrödinger equation (ACSE) [D. A. Mazziotti, Phys. Rev. Lett. 97, 143002 (2006)], it is shown, recovers both single- and multi-reference electron correlation in the chemiluminescent reaction of dioxetanone especially in the vicinity of the conical intersection where strong correlation is important. Dioxetanone, the light-producing moiety of firefly luciferin, efficiently converts chemical energy into light by accessing its excited-state surface via a conical intersection. Our previous active-space 2-RDM study of dioxetanone [L. Greenman and D. A. Mazziotti, J. Chem. Phys. 133, 164110 (2010)] concluded that correlating 16 electrons in 13 (active) orbitals is required for realistic surfaces without correlating the remaining (inactive) orbitals. In this paper we pursue two complementary goals: (i) to correlate the inactive orbitals in 2-RDMs along dioxetanone's reaction coordinate and compare these results with those from multireference second-order perturbation theory (MRPT2) and (ii) to assess the size of the active space—the number of correlated electrons and orbitals—required by both MRPT2 and ACSE for accurate energies and surfaces. While MRPT2 recovers very different amounts of correlation with (4,4) and (16,13) active spaces, the ACSE obtains a similar amount of correlation energy with either active space. Nevertheless, subtle differences in excitation energies near the conical intersection suggest that the (16,13) active space is necessary to determine both energetic details and properties. Strong electron correlation is further assessed through several RDM-based metrics including (i) total and relative energies, (ii) the von Neumann entropy based on the 1-electron RDM, as well as the (iii) infinity and (iv) squared Frobenius norms based on the cumulant 2-RDM.
Intrinsic stability and hydrogen affinity of pure and bimetallic nanowires
Authors: Soldano G. , Santos E. , Schmickler W.
Published: May 2011

A density functional theory study of the intrinsic stability of pure and bimetallic wires is presented. Several bimetallic combinations forming one-atom thick wires are studied. An explanation for the experimental instability of Cu wires in contrast to the stability of Au and Ag wires is given, which relies on the higher surface energy of the former. All the possible intercalations between Ni, Pd, Pt, Cu, Ag, and Au are studied. The bimetallic wires AuCu and AuAg were found to be the most stable ones. The reactivity of the latter two systems is also examined using hydrogen adsorption as a microscopic probe. It was found that at the inter-metal interface, up to second neighbors, Cu and Ag become more reactive and Au becomes more inert than the corresponding pure wires. These results are explained within the d-band model.
Inelastic collisions in molecular oxygen at low temperature (4 ⩽ T ⩽ 34 K). Close-coupling calculations versus experiment
Authors: Pérez-Ríos J. , Tejeda G. , Fernández J. M. , Hernández M. I. , Montero S.
Published: May 2011

Close-coupling calculations and experiment are combined in this work, which is aimed at establishing a set of state-to-state rate coefficients for elementary processes ij → ℓm in O2:O2 collisions at low temperature involving the rotational states i, j, ℓ, m of the vibrational ground state of 16O2\documentclass[12pt]{minimal}\begin{document}$(^3\Sigma _g^-)$\end{document}(3Σg−). First, a set of cross sections for inelastic collisions is calculated as a function of the collision energy at the converged close-coupled level via the MOLSCAT code, using a recent ab-initio potential energy surface for O2-O2 [M. Bartolomei et al., J. Chem. Phys. 133, 124311 (2010)10.1063/1.3479395]. Then, the corresponding rates for the temperature range 4 ⩽ T ⩽ 34 K are derived from the cross sections. The link between theory and experiment is a Master Equation which accounts for the time evolution of rotational populations in a reference volume of gas in terms of the collision rates. This Master Equation provides a linear function of the rates for each rotational state and temperature. In the experiment, the evolution of rotational populations is measured by Raman spectroscopy in a tiny reference volume (≈2 × 10−4 mm3) of O2 travelling along the axis of a supersonic jet at a velocity of ≈700 m/s. The accuracy of the calculated rates is assessed experimentally for 10 ⩽ T ⩽ 34 K by means of the Master Equation. The rates, jointly with their confidence interval estimated by Monte Carlo simulation, account to within the experimental uncertainty for the evolution of the populations of the N = 1, 3, 5, 7 rotational triads along the supersonic jet. Confidence intervals range from ≈6% for the dominant rates at 34 K, up to ≈17% at 10 K. These results provide an experimental validation of state-to-state rates for O2:O2 inelastic collisions calculated in the close-coupling approach and, indirectly, of the anisotropy of the O2-O2 intermolecular potential employed in the calculation for energies up to 300 cm−1.
Two- and three-body interactions among nanoparticles in a polymer melt
Authors: Frischknecht Amalie L. , Yethiraj Arun
Published: May 2011

We perform direct three-dimensional density functional theory (DFT) calculations of two- and three-body interactions in polymer nanocomposites. The nanoparticles are modeled as hard spheres, immersed in a hard-sphere homopolymer melt of freely jointed chains. The two-particle potential of mean force obtained from the DFT is in near quantitative agreement with the potential of mean force obtained from self-consistent polymer reference interaction site model theory. Three-body interactions among three nanoparticles are found to be significant, such that it is not possible to describe these systems with a polymer-mediated two-body interaction calculated from the potential of mean force.
Mesh-shape-and-size controlled rapid-melting growth for the formation of single-crystalline (100), (110), and (111) Ge networks on insulators
Authors: Mizushima Ichiro , Toko Kaoru , Ohta Yasuharu , Sakane Takashi , Sadoh Taizoh , Miyao Masanobu
Published: May 2011

Single-crystalline-Ge (c-Ge) networks with various crystal orientations on insulators formed on Si substrates are essential for integrating high-speed and multifunctional devices onto the Si platform. c-Ge networks are realized by rapid-melting growth of mesh-patterned amorphous-Ge over large areas (500×250 μm2) on (110) and (111) as well as (100) Si substrates by optimizing the shape and the size of the mesh. It is revealed that latent-heat generated at the growth front can be controlled by selecting mesh-shape-and-size, which suppresses the spontaneous nucleation. In addition, essential role of the growth-direction on preventing the rotational growth is clarified.
Strain compensation in AlInN/GaN multilayers on GaN substrates: Application to the realization of defect-free Bragg reflectors
Authors: Cosendey Gatien , Carlin Jean-François , Kaufmann Nils A. K. , Butté Raphaël , Grandjean Nicolas
Published: May 2011

We report on the growth conditions of AlInN layers grown on free-standing GaN substrates. We found that an average indium content of ∼20% is needed to obtain defect-free AlInN/GaN multilayers. This is larger than the commonly accepted value of 18% for lattice-matched condition. A model where tensile strain at the GaN/AlInN interface is induced by indium surface segregation occurring in AlInN layers is proposed to explain this discrepancy. A high In/Al flux ratio is shown to reduce this effect and allowed obtaining a defect-free AlInN/GaN Bragg reflector with a peak reflectivity of 99.6% suitable for vertical cavity light emitting devices.
Fingerprints of the hydrogen bond in the photoemission spectra of croconic acid condensed phase: An x-ray photoelectron spectroscopy and ab-initio study
Authors: Bisti F. , Stroppa A. , Picozzi S. , Ottaviano L.
Published: May 2011

The electronic structure of Croconic Acid in the condensed phase has been studied by comparing core level and valence band x-ray photoelectron spectroscopy experiments and first principles density functional theory calculations using the Heyd-Scuseria-Ernzerhof screened hybrid functional and the GW approximation. By exploring the photoemission spectra for different deposition thicknesses, we show how the formation of the hydrogen bond network modifies the O 1s core level lineshape. Moreover, the valence band can be explained only if the intermolecular interactions are taken into account in the theoretical approach.
Laser control of molecular excitations in stochastic dissipative media
Authors: Tremblay Jean Christophe
Published: May 2011

In the present work, ideas for controlling photochemical reactions in dissipative environments using shaped laser pulses are presented. New time-local control algorithms for the stochastic Schrödinger equation are introduced and compared to their reduced density matrix analog. The numerical schemes rely on time-dependent targets for guiding the reaction along a preferred path. The methods are tested on the vibrational control of adsorbates at metallic surfaces and on the ultrafast electron dynamics in a strong dissipative medium. The selective excitation of the specific states is achieved with improved yield when using the new algorithms. Both methods exhibit similar convergence behavior and results compare well with those obtained using local optimal control for the reduced density matrix. The favorable scaling of the methods allows to tackle larger systems and to control photochemical reactions in dissipative media of molecules with many more degrees of freedom.
Communication: A simple method for simulation of freezing transitions
Authors: Orkoulas G. , Nayhouse Michael
Published: May 2011

Despite recent advances, precise simulation of freezing transitions continues to be a challenging task. In this work, a simulation method for fluid-solid transitions is developed. The method is based on a modification of the constrained cell model which was proposed by Hoover and Ree [J. Chem. Phys. 47, 4873 (1967)]10.1063/1.1701730. In the constrained cell model, each particle is confined in a single Wigner-Seitz cell. Hoover and Ree pointed out that the fluid and solid phases can be linked together by adding an external field of variable strength. High values of the external field favor single occupancy configurations and thus stabilize the solid phase. In the present work, the modified cell model is simulated in the constant-pressure ensemble using tempering and histogram reweighting techniques. Simulation results on a system of hard spheres indicate that as the strength of the external field is reduced, the transition from solid to fluid is continuous at low and intermediate pressures and discontinuous at high pressures. Fluid-solid coexistence for the hard-sphere model is established by analyzing the phase transition of the modified model in the limit in which the external field vanishes. The coexistence pressure and densities are in excellent agreement with current state-of-the-art techniques.
Communication: Stable carbon nanoarches in the initial stages of epitaxial growth of graphene on Cu(111)
Authors: Van Wesep Robert G. , Chen Hua , Zhu Wenguang , Zhang Zhenyu
Published: May 2011

To fully exploit the device potential of graphene, reliable production of large-area, high-quality samples is required. Epitaxial growth on metal substrates have shown promise in this regard, but further improvement would be facilitated by a more complete understanding of the atomistic processes involved in the early growth stages. Using first-principles calculations within density functional theory, we have investigated the energetics and kinetics of graphene nucleation and growth on a Cu(111) surface. Our calculations have revealed an energetic preference for the formation of stable one-dimensional carbon nanoarches consisting of 3–13 atoms when compared to two-dimensional compact islands of equal sizes. We also estimate the critical cluster size that marks the transition from nanoarch dominance to island dominance in the growth sequence. Our findings may provide the structural link between nucleated carbon dimers and larger carbon nanodomes, and are expected to stimulate future experimental efforts.
Nanoscale rotational deformation in solids at high stresses
Authors: Ovid’ko I. A. , Sheinerman A. G.
Published: May 2011

A special physical mode of plastic flow and nanograin formation in nanocrystalline and polycrystalline solids deformed at high stresses is suggested and theoretically described. The mode represents the nanoscale rotational deformation (NRD) occurring through the collective events of ideal nanoscale shear in solids. We calculated its stress and energy characteristics. It is found that NRD can effectively occur in nanocrystalline and polycrystalline solids during dynamic loading.
Cold field emission from hydrogen exfoliated graphene composites
Authors: Baby Tessy Theres , Ramaprabhu Sundara
Published: May 2011

In the present letter, field emission property of highly wrinkled hydrogen exfoliated graphene (HEG) was studied using an indigenously fabricated setup under ∼10−6 mbar pressure. Graphene was coated on a flexible carbon cloth by spin coating for the field emission study. The turn on voltage obtained for a HEG field emitter is around 1.18 V/μm. Further, the field emission property of HEG was improved by decorating metal oxide over HEG. The stability of the field emitter was tested and field enhancement factor was calculated using Fowler–Nordheim plot.
Electrical passivation and chemical functionalization of SiC surfaces by chlorine termination
Authors: Schoell S. J. , Howgate J. , Hoeb M. , Auernhammer M. , Garrido J. A. , Stutzmann M. , Brandt M. S. , Sharp I. D.
Published: May 2011

We have developed a straightforward plasma-based method which yields chlorine-terminated n-type 6H-SiC surfaces. Atomic force microscopy shows that the surface roughness is not affected by the plasma processing. Additionally, x-ray photoelectron spectroscopy reveals a significant reduction in oxygen, and a corresponding rise of chlorine core level intensities, following halogen termination. Contact potential difference and surface photovoltage measurements show formation of negative surface dipoles and approximately flat band surface potentials after chlorine termination of (0001) n-type 6H-SiC (built-in voltage Vbi<20 meV). Starting from halogenated surfaces, we demonstrate both ultraviolet light-induced and thermally-induced functionalization with alkene-derived self-assembled organic monolayers.
Fast self-heating in GaN-based laser diodes
Authors: Scheibenzuber W. G. , Schwarz U. T.
Published: May 2011

We study the time evolution of the internal temperature of GaN-based laser diodes in pulsed operation using time resolved spectroscopy. Time dependent emission spectra are compared to continuous-wave measurements at different temperatures to relate changes in the longitudinal mode spectrum to the internal temperature. From the different shift in emission center and longitudinal modes, two subsystems are identified which heat up on different time scales: the charge carrier plasma and the crystal lattice. While the lattice takes several microseconds to reach thermal equilibrium, the plasma heats up within 20 ns after the onset of the electrical pulse. This behavior is attributed to the small heat capacity of the charge carrier plasma compared to the crystal lattice.
Communication: Avoiding unbound anions in density functional calculations
Authors: Kim Min-Cheol , Sim Eunji , Burke Kieron
Published: May 2011

Converged approximate density functional calculations usually do not bind anions due to large self-interaction error. But Hartree-Fock (HF) calculations have no such problem, producing negative HOMO energies. Thus, electron affinities can be calculated from density functional total energy differences using approximations such as PBE and B3LYP, evaluated on HF densities (for both anion and neutral). This recently proposed scheme is shown to work very well for molecules, better than the common practice of restricting the basis set except for cases such as CN, where the HF density is too inaccurate due to spin contamination.