Exponential Transformations for the Harmonic Chain with a Molecular Defect

The exponential transformation, developed in an earlier paper [1], is applied to the Hamiltonian of a linear harmonic chain with a molecular defect. The resulting eigenvalue equation is solved for the localized frequency. A discussion of the renormalized in-band frequencies shows that in good approximation the entire Hamiltonian is diagonalized by a single transformation. This is of great advantage, since in the classical Lifshitz formalism each single frequency has to be evaluated separately. Furthermore, a simpler transformation is discussed, which is derived from an U-matrix formalism. Numerical results of the two transformations are given for a chain with 999 lattice points and compared with the exact values from the classical Lifshitz formalism.     

Mechanism of ion beam induced mixing of layered solids

A brief review of recent experiments on interface mixing by high-energy heavy-ion bombardment indicates that observed low-fluence mixing rates through interfaces are substantially larger than what can be accounted for by collisional mixing at low and high recoil energies. Diffusion and subsequent trapping of point defects within individual damaged zones is shown to lead to gaussian relocation profiles even at moderate bombarding-ion fluences. Estimated half widths are consistent with measured mixing rates. The depth and energy dependences of these half widths differ markedly from those predicted for low-energy cascade mixing.     

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Can the night-time atmospheric oxidant NO*3 damage aromatic amino acids?

Reaction of nitrate radicals, NO*3 , with aromatic amino acids leads to irreversible oxidative functionalization at the beta-position or at the aromatic ring, suggesting that this important atmospheric oxidant could potentially cause damage to peptides lining the respiratory tract and may contribute to pollution-derived diseases.     

Rapid measurement of three-dimensional diffusion tensor

In this article, the authors demonstrate a rapid NMR method to measure a full three-dimensional diffusion tensor. This method is based on a multiple modulation multiple echo sequence and utilizes static and pulsed magnetic field gradients to measure diffusion along multiple directions simultaneously. The pulse sequence was optimized using a well-known linear inversion metric (condition number) and successfully tested on both isotropic (water) and anisotropic (asparagus) diffusion systems.     

Atomistic details of precipitates in lean Al–Mg–Si alloys with trace additions of Ag and Ge studied by HAADF-STEM and DFT

Abstract

Bonding energies and volume misfits for alloying elements and vacancies in multicomponent Al–Mg–Si alloys have been calculated using density functional theory (DFT). A detailed atomic scale analysis has been done for characteristic precipitate structures, using high-angle annular dark-field scanning transmission electron microscopy. Two new stacking configurations of the important strengthening phase β′′ were discovered in the Ge-added alloy. All three stacking variations were found to be energetically favourable to form from DFT calculations. The second stacking configuration, β₂′′, contains vacated columns in its unit cell, consequently requiring less solute to create the same volume fraction of precipitate needles. DFT suggests a lower formation enthalpy per atom for β₂′′ when Si is exchanged with Ge. In the alloy containing Ag additions, a new Q’/C-like local configuration containing Ag instead of Cu was discovered, also this phase was deemed energetically favourable from DFT.     

Hierarchical self-assembly of polycyclic heteroaromatic stars into snowflake patterns

Seeing stars: The two-dimensional patterns of the polycyclic heteroaromatic star molecules 1 on graphite vary with the side chain length. For n=12, frustrated self-assembly leads to hierarchically organized superstructures: up to 10?molecules form triangular aggregates which pack densely into hexagonal patterns with very large (15.5?nm) lattice constants.     

Direct surface force measurement in water using a nanosize colloidal probe technique

The direct force measurement between colloidal surfaces has been an essential topic in both theories and applications of surface chemistry. As particle size is decreased from micron size down to true nano size (<10 nm), surface forces are increasingly important. Nanoparticles at close proximity or high solids loading are expected to show a different behavior than what can be estimated from continuum and mean field theories. The current tools for directly measuring interaction forces such as a surface force apparatus or atomic force microscopy (AFM) are limited to particles much larger than nanosize. Here a modified colloidal probe technique is suggested using a multiwalled carbon nanotube (MWNT) to overcome this problem. Determination of zero separation in AFM is critical to extract a reliable force-separation curve when MWNT is used as a probe. Hence, a systematic approach to the data collection for a nanosize colloidal probe is proposed and a sample of a direct surface force measurement curve obtained with the MWNT probe is presented.     

Proton relaxation times in 7LiCl and 6LiCl solutions

Measurements of the proton spin-lattice relaxation times have been made as a function of concentration and temperature in aqueous solutions of ⁷LiCl and ⁶LiCl. The difference in the relaxation times for two isotopic solutions of the same concentration and temperature is small, corresponding to a difference n reciprocal relaxation times of 0·004 sec^-1. c at 25°c, where c is the molarity of the solutions. This value decreases with increasing temperature. It is shown that the difference in relaxation times arises solely from the magnetic dipole interaction between the ⁷Li ion nucleus and water protons. The concept of a long-lived, rigid hydrated complex around the Li ion is shown to be inconsistent with the results.