International Journal of Superconductivity The latest articles from Hindawi Publishing Corporation © 2015 , Hindawi Publishing Corporation . All rights reserved. Spin Flows in Magnetic Semiconductor/Insulator/Superconductor Tunneling Junction Sun, 05 Apr 2015 11:41:55 +0000 Tunneling currents along the c-axis of the majority and minority spin electrons have been studied for a magnetic semiconductor (MS)/insulator (I)/superconductor (S) tunneling junction consisting of a Ga1−xMnxAs with x = 1/32, a nonmagnetic with a realistic dimension, and a (Hg-1223) high- S. The normalized charge and spin currents, and , and the flows of the majority and minority spin electrons, and , have been calculated at a fixed external voltage , as a function of the magnetic moment μ′   per a Mn atom which is deduced from the band structure calculations. It is found that the tunneling due to the minority spin electron dominates when , but such a phenomenon is not found for . We have pointed out that the present tunneling junction seems to work as a switching device in which the ↑ and ↓ spin flows can be easily controlled by the external magnetic field. Michihide Kitamura and Akinobu Irie Copyright © 2015 Michihide Kitamura and Akinobu Irie. All rights reserved. AC Loss Measurements of a Trapezoidal Shaped HTS Coil Using an Electrical Method Tue, 02 Dec 2014 09:09:46 +0000 High temperature superconducting (HTS) coils of different shapes (typically circular or trapezoidal) wound on iron or ironless core are fundamental components of many superconducting electrical power devices. A 150-turn (75 turns/pancake) trapezoidal coil in double pancake configuration has been designed and realized in our laboratory of ENEA Frascati. Various epoxy resins and YBCO tapes have been tested in the temperature range room to liquid nitrogen, leading us to the choice of AmSC (American Superconductor) tape for the winding and araldite resin for the impregnation process. The trapezoidal shape has been chosen because of its suitable geometry for practical applications, the results being complementary to what was previously achieved on round shaped HTS coils. The AC transport current losses have been measured using a compensated electrical method and expressed in terms of a linearly frequency dependent resistance. A linear dependence of the losses resistance from frequency was expected and found in agreement with previous results. The current-voltage curve has been measured in zero externally applied field condition, the results being in good agreement with a numerical simulation. The magnetic field distributions, at different air gaps from coil top and zero externally applied filed condition, have been simulated and reported as well. G. Messina, L. Morici, U. Besi Vetrella, G. Celentano, M. Marchetti, R. Viola, and P. Sabatino Copyright © 2014 G. Messina et al. All rights reserved. Charge and Spin Currents in Ferromagnet-Insulator-Superconductor Tunneling Junctions Using Hg-1223 High- Superconductor Tue, 26 Aug 2014 00:00:00 +0000 Charge and spin currents along the -axis in ferromagnet-insulator-superconductor (F/I/S) tunneling junctions have been studied within the framework of the tunneling Hamiltonian model. As a superconductor , HgBa2Ca2Cu3O8+δ (Hg-1223) with copper-oxide high- superconductor has been selected, and as a ferromagnet F, Fe metal with bcc structure has been selected for simplicity. The electronic structures of above materials have been calculated on the basis of the band theory using the spin-polarized self-consistent-field data for the atomic orbital energies and the universal tight-binding parameters (UTBP) for the interactions. For the and defined in the present paper, which are tunneling probabilities of the majority and the minority spin electrons, it is shown that the condition means the standard F/I/S tunneling junction with a nonmagnetic insulating layer, and the condition means the F/I/S tunneling junction with a magnetic insulating layer showing a detectable magnetization. We have found that the charge current and the differential conductance nearly remain the same as the change of , but the spin current is largely changed due to the change of . As an experimental method to detect the change of the spin current, the validity of an X-ray magnetic circular dichroism (XMCD) has been pointed out. Michihide Kitamura, Yoshitaka Uchiumi, and Akinobu Irie Copyright © 2014 Michihide Kitamura et al. All rights reserved. Electrical Transport and Lowered Percolation Threshold in YBa2Cu3O7−δ-Nano-YBa2ZrO5.5 Composites Mon, 25 Aug 2014 08:15:51 +0000 We investigated the chemical reactivity and percolation characteristics of insulating nanocrystalline YBa2ZrO5.5 prepared by modified combustion route and the YBa2Cu3O7−δ superconductor composite system. Structural analysis was done by using X-ray diffraction technique, surface morphology of the samples was studied using scanning electron microscopy, and electrical transport measurements like critical transition temperatures () and self-field transport critical current () were done by using standard four-probe technique. It is found that, in YBa2Cu3O7−δ-nano-YBa2ZrO5.5 composite system, the superconductor and insulator materials coexist as separate phases without any noticeable chemical reaction even after sintering at high temperatures. Furthermore, percolation threshold and critical exponent are found to be , and . And the analysis of the current flow in the polycrystalline samples reveals weak link behavior in the majority of grain connections. Pullanhiyodan Puthiyaveedu Rejith, Sukumariamma Vidya, and Jijimon Kumpukattu Thomas Copyright © 2014 Pullanhiyodan Puthiyaveedu Rejith et al. All rights reserved. An Analysis of Strong Coupling in the High Tc Superconductors Mon, 17 Mar 2014 09:56:00 +0000 A new McMillan formula is assembled for the High Temperature Superconductors that evidences strong coupling in these materials. The McMillan-Hopfield parameter is calculated in each case, in addition to the Bergmann and Rainer enhancement frequency. The cubic symmetry of the CuO layers and the lattice cohesion and oxygen presence are suggested reasons for the effect. J. H. Turner Copyright © 2014 J. H. Turner. All rights reserved. The Stratified Superconductivity in Ba0.6K0.4BiO3 Single Crystal: Direct Measurement of Energy Gap between Homo-, and Inhomogeneous States Sun, 02 Mar 2014 08:47:15 +0000 The existence of space inhomogeneous superconductor insulator state (SISIS) found out earlier in polycrystalline samples of high- system ( K) is confirmed on single crystal. At ( K) the transition from the homogeneous superconducting state into the SISIS occurs. SISIS is characterized by the appearance of two gaps on the Fermi surface, semi- and superconducting, which are modulated in space in antiphase, the electric transport between superconducting regions being carried out due to Josephson tunneling. Thus the whole sample becomes a multiple Josephson system. Nonlinear curves are observed on single crystal at temperatures below . Dependence of curves on temperature and magnetic field, typical to a Josephson system, was found out. Besides, a step-like peculiarity at the values of voltage of the order of one and two superconducting gaps shows up. These peculiarities are suppressed by magnetic field much earlier than critical current. The new data firstly correlate with the model of SISIS and secondly permit for the first time to determining directly the energy gap between homogeneous and stratified superconductor states. L. N. Zherikhina, A. M. Tskhovrebov, L. A. Klinkova, D. A. Balaev, S. I. Popkov, K. A. Shaikhutdinov, and D. A. Velikanov Copyright © 2014 L. N. Zherikhina et al. All rights reserved. Polaronic Mechanism of Superconductivity in Cuprates Tue, 30 Jul 2013 11:44:01 +0000 A strong polaron pairing model of high-temperature cuprate superconductors is presented. The normal and anomalous one-particle Green’s functions are derived from a system with strong electron-phonon coupling. Self-consistent equation for the superconducting order parameter () is derived using Green’s function technique and following Lang and Firsov transformations. Expressions for specific heat, density of states, free energy, and critical field based on this model have been derived. The theory is applied to explain the experimental results in the system . There is convincing evidence that the theory is fully compatible with the key experiments. Pradeep Chaudhary, Anuj Nuwal, S. C. Tiwari, R. K. Paliwal, and S. L. Kakani Copyright © 2013 Pradeep Chaudhary et al. All rights reserved. Synthesis and Characterization of RuSr2R1.6Ce0.4Cu2O10 (R = Gd, Eu, and Sm) Magnetosuperconductors Mon, 29 Jul 2013 10:45:53 +0000 The discovery of the coexistence of superconductivity and weak ferromagnetism in ruthenocuprates like (Ru, R-1222; R = Eu, Gd) and RuSr2GdCu2O8 (Ru-1212) has created a tremendous interest both experimentally as well as theoretically. Here, we have prepared polycrystalline samples of RuSr2R1.4Ce0.6Cu2O10 (R = Gd, Eu, and Sm) by the standard solid state reaction method. These samples were characterized by XRD, SEM, dc resistivity, and squid measurements. All the prepared samples were single phase without any trace of impurity. From the low-temperature resistivity measurement, the samples of (R = Eu and Gd) were found to be superconducting, while the samples of RuSr2Sm1.6Ce0.4Cu2O10 show semiconducting behavior. The magnetization (M) versus field (H) hysteresis at 5 K clearly shows the ferromagnetic behavior of the samples. The zero field cooled magnetization () and field cooled magnetization () diverge at 100 K. Rajneesh Mohan, Nupinderjeet Kaur, and N. K. Gaur Copyright © 2013 Rajneesh Mohan et al. All rights reserved.