D N Zheng

High critical‐current densities in (Tl0.5Pb0.5)Sr2Ca2Cu3O9 with Tc up to 115 K

A high critical‐current density was observed in the material (Tl0.5Pb0.5)Sr2Ca2Cu3O9. Based on magnetic‐hysteresis measurements, this single (insulating) Tl‐O layer material has a Jc value of 1.24×105 A/cm2 when measured at a temperature of 77 K and a field of 1 T. This value of Jc is comparable to that measured here for YBa2Cu3O7 (1.04×105 A/cm2), having single‐conducting Cu‐O chains, but considerably greater than the value for the material Tl2Ba2Ca2Cu3O10 with double (insulating) Tl‐O layers in which no hysteresis was observed under these same conditions. Our results from specific heat measurements indicate that the (Tl0.5Pb0.5)Sr2Ca2Cu3O9 compound has a three‐dimensional character due to the enhancement of the superconducting coupling along the c axis between the Cu‐O planes and thus results in an increase in Jc compared to the two‐dimensional Bi‐ and Tl‐2212 and 2223 [e.g., (Bi or Tl)2(Sr or Ba)2CaCu2O8 and (Bi or Tl)2(Sr or Ba)2Ca2Cu3O10, respectively] compounds. We believe that the high Jc is associ...

Temperature dependent magnetic penetration depth of Co and Zn doped YBa2Cu3O7 obtained from the AC susceptibility of magnetically aligned powders

Abstract We have measured the absolute value and temperature dependence of the magnetic penetration depth λ(T) of magnetically aligned cobalt and zinc doped YBa2Cu3O7-δ (YBCO) using a commercial AC susceptometer. For pure YBCO, our values of λab(0)≅140 nm and λc(0)≅1040 nm agree with other published values. We find increasing the Co doping increase both the fluctuation effects and the anisotropy ratio λc/λab, probably due to the reduction of coupling in the c-direction, while doping with Zn substantially increases both λab and λc. Our data imply a large, highly anisotropic gap whose maximum value is 2Δ(0)≅10kBTc. All samples show a T2 term in both λab(T) and λc(T) at low temperatures (T

Critical current, magnetic irreversibility line and relaxation in a single TlO layer 1223 superconductor☆

Abstract We have carried out systematic studies on the critical current, magnetic irreversibility line and magnetic relaxation for a c -axis orientated powdered (Tl 0.5 Pb 0.5 )Sr 2 Ca 2 Cu 3 O 9 (Tl-1223) sample. This single TlO layer compound, in comparison with the double TlO layer compound Tl 2 Ba 2 Ca 2 Cu 3 O 10 (Tl-2223), exhibited a high irreversibility line, less temperature and magnetic field dependent critical currents and a smaller flux creep rate. The substantially greater flux pinning ability of Tl-1223 can be understood in terms of its structural characteristics; in Tl-1223, there is only one insulating TlO layer between superconducting CuO 2 blocks while there are two in Tl-2223. The increase in the number (and distance) of insulating layers weakens the Josephson coupling between CuO 2 blocks along the c -axis and the results in a decrease in the value of J c . This decoupling between blocks causes the vortex lattice to break into pancake-like vortices which are easily thermally activated.

Evidence for Matthiessen's rule in the normal state resistivity of zinc doped yttrium barium copper oxide

The authors have measured the electrical resistivity rho (T) for a number of sintered samples of Y1Ba2(Cu1-xZnx)3O7 with x in the range 0.01-0.10. Above Tc, d rho /dT falls smoothly with increasing bulk density and is independent of x as confirmed by measurements on alloys of equal density. Taking d rho /dT to be independent of x, they find that the extrapolated residual resistivity ( rho res) at T=O increases linearly with x in the range O<or=x<or=0.07. These facts suggest that the linear term in rho arises from an intrinsic scattering mechanism which is independent of x, and that the substituting zinc atoms act as independent scattering centres, in accordance with Matthiessens rule for normal metals.

Critical current densities of superconducting YBa2Cu4O8 synthesised by the citrate gel process

Abstract High-T c superconducting YBa 2 Cu 4 O 8 submicron powders with transition temperature around 80 K were successfully synthesized via citrate gel process at one atmosphere oxygen pressure. An intragrain critical current density of up to 1.3×10 7 A / cm 2 at 6 K and H=1 T has been measured by magnetisation method; this value is comparable to that found in sintered YBa 2 Cu 3 O 7−δ . The integrain criticalcurrent density, as measured by an a.c. inductive technique, was 10 3 A/cm 2 at 4.2 K; this is in excess of typical values for YBa 2 Cu 3 O 7−δ .

Preparation of Ti-Ca-Ba-Cu-O single crystals with Tc up to 118 K by the step-cooling method

Single crystals of the Tl-Ca-Ba-Cu-O system with superconducting transition temperature (Tc) up to 118 K have been successfully prepared by the step-cooling method. Energy-dispersive X-ray spectrometry, selected area electron diffraction and high-resolution electron microscopy were used to determine the composition and crystal structure of the superconducting phase. The crystals with chemical composition Tl2Ca2Ba2Cu3OY are based on a body-centred tetragonal unit cell with a=3.85 AA and c=35.9 AA. Large-scale single crystals of Tl2Ca3Ba2Cu4OY, containing some intergrowths with Tl2Ca2Ba2Cu3OY and Tl2Ca4Ba2Cu5OY, were also observed. Electron spin resonance measurements revealed a low-field microwave absorption below Tc.

The effect of oxygen on intergrain critical currents of YBa2Cu4O8 and Y(Ba1.6Sr0.4)Cu4O8 ceramic samples

The authors have investigated the effect of quenching on the intergrain critical current, Jc(inter), of YBa2Cu4O8 and Y(Ba1.6Sr0.4)Cu4O8 bulk samples. The quenching process was performed by quenching the samples from different annealing temperatures into liquid nitrogen. The critical current was deduced by AC inductive measurements. It was found that Jc(inter) was progressively lowered with increasing quenching temperature. For Sr-doped samples the suppression of the Jc(inter) by oxygen removal was more appreciable than for the undoped samples. The results suggest that weak links at grain boundaries are sensitive to quenching treatment although the grains themselves are thermally stable.

The effect of doping on the irreversibility line of oxide superconductors

YBa2Cu3O7 samples have been doped with cobalt and zinc. Measurements of the upper critical field Hc2 and irreversibility line were made in order to investigate the effect of doping. It is found that delta Hc2/ delta t (t=Tc/T) is reduced by doping but the irreversibility line is much more strongly affected particularly in cobalt doped sample. They attribute the lower irreversibility line in the cobalt doped sample to the higher degree of anisotropy.

Granular effects in superconducting YBa2Cu4O8 ceramic

Abstract Two characteristic transition temperatures have been observed in superconducting YBa 2 Cu 4 O 8 ceramic by means of an ac inductive method. Our susceptibility data from a powdered sample, and a bulk sample at different magnetic field strengths, show that the two temperatures correspond to superconducting transition at the intergrain region T c (inter) and at the intergrain region T c (intra) ; the latter gives a weak diamagnetic signals. We found that T c (inter) was gradually lowered when the sample was subjected to increasing quenching temperatures, while T c (intra) was unaffected by such treatment, the latter being consistent with the oxygen stability of YBa 2 Cu 4 O 8 . The small susceptibility signals occuring at T c (intra) will be discussed.

Critical current and penetration depth in (Tl0.5Pb0.5)(YxCa1-x)Sr2Cu2O7- delta

The authors report a series of Jc measurements made on (Tl0.5Pb0.5)(YxCa1-x)Sr2Cu2O7- delta , over a range of x. The results show a clear peak in intragrain Jc with x corresponding to the peak in Tc, but a continuous decrease in intergrain Jc. This lack of correspondence is unique amongst the doped materials they have studied. The behaviour of the London penetration depth with x has also been estimated. The data indicate an increase in lambda L across the range, but a coincident fall in the average particle radius makes this conclusion uncertain.