120-K Tl-based high-T_c superconducting thin films on LaAlO ₃ substrates

The preparation and properties of 120-K Tl-based high-temperature superconducting thin films on LaAlO3 substrates are reported. These films are obtained by post-annealing 2Tl: 2Ca: 2Ba: 3Cu: 10O precursor films at ≅890°C in a sealed quartz tube filled with ≅ 1-atm O2. Results of ac susceptibility measurements show a sharp drop in the real part of the susceptibility (χ′) and a sharp maximum peak in the imaginary part of the susceptibility (χ″) at a temperature close to the observed zero resistance Tc of ≅ 120 K. The broadening of the transition width in χ′ and the decrease in the temperature where χ″ maximum appears are found to occur at a slower rate with increasing ac field for these films than for the films deposited on Y-ZrO2 and MgO and having a similar zero resistance Tc. This weaker field dependence of χ′ and χ″ is shown to lead to a higher critical current density for the films deposited on LaAlO3 substrates. The higher critical current density is attributed to a stronger coupling between the grains arising from the absence of the formation of well-defined grain boundaries during film growth.