Operational definition of (brane-induced) space–time and constraints on the fundamental parameters

First we contemplate the operational definition of space–time in four dimensions in light of basic principles of quantum mechanics and general relativity and consider some of its phenomenological consequences. The quantum gravitational fluctuations of the background metric that comes through the operational definition of space–time are controlled by the Planck scale and are therefore strongly suppressed. Then we extend our analysis to the braneworld setup with low fundamental scale of gravity. It is observed that in this case the quantum gravitational fluctuations on the brane may become unacceptably large. The magnification of fluctuations is not linked directly to the low quantum gravity scale but rather to the higher-dimensional modification of Newton's inverse square law at relatively large distances. For models with compact extra dimensions the shape modulus of extra space can be used as a most natural and safe stabilization mechanism against these fluctuations.     

Directional cellular growth of Al-2?wt% Li bulk samples

Al-2 wt% Li alloy was prepared using metals of 99.99% high purity in the vacuum atmosphere. The bulk samples were directionally solidified upward with a constant growth rate, V, (∼8.30 μm/s) and different temperature gradients, G, (3.11–6.06 K/mm) and also with a constant G (6.06 K/mm) and different V (8.3–164.70 μm/s) in the directional solidification apparatus. The cellular spacings, λ, were measured from both transverse and longitudinal section of the specimens and expressed as functions of solidification processing parameters, G and V, by using a linear regression analysis. The effects of the G and V on λ, were investigated. The experimental results were compared with the current theoretical and numerical models, and similar previous experimental results.     

Structural study on the crystallization behavior of Sb3Te2 alloy for phase change memory

The atomic crystal structure of the Sb₃Te₂ binary alloy has been investigated with high‐resolution transmission electron microscopy (HR‐TEM) and fast Fourier transform patterns. As a result, there is inconsistency between the previous theoretical model and the experimental result. But, from the calculations of lattice parameters and the number of layers in the unit cell, it is found that the Sb₃Te₂ alloy can be crystallized into the Sb₂Te structure with P m1 space group and the difference between stacking sequences of the Sb₃Te₂ and the Sb₂Te structures has been discussed with the proposed atomic arrangement model of unit cells. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of growth rate and Mg content on dendrite tip characteristics of Al–Cu–Mg ternary alloys

An experimental analysis is presented to correlate the secondary dendrite arm spacing λ ₂ and dendrite tip radius R with growth rate V and Mg content C _0-Mg of Al–Cu–Mg ternary alloys. Under constant temperature gradient G (4.84±0.13 K mm⁻¹), a series of directional solidification experiments were performed at five different growth rates V (16.7–83.3 μm/s) and five different Mg contents C _0-Mg in Al–5 wt.% Cu–(0.5–5) wt.% Mg alloys. Solid–liquid interface was investigated from the longitudinal sections of the quenched samples, and λ ₂ and R were measured on the dendrite tips. The dependencies of λ ₂ and R on V and C _0-Mg were determined. The experimental results showed that the values of λ ₂ and R decrease as V and C _0-Mg increase at a constant G. The present exponent values related to V are found to be slightly lower than the values of the theoretical models and previous experimental works; however, C _0-Mg exponent values are found to be much lower than the theoretical models and previous experimental works. The ratio of the secondary dendrite arm spacing to the dendrite tip radius is 2.09±0.15, in good agreement with the scaling law. At a constant C _0-Mg, the values of VR ² were found to slightly increase with the ascending V. However, as C _0-Mg increases, the values of VR ² decrease.     

Al-Cu approximants in the alloy

The composition, with an e / a ratio of 1.86 being close to ternary Al-Cu-TM (Transition Metal) quasicrystals, has been chosen for the search of Al-Cu approximants. Phase structures and compositions were studied using TEM, X-ray diffraction and EPMA techniques. Two new phases were found: face-centered orthorhombic oF-Al_43.2Cu_56.8 (a = 0.816 ₆ , b = 1.414 ₉ , c = 0.999 ₅ nm) and body-centered orthorhombic oI-Al_41.3Cu_58.7 (oI, a = 0.408 ₃ , b = 0.707 ₄ , c = 0.999 ₅ nm). Their e / a ratios are the same as that of the Al-Cu-Fe icosahedral quasicrystal. Their space groups are probably oF-Fmm2 and oI-Imm2. Both are B2 vacancy-containing superstructures; unit cell compositions can be expressed approximately as oF-Al₃₆Cu and oI-Al₈Cu (: vacancies). They both exist in twinning variants of the types and . Such twinning modes indicate that these orthorhombic phases are the decomposition products of a high-temperature parent phase 2-Al₂Cu₃, the atomic structure of which contains pentagonal atomic arrangements. Therefore the oF, oI, and phases are all B2-based approximants corresponding to the Al-Cu-TM quasicrystals. Received: 8 July 1997 / Revised: 19 May 1998 / Accepted: 16 June 1998     

Surface density oscillations in disordered binary alloys: X-ray reflectivity study of Cu3Au(001)

3 Au(001) surface using Synchrotron radiation and show that the intensity distribution along the specular rod exhibits the same features as in the case of liquid metals which has been discussed in recent literature. The results are discussed within the framework of the distorted wave Born approximation which allows a straightforward easy-to-apply scheme to analyze such reflectivity data in the entire -range including the total external reflection regime. The data analysis gives a direct evidence for an oscillating average density profile which is analoguous to the socalled surface layering at liquid surfaces. The experimental values for the thermal roughness indicate that surface roughness is suppressed in the ordered phase and allowed to softly grow with temperature above the bulk order-disorder phase transition. The presented experimental and theoretical study of X-ray reflectivity from binary alloys demonstrates the potential of this technique for the study of surface roughening, surface layering and surface segregation phenomena at solid and liquid surfaces. Received: 14 August 1996/Accepted: 7 October 1996     

A pseudopotential study of molecular spectroscopy in rare gas matrices: absorption of NO in argon

We present a pseudopotential method to study the absorption spectroscopy of NO in an argon matrix modeled by a large albeit finite cluster. The excited states of NO are described with the virtual orbitals of a NO⁺ Hartree-Fock calculation plus a core-polarization operator to account for the electron-NO⁺ correlation. The argon atoms of the matrix are replaced by pseudopotentials for the repulsive contributions and core-polarization operators to account for matrix polarization and correlation with the excited electron. The model is shown to account for the matrix-induced transition shifts and also for the cut-off of the Rydberg series for n >3 reported in absorption experiments from the ground state. Received: 6 March 1998 / Revised: 1st June 1998 / Accepted: 16 June 1998     

Colloidal inclusions in smectic films with spontaneous bend

Inclusions in free-standing smectic films are simple model systems for two-dimensional anisotropic dispersions. From theory and experiment, different topologies of elastic distortions of the embedding liquid crystal are known. Quadrupolar and different dipolar defect configurations in the vicinity of the inclusion are possible, and these configurations determine the type of interactions between the inclusions. The quadrupolar configuration is often energetically preferred. We show, however, that dipolar director configurations around inclusions can be energetically favourable over quadrupolar arrangements in chiral smectics, as a consequence of a spontaneous-bend term in the elastic-energy formulation. As the inclusion size influences the selection of the deformation types, the corresponding spontaneous-bend constant can be estimated for the strong anchoring limit if the c -director fields around inclusions of different diameters are taken into account.     

New organic–inorganic hybrid matrices doped with rhodamine 6G as solid-state dye lasers

We report on the laser action of rhodamine 6G (Rh6G) incorporated into new hybrid organic–inorganic monolithic materials. The synthesis of these materials proceeded via the simultaneous sol-gel process of the inorganic part (tetraethoxysilane or tetramethoxysilane) and the free-radical polymerization of an organic monomer part (2-hydroxypropyl methacrylate, 2-hydroxyethyl methacrylate and a 1:1 v/v copolymer of this monomer with methyl methacrylate). The wt. % proportion of the alkoxide was systematically varied in each organic formulation, and the effect of each organic–inorganic composition on the lasing properties of Rh6G was evaluated. The laser samples were transversely pumped and the influence on the laser action of dye concentration, pump wavelength and pump repetition rate was analyzed. Lasing efficiencies of up 26% and good stabilities, with a 90% drop in the initial laser output of up to 12000 pump pulses at 2.5 Hz, were obtained when the samples were pumped at 355 nm with 5.5 mJ/pulse from the third harmonic of a Q-switched Nd:YAG laser. Received: 31 July 2002 / Revised version: 14 October 2002 / Published online: 20 December 2002 RID="*" ID="*"Corresponding author. Fax: +34-91/564-4853, E-mail: ogarcia@ctp.csic.es     

Long range interactions of the Mg+ and Ca+ ions

The polarizabilities of the low lying states of the Mg⁺ and Ca⁺ ions are evaluated by diagonalizing the semi-empirical Hamiltonians in a large dimension single electron basis. The quadrupole moment of the metastable 3d state Ca⁺ is also calculated and is within 1% of a recent experimental value while being 5% smaller than some large ab-initio calculations. In addition, the long range dispersion coefficients for these ions interacting with a number of atoms are given. Oscillator strengths are also given and generally agree with the most sophisticated ab-initio calculations. The polarizabilities and dispersion coefficients can be used to estimate the frequency shifts of the Ca⁺ 4s ↦ 3d clock transition due to background electric fields and also collisions with a buffer gas.