GROWTH-RATE DEPENDENCE OF PHYSICAL-PROPERTIES OF YBA2CU3OX SINGLE-CRYSTALS

We present magnetization measurements on YBa2Cu3Ox single crystals coo led at various rates (0.1-1.0 degrees C/h) during the growth process. The oxygen concentration dependence of the superconducting transition temperature, T-c, as determined from the low-field magnetization, is v ery different for samples grown at different cooling rates. Crystals g rown at 1.0 and 0.7 degrees C/h display the typical T-c vs. x maximum, with a maximum T-c of 93.4 K at an oxygen concentration of x = 6.94 ( oxygenation at 480 degrees C in flowing O-2). Crystals grown at the ra te of 0.3 degrees C/h show a similar maximum in T-c vs. x, but with a maximum value of T-c of only 92 K. Crystals grown at an even slower ra te (0.2 and 0.1 degrees C/h) no longer display a maximum in T-c vs. x. For these crystals, T-c monotonically decreases with decreasing oxyge n concentration. The maximum T-c of 91 K is reached at the lowest oxyg enation temperature of 375 degrees C. This difference in T-c vs. oxyge n concentration is accompanied by significant differences in the pinni ng behavior as determined from hysteresis loop measurements. The micro scopic conditions giving rise to this behavior are not understood. (C) 1998 Published by Elsevier Science B.V. All rights reserved.