S.Y. Ding

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 susceptibility and activated energy of high-Tc superconductor HgBa2Ca2Cu3O8+x

Abstract The real and imaginary parts of AC susceptibility of HgBa 2 Ca 2 Cu 3 O 8+ x were measured as functions of temperature, DC bias magnetic field and measurement frequency. The irreversibility lines were obtained from the temperatures where χ″ peaks appear in different DC fields. It is found that the temperature of maximum χ″ depends on the measurement frequency. The results were explained by a relaxation model with τ = τ 0 e U ( H , T )/ kT , U=U 0 ( 1 − T/T c )H − 1 2 . Furthermore, the effect of annealing treatment in flowing oxygen gas on the flux pinning of the sample was investigated.

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.

Oxygen nonstoichiometry, structural phase transitions and anomalous Pr ordering for PrBa2-xLaxCu3Oz solid solution

The oxygen nonstoichiometry, structure, magnetic and calorimetric data for La doped Ba-site PrBa2-xLaxCu3Oz systems (0≤x≤1 and 6.881≤z≤ 7392) are reported. The x-dependence of the oxygen content increases monotonically and two structural phase transitions are observed, confirmed by powder x-ray Rietveld analysis, from the orthorhombic 1212C-phase (space group: Pmmm) to tetragonal 1212 (P4/mmm) at x = 0.45, and then to another orthorhombic (Cmmm) phase around x = 0.7. Magnetic susceptibility and heat capacity measurements show that the anomalous Neel temperature TN(Pr) decreases monotonically from 18 K for x = 0 to 2.8 K for PrBa1.3La0.7Cu3O7.236 and to below 2 K for x≥0.8 for samples prepared under the same conditions, while TN(Pr) increases with an increase of oxygen content for the same compound. The increasing Pr-O bond length observed with decreasing TN(Pr) indicates that this unusual Pr magnetic ordering is closely related to the wave-function overlap between the Pr-4f orbital and eight O-2pπ orbital in the CuO2 bi-layer. No superconductivity has been detected for these insulating cuprates.

Effect of flux creep on Ic measurement of electric transport

The influence of flux creep on Ic measurement was studied by numerically solving the nonlinear flux creep equation at different dI/dt, Vc and n*, where dI/dt is the sweeping rate of the applied current (I), Ic, Vc and n* are the critical current, the criterion and the material parameter, respectively. It is shown that the V-I curve consists of two parts converging at Ip, Vp and (dI/dt)p at which the current fully penetrates the sample. In the segment where I Ip, the curve is a power law, VIn*, reflecting the material equation. It is suggested that the appropriate region in the V-I curve to determine n* is where I>Ip. Based on the V-I curve, it is concluded that if dI/dt>(dI/dt)p, Ic decreases with increasing dI/dt. On the other hand, if dI/dt<(dI/dt)p, Ic is independent of dI/dt and is therefore suitable. The three critical parameters ((dI/dt)p, Vp and Ip) are dependent on each other. The parabolic V-I relation can be observed in the giant flux creep state (small n*), whereas in the critical state (large n*), V in the parabolic V-I relation is too small to be detected by a standard voltmeter. This also indicates that the pulsed method may underestimate Ic in the case of high temperatures or in strong applied fields, but it will not affect Ic in the case of low temperatures, weak applied fields and strong pinning.

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.

Low field irreversibility line in (Hg0.66Pb0.34)Ba2Ca2Cu3O8+δ by third harmonic a.c. susceptibility measurements

Abstract First-harmonic χ1 and third-harmonic χ3 a.c. susceptibility measurements have been carried out on a Hg0.66Pb0.34Ba2Ca2Cu2O8+δ sample. The frequency dependence of the onset temperature of χ3 can be described well by T on (f) = C(2πf) 1 ((z − 1)V) + T g . Extrapolating to f = 0, we derived the vortex glass transition temperature Tg, and established the irreversibility line (IL) for the Pb-doped Hg-1223 sample. In the low-field region (H ⩽ 0.1 T), the power law H irr = 9.0(1 − T irr T c ) 1.95 fits the data rather well; however, deviation from the power law is observed in the higher-field region, indicating the transition in dimensionality from 3D to 2D vortices.

Field-induced thermally-activated dissipation and lock-in transition in a (YBCO)24/(PrBCO)2 multilayer thin film

We have measured the temperature dependence of the resistivity ρ of an epitaxial (YBa2Cu3O7-δ)24/(PrBa2Cu3O7-δ)2 multilayer nanometre film at several magnetic fields parallel to the c-axis and at a constant magnetic field H = 4 T. The measurements were taken for various angles α between the magnetic field and the surface of the film with H in the plane between the measuring current I and the c-axis, and for various angles θ between the magnetic field H and the surface of the film (ab-plane) with H ⊥ I. The resistive broadening depends mainly on the effective applied magnetic field He = H×(sin2θ + cos2θ/γ2)1/2 with respect to the c-axis, and shows a three-dimensional anisotropic behaviour, where γ is the anisotropy parameter. The angular dependences of the peak temperature Tp and the activation energy Ue for angle θ and α>5° are in agreement with the effective mass model, namely Tp(0)-Tp(H,θ) ∝(sin2θ + cos2θ/γ2)1/2n and Ue∝He-m with γ = 10, n = 1.31 and m = 0.87. The dissipative resistivity below Tp for various magnetic fields can be scaled onto a single curve by thermally activated flux motion. When the applied field tilts the CuO2 plane, the angular dependences of Tp and Ue reveal a lock-in transition, showing that the flux lines were trapped in the non-superconducting PrBCO layers.