High critical‐current densities in (Tl_0.5Pb_0.5)Sr₂Ca₂Cu₃O₉ with T_c up to 115 K

A high critical‐current density was observed in the material (Tl_0.5Pb_0.5)Sr₂Ca₂Cu₃O₉. Based on magnetic‐hysteresis measurements, this single (insulating) Tl‐O layer material has a J_c value of 1.24×10⁵ A/cm² when measured at a temperature of 77 K and a field of 1 T. This value of J_c 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 (Tl_0.5Pb_0.5)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 J_c 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 J_c is associated with an increase in the irreversibility field, rather than an increase in pinning centers.