Magnetic and thermal properties of the 116 K superconductor Tl-1223

Abstract

The ( Tl,Pb,Bi) 1( Sr,Ba) 2Ca 2Cu 3O 9 (Tl-1223) superconducting phase is characterized by means of X-ray diffraction, SEM and optical micrographs, AC susceptibility, resistivity, magnetization M( T, H) both in the reversible and in the irreversible regime up to 5 T, and specific heat near T c up to 14 T. Reversible magnetization data are analyzed using the London model in the low field limit, and the lower critical field is determined from the first flux penetration. Values are given for the upper critical field, μ 0 H c2c(0) = 75 T, the thermodynamic critical field, μ 0 H c(0) = 0.81 T, the lower critical field, μ 0 H c1c(0) = 0.037 T, the penetration depth, λ ab(0) = 1370 A ̊ , the Ginzburg-Landau parameter, κ( T c) = 45, κ(0) = 65, and for the mean-field specific heat jump, ΔC/ T c = 18 mJ/(K 2mol). The variation of the entropy with the field is measured near T c and agrees with magnetization data, thus demonstrating equilibrium conditions. Large magnetic hysteresis is observed only at temperatures below 20 K. The irreversibility line in the high temperature regime follows the functional dependence H irr( T)∞(1− T/ T c) 2. A systematic departure from this behaviour is observed below 70 K, possibly as a result of a phase transformation in the vortex system.