Deactivation of Pd particles supported on Nb2O5/Cu3Au(1 0 0): SFG and TPD studies from UHV to 100 mbar

Structural changes that occur on Pd-Nb₂O₅/Cu₃Au(1 0 0) model catalysts upon thermal annealing were followed by sum frequency generation (SFG) and temperature-programmed desorption (TPD) using CO as probe molecule. SFG experiments were performed both under ultrahigh vacuum and mbar pressure. Heating the catalyst to temperatures above 300 K lead to an irreversible 50% decrease in the CO adsorption capacity and modified the remaining adsorption sites. Alterations of the phase between resonant and non-resonant SFG signals upon annealing indicate a change in the electronic structure of the surface, which excludes Pd sintering or migration of Nb₂O₅ over Pd particles to cause the observed effect and rather suggests the formation of “mixed Pd-NbO_x” sites. The same changes in surface properties also occur during CO hydrogenation at 1 bar and high temperature, pointing to an involvement of “mixed Pd-NbO_x” sites in catalytic reactions.     

“Quantization” of the q-vector in microcrystals of K0.3MoO3 and NbSe3

We report steps of conductivity between discrete conducting states in microsamples of quasi-one-dimensional conductors K_0.3MoO₃ and NbSe₃. The steps reveal single phase-slip events, and the discrete states reveal “quantization” of the wave vector q of the charge-density wave. The “quantization” is observed due to the coherence of the CDW within the sample volume and tight boundary conditions at the contacts. The distribution of steps in temperature gives the temperature dependences of the q-vectors, while the steps' height provides the carriers' mobilities. For the case of NbSe₃ we give the 1st direct confirmation of the extremely high mobility of the “pocket” holes at low temperatures.     

Consequences of “one dimensional” electron energy bands

Recently a “one-dimensional” band hypothesis was invoked to explain giant low temperature specific heats in (Ti₂O₃)_x (V₂O₃)_1−x, 0.9 ? x ? 1.0. We discuss the consequences of “one-dimensionality” and show that, for Ti₂O₃, theory and resistivity experiments do not support this hypothesis.     

High resolution study of AsHD2: Ground state and the three bending fundamental bands ν3, ν4, and ν6

For the first time the infrared spectrum of the AsHD₂ molecule has been measured in the region of the bending fundamental bands ν₃, ν₄, and ν₆ on a Fourier transform spectrometer with a resolution of 0.0024 cm⁻¹ and analyzed. More than 5500 transitions with J^max = 26 have been assigned and used both to obtain “ground state combination differences” and for the determination of upper state ro-vibrational energies of the triad (001000), (000100), and (000001). Rotational parameters including centrifugal distortion coefficients up to octic terms of the ground vibrational state were calculated by fitting more than 500 “ground state combination differences” with J^max and . The obtained set of 24 parameters provides a rms-deviation of 0.00011 cm⁻¹. The upper energies were fitted with 52 parameters of an effective Hamiltonian which takes into account strong resonance interactions between all vibrational states of the triad (001000), (000100), and (000001). The rms-deviation for the energy levels considered in the fit is 0.00014 cm⁻¹.     

High resolution fourier transform spectrum of PD3 in the region of the stretching overtone bands 2ν1 and ν1+ν3

The infrared spectrum of the PD₃ molecule has been measured in the region of the first stretching overtone bands on a Fourier transform spectrometer with a resolution of 0.0068 cm⁻¹ and analyzed for the first time. More than 800 transitions with J^max=15 have been assigned to the bands 2ν₁ and ν₁+ν₃. An effective Hamiltonian was used which takes into account both the presence of resonance interactions between the states (2 0 0 0) and (1 0 1 0), and interactions of these with the third stretching vibrational state of the v=2 polyad, (0 0 2 0). A set of 44 spectroscopic parameters is obtained from the fit. This reproduces the 305 initial “experimental” upper rovibrational energies with an rms=0.0015 cm⁻¹.     

“Kerrr” black hole: The lord of the string

Kerrr in the title is not a typo. The third “r” stands for regular, in the sense of pathology-free rotating black hole. We exhibit a long search-for, exact, Kerr-like, solution of the Einstein equations with novel features: (i) no curvature ring singularity; (ii) no “anti-gravity” universe with causality violating time-like closed world-lines; (iii) no “super-luminal” matter disk.     

Analysis of the linear stability of stationary solutions in a circular laser containing a saturable absorber “LSA” for inhomogeneous broadening

In this present paper, the optical bistability in a circular laser containing a saturable absorber LSA for inhomogeneous broadening is studied. A simple mathematical model was developed to describe the action of this type of laser in a circular resonator under two conditions, when the spontaneous emission is neglected or included in the cavity. For both of these two cases, the coefficient of saturation was considered to be equal or different to unity (ζ=1 or ζ≠1). The photons intensities Q_j were determined as function of the pumping rate σ₀ of the active medium and analyzed the linear stability of the stationary solutions obtained in a circular laser containing a saturable absorber “LSA” for inhomogeneous broadening.     

The nano-structural properties of hydrogenated a-Si and Si-C thin films alloys by GISAXS and vibrational spectroscopy

Amorphous hydrogenated silicon (a-Si:H) with high hydrogen content (10-40 at.%), and non-stehiometric silicon-carbon (Si_1−xC_x) thin films with a variation of the carbon to silicon ratio up to 0.3, were deposited by using a magnetron sputtering source. The Si_1−xC_x thin films were partially crystallised after deposition by thermal annealing up to 1050 °C.The GISAXS (Grazing Incidence Small Angle X-ray Scattering) spectra of all of the prepared specimens indicate the presence of “particles” in the “bulk” of the films. For the a-Si:H samples, “particles” are most probably voids agglomerates with a variation in size between 3 and 6 nm. The mean value of the size distribution of the “particles” increases while its width slightly decreases with the hydrogen content in the film. This indicates a better structural ordering which is consistent with the results of Raman spectroscopy that show a decrease of the ratio between intensities of transversal acoustic (TA) and transversal optic (TO) phonon peaks, I_TA/I_TO, and a narrowing of the TO peak with increasing hydrogen content. These results are discussed as a consequence of the beneficial influence of hydrogen bombardment during the film growth.For Si_1−xC_x thin films, the “particles” are assumed to be SiC nano-crystals with a size between 2 and 14 nm and they are larger in films with a higher carbon concentration. Inside each of the films, the crystals are larger closer to surface and they grow faster in the direction parallel to the surface than in that which is perpendicular to it.     

Hidden symmetry of the CKM and neutrino-mapping matrices

We propose that the smallness of the light quark masses is related to the smallness of the T (i.e. CP) violation in hadronic weak interactions. Accordingly, for each of the two quark sectors (“upper” and “lower”) we construct a 3 × 3 mass matrix in a bases of unobserved quark states, such that the “upper” and “lower” basis states correspond exactly via the W^± transitions in the weak interaction. In the zeroth approximation of our formulation, we assume T conservation by making all matrix elements real. In addition, we impose a “hidden symmetry” (invariance under simultaneous translations of all three basis quark states in each sector), which ensures a zero mass eigenstate in each sector.Next, we simultaneously break the hidden symmetry and T invariance by introducing a phase factor e^iχ in the interaction for each sector. The Jarlskog invariant J_CKM, as well as the light quark masses are evaluated in terms of the parameters of the model. Comparing formulas, we find that most unknown factors drop out, resulting in a simple relation with , to leading order in χ and m_s/m_b, with A, λ the Wolfenstein parameters. (Because of the large top quark mass, the contribution from upper quark sector can be neglected.) Setting J_CKM = 3.08 × 10⁻⁵, m_b = 4.7 GeV (1s mass), m_s = 95 MeV, A = 0.818, and λ = 0.227, we find , consistent with the accepted value m_d = 3 − 7 MeV.We make a parallel proposal for the lepton sectors. With the hidden symmetry and in the approximation of T invariance, both the masses of e and ν₁ are zero. The neutrino-mapping matrix V_ν is shown to be of the same Harrison-Perkins-Scott form which is in agreement with experiments. We also examine the correction due to T violation, and evaluate the corresponding Jarlskog invariant J_ν.     

The effect of hydrogen stoichiometry on palladium strain and resistivity

The strain and the electrical resistivity of a Pd sample stressed by a constant tension have been investigated through a series of hydrogenation cycles in a continuous H stoichiometry [0?x?0.8] range. The isotropic lattice expansion for both “as drawn” and “annealed” Pd sample reveals a strain of only 1% from pure Pd to PdH_0.8 in disagreement with literature data available; the measured effect is minimum at x=0.13 (α+β phase) and then from x=0.6 (β phase) it has an exponential increase. The contribution of the mechanical tensile stress on the total relative elongation of the wire is also investigated. An increase of the Pd sample tensile strain after each hydrogenation cycle is reported for “as drawn” samples, while for “annealed” samples the reverse behaviour is observed. Moreover, annealed samples show considerably higher value of tensile strain compared to “as drawn”. The variation of mechanical strain versus H content, for both “annealed” and “as drawn”, has a maximum at x=0.52. Strain variation and resistivity variation versus H content exhibit similar behaviour.     

Significant effect of surfactant micelles on pH dependent fluorescent off-on-off behavior of Salicylaldehyde-2,4-Dinitrophenylhydrazone

The fluorescence response to pH of 2,4 dinitrophenolhydrazone in 1:1 CH₃OH:H₂O and micellar mediums—negatively charged sodium dodecylsulphate (SDS), positively charged cetyltrimethyl ammonium bromide (CTAB) and neutral Triton X-100 (TX-100), is reported. At pH 4.0 the fluorescence of the molecule can be switched “on” by selecting CTAB as the solvent. At pH 6.0 if the medium is TX-100 the fluorescence is “off”, but remains “on” for the other three solvents. At pH 8.0, fluorescence is “on” in the solvents except CTAB. SDS and TX-100 switch the fluorescence “on” at pH 13.0 but for the other two solvents the fluorescence is “off”.     

The stable structures of iron pnictides AFeAs (A = alkali and alkaline-earth metals): A first principles study

We have investigated the structural stabilities of iron arsenide compounds A_x(FeAs)_1−x (A = alkali and alkaline-earth metals) by first principles calculations. We find that (i) all of the experimental “122” type structures with composition x=1/3 are stable; (ii) all of the “111” type structures with composition x=1/2 except CsFeAs are stable; (iii) K₃FeAs with composition x=3/4 is stable. The predicted stable KFeAs, RbFeAs, SrFeAs, BaFeAs, K₃FeAs have the As-Fe-As bond angles close to the ideal tetrahedral angles, indicating that they may be superconductors.     

Structure refinements of cadmium substituted lead fluoroapatites by powder X-ray pattern fitting

Solid solutions of cadmium and lead fluoroapatite [Pb_(10−x)Cd_x(PO₄)₆F₂ (0?x?5)] were synthesized by a wet process in a basic medium. Replacement of lead by cadmium induces a linear variation of the crystallographic parameters “a” and “c” according to Végard's law. The cadmium content, as obtained from the refinement, is in agreement with the chemical analysis. The distribution of the lead and cadmium ions between two non-equivalent crystallographic sites, M(1) and M(2), was determined by the Rietveld method. The site-occupancy factors of atoms clearly indicate a preference of cadmium for site M(1) in the apatite structure in agreement with its smaller ionic radii. A progressive shift of the F⁻ ion toward the center of the triangles formed by the site M(2) metals has been observed with increasing cadmium content.     

Phantom Friedmann cosmologies and higher-order characteristics of expansion

We discuss a more general class of phantom (p < −?) cosmologies with various forms of both phantom (w < −1), and standard (w > −1) matter. We show that many types of evolution which include both Big-Bang and Big-Rip singularities are admitted and give explicit examples. Among some interesting models, there exist non-singular oscillating (or “bounce”) cosmologies, which appear due to a competition between positive and negative pressure of variety of matter content. From the point of view of the current observations the most interesting cosmologies are the ones which start with a Big-Bang and terminate at a Big-Rip. A related consequence of having a possibility of two types of singularities is that there exists an unstable static universe approached by the two asymptotic models—one of them reaches Big-Bang, and another reaches Big-Rip. We also give explicit relations between density parameters Ω and the dynamical characteristics for these generalized phantom models, including higher-order observational characteristics such as jerk and “kerk.” Finally, we discuss the observational quantities such as luminosity distance, angular diameter, and source counts, both in series expansion and explicitly, for phantom models. Our series expansion formulas for the luminosity distance and the apparent magnitude go as far as to the fourth-order in redshift z term, which includes explicitly not only the jerk, but also the “kerk” (or “snap”) which may serve as an indicator of the curvature of the universe.     

Role of the substrate thickness for the structural properties of organic-organic heterostructures

F₁₆CuPc deposited on pentacene is characterized by the coexistence of two different configurations: F₁₆CuPc is found in the standing up phase (“s-configuration”) on top of pentacene terraces and in a lying down phase (“l-configuration”) at pentacene step edges. By combining AFM and grazing incidence X-ray diffraction we show that the ratio between F₁₆CuPc in l- and s-configurations increases with thickness of the pentacene substrate film, demonstrating the role of the pentacene steps as nucleation centers for the F₁₆CuPc l-configuration. Experiments performed with ultra-thin pentacene thicknesses disclose that the F₁₆CuPc l-configuration does not grow on top of the first and second pentacene layers, pointing to the action of long-range interactions with the substrate.     

Systematic analysis of the Fourier transform spectrum of CH3OH between 400 and 950 cm

We have investigated a high-resolution Fourier transform (FT) absorption spectrum of the ¹³CH₃OH isotopomer of methanol from 400 to 950 cm⁻¹ with the “Ritz” program. We present the assignments of 7160 transitions, 3021 of which belong to A-symmetry, and 4139 to E-symmetry. These transitions occur between states labeled by K quantum numbers up to 14, and by torsional quantum numbers n up to 4. The “Ritz” program evaluated the energies of the 4684 involved levels with an accuracy of the order of 10⁻⁴ cm⁻¹. All of the assigned lines correspond to transitions involving torsionally excited levels within the ground small-amplitude vibrational state.     

“Rotating” steps in Si(0 0 1) homoepitaxy

Steps on Si(0 0 1) surfaces which are initially not aligned along the high symmetry directions of the dimer reconstruction are observed, by scanning tunneling microscopy, to “rotate” toward [1 1 0] directions during Si growth. This step “rotation” occurs due to a faceting of the step edges. A theoretical analysis of adatom incorporation into the steps shows that this kinetic instability may be caused by a suppressed mobility of the growing species along the S_A step edge.     

Theoretical research on optical properties and quantum efficiency of Ga1−xAlxN photocathodes

In order to design the optimal component structure of transmission-mode (t-mode) Ga_1−xAl_xN photocathode, the optical properties and quantum efficiency of Ga_1−xAl_xN photocathodes are simulated. Based on thin film principle, optical model of t-mode Ga_1−xAl_xN photocathodes is built. And the quantum efficiency formula is put forward. Results show that Ga_1−xAl_xN photocathodes can satisfy the need of detectors with “solar blind” property when the Al component is bigger than 0.375. There is an optimal thickness of Ga_1−xAl_xN layer to get highest quantum efficiency, and the optimal thickness is 0.3 μm. There is close relation between absorptivity and quantum efficiency, which is in good agreement with the “three-step” model. This work gives a reference for the experimental research on the Ga_1−xAl_xN photocathodes.     

Compensation of nonlinear phase noise in an in-plane-magnetized anisotropic spin-torque oscillator

A theory of generation linewidth of a spin-torque oscillator (STO) based on an in-plane-magnetized nano-pillar with an anisotropic “free” magnetic layer has been developed. It is predicted that by choosing the direction of the in-plane bias magnetic field H₀ along the “hard” anisotropy axis of the STO “free” layer and the magnitude of this field to be four times larger than the anisotropy field H_A (H₀=4H_A) it would be possible to compensate the nonlinear phase noise and to achieve the minimum value of the generation linewidth, characteristic for an auto-oscillator without a nonlinear frequency shift.