Josephson effect in graphene bilayers with adjustable relative displacement

The Josephson current is investigated in a superconducting graphene bilayer where the pristine graphene sheets can make in-plane or out-of-plane displacements with respect to each other. The superconductivity can be of intrinsic nature, or due to a proximity effect. The results demonstrate that the supercurrent responds qualitatively differently to relative displacement if the superconductivity is due to either intralayer or interlayer spin-singlet electron-electron pairing, thus providing a tool to distinguish between the two mechanisms. Specifically, both the AA and AB stacking orders are studied with antiferromagnetic spin alignment. For the AA stacking order with intralayer and on-site pairing no current reversal is found. In contrast, the supercurrent may switch its direction as a function of the in-plane displacement and out-of-plane interlayer coupling for the cases of AA ordering with interlayer pairing and AB ordering with either intralayer or interlayer pairing. In addition to sign reversal, the Josephson signal displays many characteristic fingerprints which derive directly from the pairing mechanism. Thus, measurements of the Josephson current as a function of the graphene bilayer displacement open up means for achieving a deeper insight of the superconducting pairing mechanism

Inherent electron and hole trapping in amorphous phase-change memory materials : Ge2Sb2Te5

While the amorphous state of a chalcogenide phase-change material is formed inside an electronic-memory device via Joule heating, caused by an applied voltage pulse, it is in the presence of excess field-induced electrons and holes. Here, hybrid density-functional-theory calculations for glassy Ge2Sb2Te5 demonstrate that extra electrons are trapped spontaneously, creating deep traps in the band gap. Hole self-trapping is also energetically favourable, producing states around midgap. The traps have a relatively low ionization energy, indicating that they can easily be thermally released. Near-linear triatomic Te-Ge/Sb-Te/Ge/Sb environments are the structural motifs where the extra electrons/holes are trapped inside the glass network, highlighting that the intrinsic axial bonds of octahedral-like sites in amorphous Ge2Sb2Te5 can serve as charge-trapping centres. Trapping of two electrons in a chain-like structure of connected triads results in breaking of some of these highly polarizable long bonds. These results establish the foundations of the origin of charge trapping in amorphous phase-change materials, and they may have important implications for our understanding of resistance drift in electronic-memory devices and of electronic-excitation-induced athermal melting.publishedVersionPeer reviewe

Three in One: Three Different Molybdates Trapped in a Thiacalix[4]arene Protected Ag<inf>72</inf> Nanocluster for Structural Transformation and Photothermal Conversion

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Photoelectron spectra of aluminum cluster anions: Temperature effects and ab initio simulations

Photoelectron (PES) spectra from aluminum cluster anions (from 12 to 15 atoms) at various temperature regimes, were studied using ab-initio molecular dynamics simulations and experimentally. The calculated PES spectra, obtained via shifting of the simulated electronic densities of states by the self-consistently determined values of the asymptotic exchange-correlation potential, agree well with the measured ones, allowing reliable structural assignments and theoretical estimation of the clusters' temperatures.Comment: RevTex, 3 gif figures. Scheduled for Oct 15, 1999, issue of Phys. Rev. B as Rapid Communicatio