Transition from Mott insulator to superconductor in GaNb4_{4}Se8_{8} and GaTa4_{4}Se8_{8} under high pressure

Electronic conduction in GaM$_{4}$Se$_{8}$ (M=Nb;Ta) compounds with the fcc GaMo$_{4}$S$_{8}$-type structure originates from hopping of localized unpaired electrons (S=1/2) among widely separated tetrahedral M$_{4}$ metal clusters. We show that under pressure these systems transform from Mott insulators to a metallic and superconducting state with T$_{C}$=2.9 and 5.8K at 13 and 11.5GPa for GaNb$_{4}$Se$_{8}$ and GaTa$_{4}$Se$_{8}$, respectively. The occurrence of superconductivity is shown to be connected with a pressure-induced decrease of the MSe$_{6}$ octahedral distortion and simultaneous softening of the phonon associated with MSe-bonds.Comment: 10 pages, 5 figure

Short and canonical GRBs

Within the "fireshell" model for the Gamma-Ray Bursts (GRBs) we define a "canonical GRB" light curve with two sharply different components: the Proper-GRB (P-GRB), emitted when the optically thick fireshell of electron-positron plasma originating the phenomenon reaches transparency, and the afterglow, emitted due to the collision between the remaining optically thin fireshell and the CircumBurst Medium (CBM). We outline our "canonical GRB" scenario, with a special emphasis on the discrimination between "genuine" and "fake" short GRBs.Comment: 4 pages, 3 figures, in the Proceedings of the "Gamma Ray Bursts 2007" meeting, November 5-9, 2007, Santa Fe, New Mexico, US

Pressure-induced quantum phase transition in Fe₁−ᵪCoᵪSi (x = 0.1,0.2)

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Pressure driven collapse of the magnetism in the Kondo insulator UNiSn

The effect of pressure on the electronic and magnetic properties of the antiferromagnetic (TN~43 K) narrow gap semiconductor UNiSn has been investigated by 119Sn Mössbauer spectroscopy and nuclear forward scattering of synchrotron radiation, electrical resistance, and x-ray diffraction. We show that the decrease of the semiconducting gap which leads to a metallic state at p~9 GPa is associated with an enhancement of TN. At higher pressures, both TN and the transferred magnetic hyperfine field decrease, with a collapse of magnetism at ~18.5 GPa. The results are explained by a volume-dependent competition between indirect Ruderman-Kittel-Kasuya-Yosida interaction and the 5f-ligand hybridization

Two-band second moment model and an interatomic potential for caesium

A semi-empirical formalism is presented for deriving interatomic potentials for materials such as caesium or cerium which exhibit volume collapse phase transitions. It is based on the Finnis-Sinclair second moment tight binding approach, but incorporates two independent bands on each atom. The potential is cast in a form suitable for large-scale molecular dynamics, the computational cost being the evaluation of short ranged pair potentials. Parameters for a model potential for caesium are derived and tested

HIGH-PRESSURE MÖSSBAUER STUDIES ON 151Eu IN SELECTED Eu INTERMETALLIC COMPOUNDS

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The Stability of the Ferromagnetic State in La(Fe0.86Al0.14)13 Under High Pressure

The ferromagnetic (FM) La(Fe0.86Al0.14)13 Invar alloy displays a pressure-induced FM to antiferromagnetic (AF) phase transition at a pressure as low as p ~< 0.1 GPa. A quantitative analysis of the results shows that the FM → AF magnetic phase transition recently observed in La(FexAl1-x)13 at ambient pressure is governed by the decrease of the unit cell volume rather than the increase of the number of Fe nearest neighbor atoms.