High-resolution visible laser spectroscopy of HfF

The electronic spectrum of hafnium monofluoride has been investigated from 415 to 725 nm using a laser-ablation/molecular beam laser-induced fluorescence spectrometer. Several electronic systems were observed and data have been recorded at both low and high resolution. High resolution rotational analyses of the [17.4]1.5-X1.5 (0-0), [17.9]2.5-X1.5 (0-0), [19.7]0.5-X1.5 (0-0), [20.0]0.5-X1.5 (0-0), [21.1]2.5-X1.5 (0-0), [22.3]1.5-X1.5 (0-0), and [23.3]0.5-X1.5 (0-0) subbands have been carried out, resulting in accurate values for the ground and excited state effective rotational constants. Furthermore, the rotational analysis of the subbands assigned as [17.4]1.5-X1.5 (1-0) and [17.9]2.5-X1.5 (1-0) allows us to determine values of 589.7569(6) and 588.9076(6) cm⁻¹ for ΔG^′_1/2 [17.4] and ΔG^′_1/2 [17.9], respectively. From dispersed fluorescence data we find that ΔG^′′_1/2=670(13) cm⁻¹ for the ground state and that another low-lying electronic state lies at ∼2850 cm⁻¹. The data also suggests that a second low-lying electronic state lies at ∼5200 cm⁻¹ above the ground state.     

Quantum phase transitions in cascading gauge theory

We study N=1 supersymmetric SU(K+P)×SU(K) cascading gauge theory of Klebanov et al. (2000) [1] and [2] on R×S³ at zero temperature, and at the origin of the baryonic branch. A radius of S³ sets a compactification scale μ. An interplay between μ and the strong coupling scale Λ of the theory leads to an interesting pattern of quantum phases of the system. For μ?μχSB=1.240467(8)Λ the vacuum state of the theory is chirally symmetric. At μ=μχSB the theory undergoes the first-order transition to a phase with spontaneous breaking of the chiral symmetry. We further demonstrate that the chirally symmetric state of cascading gauge theory becomes perturbatively unstable at scales below μc=0.950634(5)μχSB. Finally, we point out that for μ<1.486402(5)Λ the stress-energy tensor of cascading gauge theory can source inflation of a closed Universe.     

High-order incompressible large-eddy simulation of fully inhomogeneous turbulent flows

The subgrid-scale (SGS) eddy-viscosity model developed by Vreman [Phys. Fluids 16 (2004) 3670] and its dynamic version [Phys. Fluids 19 (2007) 065110] are tested in large-eddy simulations (LES) of the turbulent flow in an Re = 12,000 lid-driven cubical cavity by comparison to the direct numerical simulation (DNS) data of Leriche and Gavrilakis [Phys. Fluids 12 (2000) 1363]. This appears to be the first test of this class of model to flows without any homogeneous flow directions, which is typical of flows in complex geometries. Additional LES predictions at Re = 18,000 and Re = 22,000 are compared to the DNS data of Leriche [J. Sci. Comp. 27 (2006)]. The new LES framework yielded excellent agreement for both the mean velocity and Reynolds stress profiles and matches DNS data better than the more traditional Smagorinsky-based SGS models.     

Matrix-enhanced cluster-SIMS

We have used 23 keV C₆₀⁺ projectiles in the event-by-event bombardment and detection mode to investigate the emission of the gramicidin S [M − H]⁻ ion embedded in a matrix of sinapic acid. We have observed an increase in the gramicidin S [M − H]⁻ ion of approximately eight times by controlling the ratio of gramicidin S to sinapic acid. The maximum of the gramicidin S [M − H]⁻ yield occurs at a matrix/analyte ratio of 10:1. This ratio is different from those typically used in matrix-assisted laser desorption/ionization.     

Deformation of single crystal sample using D-DIA apparatus coupled with synchrotron X-rays: In situ stress and strain measurements at high pressure and temperature

We present a technique for high pressure and high temperature deformation experiment on single crystals, using the Deformation-DIA apparatus at the X17B2 beamline of the NSLS. While deformation experiments on polycrystalline samples using D-DIA in conjunction with synchrotrons have been previously reported, this technical paper focuses on single crystal application of the technique. Our single crystals are specifically oriented such that only [1 0 0] slip or [0 0 1] slip in (0 1 0) plane is allowed. Constant applied stress (sigma <300 MPa) and specimen strain rates were monitored using in situ time-resolved X-ray diffraction and radiography imaging, respectively. Rheological properties of each activated slip system in the crystals can be revealed using this technique. In this paper, we describe the principle of sample preparation (e.g. [1 1 0]_c and [0 1 1]_c orientations) to activate specific slip systems (i.e. [1 0 0](0 1 0) and [0 0 1](0 1 0), respectively), stress measurement and procedures of the deformation experiments.     

First-principles study of elastic and thermo-physical properties of kesterite-type Cu2ZnSnS4

The lattice constants and elastic constants of the kesterite-type Cu₂ZnSnS₄ have been calculated using density-functional theory (DFT). The calculated lattice constants are in good agreement with the experimental data. The calculated elastic constants indicate that the bonding strength along the [1 0 0] and [0 1 0] directions is as strong as the one along the [0 0 1] direction. The high B/G ratio shows that the kesterite-type Cu₂ZnSnS₄ compound has ductile behavior. Finally, using the Debye model, the volume, bulk modulus and heat capacity as a function of temperature for the kesterite-type CZTS have been estimated at different pressures. The Debye temperature and Gruneisen parameter are 157 K and 2.28 at 300 K temperature, respectively. The present results can give some information for the design of the kesterite-type CZTS compounds, and these can also be used to stimulate future experimental and theoretical work.     

Spectroscopy of NdO: New results

¹⁴²NdO molecules have been produced by heating ¹⁴²Nd₂O₃ to about 2100 K in a vacuum furnace in the presence of argon gas. A ring dye laser operating with DCM dye has been used to excite ¹⁴²NdO transitions in the 636-666 nm spectral region, and induced fluorescence has been spectroscopically analysed at high resolution with a Fourier transform spectrometer. Contributions from thermal emission have been simultaneously observed. Two new low-lying electronic states have been detected, at energies of about 2708 and 4139 cm⁻¹, designated as [2.7], most probably observed at ν = 1, and [4.1], likely to be (2)6 (observed at ν = 0). The ν = 1 level of the (1)6 state, already known at ν = 0, has been observed for the first time. Most levels pumped by the laser, between 14 000 and 17 400 cm⁻¹, could be identified from earlier work. In addition, by studying in more detail recently obtained fluorescence spectra [J. Mol. Spectrosc. 225 (2004) 132] spectroscopic constants have been improved for a number of states. Finally, from thermal emission spectra, rotational analyses of the 0-0 bands of two new systems, [16.4] − (2)5 and [14.1] − X4, and reanalyses at higher resolution of the 0-0 bands of the systems V, VII, VIII, and X have been carried out. A consistent set of spectroscopic constants of the levels of ¹⁴²NdO characterized as yet is presented.     

Effect of deposition pressure on structural, optical and electrical properties of zinc selenide thin films

ZnSe thin films have been prepared by inert gas condensation method at different gas pressures. The influence of deposition pressure, on structural, optical and electrical properties of polycrystalline ZnSe films have been investigated using X-ray diffraction (XRD), optical transmission and conductivity measurements. The X-ray diffraction study reveals the sphalerite cubic structure of the ZnSe films oriented along the (1 1 1) direction. The structural parameters such as particle size [6.65-22.24 nm], strain [4.01-46.6×10⁻³ lin⁻² m⁻⁴] and dislocation density [4.762-18.57×10¹⁵ lin m⁻²] have been evaluated. Optical transmittance measurements indicate the existence of direct allowed optical transition with a corresponding energy gap in the range 2.60-3.00 eV. The dark conductivity (σ_d) and photoconductivity (σ_ph) measurements, in the temperature range 253-358 K, indicate that the conduction in these materials is through an activated process having two activation energies. σ_d and σ_ph values decrease with the decrease in the crystallite size. The values of carrier life time have been calculated and are found to decrease with the reduction in the particle size. The conduction mechanism in present samples has been explained, and the density of surface states [9.84-21.4×10¹³ cm⁻²] and impurity concentration [4.66-31.80×10¹⁹ cm⁻³] have also been calculated.     

Development of a miniature STM holder for study of electronic conductance of metal nanowires in UHV-TEM

A new miniature scanning tunneling microscope (STM) holder was developed in order to simultaneously investigate electronic conductance and structure of nanowires in an ultra high-vacuum electron microscope (UHV-TEM). A thin gold wire held between the STM tip and substrate stage of the specimen holder was stretched to form a suspended gold nanowire. The new TEM-STM system allowed us to measure electronic conductance at intervals of 20 ms, and to record high-resolution TEM images on videotape at 30 fps. Suspended gold nanowires formed from [1 1 0] oriented electrodes were well-elongated. In contrast, [1 0 0] and [1 1 1] electrodes produced nanowires with short necks. Electronic conductance was found to change as nanowire structure changed, with conductance quantization in units of 2e²/h, where e is the electron charge and h is Planck’s constant, only being exhibited for well-elongated nanowires.     

Effect of C layer on the structures and magnetic properties of (0 0 1)-oriented [C/CoPt]n/Ag films

CoPt/Ag and [C/CoPt]n/Ag thin films have been prepared onto the glass substrates by magnetron sputtering. We investigated the evolution of texture and magnetic properties of CoPt/Ag and [C/CoPt]n/Ag films. The results show that C-doping plays an important role in improving (0 0 1) texture, improving the order parameter S, reducing the intergrain interactions, and making the magnetization reversal mechanism more close to Stoner-Wolfarth rotational mechanism. The growth mechanism of (0 0 1) texture also seems to be related strongly to the films thickness. Our results show that the highly (0 0 1)-oriented films with ordered fct phase have a significant potential for the perpendicular media of extremely high-density recording.     

The crystallographic texture and magnetic anisotropy of melt-spun CeCo5.4 ribbon

CeCo_5.4 ribbons have been prepared by melt spinning at wheel speeds v=5, 15, 25 and 35 m/s. The ribbons are essentially single 1:5 phase, and have significant crystallographic texture and magnetic anisotropy. The ribbon's longitudinal direction is easy direction, and normal to the ribbon plane direction is hard direction. For v=5 m/s [1 1 1]-axes of the grains near non-contact surface of the ribbon are normal to the ribbon plane. With increase of v, [1 1 0] and [2 0 0]-axes of the grains near non-contact surface rotate toward the normal direction and the c-axes parallel to the ribbon plane. The anisotropy increases up to v=25 m/s and then decreases. The grains near contact-wheel surface are randomly oriented for all v. The coercivity increases with increase of v due to decrease of the grain size. The values of coercivity are smaller in the easy direction and are larger in the hard direction, meaning that the coercivity mechanism is mainly characterized by domain wall pinning.     

A comparative study of fibre-textured and epitaxial Nb-doped Pb(Zr0.53Ti0.47)O3 thin films on different substrates

Fibre-textured and epitaxial Nb-doped Pb(Zr_0.53Ti_0.47)O₃ (PNZT) thin films were grown on the different substrates by a sol-gel process. The [1 0 0]- and [1 1 1]-fibre-textured polycrystalline PNZT films were obtained on platinized silicon substrates by introducing PbO and TiO₂ seeding layers, while the [0 0 1]- and [1 1 1]-oriented epitaxial PNZT films were formed directly on Nb-doped SrTiO₃ (Nb:STO) single-crystal substrates with (1 0 0) and (1 1 1) surfaces, respectively. The preferential orientation and phase structure of the fibre-textured and epitaxial PNZT films, as well as their influences on the electrical properties were investigated. Higher remnant polarization (P_r) and piezoelectric coefficient (d₃₃) were obtained for the epitaxial PNZT films on Nb:STO substrates than that for the fibre-textured ones on platinized silicon substrates. For both fibre-textured and epitaxial cases, the PNZT films with [1 0 0]/[0 0 1] orientations show higher piezoelectric responses than [1 1 1]-oriented ones, whereas better ferroelectric properties can be obtained in the latter. The intrinsic and extrinsic contributions were discussed to explain the difference in electrical properties for differently oriented fibre-textured and epitaxial PNZT films on different substrates.     

Ab initio potential energy surface and pressure broadening coefficients for the He-CH3F complex

Symmetry-adapted perturbation theory has been applied to compute the He-CH₃F potential with the CH₃F molecule assumed rigid. The potential has a global minimum of −48.9 cm⁻¹ at the center of mass separation of 7.2 bohr with the helium atom lying along the C-F bond on the hydrogen’s side. The computed points were fitted to an analytic energy surface with a correct asymptotic behaviour. This potential has been used to compute the pressure broadening (PB) coefficients for the (j, k) = (0, 0) → (1, 0) and (1, 0) → (2, 0) rotational transitions of CH₃F perturbed by helium for a wide range of temperatures. Close-coupling results are compared with the experimental data of Willey et al. [J. Chem. Phys. 97 (1992) 4723], Beaky et al. [J. Mol. Struct. 352/353 (1995) 245] and infinite order sudden results are compared with those of Grigoriev et al. [J. Mol. Struct. 186 (1997) 48] for the ν₆ band of CH₃F perturbed by helium at room temperature. To our knowledge, present work is the first attempt of making fully ab initio calculations of collisional cross-sections and pressure broadening coefficients for this simple symmetric top system at low and room temperature.     

L10 FePt thin films with [0 0 1] crystalline growth fabricated by SiO2 addition—rapid thermal annealing and dot patterning of the films

FePt films that have a high degree of order S in their L1₀ structure (S>0.90) and well-defined [0 0 1] crystalline growth perpendicular to the film plane were fabricated on thermally oxidized Si substrates by the addition of an oxide and successive rapid thermal annealing (RTA). The mechanism of L1₀ ordering and [0 0 1] crystalline growth perpendicular to the film plane arising through the oxide addition and RTA process is also discussed. The L1₀ ordering (S>0.90) and the [0 0 1] crystalline growth were achieved by (1) lowering the activation energy due to in-plane tensile stress and the initiation of L1₀ ordering at a low temperature, (2) [0 0 1] crystalline growth through in-plane tensile stress, and (3) enhancement of atomic diffusion via the addition of an oxide and the resultant lowering of the ordering temperature. Effect (1) was observed in the case of SiO₂ addition, effect (2) was generally observed in the case of oxide addition and the RTA process, and effect (3) was prominent in the case of ZnO addition. With the addition of ZnO, the L1₀ ordering started at below 400 °C and was completed at 500 °C. Finally, dot patterns were successfully fabricated down to a diameter of 15 nm using electron beam lithography, and the magnetic state of the dot pattern was observed by magnetic force microscopy.     

Scanning tunneling microscopy images of Al0.2GA0.8As-{1 1 0}: Influence of applied bias voltage and comparison between filled- and empty-states images

Bias-voltage dependent images of Al_0.2Ga_0.8As-{1 1 0} are presented. The images show both the filled- and empty-states of the surface. Apart from voltage-dependent changes in the apparent direction of the atomic rows also observed for binary III-V semiconductors, bright and dark areas about 2 nm in diameter appear at small voltage in filled-states images, and small ridges along [−1 1 0] appear at small voltage in empty-states images. The spatial extent of the bright and dark areas observed in filled-states images is thought to be determined by the electron-electron interaction. It is also shown that when a given patch of Al_0.2Ga_0.8As-{1 1 0} surface is imaged simultaneously in the filled- and empty-states mode, the locations and spatial extent of the alloy-related minima (the “dark patches”) do not coincide. This casts doubt on the assumption that a locally decreased tunneling probability represents an increased local content of Al.     

The influence of the cholesterol microenvironment in tissue sections on molecular ionization efficiencies and distributions in ToF-SIMS

High-resolution images of cholesterol were obtained from Lymnaea stagnalis nervous tissue using metal-assisted (MetA) time-of-flight secondary ion mass spectrometry (ToF-SIMS). The spatial distributions of different pseudomolecular ions of cholesterol [M − H]⁺, [M − OH]⁺ and [2M + Au]⁺, illustrate the influence of the tissue microenvironment on the ionization efficiencies of these ions. These biological matrix effects result in differences in localizations of molecular ions derived from the same molecular species.     

The permanent electric dipole moment of holmium monoxide, HoO

The high resolution molecular beam laser-induced fluorescence (LIF) spectrum of the (0, 0) band of the [17.6]7.5-X₁8.5 system of holmium monoxide, HoO, has been recorded field free and in the presence of a variable static electric field. The rotational constant, B₀, and magnetic hyperfine structure constant, h, for the v = 0 levels of the [17.6]7.5 and X₁8.5 states were obtained by assigning and fitting the field-free spectral features. The Stark shifts and splittings in the LIF spectra were analyzed to produce values for the magnitude of the permanent electric dipole moments, |μ|, of 3.96(6) and 4.80(5) D for [17.6]7.5, v = 0 and X₁8.5, v = 0, respectively.     

Effect of uniaxial stress cooling on the neutron magnetic scattering in a ZnCr2O4 single crystal

For the determination of the magnetic structure, elimination of the magnetic domains is indispensable and the appropriate stress will be effective when the crystal has a structural transformation at T_N. The effect of uniaxial stress cooling on the neutron magnetic scattering in a ZnCr₂O₄ single crystal was studied. The pressure axis was set parallel to the [1 0 0] or [0 0 1], parallel or perpendicular to the diffraction plane, (0 0 1). The [1 0 0] pressure cooling increases one half-integer spots along the [1 0 0], whereas decreases others. The [0 0 1] pressure cooling increases two half-integer spots in the (0 0 1) plane, whereas decreases one half-integer spots.     

Spontaneously broken quark helicity symmetry

We discuss the origin of chiral-symmetry breaking in the light-cone representation of QCD. In particular, we show how quark helicity symmetry is spontaneously broken in SU (N) gauge theory with massless quarks if that theory has a condensate of fermion light-cone zero modes. The symmetry breaking appears as induced interactions in an effective light-cone Hamiltonian equation based on a trivial vacuum. The induced interaction is crucial for generating a splitting between pseudoscalar and vector meson masses, which we illustrate with spectrum calculations in some 1 + 1-dimensional reduced models of gauge theory.