A new delayed proton precursor: Chromium-45

The new $\text{T}{}_{\mathrm{z}}\text{= ?}\text{3}\text{2}$ isotope, ⁴⁵Cr, has been produced by the ³²S(¹⁶O, 3n)⁴⁵ Cr reaction. Its half-life was measured to be 50 ± 6 ms. A single β-delayed proton group with center-of-mass energy 2.10 ± 0.05 MeV was observed, corresponding to the decay of the lowest $\text{T}{}_{\mathrm{z}}\text{=}\text{3}\text{2}$ state in ⁴⁵V.     

Beam propagation through parabolic-index waveguides with distorted optical axis

An integral expression is derived for the fields in parabolic-index waveguides with distorted optical axis. It is based on well-known ray matrix techniques and generalized Huygens integrals. The distortions give rise to beam oscillations which in turn are identified as the excitation mechanism for radiation modes. The model gives some better understanding of the transition regions between waveguides. On leave from the Department of Electronics onthe Weizmann Institute of Science, Rehovot, Israel     

Delicious not siliceous: expanded carbohydrates as renewable separation media for column chromatography

Expansion of native corn starch produces a high surface area mesoporous material capable of acting as a novel stationary phase for separating various mixtures of compounds.     

Polymerization in liquid crystals—XI: Copolymerization of N-acryloyl piperidine with cholesteryl acrylate and phenyl-p-acryloyloxy benzoate

N-acryloyl piperidine (AP) was copolymerized with mesogenic monomers [such as cholesteryl acrylate (CA) and phenyl-p-acryloyloxy benzoate (PAB)] in isotropic solution using radical initiation (A_AP = 1.1, r_CA = 0.16, and r_AP = 0.81, r_PAB = 0.16. The copolymers are anisotropic having minimum values of transition and clearing points with composition. The composition corresponding to the minimum is a boundary between two structures as indicated by wide-angle X-ray scattering. Distribution of the gyration radii determined by GPC in tetrahydrofuran is identical to that of solid polymers measured by small-angle X-ray scattering, except that the average value of the latter is about 10^?6 cm less than that of the former. It was established that AP as a non-mesogenic comonomer did not suppress the mesomorphism of the copolymer and in fact formed mesomorphic homopolymers. In the solid polymer, the macromolecule which is anisotropic in geometry was found to form the structural unit having a minimum of the melting and clearing point with composition, as for a eutectic system.     

A new N = Z isotope: Krypton 72

Beta-delayed γ-rays have been observed from the decay of ${}^{72}\text{Kr}\text{(τ}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 16.7 ± 0.6}\text{s}\text{)}$. A decay scheme is proposed based on γ-γ and β⁺-γ coincidence measurements. The total decay energy was measured to be Q_EC = 5057 ± 135 keV. The value is compared with mass predictions.     

Enhancement of the critical temperature of HgBa₂CuO(4+δ) by applying uniaxial and hydrostatic pressure: implications for a universal trend in cuprate superconductors

It is well known that the superconducting transition temperature (T(c)) of cuprate superconductors can be enhanced by varying certain structural and electronic parameters, such as the flatness of the CuO? planes or their doping level. We determine the uniaxial and hydrostatic pressure derivatives of T(c) in the structurally simple tetragonal compound HgBa?CuO(4+δ) near optimal doping. Our results provide experimental evidence for two further methods to enhance T(c): (i) reducing the area of the CuO? planes, and (ii) increasing the separation of the CuO? planar groups. T(c) is found to couple much more strongly to the ratio c/a of the lattice constants than to the unit cell volume. A comparison with prior results for structurally more complicated cuprates reveals a general trend of uniaxial pressure derivatives with T(c).     

Structural phase transitions in ionic molecular solids

An overview is presented of our studies on the nature of structural instabilities in relatively complex ionic solids. These are based on parameter-free interionic potentials based on the Gordon-Kim modified electron gas formalism extended to molecular ions.

We describe the manner in which there emerge from these studies quite general concepts of “size” and “shape” as structural determinants. In particular, we discuss how these, and the approximate symmetries that they can produce, can provide a relatively simple structure-based explanation of the origins of incommensurate phases in these systems. However, we also emphasize that the existence of such symmetries does not guarantee an incommensurate phase. This can only be realized if long-range correlations are sufficiently strong to overcome random local disordering. Thus, either the molecular units are partially linked and/or there exist long-range Coulomb interactions between individual units.     

Vibronic origins of high T c

In this paper we present a detailed development of our vibronic mechanism for the explanation of high temperature superconductivity. We first review the evidence for believing that some unified mechanism for “low” and “high” T _c is required. We then develop the case for the existence of the double-well motions, required by our theory, in these systems, and proceed to develop our ideas further to the point of unifying, the T _c ～ 100 K, and the T _c ～ 40 K systems. We also advance a possible explanation of the anomalous isotope effects in these systems, and conclude with an extended discussion of non-cuprate systems and general criteria for high T _c theories, including the gap to kT _c ratio.