Phase coexistence and critical temperatures of the (Bi, Pb)2Sr2Ca2Cu3Ox phase under partial pressures of oxygen between 10−3 and 0.21 bar with and without additions of silver

We have investigated the stability of the (Bi, Pb)2Sr2Ca2Cu3Ox phase for the stoichiometry (Bi: Pb: Sr:Ca: Cu = 1.72: 0.34: 1.83: 1.97: 3.13), subjecting it to temperatures between 700 and 850 °C under various oxygen partial pressures. A narrow region was found in which Bi, Pb(2223) was the only superconducting phase. This region follows closely the thermal decomposition line. X-ray pure Bi, Pb(2223) will partially decompose if treated outside of the stability region. For a given oxygen partial pressure, the Bi, Pb(2223) phase tends to coexist with the 2201 phase for temperatures above, and the 2212 phase for temperatures below this region. At even lower temperatures an additional lead-rich phase appears. Critical temperatures Tc vary little with treatment and range between 108.5 K and 110.8 K. If 10% silver is added to the starting powder, the phase coexistence regions shift. Silver does not seem to have a significant effect on the absolute values of the critical temperature. The Bi, Pb(2223) thermal decomposition temperature for a given oxygen pressure is lowered by at least 10 K by the presence of A

Central peak position in magnetization loops of high-TcT_c superconductors

Exact analytical results are obtained for the magnetization of a superconducting thin strip with a general behavior J_c(B) of the critical current density. We show that within the critical-state model the magnetization as function of applied field, B_a, has an extremum located exactly at B_a=0. This result is in excellent agreement with presented experimental data for a YBCO thin film. After introducing granularity by patterning the film, the central peak becomes shifted to positive fields on the descending field branch of the loop. Our results show that a positive peak position is a definite signature of granularity in superconductors.Comment: $ pages, 6 figure

Possible superconductivity above 25 K in single crystalline Co-doped BaFe2_{2}As2_{2}

We present superconducting properties of single crystalline Ba(Fe$_{0.9}$Co$_{0.1}$)$_{2}$As$_{2}$ by measuring magnetization, resistivity, upper critical field, Hall coefficient, and magneto-optical images. The magnetization measurements reveal fish-tail hysteresis loop at high temperatures and relatively high critical current density above $J_{c}=10^{5}$ A/cm$^{2}$ at low temperatures. Upper critical field determined by resistive transition is anisotropic with anisotropic parameter $\sim$ 3.5. Hall effect measurements indicate that Ba(Fe$_{0.9}$Co$_{0.1}$)$_{2}$As$_{2}$ is a multiband system and the mobility of electron is dominant. The magneto-optical imaging reveals prominent Bean-like penetration of vortices although there is a slight inhomogeneity in a sample. Moreover, we find a distinct superconductivity above 25 K, which leads us to speculate that higher transition temperature can be realized by fine tuning Co-doping level.Comment: 4 pages, 5 figure

Self-organized current transport through low angle grain boundaries in YBa2_2Cu3_3O7−δ_{7-\delta} thin films, studied magnetometrically

The critical current density flowing across low angle grain boundaries in YBa$_2$Cu$_3$O$_{7-\delta}$ thin films has been studied magnetometrically. Films (200 nm thickness) were deposited on SrTiO$_3$ bicrystal substrates containing a single [001] tilt boundary, with angles of 2, 3, 5, and 7 degrees, and the films were patterned into rings. Their magnetic moments were measured in applied magnetic fields up to 30 kOe at temperatures of 5 - 95 K; current densities of rings with or without grain boundaries were obtained from a modified critical state model. For rings containing 5 and 7 degree boundaries, the magnetic response depends strongly on the field history, which arises in large part from self-field effects acting on the grain boundary.Comment: 8 pages, including 7 figure

Experimental evidence for fast cluster formation of chain oxygen vacancies in YBa2Cu3O7-d being at the origin of the fishtail anomaly

We report on three different and complementary measurements, namely magnetisation measurements, positron annihilation spectroscopy and NMR measurements, which give evidence that the formation of oxygen vacancy clusters is on the origin of the fishtail anomaly in YBa2Cu3O7-d. While in the case of YBa2Cu3O7.0 the anomaly is intrinsically absent, it can be suppressed in the optimally doped state where vacancies are present. We therefore conclude that the single vacancies or point defects can not be responsible for this anomaly but that clusters of oxygen vacancies are on its origin.Comment: 10 pages, 4 figures, submitted to PR

Do columnar defects produce bulk pinning?

From magneto-optical imaging performed on heavy-ion irradiated YBaCuO single crystals, it is found that at fields and temperatures where strong single vortex pinning by individual irradiation-induced amorphous columnar defects is to be expected, vortex motion is limited by the nucleation of vortex kinks at the specimen surface rather than by half-loop nucleation in the bulk. In the material bulk, vortex motion occurs through (easy) kink sliding. Depinning in the bulk determines the screening current only at fields comparable to or larger than the matching field, at which the majority of moving vortices is not trapped by an ion track.Comment: 5 pages, 5 figures, submitted to Physical Review Letter

Low field vortex dynamics over seven time decades in a Bi_2Sr_2CaCu_2O_{8+\delta} single crystal for temperatures 13 K < T < 83 K

Using a custom made dc-SQUID magnetometer, we have measured the time relaxation of the remanent magnetization M_rem of a Bi_2Sr_2CaCu_2O_{8+\delta} single crystal from the fully critical state for temperatures 13 K < T < 83 K. The measurements cover a time window of seven decades 10^{-2} s < t < 10^5 s, so that the current density j can be studied from values very close to j_c down to values considerably smaller than j_c. From the data we have obtained: (i) the flux creep activation barriers U as a function of current density j, (ii) the current-voltage characteristics E(j) in a typical range of 10^{-7} V/cm to 10^{-15} V/cm, and (iii) the critical current density j_c(0) at T = 0. Three different regimes of vortex dynamics are observed: For temperatures T < 20 K the activation barrier U(j) is logarithmic, no unique functional dependence U(j) could be found for the intermediate temperature interval 20 K < T < 40 K, and finally for T > 40 K the activation barrier U(j) follows a power-law behavior with an exponent mu = 0.6. From the analysis of the data within the weak collective pinning theory for strongly layered superconductors, it is argued that for temperatures T < 20 K pancake-vortices are pinned individually, while for temperatures T > 40 K pinning involves large collectively pinned vortex bundles. A description of the vortex dynamics in the intermediate temperature interval 20 K < T < 40 K is given on the basis of a qualitative low field phase diagram of the vortex state in Bi_2Sr_2CaCu_2O_{8+\delta}. Within this description a second peak in the magnetization loop should occur for temperatures between 20 K and 40 K, as it has been observed in several magnetization measurements in the literature.Comment: 12 pages, 10 figure

Scaling of the Equilibrium Magnetization in the Mixed State of Type-II Superconductors

We discuss the analysis of mixed-state magnetization data of type-II superconductors using a recently developed scaling procedure. It is based on the fact that, if the Ginzburg-Landau parameter kappa does not depend on temperature, the magnetic susceptibility is a universal function of H/H_c2(T), leading to a simple relation between magnetizations at different temperatures. Although this scaling procedure does not provide absolute values of the upper critical fieldH_c2(T), its temperature variation can be established rather accurately. This provides an opportunity to validate theoretical models that are usually employed for the evaluation of H_c2(T) from equilibrium magnetization data. In the second part of the paper we apply this scaling procedure for a discussion of the notorious first order phase transition in the mixed state of high temperature superconductors. Our analysis, based on experimental magnetization data available in the literature, shows that the shift of the magnetization accross the transition may adopt either sign, depending on the particular chosen sample. We argue that this observation is inconsistent with the interpretation that this transition always represents the melting transition of the vortex lattice.Comment: 18 pages, 12 figure

The pseudogap in high-temperature superconductors: an experimental survey

We present an experimental review of the nature of the pseudogap in the cuprate superconductors. Evidence from various experimental techniques points to a common phenomenology. The pseudogap is seen in all high temperature superconductors and there is general agreement on the temperature and doping range where it exists. It is also becoming clear that the superconducting gap emerges from the normal state pseudogap. The d-wave nature of the order parameter holds for both the superconducting gap and the pseudogap. Although an extensive body of evidence is reviewed, a consensus on the origin of the pseudogap is as lacking as it is for the mechanism underlying high temperature superconductivity.Comment: review article, 54 pages, 50 figure