X.X. Yao

The synthesis and characterization of HgBa2Ca2Cu3O8+δ superconductors with substitution of Hg by Pb

Abstract We have successfully fabricated ceramic superconductors of Hg-1223 with substitution of Pb for Hg, i.e. Hg1−xPbxBa2Ca2Cu3Oy, by using encapsulation and short time annealing techniques. Samples fabricated in this manner may have nearly single phase structure and some enhancement of Tc can be achieved through this method. The materials of this ceramic with x ranging from 0 to 0.5 exhibit high-temperature superconductivity at temperatures falling in the range from 103 to 135 K. The sample with optimum doping of Pb of x = 0.34 and ° = 0.40 has a highest Tc of 135 K, and its diamagnetic onset temperature Tcdia = 143 K. The composition of this sample has been found experimentally and is specified by a formula of Hg0.66Pb0.34Ba2Ca1.98O8.4. The valence number for the copper constituent is given by V Cu = 2.30 . X-ray diffraction (XRD), transmission electron microscope (TEM) and electron probe microanalyzer (EPMA) analyses have been carried out for detailed investigation. Results indicate that the critical temperature for samples which were post-annealed with either oxygen flow or in high pressure of oxygen gas was increased, while that for those annealed in argon flow was decreased. Furthermore, the as-prepared samples were slightly in the under-doped state with respect to the vacancy. It was shown that Tc parabolically depends on the valence state of copper.

HgBa2CaCu2Oy superconducting thin films prepared by laser ablation

Abstract HgBa 2 CaCu 2 O y superconducting thin films were prepared on SrTiO 3 (100) and LaAlO 3 (100) substrates using laser ablation and post-Hg-vapor annealing. X-ray diffraction and electromagnetic measurements show that these films are dominated by c -axis-oriented Hg-1212 phase. For the Hg-1212/SrTiO 3 sample, a zero-resitance superconducting transition temperature ( T c0 ) of 110 K and critical current density ( J c ) of up to 10 6 A/cm 2 at 77 K and zero field have been obtained. The Hg-1212/LaAlO 3 sample exhibits slightly lower T c0 ) ( = 100 K) and J c (= 2 × 10 5 A/cm 2 ). Our investigations suggest that by using superconducting HgBaCaCuO target mercury can be incorporated into the films easily during post annealing.

Effect of current sweep rate on critical current of superconducting Ag-Bi-2223 tapes

Abstract The effect of sweep rate of applied current (d I /d t ) on critical current ( I c ) of Ag-sheathed Bi 2 Pb x Sr 2-x Ca 2 Cu 3 O y (Ag-Bi-2223) tapes was studied by the four-probe measurement method of V – I curves. It was found that the V – I curves were affected apparently by d I /d t , resulting I c , decreases with increasing d I /d t for all critical current criterions used here. This result implies that the I c determined at very large d I /d t supplied, for example, by the pule current system will be significantly smaller than the I c measured at small d I /d t usually used in the d.c. method. The possible influences of the present results on published V – I data and I c measurement technique are discussed. We have shown that the ohmic heat at the current contact leads of our Ag-sheathed Bi-2223 samples immersed in liquid nitrogen is negligible.

AC response of high temperature superconductors in the flux creep region

Abstract By numerically solving the flux creep equation, we have investigated the ac response of a high temperature superconducting slab in the flux creep region. We have shown that in the vortex glass state, where the U ( j ) relationship is a highly nolinear one, U ( j ) = ( U 00 U ( H , T )/ μ )[( j c / j ) μ − 1], the straight-line-approximation provides a good description of the flux density profile inside the slab except when both μ and U 00 / k B T c are small enough. We have also shown that in the vortex liquid state where the activation energy U depends linearly on the current density j , the straight-line-approximation breaks down when U 00 / k B T c U 00 / k B T c > 500.

Effect of electric field relaxation on the V-I curve

The effects of sweeping rate of applied current (dI/dt) on the characteristic (V - I) and electric field relaxation (V - t) curves have been investigated numerically as well as experimentally. The calculation is based on the nonlinear electric field diffusion equation, while the experiment is conducted by electric transport measurements of V - I and V - t curves on three Ag-Bi-2223 tape samples. It is found that the V - I curves shift toward smaller current with increasing dI/dt and V decays with time apparently. We show that a certain dI/dt causes a highly spatially varied electric field which is the natural result of the fact that flux lines can only diffuse over barriers with certain velocity. We also show that the electric field diffusion causes the two phenomena in the bulk sample: (1) the V - I curve is affected by the sweeping rate of the applied current; (2) the resistance relaxes with time.

Harmonic ac susceptibilities for textured YBa2Cu3O7

Abstract Harmonic ac susceptibilities were measured as a function of temperature ( T ), ac field frequency ( f ) and amplitude ( B ac ), and dc field ( B dc ) for bulk textured YBa 2 Cu 3 O 7 . The higher harmonics consist mainly of odd ones in strong dc fields ( B dc >50 mT) and depend on B dc , B ac and f . It is confirmed that the origin of harmonic generation for the textured sample is thermally activated flux diffusion over barriers U which depend nonlinearly on current density ( j ). The high nonlinear flux diffusion induces a self-organized Bean critical state in which the straight line approximation is a good description of the field profile except that the current density is j ( f ) instead of the critical value j c . Thus the frequency dependent Bean critical state accounts for the origin of harmonic generation and explains the present and other experiments well.

Microstructure, magnetization and dc transport properties of MTG-YBa1.8 Na0.2Cu3Oy crystal

The Na-doping MTG-YBCO crystal with the nominal composition YBa1.8Na0.2Cu3Oy + 40?mol% Y211 has been successfully synthesized by the melted-textured-growth (MTG) method with top seed. The microstructure photographs show that the crystal has a good layered structure with a number of twins. There is a second peak in the magnetization curve for the magnetic field parallel to the c-axis and at 77?K. The field of zero resistance temperature Tc0 shows a power law, H = H0 (Tc0(0)-Tc0(H))n with n = 1.52 for H?ab and n = 1.41 for H?c, respectively. The thermally activated flux creep model can scale the dissipative resistivity below the peak temperature Tp. The temperature and field dependences of the effective activation energy can be described by the formula U = U0(1-T/Tp)mH-?, where m = 1.75, ? = 0.75 for H?ab and m = 2.5, ? = 1.52 for H?c, respectively. The effect of Na-doping induced disorder on flux pinning has been discussed.

Logarithmic U(j) relationship in melt-textured growth YBa2Cu3O6+x by AC susceptibility measurements

Complex AC susceptibility measurements have been performed systematically on a melt-textured growth YBa2Cu2O6+x sample. The activation energy has been determined to be U(T, H, j) = U0[1 − (T/Tx)2]32(H0/H)n ln(j0/j), with n=0.46 and n=0.59 for H∥ab and H∥c, respectively. The expressions U(H)∼H−0.5 and U(j)∼ln(j0/j) obtained by the AC technique in this work agreee basically with those observed in YBa2Cu3O7 samples by transport or magnetic measurements, which may result from vortex pinning by the planar defects in the sample. We have also shown that the limited time window in conventional magnetic-relaxation measurements can be extended to much smaller values by the AC technique, which is in the range of 4 × 10−5 s−3 × 10−3 s in this work.

Dependence of flux-creep activation energy upon current density in HgBa2Ca2Cu3Oy

Abstract We have investigated the magnetic relaxation on a HgBa 2 Ca 2 Cu 3 O y superconductor over the range 4.2–60 K. Using a linear relation between U ( J ) and kT ln(1 + t / t 0 ), we extracted the effective activation energy and the time-window constant t 0 . A nonlinear dependence of U eff on J , U eff ∝ J − μ , was found. μ is about 0.98 in the temperature range 4.2–60 K, close to the value of 7 9 in the regime of flux creep in large vortex bundles predicted by the three-dimensional collective-creep theory. The time scale t 0 depends exponentially on the initial conditions. The relationship between U ( J ) and the magnetic relaxation curve is discussed. With logarithmic accuracy one can construct the U ( J ) curve simply using several long-time relaxation curves at different temperatures.

Frequency dependence of ac susceptibility in high temperature superconductors

Abstract By numerically solving the flux creep equation, we have investigated the ac susceptibilities of a superconducting slab, for the situation where the activation energy U depends explicitly on current density j as U ( j ) = ( U 0/μ )(( j c / j ) μ − 1). The calculated fundamental response (χ″ 1 ( T ) versus T curves) and third harmonic response (|χ 3 (T)| versus T curves) exhibit a strong dependence of the peak height and the peak position on frequency which compare well with the experimental data reported. The extensively used critical state model breaks down in explaining these results and therefore magnetic relaxation effects must be taken into account.