Hydrogen-rich compounds hold promise as high-temperature superconductors under high pressures. Recent theoretical hydride structures on achieving high-pressure superconductivity are composed mainly of H 2 fragments. Through a systematic investigation of Ca hydrides with different hydrogen contents using particleswam optimization structural search, we show that in the stoichiometry CaH 6 a body-centered cubic structure with hydrogen that forms unusual "sodalite" cages containing enclathrated Ca stabilizes above pressure 150 GPa. The stability of this structure is derived from the acceptance by two H 2 of electrons donated by Ca forming an "H 4 " unit as the building block in the construction of the three-dimensional sodalite cage. This unique structure has a partial occupation of the degenerated orbitals at the zone center. The resultant dynamic Jahn-Teller effect helps to enhance electronphonon coupling and leads to superconductivity of CaH 6 . A superconducting critical temperature (T c ) of 220-235 K at 150 GPa obtained from the solution of the Eliashberg equations is the highest among all hydrides studied thus far.calcium hydride | sodalite structure

Rods of a visible-light-cured dental composite resin were photo-polymerized and immersed in water at 37 degrees C for 7 days. The residual monomers (TEGDMA and Bis-GMA) trapped in the set composite and those eluted into water were analysed by gas-liquid chromatography. It became evident that minor amounts of the residual monomers dissolved in water, but that most residual monomers remained in the set composite. Extension of the irradiation period contributed to the significant reduction in the residual monomer level and its elution into water.

From first-principles calculations, a high-pressure metallic phase of SnH(4) with a novel layered structure intercalated by "H(2)" units is revealed. This structure is stable at pressure between 70 and 160 GPa. A remarkable feature of this structure is the presence of soft modes in the phonon band structure induced by Fermi surface nesting and Kohn anomalies that lead to very strong electron-phonon coupling. The application of the Allen-Dynes modified McMillan equation with the calculated electron-phonon coupling parameter lambda shows that a superconducting critical temperature close to 80 K can be achieved at 120 GPa.

We investigate cascades of isochronous pitchfork bifurcations of straight-line librating orbits in some two-dimensional Hamiltonian systems with mixed phase space. We show that the new bifurcated orbits, which are responsible for the onset of chaos, are given analytically by the periodic solutions of the Lamé equation as classified in 1940 by Ince. In Hamiltonians with C 2v symmetry, they occur alternatingly as Lamé functions of period 2K and 4K, respectively, where 4K is the period of the Jacobi elliptic function appearing in the Lamé equation. We also show that the two pairs of orbits created at period-doubling bifurcations of island-chain type are given by two different linear combinations of algebraic Lamé functions with period 8K.

A combination of static and dynamical first-principles electronic calculations of silane, SiH4, at high pressure has revealed a novel monoclinic structure with C2/c symmetry. This highpressure phase is metallic and composed of layers of SiH4 bridged by H bonds. Perturbative linear response calculations at 90 and 125 GPa predict large electron-phonon couplings yielding an electron-phonon coupling parameter λ close to 0.9. The application of McMillan equation gives a superconducting critical temperature (Tc) between 45 and 55 K.

We develop uniform approximations for the trace formula for non-integrable systems in which SU(2) symmetry is broken by a non-linear term of the Hamiltonian. As specific examples, we investigate Hénon-Heiles type potentials. Our formalism can also be applied to the breaking of SO(3) symmetry in a three-dimensional cavity with axially-symmetric quadrupole deformation.PACS number: 03.65.SqNovember 5, 1998 J. Phys. A, in print * ) Present address:

Structural phase transitions and superconducting properties in three phases ͑9R, fcc, and cI16͒ of solid Li are investigated using a pseudopotential plane-wave method based on density functional perturbation theory. In particular, it is shown that phonon softening is responsible for a pressure-induced fcc→ cI16 transition as well as for a significant enhancement of electron-phonon coupling and superconducting transition temperature T c preceding this structural transformation. The nature of superconductivity in the fcc and cI16 phases is examined by solving the Eliashberg equations with the spectral function ␣ 2 F͑͒ obtained from first-principles calculations and by evaluating the functional derivative ␦T c / ␦␣ 2 F͑͒. The calculated T c reaches a maximum at pressure close to the fcc→ cI16 transition and is significantly reduced in the cI16 phase, in agreement with the trend observed experimentally. The variation in T c as a function of pressure is explained in terms of the functional derivative and shifts of the spectral weight.

We report self-consistent calculations of the microscopic electronic structure of the so-called giant vortex states. These multiquantum vortex states, detected by recent magnetization measurements on submicron disks, are qualitatively different from the Abrikosov vortices in the bulk. We find that, in addition to multiple branches of bound states in the core region, the local tunneling density of states exhibits Tomasch oscillations caused by the single-particle interference arising from quantum confinement. These features should be directly observable by scanning tunneling spectroscopy.S uperconducting vortices are topological singularities in the order parameter (1). In a bulk system, each vortex carries a single flux quantum, whereas vortices with multiple flux quanta are not favorable energetically (2). In small superconductors, however, the situation may be different. Today's nanotechnology can provide valuable insight into the nature of mesoscopic superconductors, whose linear dimensions can be comparable to the coherence length or the inter-vortex distance of the Abrikosov lattice. The following question then arises naturally: does single-quantum vortex matter survive the limit of decreasing sample size? More than 30 years ago, Fink and Presson (3) gave the intriguing answer ''not always'' in their pioneering work on a related system: a thin cylinder in parallel field. They have shown it theoretically, within the framework of the phenomenological Ginzburg-Landau (GL) theory, and also provided experimental evidence (4) for the existence of an enormous superfluid eddy current on the surface of a thin cylinder. They called this state a giant vortex state.Although the work of Fink and Presson was largely forgotten for the next several decades, it nevertheless anticipated the present excitement in the field of nanoscale superconductivity. With recent advances in the controlled fabrication and study of nanometer-scale superconductors, the concept of giant vortex was brought back to focus by Moshchalkov and coworkers a few years ago (5). Their experiments on mesoscopic squares and square rings have indeed revealed that small superconductors do not always favor many-vortex states reminiscent of the Abrikosov vortex lattice. The measured H-T phase boundaries of these small structures were explained in terms of giant vortex states in the GL picture (5). Subsequent experiments on submicron disks (6) have further shown the existence of giant vortex states inside the phase boundaries. Within the GL framework (6-10) some of the abrupt changes in the magnetization observed have been attributed, e.g., to the collapse of a multivortex state into a giant vortex, or to transitions among different giant vortex states.This phase of vortex matter has a single vortex occupying the sample, carrying multiple f luxoid (3, 11, 12) quanta. Such a state has no immediate analogue in bulk systems, and would only be similar to vortex states predicted for artificially patterned structures (13,14). Moshchalkov et al. (15) have also sugge...