Vibrations and thermodynamics of clusters of polycyclic aromatic hydrocarbon molecules: the role of internal modes

The vibrational spectra of clusters of coronene molecules are theoretically calculated using a mixed quantum/classical scheme, each molecule being described by a tight-binding Hamiltonian, the intermolecular forces being provided by explicit Lennard-Jones and point charge sites. The normal modes of vibrations are shown to exhibit significant variations upon clustering. In particular, for large clusters intra- and intermolecular modes tend to mix and fill the mid-infrared range. We also calculate the heat capacity of the (C24H12)8 cluster as a function of temperature, emphasizing the isomerizations that take place during melting. Quantum delocalization effects, as obtained from the Pitzer-Gwinn semiclassical approximation, are important enough to wash out all signatures of the structural transitions on the caloric curve. On the basis of a simple two-state model we estimate that clusters containing about 300 molecules are required for melting to be detected on the caloric curve.     

Self-consistent field tight-binding model for neutral and (multi-) charged carbon clusters

A semiempirical model for carbon clusters modeling is presented, along with structural and dynamical applications. The model is a tight-binding scheme with additional one- and two-center distance-dependent electrostatic interactions treated self-consistently. This approach, which explicitly accounts for charge relaxation, allows us to treat neutral and (multi-) charged clusters not only at equilibrium but also in dissociative regions. The equilibrium properties, geometries, harmonic spectra, and relative stabilities of the stable isomers of neutral and singly charged clusters in the range n=1-14, for C(20) and C(60), are found to reproduce the results of ab initio calculations. The model is also shown to be successful in describing the stability and fragmentation energies of dictations in the range n=2-10 and allows the determination of their Coulomb barriers, as examplified for the smallest sizes (C(2) (2+),C(3) (2+),C(4) (2+)). We also present time-dependent mean-field and linear response optical spectra for the C(8) and C(60) clusters and discuss their relevance with respect to existing calculations.     

A generalization of an integral of Ramanujan

This paper considers a generalization of an integral introduced by S. Ramanujan in his third notebook. Ramanujan’s integral is itself a version of the dilogarithm,

Conservative Velocity Interpolation for PDF Methods

In many particle methods the accurate interpolation of a velocity field represented on a computational grid to arbitrary positions is crucial [2, 5]. Here, the importance of mass conservation and order of the interpolation scheme were analyzed. Initially equally distributed particles were tracked in a stationary, incompressible 2d flow field using different interpolation schemes. It could be demonstrated that especially mass conservation is of great importance, in particular in the case of complex flow patterns. The ideas presented in this paper are more general and the methods can be extended for unsteady, compressible 3d flow problems. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Accurate power supply and ground plane pair models [for MCMs]

In this paper, we derive a theory and method for the use of two-dimensional (2-D) discrete transmission lines (TL's) or discrete coupled transmission lines (CTL's) in modeling power supply and ground planes accurately. If the stray coupling between power or local ground and global ground is not significant, the discrete TL's model is used. Otherwise the discrete CTL's model is used. An arbitrarily shaped plane pair is discretized into a 2-D TL or CTL array by an automatic mesh algorithm. The equivalent distributed circuit, including skin loss effect at high frequencies, represents this power ground plane pair. The theory is extended to be applicable to a generic multiple dielectric layer structure. The model computation results are in excellent agreement with S parameter measurements for practical frequency ranges, including the first major resonant nulls and peaks. The null or peak of the S parameter frequency response represents the test port interaction with the resonant standing wave of these planes at that frequency. The resultant S parameter data of these models can be condensed into a simpler N port equivalent circuit to represent a larger hierarchical power and ground plane network for fast simulation     

A low-noise, low-power VCO with automatic amplitude control forwireless applications

Voltage controlled oscillators (VCOs) used in portable wireless communications applications, such as cellular telephony, are required to achieve low phase-noise levels while consuming minimal power. This paper presents the design challenges of a VCO with automatic amplitude control, which operates in the 300 MHz to 1.2 GHz frequency range using different external resonators. The VCO phase noise level is -106 dBc/Hz at 100-KHz offset from an 800-MHz carrier, and it consumes 1.6 mA from a 2.7-V power supply. An extensive phase-noise analysis is employed for this VCO design in order to identify the most important noise sources in the circuit and to find the optimum tradeoff between noise performance and power consumption     

Characterization of the CH4/H2/Ar high density plasma etching process for HgCdTe

High density plasma etching of mercury cadmium telluride using CH₄/H₂/Ar plasma chemistries is investigated. Mass spectrometry is used to identify and monitor etch products evolving from the surface during plasma etching. The identifiable primary etch products are elemental Hg, TeH₂, and Cd(CH₃)₂. Their relative concentrations are monitored as ion and neutral fluxes (both in intensity and composition), ion energy and substrate temperature are varied. General insights are made into surface chemistry mechanisms of the etch process. These insights are evaluated by examining etch anisotropy and damage to the remaining semiconductor material. Regions of process parameter space best suited to moderate rate, anisotropic, low damage etching of HgCdTe are identified.     

The asymmetric single-impurity Anderson model – the modified perturbation theory

We investigate the single-impurity Anderson model by means of the recently introduced modified perturbation theory. This approximation scheme yields reasonable results away from the symmetric case. The agreement with exactly known results for the symmetric case is checked, and results for the non-symmetric case are presented. With decreasing conduction band occupation, the breakdown of the screening of the local moment is observed. In the crossover regime between Kondo limit and mixed-valence regime, an enhanced zero-temperature susceptibility is found.     

TeV γ-ray astronomy

The TeV γ-ray sky displays at this time 9 sources comprising 6 supernova remnants (SNR) and 3 active galactic nuclei (AGN). They have been discovered with AIR ERENKOV TELESCOPES since 1989, and more sources are sure to be found. They are related to violent astrophysical events and promise to be used as unique tools for cosmological questions like the universal infrared photon field and the intergalactic magnetic field strength.