Barkas effect for antiproton stopping in H2

We report the stopping power of molecular hydrogen for antiprotons of kinetic energy above the maximum (approximately 100 keV) with the purpose of comparing with the proton one. Our result is consistent with a positive difference in antiproton-proton stopping powers above approximately 250 keV and with a maximum difference between the stopping powers of 21%+/-3% at around 600 keV.     

Evidence of SiO at the Si-oxide interface by surface soft X-ray absorption near edge spectroscopy

Using synchrotron radiation a new surface sensitive spectroscopy has been applied to determine the local structure of the first surface oxide layer formed on the Si(111) surface. The Surface Soft X-ray Absorption (SSXA) spectra have been measured. From the analysis of the X-ray Absorption Near Edge Structures (XANES) we have extracted structural information. We have first determined that bulk amorphous SiO has a characteristic microsopic structure, which cannot be described by the random alloy or microcrystalline (Si + SiO₂) mixture models. The oxide layer formed on the Si(111) surface by ground-state molecular excitation in ultra high vacuum at temperatures (～700°C) approaching the oxide dissociation point has this unique SiO local structure. Such SiO layer not formed at room temperature is expected to be present in the SiSiO₂ interface grown at high temperature. An electronic transition to empty states at the SiSiO₂ interface has been observed.     

Surface doping of semiconductors by pulsed-laser irradiation in reactive atmosphere

Intense pulsed-laser irradiation in a suitable chemical atmosphere can produce a significant incorporation of chemical species from the environment to the surface molten layer. This process has been used to produce p-n junctions in silicon and GaAs irradiated, respectively, in PCl₃ and SiH₄ atmospheres. A modelling of the incorporation process, taking into account the solid-liquid-solid transition of the surface layer, has been developed following both a numerical and a semi-analytical approach. The modelling of the doping process gives results in a reasonably good agreement with the experimental doping profiles, obtained by irradiating Si samples in PCl₃ atmosphere.     

Evidence for the strain critical point in high Tc superconductors

We report experimental evidence for the phase diagram of doped cuprate superconductors as a function of the micro-strain of the planar Cu-O bond length, measured by Cu K-edge EXAFS, and hole doping . The local lattice distortions are measured by EXAFS and the charge ordering is measured by synchrotron radiation diffuse X-ray diffraction. This phase diagram shows a QCP at P() where for charge-orbital-spin stripes and free carriers co-exist. The superconducting phase occurs in the region of critical fluctuations around this QCP. The function of two variables shows its maximum at the strain QCP. The critical fluctuations near this strain QCP give the self-organization of a metallic superlattice of quantum wires “superstripes" that favors the amplification of the critical temperature. Received 25 September 2000     

Two-prong πHe interactions at 106 MeV

π^-4He interactions at 106MeV, with two secondary charged particles in the final state, have been studied using a self-shunted streamer chamber in a 0.65T magnetic field and equipped with two CCD videocameras for obtaining digitized stereo-images of nuclear events occurring within the fiducial volume of the streamer chamber. Branching ratios for the various interaction channels are deduced. Evidence is presented for a reaction channel involving a γ produced in the final state, as well as for the first observation of direct Δ^- -resonance production. Cross-section distributions for the elastic and quasi-elastic interaction channels are also reported.     

Theory of rumour spreading in complex social networks

We introduce a general stochastic model for the spread of rumours, and derive mean-field equations that describe the dynamics of the model on complex social networks (in particular, those mediated by the Internet). We use analytical and numerical solutions of these equations to examine the threshold behaviour and dynamics of the model on several models of such networks: random graphs, uncorrelated scale-free networks and scale-free networks with assortative degree correlations. We show that in both homogeneous networks and random graphs the model exhibits a critical threshold in the rumour spreading rate below which a rumour cannot propagate in the system. In the case of scale-free networks, on the other hand, this threshold becomes vanishingly small in the limit of infinite system size. We find that the initial rate at which a rumour spreads is much higher in scale-free networks than in random graphs, and that the rate at which the spreading proceeds on scale-free networks is further increased when assortative degree correlations are introduced. The impact of degree correlations on the final fraction of nodes that ever hears a rumour, however, depends on the interplay between network topology and the rumour spreading rate. Our results show that scale-free social networks are prone to the spreading of rumours, just as they are to the spreading of infections. They are relevant to the spreading dynamics of chain emails, viral advertising and large-scale information dissemination algorithms on the Internet.     

Measurement of the antiproton-nucleus annihilation cross section at 5.3 MeV

Antiproton annihilation cross sections on medium-heavy and heavy nuclear targets are measured for the first time at 5.3 MeV kinetic energy at the Antiprotons Decelerator facility of CERN. The results agree with the expected behavior from the black-disk model with the Coulomb corrections.     

Combining Solid-State NMR with Structural and Biophysical Techniques to Design Challenging Protein-Drug Conjugates

Several protein-drug conjugates are currently being used in cancer therapy. These conjugates rely on cytotoxic organic compounds that are covalently attached to the carrier proteins or that interact with them via non-covalent interactions. Human transthyretin (TTR), a physiological protein, has already been identified as a possible carrier protein for the delivery of cytotoxic drugs. Here we show the structure-guided development of a new stable cytotoxic molecule based on a known strong binder of TTR and a well-established anticancer drug. This example is used to demonstrate the importance of the integration of multiple biophysical and structural techniques, encompassing microscale thermophoresis, X-ray crystallography and NMR. In particular, we show that solid-state NMR has the ability to reveal effects caused by ligand binding which are more easily relatable to structural and dynamical alterations that impact the stability of macromolecular complexes.