High critical-current densities in (Tl0.5Pb0.5)Sr2Ca2Cu3O9 with T c up to 115 K

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Abstract
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 associated with an increase in the irreversibility field, rather than an increase in pinning centers.