EFFECT OF FLUORINE ON THE CRITICAL-CURRENT DENSITY OF THALLIUM-BASED HIGH T-C SUPER CONDUCTORS

(Tl(0.5)s, Pb-0.5)Sr1.6Ba0.4Ca2Cu3Oy/F-x high temperature superconduct ors are reported with transition temperatures above 125 K. Samples wit h different fluorine contents (0 less than or equal to x less than or equal to 3) have been prepared. The addition of fluorine has little ef fect on the transition temperature, but remarkably increases the width of the hysteresis loops (which is proportional to J(c)). The width of the hysteresis loops at 4.2 K was found to increase with x reaching a n optimum value at x = 2.2. The maximum width of the hysteresis loops is about 3 times larger than that for fluorine free material. Upon fur ther increases in fluorine content the width of the hysteresis loops s tarts decreasing.