Electronic structure,electrical and magnetic properties of RMo(8)O(14) compounds (R = La,Ce, Pr,Nd, Sm) containing bicapped Mo(8) clusters

Magnetic and electrical resistivity properties of RMo(8)O(14) (R = La, Ce, Pr, Nd, Sm) compounds containing different bicapped-octahedral Mo(8) clusters are discussed. Extended Hückel (EH) molecular calculations were carried out in order to study the influence of the position of metal capping atoms on the electronic structure of different Mo(8) isomers. Different optimal metal electron counts are possible for these clusters. Periodic density functional calculations confirm the molecular character of these compounds and allow the understanding of their semiconducting and magnetic properties.     

Superconductivity in MgB2: clean or dirty?

A large number of experimental facts and theoretical arguments favor a two-gap model for superconductivity in MgB2. However, this model predicts strong suppression of the critical temperature by interband impurity scattering and, presumably, a strong correlation between the critical temperature and the residual resistivity. No such correlation has been observed. We argue that this fact can be understood if the band disparity of the electronic structure is taken into account, not only in the superconducting state, but also in normal transport.     

Enhancement of proton acceleration by hot-electron recirculation in thin foils irradiated by ultraintense laser pulses

MeV-proton production from solid targets irradiated by 100-fs laser pulses at intensities above 1x10(20) W cm(-2) has been studied as a function of initial target thickness. For foils 100 microm thick the proton beam was characterized by an energy spectrum of temperature 1.4 MeV with a cutoff at 6.5 MeV. When the target thickness was reduced to 3 microm the temperature was 3.2+/-0.3 MeV with a cutoff at 24 MeV. These observations are consistent with modeling showing an enhanced density of MeV electrons at the rear surface for the thinnest targets, which predicts an increased acceleration and higher proton energies.     

Geometry of the valence transition induced surface reconstruction of Sm(0001)

We present a structural determination of the surface reconstruction of the Sm(0001) surface using surface x-ray diffraction, scanning tunneling microscopy, and ab initio calculations. The reconstruction is associated with a large (22%) expansion of the atomic radius for the top monolayer surface Sm atoms. The mechanism driving the surface reconstruction in Sm is unique among all elements and is connected to the strong correlations of the 4f electrons in Sm and the intermediate valence observed in certain Sm compounds. The atoms constituting the top monolayer of Sm(0001) have vastly different chemical properties compared to the layer underneath and behave as if they were an adsorbate of a different chemical species.     

Phase fluctuations in atomic Bose gases

We improve on the Popov theory for partially Bose-Einstein condensed atomic gases by treating the phase fluctuations exactly. As a result, the theory becomes valid in arbitrary dimensions and is able to describe the low-temperature crossover between three-, two-, and one-dimensional Bose gases, which is currently being explored experimentally. We consider both homogeneous and trapped Bose gases.     

Self-consistent electronic structure of Si,Ge and diamond by the LMTO-ASA method

We have investigated the accuracy of the self-consistent LMTO-ASA method with interstitial spheres for open structure materials by studying the electronic structure of Si, Ge and diamond. Our band structures agree in detail with the results of the best state of the art self-consistent calculations. In addition we obtain quantitative agreement with the observed cohesive properties for all three systems. The success of these calculations suggests that the method should be useful for other open structure materials.     

Determining lower and upper bounds on probabilities of atomic propositions in sets of logical formulas represented by digraphs

In this paper we consider the problem of determining lower and upper bounds on probabilities of atomic propositions in sets of logical formulas represented by digraphs. We establish a sharp upper bound, as well as a lower bound that is not in general sharp. We show further that under a certain condition the lower bound is sharp. In that case, we obtain a closed form solution for the possible probabilities of the atomic propositions.The second author is partially supported by ONR grant N00014-92-J-1028 and AFOSR grant 91-0287.     

Some results on Borel structures with applications to subseries convergence in abelian topological groups

We show that in an abelian topological group subseries convergence depends only on the Borel field generated by the topology. We also prove a result about measurability of a limit for a pointwise converging sequence of measurable mappings into an analytic topological space.