Computational and experimental study of the cluster size distribution in MAPLE

A combined experimental and computational study is performed to investigate the origin and characteristics of the surface features observed in SEM images of thin polymer films deposited in matrix-assisted pulsed laser evaporation (MAPLE). Analysis of high-resolution SEM images of surface morphologies of the films deposited at different fluences reveals that the mass distributions of the surface features can be well described by a power-law, Y(N) ∝ N^−t, with exponent −t ≈ −1.6. Molecular dynamic simulations of the MAPLE process predict a similar size distribution for large clusters observed in the ablation plume. A weak dependence of the cluster size distributions on fluence and target composition suggests that the power-law cluster size distribution may be a general characteristic of the ablation plume generated as a result of an explosive decomposition of a target region overheated above the limit of its thermodynamic stability. Based on the simulation results, we suggest that the ejection of large matrix-polymer clusters, followed by evaporation of the volatile matrix, is responsible for the formation of the surface features observed in the polymer films deposited in MAPLE experiments.     

Causal particle detectors and topology

We investigate particle detector responses in some topologically non-trivial spacetimes. We extend a recently proposed regularization of the massless scalar field Wightman function in four-dimensional Minkowski space to arbitrary dimension, to the massive scalar field, to quotients of Minkowski space under discrete isometry groups and to the massless Dirac field. We investigate in detail the transition rate of inertial and uniformly accelerated detectors on the quotient spaces under groups generated by (t, x, y, z) ? (t, x, y, z + 2a), (t, x, y, z) ? (t, −x, y, z), (t, x, y, z) ? (t, −x, −y, z), (t, x, y, z) ? (t, −x, −y, z + a) and some higher dimensional generalizations. For motions in at constant y and z on the latter three spaces the response is time dependent. We also discuss the response of static detectors on the RP³ geon and inertial detectors on RP³ de Sitter space via their associated global embedding Minkowski spaces (GEMS). The response on RP³ de Sitter space, found both directly and in its GEMS, provides support for the validity of applying the GEMS procedure to detector responses and to quotient spaces such as RP³ de Sitter space and the RP³ geon where the embedding spaces are Minkowski spaces with suitable identifications.     

Phase transition in Ising, XY and Heisenberg magnetic films

The phase transition and magnetic properties of a ferromagnet spin-S, a disordered diluted thin and semi-infinite film with a face-centered cubic lattice are investigated using the high-temperature series expansions technique extrapolated with Padé approximants method for Heisenberg, XY and Ising models. The reduced critical temperature of the system τ_c is studied as function of the thickness of the thin film and the exchange interactions in the bulk, and within the surfaces J_b, J_s and J_⊥, respectively. It is found that τ_c increases with the exchange interactions of surface. The magnetic phase diagrams (τ_c versus the dilution x) and the percolation threshold are obtained. The shifts of the critical temperatures T_c(l) from the bulk value (T_c(∞)/T_c(l) − 1) can be described by a power law l^−λ, where λ = 1/υ is the inverse of the correlation length exponent.     

Single crystal EPR and optical absorption study of Cr doped l-histidine hydrochloride monohydrate

EPR study of the Cr³⁺ ion doped l-histidine hydrochloride monohydrate single crystal is done at room temperature. Two magnetically inequivalent interstitial sites are observed. The hyperfine structure for Cr⁵³ isotope is also obtained. The zero field and spin Hamiltonian parameters are evaluated from the resonance lines obtained at different angular rotations and the parameters are: D=(300±2)×10⁻⁴ cm⁻¹, E=(96±2)×10⁻⁴ cm⁻¹, g_x=1.9108±0.0002, g_y=1.9791±0.0002, g_z=2.0389±0.0002, A_x=(252±2)×10⁻⁴ cm⁻¹, A_y=(254±2)×10⁻⁴ cm⁻¹, A_z=(304±2)×10⁻⁴ cm⁻¹ for site I and D=(300±2)×10⁻⁴ cm⁻¹, E=(96±2)×10⁻⁴ cm⁻¹, g_x=1.8543±0.0002, g_y=1.9897±0.0002, g_z=2.0793±0.0002, A_x=(251±2)×10⁻⁴ cm⁻¹, A_y=(257±2)×10⁻⁴ cm⁻¹, A_z=(309±2)×10⁻⁴ cm⁻¹ for site II, respectively. The optical absorption studies of single crystals are also carried out at room temperature in the wavelength range 195-925 nm. Using EPR and optical data, different bonding parameters are calculated and the nature of bonding in the crystal is discussed. The values of Racah parameters (B and C), crystal field parameter (Dq) and nephelauxetic parameters (h and k) are: B=636, C=3123, Dq=2039 cm⁻¹, h=1.46 and k=0.21, respectively.     

A theoretical and matrix-isolation FT-IR investigation of the conformational landscape of N-acetylcysteine

The conformational landscape of N-acetylcysteine (NAC) has been investigated by a combined experimental matrix-isolation FT-IR and theoretical methodology. This combination is a powerful tool to study the conformational behavior of relatively small molecules. Geometry optimizations at the HF/3-21 level resulted in 438 different geometries with an energy difference smaller than 22 kJ mol⁻¹. Among these, six conformations were detected with a relative energy difference smaller than 10 kJ mol⁻¹ at the DFT(B3LYP)/6-31++G∗∗ level of theory. These were finally subjected to MP2/6-31++G∗∗ optimizations which resulted in five minima. The vibrational and thermodynamical properties of these conformations were calculated at both the DFT and MP2 methodologies. Experimentally NAC was isolated in an argon matrix at 16 K after being sublimated at 323 K. The most stable MP2 form appeared to be dominant in the experimental spectra but the presence of three other conformations with ΔE_MP2 < 10 kJ mol⁻¹ was also demonstrated. The experimentally observed abundance of the H-bond containing conformations appeared to be in good accordance with the predicted MP2 value.     

Effect of heating rates on microstructure and superconducting properties of pure MgB2

The influence of different heating rates, ranging from 5 to 30 K min⁻¹, on the microstructure and superconducting of the MgB₂ bulk was investigated. No obvious variation in the grain size was found for the samples heated from 5 K min⁻¹ to 20 K min⁻¹ except for the changes in morphologies. Moreover, the grain refinement was obtained under the heating rate of 30 K min⁻¹. The critical current density (J_c) suggested that the 5 K min⁻¹ sample had the best performance in high field. Here, the differential thermal analysis (DTA) was employed to analyze the kinetics of MgB₂ phase formation with the different heating rates. The results showed that the large amount of MgB₂ formed at low temperature, which lead to compact structures under the slow heating rate. The fast heating rate would promote the evaporation of Mg at high temperature, which was considered to generate the vacancy and impurities in the sample.     

Preparation and characterization of CdS/Si coaxial nanowires

CdS/Si coaxial nanowires were fabricated via a simple one-step thermal evaporation of CdS powder in mass scale. Their crystallinities, general morphologies and detailed microstructures were characterized by using X-ray diffraction, scanning electron microscope, transmission electron microscope and Raman spectra. The CdS core crystallizes in a hexagonal wurtzite structure with lattice constants of a=0.4140 nm and c=0.6719 nm, and the Si shell is amorphous. Five Raman peaks from the CdS core were observed. They are 1LO at 305 cm⁻¹, 2LO at 601 cm⁻¹, A₁-TO at 212 cm⁻¹, E₁-TO at 234 cm⁻¹, and E₂ at 252 cm⁻¹. Photoluminescence measurements show that the nanowires have two emission bands around 510 and 590 nm, which originate from the intrinsic transitions of CdS cores and the amorphous Si shells, respectively.     

Electrochemical investigation of the surface energy: Effect of the HF concentration on electroless silver deposition onto p-Si (1 1 1)

Electroless silver deposition onto p-silicon (1 1 1) from 0.005 mol l⁻¹ AgNO₃ solutions with different HF concentration was investigated by using an electrochemical direct current polarization method and open circuit potential-time (Ocp-t) technique. The fact that three-dimensional (3D) growth of silver onto silicon is favored with increasing the HF concentration was ascribed to the drop of the surface energy and approved by electrochemical direct current polarization, Ocp-t technique and atomic force microscopy (AFM). The drop slope of open-circuit potential, K_{−ΔE(OCP)/t}, was educed from the mixed-potential theory. K_{−ΔE(OCP)/t} as well as the deposition rate determined by an inductively coupled plasma atomic emission spectrometry (ICP-AES), increased with the HF concentration, yet was not a linear function. Results were explained by the stress generation and relaxation mechanisms.     

Studies of the phosphorescence of polycrystalline hafnia

Persistent phosphorescence induced by ultraviolet light in polycrystalline HfO₂ and enhancement of the phosphorescence by sintering are investigated. The phosphorescence afterglow emission is in the 1.8-3.2 eV spectral range, with a peak at 2.53 eV. The afterglow intensity is significantly increased by sintering in either inert atmosphere or air. The afterglow light sum measured at room temperature for samples sintered at 1500 °C is more than an order of magnitude higher than that before sintering. In the temperature range −50 to 200 °C, three thermoluminescence (TL) peaks are observed near −10, 30, and 100 °C. The relative contribution of the low-temperature TL peak to the total TL intensity decreases after sintering, and this effect is more pronounced upon sintering in inert atmosphere. Conversely, the contribution of the TL peak near 100 °C increases after sintering. The enhancement of the afterglow by sintering is associated with the observed increase in the intensity of TL peaks at and above room temperature and attributed to an increase in the number of deep charge traps. The room-temperature afterglow time decay has a form consistent with the second-order mechanism, ∝(t₀+t)⁻ⁿ, and the best-fit values of both fitting parameters t₀ and n tend to increase with the sintering temperature.     

Effect of the spiral phase element on the radial-polarization (0, 1) LG beam

Radially-polarized beams can be strongly amplified without significant birefringent-induced aberrations. However, radially-polarized beam is a high-order beam, and therefore has to be transformed into a fundamental Gaussian beam for reduction the beam-propagation factor M². In effort to transform the radially-polarized beam to a nearly-Gaussian beam, we consider effect of a spiral phase element (SPE) on the Laguerre-Gaussian (LG) (0, 1)^∗ beam with radial polarization, and compare this with the case when the input beam is a LG (0, 1)^∗ beam with spiral phase and uniform or random polarization. The LG (0, 1)^∗ beam with radial polarization, despite its identity in intensity profile to the beam with spiral phase, has distinctly different properties when interacting with the SPE. With the SPE and spatial filter, we transformed the radially-polarized (0, 1)^∗ mode with M² = 2.8 to a nearly-Gaussian beam with M² = 1.7. Measured transformation efficiency was 50%, and the beam brightness P/(M²)² was practically unchanged. The SPE affects polarization state of the radially-polarized beam, leading to appearance of spin angular momentum in the beam center at the far-field.     

Annealing induced microstructural evolution of electrodeposited electrochromic tungsten oxide films

A significant influence of microstructure on the electrochromic and electrochemical performance characteristics of tungsten oxide (WO₃) films potentiostatically electrodeposited from a peroxopolytungstic acid (PPTA) sol has been evaluated as a function of annealing temperature. Powerful probes like X-ray diffractometry (XRD), transmission electron microscopy (TEM), UV-vis spectrophotometry, multiple step chronoamperometry and cyclic voltammetry have been employed for the thin film characterization. The as-deposited and the film annealed at 60 °C are composed of nanosized grains with a dominant amorphous phase, as well as open structure which ensues from a nanoporous matrix. This ensures a greater number of electroactive sites and a higher reaction area thereby manifesting in electrochromic responses superior to that of the films annealed at higher temperatures. The films annealed at temperatures ≥250 °C are characterized by a prominent triclinic crystalline structure and a hexagonal phase co-exists at temperatures ≥400 °C. The deleterious effect on the electrochromic properties of the film with annealing is ascribed to the loss of porosity, densification and the increasing crystallinity and grain size. Amongst all films under investigation, the film annealed at 60 °C exhibits a high transmission modulation (ΔT ∼ 68%) and coloration efficiency (η ∼ 77.6 cm² C⁻¹) at λ = 632.8 nm, charge storage capacity (Q_ins ∼ 21 mC cm⁻²), diffusion coefficient (6.08 × 10⁻¹⁰ cm² s⁻¹), fast color-bleach kinetics (t_c ∼ 275 s and t_b ∼ 12.5 s) and good electrochemical activity, as well as reversibility for the lithium insertion-extraction process upon cycling. The remarkable potential, which the film annealed at 60 °C has, for practical “smart window” applications has been demonstrated.     

Measurement of K production in the reaction Kp → Kp

The K^∗− spectrum in the reaction K⁻+p → K^∗−+p has been measured at beam momenta 10.9, 13.4 and 15.9 GeV/c using the missing mass technique. Production of the L(1770), and a Q-K^∗(1420) enhancement are observed. Differential cross sections in the range of momentum transfer 0.12 < | t_pp | < 0.40 (GeV/c)² are given. The L meson is observed with a width Γ = 100 ± 26 MeV. The mass spectrum between the L and 2.5 GeV does not show significant structure.     

A parallel algorithm for solving the 3d Schrödinger equation

We describe a parallel algorithm for solving the time-independent 3d Schrödinger equation using the finite difference time domain (FDTD) method. We introduce an optimized parallelization scheme that reduces communication overhead between computational nodes. We demonstrate that the compute time, t, scales inversely with the number of computational nodes as t ∝ (N_nodes)^{−0.95 ± 0.04}. This makes it possible to solve the 3d Schrödinger equation on extremely large spatial lattices using a small computing cluster. In addition, we present a new method for precisely determining the energy eigenvalues and wavefunctions of quantum states based on a symmetry constraint on the FDTD initial condition. Finally, we discuss the usage of multi-resolution techniques in order to speed up convergence on extremely large lattices.     

Electrical transport properties of CaMnO3 thermoelectric compound: a theoretical study

The electrical transport coefficients of anti-ferromagnetic CaMnO₃ have been investigated by density functional theory calculation within generalized gradient approximation. The calculated transport coefficients exhibit the anisotropic nature, in agreement with its electronic states. The transport property results reveal the stronger carrier transport along the O1–Mn–O1 plane within the O–Mn–O octahedron, indicating that the Mnd and O1p orbitals are mainly responsible for electrical transport. The maximum power factor values as a function of relaxation time reach 8.4×10²³ Wm⁻¹ K⁻² s⁻¹, 7.9×10²³Wm⁻¹ K⁻² s⁻¹ and 4.9×10²³ Wm⁻¹ K⁻² s⁻¹ within c, a and b direction, respectively. The dimensionless figure of merit ZTxx, ZTyy as well as ZTzz is estimated with 1.28, 0.8 and 1.37 at 1000 K, respectively.     

On a possible variation of the proton-to-electron mass ratio: H2 spectra in the line of sight of high-redshift quasars and in the laboratory

Recently the finding of an indication for a decrease of the proton-to-electron mass ratio μ = m_p/m_e by 0.002% in the past 12 billion years was reported in the form of a Letter [E. Reinhold, R. Buning, U. Hollenstein, P. Petitjean, A. Ivanchik, W. Ubachs, Phys. Rev. Lett. 96 (2006) 151101]. Here we will further detail the methods that led to that result and put it in perspective. Laser spectroscopy on molecular hydrogen, using a narrow-band and tunable extreme ultraviolet laser system at the Laser Centre Vrije Universiteit Amsterdam, results in transition wavelengths of spectral lines in the Lyman and Werner band systems at an accuracy of (4-11) × 10⁻⁸, depending on the wavelength region. This corresponds to an absolute accuracy of 0.000004-0.000010 nm. A database of 233 accurately calibrated H₂ lines is presented here for future reference and comparison with astronomical observations. Recent observations of the same spectroscopic features in cold hydrogen clouds at redshifts z = 2.5947325 and z = 3.0248970 in the line of sight of two quasar light sources (Q 0405−443 and Q 0347−383) resulted in 76 reliably determined transition wavelengths of H₂ lines at accuracies in the range 2 × 10⁻⁷ to 1 × 10⁻⁶. Those observations were performed with the Ultraviolet and Visible Echelle Spectrograph at the Very Large Telescope of the European Southern Observatory at Paranal, Chile. A third ingredient in the analysis is the calculation of an improved set of sensitivity coefficients K_i, a parameter associated with each spectral line, representing the dependence of the transition wavelength on a possible variation of the proton-to-electron mass ratio μ. The new model for calculation of the K_i sensitivity coefficients is based on a Dunham representation of ground state and excited state level energies, derived from the most accurate data available in literature for the ground electronic state and the presently determined level energies in the and C¹Π_u states. Moreover, the model includes adiabatic corrections to electronic energies as well as local perturbation effects between B and C levels. The full analysis and a tabulation of the resulting K_i coefficients is given in this paper. A statistical analysis of the data yields an indication for a variation of the proton-to-electron mass ratio of Δμ/μ = (2.45 ± 0.59) × 10⁻⁵ for a weighted fit and Δμ/μ = (1.99 ± 0.58) × 10⁻⁵ for an unweighted fit. This result, indicating the decrease of μ, has a statistical significance of 3.5σ. Mass-variations as discussed relate to inertial or kinematic masses, rather than gravitational masses. Separate treatment of the data gives a similar positive result for each of the quasars Q 0405−443 and Q 0347−383. The statistical analysis is further documented and possible systematic shifts underlying the data, with the possibility of mimicking a non-zero Δμ/μ value, are discussed. The observed decrease in μ corresponds to a rate of change of d lnμ/dt = −2 × 10⁻¹⁵ per year, if a linear variation with time is assumed. Experiments for detecting a possible variation of μ in the modern epoch via ultra-precision experiments on H₂ quadrupole transitions are proposed.     

High resolution infrared spectroscopy of [1.1.1]propellane: The region of the ν9 band

The region of the infrared-active band of the ν₉ CH₂ bending mode [1.1.1]propellane has been recorded at a resolution (0.0025 cm⁻¹) sufficient to distinguish individual rovibrational lines. This region includes the partially overlapping bands ν₉ (e′) = 1459 cm⁻¹, 2ν₁₈ (l = 2, E′) = 1430 cm⁻¹, ν₆ + ν₁₂ (E′) = 1489 cm⁻¹, and ν₄ + ν₁₅ (A₂″) = 1518 cm⁻¹. In addition, the difference band ν₄ − ν₁₅ (A₂″) was observed in the far infrared near 295 cm⁻¹ and analyzed to give good constants for the upper ν₄ levels. The close proximities of the four bands in the ν₉ region suggest that Coriolis and Fermi resonance couplings could be significant and theoretical band parameters obtained from Gaussian ab initio calculations were helpful in guiding the band analyses. The analyses of all four bands were accomplished, based on our earlier report of ground state constants determined from combination differences involving more than 4000 pairs of transitions from five fundamental and four combination bands. This paper presents the analyses and the determination of the upper state constants of all four bands in the region of the ν₉ band. Complications were most evident in the 2ν₁₈ (l = 2, E′) band, which showed significant perturbations due to mixing with the nearby 2ν₁₈ (l = 0, A₁′) and ν₄ + ν₁₂ (E′) levels which are either infrared inactive as transitions from the ground state, or, in the latter case, too weak to observe. These complications are discussed and a comparison of all molecular constants with those available from the ab initio calculations at the anharmonic level is presented.     

Heat capacities of the electron acceptor 7,7,8,8-tetracyano- quinodimethane (TCNQ) and its radical-ion salt NH4(TCNQ) showing spin-Peierls transition

Heat capacities of the electron acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its radical-ion salt NH₄-TCNQ have been measured at temperatures in the 12-350 K range by adiabatic calorimetry. A λ-type heat capacity anomaly arising from a spin-Peierls (SP) transition was found at 301.3 K in NH₄-TCNQ. The enthalpy and entropy of transition are Δ_trsH=(667±7) J mol⁻¹ and Δ_trsS=(2.19±0.02) J K⁻¹ mol⁻¹, respectively. The SP transition is characterized by a cooperative coupling between the spin and the phonon systems. By assuming a uniform one-dimensional antiferromagnetic (AF) Heisenberg chains consisting of quantum spin (S=1/2) in the high-temperature phase and an alternating AF nonuniform chains in the low-temperature phase, we estimated the magnetic contribution to the entropy as Δ_trsS_mag=0.61 J K⁻¹ mol⁻¹ and the lattice contribution as Δ_trsS_lat=1.58 J K⁻¹ mol⁻¹. Although the total magnetic entropy expected for the present compound is R ln 2 (=5.76 J K⁻¹ mol⁻¹), a majority of the magnetic entropy (∼4.6 J K⁻¹ mol⁻¹) persists in the high-temperature phase as a short-range-order effect. The present thermodynamic investigation quantitatively revealed the roles played by the spin and the phonon at the SP transition. Standard thermodynamic functions of both compounds have also been determined.     

Probing dense baryonic matter with strangeness

Strange particles allow to probe the properties of hot and dense baryonic matter. At incident energies close to the production threshold medium effects on the strange hadrons are magnified. Available data on K^0,+,−K^∗ (892), Φ, Λ, and Σ^∗ (1385) are summarized for the system Al+Al at an incident energy of 1.9 AGeV and compared to statistical model calculations. Results of searches for multi-baryonic strange bound states in the Λp-channel are presented.     

Etch front roughening in wrinkly metal

Scaling behavior of wrinkle evolution due to chemical etching of Au on stretchable substrate is studied by atomic force microscopy. The surfaces were etched with a small drop of KI solution. Scaling exponent α decreased with the etch time, from 0.93 to 0.62. For dynamic exponent β up to 30 min we find it to be 0.16 ± 0.05. Within the same time frame, the wavelength increases as ∼t^0.23+−0.05 but drops at 45 min as ∼t^{−1.22+−0.42} and saturates. At this stage, we observe percolation island type features along with some wrinkles. The measured value of α at this stage is 0.62 ± 0.02; consistent with the percolation model. The wavelength coarsening at this stage suggests the existence of chemically driven spinodal decomposition. Our obtained value of α and β is consistent with the value of z obtained from correlation length which is given by ξ = t^1/z. We find z = 4.87 ± 0.50. Our experiment suggests wrinkle formation in thin films on viscoelastic surface occurs in three stages.     

Nanostructuring of Fe films by oblique incidence deposition on a FeSi2 template onto Si(1 1 1): Growth, morphology, structure and faceting

The growth of thin Fe films deposited at oblique incidence on an iron silicide template onto Si(1 1 1) single crystal has been investigated as a function of Fe thickness (0 < t_Fe ? 180 monolayers (MLs)) and incidence angle (0 ? θ ? 80°). The growth mode is determined in situ by means of scanning tunnelling microscopy (STM) and low energy electron diffraction (LEED). Stripes oriented perpendicularly to the incident atomic flux are formed for θ ? 30°. Self-correlation functions are used to extract characteristic lengths from STM images. The correlation lengths in the direction of the incident flux (ξ_x) and perpendicular to the atomic flux (ξ_y) grow with different powers versus time (ξ_x∝t^σ and ξ_y∝t^ρ, with σ = 0.34 ± 0.03 and ρ = 0.67 ± 0.03) following the exact solution of the (1 + 1) dimensional Kardar-Parisi-Zhang (KPZ) equation. The root mean square roughness follows also a scaling law for t_Fe < 120 ML leading to a growth exponent β = 0.73 ± 0.02. Shadowing and steering effects are discussed on the basis of our STM data.