PREPARATION OF ONE TO 3 UNIT-CELL THICK YBA2CU3O7-DELTA POWDERS AND INVESTIGATION OF THEIR MAGNETIC AND MICROWAVE PROPERTIES

Nanocrystalline YBa2Cu3O7-delta powders have been prepared by a citrat e gel modification of the sol-gel technique. Using a relatively low te mperature, T = 790 degrees C, and (i) a process of extracting oxygen a nd reoxidizing the precursor powder or (ii) a sequence of vacuum annea ling and prolonged heating in air, it was possible to avoid sintering of the particles. According to x-ray powder diffraction analysis the p articles have orthorhombic structure pertinent to the YBa2Cu3O7 phase. Measurements made by atomic force microscopy show that they have flat shape with two maxima in the thickness distribution, one at 1.2 nm (a pproximate to 1c) and the other at 3.8 nm (approximate to 3c), corresp onding to one or three orthorhombic unit cell lengths of the crystal. The average width of the particles is 40 nm. A similar value could be obtained from XRD measurements and the width of microwave absorption l ine around zero applied field. Superconducting properties of the powde rs were investigated with magnetization measurements.