MORPHOLOGY OF THE TWIN STRUCTURE AND ELECTRICAL-CONDUCTIVITY OF YBA2CU3O7-X SINGLE-CRYSTALS

Resistivity rho(ab) of superconducting YBa2Cu3O7-x crystals with high oxygen content (x greater than or similar to 6.98) is measured in the temperature range 90-290 K. A correlation is established between the n umber of morphologically different twin substructures and the number o f anomalies on the dependence rho(ab)(T) in the range of transition to the superconducting phase. It is also established that the temperatur es of the transition to the normal (nonsuperconducting) phase in morph ologically different twin substructures form a discrete sequence with periods of similar to 0.3 K (with respect to temperature) and x simila r to 0.003 (with respect to oxygen deficit). The experimental dependen ces are interpreted in terms of the concept of normal and oxygen-defic ient lamellae, whose ratio obeys the law of integers.