Repulsion-Sustained Supercurrent and Flux Quantization in Rings of Symmetric Hubbard Clusters

We test the response to a threading magnetic field of rings of 5-site $C_{4v}$-symmetric repulsive Hubbard clusters connected by weak intercell links; each 5-site unit has the topology of a CuO$_{4}$ cluster and a repulsive interaction is included on every site. In a numerical study of the three-unit ring with 8 particles, we take advantage of a novel exact-diagonalization technique which can be generally applied to many-fermion problems. For O-O hopping we find Superconducting Flux Quantization (SFQ), but for purely Cu-Cu links bound pair propagation is hindered by symmetry. The results agree with W=0 pairing theory.Comment: 4 pages, 2 figure

Supermassive Black Hole Binaries: The Search Continues

Gravitationally bound supermassive black hole binaries (SBHBs) are thought to be a natural product of galactic mergers and growth of the large scale structure in the universe. They however remain observationally elusive, thus raising a question about characteristic observational signatures associated with these systems. In this conference proceeding I discuss current theoretical understanding and latest advances and prospects in observational searches for SBHBs.Comment: 17 pages, 4 figures. To appear in the Proceedings of 2014 Sant Cugat Forum on Astrophysics. Astrophysics and Space Science Proceedings, ed. C.Sopuerta (Berlin: Springer-Verlag

Differential effective medium theory of sedimentary rocks

We show that both the electrical and acoustic properties of fluid-saturated sedimentary rocks can be described within the unified framework of differential effective medium theory. Calculations based on the differential effective medium picture of rock microstructure yield predictions of sonic travel times and acoustic attenuation in good agreement with experimental data. In particular, the theory shows that the large frequency peak in attenuation and its associated velocity dispersion observed in sandstones are characteristic of a composite system containing fluid-filled microcracks

A generalized differential effective medium theory

A generalization of the Differential Effective Medium approximation (DEM) is discussed. The new scheme is applied to the estimation of the effective permittivity of a two phase dielectric composite. Ordinary DEM corresponds to a realizable microgeometry in which the composite is built up incrementally through a process of homogenization, with one phase always in dilute suspension and the other phase associated with the percolating backbone. The generalization of DEM assumes a third phase which acts as a backbone. The other two phases are progressively added to the backbone such that each addition is in an effectively homogeneous medium. A canonical ordinary differential equation is derived which describes the change in material properties as a function of the volume concentration φ of the added phases in the composite. As φ→ 1, the Effective Medium Approximation (EMA) is obtained. For φ < 1, the result depends upon the backbone and the mixture path that is followed. The approach to EMA for φ ≊ 1 is analysed and a generalization of Archie's law for conductor-insulator composites is described. The conductivity mimics EMA above the percolation threshold and DEM as the conducting phase vanishes. © 1985

Effective-medium theories for two-phase dielectric media

Two different effective-medium theories for two-phase dielectric composites are considered. They are the effective medium approximation (EMA) and the differential effective medium approximation (DEM). Both theories correspond to realizable microgeometries in which the composite is built up incrementally through a process of homogenization. The grains are assumed to be similar ellipsoids randomly oriented, for which the microgeometry of EMA is symmetric. The microgeometry of DEM is always unsymmetric in that one phase acts as a backbone. It is shown that both EMA and DEM give effective dielectric constants that satisfy the Hashin-Shtrikman bounds. A new realization of the Hashin-Shtrikman bounds is presented in terms of DEM. The general solution to the DEM equation is obtained and the percolation properties of both theories are considered. EMA always has a percolation threshold, unless the grains are needle shaped. In contrast, DEM with the conductor as backbone always percolates. However, the threshold in EMA can be avoided by allowing the grain shape to vary with volume fraction. The grains must become needlelike as the conducting phase vanishes in order to maintain a finite conductivity. Specifically, the grain-shape history for which EMA reproduces DEM is found. The grain shapes are oblate for low-volume fractions of insulator. As the volume fraction increases, the shape does not vary much, until at some critical volume fraction there is a discontinuous transition in grain shape from oblate to prolate. In general, it is not always possible to map DEM onto an equivalent EMA, and even when it is, the mapping is not preserved under the interchange of the two phases. This is because DEM is inherently unsymmetric between the two phases

GENERALIZED DIFFERENTIAL EFFECTIVE MEDIUM THEORY FOR COMPOSITES.

A generalization of the Differential Effective Medium approximation (DEM) is obtained in the context of the effective medium problem for a two phase composite. The approach to EMA is analyzed and a generalization of Archie's law for conductor-insulator composites is described. The conductivity mimics EMA above the percolation threshold and DEM as the conducting phase vanishes. The effects of grain shape are also considered. The Hashin-Shtrikman bounds on the mechanical and electrical properties can be realized by ordinary DEM with plate-like grains

3d2 Configuration In Six-times-ionized Argon, Ar Vii

The transition array 2p63p3d-2p63d2 has been identified in the spectrum of magnesium-like Ar VII produced by electrical discharges in the vacuum-ultraviolet region. Twenty-five new transitions have been identified as combinations of levels of this transition array. From these transitions we have determined the levels of the 3d2 configuration. It was also possible to determine the 2p63p3d1D 2 0 level that was missed in the early research with the Ar VII spectrum. Hartree-Fock calculations with relativistic corrections were used to predict energy levels and transitions. Isoelectronic comparisons along the Mg I sequence are used to support the experimental results. © 2001 Optical Society of America.181117181721Trigueiros, A.G., Mania, A.J., Gallardo, M., Almandos, J.G.R., Atomic transitions for the Ar VII spectrum in the vacuum ultraviolet (1997) J. Opt. Soc. Am. B, 14, pp. 2463-2468Zetterberg, P.O., Magnusson, C.E., The spectrum and term system of P IV (1977) Phys. Scr., 15, pp. 189-201Joelsson, I., Zetterberg, P.O., Magnusson, C.E., The spectrum and term system of S v (1981) Phys. Scr., 23, pp. 1087-1095Jupén, C., Fremberg, J., (1983), University of Lund, S-22100 Lund, Sweden (personal communication)Redfors, A., The 3d2 configuration in Ca IX-Zn XIX (1988) Phys. Scr., 38, pp. 702-706Churilov, S.S., Levashov, V.E., Wyart, J.P., Extended analysis of the 3p3d-3d2 transitions in the sequence K VIII-Cu XVII and isoelectronic trends in Mg-like ions through Kr xxv (1989) Phys. Scr., 40, pp. 625-633Froese-Fischer, C., Godefroid, M., Lifetime trends for the n = 3 singlet states in the Mg sequence (1982) Nucl. Instrum. Methods Phys. Res., 202, pp. 307-322Froese-Fischer, C., Godefroid, M., Short-range interactions involving plunging configurations of the n = 3 complex in the Mg sequence (1982) Phys. Scr., 25, pp. 394-400Reistad, N., Brage, T., Ekberg, J.O., Engström, L., J-dependent 3s4p 3Pj Lifetimes in Mg-like sulphur and chlorine (1984) Phys. Scr., 30, pp. 249-254Baluja, K.L., Hilbbert, A., Energy and oscillator strengths for allowed transitions in S v, Cl VI, and Fe xv (1985) Nucl. Instrum. Methods Phys. Res. B, 9, pp. 477-486Aashamar, K., Luke, T.M., Talman, J.D., Energy levels in the magnesium sequence, Mg I-Mn XIV, calculated in the multiconflguration optimized potential model (1986) Phys. Scr., 34, pp. 386-393Fawcett, B.C., Calculated oscillator strengths and wavelengths for allowed transitions within the third shell for ions in the Mg-like isoelectronic sequence between S V and Ni XVII (1983) At. Data Nucl. Data Tables, 28, pp. 579-596Fawcett, B.C., Calculated oscillator strengths, wavelengths and energy levels for allowed 3-3 and 3-4 transitions for Fe XV with isoelectronic comparisons between Dirac-Fock and Hartree-Fock (1986) Phys. Scr., 84, pp. 331-336Brage, T., Hibbert, A., Plunging configurations and J-dependent lifetimes in Mg-like Ions (1989) J. Phys. B, 22, pp. 713-726Gallardo, M., Bredice, F., Raineri, M., Reyna Almandos, J.G., Light source for obtaining spectra of highly ionized gases (1989) Appl. Opt., 28, pp. 4513-4515Radziemski, L.J., Kaufman, V., Wavelengths, energy levels, and analysis of neutral chlorine (Cl I) (1969) J. Opt. Soc. Am., 59, pp. 424-443Cowan, R.D., (1981) The Theory of Atomic Structure and Spectra, , U. California Press, Berkeley, CalifEdlén, B., Atomic spectra (1964) Encyclopedia of Physics, 27, p. 80. , S. Flügge, ed. (Springer-Verlag, Berlin

Microstructure and Residual Stress Formation during Friction Stir Welding of Semi Solid ZK60 Magnesium Alloy

In this work, we report on the friction stir weldability of a semi-solid cast ZK60 alloy modified with 1.5 wt% mischmetall in the lap-joint configuration using a 120WV4 steel tool with concave shoulder and conical pin. The coarser solidification microstructure in the semi-solid cast ZK60-1.5%RE alloy requires low strain rates and increased heat input to produce lap-joints without inner defects. This was achieved with 250 rpm tool rotation and 50 mm/min welding speed. Friction stir welding results in a very fine grained microstructure in the stir zone probably due to dynamic recrystallization. In the thermomechanically affected zones dynamic recrystallization seems to occur within the solute enriched intergranular zones. The distribution of longitudinal residual stresses exhibit stress maxima at both thermomechanically affected zones. A compression peak is observed at the retreating side, whereas a tensile stress maximum occurs at the advancing side.</jats:p