High-resolution gamma-ray radiography produced by a laser-plasma driven electron source

An electron beam from a laser-plasma accelerator is converted into a gamma-ray source using bremsstrahlung radiation in a dense material. The gamma-ray beam has a pointlike source size because it is generated by a high quality electron beam with a small source size and a low divergence. Using this gamma-ray source, the radiography of complex and dense objects with submillimeter resolution is performed. It is the first evidence of a gamma-ray source size of a few hundreds micrometers produced with laser-driven accelerators. This size is consistent with results from Monte Carlo simulations.     

Phase separation in the two-dimensional bosonic Hubbard model with ring exchange

We show that soft-core bosons in two dimensions with a ring exchange term exhibit a tendency for phase separation. This observation suggests that the thermodynamic stability of normal Bose liquid phases driven by ring exchange should be carefully examined.     

Folding and unfolding of an elastinlike oligopeptide: "inverse temperature transition," reentrance, and hydrogen-bond dynamics

The temperature-dependent behavior of a solvated oligopeptide, GVG(VPGVG), is investigated. Spectroscopic measurements, thermodynamic measurements, and molecular dynamics simulations find that this elastinlike octapeptide behaves as a two-state system that undergoes an "inverse temperature" folding transition and reentrant unfolding close to the boiling point of water. A molecular picture of these processes is presented, emphasizing changes in the dynamics of hydrogen bonding at the protein/water interface and peptide backbone librational entropy.     

Spectroscopy of 40Ca and negative-parity bands

We have studied the reactions as well as the binary channel , in order to search for deformed states, which form rotational bands in ⁴⁰Ca. We observe positive- and negative-parity bands. The negative-parity band is proposed to be a partner of an inversion doublet with the positive-parity states being based on 4p-4h configurations. The properties of the positive-parity states are discussed on the basis of the shell model and the parity doublet on the basis of a cluster model with intrinsic reflection asymmetric shapes.Received: 2 May 2003, Revised: 23 September 2003, Published online: 20 January 2004PACS: 21.10.-k Properties of nuclei; nuclear energy levels - 21.60.Gx Cluster models     

Detaching thiolates from copper and gold clusters: which bonds to break?

The interaction of alkanethiolates with small coinage metal clusters of copper and gold was studied based on density functional theory with a focus on the metal-thiolate junction. Calculation of fragmentation energies indicate that for Cu(n)-thiolate (n = 1,3,5,7, and 9) there is a progressive lowering in energy for the fragmentation of the S-C bond in the thiolate from a value of 2.9 eV for n = 1 to 1.4 eV for n = 9. The detailed electronic origins of this specific weakening are attributed to a polarization of electron density in the S-C bond as induced by bonding with the Cu(n) cluster. For the gold analogues, this effect is not observed and fragmentation at the S-C bond experiences only a slight 10% destabilization as n increases from 3 to 9. The relativistic origin of this difference between Cu and Au is discussed, and an analysis of bonding considerations is presented.     

Positive electron affinity of fullerenes: Its effect and origin

The universal variation pattern of the total energy of various fullerenes including single-walled carbon nanotubes with respect to their extra charge is revealed by the density-functional-theory calculations. The parabolic energy-charge curve with its lowest energy value corresponding to a negatively charged fullerene indicates that these carbon materials have positive electron affinity and are not in the most stable state. The positive electron affinity seems to originate from the pi-electrons and is found to be related to the aggregation property of fullerenes.     

Numerical predictions and experiments on the free-plate edge mode

In this paper, te edge mode variation is studied with three different methods: the reciprocal work method, already used by Torvik [J. Acoust. Soc. Am. 41 (1967) 346] to model this phenomenon, the S-parameter method and a finite element model that are applied for the first time to the study of the edge resonance. Moreover, laser probe measurements of the edge mode have also been performed and compared to the numerical predictions. The good agreement between the numerical predictions and the experimental data allows full understanding of the resonant phenomenon. The edge resonance is linked to the strong increase in amplitude of two complex Lamb waves, and the edge mode is proved to radiate into the plate as the first symmetrical Lamb mode S(0). Displacements at the edge and away from the edge have been computed and measured to evaluate the spatial and temporal behaviour of the edge mode. The dependence of the edge resonance frequency and amplitude on the Poisson coefficient has also been studied.     

Theoretical Study of the Electronic Structure of the KRb Molecule

Upon request of experimentalists now engaged in high-resolution spectroscopic investigations of the molecule KRb, we have determined the potential energy of electronic states (2S+1)Lambda((+)) correlating up to the limit K(5p) + Rb(5s) and of electronic states Omega((+/-)) correlating up to the limit K(4s) + Rb(4d(2)D(3/2)) in a large range of internuclear distance R. For the five states so far observed, the agreement between calculated and experimental molecular constants is good with DeltaR(e) < 0.08 ?, Deltaomega(e) < 6 cm(-1), and DeltaT(e) < 140 cm(-1). Extensive numerical data for energies versus R have been listed in a data base available at http://www.idealibrary.com. Copyright 2000 Academic Press.     

Ab initio simulation of phase transitions and dissociation of H2S at high pressure

By ab initio constant pressure molecular dynamics, we have identified the structure of phase V and phase VI of H2S at 35 and 65 GPa, respectively. The theoretical IR spectra of both phases are consistent with experimental findings and support our proposed structural models. We find that phase V is characterized by the presence of charged SH+3 and SH- species which are created and destroyed dynamically, whereas phase VI is no longer a molecular phase but consists of sheets of S with the majority of H intercalated between the layers. The stability of the two phases with respect to dissociation into elemental crystalline hydrogen and sulfur is discussed.     

Structural principles and amorphouslike thermal conductivity of Na-doped Si clathrates

The postulated low thermal conductivity and the possibility of altering the electronic conductivity of metal-doped clathrates with semiconducting host elements have stimulated great interest in exploring these compounds as promising thermoelectric materials. The electronic and thermal properties of the prototypical Na xSi (46) system are studied in detail here. It is shown that, despite the fact that the Na/Si clathrate is metallic, its thermal conductivity resembles that of an amorphous solid. A theoretical model is developed to rationalize the structural stability of the peculiar structural topology, and a general scheme for rational design of high efficiency thermoelectric materials is presented.