M.A.-K. Mohamed

Oxygen isotope effect in superconducting (Y1-xPrx)Ba2Cu3O7-δ

Abstract Oxygen isotope effect studies were performed on the (Y 1−x Pr x ) Ba 2 Cu 3 O 7−δ system for 0⩽x⩽0.5. With increasing x, and falling transition temperature, we find a large isotope effect, the coefficient α 0 is approaching 0.5. The results show the importance of the electron-phonon interaction for the superconductivity of these compounds.

The oxygen isotope effect in superconducting YBa2Cu3O7−δ

Abstract We report on measurements of the effect of 18O-16O substitution on the superconducting properties of Y1Ba2Cu3O7−δ. Substitutions were made by gas-exchange alone, and by gas exchange in conjunction with substituted BaCO3 and CuO. From Raman scattering we find that gas-exchange alone leads to lower substitution levels, particularly in the bridging oxygen O(4) position. Gas-exchanged samples give an isotope effect of 0.3 to 0.4 K. Use of substituted materials leads to samples with an isotope effect of up to 0.9 K in the resistance transition and 1.7 K in in the magnetically determinded transition.

The oxygen isotope effect in Pr, Ca, and Zn substituted YBa2Cu3O7−δ and EuBa2Cu3O7−δ

Abstract The oxygen isotope effect α was studied for Pr and Ca substituted YBCO, and for Zn substituted EuBCO and YBCO, α increases to values near 0.5 for the highest Pr concentrations, and lowest transition temperatures. In the Zn substituted system α remains small to the lowest transition temperatures. It is shown that the isotope exponent is a function of mobile hole concentration, it decreases with increasing hole concentration and remains low for over-doping.

Oxygen isotope effect in the superconductor YBa2Cu3O7-x

Abstract Shifts in the superconducting transition in 18 O substituted YBa 2 Cu 3 O 7-x are reported. Shifts as large as 1.31 K (resistively) and 1.70 K (magnetically) are found.

Anderson flux creep model and relaxation of persistent current in grain-aligned Y1Ba2Cu3O7−δ

Abstract Flux-creep-induced relaxation of persistent current was investigated close to T c in a ring-shaped grain-aligned Y 1 Ba 2 Cu 3 O 7−σ (YBCO) with the c-axis perpendicular to the rings plane. The results revealed two distinct relaxation regimes: a steady-state logarithmic relaxation of the persistent current from an initial value J o ≅ J c with the energy barrier U eff (J) = U o (1−J/J c ) where U o ≅ 0.4 eV, and a non-steady-state (non-lo garithmic) one from J o c , with U eff (J) = constant at fixed value of time.

Specific heat measurements on multiphase BiSrCaCu oxide and BiPbSrCaCu oxide superconductors

Abstract Specific heat measurements on a Bi 1 Sr 1 Ca 1 Cu 2 O x and Bi 0.8 Pb 0.2 Ca 1 Cu 1.4 O x sample in the range 7 to 200 K are reported. There are three superconducting phases near 102, 79 and 64 K. Addition of Pb leads to an increase in the amount of 110 K phase, and a rise of the transition temperature of all phases. A fit to the low temperature data shows that the linear temperature term in the specific heat is absent.

Josephson junction network and Josephson flux pinnning in Y1Ba2Cu3O7−δ granular superconductors

Abstract The transport and magnetic properties of the Josephson junction network have been studied in granular Y 1 Ba 2 Cu 3 O 7−δ (YBCO) and Y 1 Ba 2 Cu 3 O 7−δ /Ag (YBCO/Ag) superconductors. The junction network in YBCO is of an SIS type, however, the junction network in YBCO/Ag is either an SIS or SNS type depending on the samples preparation procedure. The SIS-type junction network is characterized by strong Josephson flux pinning in contrast to the SNS-type network which exhibits weak pinning of Josephson vortices.

Trapped flux, persistent current and diamagnetic shielding in a ring and a disk of Y1Ba2Cu3O7

Abstract Trapped flux, persistent current, diamagnetic shielding and Meissner effect in a ring and a disk of Y 1 Ba 2 Cu 3 O 7 have been studied by the measurement of the distribution of magnetic field across the sample at 77K in applied magnetic field up to 1000 G. It was observed that penetration of magnetic flux into a disk depends on applied field only. The penetration of flux into the ring depends on both applied magnetic field and time. The dependence of the decay rate of trapped field on an initial trapped field revealed the vortex-persistent current interaction in the ring. The vortex motion in the ring is the combination of flux-creep and flux-flow processes.

Studies of the superconducting properties of YBCO/Ag/YBCO proximity junctions using a ceramic composite ring

Abstract We have studied the properties of YBCO/Ag/YBCO proximity interfaces in a Y 1 Ba 2 Cu 3 O 7−δ composite ring close to T c ; the junction critical current as a function of temperature (76–85K) and magnetic field (up to 50G). If a SNS junction is a part of a composite superconducting ring, the magnitude and relaxation of a macroscopic persistent current circulating around the ring provides a direct evidence that the junctions are really superconducting. We manufactured a SQUID-like ceramic ring containing two parallel SNS junctions and measured profiles of the trapped magnetic field due to the persistent current generated in the zero-field-cooled ring. The junction critical current was found to be in reasonable agreement with a formula for a SNS junction: I c (T)= I c ( O ) [1−(T/T c )] 2 .

Critical state in disk and ring of Y1Ba2Cu3O7−δ ceramic superconductor

Abstract We have studied the critical state in the zero-field-cooled disk-shaped superconducting ceramics of Y1Ba2Cu3O7−δ at 77 K over an applied magnetic field range 0–600 G. The measurements of the diamagnetic shielding field and the trapped field profiles have been performed across the superconducting ceramic disk and pressed-powder disk, in order to determine the intergrain component of shielding and trapped field. We have applied the critical state model of Daumling and Larbalestier to explain the dependence of the intergrain component of shielding in the ceramic disk of Y1Ba2Cu3O7δ on an applied magnetic field. Good agreement between this model and the experimental data on the intergrain shielding of the ceramic disk of Y1Ba2Cu3O7−δ has been obtained for applied fields above 100 G at which the transport critical current density shows weak dependence on applied field. Similar results were obtained for ceramic rings of Y1Ba2Cu3O7−δ.