Morphology and Orientation Selection of Non-Metallic Inclusions in Electrified Molten Metal

The effect of electric current on morphology and orientation selection of non-metallic inclusions in molten metal has been investigated using theoretical modelling and numerical calculation. Two geometric factors, namely the circularity (fc) and alignment ratio (fe) were introduced to describe the inclusions shape and configuration. Electric current free energy was calculated and the values were used to determine the thermodynamic preference between different microstructures. Electric current promotes the development of inclusion along the current direction by either expatiating directional growth or enhancing directional agglomeration. Reconfiguration of the inclusions to reduce the system electric resistance drives the phenomena. The morphology and orientation selection follows the routine to reduce electric free energy. The numerical results are in agreement with our experimental observations

Isotope effects and possible pairing mechanism in optimally doped cuprate superconductors

We have studied the oxygen-isotope effects on T_{c} and in-plane penetration depth \lambda_{ab}(0) in an optimally doped 3-layer cuprate Bi_{1.6}Pb_{0.4}Sr_{2}Ca_{2}Cu_{3}O_{10+y} (T_{c} \sim 107 K). We find a small oxygen-isotope effect on T_{c} (\alpha_{O} = 0.019), and a substantial effect on \lambda_{ab} (0) (\Delta \lambda_{ab} (0)/\lambda_{ab} (0) = 2.5\pm0.5%). The present results along with the previously observed isotope effects in single-layer and double-layer cuprates indicate that the isotope exponent \alpha_{O} in optimally doped cuprates is small while the isotope effect on the in-plane effective supercarrier mass is substantial and nearly independent of the number of the CuO_{2} layers. A plausible pairing mechanism is proposed to explain the isotope effects, high-T_{c} superconductivity and tunneling spectra in a consistent way.Comment: 5 pages, 4 figure

The Isotope Effect in Superconductors

We review some aspects of the isotope effect (IE) in superconductors. Our focus is on the influence of factors not related to the pairing mechanism. After summarizing the main results obtained for conventional superconductors, we review the effect of magnetic impurities, the proximity effect and non-adiabaticity on the value of the isotope coefficient (IC). We discuss the isotope effect of $T_c$ and of the penetration depth $\delta$. The theory is applied to conventional and high-$T_c$ superconductors. Experimental results obtained for YBa$_2$Cu$_3$O$_{7-\delta}$ related materials (Zn and Pr-substituted as well as oxygen-depleted systems) and for La$_{2-x}$Sr$_x$CuO$_4$ are discussed.Comment: 31 pages, 10 figures. Review article to appear in "Pair Correlation in Many Fermions Systems", Plenum Press 199

MNK1 and MNK2 mediate adverse effects of high-fat feeding in distinct ways

The MAP kinase-interacting kinases (MNK1 and MNK2) are non-essential enzymes which are activated by MAP kinases. They are implicated in controlling protein synthesis. Here we show that mice in which the expression of either MNK1 or MNK2 has been knocked out (KO) are protected against adverse effects of high-fat feeding, and in distinct ways. High-fat diet (HFD)-fed MNK2-KO show less weight gain than wild-type animals, and improved glucose tolerance, better insulin sensitivity and markedly diminished adipose tissue inflammation. This suggests MNK2 plays a role in adipogenesis and/or lipogenesis and in macrophage biology. MNK1-KO/HFD mice show better glucose tolerance and insulin sensitivity, but gain weight and show similar adipose inflammation to WT animals. These data suggest MNK1 participates in mediating HFD-induced insulin resistance. Our findings reveal distinct roles for the MNKs in a novel area of disease biology, metabolic dysfunction, and suggests they are potential new targets for managing metabolic disease

Electronic structure study of double perovskites A2A_{2}FeReO6_{6} (A=Ba,Sr,Ca) and Sr2M_{2}MMoO6_{6} (M=Cr,Mn,Fe,Co) by LSDA and LSDA+U

We have implemented a systematic LSDA and LSDA+U study of the double perovskites $A_{2}$FeReO$_{6}$ (A=Ba,Sr,Ca) and Sr$_{2}$$M$MoO$_{6}$ (M=Cr,Mn,Fe,Co) for understanding of their intriguing electronic and magnetic properties. The results suggest a ferrimagnetic (FiM) and half-metallic (HM) state of $A_{2}$FeReO$_{6}$ (A=Ba,Sr) due to a pdd-$\pi$ coupling between the down-spin Re$^{5+}$/Fe$^{3+}$ $t_{2g}$ orbitals via the intermediate O $2p_{\pi}$ ones, also a very similar FiM and HM state of Sr$_{2}$FeMoO$_{6}$. In contrast, a decreasing Fe $t_{2g}$ component at Fermi level ($E_{F}$) in the distorted Ca$_{2}$FeReO$_{6}$ partly accounts for its nonmetallic behavior, while a finite $pdd$-$\sigma$ coupling between the down-spin Re$^{5+}$/Fe$^{3+}$ $e_{g}$ orbitals being present at $E_{F}$ serves to stabilize its FiM state. For Sr$_{2}$CrMoO$_{6}$ compared with Sr$_{2}$FeMoO$_{6}$, the coupling between the down-spin Mo$^{5+}$/Cr$^{3+}$ $t_{2g}$ orbitals decreases as a noticeable shift up of the Cr$^{3+}$ 3d levels, which is likely responsible for the decreasing $T_{C}$ value and weak conductivity. Moreover, the calculated level distributions indicate a Mn$^{2+}$(Co$^{2+}$)/Mo$^{6+}$ ionic state in Sr$_{2}$MnMoO$_{6}$ (Sr$_{2}$CoMoO$_{6}$), in terms of which their antiferromagnetic insulating ground state can be interpreted. While orbital population analyses show that owing to strong intrinsic pd covalence effects, Sr$_{2}M$MoO$_{6}$ (M=Cr,Mn,Fe,Co) have nearly the same valence state combinations, as accounts for the similar M-independent spectral features observed in them.Comment: 21 pages, 3 figures. to be published in Phys. Rev. B on 15th Se

Point-contact Andreev-reflection spectroscopy in ReFeAsO_{1-x}F_x (Re = La, Sm): Possible evidence for two nodeless gaps

A deep understanding of the character of superconductivity in the recently discovered Fe-based oxypnictides ReFeAsO1-xFx (Re = rare-earth) necessarily requires the determination of the number of the gaps and their symmetry in k space, which are fundamental ingredients of any model for the pairing mechanism in these new superconductors. In the present paper, we show that point-contact Andreev-reflection experiments performed on LaFeAsO1-xFx (La-1111) polycrystals with Tc ~ 27 K and SmFeAsO0.8F0.2 (Sm-1111) ones with Tc ~ 53 K gave differential conductance curves exhibiting two peaks at low bias and two additional structures (peaks or shoulders) at higher bias, an experimental situation quite similar to that observed by the same technique in pure and doped MgB2. The single-band Blonder-Tinkham-Klapwijk model is totally unable to properly fit the conductance curves, while the two-gap one accounts remarkably well for the shape of the whole experimental dI/dV vs. V curves. These results give direct evidence of two nodeless gaps in the superconducting state of ReFeAsO1-xFx (Re = La, Sm): a small gap, Delta1, smaller than the BCS value (2Delta1/kBTc ~ 2.2 - 3.2) and a much larger gap Delta2 which gives a ratio 2Delta2/kBTc ~ 6.5 - 9. In Sm-1111 both gaps close at the same temperature, very similar to the bulk Tc, and follow a BCS-like behaviour, while in La-1111 the situation is more complex, the temperature dependence of the gaps showing remarkable deviations from the BCS behaviour at T close to Tc. The normal-state conductance reproducibly shows an unusual, but different, shape in La-1111 and Sm-1111 with a depression or a hump at zero bias, respectively. These structures survive up to T* ~ 140 K, close to the temperatures at which structural and magnetic transitions occur in the parent, undoped compound.Comment: 10 pages, 7 color figures, Special Issue of Physica C on Superconducting Pnictide

Persistence for stochastic difference equations: A mini-review

Understanding under what conditions populations, whether they be plants, animals, or viral particles, persist is an issue of theoretical and practical importance in population biology. Both biotic interactions and environmental fluctuations are key factors that can facilitate or disrupt persistence. One approach to examining the interplay between these deterministic and stochastic forces is the construction and analysis of stochastic difference equations $X_{t+1}=F(X_t,\xi_{t+1})$ where $X_t \in \R^k$ represents the state of the populations and $\xi_1,\xi_2,...$ is a sequence of random variables representing environmental stochasticity. In the analysis of these stochastic models, many theoretical population biologists are interested in whether the models are bounded and persistent. Here, boundedness asserts that asymptotically $X_t$ tends to remain in compact sets. In contrast, persistence requires that $X_t$ tends to be "repelled" by some "extinction set" $S_0\subset \R^k$. Here, results on both of these proprieties are reviewed for single species, multiple species, and structured population models. The results are illustrated with applications to stochastic versions of the Hassell and Ricker single species models, Ricker, Beverton-Holt, lottery models of competition, and lottery models of rock-paper-scissor games. A variety of conjectures and suggestions for future research are presented.Comment: Accepted for publication in the Journal of Difference Equations and Application

Subsurface Flows in and Around Active Regions with Rotating and Non-rotating Sunspots

The temporal variation of the horizontal velocity in subsurface layers beneath three different types of active regions is studied using the technique of ring diagrams. In this study, we select active regions (ARs) 10923, 10930, 10935 from three consecutive Carrington rotations: AR 10930 contains a fast-rotating sunspot in a strong emerging active region while other two have non-rotating sunspots with emerging flux in AR 10923 and decaying flux in AR 10935. The depth range covered is from the surface to about 12 Mm. In order to minimize the influence of systematic effects, the selection of active and quiet regions is made so that these were observed at the same heliographic locations on the solar disk. We find a significant variation in both components of the horizontal velocity in active regions as compared to quiet regions. The magnitude is higher in emerging-flux regions than in the decaying-flux region, in agreement with earlier findings. Further, we clearly see a significant temporal variation in depth profiles of both zonal and meridional flow components in AR 10930, with the variation in the zonal component being more pronounced. We also notice a significant influence of the plasma motion in areas closest to the rotating sunspot in AR 10930 while areas surrounding the non-rotating sunspots in all three cases are least affected by the presence of the active region in their neighborhood.Comment: Solar Physics (in press), includes 11 figure

Symmetries of Pairing Correlations in Superconductor-Ferromagnet Nanostructures

Using selection rules imposed by the Pauli principle, we classify pairing correlations according to their symmetry properties with respect to spin, momentum, and energy. We observe that inhomogeneity always leads to mixing of even- and odd-energy pairing components. We investigate the superconducting pairing correlations present near interfaces between superconductors and ferromagnets, with focus on clean systems consisting of singlet superconductors and either weak or half-metallic ferromagnets. Spin-active scattering in the interface region induces all of the possible symmetry components. In particular, the long-range equal-spin pairing correlations have odd-frequency s-wave and even-frequency p-wave components of comparable magnitudes. We also analyze the Josephson current through a half-metal. We find analytic expressions and an interesting universality in the temperature dependence of the critical current in the tunneling limit.Comment: 20 pages, 5 figures, added citations, corrected typo