Chemical ordering in Y66(FexMn1-x)34 metallic glasses

The compositional dependence of the 57Fe Mossbauer effect spectra for the metallic glasses Y66(FexMn1-x)34 has been studied. For high concentrations of Fe the spectra consist of two symmetric peaks with the spectra becoming progressively more complex as the Fe content decreases. The smooth variation of the parameters derived from the spectra indicates that Fe and Mn perform the same functions in the glass. The author finds evidence for chemical ordering in these glasses and demonstrates the existence of two distinct atomic sites for Fe. Site 1 is attributed to Fe atoms having predominantly Y near neighbours and site 2 is identified as Fe atoms having a larger number of Fe (Mn) near neighbours

Superconductivity and the electronic structure of Zr- and Hf-based metallic glasses

The results of a comprehensive study of the superconducting transition temperatures of Zr- and Hf-based metallic glasses are reported. The microscopic origins of superconductivity in these glasses are discussed in terms of recent ultraviolet photoelectron spectroscopy (UPS) measurements and calculations based on the renormalized atom technique. These calculations accurately predict the UPS spectra and the results of low-temperature heat-capacity measurements. The complete description of the electronic structure afforded by these calculations allows, for the first time, a consistent picture of the variation of Tc with X in the glasses Zr1-yXy (X=3d or 4d transition metals). In addition, the dependence of Tc with composition (y) can be understood in terms of the X d subband positions relative to EF. The results reported here support our recent contention that the strong depression of Tc observed for X=Fe, Mn, Cr, and V glasses is related to the formation of localized magnetic moments and spin fluctuations. An alternate explanation for the low Tc of X=V and Cr glasses based on the idea of an atomic-structure change is also discussed

Vortex dynamics and upper critical fields in ultrathin Bi films

Current-voltage (I-V) characteristics of quench condensed, superconducting, ultrathin $Bi$ films in a magnetic field are reported. These I-V's show hysteresis for all films, grown both with and without thin $Ge$ underlayers. Films on Ge underlayers, close to superconductor-insulator transition (SIT), show a peak in the critical current, indicating a structural transformation of the vortex solid (VS). These underlayers, used to make the films more homogeneous, are found to be more effective in pinning the vortices. The upper critical fields (B$_{c2}$) of these films are determined from the resistive transitions in perpendicular magnetic field. The temperature dependence of the upper critical field is found to differ significantly from Ginzburg-Landau theory, after modifications for disorder.Comment: Phys Rev B, to be published Figure 6 replaced with correct figur

Rings and rigidity transitions in network glasses

Three elastic phases of covalent networks, (I) floppy, (II) isostatically rigid and (III) stressed-rigid have now been identified in glasses at specific degrees of cross-linking (or chemical composition) both in theory and experiments. Here we use size-increasing cluster combinatorics and constraint counting algorithms to study analytically possible consequences of self-organization. In the presence of small rings that can be locally I, II or III, we obtain two transitions instead of the previously reported single percolative transition at the mean coordination number $\bar r=2.4$, one from a floppy to an isostatic rigid phase, and a second one from an isostatic to a stressed rigid phase. The width of the intermediate phase $~ \bar r$ and the order of the phase transitions depend on the nature of medium range order (relative ring fractions). We compare the results to the Group IV chalcogenides, such as Ge-Se and Si-Se, for which evidence of an intermediate phase has been obtained, and for which estimates of ring fractions can be made from structures of high T crystalline phases.Comment: 29 pages, revtex, 7 eps figure