38 research outputs found
Phase coexistence and critical temperatures of the (Bi, Pb)2Sr2Ca2Cu3Ox phase under partial pressures of oxygen between 10â3 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
Self-organized current transport through low angle grain boundaries in YBaCuO thin films, studied magnetometrically
The critical current density flowing across low angle grain boundaries in
YBaCuO thin films has been studied magnetometrically.
Films (200 nm thickness) were deposited on SrTiO 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
Possible superconductivity above 25 K in single crystalline Co-doped BaFeAs
We present superconducting properties of single crystalline
Ba(FeCo)As 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
A/cm at low temperatures. Upper critical field determined by resistive
transition is anisotropic with anisotropic parameter 3.5. Hall effect
measurements indicate that Ba(FeCo)As 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
Central peak position in magnetization loops of high- 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
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
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
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
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
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