Enhancing the superconducting transition temperature of BaSi2 by structural tuning

We present a joint experimental and theoretical study of the superconducting phase of the layered binary silicide BaSi(2). Compared with the AlB(2) structure of graphite or diboridelike superconductors, in the hexagonal structure of binary silicides the sp(3) arrangement of silicon atoms leads to corrugated sheets. Through a high-pressure synthesis procedure we are able to modify the buckling of these sheets, enhancing the superconducting transition temperature from 6 to 8.9 K when the silicon planes flatten out. By performing ab initio calculations based on density-functional theory we explain how the electronic and phonon properties are strongly affected by changes in the buckling. This mechanism is likely present in other intercalated layered superconductors, opening the way to the tuning of superconductivity through the control of internal structural parameters.     

Longitudinal smoothness of the holonomy groupoid

Iakovos Androulidakis and Georges Skandalis have defined a holonomy groupoid for any singular foliation. This groupoid, whose topology is usually quite bad, is the starting point for the study of longitudinal pseudodifferential calculus on such foliation and its associated index theory. These studies can be highly simplified under the assumption of the holonomy groupoid being longitudinally smooth. In this note, we rephrase the period bounding lemma that asserts that a vector field on a compact manifold admits a strictly positive lower bound for its periodic orbits in order to prove that the holonomy groupoid is always longitudinally smooth.     

Operator algebras and a theorem of Dieudonne

LetA be aC*-algebra with second dualA″. Let (φ _n)(n=1,...) be a sequence in the dual ofA such that limφ _n(a) exists for eacha εA. In general, this does not imply that limφ _n(x) exists for eachx εA″. But if limφ _n(p) exists whenever p is the range projection of a positive self-adjoint element of the unit ball ofA, then it is shown that limφ _n(x) does exist for eachx inA″. This is a non-commutative generalisation of a celebrated theorem of Dieudonné. A new proof of Dieudonné’s theorem, for positive measures, is given here. The proof of the main result makes use of Dieudonné’s original theorem.     

K-duality for pseudomanifolds with isolated singularities

We associate to a pseudomanifold X with a conical singularity a differentiable groupoid G which plays the role of the tangent space of X. We construct a Dirac element and a dual Dirac element which induce a K-duality between the C∗-algebras C∗(G) and C(X). This is a first step toward an index theory for pseudomanifolds.     

Bi-Directional Ion Emission from Massive Gold Cluster Impacts on Nanometric Carbon Foils

Carbon cluster emission from thin carbon foils (5-40 nm) impacted by individual Au(n) (+q) cluster projectiles (95-125 qkeV, n/q = 3-200) reveals features regarding the energy deposition, projectile range, and projectile fate in matter as a function of the projectile characteristics. For the first time, the secondary ion emission from thin foils has been monitored simultaneously in both forward and backward emission directions. The projectile range and depth of emission were examined as a function of projectile size, energy, and target thickness. A key finding is that the massive cluster impact develops very differently from that of a small polyatomic projectile. The range of the 125 qkeV Au(100q) (+q) (q ≈ 4) projectile is estimated to be 20 nm (well beyond the range of an equal velocity Au(+)) and projectile disintegration occurs at the exit of even a 5 nm thick foil.     

Radial oscillations of neutron stars in strong magnetic fields

The eigen frequencies of radial pulsations of neutron stars are calculated in a strong magnetic field. At low densities we use the magnetic BPS equation of state (EOS) similar to that obtained by Lai and Shapiro while at high densities the EOS obtained from the relativistic nuclear mean field theory is taken and extended to include strong magnetic field. It is found that magnetized neutron stars support higher maximum mass whereas the effect of magnetic field on radial stability for observed neutron star masses is minimal.